ISSN:0003-2654    

DOI:10.1039/AN97500FX041

出版商:RSC    

年代:1975

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN97500BX043

出版商:RSC    

年代:1975

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN97500FP113

出版商:RSC    

年代:1975

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN97500BP117

出版商:RSC    

年代:1975

数据来源: RSC

摘要:

NOVEMBER, 1975 The Vol. 100, No. 11 96 Thiazolylazo Dyes and Analytical Chemistry A Review" HAvard R. Hovind Their Applications in Institute of Pharmacy, University of Oslo, Box 1068, Blindern, Oslo 3, Norway Summary of Contents Introduction Thiazolylazophenol dyes Preparation and purification General properties Complexing properties Applications in spectrophotometry Extraction - photometric applications Applications as indicators Other applications in analytical chemistry Preparation and purification General properties Spectrophotometric investigations Extraction - photometric applications Applications as indicators Thiazolylazonaphthol dyes Conclusions Introduction The following survey is concerned with the properties and analytical applications of the azo dyes that are prepared from 2-aminothiazole derivatives by diazotising them and coupling the product with phenolic substances.The compounds most frequently used are 4-(2'-thiazolyl- azo)resorcinol (TAR) and 1-(2'-thiazolylazo)-2-naphthol (TAN), which represent the two main classes of thiazolylazo dyes. A large number of derivatives of these two compounds have been prepared and investigated for analytical purposes. These dyes have attracted much attention as they are sensitive chromogenic reagents in addition to being interesting complexing agents, and have been used as reagents for spectro- photometric and extraction - photometric determinations of many metal ions. Some of them have also proved to be particularly useful as indicators in complexometric titrations, owing to the low stabilities of the metal - indicator complexes.This review deals with the thiazolylazophenol and the thiazolylazonaphthol derivatives separately and covers the literature from 1966 up to the end of 1974, the earlier literature having been reviewed by Navriiti1,l and by Anderson and Nickles2 Other reviews relating to this subject, by Busev, Ivanov and Krysina,3 and Wada,* have appeared. Related com- pounds that are applied only for the purpose of dyeing fibres are not considered here. Thiazolylazophenol Dyes Preparation and Purification These compounds are easily prepared as, compared with the analogous 2-aminopyridines,6 no special conditions are required for the diazotisation of 8-aminothiazole and its derivatives.6 The diazotisation reaction occurs readily in concentrated solutions of hydrochl~ric,~-l~ phosph0ricl&~3 and s u l p h ~ r i c ~ ~ - ~ ~ acids, or in a mixture of dimethylformamide and sulphuric acid.27-29 The diazotate formed is so reactive that it has to be coupled with the phenol in * Reprints of this paper will be available shortly.For details see summaries in advertisement pages. 769770 HOVIND : THIAZOLYLAZO DYES AND THEIR Analyst, Vol. 100 cold acidic solution, which often results in the immediate production of the dye in a crystalline state. However, these products have to be freed from impurities such as inorganic salts, organic raw materials that result from the synthesis, other azo dye derivatives and an undefined water content. Usually the dye is purified by repeated crystallisation from ethanol; TARK has been recrystallised from butan-1-01.11 Dissolution of the dye in alkali-metal hydroxide solution, followed by extraction with diethyl ether and subsequent re-precipitation with dilute hydro- chloric acid, has proved to be useful for the purification of TAR,30 and TAMP has been puri- fied by sublimation under reduced pressure.31 The water content is removed by drying the dye over calcium chloride in a desi~cator,~~-3* or by drying the dye to constant mass at 120 0C.35936 The azo dye can be separated from impurities such as phenols and other azo dye derivatives by paper ~hromatography,~~~J~~~~~~-38 by thin-layer chromatography on silica ge116933939-44 or by chromatography on aluminium 0xide.~5 These methods, in addition to potentiometric titration of the azo group with chromium(I1) ~hloride,9~13~28~32-36~3*~39~~~~47 and photometric titration with copper(I1) sulphate in aqueous ethanolic s o l ~ t i o n , ~ ~ ~ ~ ~ have also been used for control of the purity of the dyes.General Properties The thiazolylazophenol derivatives surveyed here are listed in Table I. Nearly all of these compounds are red, violet or brownish in their crystalline state, and only a few are readily soluble in water, most of them being only partly soluble or insoluble. However, their solu- bility can be increased by the addition of organic solvents such as ethanol, dimethylfonnamide and acetone. It is greater in less polar solvents, e.g., chloroform, cyclohexanone and butan-1- 01, than in water, and this property is used as the basis of extraction procedures. Of the compounds cited, TAR is the most widely used reagent ; it is soluble in strongly acidic solution, giving a red cation with an absorption maximum at 488 nm, and in strongly alkaline solution, giving a red - violet anion with an absorption maximum at 510 nm ( E = 34 500).To neutral and dilute acidic solutions TAR imparts a yellow colour, and to dilute solutions of alkali an orange - red colour. These hypsochromic and bathochromic shifts, which are due to protonation and ionisation, respectively, correspond to the acid - base equilibria48 488 nm 0- 410-440 nm 11 HO, 510 nm 470 nm The acid-dissociation constants have been determined for most of these compounds, and the results obtained are listed in Table 11. Have1 and K u b h used a computer program for the calculation of the dissociation constant of TAMP.49 As the o-hydroxy group of the TAR derivatives is involved in intramolecular bonding to the azo group, the first proton to be dissoci- ated is from the p-hydroxy group.50 who investigated the intramolecular hydrogen bonding in some azo dyes, related the dissociation constant to the electrostatic effects of the ring substituent on the electronegativity of the azo nitrogen acting as a proton acceptor.The deprotonation of the o-hydroxy group usually proceeds more slowly when the hydrogen bond becomes stronger. Addition of different groups to the phenolic nucleus of the dye affects the acidic properties of the hydroxy group@); thus, a nitro group intensifies the acidic properties of TAR.l2 The effect of substitution in the thiazole ring is usually less significant, except for the introduction of bromine in the 5-p0sition,~~ which also intensifies the acidic properties of the compound. A decrease in the absorptivity when the pH is constant and the ionic strength of the solution is changed shows the absence of intermolecular association of the dyes in solution.9 Inskeep etNovember, 1975 APPLICATIONS IN ANALYTICAL CHEMISTRY.A REVIEW TABLE I SURVEY OF THIAZOLYLAZOPHENOLS A B 771 Abbrevi- ation TAR MeTAR KTAR PhTAR p-NTAR BrTAR STAR PBTAR TAR-R TARK STARK TARN STARN TAO TAPhl O-TAP TAMR TAAMP TAM TAAR NTAAR TAAC MeTAAC TAH TAC TAMP TAEP TACl TABr Position of substituent group r h \ A OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH B H H H H H H H H H H H NO2 NO2 H H H H H H H H H H H H H H H H C OH OH OH OH OH OH OH OH OH OH OH OH OH OH OH H OCH, NH2 N(CHd2 D E H H H H H H H H H H H H H H H H COOH H H H H H H CH3 H OH H H H i r H H H H N(C2H6)2 N(C2H.5)2 NHC2H, CH, NHC2H, CH, H OH H CH, H OC2HE.H OCH, H c1 H Br H H H H H H H H H H Q CH3 H COOH ‘13~45 CIOH, H H C6H6 H H H H H H H H H H H H H H CH, H H H H H H R Systematic name Reference* H 4- (2’-Thiazolylazo) resor- H 4-(4‘-Methyl-2’-thiazolyl- H 4-( 4’-Carboxy-2’-thiazolyl- H 4-( 4’-Phenyl-2’-thiazolyl- H 4- (4‘-8-Naphthyl-2’-thiazol- Br 4-( 5’-Bromo-2’-thiazolyl- SO,H 4-( 5’-Sulpho-2’-thiazolyl- C,H,CO 4-(4’-Phenvl-5’-benzovl- cinol 9 azo) resorcinol - azo) resorcinol 13 azo)resorcinol - ylazo) resorcinol - azo) resorcinol 14 azo) resorcinol 12 H H SO ,H H SO,H H H H H H H H NO2 H H H H H H H H 2’-thiazolylazo)resorcinol - 4- (2’-Thiazolylazo) -6-alkyl- 5- (2’-Thiazolylazo) -2,4-di- 5- (5’-Sulpho-2’-thiazolylazo) - 2,4-dihydroxybenzoic acid - 4- (2’-Thiazolylazo) -2-nitro- resorcinol 12 4- (5’-Sulpho-2’-thiazolylazo) - 2-nitroresorcinol - 4-( 2’-Thiazolylazo) orcinol - 4- (2’-Thiazolylazo) phloro- glucinol - 2- (2’-Thiazoly1azo)phenol 24 2- (2’-Thiazolylazo) -5-methoxy- phenol - 2- (2’-Thiazolylazo) -5-amino- phenol - 2-( 2’-Thiazolylazo) -5-dimethyl- aminophenol - 2- (2‘-Thiazolylazo) -5-diethyl- aminophenol 10 2-( 5’-Nitro-2’-thiazolylazo) - Ei-diethylaminophenol - 2-(2’-Thiazolylazo) -&ethyl- amino-p-cresol 10 2-( 4’-Methyl-2’-thiazolylazo) - 5-eth ylamino-p-cresol - 2- (2’-Thiazolylazo) hydro- 2-(2’-Thiazolylazo) -p-cresol - 2-(2‘-Thiazolylazo) -4-methoxy- 2- (2’-Thiazolylazo) -4-ethoxy- 2-( 2’-Thiazolylazo) -4-chloro- 2- (2’-Thiazolylazo) -4-bromo- resorcinol (n = 1-6) 7 hydroxybenzoic acid 11 - quinone phenol - - phenol phenol phenol 24 - * References only to those publications which give a description of the synthesis of the compound.continued772 HOVIND : THIAZOLYLAZO DYES AND THEIR TABLE I-cofitinnued Position of substituent group Abbrevi- , A \ ation A TAI OH TADCl OH TACMe OH TAMe346 OH TAMe345 OH $-TAP H TAPC H PBTAPC H CETAPC H B H c1 CH, CH3 H H OH OH OH C H H H H CH, OH OH OH OH D E I H c1 H CH, H CH, CH3 CH, CH, H H H H H H H H Q H H H H H H H Cf3H6 HOCH, R H H H H H H H C,H6C0 C2H602C A B E D Position of substituent group A Abbrevi- I - ation A BTAR OH BrBTAR OH BrBTAO OH BTAC OH BrBTAC OH MeBTAC OH MOBTAC OH BTAAC OH BTAMP OH BTACl OH BTADCl OH B H H H H H H H H H H c1 C D E OH H H OH H H OH H CH3 H CH, H H CH3 H H CH, H H CH, H NHC,H, CH, H H OCH, H H C1 H H C1 H Q Analyst, Vol.100 Systematic name Reference phenol 24 2-(2’-Thiazolylazo) -4-iodo- 2- (2’-Thiazolylazo) -4,6- 2-(2’-Thiazolylazo) -4,6- 2-(2’-Thiazolylazo)-3,4,6- 2-(2‘-Thiazolylazo)-3,4,5- 44 2’-Thiazoly1azo)phenol - 4-(2’-Thiazolylazo)pyrocatechol- 4- (4’-Phenyl-S’-benzoyl-2’- thiazo1ylazo)pyrocatechol 16 4- (4’-Hydroxymethyl-5’-carb- ethoxy-2’-thiazolylazo) - dichlorophenol - dimethylphenol - trimethylphenol - trimethylphenol - pyrocatechol 19 R H Br Br H Br CH, OCH, H H H H Systematic name Reference 4-(2’-Benzothiazolylazo)resorcinol 28 4- (6’-Bromo-2’-benzothiazolylazo) - resorcinol 28 4- (6’-Bromo-2’-benzothiazolylazo) - orcinol 27 2-(2’-Benzothiazolylazo) -p-cresol 25 2- (6’-Bromo-2’-benzothiazolylazo) - 2-(6 -Methyl-2’-benzothiazolyl- 2- (6’-Methoxy-2‘-benzothiazolyl- 2- (2’-Benzothiazolylazo) -&ethyl- 2- (2‘-Benzothiazolylazo) -4-methoxy- 2- (2’-Benzothiazolylazo) -4-chloro- 2- (2’-Benzothiazolylazo) -4,6- p;cresol 25, 27 azo) -p-cresol 25 azo) -p-cresol 25 amino-p-cresol 29 phenol 16 phenol 15 dichlorophenol 26 Position of substituent group Abbrevi- A \ ation A B C D E Q R Systematic name Reference HBTAR OH H OH H H H 2H 4-(4‘,5‘,6‘,7‘-Tetrahydro-Z‘- benzothiazolylazo) - resorcinol 18 couttinzuedNovember, 1975 APPLICATIONS I N ANALYTICAL CHEMISTRY.A REVIEW TABLE I-continued Position of substituent group A Abbrevi- 7 ation A DHBTAR OH DTAR OH p-DHBTAP H fi-DTAP HBTAPC DTAPC HBTAAR DHBTAC DTAC DTAF DTACl DTABr DTAO DTAPhl DTADM Compound TAR H H H OH OH OH OH OH OH OH OH OH B H H H H OH OH H H H H H H H H H C OH OH OH OH OH OH D H H H H H H H H H H H OH OH CHI E H H H H H H H H H H H H CH3 OH H TABLE I1 R 2H 0 2H 0 2H 0 2H 2H 0 0 0 0 0 0 0 773 Systematic name Reference 4-( 5’, 5’-Dimethyl-4’,5’,6’, 7‘- tetrahydro-2’- benzothiazolylazo) - resorcinol 4- (5’,5’-Dimethyl-7’-oxo- 4’,5’,6’,7‘-tetrahydro-2’- benzothiazolyl- azo) resorcinol 4- (5’, 5’-Dimethyl-4’, 5’, 6’, 7‘- tetrahydro-2’- benzothiazolylazo) - phenol 4- (5’,5‘-Dimethyl-7’-oxo- 4’,5’,6’, 7’-tetrahydro-2’- benzothiazolylazo) - phenol 4-( 4’, 5’, 6’, 7’-Tetrahydro-2’- benzothiazolylazo) - pyrocatechol 4-( 5’, 5’-Dimethyl-7’-oxo- 4’,5’,6’,7‘-tetrahydro-2‘- benzothiazolyl- azo) pyrocatechol 2-(4’,5’,6’,7’-Tetrahydro-2’- benzothiazolylazo) -5- diethylaminophenol 4-( 5’, 5’-Djmethyl-4’,5’,6’,7’- tctrahydro-2’- benzothiazolylazo) -p-cresol 4’,5‘,6’, 7’-tetrahydro-2‘- benzothiazolyl- azo) -9-cresol 4‘,5‘,6‘,7‘-tetrahydro-2‘- benzothiazolyl- azo) -4-fluorophenol 2- (5’,5’-Dirnethyl-7’-0~0- 4‘,5‘,6‘,7‘-tetrahydro-2‘- benzothiazolyl- azo) -4-chlorophenol 2- (5‘, 5’-Dimethyl-7’-oxo- 4’, 6’, 6’, 7’- tetrahydro-2‘- benzothiazolyl- azo) -4-bromophenol 4- (5‘, 5‘-Dimethyl-7‘-oxo- 2- (5’, 5’-Dimethyl-7’-oxo- 2- (5’, 5’-Dime thyl- 7’-oxo- 8 21 8 21 17 - 20 - 21 23 23 23 4‘, 5‘, 6’, 7‘- tetrahydro-2‘- benzothiazolyl- 4‘,5‘, 6’,7’-tetrahydro-2’- azo) phloroglucinol 2- (5’, 5’-Dimethy]-7’-oxo- 4‘,5’, 6’, 7’-tetrahydro-2’- benzothiazol ylazo) - azo) orcinol 21 benzothiazolyl- - 4-( 5’, 5’-Dimethyl-7’-oxo- 4,5-dimethylphenol 22 ACID-DISSOCIATION CONSTANTS OF THIAZOLYLAZOPHENOLS Spectrophotometric method used, except where otherwise indicated. PKNH PKn-OH PKO-OH Solvent P Reference 0.96 6.23 9.44 Water 0.1 48 - 7.45 - Water 0.05 7 1.65 f 0.11 7.37 f 0-03 12.80 f 0.04 50% dioxan* 0.01 30 - 6-53 10.76 50% methanol* 0.1 69 * Potentiometric method.continued774 Compound TAR KTAR PhTAR p-NTAR RrTAR STA4R PBTAR TAR-Me TAR-Et T14R-Bu TAR-Pr TAR-Pe TAR-He TAliK STARK TARN STARN BTAR BrBTAR HBTAR DHBTAR TAPC PBTAPC HBTAPC DTAPC O-TAP TAMP TAMP TAAR TAAC TAC TAMR TACl TABr 0.45 f 0.10 - 1-33 - 2-41 - 1 f 0.03 0.47 - 0.97 -0.51 - -0.3 -& 0.03 - < 0.3 - - 0.68 0.00 f 0.04 HOVIND: THIAZOLYLAZO DYES AND THEIR TABLE II-contiwed pK,-OE 6-7 6-15 f 0.22 - 6.2 6.0 6.23 - 6.56 6-23 6.17 6-15 & 0.22 5.90 6-3 6-7 5.95 f 0.02 5.85 f 0.04 7-09 rf 0.15 5.94 5.52 7.61 7-63 7.70 8.04 8-75 8-98 7.90 f 0.21 7.40 7.48 f 0.19 7.93 f 0-04 6-95 f 0.19 5.79 & 0.08 - - - - 5.85 f 0.34 7.05 f 0.02 6.47 7.36 6.95 f 0.05 6-24 -& 0.01 7.20 f 0.02 7.24 f 0.02 7.8 1 5-60 - - - - - - - - - - - - - - - - - - - - - - - - - - * DMF = dimethylformamide. t P K , , , .- ~ ~ . P K o - O H 10.8 9.67 f 0.15 9-68 f 0.19 9.4 9.3 9.44 9.15 10.54 9.44 9.49 9.68 f 0.19 9.80 9.6 10.8 10.50 9.93 f 0.17 8-72 f 0.03 - - 11.68 10.03 f 0.10 - - - - - 9-98 12.70 f 0.14 11.6 - - - 10.27 f 0.23 10.6 10.12 10.01 f 0.52 9.91 11.93 f 0.03 11-20 11.14 f 0.077 1.0-26 f 0.02t - - - 11.99t - 7.36 7.4 8.13 & 0.02 8.12 & 0.05 8.45 7.90 8.303 & 0.017 9.07 7.83 f 0.03 8.38 f 0.03 8.16 f 0.03 8.92 f 0-03 7.83 9.32 9.16 8.95 f 0.1 8.31 8.36 0.02 6.70 f 0.04 7.08 f 0.04 7-36 7.2 1 7.2 7-35 7.4 Solvent 50% methanol Water Water Water Water 30% DMF* Water 30% ethanol Water 2% dioxan 20% acetone Aqueous ethanol Water 20% dioxan Water 20% acetone Water Conc.H,SO, Aqueous ethanol 36% ethanol Water Water Water Water Water Water Water 20% acetone 20% ethanol Conc. H,SO, Water Conc. H,SO, 50% methanol Water Water 30% ethanol 40% ethanol Aqueous ethanol 2% ethanol Aqueous ethanol Butan- 1-01 10% ethanol 40% ethanol 10% ethanol 10% dioxan 10% dioxan Water Water 30% ethanol 30% dioxan 30% ethanol 30% ethanol Water 30% DMF 30% methanol 30% ethanol 10% dioxan Aqueous ethanol Aqueous ethanol 50% methanol 10% dioxan Aqueous dioxan Water Water 30% ethanol 10% dioxan 10% dioxan 10% dioxan 10% dioxan 35-40% DMF 35-40% DMF Analvst, Vol.100 J P 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.01 0.1 0.1 0.1 0.1 0.2 - - - - - 0.1 0.05 0.1 0.05 0.05 0.05 0.05 0.05 0.1 0.1 0.2 0.1 0.1 0.1 0.1 0.1 0.01 0.1 - - - - 0.5 0.5 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 - - - 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0-1 Reference 69 51 9 35 55 68 47 39 39 52 14 13 55 55 47 14 12 81 105 89 7 51 7 7 7 7 7 11 83 81 12 81 28 69 59 25 59 18 54 61 72 16 16 17 45 75 24 79 37 38 38 99 49 41 42 42 42 42 31 10 10 69 117 107 37 38 38 24 79 24 79 continuedNovember, 1975 APPLICATIONS IN ANALYTICAL CHEMISTRY.A REVIEW TABLE II-continued - TADCl - TACMe - TAMe346 - TAMe345 - DHBTAC - 0.09 HBTAAR 2.96 0.03 DTADM - DTAF - DTACl - DTABr - PKo-OH 7.25 7.3 5-3 10.89 10.30 9.67 10.80 f 0.02 7.77 f 0.03 7-35 6-93 6-80 8.82 f 0.01 Solvent 10% dioxan 10% dioxan 10% dioxan Aqueous dioxan Aqueous dioxan Aqueous dioxan Aqueous ethanol 38:/0 ethanol 4004 ethanol 48% ethanol 48% ethanol 48% ethanol 775 p Reference 0.1 24 0.1 79 0.1 118 0.1 107 0.1 78 0.1 78 0.1 80 - 20 0.1 22 0.1 23 0.1 23 0- 1 23 Complexing Properties The dyes give coloured complexes with most metals, stable chelates being formed, especially with some of the transition metals. In acidic and slightly acidic solutions, the metals form complexes with the metal to ligand ratio of 1 : 1 or a mixture of complexes with ratios 1 : 1 and 1 : 2; in alkaline solutions, the equilibrium is usually displaced towards the 1 : 2 complex.In addition to the normal complexes, some metals form protonated complexes of the type MRH32935936p48s52 with TAR derivatives in acidic solutions, and an increasing amount of M(RH), is formed when the pH is r a i ~ e d . ~ ~ ~ ~ ~ ~ ~ 3 , 5 3 , ~ ~ In organic solvents the most common composition of the complexes is 1:2. Minczewski et aL55 reported that lanthanum(II1) and TAR formed a complex with a metal to ligand ratio of 1 : 3 in neutral and slightly alkaline solution. Under the conditions of the sys- tems investigated, 1 : 3 complexes are also formed between thorium(1V) and TAR,56 scandium- (111) and TAR,57 cobalt(II1) and TAAR,58 BTAR and BrBTAR,59 and germanium(1V) and gallium(II1) with PBTAPC.60961 Gusev et al.reported a complex with the strange composi- tion of 1 :1-5 between vanadium(V) and TAAR,1° a possible equilibrium between the 1:2 and 1 : 4 complexes for cadmium(I1) and BrBTAC2 and for TAC and BTAC with ~ i n c ( I I ) . ~ ~ Mixed complexes have also been reported. With an excess of the second ligand, titanium- (IV) formed a ternary complex with TAR and the acetate ion or the hydrogen peroxide molecule, the composition being 1 : 1 with respect to TAR.55 Analogous complexes have been reported for thallium(II1) and TAAC, with the acetate ion as the second ligand,1° for molyb- denum(V1) and TAR, with hydroxylamine as the second ligand,64 and for antimony(III), which could be extracted as the HR[SbI,]R complex with TAAC.1° Gallium(II1) formed a mixed hydroxy complex of the type Ga(OH)(HR), with HBTAPV5@ and HBTAR67 in aqueous ethanolic solution, and probably a complex of the type Ga(OH)(HR)+ in slightly acidic solution.36 The complexing reaction with some of the transition elements is very slow because of the slow substitution of the aquo-complexes of these metals. Thus Nickless et al.69 showed that TAR reacts slowly with platinum, palladium, gold, rhodium, chromium, osmium, ruthenium and iridium ions, the complexes being formed only on standing for several days or on heating, which was confirmed by Busev et ~ 1 .~ ~ for the rhodium(II1) ion, maximum absorption being reached after heating for 15 min in boiling water; on further heating the complex decomposed.The increased reaction rate at elevated temperature is made use of in the titration of nickel(I1) with solutions of TAR,S2 T A P and TAMP,37 the sample solution being heated to 60-70 "C before reaching the end-point. Evidence has been presented which suggests that TAR acts as a terdentate ligand towards metal ions.30 For a thiazolylazo dye with a hydroxy group in the ortho position relative to the azo group, the metal ion is bonded to the oxygen of the hydroxy group, the azo group and the nitrogen atom of the thiazole ring, forming two five-membered rings. The $-hydroxy derivative, $-TAP, on the other hand, formed appreciably strong complexes only with certain heavier metals and scarcely any with metals of the first transition series.69 The pyrocatechol derivatives, however, form stable complexes with the metal ions gallium(III),16~61~s5 alumin- ium( 111) ,16JlY72 germanium( IV) ,16960 tungsten(V1) ,16973 tin( IV)74 and antimony(II1) .74 Other metal ions also form complexes with these ~ o m p o u n d s .~ ~ , ~ ~ ~ ~ ~ Ruthenium( IV) formed an oxychloride complex with TAR.68776 HOVIND : THIAZOLYLAZO DYES AND THEIR Analyst, Vol. 100 Some metal ions form complexes with TAR and its derivatives at a pH that is too low to permit dissociation of the 9-hydroxy group, and a proton is displaced from the o-hydroxy group, although this would be the last proton to dissociate. According to Chalmer~,~~ there k211 are two ways of ascribing dissociation constants to such compounds, with slightly different results.As HR-* has conventionally been taken to be the chelating species, k2* would need to be known for the stability constant of the complexes to be calculated. Some authors have used k , as an approximation to k2*, while Cha1me1-s~~ has stated that the use of k , is to be pre- ferred as the difference between these two constants is due entirely to the inductive effect of the dissociated p-hydroxy group. A third point of view is expressed by C0rsini,~6 using the normal dissociation constant for the calculation of stability constants of complexes. Usually the least electropositive elements form the strongest complexes, because of the covalent character of the bonding, and the Irving-Williams’ order is obeyed for the first transition series.69 The stability constants of the 1 :1 complexes with the rare earths increase with the atomic number of the metal ion.The thermodynamic data indicate that the lanthan- ide complexes of TAC, TAMP and TACl are of the inner-sphere type, while transition metal complexes usually are of the outer-sphere type.,l~~~ TAC analogues, substituted in the 3-position in the phenolic ring, were reported to give less stable complexes because of steric hindrance in the formation of the complex by, for example, the methyl group in the 3-positi0n.~~ Of the closely related derivatives o-TAP, TAC1, TABr and TAI, only TACl formed complexes that were stable enough to be practically suitable for analytical purpose^.^^^^^ Applications in Spectrophotometry TAR appears to be similar in character to the well known reagent PAR [4-(2-pyridylazo)- resorcinol]. However, as a result of the lower basicity of the thiazole nucleus, substitution of the pyridine ring with a thiazole ring will lead to a shift of the reaction for the formation of the complex towards the more acidic region. The resulting dye is therefore more reactive against metal ions in acidic solutions than the pyridyl analogues.The thiazolylazo dyes form coloured complexes with many metals in acidic solutions. Armeanu and Dragusin26 have shown that cadmium forms a 1 : 2 complex with BTADCl at pH 5.5, which is one of the few cadmium complexes with maximum absorption in acidic solution. Some other metal ions can even be determined in highly acidic solutions.Thus thallium(II1) forms a 1 : 1 complex with TAR at pH 1-2,35 mercury(I1) a blue complex with TAMP a t pH 1.5-2,41 gallium(II1) a strongly coloured complex ( E = 69 000) with BTAAC at pH 3,29 and copper(I1) complexes with DHBTAR54 and DHBTACBo at pH below 1. Thus palladium(I1) forms blue complexes with STAR, STARK and STARN in highly concentrated sulphuric acid solutions,Bl the colour change being greater than that which occurs in weakly acidic solution.82 Very contrasting colour changes have also been observed for the complex formed between copper(I1) and DTADM,22 DTAF, DTACl and DTABr2, in acidic solutions. The colour change of the uranium(V1) complex with TAMR3s is greater than those with and TARK83 and, in spite of its weaker colour intensity, the use of the former dye is therefore preferred in the photometric determination of uranium(V1).In regard to the reactivity against metal ions, Shimidzu and Uno15 have shown that the 2-benzothiazolylazo derivatives were superior to the 4- and 7- derivatives, the colour change being greatest for BTAMP and BTAC1. Benzothiazolylazo dyes, in contrast to the thiazolyl- azo derivatives, do not form coloured complexes with niobium(V) in any media,s4 which result was associated with the redistribution of the electron density in the heterocyclic portion of the molecule under the effect of the adjoining benzene ring. Moreover, the colour intensity of the Some of the reactions for complex formation lead to very contrasting colour changes.November, 1975 APPLICATIONS IN ANALYTICAL CHEMISTRY.A REVIEW 777 cobalt(I1) complexes with TAR34s47 and BrTAR47 was nearly twice as great as that with BTAR and BrBTAR59 In the study of vanadium(V) complexes with some organic reagents, Shalamovs5 indicated that the presence of a methyl group in the para position relative to the hydroxy group de- creases the selectivity and sensitivity of the complexing reaction ; TAACs5 and TAARs6 are still being used for the determination of trace amounts of vanadium. On the other hand, Maltseva and Elinsons4 concluded that these two dyes were not only more selective reagents for niobium(V) in acidic solution, but were also more sensitive than the analogous pyridylazo compounds, the absorptivity of the former compounds being twice that of the latter. I t has also been shown that the determination of niobium(V) with TAAR is more sensitive than that with thioc~anate,~~ and Sommer and Ivanov pointed out that TAR was a more selective reagent for uranium(V1) than PAR, but that the former was slightly less sensitive.32 The pyrocatechol derivatives are shown to be highly sensitive and selective reagents for germanium(IV),60 gallium(II1) ,61 al~miniurn(III)~~s~~ and t~ngsten(VI),~~ the E value for the tungsten complex with PBTAPC being 70 000.TAAR and TAAC are highly sensitive re- agents for cobalt(II), the values for E being 64 000 and 57 800, respectively.88 Gallium(II1) forms strongly coloured complexes with some of the dyes, the value for E being more than 65 OO0,20s29989 and can therefore be determined in concentrations down to about 10 p.p.b.(parts per lo9) with HBTAR1ss67 and PBTAR.sg The determination of scandium(II1) with TAR is reported to be more satisfactory than with any of the earlier reagents,57 and that of nickel(I1) with STARN is four times as sensitive as the method in which dimethyl- glyoxime is used.g0 In the colorimetric determination of unesterified fatty acids, the colour intensity with TAC was twice that given with the usual copper(I1) reagents,91 and TAC has therefore been used for the determination of serum fatty acids.92 HniliCkovA and S ~ m m e r ~ ~ showed that the choice of buffer solution is very important as it affects the sensitivity of the method, and the common buffer solutions (acetate, hexamine, biphthalate and formate) decreased the absorptivity in acidic solutions, while phosphate buffer did not.Serious interference from copper in the determination of different transition metals is masked by thiosemicarbazide, for example, in the determination of zinc in potable watersg3 In the determination of cobalt (11) with TAR, copper can be masked with sodium thio~ulphate,3~ and can also be separated electrolytically.94 In the determination of uranium(V1) with TAR, several interfering ions were masked with sodium fluoride, cyclohexanediaminetetraacetic acid or 5-sulphosalicylic a ~ i d . ~ ~ s ~ ~ The metal ion to be determined can also be separated initially, e.g., cobalt (11) in nickel ores, can be separated with aluminium hydroxide,94 molyb- denum(V1) in copper ores with benzoin ~ - o x i m e ~ ~ and small amounts of scandium in silicate rocks by ion-exchange chr~matography.~~ In a field method for the determination of zinc with TAR, ion exchange is applied in order to remove iron c~ntamination.~~ For the deter- mination of niobium in steel, the interferences are removed by extraction with isobutyl methyl ketone.97 An analogous series of thiazolylazo derivatives have been introduced by Budesinsky and V r z a l o ~ a , ~ ~ who proposed thiazolylazothiophenols as spectrophotometric and radiometric reagents for trace metals.The applications of these compounds for spectrophotometric investigations are summarised in Table 111, and it can be seen that the dyes are useful reagents, finding applications for a wide range of metal ions. TABLE I11 SURVEY OF SPECTROPHOTOMETRIC DATA Limit of determination, Compound Ion Llax.E pH p.p.m. Remarks TAR cua+ 560 19 400 2-7-3.7 1.4-5-2 CuRII+, Kstab. detd. 510 31 100 6.1 - CUR 515 29200 6.2 - - 540 30800 8-35 - BiR+ 530 30 400 6.1-8.1 - PbR BiS+ 540 22 400 2.8-4.0 3.8-8-3 BiRH2+, Kstab. detd. PbZ+ 540 20 000 3.6-4.1 - PbRH+, Kstab. detd. Reference 48 48 55 48 48 48 48 continued778 HOVIND : THIAZOLYLAZO DYES AND THEIR TABLE III-continued Limit of determination, Analyst, "01. 100 Compound TAR cont. MeTAR KTAR BrTAR STAR PBTAR TA4RK TARN STARK STARN TAO TAM TAMR TAAR Ion Zn2+ U0,2+ R h 3 + Ni2+ La3+ vo,+ Ins+ Fe3+ Th4+ TIS+ co2+ Ga3+ R u I V OSIV OsVIlI sc3+ Co2+ 1n3+ Co2+ Ni2+ Zn2+ Cd2+ Pda+ Gas+ Pd2+ Ni2+ 1n3+ uo22+ Pd2+ Ni2+ Zn2+ Cd2+ Pd2+ Pd2+ Ni2+ COB+ vo,+ Nbv cu2+ uo 2+ NbB VO2+ co2+ Pb2+ h a x .510 510 530 530 540 560 510 500 540 520 546 540 530 530 540 540 540 520 615 520 510 510 525 570 550 550 540 530 500 600 500 510 570 500 505 505 580 620 520 620 530 535 535 600 615 510 505 640 630 510 520 540 605 535 610 590 590 580 580 580 580 575 € 32 000 35 400 - - 33 000 - - 31 500 35 200 26 500 26 600 23 500 28 000 22 600 15 000 24 000 19 000 31 500 20 000 56 000 56 000 35 000 15 600 15 500 22 200 50 600 29 400 37 600 15 000 40 000 59 000 44 200 24 200 23 900 7440 9300 68 500 14 700 28 000 22 000 24 000 12 500 62 000 33 600 32 380 6940 12 900 65 000 42 000 13 000 48 000 30 800 14 000 - - - 35 000 35 000 35 000 69 000 64 000 8000 P H 4.8 7.0 7.4-8.4 7.5 7.5-8.0 7.5-8.0 6.0-6.6 7.3 < 4.5 7.0 5.0 > 3.3 4.5 5.2-5.3 5.7-6.3 4.8 6.5 3.9 7.4 1-2 7.5-8.0 6.5-8.0 5.0-5.2 5.0-7.2 5.7-7.2 7.5-9-0 8.1 - 5-6 3.0 5.5 6.5-8.5 7.5-8.0 6.5 8.0 8.2 5.0 3.5-9.3 3-7 4.0 5-8 6.5 5.6 5-8 7.3 7-7 - - - 5.5-6.5 6-8 5.0-5.5 3.2-3.8 - 4.2-5.0 4.0-54 5.0 3.2-5.0 5.5 6.0 5-9 4.6 p.p.m.- - - < 1.0 04-48 - 0.5-3.7 - - - - - - < 2.4 - - - - 0.1-0.4 <9 0.5-3.0 0.2-2-0 0.1-1.6 0.3-4.5 0'6-9.5 0-4-6.5 0.1-1.6 - - - - 0-2-2.0 0.08-0.5 - - - - 0.8 0*01-0.14 - - - - - - - - 0.8 0.8 - - - 1.2 - 0.5-9.5 < 1.5 0.1-1.2 < 1.8 0*1-1*0 0.6-10 - - Remarks Reference ZnRH+, Kstab. detd. ZnR ZnO detd. in air, 20 mg m--3 Zn detd. in potable water &tab. detd. 0.04-0*33% in ores detd. Solution heated NiRH+, Kstab. detd. - - - Kstab. detd. Kstab. detd. Ternary complex with acetate Ternary complex with H202 1 : 2 complex 1 : 3 complex, Kstab.detd. Kstab. detd. Kstab. detd. Kstab. detd. Kstab. detd- As oxychlorjde Kstab. detd. &tab. detd. 1 : 3 complex, better than earlier reagents Ternary with hydroxyl- amine, Kstab. detd. - - - 1: 1 complex 1 : 2 complex, Kinstab. detd. Kstab. detd. After separation Kinstab. detd. Kinstab. detd. Kinstab. detd. Rehaviour in H,SO, studied - - Kstab. detd. Kstab. detd. Kstab. detd. Kinstab. detd. Kinstab. detd. Kinstab. detd. Kinstab. detd. Studied in H2S0, Studied in H2S0, Icinstab. detd. 1 : 1 complex, detd. in steel Complexes with many Kstab. detd. Better than PAR and thiocyanate Detd. in alloys 1 : 1.5 complex 1 : 3 complex 1 : 2 complex 1.2-12 pg - metals studied - 48 48 96 93 32 95 46 55 39 55 55 53 55 36 55 55 55 55 56 35 34 47 36 68 68 68 57 64 88 13 13 47 94 82 82 82 82 81 89 11 83 83 83 82 82 82 82 81 81 90 88 85 97 61 38 87 84 86 10 58 88 121 comtimuedNovember, 1975 APPLICATIONS IN ANALYTICAL CHEMISTRY.A REVIEW 779 Compound TAAC TAC TAMP TAPC PBTAPC CETAPC BTAR BrBTAR 6R-BTAC BTAAC BTAMP BTADCl HBTAR DHBTAR HBTAPC DTAPC DHBTAC DTAF DTACl DTRBr HBTAAR DTADM Ion Nbv co2+ Gas+ Sb3+ Hge+ Cu2+ Ln3+ vo,+ 1n3+ Hg2+ LrP+ A l 3 + Sn4+ Sb3+ w0,a- - coa+ co2+ - Ga3+ Zn2+ Cda+ Ga3 + cue+ I Ga3+ Gas+ cu*+ Nie+ cu2+ CU2+ cue+ Gaa+ cue+ co2+ Ni2+ - hmax. 560 560 570 550 590 545 602 612 610 -565 628 615 -555 510 630 480 540 510 510 - - - 560 620 550 550 575 - 520 - - 615 610 640 640 640 540 630 610 630 TABLE III-continued Limit of determination. E 26 000 57 800 28 000 65 000 38 500 37 000 10 300 21 000 - - 8-13 000 14 000 32 900 49 400 5-6000 - - 70 000 30 000 36 000 - - 69 000 38 800 53 600 41 000 - - 50 600 - - 17 200 32 800 21 200 21 000 20 400 28 500 44 400 40 000 - PH 5.0 6-8 3.0-4.0 - - - 4.5-8.8 4.6-8.8 8.2-8.3 - - - - - 6.0 5.0-6.6 5.0-5.5 4.0-5.5 4-0-6-5 - 3.5-7.0 I 3.0 9-10 5.5 6.5-7.3 < I - 6.7-7.2 - - < I 2-5 4.0 4.0 4.0 5.9 >2 >2 >2 Remarks 0.5-6% in alloys Kinstab.detd. Kinstab. detd. - - - Fatty acid detd. Fatty acid detd. in serum Kstab. detd. Hg( RH) predominates In 50% ethanol Kstab. detd. Kinetab. detd. - - 1-35 pg Many metals react Klnstab. detd. Kinstab. detd. R = H, Br, CH,, OCH,, Many metals react - - In 85% acetone Qualitative reactions GaOH(HR),, Kinstab. detd. Kstab. detd. for protonated complex, also for Pb, Pb, Ni, Zn, Cd Kinstab.detd. Many metals react Kinstab. detd. Kinstab. detd., also Co, Pb, Zn, Cd Also Ni Also Ni Also Ni 1 : 1 complex, Kstab. detd. 1 : 2 complex, Kstab. detd. 1 : 2 complex, Kstab. detd. - - Reference 84 88 85 29 29 104 117 117 91 92 77 37 41 31 72 74 74 73 19 59 59 25 29 63 26 18 67 54 65 66 75 80 80 23 23 23 20 22 22 22 Extraction - Photometric Applications Many of the thiazolylazo dyes and their metal complexes have rather limited solubility in aqueous solution, and in extraction procedures use is made of their much greater solubility in certain organic solvents. Usually chloroform is applied as the extraction rnedi~rn,~,~O,~Q,6~163, other ~ o 1 ~ e n t ~ ~ , ~ ~ ~ ~ 8 , ~ ~ ~ ~ ~ ~ ~ ~ ~ have also been used. The choice of solvent is often very important : pentan-1-01 or isobutyl methyl ketone is used for the extraction of uranium(V1) with TAMP, chloroform being less suitable because with this solvent the complex collects partly at the interf ace.38 The pH of the aqueous phase must also be adjusted to an appropriate value as the amount of complex extracted is usually negligible when the solution is fairly acidic, although there are a few exceptions ; the extraction procedure is then selective.Thus the palladium(I1) complex with TARK is extracted into ethyl acetate at pH below 2,ll the thallium(II1) complex with TAAC into chloroform at pH 1.6,1° the antimony(II1) complex with BTAAC into chloroform at a pH of about 1,29 the palladium(I1) complex with TAR into amyl alcohol from sulphuric acid solutionslO* and some different metal ions are extracted with HBTAPC at a pH of about 99-104 but butan-1-o1,16,60,61,71,105 benzene,10,62,78,106,107 carbon tetrachloridel0,62,107 and Some780 HOVIND : THIAZOLYLAZO DYES AND THEIR Analyst, Vol.100 1.17 In very alkaline solutions the absorbance may decrease owing to hydrolysis of the complex. In order to increase the efficiency of extraction, the addition of a strong electrolyte such as potassium nitrate can be made.38 In certain instances the extraction procedure is made more selective by the addition of suitable masking agents. Thus iron and niobium are masked by cyanide and triethanolamine in the determination of vanadium with TAM.103 A specific method for the determination of cobalt is based on the fact that the cobalt - TAR complex cannot be re-extracted into the aqueous phase in the presence of EDTA or ~yanide.10~ More- over, cadmium is separated directly from aqueous solutions by extraction with suitable solutions of tet raalkylammonium salt s.l10 The reagents used for the adjustment of pH of the aqueous phase may have a significant effect on the extraction rate of the complex.KasiuralOO observed that the extraction curves for some metal ions from ammonia and sodium hydroxide solutions were very different, and a markedly low rate was observed for the extraction of the cobalt(I1) and nickel(I1) complexes with TAAR30 and of the nickel(I1) complex with TAMlo0 from sodium hydroxide solutions. TACMe was shown to be a more sensitive reagent for nickel than TAC, but the slow extraction rate of the former is disadvantageo~s.~~~ The quantitative interpretation of the distribution of the cadmium(I1) - TAR complexes between the aqueous and organic phases made by Frei and NavrAtilllO indicated that two groups of extractable species were formed: (i) uncharged complexes of the type CdR, which are poorly extracted because of the water molecules that occupy free co-ordination sites, al- though they can be extracted if the water molecules are replaced by a suitable organic reagent or by solvent molecules; and (ii) complexes CdR22-, the charge on which can be eliminated by the addition of a suitable organic cation, e.g., tetraalkylammonium salts, the complex then being easily extracted as an ion-association complex.Other metals are also extracted as ion- association complexes.10g The metal ions are usually extracted as the 1 : 2 complexes, but gallium(II1) and germanium- (IV) are extracted as 1 : 3 complexes with T A P P and PBTAPC,60 respectively.Some metals are also extracted as 1 : 1 complexes, e.g., palladium(I1) with TAR,loscopper(II) with PBTAR105 and TAMP,lo6 mercury(I1) with TAMP99 and vanadium(V) with TAM.lo3 Owing to the preferential dissociation of the 9-hydroxy group, the dihydroxy derivatives form ionic complexes with many metals, and therefore TAR and its derivatives have seldom been used for extraction purposes. An exception is palladium(II), which is extracted from sulphuric acid solutions with TARK into ethyl acetate,ll and with TAR108 or HBTAPC17 into pentan-1-01. Generally, polar solvents such as pentan-1-0P~~J~~ and but an-1-01105 are suitable for the extraction of TAR - metal complexes.The solubility of the reagent in aqueous solution is reduced by the introduction of an alkyl group into the benzene ring in order to make the resorcinol nucleus more hydrophobic. With greater length of side-chain the solubility in aqueous solution is diminished, and the reagent becomes suitable for the extraction of heavy metals. Thus, the zinc(I1) complex with the 6-hexyl derivative of TAR has a high distribution coefficient and is readily soluble in organic solvents .7 Many of the complexes have their maximum absorption between 520 and 580 nm, but some of them absorb at longer wavelengths: palladium(I1) - TARK at 650 nm,ll uranium(V1) - TAMP at 610 nm,38 mercury(I1) - TAMP at 630 nm,98 cadmium(I1) - BTAC at 620 nm,62 nickel(I1) - TAC at 610 nm and nickel(I1) - TACMe at 620 nm.lo7 These metals can therefore be determined in the presence of other metal ions as the spectra of their complexes do not over- lap. Nickel(I1) forms a strongly coloured complex with TAM, with a value for E of 75 900.101 Also, PBTAPC forms complexes with high absorptivities with some metals.16~60~71 An analogous dye, 5-(2’-thiazolylazo)-2,6-dihydroxypyridine, has been proposed as a re- agent for the extraction of palladium(I1) from acidic solutions.lll 4-(6’-Methoxy-3’-methyl- 2’-benzothiazolylazo)-N-methyldiphenylarnine has been used as a reagent for the extraction of some metal ions, which were extracted as ion-association complexes, e.g., R(SbC1,) ,1129113 R[Zn(SCN) 3] ,1l3 R(1n14),l13 R(HgBr3)113s114 and R(FeC14),1151116 the iron complex being extracted into a mixture of benzene and cyclohexanone.l16 The papers dealing with these compounds for extraction procedures are summarised in Table IV.Reagents with only one hydroxy group in the benzene ring have primarily been used and seem to be more efficient than the dihydroxy derivatives, probably owing to the formation of uncharged complexes.November, 1975 Compound TAR PBTAR TAR-He TARK TAM TAAR TAAC MeTAAC TAC TAMP TACMe TAPC PBTAPC BTAC BrBTAC MeBTAC MOBTAC BTAAC HBTAPC TAMe345 TAMe346 Ion Pd2+ co2+ Cd2+ CU2+ Co2+ Ni2+ Zn2+ Pd2+ Ni2+ vo,+ - Hg2+ Sb3+ ~ 1 3 + Hga+ Zn2.t 1n3+ 1n3+ uo,2+ Ni2+ Hg2+ CU2+ Ni2+ Ga3+ - Ge4+ ~ 1 3 + Cd2+ Cd2+ Cd2+ Cd2+ Sb3+ - Ni2+ Ni2+ APPLICATIONS I N ANALYTICAL CHEMISTRY.A REVIEW TABLE IV EXTRACTION - PHOTOMETRIC STUDIES Limit of determination. 781 Xmax. 650 570 - 520 520 520 - 650 520 595 -_ 580 530 580 580 545 - 560 617 610 630 - 624 570 - 540 580 610 610 610 610 600 - 605 620 E 8250 25 000 - 64 000 62 000 59 000 - 7100 75 900 42 000 - 19 000 30 500 30 000 9000 37 000 - 25 000 30 200 14 000 27 900 - 33 100 66 500 - 62 000 67 500 45 000 49 200 49 000 38 500 37 000 - 23 000 19 500 PH - 4.5-5.0 - 2.5-7.5 3-3-7.8 3.3-6.7 - <2 - 3-5-4-3 - 7.0 1.6 - 7.0 5.2 - 5.2 6.9 6.5-7.5 - 7.1-9.1 6.7 4-6 - 4-7 5.63 - - - - -1 1.05 - - p.p.m. - - > 0.06 - - - - 2-1 1 -0.4 < 1.2 - 0.15-10 1.2-3.5 0.24-7 0.45-10 0.15-5 - 0.15-5 (5 - - - - - - I - - 0.1-3 - - 0.2-0.4 - - - Remarks Reference From 1 M H2S04 into amyl alcohol, concentration from Into amyl alcohol, concentration Into amyl alcohol, two types Into butan-1-01, 1:l complex 105 Into butan-1-01 105 Into butan-1-01 105 Into methylcyclohexanone or chloroform 7 6 to 95 pg ml-l from 0.1 to 10 pg ml-l of species extracted 110 108 109 Into ethyl acetate 11 From an alkaline medium into chloroform, Kinstab.detd. 101 Into chloroform, iron masked 103 Many metals extracted from NaOH or NH, into chloroform, Kinstab. detd. 100 Into chloroform 102 Into chloroform 10 As HR[SbI,]R complex from 0.5-2 N acid into chloroform 10 Into chloroform 102 Into chloroform 104 Into chloroform or dichloroethane 63 Into chloroform 104 Into benzene 107 Into isobutyl methyl ketone or pentan- 1-01 38 Into chloroform, 1 - 7 x Into benzene 106 Into benzene, slow extraction 107 Jnto butan-1-01, 1:3 complex 61 Many metals extracted into butan- 1-01 16 Into butan-1-01, 1:3 complex 60 Into butan-1-01, Kinstab.detd. 71 Into chloroform 62 Into chloroform 62 Into chloroform 62 Into chloroform 62 Into chloroform 29 Into pentan-1-01, many metals extracted 17 Into benzene 78 Into benzene 78 M, 1 :2 complex 99 Applications as Indicators The most common application is the direct titration of the metal against EDTA solution with the dye as indicator,the metal being displaced from its dye complex at the end-point.24~29,37,70~7g~ 99,104,117--123 The blue, green or violet colour of the solution due to the complex changes to that of the free ligand (yellow or orange). The rate of the colour change of the indicator near the equivalence point is governed by the rate of the substitution reaction between the metal - indicator complex and the titrant ; at room temperature this reaction is often slow, but its rate can be increased by heating the solution to 60-70 "C near the end-p0int.37,5~,70,123 Addition of an organic solvent can also improve the r n e t h ~ d .~ ~ , ~ ~ ~ With TAC as indicator, the presence of 1 ,lo-phenanthroline permits the titration of nickel to be performed at room temperature.123782 HOVIND : THIAZOLYLAZO DYES AND THEIR Analyst, Vol. 100 Therefore, buffer solutions used in the chelatometric titrations must be chosen with due regard to their effect on the rate of colour change of the indicator. With the indirect chelatometric method an excess of EDTA is added to the solution, and the uncomplexed EDTA is back-titrated with a standard solution of metal ions; usually copper(I1) sulphate is used.Copper(I1) produces a sharp and well defined end-point, the colour change being from red to blue. In this way tin is determined in organotin compounds after decom- position of the organic matter124; aluminium can also be determined by this method.125 An analogous method is used for the indirect determination of glutamic acid in the presence of small amounts of natural amino-acids, the acid being titrated against cadmium(I1) with TAR as indicator. At the end-point, when the cadmium - dye complex is formed,l26 the colour changes from yellow to pink. TABLE V THIAZOLYLAZOPHENOLS APPLIED AS INDICATORS Compound TAR TAMR TAAMP TAAR TAAC MeTAAC TAC TAMP TACl TADCl Ion Tla+ Gaa+ Ni2+ Cd2+ Hgz+ Ma+ - MoO*2- Pod3- cua+ In8+ Pb2+ ~ 1 3 + 1n3+ 1n3+ Ina+ Hgz+ Hgz+ Niz+ - Sn4+ Cu2+ In3+ Bia+ Hg2+ Hg2+ ~ 1 3 + Pod3- Hg2+ Hg2+ Niz+ Ni2+ Ni2+ Hg2+ PH 1*2-5.5 3.5-4-0 44-6.5 9.0 3.3-3.5 4.2-7.5 3.5-6.5 5.5-6.6 6.7-7.3 6.0 6.0 1.2-5.5 - 5.2 5-2 5.2 7.0-8.3 1.3-7.0 10 - 3-5 4.5 4.5 1.5-2.0 - - 1-0-1.8 3.0 57-6.7 6.7-7.3 6.7-8.7 6.7-8.7 - - 5.5-7.5 Limit of determination 10-200 p.p.m.10-200 p.p.m. - 0.001-0.02 M - 0.1-3 x 10-4 M 0.9-47 mg 44-19 mg 3-30 mg 6-35 mg - 3 mg - - 3-30 mg 6-35 mg 10.5-63 mg 34-135 mg - 94-47 mg 4.8-20 mg (210 mg 10-210 mg - 50-300 p.p.m. 50-300 p.p.m. Colour change - Lilac to green Lilac to green Yellow to pink Yellow to red Yellow to red - Red to yellow Yellow to red violet - green Yellow to red Red to yellow Red to yellow Violet to yellow Violet to orange Violet to orange Violet to orange Blue to yellow Yellow to green - Blue - violet to red Yellow to blue Blue - green to Blue - green to Green to orange Green to yellow Blue to orange Yellow to pink Yellow to blue Yellow to blue - Yellow to blue - Blue to orange Blue to orange Blue to yellow - Blue to yellow - Blue to yellow - Violet to orange yellow yellow green green red red red Remarks Reference EDTA direct Cu - EDTA - TAR indicator Cu - EDTA - TAR indicator Indirect determination of glutamic acid Mercurimetric titration of C1-, Br-, SCN- Back-titration of EDTA with Cua+ Cu - EDTA - TAR indicator.Many metals Precipitation titration with Pb2+ Precipitation titration with Pb2+ EDTA direct EDTA direct EDTA direct EDTA direct EDTA direct EDTA direct EDTA direct EDTA direct Mercurimetric titration of C1-, Br-, SCN- EDTA direct EDTA direct, for many metals, rate of colour change Back-titration of EDTA with Cu2+ EDTA direct EDTA direct EDTA direct EDTA direct EDTA direct Mercurimetric titration of C1-, Br-, SCN- Precipitation titration with Pba+ Precipitation titration with Pb2+ EDTA direct EDTA direct EDTA, direct in acidic medium EDTA direct EDTA direct EDTA direct 119 127 52 126 44 125 128 129 129 37 37 37 121 104 29 104 117 44 70 123 124 37 37 37 37 99 43 129 129 120 122 24 79 122 118November, 1975 APPLICATIONS IN ANALYTICAL CHEMISTRY.A REVIEW 783 The replacement titration is analogous to the direct method, but the indicator used is a metal - dye complex, usually copper(I1) - TAR.52*127J28 When the end-point is reached, the EDTA reacts with the copper in the indicator, causing the colour to change to yellow - green.This method gives a sharp end-point and can even be used for metal ions that do not react with TAR. Again, the titration must be performed very slowly, or at elevated temperature, be- cause of the slowness of the indicator transition at room temperature. TAR and TAMP have been used for the determination of molybdate and phosphate ions, lead(I1) being the reagent used for their pre~ipitation.~~~ Owing to interferences from other ions, however, molybdenum must be pre-separated. TAMP,43 TAR and TAC44 have also been shown to be suitable end-point indicators for the mercurimetric determination of chloride, bromide and thiocyanate.Table V summarises the applications of thiazolylazophenols as indicators. Other Applications in Analytical Chemistry TAR has been used as a spray reagent for the determination of metal ions on thin-layer chromatograms, the visual limit of determination being 0.1-0-01 pg per spot.130 When a reflectance spectroscopic method was used, the calibration graph was found to be linear up to 1 pg per sp0t.1~1 Kamada et recorded nuclear magnetic resonance spectra of some 0-hydroxy derivatives and studied the effects of different substituents and of solvents133; they also correlated the nuclear magnetic resonance chemical shifts with the electronic spectra.la4 Florence et ~ 1 . l ~ ~ studied the polarographic behaviour of some azo compounds and concluded that while the disproportionation of the heterocyclic dyes was base catalysed, that of the aromatic azo dyes was acid catalysed. The effect of nickel(I1) and iron(I1) on the half-wave potential was studied for analytical purposes.As aluminium did not cause a shift, nickel(I1) could be determined in aluminium without the need for pre-separation. Thiazolylazonaphthol Dyes Preparation and Purification These compounds have been prepared by methods analogous to those for preparing the phenol derivatives, the diazotisation being performed in a solution containing a high con- centration of hydrochl~ric,l~&~~~ s ~ l p h ~ r i ~ ~ ~ ~ ~ ~ ~ , ~ ~ ~ or p h o s p h o r i ~ l , ~ ~ ~ * ~ ~ acid, or in a mixture of sulphuric acid and dimethylf~rmamide.~~~~J~~J~~ BTAN has been prepared by allowing benzothiazole to react with hydroxylamine in the presence of alkali,147 and a-BTAN by heating 2-hydrazinobenzothiazole and 1,2-naphthoquinone in acetic acid.148 The azo compounds are usually purified by recrystallisation from ethanol, but DTAN was recrystallised from dimethylf~rmarnide,~~~ 6-R-BTAN was purified by dissolution in dimethyl- formamide and re-precipitation by the addition of water27 and $-TAN was dissolved in light petroleum and precipitated by the addition of water and ethanol.146 The purity of the compounds was controlled by traditional methods such as paper chromato- g r a p h ~ ~ ~ and electrophore~is,~~~ by c h r o m o m e t r i ~ ~ , ~ ~ ~ - ~ ~ ~ or vanadat~metricl~' titration, by spectrophotometric titration with ~ o p p e r ( I I ) ~ , ~ ~ or by photometric determination with pal- ladium(I1) in 2 N sulphuric acid.150 General Properties Most of these compounds are violet, red or yellowish powders, which are sparingly soluble in water, but readily soluble in organic solvents such as methanol, ethanol, acetone and dimethylformamide, and in mixtures of water and organic solvents.Hydroxy and sulphonic acid groups have been introduced into some of the new reagents in order to render them soluble in water; chromotropic acid and some of the sulphonic acid derivatives are examples of this effect. Neutral aqueous solutions of TAN and most of its derivatives and solutions of them in organic solvents are red in colour, with an absorption maximum at 490 nm.In acidic solutions a yellow cation is formed (A max. 440 nm), and in an alkaline medium the hydroxy group is dissociated to give a violet anion (h mBx. 530 nm). Other groups with acid - base properties, substituted in the molecule, may alter this pattern slightly. The compound BrBTAN be- haves anomalously as the spectral characteristics did not vary over the pH range 0.2-13.0.152 The derivatives studied below are listed in Table VI.784 HOVIND : THIAZOLYLAZO DYES AND THEIR TABLE VI SURVEY OF THIAZOLYLAZONAPHTHOLS Analyst, VoL. 100 Abbrevi- ation TAN MeTAN KTAN BrTAN TANK TANBr TANGS TAN36S BrTAN36S MTAN36S P-TAN Abbrevi- ation BTAN BrBTAN MeBTAN MBTAN CIBTAN HOBTAN NBTAN BTAN6S P-BTAN D Position of substituent group r I A OH OH OH OH OH OH OH OH OH OH H B C D Q R Systematic name Reference H H H H COOH H H SO ,H S03H SO,H H H H H H H H H H H H OH H H H H H Br SO,H S03H SO 3H SO ,H H H H CH, H COOH H H Br H H H H H H H H H Br H CH30 H H c Position of substituent group A OH OH OH OH OH OH OH OH H 1- (2’-Thiazolylazo)-2- 1 - (4‘-Methyl-2’-thiazolylazo) - 1- (4’-Carboxy-2’-thiazolylazo) - l-(5’-Bromo-2’-thiazolylazo) - 1-(2’-Thiazolylazo) -2- 1-( 2’-Thiazolylazo) -6-bromo- 1-(2’-Thiazolylazo) -2- 1-(2’-Thiazolylazo) -2- naphthol 9, 14 2-naphthol - 2-naphthol 139, 161 2-naphthol 14 hydroxy-3-naphthoic acid 142 2-naphthol - naphthol-6-sulphonic acid - naphthol-3,6-disulphonic acid 138 l-(5’-Bromo-2’-thiazolylazo) - 2-naphthol- 3,6-disulphoni c acid 138 1-( Y-Methoxy-2’- thiazol ylazo) -2-naphthol- 1-(2’-Thjazolylaz0)-4- 3.6-disulphonic acid - naphthol 146 -- B C R H H H H H Br H H CH, H H OCH, H H C1 H H OH H H NO, H S03H H O H H H Systematic name Reference 1- (2’-Benzothiazolylazo) -2-naphthol 1- (6’-Bromo-2’-benzothiazolylazo) -2-naphthol 1- (6’-Methyl-2’-benzothiazolylazo) -2-naphthol 1 - (6’-Methoxy-2’-benzothiazolylazo) -2- 1 - (6’-Chloro-2’-benzothiazolylazo) -2-naphthol 1- (6‘-Hydroxy-2’-benzothiazolylazo)-2- 1- ~6’-Nitro-2’-benzothiazolylazo) -2-naphthol 1- (2’-Benzo thiazolylazo) -2-naphthol- 1 - (2’-Benzothiazolylazo) -4-naphthol naphthol naphthol 6-sulphonic acid 27, 148 27 27 2 7 2 7 27 27 - 148 continuedNorvmbee, 1975 APPLICATIONS IN ANALYTICAL CHEMISTRY. A REVIEW TABLE VI-continued 785 Position of substituent group Abbrevi- t-A--, DTAN OH H 1-( i”-Ox0-5’,5’-dimethy1-4’,5’, 6’,7’-tetrahydro- DTANOH OH OH 1-(7’-0~0-5’,5’-dimethyl-4’,5’, 6’,7’-tetrahydro- p-DTAN H OH 1-( 7’-0xo-5’, 5’-dimethyl-4’,5’, 6’, 7’-tetrahydro- ation A B Systematic name 2’-benzothiazolylazo) -2-naphthol 2’-benzothiazolylazo) -2,4-dihydroxynaphthalene 2'-benzothiazol ylazo) -4-naphthol Position of substituent group Abbrevi- (-*-----, ation a R Systematic name TANC H H 2-( 2’-Thiazolylazo)chromotropic acid PTANC C,H, H 2-( 4’-Phenyl-2’-thiazolylazo)chromotropic acid BrTANC H Br 2- (5’-Bromo-2’-thiazolylazo) chromotropic acid DMTANC CH, CH, 2-(4’,5’-Dimethyl-2’-thiazolylazo)chromotropic acid Abbrevi- Substitution ation R OH OH SO3H R HO,S Systematic name BTANC H 2-(2’-Benzothiazolylazo) chromotropic acid BrBTANC Br 2- (6’-Bromo-2‘-benzo thiazolylazo) chromotropic acid EBTANC C,H,O 2-(6’-Ethoxy-2’-benzothiazolylazo)chromotropic acid MeBTANC CH, 2-(6’-Methyl-2’-benzothiazolylazo)chromotropic acid MBTANC CH,O 2-(6’-Methoxy-2’-benzothiazolylazo)chromotropic acid NBTANC NO, 2- (6’-Nitro-2’-benzothiazolylazo) chromotropic acid IBTANC I 2- (6’-Iodo-2’-benzothiazolylazo)chromotropic acid Reference 21, 143 21 21 Reference 137 136, 137 136, 137 136, 137 Reference - 136, 137 137 Abbreviation Systematic name Reference or-TAN 2-(2’-Thiazolylazo)-l-naphthol 146 a-BTAN 2- (2’-Benzothiazolylazo) - l-naphthol 148 continued786 HOVIND : THIAZOLYLAZO DYES AND THEIR TABLE VI-continued Abbreviation Systematic name Analyst, Vol.100 Reference 2-( 7'-0x0-5',5'-dimethyl- 4',6',6',7'-tetrahydro-2'- benzothiazolylazo) chromo- tropic acid 21 2,7-Bis( 2'-thiazolylazo)- chromotropic acid 170 5-(4', 5',6',7'-Tetrahydro-2 - benzothiazolylazo) - quinolin-8-01 180 K a w a ~ e l ~ ~ studied the visible and infrared spectra of the three isomers BTAN, a-BTAN and e-BTAN, and revealed that the equilibrium was shifted considerably towards the hydrazone form for 9-BTAN only.For this compound the visible spectra shifted to longer wavelengths when the dielectric constant of the solvent was increased, while the two other isomers were independent of the solvent and the temperature. Based on X-ray evidence, K ~ r a h a s h i l ~ ~ showed that the azo and the hydrazone tautomers co-exist in the crystalline state in TAN, and that the azo tautomer is predominant. In Table VII the published values of the acid-dissociation constants of the thiazolylazonaph- tho1 derivatives are listed.An extremely high value of 13.3 is observed for BrBTAN.152 The acid-dissociation constant is distinctly affected by the location of the hydroxy group on the naphthol nucleus, the pKoH value increasing in the substitution series as follows: para < alpha < beta.146J48J51J54 Introduction of other groups with an acid - base character into the naphthol nucleus also affects the dissociation of the hydroxy group; for example, the pK,-o~ value of the chromotropic acid derivatives is lowered by 1-3 units compared with TAN.137~~38 The p K o ~ value increases and the pKNH value decreases with increasing concentration of organic solvent in the aqueous solution, but both constants decrease when the ionic strength of the solution is increased.This phenomenon was studied by Ivanov ct al.,151 who derived correlation equations for the calculation of the conventional ionisation constants of TAN in mixtures of water and different organic solvents at ionic strengths of not more than 0.5. The thiazolylazonaphthol derivatives form coloured complexes with most of the transition metals, the metal to ligand ratio usually being 1 : 1 in acidic media and 1 : 2 when the pH is raised. Unlike the phenol derivatives, no complex with a greater metal to ligand ratio has been reported for these derivatives. Heterocyclic o-hydroxy azo dyes have generally been considered as terdentate ligands,2 but there are some discrepancies in the literature. Some experimental results have been taken as evidence for the existence of terdentate ligands; thus the 1 : 1 complex between copper(I1) and DTAN is considered to be planar in solution, a solvent molecule taking the fourth co- ordination position.154 The crystal structure of FeII(TAN), is also in accordance with this assumption, as iron is bonded octahedrally to two phenolic oxygen atoms, two azo-nitrogen atoms adjacent to the naphthol ring and two thiazole-nitrogen atoms, forming two five- membered rings with each TAN m01ecule.l~~ This finding is also confirmed for the complexes PdCl(TAN) 156 and CO=I(TAN)~CIO Zuchenko et a1.,l5* on the other hand, found it to be likely that both octahedral and tetra- hedral structures are possible for the complexes formed by copper(II), nickel(I1) and cobalt(I1) with BTAN, and concluded that the complexes were probably tetrahedral in structure with no bond to the heterocyclic nitrogen atom in solution.This finding was confirmed by GarnovskiiNovember, 1975 APPLICATIONS IN ANALYTICAL CHEMISTRY. A REVIEW 787 et ~ 1 . l ~ ~ for the complexes of MBTAN with nickel(I1) and zinc(I1) in solution; the cryoscopic measurements indicated that the complex molecule is monomeric, excluding an octahedral polymeric structure. Gusev et ~1.146 indicated that the 1 : 1 complex formed between copper(I1) and a-TAN in aqueous dioxan has a tetrahedral structure with a nitrate ion in the 4-position. In nearly neutral solution the octahedral 1 : 2 complex is formed, in which the copper ion is bonded to both the azo and the heterocyclic nitrogen atoms.The metal is bonded to the oxygen atom by salt formation, with a complex-forming bond to the nitrogen atoms. The occurrence of a bathochromic shift was taken as evidence for the ionic character of the metal - oxygen bond, The stability constants of the complexes are noticeably lower in aqueous media than in aqueous organic but the lower solubility of the complexes in water, especially those of the benzothiazolylazo derivatives, rendered the use of aqueous organic solvents necessary in order to prevent their precipitation. hER < h R - < hMeR*152 TABLE VII ACID-DISSOCIATION CONSTANTS OF THIAZOLYLAZONAPHTHOLS Spectrophotometric method used. PKOH 8.94 f 0.15 8.94 f 0.15 8.71 8.94 0-05 9.43 f 0.17 8-25 & 0.21 8-50 rt_ 0-04 9.76 & 0.05 10.4 11.5 9-83 10.00 9.83 10.4 11.5 8.44 8-38 8.74 f 0.03 13.30 & 0.3 8-41 8-46 f 0.02 7.70 f 0.04 6.48 f 0.11 6-94 f 0.02 5.70 f 0.03 6.71 f 0.08 7.43 f 0.01 7.7 7.2 6.0 7.22 0.03 7.17 f 0.04 Solvent 20% acetone 20% acetone Water 30% acetone Aqueous ethanol 40% acetone 80% acetone Aqueous ethanol 20% dioxan 40% dioxan 5% dioxan 40% DMF 5% dioxan 20% dioxan 50% dioxan 50% methanol Water 50% dioxan Water Water 30% ethanol 40% ethanol 20% ethanol 50% dioxan 40% ethanol 20% ethanol 50% dioxan Water 20% acetone Water 57.6% ethanol 57.6% ethanol PKCOOH Compound TAN PKNH Reference 9 14 151 172 140 14 172 162 141 141 142 184 183 183 183 69 69 148 152 69 143 154 146 148 154 146 148 138 138 137 144 144 - 2.37 1.72 & 0.04 0.58 f 0.03 1.32 f 0.06 - - -0.5 0.68 0.68 0.5 - - - MeTAN BrTAN KTAN TANK - 3.65 f 0.02 3.68 3.45 3.41 3-45 3.68 3.92 - - TANGS BTAN BrBTAN BTAN6S DTAN - 1.10 f 0.05 1.20 f 0.16 - - $-TAN P-BTAN p-DTAN a-TAN a-BTAN TAN36S BrTAN 3 6 S DMTANC HBTAQ Spectrophotometric Investigations In aqueous dimethylformamide TAN forms only one complex with rhodium( 111) ,150 osmium(1V) and osmium(VIII),1*9 and the solutions must be heated in boiling water in order to complete the reaction because of the slow substitution of the aquo-complexes. In oxygen-free ethanolic solutions copper(1) reacts instantaneously with TAN to form a mixed complex, with the chloride ion as the second ligand.160 Owing to the much lower stability of their complexes, $-TAN146 and p-DTAN154 are less suitable as complexing agents, compared with the corresponding o-hydroxy derivatives; a-TAN, on the other hand, is superior to TAN in its high sensitivity and distinctness of colour change.146788 HOVIND : THIAZOLYLAZO DYES AND THEIR Analyst, Vol. 100 With KTAN two complexes are formed with indium(III), the 1 : 2 complex being a mixed one of the type In(OH)R,,lG1 and interferences from gallium, aluminium and scandium can be eliminated by the addition of fluoride. Ytterbium(II1) has been determined at 230 nm with this reagent.13g The tY-R-BTANC derivatives have been proposed as reagents for cadmium, but are not sufficiently selective for this ion, as many other metals also give colour reactions with them in neutral s o l ~ t i o n s . ~ ~ ~ J ~ ~ BrBTAN, however, which forms a red - violet complex with cad- miuni(I1) in a weakly alkaline medium, is a very sensitive reagent for this ion (E = 51 goo), and cadmium in lead - zinc ores can be determined with it after preliminary separation by ion exchange.15, Dyes containing a sulphonic acid group in the 8-position of the naphthol ring proved to be less sensitive towards metal ions.69 When TAN36S and BrTAN36S react with palladium(II), very contrasting colour changes are observed, maximum absorption occurring at 654 nm and 678 nm, respectively.138 Gallium- (111) and indium(II1) also react with TAN36S in moderately acidic media. These last two TABLE VIII SPECTROPHOTOMETRIC DATA FOR THIAZOLYLAZONAPHTHOLS Limit of determination, Compound TAN MeTAN KTAN TANK TAN36S a-TAN $-TAN BrBTAN DTAN TANC PTANC fi-DTAN DMTANC BrTANC BTANC BrBTANC EBTANC IBTANC MBTAN36S Cd2+ HBTAQ Ni2+ I on cu+ CuZ+ Rh3+ OSIV OSVIIl Cu2+ Yb3+ 1n3+ ~ 1 3 + CuZ+ Ni2+ Ga3+ 1n3+ ~ 1 3 + Ni2+ cu2+ Cu2+ Cd2+ cu2+ cu2+ Pd2+ ~ 1 3 + Zr1v ThIV A l 3 + Gas+ ThIv ZrIv Pd2+ Cd2+ Cd2+ Cd2+ Cd2+ ~ 1 3 + h l t I X .580 580 620 560 560 590 230 570 570 598 580 590 570 580 570 570 580 570 580 590 590 610 610 540 590 600 600 654 630 630 620 630 630 630 620 620 630 600 678 620 600 620 590 580 E PH 20 000 3.7-6.2 35 300 1.5-3.5 9800 6.3-7.5 15 300 6.7 15 700 7.0 39 500 2.0 15 000 3 32000 5 - 5-6 23 500 2-3 I > 1.5 - >3 23 300 3.0-4.0 - 3.5-4.0 - - 24 700 3.0-4.0 - 3.0-3.5 29 800 2.3-2.8 - - - - 24 100 1.0-5.3 46 800 5.5-6.0 57500 - 51 900 - - 3.5 12400 (7 39 800 1.8-2.5 - - 45 000 1.5-3.0 24 000 1.0-2-5 22 000 1.0-1.4 30 000 2.8-3.4 45 000 1.5-3.0 45 000 1.0-3.0 24500 3-4 27 000 1.8-2.9 27 500 0.1-1.0 30 600 1.0-2.5 12750 - 8750 -7.2 26 000 1.0-2.5 9750 -7.2 9900 -7.2 13800 <7 110 800 6.2-6.8 550 .p.p.m. Remarks Reference 0.3-1.8 1 : 1 complex, in oxygen-free ethanol, Kstab. detd. - 1 : 2 complex 0.8-4.0 - 0.6-10 - 0.6-10 Kstab. detd. - - - - - - Katab. detd. - Kstab. detd. 0.2-2.5 Kstab. detd: 0.2-2.51 Tartaric acid destroys Mixture of 1: 1 and 1: 2 complexes Kstab. detd., 1: 1 complex Kstab. detd., 1 : 2 complex 2-11 mg, Kstab. detd. - the complex Kstab. detd. Together with GaS+ Together with Ga3+ Kstab. detd. Small amounts in soil 1 : 1 complex, CuRNO,, tetrahedral 1 : 2 complex, octahedral In weakly alkaline solution 1 : 1 complex 1: 1 complex 1 : 2 complex Colour stable for 3 h - - - - - - - - - - - 1 : 1 complex Not selective Not selective Not selective 1 : 4 complex, Kinstab.detd. - - 160 167 150 149 149 167 139 161 161 184 183 183 163 164 165 163 164 165 163 166 146 146 167 146 152 154 154 138 145 137 168 169 145 137 137 137 137 137 138 147 147 137 147 147 181November, 1975 APPLICATIONS IN ANALYTICAL CHEMISTRY. A REVIEW 789 ions can be determined together, then the gallium complex is destroyed by the addition of tartaric acid and the indium determined, the gallium content being taken as the difference.163-165 TAN36S has also been used for the determination of small amounts of nickel in fertilisers and soi1.166 Many chromotropic acid derivatives have been proposed as photometric reagents in acidic media, contrasting colour changes being obtained especially with aluminium, zirconium, thorium and gallium ions,137 and some of these dyes have proved to be sensitive reagents for aluminium(II1) at pH 1-3.1379145 Copper(I1) can be determined with TAN, MeTAN or a-TAN at a pH of about 2.167 Zirconium(1V) forms a 1 : 2 complex with PTANC at pH 1-1.4, which has been used for its detemination in alloys and glasses.168 This reagent is also used for the determination of thorium( IV) in pure magnesium and in a110ys.l~~ The bisazo derivative, BITANC, has proved to be a useful analytical reagent for copper(II), thorium(1V) and rare earths.170 The applications of these compounds in spectrophotometric investigations are summarised in Table VIII.Extraction - Photometric Applications Owing to the low solubility of the metal complexes of TAN in aqueous solutions, they can be extracted rather easily into organic solvents, usually chloroform, but benzene has also been used for this p u r p o ~ e .~ ~ ~ ~ ~ ~ ~ In aqueous solutions the complexes MnR+ and MmR2 are formed stepwise, but generally only the 1 : 2 complex is extracted into the organic phase. As cobalt can readily be oxidised to cobalt(II1) in the presence of azo dye~,~8 the complex formed was studied in the presence and absence of ascorbic acid, but there was no evidence of oxidation of the cobalt with TAN,171 in contrast to the effect of TAR. Cobalt, unlike other cations, cannot be re-extracted into the aqueous phase when EDTA or cyanide is added to the system,lOS and, by using these masking agents, cadmium can be determined in the presence of cobalt.1'1 Complex formation between cobalt(II1) and TAN or BrTAN gives rise to a much more contrasting colour change than that which occurs with cobalt (11) Wada and Nakaga~al'~ have evolved a method for the determination of nickel in mixtures of metal ions.Cobalt(I1) is oxidised to its tervalent state and masked by the addition of ammonia, iron is masked with pyrophosphate, the solution is then extracted and copper, zinc, cadmium, manganese and chromium are removed from the extract by back-extraction with diethyldithiocarbamate and then with 0.01 M hydrochloric acid. Nickel is best extracted with chloroform from a solution that is made slightly alkaline with ammonia, and can be determined in iron and aluminium oxides and in indium metal when pre-separated with dimethy1glyo~ime.l~~ The kinetic study of the extraction of nickel( 11) from aqueous solution showed that the formation of the 1: 1 complex was the rate-determining step.175 Busev et aZ.176 extracted palladium( 11) with TAN into chloroform from solutions containing mixtures of transition metals.They also proved that BrTAN is more sensitive and selective for this purpose, as 20-fold concentrations of other platinum metals do not interfere.177 Mercury was extracted as a complex with TAN or MeTAN for its determination in cinnabar,l78 and the 1 :1 complex of rhodium with TAN was extracted into chloroform.150 cc-BTAN proved to be superior to BTAN in its high sensitivity and distinctness of colour change, the absorption maximum being at a wavelength 20 nm greater than that given with the latter.148 It is also a very sensitive reagent for zinc, the absorptivity of the complex formed being about 70000.148 For the complexes of zinc(I1) with BrBTAN, the effect of extractable salts on the protolysis of neutral complexes during re-extraction was studied by Dziomk0.17~ HBTAQ is a related compound that proved to be a very sensitive reagent for nickel(II), as the absorptivity of the 1: 4 complex is as great as 110 800 in a weakly acidic The applications of these compounds in extraction procedures are summarised in Table IX.medium .180,181 Applications as Indicators These derivatives have mostly been used as indicators for direct titration with EDTA solution, the colour changing from that of the metal - indicator complex (blue or violet) to that of the dye itself (orange or yellow). At pH 3 only a few cations react with TAN, which acts as a relatively selective reagent for the chelatometric titration of copper and bismuth, and if a back-titration is carried out, lead790 Compound TAN MeTAN BrTAN BrBTAN cc-BTAN HOVIND THIAZOLYLAZO DYES AND THEIR Analyst, Vol.100 TABLE IX EXTRACTION - PHOTOMETRIC STUDIES OF THIAZOLYLAZONAPHTHOLS Ion Amax. cQ'+ 677 670 655 coa+ 620 Cd4+ 578 Pda+ 676 Ni'+ 695 592 590 R h 3 + 630 Hg2+ 580 Hga+ 580 CO'+ 570 coa+ 680 Pda+ 690 Zna+ - Zna+ 621 37 000 4.5-6.0 32500 4-7 25 500 2-6-6.0 - - 10200 6-5-13 46000 9-10 38 000 >7*0 11 900 - - 5.8-7.1 5000 5.5-8.0 5800 6.0-8.0 47 100 4-7 29 600 2'5-6.0 14000 (2.5 -- 4.7-5.5 70000 - Limit of determination, p.p.m.- 0.06-2 - - - 1-6 - - - - 1-5 1-8 - - - - I Remarks Into benzene, 2.5 x lo-' M, Into benzene Into chloroform Into chloroform Into benzene, 7-1 x 10-6 M, Ketab. detd. Into chloroform Into chloroform, 1-20 pg Into chloroform, 1-10 pg Into chloroform Into chloroform, Kstab. detd. Into chloroform Into chloroform Into chloroform Into chloroform Into chloroform, 1-10 p g Into chloroform Into chloroform Ketsb. detd. Reference 171 109 172 172 171 176 173 174 175 160 178 178 172 172 177 179 148 and nickel can also be determined a t this pH.182 Aluminium can also be determined by back- t i t r a t i ~ n . l ~ ~ After destruction of organic matter, tin in organotin compounds was determined by back-titration of EDTA with a copper solution, with TAN as indicator.124 As the pKoH value of TANK is greater than that for TAN, the former can be used as a metal- ion indicator in alkaline solutions at pH values of up to 9, and a sharp end-point is attained in the titration of copper and nickel with EDTA s01ution.l~~ Also, the use of this compound as an indicator is favoured because the formation constants of the 1 : 2 complexes are smaller than those for other thiazolylazo dyes, and this factor is advantageous for the titration of nicke1.183 TANK can also be used for the direct complexome'tric titration of indium(III)142 and thallium- (III).lM Addition of another blue dye such as methylene blue or bromocresol green improves the contrast of the colour change.lE3 TAN36S can be satisfactorily used as an indicator for the direct titration of thallium(II1) with EDTA solution,1f9~163 and for the determination of gallium and indium together.163P4 This dye has also been used for the mercurimetric titration of chloride, bromide and thio- cyanate ions.42 With a-TAN, thallium and bismuth are determined together, thallium(II1) is then reduced with ascorbic acid and the bismuth determined, the thallium content being determined by differenceale' This dye has also been used for the complexometric determination of copper in dural~minium,~~~ while some chromotropic acid derivatives proved to be useful as indicators for the potentiometric determination of aluminium, copper and vanadium.136 DTAN and p-DTAN are suitable indicators for strongly ionic acid - base titrations at pH 8.0-9.4 and 4.9-6.0, re~pectively.l~~J~~ The applications as indicators are summarised in Table X.The properties and analytical applications of thiazolylazophenol and thiazolylazonaphthol derivatives, including a few related compounds, have been reviewed. From Tables 111, IV, VIII and IX, it can be seen that these compounds have been used as sensitive spectrophotometric and extraction - photometric reagents for most metal ions in aqueous and organic solutions. However, a number of these dyes have very little practical application for analytical purposes because they show only slight differences in their properties towards metal ions, despite the variety of substituent groups and their locations. There are also some disadvantages in using these reagents.The considerable absorbance of the reagent itself, especially that of the first anionic form of the TAR derivatives, which appears in the solution from about pH 5 , can often cause complications in the determination of metal ions in slightly acidic and alkaline solutions. However, the large bathochromic shifts caused by the complexing reaction with some metal ions sometimes reduce this problem. ConclusionsNovember, 1975 APPLICATIONS IN ANALYTICAL CHEMISTRY. A REVIEW 79 1 These dyes form more or less stable complexes with nearly all of the metals, the exceptions being the alkali metals and, in part, the alkaline earths. The ions of copper, nickel and cobalt, especially, form stable complexes with the dyes, and these ions may interfere seriously in the determinations of other elements.Therefore, it may be necessary to make use of masking agents or separation procedures in order to improve the selectivity of the reagent with respect to a certain ion. Depending on pH, different complexes may be formed between a metal ion and a reagent. In acidic media, the 1 :1 complex is usually formed, and the pH must be raised in order that complexes with higher metal to ligand ratios may be formed and thus improve the sensitivity of the reaction. Addition of an organic solvent may also have this effect, but, on the other hand, the number of metal ions that form complexes with the dye increases when the pH is raised. The increase in sensitivity of the reaction may therefore be made at the expense of selectivity. Compound TAN TANK TAN36S a-TAN DTAN P-DTAN Ion cu2+ Pb2+ ~ 1 3 + AP+ Sn4+ cue+ Nie+ ~ 1 3 + ~ 1 3 + 1n3+ Ga3+ Hga+ cu2+ ~ 1 3 + - Cu*+ - TABLE X THIAZOLYLAZONAPHTHOLS AS INDICATORS Limit of p1-l.determination Colour change Remarks 3 3 1.2-5.5 3-5 3.2-7.5 3.1-8.0 - 3-8 4.5-8-5 6-9 - 1 *2-5.5 3-0 3.0 - - - 2.7-3.0 1.8 8.00-9.40 3.5 4.90-6.00 3-60 mg Violet to yellow - Yellow to red - violet 10-4-10-6 M - 6-60 mg Yellow to violet - Yellow to red - I Blue - violet to - Violet to yellow - Violet to yellow - Blue - violet to - Violet to yellow violet yellow yellow yellow 1-8 p.p.m. Blue - violet to 10-4-10-6 M - 2-10 mg Violet to yellow 2-10 m p Violet to vellow EDTA direct Back-titration of EDTA EDTA direct Back-titration of EDTA with Cu2+ Back-titration of EDTA with Cu2+ EDTA direct EDTA direct EDTA direct EDTA direct EDTA direct EDTA direct EDTA direct EDTA direct EDTA direct 2-10 m i Red to orange EDTA direct 2-10 rng Orange to vellow EDTA direct Reference 182 182 119 124 125 141 142 183 141 183 184 119 163 164 163 164 - Violei to yhlow hlercurimetric determination of C1-, Br-, SCN- 44 - Blue to orange EDTA direct 146 - Blue - violet to EDTA direct, orange together with Bia+ 167 Pink to blue Acid - base indicator 143, 154 - Blue to orange EDTA direct 154 - Orange to blue Acid - base indicator 164 - It is a great advantage that the bathochromic shift of the complexing reaction is often con- siderably larger for some metal ions than others, as the former can then be determined in the presence of the latter without a preliminary separation.In some instances the use of highly sensitive reagents is necessary, but usually the most important aim in the search for new reagents is to prepare a reagent with highly selective properties with respect to certain ions. However, it must be admitted that, in most instances, the selectivity of the reagents is not high enough to enable any of the elements to be determined in the presence of the others. Therefore, one or more interfering elements must often be removed or masked in the analytical procedures. Consequently, the selectivity of the reagent with respect to a certain metal ion is governed by the extent of the bathochromic shift of the complex formation and the pH at which the reaction can take place quantitatively, the sensitivity being reflected by the value for the absorptivity of the complex.These aspects should be kept in mind when the analytical useful- ness of the reagents is considered. Thus TAR, which forms complexes with nearly all of the792 HOVIND : THIAZOLYLAZO DYES AND THEIR Analyst, Vol. 100 metals, is most sensitive as a photometric reagent for cobalt(I1) and scandium(III), but its selectivity is poor. However, the selectivity for thorium(1V) is much better, leading to a very marked colour change on complexation, and for thallium(III), which forms a complex with TAR at very low pH. Copper(I1) ions react with almost all of the dyes reviewed here, but only a-TAN, TAC and the tetrahydrobenzothiazolylazo derivatives give rise to sufficiently great colour changes. As the complexes are formed in rather acidic media, these dyes are to some extent selective as re- agents for copper(I1) ions.The most sensitive reagent for copper is PBTAR, but cobalt and nickel also form strongly coloured complexes with this reagent at the same pH. The highest selectivity is obtained with DHBTAR and a-TAN. Cobalt(I1) and nickel(I1) ions also form stable complexes with most of these dyes. The absorptivities of the cobalt complexes are generally considerable, and the most sensitive reagent for cobalt is TAAR. However, the complexes are formed quantitatively only in a weakly acidic or alkaline medium, SO that none of these dyes are selective as reagents for cobalt. The highest selectivity is attained by oxidation of cobalt to cobalt(III), followed by an extraction - photometric determination of the BrBTAN complex. The most selective reagents for nickel are the alkyl-substituted phenol derivatives, while TAM, STARN and TARN are more sensitive.The greatest sensitivity is attained by the extraction - photometric determination of the nickel(I1) ions with HBTAQ, but this reagent is also sensitive towards cobalt and silver. The benzothiazolylazo derivatives constitute selective reagents for the extraction - photo- metric determination of zinc and cadmium. Thus the best selectivity for zinc is attained with BTAMP and a-BTAN, the latter also being a very sensitive reagent. Cadmium is best determined with BrBTAN. The complexes with mercury( 11) generally have rather low absorptivities. TAMP, however, constitutes an exception and is the most suitable of these re- agents for the determination of mercury, both the selectivity and the sensitivity being accept- able. The colour intensity of the palladium(I1) complexes is generally weak compared with that of the other elements, but the reduced sensitivity is partly compensated for by the very high selectivity caused by the large bathochromic shifts of the complexing reaction, especially with TANC, BrTANC, TAN and BrTAN.Of these dyes BrTAN is the most sensitive and selective reagent for the extraction - photometric determination of palladium(I1) ; only large excesses of other platinum metals cause interference. The chromotropic acid derivatives are sensitive photometric reagents for aluminium in acidic media, while the cresol and pyrocatechol derivatives are among the most satisfactory reagents for the other B sub-group metals.Also, transition metals that form oxy-anions have been determined with these reagents. For the determination of transition metals of higher valency, some substituted aminophenol and chromotropic acid derivatives have proved to be useful, but the selectivity is dependent on the use of masking agents and methods of separation. The stability constants of the metal - dye complexes are lower than those of the EDTA chelates, and some of these dyes have been used as indicators for the direct complexometric titration of B sub-group metals and, in particular, of the transition metals of Groups I and 11. Because of the slow indicator change near the end-point, heating of the solution or the addition of reagents is often required in order to increase the reaction rate.The indirect method of back-titration of the excess of EDTA with standard copper(I1) solution has been performed only for some B sub-group metals, and in the replacement titrations of some metal ions only copper(I1) - TAR has been used as the indicator. The most satisfactory metallochromic indicators are those with which the dissociation of a proton from the phenolic group occurs at a higher pH, and the 1 : 2 complexes are less easily formed. In this respect TANK is highly favoured as an indicator, especially for the determina- tion of nickel. Also, TAR and some other phenol derivatives have been used for the complexo- metric determination of some metals. As none of these reagents are selective, the application of masking agents is necessary in some instances, and the carrying out of the titration at a lower pH is often preferred in order to avoid interferences from other metal ions.f i e most important dyes for use with determinations in aqueous solutions are those which contain one or more groups such as the hydroxy and sulphonic acid groups, which confer the property of solubility in water. Thus TAR and its derivatives, and chromotropic or other sulphonic acid derivatives, have been used for photometric determinations in aqueous solutionsNovember, 1975 APPLICATIONS IN ANALYTICAL CHEMISTRY. A REVIEW 793 and also as metallochromic indicators. For solvent-extraction procedures, the less water- soluble benzothiazolylazo dyes have proved to be useful ; also, the alkyl-substituted phenol derivatives and TAN are suitable for the extraction of some metal ions.The colour change with the naphthol dyes is generally greater than that with the phenol dyes. The thiazolylazo dyes thus include a relatively large class of analytical reagents with a wide range of applications. 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ISSN:0003-2654    

DOI:10.1039/AN9750000769

出版商:RSC    

年代:1975

数据来源: RSC

摘要:

AIzalyst, November, 1975, Vol. 100, $9. 797-805 797 The Spectrophotometric Characteristics of Aqueous Solutions of a- and P-Molybdosilicic Acids Victor W. Truesdale and Christopher J. Smith Institute of Hydrology, Maclean Building, Crowmarsh Giflord, Wallingford, Oxfordshire, OX10 8 B B Knowledge of the spectrophotometric properties of a- and ,%molybdosilicic acids is required for analytical and structural chemical studies. These pro- perties have been studied under various conditions. The conditions with respect to pH and molybdate concentration used for the formation of, and during the subsequent investigation of, the acids were studied. At a given pH and molybdate concentration the shapes of the absorption spectra of the two acids are very similar. That part of the spectrum between 370 and 400 nm is independent of molybdate concentration and pH.However, that between 290 and 370nm is critically dependent on both of these variables. These results suggest that the molybdosilicic acids form charge-transfer or other complexes with molybdate species in solution. The implications of these findings, which conflict with the results obtained by most previous workers, are discussed. The literature contains little reliable information concerning the absorption spectra of a- and /?-molybdosilicic acids. There are a t least two reasons why this situation is sur- prising. Firstly, much silicate analysis is conducted colorimetrically by making use of the yellow colour of the molybdosilicic acid. Clearly, a knowledge of the absorption spectra of these acids would be of assistance in such analyses.Secondly, the absorption spectra of the two acids are likely to give valuable information to the structural chemist. So far, studies on the structure of these acids have been hampered by the fact that pure solid samples of the acids are difficult to prepare,lV2 and as a result, w o r k e r ~ ~ ~ ~ have relied mostly upon “solution methods,” eg., polarography, in order to gain information about these molecules. Nevertheless, the approach by use of the absorption spectra, which has proved invaluable in structural studies of many other compounds, has not been exploited. Our attention was drawn to the unreliability and dearth of literature on this subject while conducting studies of the rate at which the p-acid is transformed into the a-acid.We thought it to be expedient to measure the absorbance of molybdosilicic acid solutions at a wavelength of approximately 320 nm, at which wavelength the literature3+ indicated that the a-acid would have a higher absorbance than the /3-acid. In practice, however, the converse was found to be true. As the results of this preliminary work conflicted with those of most previous work on this topic we investigated the problem further and report our findings below. Our experience with these spectrophotometric studies suggests that Strickland’s3 acid to molybdate ratio concept is probably responsible for the lack of use of the ultraviolet - visible spectra of the acids. In an earlier paper’ we explained how this concept had confused workers about the conditions under which these acids are formed.It was shown that the conditions for their formation should be described rigorously in terms of pH and molybdate concen- tration present in the reaction mixture, and not in terms of acid to molybdate ratios. We presume that the confusion surrounding these conditions has been extended to the conditions used during determinations of the absorption spectra of these acids. Thus, it seems that once workers had adopted the acid to molybdate ratio for the conditions for formation, it was difficult to return to the logical use of pH and molybdate concentration as essential controls of solution conditions during subsequent investigations, Experimental Apparatus The apparatus employed was similar to that used previously.’ Absorbances were measured by means of either a Hilger and Watts Uvichem 1600 or a Unicam SP1800 spectrophotometer.An EIL, Model 23A, direct-reading pH meter was used, which was standardised a t pH 4.0798 TRUESDALE AND SMITH : SPECTROPHOTOMETRIC CHARACTERISTICS AnaEyst, VOl. 100 by means of Soloid buffer tablets (Burroughs Wellcome & Co.) and at pH 1.0 by using a mixture of 25 ml of 0.20 M potassium chloride solution and 67 ml of 0.2 M hydrochloric acid.8 No correction has been applied for differences in ionic strength that exist between standards and other solutions. The reagents were prepared in the way described earlier.’ As practised in earlier work7 reaction mixtures were not made up to a final fixed volume, but instead, reliance was placed on the high precision afforded by the use of Zipette syringe pipettes (1-25 ml) and Eppendorf pipettes (0-1 ml).Method In principle, the approach used previously7 was adopted here. Unless stated otherwise, the molybdosilicic acids were prepared under one set of conditions before various reagents were added in order to produce a second set of conditions under which the absorption spectrum was measured. Solutions (26.0ml) of the acids were prepared by adding 1.00ml of a 100 mg 1-1 silicate-silicon solution to 25.0 ml of acidified molybdate solution. The latter solution was prepared by mixing appropriate amounts of a 0.25 M molybdate-molybdenum stock solution with distilled water and 0.5 or 5-0 N sulphuric acid. The pH of the solution was set at 4.0 for a-acid production and at 1.8 for production of the P - a ~ i d .~ Blank solutions (0-0 mg 1-1 of silicon) were prepared by substituting distilled water for the silicate standard. The absorbance (at 390 nm) of the molybdosilicic acid was allowed to attain a steady value (requiring usually from 5 to 30 min). Depending upon the objective of the experiment, up to 5.00ml of sulphuric acid, sodium acetate or sodium chloride solution were added to produce the new conditions. Additional distilled water was added to give a final volume of 31.0 ml. The absorption spectra were then measured in 1-cm cells with the blank solution in the reference beam. Results The objective was to examine the absorption spectra of a- and p-molybdosilicic acids at various pH values and molybdate concentrations.This was shown to be a relatively simple operation provided that three problems are overcome. Firstly, the effect of a change in the pH of the reaction mixture on the absorbance of the background molybdate solution (blank) must be considered. Secondly, when dealing with the 13-acid, the rate of the reaction by which the /3-acid is transformed into the a-acid must be assessed. Thirdly, the effect that temperature exerts upon the absorbance of the background molybdate solution (blank) must be controlled. The results of studies of these three problems are reported, followed by a description of the spectra obtained. Effect of pH on Molybdate Spectra The necessity to form the molybdosilicic acid at a particular pH before adjusting the pH to the value at which the spectrum was to be determined has already been expressed.With the a-acid this requirement presents no difficulty; acid is added to the reaction mixture in which the a-acid has formed (and to a corresponding blank) and the absorption spectrum obtained. In contrast, when the pH of a reaction mixture in which the /3-acid has formed (pH below 143) is increased, the resulting mixture (and corresponding blank) has an absorbance (370-300 nm) that is variable over a period of time. From Collin, Lagrange and Schwing’ss work, it seems that the decondensation of MO,O,,~- molybdate species is responsible for these changes. Several preliminary experiments were performed in order to ensure that the interference from this decondensation reaction would not distort the absorption spectrum of the /3-acid.An example of the over-all change in absorbance that accompanies decondensation is given in Table I, where the absorption spectra of 0.025 M molybdate-molybdenum solution at pH values of 1.5 and 2.5 are presented. It can be seen that this pH change induces rela- tively large changes in absorbance in the wavelength range 320-370 nm. Other experiments have shown that after the pH of a molybdate solution is raised from 1.5 (by adding sodium acetate) the absorbance (320-370 nm) decreases exponentially with time. This first-order behaviour of the decondensation reaction, in the pH range 14-26, is similar to that found previouslyg at a pH of approximately 10. Another experiment (Table 11) showed that although the magnitude of the rate constant was similar when the reaction was studied at any wavelength between 320 and 370nm, it nevertheless attained a maximum value atNovember, 1975 OF AQUEOUS SOLUTIONS OF a- AND /~-MOLYBDOSILICIC ACIDS 799 TABLE I ABSORBANCE (1-Cm CELLS) OF 0.025 M MOLYBDATE SOLUTION AT VARIOUS WAVELENGTHS, AND RELATIVE TO DISTILLED WATER Absorbances above 1.60 were obtained by differential spectrophotometry.Wavelengthlnm . . 320 330 340 350 360 370 380 390 Absorbance at pH 1-5 3.03 2.85 2.14 1.11 0.450 0.180 0.073 0.041 Absorbance at pH 2.5 2.68 1.77 0.73 0.28 0.110 0.047 0.024 0.023 Difference 0.36 1.08 1-41 0.83 0.340 0.133 0.049 0.018 approximately 340 nm, the wavelength at which the absorption spectra for molybdate at pH values of 1.5 and 2.5 differ most. Further experimentation showed that these changes in molybdate speciation affect the molybdosilicic acid solutions and their corresponding blanks to the same extent. The rate constants in both molybdosilicic acid solution and the corresponding blank agreed to within 6 per cent.The rate constant (observed at 350nm and 17 "C) was found to be at a minimum at pH 1.9 and to increase rapidly as the pH was increased to above 2.5 (Table 111). The rate constants supply an estimate of the time required for these changes to occur (Table 111). TABLE I1 FIRST-ORDER RATE CONSTANTS FOR THE DECONDENSATION OF M0,0,,6- AT pH 2.5 AND APPROXIMATELY 17 "c, AS OBSERVED AT FIVE DISCRETE WAVELENGTH SETTINGS Wavelength/nm . . .. . . 320 330 340 350 360 Rate constant/min-l (f 0.02) . . 0-86 0.99 1.08 0.92 0.89 The results of the above experiments show that interference by the decondensation reaction can be avoided by allowing the reaction to approach equilibrium in each molybdosilicic acid solution and its corresponding4dank.Although, in principle, it would be possible to avoid this interference by initiating the decondensation reaction simultaneously in both molybdo- silicic acid solution and blank, in practice this would be difficult to achieve. Consequently, we feel that the former approach is more suitable. Fortunately, the fastest decondensation rates, and therefore the shortest reaction times, occur at the highest pH values. It is therefore unnecessary to allow a considerable time lapse at the high pH values, at which the transformation of the /&acid into the a-acid also occurs more rapidly.TABLE I11 VARIATION OF THE RATE OF DECONDENSATION OF M0,0,,6- SPECIES WITH pH AT 17.0 "c Starting solution prepared a t pH 1.50, 0.025 M in molybdenum. Final pH . . . . 1.70 1.80 1.89 2-02 2.30 3.00 3-41 3.72 3.80 Rate constant/min-l . . 0.95 0.77 0.78 0.64 1.09 1-83 2-96 4.61 >5*0 Time for 99.8 per cent. reaction/miii . . 6.5 8.1 8.0 9.7 5.7 3.4 2.1 1.4 <1*2 Transformation .of 18- into a-Molybdosilicic Acid The transformation of p-molybdosilicic acid into the a-acid presents another problem in the accurate measurement of the absorption spectrum of the /%acid at high pH values. The results of our unpublished studies of this reaction show that, under the conditions used here, an exponential rate law applies, and that the rate increases with increasing pH.At a tem- perature of 17 "C, and pH values greater than 3-0, the rate is sufficiently high to introduce errors into these experiments. We estimate that the maximum standing time of 5 min used in our experiment will have allowed up to 11 and 16 per cent. of the 18-acid to become trans- formed at pH 3-0 and 4.0, respectively. However, although producing some inaccuracy, transformation to this extent does not affect the essence of our conclusions.800 TRUESDALE AND SMITH : SPECTROPHOTOMETRIC CHARACTERISTICS AIz&!yd, VOl. 100 Effect of Temperature on Molybdate Spectra Experience has shown that the absorbance (320-390 nm) of an acidified molybdate solution, relative to distilled water, decreases markedly with increase in temperature of the solution.Thus, when replicate aliquots of an acidified molybdate solution are placed in both reference and sample beams of a spectrophotometer, an apparent absorption spectrum is generated if the two aliquots are not isothermal. This apparent absorption spectrum is similar to that of molybdosilicic acid when the colder solution is in the sample beam. The height of the peak produced by a given temperature difference decreases as the pH is increased from 1.0 to 4.0. We prevented an error of this kind from distorting the molybdosilicic acid absorption spectra by taking measurements at a standard temperature (17.0 & 0.5 "C). 1.4 0 -2 290 300 32 0 340 360 380 400 Wavelength/nm Fig. 1. The absorption spectrum (l-cm cell) of a-molybdo- silicic acid a t various pH values (0.008 M molybdate-molyb- denum concentration, 32 mg 1-1 of silicon as the ct-acid).pH: A, 1.43; B, 0.80; C, 2.24; and D, 4.3. Absorption Spectrum of a-Molybdosilicic Acid Experiments showed (Figs. 1 and 2) that the part of the spectrum between 370 and 400 nm is unaffected by a change either in pH from 4.0 to 1.0, or in molybdate-molybdenum con- centration from 0.004 to 0.040~. In contrast, the part of the spectrum between 290 and 370nm is sensitive to changes in both of these variables. 1.61 I 290 300 320 340 360 380 400 Wavelengthhm Fig. 2. The absorption spectrum of a-molybdo- silicic acid a t two molybdate concentrations. In each instance 32 mg 1-1 of silicon were present. Lower curve refers to 0.040 M molybdate concentration and the upper curve to 0.004 M molybdate concen- tration.Both solutions a t pH 4.0.November, 1975 OF AQUEOUS SOLUTIONS OF 01- AND /3-MOLYBDOSILICIC ACIDS 801 From Fig. 1 it can be seen that the peak absorbance of a-molybdosilicic acid in 0.008 M molybdate-molybdenum solution shifts to longer wavelengths and is decreased in height when the pH is lowered from 4.0 to 1-43. However, the results also show that further decreases in pH below 1.43 reverse this trend. This finding is confirmed by other, more detailed, results (Fig. 3) which show how the absorbance (325 nm) of a-molybdosilicic acid solution varies with change in pH. The changes in absorbance illustrated in Fig. 3 reflect the shifts in the position and size of the absorption peak. The reversal of this shift is marked by a well defined minimum absorbance at pH 1.5.However, the absence of a distinct minimum in the curves for 0-036 and 0-040 M molybdate solution does not indicate a change in the over-all pattern of behaviour. At these higher molybdate concentrations, the peak absorbance is shifted so far towards higher wavelengths that at 325nm only one of the tails of the spectrum is observed. la6* 1 .b - A - c 3 1.2- .- 1.0- % 0 . 8 - F i m Q Y $ 0 . 6 - $ d 0.4 - a 0.2 - 0 1.0 2 -0 3.0 4-0 5.0 PH Fig. 3. Variation of absorbance (325 nm, l-cm cell) of rnolybdosilicic acid solutions with pH and molybdate concentration. Continuous lines refer to the a-acid. Molybdate-molybdenum concentrations are : A, 0.040; B, 0.036; C, 0.016; D, 0-012; E, 0.008; and F, 0.004 M.Broken lines refer to the ,&acid; the molybdate concentration of any /3-acid solution is the same as that used to obtain the graph for the a-acid occurring immediately below the graph for the /3-acid. In all instances 32 mg 1-1 of silicon were present. Fig. 2 shows the change in the absorption spectrum of a-molybdosilicic acid that accom- panies a change in background molybdate-molybdenum concentration from 0.004 to 0.040 M at pH 4.0. It can be seen that although the position and height of the peak changes, the shape is similar in the two instances. Other results (Table IV) show that these changes in the position and height of the peak occur gradually, and that there is no reversal in trend as is observed with pH. Further, the results given in Fig. 3 demonstrate that this trend is apparent at all pH values between 0-7 and 4-0.TABLE IV VARIATION IN THE PEAK ABSORBANCE AND ITS WAVELENGTH (1-CM CELL) OF 01-MOLYBDOSILICIC ACID, AT pH 4.0, PRODUCED BY CHANGE IN THE CONCENTRATION OF MOLYBDATE IN THE MIXTURE In each instance 32 mg 1-I of silicon were present as the u-acid. Molybdate-molybdenum concentration/M 0.004 0.008 0.012 0.016 0.020 0.036 0.040 Peak absorbance . . .. .. . . 1.51 1.48 1.39 1-37 1.23 1.10 1-10 Wavelength of peak absorbance . . .. 322 322 325 328 329 334 335802 TRUESDALE AND SMITH : SPECTROPHOTOMETRIC CHARACTERISTICS Analyst, VoZ. 100 Absorption Spectrum of j?-Molybdosilicic Acid The results obtained at a constant molybdate concentration showed that pH-induced changes in the #I-molybdosilicic acid spectrum are similar to those observed with the a-acid.In fact, a facsimile of Fig. 1 was obtained when the same range of concentrations was covered. This similarity of behaviour led us to compare the spectra of the two acids under identical con- ditions of molybdate concentration and pH. It was found that at wavelengths between 290 and 400nm the absorptivity of the P-acid is always higher than that of the a-acid. Moreover, the absorption spectrum of the p-acid mimicked that of the a-acid; the spectra do not intersect. An example of the results that we obtained is presented in Fig. 4. Further confirmation of this behaviour is supplied in Fig. 3, which shows that the reversal, at pH 1.5, of the pH-induced changes in absorbance of the a-acid is also imitated by the /?-acid.Wavelengthhm Fig. 4. The absorption spectra of a- and p-molybdosilicic acids at pH 1.38 and in 0.040 M molybdate-molybdenum solu- tions. Both solutions contained 32 mg 1-l of silicon as molybdo- silicic acid. Effect of Sodium Chloride on the Spectra of the Two Acids The effect upon the spectra of increasing the ionic strength of the solution containing a molybdosilicic acid was investigated. Up to 5-00 ml of 5 N sodium chloride solution were added to 26-0 ml of each molybdosilicic acid solution, prepared in the way described above. The behaviour of both a- and p-acids towards the sodium chloride is identical, For each acid at pH 4.0 the salt did not seem to have any effect; the spectra obtained after addition of either distilled water or 0.8 M sodium chloride solution differed by not more than 2 per cent.In contrast, at pH 1.6 the absorbance (between 290 and 350 nm) of both acids was enhanced when the sodium chloride was added. In each instance the peak absorbance and its wave- length were changed by 7.0 -& 0.5 per cent. and 1 nm, respectively, by the addition of sodium chloride. Correction of Earlier Work As stated above, contrary to the findings of Garrett and Walker,4 the absorption, spectra of the a- and j?-acids do not intersect when they are determined under conditions that are identical with respect to pH and molybdate concentration. As a result, the predictions regarding the variation of absorbance (at approximately 330 nm or less) of mixed solutions of molybdate and silicate, made in our earlier paper,' are incorrect.As this contradiction might be confusing, and thereby inadvertently detract from the over-all value of our previous work,' we felt that it was necessary to rationalise it. Accordingly, we repeated the experiment in which the molybdosilicic acids were formed at various pH levels by mixing molybdate and silicate solutions. However, in this second experiment the absorbance was measured at 335 nm instead of 390 nm. A slightly higher silicate-silicon concentration was adopted so as to enable spectrophotometer cells of shorter path length (1 cm) to be used. This step was necessary in order to maintain the absorbance of the blank (acidified molybdate solution) at a tolerable level. The results of the experiment are presented (Fig. 5) together with those at 390nm for comparison.It can be seen that at 335 nm the plateaux for 18- and a-acids, at pH values between 1.0 and 1.8, and 3.8 and 4-8, respectively, are absent, Indeed, over the pH range appropriate to /?-acid formation the absorbance changes markedly. These results can be explained in the following way. Firstly, from the above it can be seen that at 390nm theNovember, 1975 OF AQUEOUS SOLUTIONS OF a- AND /~-MOLYBDOSILICIC ACIDS 803 absorptivity of both acids is independent of pH. Therefore, the trends in the graph for 390 nm (Fig. 5) merely reflect changes in molybdosilicic acid speciation. In contrast, the absorptivity of both acids at 335 nm changes considerably with pH. Therefore, the trends in this graph (Fig. 5) for 335 nm reflect changes in both absorptivity and speciation.Thus, for example, the results show that at pH values between 1.5and 2.2 the changes in absorptivity (335 nm) of the acids exert greater control over the absorbance (at 335 nm) than does speciation. 01 I I I I 1.0 2.0 3.0 4.0 5.0 PH Fig. 5. The absorbance (335 nm, 1-cm cell) of yellow molybdosilicic acid solutions formed at various pH values (silicate-silicon content, 3.0 mg 1-1 ; molybdate-molybdenum concentration, 0.025 M). Thirty minutes allowed for formation of the molybdosilicic acids. For comparison, the earlier results' obtained by the same method, but at 390 nm, are shown in the lower graph. Discussion The similarity between the absorption spectra of the two molybdosilicic acids is striking. Although the wavelength of maximum absorbance changes with pH, it changes to the same extent with both acids.This fact in itself suggests that the basic structure of these two compounds is similar. From this and other evidence3 it seems likely that the compounds are isomeric. However, studies carried out concurrently with this work suggest that they are not optical isomers. Tests performed with the a-acid (pH 4.0, 0-025 M in molybdate-molyb- denum) and the /?-acid (pH 1-5, 0.025 M in molybdate-molybdenum) formed in a 10 mg 1-1 silicate-silicon solution did not show any evidence of optical activity. Measurements were made at 18 "C in a polarimeter with a 20-cm cuvette and using the sodium D-line. The results presented above are important in the analytical chemistry of silicates and show that there is no wavelength between 290 and 400nm at which the absorptivities of the two acids are equal under any single set of conditions.Therefore, it is not possible to develop a colorimetric procedure involving the yellow acids that is independent of the form of the acid produced, as has been suggested previously.* This finding emphasises, therefore, that the conditions under which mixtures of the acids are formed should be avoided in an analytical procedure. The results also show that, for analytical purposes, wavelengths greater than 370nm are the most suitable. At shorter wavelengths the absorbance is critically dependent on pH and molybdate concentration and, consequently, more susceptible to errors made during the dispensing of reagents. Also, the magnitude of the molybdate blank increases markedly as the wavelength is shortened, making the determination of low concentrations of silicate more inaccurate and perhaps impossible, It has been shown that the absorption spectra of the molybdosilicic acids are modified by changes in both the concentration of molybdate-molybdenum and pH of the reaction804 TRUESDALE AND SMITH : SPECTROPHOTOMETRIC CHARACTERISTICS Analyst, VoZ. IOU mixture.These changes are not merely “non-specific” salt effects, as the addition of much larger amounts of sodium chloride imposed relatively insignificant changes. At pH values above 1.5, a decrease in pH decreases the total absorption. This acid-induced change is therefore similar to that experienced when the molybdate concentration is raised.In contrast, at pH values below 1.5, decreases in pH have the opposite effect; they raise the total absorp- tion. These results are interesting because they imply that, in solution, the molybdosilicic acids interact with both protons and molybdate species. We feel that these phenomena can be explained most simply in terms of the formation of a charge-transfer or other complex composed of a molybdosilicic acid and a molybdate species. Thus, two equations are required in order to define the system; one for the molybdate species and the other for the complex: (2n-m)- m n + m H+ +- Mono 4n-m + H,O .. - * (1) ( 8) and (2n-m)- M O ” O ( 4 . 2 ) + molybdosilicic acid + complex for a single set of m and n values, and each acid. Previous ~0rklO-l~ (see below) gives the values 1, 7 or 8 for n.The measured absorption spectrum is therefore believed to be the sum of the spectra of the complexed and the uncombined molybdosilicic acid. The above model is consistent with the observed behaviour provided that the total absorp- tion (between 290 and 400nm) of the complex is less than that of the corresponding free molybdosilicic acid under any given set of conditions. This constraint is imposed by the fact that, at constant pH, an increase in molybdate-molybdenum concentration of the solution reduces the total absorption. Extra molybdate would shift both equilibria to the right and thereby increase the amount of complex formed. The model can also account for the observed behaviour of the spectra towards changes in the pH of the reaction mixture down to pH 1.5.An increase in the acid concentration shifts both equilibria to the right, thereby increasing the concentration of the complex. At pH values below 1.5, a t which an increase in acid concentration increases the total absorption, the amount of complex formed becomes progressively less because molybdate-molybdenum is re-directed to form other species that do not interact with the molybdosilicic acids. The significance of the turning point at pH 1-5, therefore, is that the process by which the absorption is lowered by a change in pH exactly balances the effect of the process by which the absorption is raised by a change in pH. It is important not to interpret the turning point as indicating that a new molybdate species starts to form at pH 1.5.The model is consistent with the views of who have studied the speciation of molybdate. .These authors seem to be unanimous in their view that when a molybdate solution is acidified, a number of molybdate species are formed by the following transitions: MOO,” -+ M O , O ~ ~ ~ - -+ Mo,O,,*- -+ larger condensed units As Lindquistlo suggested that the second of these transitions occurs at pH values between 2-9 and 1.5, it seems likely that n in equations (1) and (2) will have the value of 7 or 8. The discovery of the interaction between molybdosilicic acids and molybdate species in solution raises the question of whether the reactivity of the acids is dependent upon their forming these complexes. Such considerations will probably supply the reasons for the difficulties experienced in preparing pure solid samples of the acids. References 1. 2, North, E. 0.. and Haney, W., in Booth, H. S., Editor, “Inorganic Syntheses,” McGraw-Hill Book Co., New York, 1939, p. 127. Tsigdinos, G. A., and Hallada, C. J., in Mitchell, P. C. H., Editor, “Chemistry and Uses of Molyb- denum,” Symposium Volume, University of Reading, September, 1973, Climax Molybdenum Co. , London. 3. 4. 5. 6. 7. Strickland, J. D. H., J . Am. Chem. Soc., 1952, 14, 862, 868 and 872. Garrett, H. E., and Walker, A. J., Analyst, 1964, 89, 642. Langer, K., 2. Analyt. Chem., 1969, 245, 139. Ringbohm, A., Ahlers, P. E., and Siitonen, S., Analytica Chinz. Acta, 1959, 20, 78. Truesdale, V. W., and Smith, C. J., Analyst, 1975, 100, 203.November, 1975 OF AQUEOUS SOLUTIONS OF U- AND /~-MOLYBDOSILICIC ACIDS 805 Weast, R. C., Selby, S. M., and Hodgman, C. D., Editors, “Handbook of Chemistry and Physics,” Collin, J. P., Lagra;flge, P., and Schwing, J. P., in Mitchell, P. C. H., Editor, “Chemistry and Uses Symposium Volume, University of Reading, September, 1973, Climax Molyb- Lindquist, I., Nova Acta R. SOG. Scient. Upsal., 1950, Series 4, 15, 1. Lindquist, I., Acta Chem. Scand., 1951, 5, 568. Sasaki, Y., and Sillkn, L. G., Acta Chem. Scand., 1964, 18, 1014. Sasaki, Y., Lindquist, I., and Sillen, L. G., J . Inorg. Nucl. Chem., 1959, 9, 93. 8. 9. 46th Edition, Chemical Rubber Co., Cleveland, Ohio, 1965-66, p. D73. of Molybdenum, denum Co., London. 10. 11. 12. 13. Received May lst, 1976 Accepted June 17th 1976

ISSN:0003-2654    

DOI:10.1039/AN9750000797

出版商:RSC    

年代:1975

数据来源: RSC

摘要:

806 Analyst, November, 1975, Vol. 100, p p , 806-809 Gasometric Micro-determination of Amino, Amide, Anilide, Oxime, Nitro and Nitroso Groups in Organic Compounds Saad S. M. Hassan and K. A. Tollan Research Microanalytical Laboratory, Department of Chemistry, Faculty of Scielace, A i n Shams University, Cairo, Egypt A simple gasometric micro-method is described for the rapid determination of the amino group in aromatic amines, a-amino-acids and carboxamides. The method is based on a deamination reaction whereby 1 mol of nitrous oxide is liberated per mole of compound. Anilides are first hydrolysed with hydrochloric acid, and nitro and nitroso compounds and oximes are reduced with zinc and hydrochloric acid to the corresponding amino compounds, and are then determined in a similar manner.Results within &0-2 per cent. absolute of the theoretical group-nitrogen content were obtained for most of the compounds under investigation. Gasometric determination of the amino group in organic compounds by reaction with nitrous acid has certain disadvantages in that most primary aromatic amines do not react quantitatively, a variety of non-nitrogenous compounds interfere and the decomposition products of nitrous acid introduce serious err0r.l However, it has been reported that some amino compounds quantitatively reduce the nitrate ion to nitrous oxide.- This reaction was used successfully by Renard and Deschamps5 for the gasometric determination of some a-amino-acids, and in the present investigation it was applied to the micro-determination of aromatic amines, a-amino-acids and carboxamides.Use of the reaction was also extended to the determination of a variety of nitrogen-containing functional groups, which produce the amino group either by hydrolysis as anilides, or after reduction as nitro and nitroso com- pounds and oximes. In addition to the simplicity and accuracy of the method, it is free from the demerits of the gasometric methods that are based on reaction with nitrous acid. In terms of equip- ment required and analysis time, the present method is superior to the available titrimetric or spectrophotometric methods for the determination of these functional groups.1 Experimental Reagents All reagents were of analytical-reagent grade unless otherwise specified. Commercial grade amino, nitro, nitroso and anilide compounds were purchased from BDH and purified by repeated crystallisation. cc-Amino-acids (chromatographic grade), carbox- amides and oximes were used without further purification.Apparatus The apparatus described previously2 was used. Determination of the Amino Group in Aromatic Amines, a-Amino-acids and Car- boxamides Weigh accurately 3-5 mg of the sample into the reaction vessel and add 1 ml of 11 N hydro- chloric acid. Connect the reaction vessel between a Dewar flask filled with solid carbon dioxide and a micro-nitrometer filled with 50 per cent. potassium hydroxide solution. Displace the air in the apparatus with carbon dioxide at the rate of 100 bubbles per ininute, or until no further air bubbles are collected in the nitrometer.Add 5 ml of a 1 + 1 mixture of 16 N nitric acid (70 per cent.) and 11 N hydrochloric acid (35 per cent.) by use of the funnel. Gently heat the reaction mixture until no gas bubbles are evolved (10 min) by using a micro- burner. Sweep out the nitrous oxide with carbon dioxide and allow the gas to stand in theHASSAN AND TOLLAN 807 nitrometer for 2 min. Carry out a blank determination and calculate the group-nitrogen content: 14.01 x 273 (V - V") (P - PO) 22.4 x 760 (273 + I') W Group-N (yo) = where V is the volume of nitrous oxide liberated, Vo is the blank value, P mmHg is the atmospheric pressure and Po mmHg the water vapour pressure at temperature T "C, which is the average room temperature and also the temperature of the potassium hydroxide solution contained in the nitrometer, and Wmg is the mass of sample.Determination of the Anilide Group Weigh accurately 3-5 mg of the sample of anilide into the reaction vessel. Add 2 ml of 11 N hydrochloric acid, gently heat the reaction mixture to dryness and repeat four times. Cool, and complete the procedure as above. Determination of the Nitro, Nitroso and Oxime Groups Introduce 1 ml of glacial acetic acid and, if necessary, warm the vessel in order to dissolve the sample. Cool the solution, add about 100 mg of zinc powder and then add 0.2 ml of 11 N hydrochloric acid in order to start the reduction. When the evolution of gas slows down, add more acid (about 2 ml are needed). Heat the reaction vessel over a micro-burner, cool and complete the procedure as above. Weigh accurately 3-5 mg of the compound into the reaction vessel.Results and Discussion Certain pure compounds containing the various nitrogen groups under investigation were analysed and the results obtained are given in Table I. Most of the results obtained for amines, a-amino-acids, monoamides, diamides, anilides, monooximes, dioximes and mono- nitro, dinitro and nitroso compounds are within Zt0-2 per cent. absolute of the theoretical nitrogen content, although there are certain exceptions (e.g., 4-aminophenol, glycine, aspartic acid, 3-nitroaniline and 3,5-dinitrobenzoic acid). The over-all relative standard deviation is 3 1 per cent. and the mean recovery is 99.6 per cent. for sample sizes of 10-50 pmol. Aromatic Amines Primary aromatic amines react with a 1 + 1 mixture of 70 per cent.nitric acid and 35 per cent. hydrochloric acid to give 1 mol of nitrous oxide per mole of amino group. This reaction may proceed via nitration of the amino-nitrogen to give the nitramine group, which de- composes to nitrous oxide : Ar-NH, + HNO, + Ar-N-N=O + Ar-N=N-OH + Ar-OH + N,O 1 1 I I H 6 The quantitative formation of nitramines, by the reactions of some amino compounds with nitrate ions, and their quantitative decomposition into nitrous oxide have been reported.2-6 Unsubstituted arylamines and amines substituted with electron-attracting or electron- repelling groups are analysed satisfactorily. The amino group in a-amino-acids is similarly determined, as shown by Renard and De~champs.~ Although a reaction time of 10 min is found to be sufficient with many cc-amino-acids, serine, leucine and glutamic acid (sodium salts) require a longer reaction time (about 30 min) €or quantitative liberation of nitrous oxide.Carboxamides and Anilides oxide, is probably a route that competes with the N-nitration reaction: Hydrolysis of carboxamides to form ammonium nitrate, which decomposes to give nitrous R-CONH, + HNO, + H2O + R-COOH + [NH,NOJ .1 N,O + 2H20808 HASSAN AND TOLLAN : GASOMETRIC MICRO-DETERMINATION OF Analyst, VoZ. 100 Anilides, after hydrolysis to the corresponding primary amines, quantitatively undergo the deamination reaction. An amount of nitrous oxide equivalent to the anilide-nitrogen and the amino-nitrogen is simultaneously recovered from compounds containing both anilide and amino groups (e.g., 4-acetamidoaniline) .TABLE I GASOMETRIC MICRO-DETERMINATION OF NITROGEN-CONTAINING GROUPS I N ORGANIC COMPOUNDS Compound 4-Aminobenzoic acid 4-Aminoacetophenone 4-Aminophenol 2- Aminophenol 3- Aminophenol Sulphanilic acid 4-Aminosalicylic acid (sodium salt) 4-Aminop yridine Anilinium sulphate Anilinium chloride Aromatic amines u-A mino-acids Glycine Glycine methyl ester L-a- Alanine DL-Threonhe DL-T yrosine 2-Amino-2-methyl- propanoic acid a-Phenylalanine L-Cystine L-Cysteine hydro- chloride Aspartic acid (sodium salt) hydrochloride Group-nitrogen content, per cent. 7- Calcu- lated 10.2 10.4 12.8 12-8 12.8 8.1 6.6 14.9 9.9 10.8 18.7 11.2 15.7 11.8 7.7 13.6 8.5 11.7 8.9 10.5 Found 10.3 10.2 10.4 10.3 12.4 12.5 12.8 12-8 12.8 12.8 8.1 8.1 6.7 6.7 15.0 14.9 9.8 9.8 10.6 10.7 18.3 18.3 11.2 11-1 15.7 15.7 11.7 11.8 7-7 7.7 13.6 13.7 8-4 8.5 11.8 11.7 8.7 8.7 10.2 10-2 Re- covery, Per cent. 100.8 100.0 100.0 99.0 96-9 97.6 100.0 100.0 100.0 100.0 100.0 100.0 101.5 101.5 100.7 100.0 99.0 99.0 98.1 99.1 97.9 97.9 100.0 99.1 100.0 100.0 99.2 100.0 100.0 100.0 100.0 100-7 98.8 100.0 100.8 100.0 97.8 97-8 97.1 97.1 Compound A mides Benzamide Acetamide Urea Oxamide Anilides Acetanilide 4-Acetamidoaniline Benzanilide Group-nitrogen content, per cent.* Calcu- lated 11.6 23.7 46-7 31.8 10.4 18*6* 7.1 Nitro and nitroso compounds 4-Nitrobenzoic acid 7.9 2-Nitrophenol 10.0 3-Nitrophenol 10.0 3-Nitroaniline 20.0t 4-Hl-droxy- 3-ni tro- 5.3 phenylarsonic acid 3,5-Dinitrobenzoic acid 13.2 l-Nitroso-2-naphthol 8.1 4-Nitrosodimethyl- 9.3 aniline Nitroso-R salt 3.7 Oximes Salicylaldehyde oxime 10-2 Dirnethylglyoxal 24.1 C yclohexanedione 19.7 Benzoin a-oxime 6.2 dioxime dioxime Found 11-5 11.7 23.6 23.6 46.7 46.5 31.7 31.7 10.2 10.3 18.7 18.6 7.0 7.0 7.7 7.7 10.1 10.1 9.8 9.8 20.0 20.3 5.4 5.4 12.9 12.9 8.0 8.0 9.1 9.2 3.7 3.7 10.1 10.1 23.9 24.2 19-7 19.7 6.2 6.1 Re- covery, Per cent.99.1 100.9 99.6 99-6 100.0 99.6 99.7 99.7 98-1 99.0 100-5 100.0 98.6 98.6 97.5 97.5 101.0 101.0 98.0 98.0 100.0 101.6 101.9 101.9 97.7 97.7 98.8 98.8 97.8 98.9 100.0 100.0 99.0 99.0 99.1 100.4 100.0 100.0 100.0 98.4 * Anilide-nitrogen and amino-nitrogen. t Nitro-nitrogen and amino-nitrogen. Nitro and Nitroso Compounds Aromatic nitro and nitroso compounds are quantitatively reduced to the amines with zinc powder and 11 N hydrochloric acid.Glacial acetic acid is a suitable solvent for many nitro and nitroso compounds. Use of iron or tin powder is not recommended because after theNovember, 1975 NITROGEN-CONTAINING GROUPS IN ORGANIC COMPOUNDS 809 reduction step appreciable amounts of iron(I1) and tin(I1) ions are formed and these undergo a redox reaction with the nitric acid during the deamination step to give nitric oxide.7 Oximes nitric acid, quantitatively produces 1 mol of nitrous oxide per mole of oxime group. possible that the reaction proceeds according to the following equations : Reduction of oximes with zinc powder and hydrochloric acid, followed by reaction with It is 2H 2H R-CH=N-OH -+ R-CH2-NHOH -+ R-CH2-NH2 + HZO R-CH2-NHZ + HNO, -+ R-CH2-N-N=O + H,O 1 . 1 H O 11 R-CH,-N=N-OH + R-CH2-OH + NZO 4 0 References 1. 2. 3. 4. 5. 6. 7 . Cheronis, N. D., and Ma, T. S., “Organic Functional Group Analysis by Micro and Semimicro Hassan, S. S. M., Mikrochim. Acta, 1970, 1109. Hassan, S . S. M., Analytica Chim. Acta, 1971, 53, 449. Hassan, S. S . M., Analytica Ckim. Acta, 1972, 58, 480. Renard, M., and Deschamps, P., Mikrochemie, 1951, 36/37, 665. Franchimont, A., Recl Trav. Chim. Pays-Bas Belg., 1898, 17, 287. Awad, W. I., and Hassan, S. S. M., Talanta, 1969, 16, 1393. Methods,” Interscience Publishers, New York, 1964, p. 232. Received September 27th, 1974 Amended June 9th, 1976 Accepted June 16th, 1976

ISSN:0003-2654    

DOI:10.1039/AN9750000806

出版商:RSC    

年代:1975

数据来源: RSC

摘要:

810 Analyst, November, 1975, Vol. 100, $p. 810-816 Automated Thermometric Analysis for the Measurement of Substances in High Concentration F. Hagedorn, G. Peuschel and R. Weber Kdi-Chemie A .G., Hanovcr, West Germany Technicon International Division S.A ., Geneva, Switzerland A technique has recently been described by the authors for the automated thermometric analysis of solutions of medium to high concentrations of analyte, using a novel micro-scale flow cell with vigorous mechanical stirring. With this technique, fully automated analysis involving precipitation reac- tions is possible, using enthalpimetric principles. Coefficients of variation are better than f l . 0 per cent. In this paper, experiments are described that were carried out in order to define the limitations of this system, and the reasons for the improved performance compared with the many other types of thermometric analysers proposed in recent years are discussed.Use has been made of the heat of reaction in analytical methods for well over 60 years.1 However, the measurement of enthalpy changes has not produced a single widely accepted instrumental technique for analysis, although every compound undergoes one or more reactions with significant heat changes, temperature differences can be readily measured to 0401 “C and interferences compared with those occurring in colorimetry are negligible. We believe that a combination of historic and fortuitous factors are to blame for this situation. In most applications, the users have obviously been impressed with the precision of tem- perature measurements, and have sought to develop micro-techniques in competition with colorimetry.Inevitably, these techniques have been cumbersome when used in attempts to compensate for the greatest interferent, the ambient temperature, and have suffered further from such predictable variables as heats of dilution due to electrolytes that are present. The high precision of temperature measurements at present obtainable seems to us to be more appropriately of use in the analysis of solutions of relatively high concentration in which temperature changes are large. Heats of reaction are linear with concentration over very wide concentration ranges and can be chosen to be several orders of magnitude greater than the heats of dilution or the changing heat capacity.Thermometric analysis of con- centrated solutions can offer high precision with relatively small dilutions. The accurate and high dilution of such samples required for colorimetric analysis is prone to error, tedious and costly in routine analysis. The development of fully automated thermometric analysis has suffered from the historic use of the adiabatic method. Many users of the technique have ignored or inadequately applied the most important asset of automated analysis, precise timing. Precise timing leads naturally to the concept that has been called, perhaps inappropriately, “steady state” as compared with the conventional “equilibrium state.” A “steady-state” measurement can best be defined as one made at a precise moment in the course of a reaction,whereas“equili- brium-state” measurements are made throughout the course of a reaction. For example, an instrument may be used for measurements on a series of tubes in which sucrose is being hydrolysed.If each tube is examined 30 s after the commencement of the reaction and if each tube contains exactly the same concentration of reactants maintained at the same tempera- ture, the instrument wiIl record the same result, Le., the instrument will indicate “steady state.” Such a recording will obviously differ from that made with a single tube until the reaction is complete or has reached equilibrium. The obvious advantage of “steady-state” measurements is speed of analysis and, therefore, frequent use of calibrating standards in order to increase precision.By limiting our application of thermometric methods to the analysis of substances thatHAGEDORN, PEUSCHEL AND WEBER 81 1 are present in high concentrations and by applying the concepts of “steady state,” one of us (F.H.) developed a new cell for thermometric analysis that will handle samples at the rate of 2040 per hour, which, for the first time, incorporates vigorous mechanical stirring. Many previous attempts”’ at developing a fully automated system have not led to any generally accepted instrumental technique. The use of the total system for analysis, which involves redox reactions, heats of dilution, precipitation reactions as well as acid - base titration, is described elsewhere. In this paper the limitations of the technique, as experimentally determined, are examined in detail. Apparatus As a full description of the apparatus has been given el~ewhere,~~~ and its use in many industrial analyses will be described elsewhere,lO only a brief description is given here.The apparatus (Fig. 1) comprises the following : an AutoAnalyzer* with sampler IV; peristaltic pump 111; recorder; and thermostat that is speciallyconstructed, being constant to &0.005 “C and holding the thermometric flow cell. Experimental Sampler a Waste r-------------- Reagent Air removal Air removal Recorder Fig. 1. Schematic diagram of analytical flow system. A thermistor from the flow cell forms one arm of a standard Wheatstone bridge and the out-of-balance signal is amplified for recording. Samples are loaded into the sampler tray and sequentially aspirated by the peristaltic pump.The sample, segmented with air, and reagents pass through a 28-turn mixing coil in the thermostat, each at the rate of about 2.25 ml min-1. The solutions are de-bubbled just before entry into the flow cell and flow through the 1.0-ml flow cell at the rate of about 3-5 ml min-l. The heat change in the rapidly stirred solution is measured with the thermistor. Cell (Figs. 2 and 3) The recurrent theme in discussions of thermometric cell efficiency is the need for adequate mixing when analysing concentrated solutions or those involving precipitation reactions. All designs to date have failed in this respect and, with one exception,* all apparatus has depended on vortex or turbulence mixing and the analysis of dilute solutions.The most important feature of the present cell design is high-speed mechanical stirring, without which analysis at a fast rate would obviously be impossible. The walls and bottom of the cell, 10, are constructed of Teflon? while the cover, 12, is made of Perspex. A thin * Technicon Corporation, Tarrytown, New York, USA. t E. I. du Pont de Nemours Trade Name.812 HAGEDORN et al. : AUTOMATED THERMOMETRIC ANALYSIS FOR Analyst, VoZ. 100 stainless-steel cap, 11, is pressed into the inside dome of the Perspex cover. The thermistor, 13,” is ground to the steel surface of the cap so as to make intimate contact. The stirrer, 14, also made of Teflon, fits into the body of the cell with over-all tolerances not exceeding 0.5 mm. For reactions involving precipitates, the dimensions are critical.Spaces that are too wide or too narrow result in retention of precipitates within the cell. The stirrer comprises a circular disc, 9, in which is embedded a magnet, 8. Four stirrer vanes, 7, are set at right-angles to each other. The magnet is rotated by a second magnet, 6, which is turned at 1OOOrevmin-l by a water turbine, 5, driven by the water-circulating pump of the thermostat. Reagents and sample enter via nipples, 4 and 3, and exit via the tube, 2, in the cover. Tube 1, which forms the axis of the stirring vanes, has perforations that provide access into each of the four chambers formed by the vanes in order to allow the exit of occasional bubbles of air. The outside of the cell is covered with a thin sheet of stainless steel so that air bubbles do not occlude to the hydrophobic surface, thus altering the heat transfer characteristics.Fig. 2. Schematic diagram of flow cell. The improvements over previous designs, which may not be obvious, are : 1. A constant stirring speed and a constant large flow-rate through the cell give rise to a constant and uniform temperature within the flow cell, that is, a stable base-line. 2. A drive mechanism for the cell stirrer which, effectively, transmits no heat to the cell. 3. Use of a single high-stability thermistor instead of the usual pair reduces noise. 4. Placing the thermistor against the metal cap rather than in a glass tube in the solution itself, substantially reduces noise from flow irregularities and increases the speed of response.Use of true “steady-state” conditions. Procedure A background or base solution, the concentration of which is the lowest to be expected during the analysis, is pumped continuously into the wash receptacle of the sampler. Hence, there is a continuous reaction in the thermometric cell. The first five solutions in the sampler tray are calibration solutions followed by samples, with every 20th position occupied by * Precision thermistor, 1-0 kQ (H. Knauer & Co. GmbH, 1 Berlin 37, Zehlendorf-Strasse 635 No. 38, Germany).Fig. 3. Photograph of cell-flowNovember, 1975 THE MEASUREMENT OF SUBSTANCES IN HIGH CONCENTRATION 813 a standard. The sampling rate is 20-40 samples per hour. Peak heights generated by the samples are compared with those of the calibration solution for quantitative evaluation.AnalaR-grade reagents are used throughout. Results Linearity Almost all examples cited in the literature have limited linearity of response. However, we have not experienced this difficulty in the determinations listed in Table I. On the other hand, completely non-linear responses were recorded for : concentrated calcium chloride solution (400-600 g 1-l), using heat of dilution (precision & 2 standard deviations on dupli- cates & 3-4 g 1-1) ; and formaldehyde (0-5 per cent. solution), using its reaction with ammonia to form hexamethylenetetramine (precision on duplicates & 0.015 per cent.). TABLE I LINEARITY AND PRECISION FOR THERMOMETRIC DETERMINATIONS Determination Ammonia-nitrogen : Magnesium : Sodium chloride : Potassium : Chloride : formaldehyde reaction .. .. precipitation . . .. .. ammonium magnesium phosphate heat of dilution . . .. .. perchlorate precipitation . . silver chloride precipitation . . heat of dilution . . .. .. barium sulphate precipitation . . hydrogen peroxide reaction . . iodide reduction reaction * . acid titration . . .. .. Alcohol : Sulphate : Sodium sulphide : Hydrogen peroxide : Sodium hydroxide : Concentration range for linear response 5-7% o-S% (as Mg) 160-240 g 1-l 4663% (as K,O) 0-4% (Cl) 40-60 VO~.-% 0-9% (SO,) 110-190 g 1-1 06-5-0 % 10-30 g 1-I Precision on duplicate analysis ( f 2 standard deviations) Matrix Fertilisers f 0.02 % Fertilisers f 0.04% Brine & l - 1 g 1-1 Fertilisers &0-2% Kieserite f 0.03% concentrates Spirits &@3% Fertilisers ZkO.1% Aqueous f0.6 g 1-1 Aqueous f 0.02% Aqueous f0-1 g 1-1 While non-linearity is to be expected for concentrated calcium chloride solution, we cannot at present account for the result with formaldehyde.As a further test for linearity and hence the magnitude of interferences from heats of dilution and association, we titrated solutions containing sodium hydroxide at 0-200 g 1-1 concentration and sodium carbonate at 30 g 1-1 concentration with sodium hydrogen car- bonate. The initial solution was automatically diluted ten-fold on the continuous-flow manifold and reacted in the thermometer cell with a 0.75 N solution of sodium hydrogen carbonate, showing linearity with concentrations of sodium hydroxide between 0 and 0.4 N (160g1-1) (Fig.4).. We would have expected such a titration to be non-linear if heats of dilution and varying dissociation constants played a significant role. Sensitivity Whether an analysis is possible depends ultimately on the sensitivity of the analytical system and should be predictable from a table of heats of reaction. The limit for precise titration (&1 per cent.) was obtained when 0.025 N hydrochloric acid was titrated with 0.5 N sodium hydroxide solution. Fig. 5 shows the curves obtained. Assuming the heat of neutralisation to be approximately 13 kcal mol-l and a full-scale deflection (f.s.d.) for the titration of 0.025 N hydrochloric acid, then the minimum net heat change required will be 0-3 kcal 1-1 for a precise analysis (&l per cent. f.s.d.).81 4 HAGEDORN et al.: AUTOMATED THERMOMETRIC ANALYSIS FOR Analyst, Vol. 100 Response, chart divisions Fig. 4. Titration of sodium hydroxide with 0.76 N sodium hydrogen carbonate solution. Carry-over Precipitation reactions obviously exhibit the worst carry-over effect, i.e., cross-contamina- tion between samples. Fig. 6 shows the carry-over effect when an analysis for 15 per cent. of sulphate is followed by a 3 per cent. analysis and vice versa at the rate of 20, 30 and 40 samples per hour. (Only at the rate of 40 samples per hour is there any appreciable carry-over, viz., h0.3 per cent.) The heat of reaction is very low and hence the carry-over is more severe than that obtained with reactions that exhibit large heat changes, Interferences Heats of dilution, and heats of association and dissociation, are recognised as the obvious interferences in thermometric analysis.However, data on the extent of such interferences are very sparse, even though great efforts have been made to eliminate theml1-l3 or to compensate for them14 instrumentally or by the use of titrants with low heats of dilution. In the references cited, the problem has been greater than we have experienced as inadequate stirring has required the use of low concentrations. Working with high concentrations of matrix materials has another disadvantage: the rate of change of heats of dilution E 8 8. 2 e L as CT 0.015 N ’ 30 samples per hour ’ ,Om025 N Fig. 6. Titration curve of 0.026 N hydro- chloric acid with 0.5 N sodium hydroxide solution and reproducibility.November, 1973’ THE MEASUREMENT OF SUBSTANCES I N HIGH CONCENTRATION 815 I 0 P f! P 8 L Q) pc 40 samples 30 samples 20 samples per hour per hour 1 ‘hour 15% so, 3 3% so, Fig.6. Carry-over in thermometric analyses for sulphate (15 per cent. sulphur dioxide followed by 3 per cent. sulphur dioxide a t various analysis rates). increases markedly with increasing concentration of salts for most solutions (Fig. 7). However, in nearly every instance encountered, the interferences from high salt concentrations can be adequately compensated for by making up the standards in the same concentration of matrix substances and diluting to the concentration at which the [rate of change flattens out Moles of water per mole of MgCI, Fig. 7. Heat of dilution for magnesium chloride solution.An extreme and very unusual example is the thermometric analysis for sulphate in a magnesium brine. The brine can differ greatly in composition, the concentration of mag- nesium chloride varying from 280 to 430 g 1-1 and that of the magnesium sulphate from 41 to 77 g 1-l. From literature values for the heats of dilution* the maximum difference is about 160 call-1 when the sample, during reaction, is diluted from 2.0 to 3.5 ml by the reactant barium chloride. On the other hand, the heat of reaction of SO:- ion with barium chloride at 11 per cent. concentration is 6000 call-1 while that at 6 per cent. concentration is about 3000 call-1. Hence, the above difference in the heats of dilution will lead to a maximum error of 5-6 per cent. relative if a base solution containing one extreme of brine is used, but obviously only 2-3 per cent.(maximum relative error) if a mean concentration is continuously aspirated as the base solution. However , techniques will be described in subsequent publications which can reduce the errors in such extreme instances to insignificant values. The anomalous effects arising from increasing ionic strength, reported by McLean and Penketh,s are probably due to inadequate mixing. The authors give as the change in * The heat of dilution should, theoretically, be determined as a function of total ionic strength. This refinement, however, will have only a secondary effect and will not significantly alter the argument.816 HAGEDORN, PEUSCHEL AND WEBER temperature, AT, of the flow-through cell: where C = Molar concentration of reactive species in sample line to cell.Fo = Flow-rate in sample line to cell (1 min-l). AH = Heat of reaction (kcal mol-l). C, = Specific heat of cell outflow (kcal k g l K-l). FF = Flow-rate out of cell (1 min-l). d~ = Density of flow out of cell (kg 1-l). Cp' = Heat capacity of cell (kcal K-l). E = Cell efficiency factor ( i e . , degree of mixing, heat transfer, etc.). The above authors have obviously taken the same precautions in bringing all reactants to the same temperature and calibrating with standard solutions as we have done in this work. Hence, it must be assumed that the cell factor E will be very dependent on mixing and that the efficiency of mixing will itself depend on the viscosity, density, instantaneous temperatures and concentration gradients unless good mechanical stirring is used. Work is currently continuing in three centres in applying the above thermometric technique to industrial analytical problems involving solutions of medium and high concentrations. AT = CFoAH(CpFpdn + Cp')E 1. 2. 3. 4. 5. 6. 8. 9. 10. 11. 12. 13. 14. References Bell, J. M., and Cowell, C. F., J . Am. Chem. Soc., 1913, 35, 49. Lewis, C. D., Abstracts of Papers, 132nd Meeting of the American Chemical Society, New York, Crespin, G., Lapis, 1964, No. 71, 24. Priestley, P. T., Sebborn, W. S., and Selman, R. F. W.. Analyst, 1965, 90, 689. McLean, W. R., and Penketh, G. E., Talanta, 1968, 15, 1185. StrAfelda, F., and KraftovA, J., Colllz Czech. Chem. Commun., 1968, 33, 3694. Peuschel, G., and Hagedorn, F., British Patent 1 291 785, 1973. Peuschel, G., and Hagedorn, F., Kali Steinsalz, 1972, 6, 4. Peuschel, G., and Hagedorn, F., 2. Analyt. Chem., in the press. Tyson, B. C., McCurdy, W. H., and Becker, C. E., Analyt. Chem., 1961,33, 1640. Keily, H. J., and Hume, D. N., Analyt. Chem., 1964, 36, 543. Rondeau, J., Legrand, M., and Paris, R. A., C. R. Hebd. Se'anc. Acad. Sci., Paris, 1966,263, 679. Saj6, I., and Sipos, B., 2. Analyt. Chem., 1966, 222, 23. Received July 14th, 1976 Accepted September 15th. 1975 1957, p. 7B.

ISSN:0003-2654    

DOI:10.1039/AN9750000810

出版商:RSC    

年代:1975

数据来源: RSC

摘要:

Analyst, November, 1975, Vol. 100, pp. 817-821 81 7 Use of a Clean-up Method to Improve Specificity in the Analysis of Foodstuffs for Volatile Nitrosamines E. A. Walker, M. Castegnaro and Brigitte Pignatelli Unit of Environmental Carcinogens, International Agency for Research on Cancer, 150 Cows Albert Thomas 69008 Lyon, France An adsorption chromatography system is described for the clean-up of nitrosamines extracted from foodstuffs. The method is applicable to a wide range of food types and, by standardisation of technique and analysis of individual fractions of the eluate, improves specificity in the detection of nitrosamines by means of gas chromatography. Analysis for trace constituents of food and beverages by means of gas chromatography presents problems of specificity. Owing to the large number of volatile compounds present in all natural products at the trace level, retention data often do not give reliable identification, even with the use of high-efficiency capillary columns and extensive clean-up procedures.Generally, mass spectrometry is required for this identification and in the instance of nitrosamines is best performed using high resolution, a facility that is available in a limited number of laboratories. It is therefore important that the need to use mass spectrometry is minimised by the use of methods of analysis that limit the number of false positive results. Detection by gas chromatography, with nitrogen-selective detectors such as the Coulson electrolytic conductivity1,2 and the flame thermionicl~~ detectors, permits direct determination of nitros- amines provided that the clean-up procedures are adequate.Recently, a nitrosamine- selective detector has been devised, which, in principle, eliminates the need for any clean-up pr~cedure.~ This detector, however, is not yet commercially available. The established alternative routes via formation of suitable derivatives for detection by electron capture include cleavage of the nitrosamines and determination of the resulting amines as the hepta- fluor~butanoyl~,~ derivatives and oxidation of the nitrosamines to nitrarnine~.~~~ Each method has its own advantages and disadvantages but it is generally found most satisfactory to concentrate on one particular method as the quality and reliability of results depend mainly on experience in a technique, as illustrated by a collaborative study of analysis for nitrosaminess in cooked meat.In this laboratory, all of the above methods have been tried, but the method involving oxidation and detection as nitramines has proved to be generally satisfactory for our particular work. In the course of analysis of a wide range of dietary items taken during an epidemiological study, a systematic column clean-up procedure was developed, which allowed a range of volatile nitrosamines to be determined in all the samples analysed with a high degree of specificity above the 5 pg kg-l level. Although nitrosamines could be detected down to the 0.1 pg k g l level by a 10-fold concentration of the nitramine solution and a corresponding reduction in attenuation of the electron-capture detector signal, at levels below 5 pg kg-1 there was some uncertainty with regard to nitrosodimethylamine.The method depends upon an initial separation of nitrosamines into two fractions on an alumina column and subsequent separation of the nitramines on the same type of column. By careful standardisation of the procedure, the nitramines are found in specific fractions of the eluate. Using this approach, interference was reduced to a minimum and specificity increased correspondingly. When a nitramine is detected by electron capture a further gas-chromatographic analysis is then performed by using a Coulson electrolytic conductivity detector (selective for nitrogen), A positive result therefore requires the presence of a nitramine in the correct fraction and agreement in response between the two dissimilar modes of detection.In 300 samples analysed in the dietary survey, no evidence was found of the nitrosamines used in the standard reference, with the exception of nitrosodimethylamine, which was generally present at a level of the order of 1 pg k g l or less. In these instances, it has not been possible to obtain mass-spectrometric confirmation of nitrosodimethylamine by examina- Several approaches to screening analysis for nitrosamines can be employed.818 WALKER et al.: CLEAN-UP METHOD TO IMPROVE SPECIFICITY AIzaZyst, VoZ. 100 tion of fraction B (Fig. 1) with mass spectrometry - gas chromatography using a resolution of 10 000. Each different type of sample was also checked by spiking at the 10 pg k g l level with a mixture of nitrosamines.For one sample, indicated to contain 7 pg k g l of nitro- sodimethylamine, confirmation by mass spectrometry was possible. A flow diagram of the clean-up method is given in Fig. 1. Experimental Nitrosodipropylamine, which has not been reported in food, was employed as an internal standard by spiking the sample at the 10 pg kg-l level before extraction. As an internal standard, it has the added advantage of being split approximately equally between the two nitrosamine fractions A and B, as indicated in Fig. 1. It thus gives an indication of the proper functioning of the clean-up procedure. The presence of a significantly higher propor- tion of nitrosodipropylamine in fraction A indicates deterioration in the alumina, probably due to the pick-up of atmospheric moisture, and the need to prepare fresh alumina.Details of the preparation of the alumina column, which consists of 3 g of basic alumina, activity 11, 3 g of neutral alumina, activity 111, and 1 g of anhydrous sodium sulphate, were given in a previous paper.*lO One minor modification has, however, been found advantageous. As alumina tends to adsorb organic materials from a laboratory atmosphere, a pre-wash is performed with diethyl ether - pentane (1 + 1) before baking in the oven a t 200-220 "C. Although several other types of column have been tried for the initial nitrosamine clean-up, none was found to offer any advantage, while the use of the same type of column for both parts of the clean-up procedure minimises procedural detail.Fractions A and B from the nitrosamine separation are oxidised separately by the method of Sen and Dalpe,' using the procedure given in detail by Castegnaro et aZ.,1° with the modifica- tion that slightly larger volumes of oxidant (vix., 9 ml of trifluoroacetic acid and 7 ml of 50 per cent. hydrogen peroxide solution) were found to give more reproducible results. Using 85-90 per cent. hydrogen peroxide solution, Tellingll has found that while the reaction is somewhat faster, nitrosopiperidine is destroyed. We have found no evidence of this problem when using 50 per cent. hydrogen peroxide solution. The final concentrate, adjusted to 5 ml with pentane, is added to the top of a freshly prepared alumina column and eluted successively with 5, 25 and 80 per cent.diethyl ether in pentane and finally with 100 per cent. diethyl ether. The eluate is collected in 5-ml fractions, each of which is analysed separately using 5-p1 aliquots. Fig. 1 indicates the fraction in which the nitramines are typically found. However, it should be noted that while the order is unchanged, the elution volume varies slightly with different batches of the prepared alumina and it is essential to run a standard solution of nitramines (prepared by oxidation of a solution of the appropriate nitrosamines) for each fresh batch of alumina. All fractions containing nitramines can be bulked and reduced to 5ml in volume prior to a more convenient quantitative determination by means of a further gas-chromatographic step. Finally, where there is evidence of nitrosamines, the solution is reduced to one fifth of its volume and gas chromatography with a Coulson detector is carried out.Discussion The advantage of analysing the individual fractions is indicated in Fig. 2, which shows a final chromatogram obtained from a sample of bread that had been spiked at the 10 pg kg-1 level with eight nitrosamines. The apparent recovery of nitropiperidine is over 200 per cent. but this apparent excess was in fact due to an interfering material found in the nitrosodiethyl- amine fraction, which had the same retention time as the piperidyl compound. Analysis of the nitrosodiethylamine fraction thus allows appropriate correction and elimination of a false positive result. Similarly, in the initial separation of the nitrosamines, for many samples (particularly those originating from wheat, such as flour, bread and cooked products con- taining flour) the nitramine fractions 7, 8 and 9, obtained after oxidation of the nitrosamines in fraction A, contain a false peak corresponding to the N-nitrodimethylamine that gives corresponding responses with both electron-capture and Coulson detectors. Likewise, a * Since this work was carried out, the manufacturers of the basic alumina of activity I (Merck) that is used to prepare both types of alumina employed in the column have modified their product.As a result, when using current batches of basic alumina it is necessary to decrease the amount of water added10 in order to produce a basic alumina suitable for column separation from 3 g to 1.9 g per 100 g of basic alumina.November, 1975 IN THE ANALYSIS OF FOODSTUFFS FOR VOLATILE NITROSAMINES nitraminesjO except that final with hexane Elution with- Nitrosamines volume adjustment is made Adsorption ChromatographY (c, H, ),o + pentane m I 1 + 3 25 1 + 1 15 3 + 1 15 1st fraction (A), 20 ml t Nitramines Adsorption chromatography analysis of each 5-ml Fraction: 1 2 3 4 5 6 - NDBA 2nd fraction (B), 55 ml N i tra mi nes 1 Elution with- (C,H,),O t pentane rnl Adsorption chromatography 1 + 19 25 I .- 1 + 3 30 4 + 1 10 10 analysis of each 5-ml fraction pure (C, H, 1, 0 3 4 5 6 7 8 9 10 NDPA N Pyr w v / N d E A 819 + - NPip NDMA Y V NMPA NMEA capture detector Gas chromatography - Coulson Quantitative analysis Fig. 1. Flow diagram showing the clean-up procedures used in the analysis of foodstuffs for nitrosamines.Nitramines : N-nitrodibutylamine (NDBA) ; N-nitrodipropylamine (NDPA) ; N-nitro- pyrrolidine (NPyr) ; N-nitrodiethylamine(NDEA) ; N-nitropiperidine (N Pip) ; N-nitrodimethylamine (NDMA) .820 WALKER et al.: CLEAN-UP METHOD TO IMPROVE SPECIFICITY Analyst, VoZ. 100 fraction is frequently to be found following fraction B in the nitrosamine separation that also gives a false response for nitrosodimethylamine. The procedure outlined thus eliminates these interferences. A 30 20 10 0 Time/min Fig. 2. Chromatogram for a sample of bread spiked a t the 10 pgkg-l level with eight nitrosamines. Chromatographic conditions : column, 3 m x $ in o.d., 10 per cent. Carbowax 20M on Chromosorb W; column temperature, 140 "C; carrier gas, nitrogen a t a flow-rate of 40 ml min-* ; detector, nickel-63, electron cap- ture.Peaks (recovery, per cent., in parentheses) : A, N-nitrodimethylamine (91) ; B, N-nitro- methylethylamine (77) ; C, N-nitrodiethylamine (87) ; D, N-nitrodipropylamine (90) ; E, N-nitro- methylpentylamine (1 10) ; F, N-nitropiperidine (232) ; G, N-nitrodibutylamine (100) ; H, N-nitro- pyrrolidine ( 107). As will be observed in Fig. 2, four peaks, not representing nitrosamines, are given by the final bulked solutions that elute after N-nitrodimethylamine. These four peaks, however, are consistently present in the prepared standard solutions of nitramines that have passed through the separation procedure, and are thus well-defined artifacts of the method and present no major problem in the analysis.The method has been applied in a complete dietary survey covering cooked meals containing meat, fish, eggs, vegetables, herbs, bread, rice, wheat flour, and fruit juices and individual ingredients of the meals. Recoveries from samples spiked with nitrosamines were normally greater than 60 per cent. The method was found to work satisfactorily both for extraction of nitrosamines by steam distillation12 and continuous liquid extraction of a digest with methanolic potassium hydroxide.13 Conclusion Although analysis of individual fractions increases the work involved in a screening analysis, the greatly decreased number that require confirmation by mass spectrometry counter- balances this drawback and reduces competitive demands on mass spectrometry, thus con- tributing to considerable economy.November, 1975 IN THE ANALYSIS OF FOODSTUFFS FOR VOLATILE NITROSAMINES 821 1 .2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. References Palframan, J. F., Macnab, J., and Crosby, N. T., J . Clzvomat., 1973, 76, 307. Rhoades, J. W., and Johnson, D. E., J . Chromat. Sci., 1970, 8, 616. Howard, J. W., Fazio, T., and Watts, J. O., J . Ass. Off. Analyt. Chem., 1970, 53, 269. Fine, D., and Rufeh, F., in Bogovski, P., and Walker, E. A., Editors, “N-Nitroso Compounds in the Environment,” IARC Scientific Publication No. 9, International Agency for Research on Cancer, Lyon, 1974, p, 40. Eisenbrand, G., in Bogovski, P., Preussmann, R., and Walker, E. A., Editors, “N-Nitroso Com- pounds, Analysis and Formation,” IARC Scientific Publication No. 3, International Agency for Research on Cancer, Lyon, 1972, p. 64. Alliston, T. G., Cox, G. B., and Kirk, R. S., Analyst, 1972, 97, 915. Sen, N. P., and Dalpe, C., Analyst, 1972, 97, 216. Althorpe, J., Goddard, D. A., Sissons, D. J., and Telling, G. M., J . Chromat., 1970, 53, 371. Walker, E. A., and Castegnaro, M., in Bogovski, P., and Walker, E. A., Editors, “N-Nitroso Com- pounds in the Environment,” IARC Scientific Publication No. 9, International Agency for Research on Cancer, Lyon, 1974, p. 57. Castegnaro, M., Pignatelli, B., and Walker, E. A., Analyst, 1974, 99, 156. Telling, G. M., J , Chromat., 1972, 73, 79. Crosby, N., Foreman, J. K., Palframan, J . F., and Sawyer, R., i n Bogovski, P., Preussmann, R., and Walker, E. A., Editors, “N-Nitroso Compounds, Analysis and Formation,” IARC Scientific Publication No. 3, International Agency for Research on Cancer, Lyon, 1972, p. 38. Fazio, T., Howard, J. W., and White, R., in Bogovski, P., Preussmann, R., and Walker, E. A., Editors, “N-Nitroso Compounds, Analysis and Formation,” IARC Scientific Publication No. 3, International Agency for Research on Cancer, Lyon, 1972, p. 16. Received A p r i l llth, 1975 Accepted May 28th, 1975

ISSN:0003-2654    

DOI:10.1039/AN9750000817

出版商:RSC    

年代:1975

数据来源: RSC

摘要:

822 Analyst, November, 1975, Vol. 100, pp. 822-826 Determination of Residues of Carboxylic Acids (Mainly Galacturonic Acid) and Their Degree of Esterification in Industrial Pectins by Colloid Titration with Cat-Floc Atsunobu Mizote, Hiroyuki Odagiri Kyoji Thei and Kazuko Tanaka Sanslzo Co. Ltd., The Second Kitahama Building, 68, Kyobashi 3-chome, Higashiku, Osaka, Japan Faculty of Science, Okayama University, Tsushima, Okayama, Japan The determination of residues of carboxylic acids (mainly galacturonic acid) in industrial pectins is carried out by colloid titration using a new titrant, Cat-Floc (Poly-NN-dimethyldiallylammonium chloride). The degree of esterification of the methyl ester of galacturonic acid is determined in the same way after hydrolysing the ester with sodium hydroxide in the presence of Cat-Floc.The same samples were analysed by the method of acid - base titration and by colloid titration, and the results obtained are compared and discussed. Pectins are widely present in all terrestrial plants, in which they function as intercellular cementing material. Industrial pectins are mainly extracted from citrus peel, and are used in the food industry in the manufacture of jellies, jams, marmalades, etc., and also for medical purposes and in cosmetics. Galactose, arabinose and galacturonic acid, with some rhamnose, are the units with which the structure of the pectins is formed, the main constituent of pectin being pectic acid, which is a polygalacturonic acid. The latter is often partly methylated, and its relative molecular mass may be as high as 100 000 or more.The content of pectin is expressed in terms of the total pectic substances determined as galacturonic acid. The percentage of galacturonic acid units esterified with methanol is called the degree of esterification, which greatly influences the gelling properties and the solubility of pectins. Thus, knowledge of the content of polygalacturonic acid and the degree to which it is esterified is essential for the evaluation of pectins. The acid - base titration method is widely accepted as the standard method for evaluating industrial pectins for commercial purposes. Okimasul has, however, successfully determined the content of the pectin and the degree of esterification by colloid titration. T8ei and Kawada2 have also determined these values by colloid titration using a new titrant, Cat-Floc, and found that the degree of esterification can effectively be determined by hydrolysing the pectin with sodium hydroxide in the presence of Cat-Floc.In this work, the content of carboxylic acid residues and their degree of esterification were determined on the same samples of pectin by the acid - base and colloid titration methods, and the results obtained are compared and discussed. As the carboxylic acids occur mainly as galacturonic acid in pectin, it is convenient to refer to them as galacturonic acid. Colloid Titration Reagents About 0.4 g of potassium poly(viny1 sulphate) (Wako Pure Chemical Industries Ltd.) is dissolved in 1 1 of distilled water. The solution is standardised by titration with 0.0025 N standard zephiramine solution (see below), using toluidine blue as indicator. The concentrated solution of Cat-Floc (Calgon Corporation, U.S.A.) contains 15 per cent.of Poly-NN-dimethyldiallylammonium chloride and 2-5 ml are diluted to 1 1 with distilled water to give a solution that is about 0-0025 N. This solution is stan- dardised against 0.0025 N potassium poly(viny1 sulphate) solution. Standard zephiramine solution. Zephiramine (tetradecyldimethylbenzylammonium chloride) Experimental Potassium poly (vinyl sulphate) solution, approximately 0-0025 N. Cat-Floc so1.utio.n.MIZOTE, ODAGIRI, T ~ E I AND TANAKA 823 (Dojindo Laboratories, Japan) is dried thoroughly in a vacuum. A 0.1-g amount is accurately weighed into a 100-ml calibrated flask, dissolved in water and the solution made up to the mark.Purification and Preparation of Industrial Pectins Five types of industrial pectins, rapid set, slow set, medium rapid set, lowlhigh methoxy and low methoxy pectins, were used for the determination of the polygalacturonic acid content and its degree of esterification. For convenience in industrial use, a certain amount of sugar is always added to pectin in order to standardise the gelling capability of the pectin. In order to remove the sugar, 1.5 g of the sample are washed three times by decantation with 50 ml of 80 per cent. methanol, followed by washing with diethyl ether; the sample is then dried at 60 "C in a vacuum. About 0.15 g of the sample is accurately weighed, dissolved in water in a 250-ml calibrated flask and the volume made up to the mark.A 5-ml aliquot is used for the colloid titration. Determination of Carboxylic Acids in Pectins To 5 ml of pectin solution, 10 ml of 0.0025 N Cat-Floc solution are added so as to form a flocculent precipitate. After adjusting the pH to a fixed value by the addition of sodium hydroxide or ammonia solution, the solution is titrated immediately with 0.0025 N potassium poly(viny1 sulphate) solution in order to prevent the hydrolysis of the methyl ester of poly- galacturonic acid. The solution should be titrated slowly so that a distinct flocculent precipitate appears and the end-point is determined by the change of colour of the toluidine blue indicator from blue to red - violet. If the titration is carried out too fast, the solution remains turbid and the colour change at the end-point becomes indistinct or, after the colour changes from blue to red - violet, it sometimes becomes blue again within a few minutes.Determination of the Methyl Ester of Polygalacturonic Acid in Pectins To 5 ml of pectin solution, 10 ml of 0-0025 N Cat-Floc solution, and sufficient sodium hydroxide solution to adjust the pH to 12.5, are added, with stirring. The pectin is hydrolysed at 0 "C for 60 min and the solution is then titrated as described above. The value obtained indicates the sum of polygalacturonic acid and its methyl ester. The ester content is calcu- lated by deducting from this value the value obtained for the carboxylic acid content. When the pectin solution is hydrolysed with the sodium hydroxide alone, the rate of hydrolysis is much slower than when an excess of Cat-Floc is present.Although the elimina- tion reaction of the pectin occurs at higher pH and higher temperat~re,~ the hydrolysis proceeds quantitatively at 0 "C and pH 12.5 in the presence of Cat-Floc. Determination of Pectin by Acid - Base Titration The polygalacturonic acid content and its degree of esterification in industrial pectins are determined by acid - base titration. This method, which is accepted as a standard method for industrial and trade purposes is as follow^.^ A 2-g amount of pectin is added, with stirring, to 200 ml of the mixture water - concen- trated hydrochloric acid - propan-2-01 (90 + 10 + 100) in a 500-ml flask and the mixture left for 15 min. I t is then filtered on a 9-cm diameter Biichner funnel, suction stopped and about 300 ml of 65 per cent.propan-2-01 are added to the pectin in the funnel with stirring. Suction is continued and the process repeated until the filtrate is free from chloride. The pectin is then washed with absolute propan-2-01 and finally with diethyl ether in order to facilitate drying of the pectin. The funnel containing the pectin is kept in a drying oven until the pectin is dry. A 0.5-g amount of the dried pectin is weighed into a 250-ml beaker and moistened with about 1 ml of 65 per cent. propan-2-01; 100 ml of distilled water are added and the beaker is placed on a magnetic stirrer and stirred until all of the pectin has dissolved (about 15 min). The solution is then titrated with 0.1 N sodium hydroxide solution to pH 7.5, while stirring, the volume of titrant consumed (a ml), which corresponds to the polygalacturonic acid, being noted.The stirring is continued, 30 ml of 0.1 N sodium hydroxide solution are added and the beaker is covered and left for exactly 30 min. An amount of dilute sulphuric acid equivalent824 AnaZyst, VoZ. 100 to 30ml of 0.1 N sodium hydroxide solution is added and, while stirring is continued, the mixture is titrated with 0.1 N sodium hydroxide solution to pH 7.5, the volume of titrant consumed again being noted ( b ml). The degree of esterification ( E ) is calculated as follows: MIZOTE ef al.: DETERMINATION OF GALACTURONIC ACID IN lOOb a + b E = - c c, \ 0 0 a W 1.0- 8 *g 2.0 0 -0 .- Results and Discussion Colloid Titration Curves of Pectins The colloid titration curves of rapid set, slow set and medium rapid set pectins are shown in Figs.1 , 2 and 3, respectively. The values for these curves are constant between pH 4.5 and 9, and, within experimental error, are coincident with those obtained by the acid - base titration. 0 I I I I I I - w 1 I I I I I I PH 3 4 5 6 7 8 9 PH Fig. 1. Colloid titration curves for rapid set pectin: (a), purified with 80 per cent. methanol; and (b), purified with water - propan-2-01 - hydrochloric acid. The arrow on the ordinate indicates the value obtained by acid - base titration. pH adjusted: 0, with NaOH solution; and 0, with ammonia solution. The colloid titration curves of low/high methoxy and low methoxy pectins are shown in Figs.4 and 5, respectively. The value given by colloid titration for a sample purified by the water - hydrochloric acid - propan-2-01 procedure agrees with that obtained by acid - base titration, but the value for low/high methoxy pectin purified with 80 per cent. methanol is Fig. 2. Colloid titration curves for slow set pectin. Symbols and conditions as for ~ i ~ . 1. 5 2.01 m 6 3 4 5 6 7 8 9 10 S l . O t I I I I I I I G 0 1 I g1.ot I I , I I 3 4 5 6 7 8 9 10 PH PH Fig. 3. Colloid titration curves for medium rapid Fig. 4. Colloid titration curves for low/high methoxy pectin. Symbols and conditions as for Fig. 1. set pectin. Symbols and conditions as for Fig. 1.November, 1975 INDUSTRIAL PECTINS BY COLLOID TITRATION WITH CAT-FLOC 825 lower than that indicated in Fig.4 for acid - base titration. The difference between (a) and (b) in Figs. 4 and 5 can be explained as follows. A part of the carboxylic acid forms a salt with some metal ions and the metal ions cannot be removed by washing with 80 per cent. methanol. Thus, the number of milliequivalents of pectin per unit mass becomes smaller than when elimination of the metal ions is effected by treatment with water - hydrochloric acid - propan- 2-01. Also, the sample purified with 80 per cent. methanol is less soluble in water than that purified with water - hydrochloric acid - propan-2-01 because insoluble pectin consists, at least partly, of the calcium or magnesium salt of pectic acid. Insoluble Matter in Low/High Methoxy and Low Methoxy Pectins The presence of insoluble matter is observed in solutions of low/high methoxy and low methoxy pectins, being gel particles dispersed in a hydrophilic and as these particles have electric charges, they are also titrated by the colloid titration method.For example, of the colloid titration curves for low/high methoxy pectin illustrated in Fig, 6, curve 1 was obtained with the solution containing gel particles that had been uniformly distributed by shaking well, curve 2 with the filtrate after passing the solution through a filter-paper and curve 3 with the supernatant liquid. The carboxylic acid contents of the solution giving curves 1 and 3 were 1.90 and 1.17 mequivg-l, respectively. It follows, therefore, that the carboxylic acid content in the insoluble matter is higher than that in the soluble portion of the pectin; with the methyl ester the reverse is true.Thus, pectins that have a high carboxylic acid content tend to be insoluble in water, but those with a high methyl ester content are liable to be soluble in water. - - L r " .. " ., .o - 3 I I I I I I 1 2 3 4 5 6 7 a 9 10 CI I I I I I I I S 3 4 5 6 7 8 9 10 d PH Fig. 5. Colloid titration curves for low meth- oxy pectin. Symbols and conditions as for Fig. 1. Fig. 6. Colloid titration curves for low/high methoxy pectin. Samples purified with 80 per cent. methanol. Curves : 1, solution containing insoluble matter; 2, solution filtered through filter-paper ; and (3), supernatant liquid after sedimentation of insoluble matter. Comparison of Colloid Titration with Acid - Base Titration The same samples were analysed by both the colloid and acid - base titration methods and the values obtained are listed in Table I.Low/high methoxy and low methoxy pectins always contain insoluble matter, and even the results from the same sample solution vary. Some variation in results arising from the TABLE I GALACTURONIC ACID CONTENT IN INDUSTRIAL PECTINS Content of galacturonic acid/mequiv 8-1 f A > Colloid titration r A \ Pectin Methanol (80%) Water - propan-2-01 - HC1 Acid - base titration Rapid set . . .. 1.42 1.47 1.46 Slow set. . .. .. 1.70 1-80 1.71 Medium rapid set . . 1.43 1.47 1.50 Low/high methoxy . . 1.73 2-00 1-92 Low methoxy . . .. 2.35 2.40 2-37826 MIZOTE, ODAGIRI, TGEI AND TANAKA sampling is also observed in rapid set, slow set and medium rapid set pectins, which contain a small amount of insoluble matter.The results obtained for the degree of esterification are shown in Table 11. The colloid titration was carried out successfully on the same sample solution with a deviation of &0.02 mequiv g-l. TABLE I1 DEGREE OF ESTERIFICATION OF INDUSTRIAL PECTINS Degree of esterification, per cent. f A \ Colloid titration r \ A Pectin Methanol (80%) Water - propan-2-01 - HCl Acid - base titration Rapid set . . .. 69 69 69-3 Slow set. . .. .. 65 67 63.0 Medium rapid set . . 66 69 67.8 Lowlhigh methoxy . . 50 45 50.9 Low methoxy . . .. 45 44 41.4 With the colloid titration only a small amount of sample is needed compared with acid - base titration. The method enables only polyelectrolytes to be titrated, but not acidic or basic material; on the other hand, with the acid - base method acidic and basic materials can be titrated. Degree of Polymerisation of Pectins for Colloid Titration In the colloid titration, the reaction between the polycation and polyanion becomes more quantitative as the degree of polymerisation increases.The degree of polymerisation of the pectins used in this work was more than several hundreds. In order to determine the mini- mum degree of polymerisation that would give quantitative results, the colloid titration was carried out for the polymerisation degrees 1,2,3,4 and 6.4. Galacturonic acid (polymerisation degree 1) and the dimer do not react with Cat-Floc. That with the polymerisation degree 3 reacts slightly with Cat-Floc, the end-point of the colloid titration being indistinguishable, but that with the polymerisation degree 6.4 gives an almost quantitative result by colloid titration. Okimasul stated that the colloid titration using methylglycolchitosan as a polycation is not successful when the mean relative molecular mass of the pectins is in the lower range. On comparing his results with those obtained in this work, it appears that pectin combines more strongly with Cat-Floc than with methylglycolchitosan, with the implication that the degree of polymerisation of Cat-Floc is higher than that of methylglycolchitosan. The authors thank Professor Dr. J. Ozawa and Dr. C. Hatanaka of the Institute for Agricultural Biology of Okayama University for their kind discussion and gift of pure pectins of low relative molecular mass. References 1. 2. 3. 4. 5. Okimasu, S., Bull. Agric. Chem. SOC. Japan, 1956, 20, 29. Taei, K., and Kawada, K., Ja#an Analyst, 1972, 21, 1510. Hatanaka, C., and Ozawa, J . , Ni+pon NGgeikagaku Kaishi, 1966, 40, 421. “Food Chemicals Codex,” Second Edition, National Research Council, Washington, D.C., 1972, p. 580. Whistler, R. L., and Smart, C. L., “Polysaccharide Chemistry,” Academic Press, New York, 1963, Received May 5th, 1975 Accepted J u H e 4th, 1975 p. 186.

ISSN:0003-2654    

DOI:10.1039/AN9750000822

出版商:RSC    

年代:1975

数据来源: RSC