ISSN:0003-2654    

DOI:10.1039/AN95075FX033

出版商:RSC    

年代:1950

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN95075BX035

出版商:RSC    

年代:1950

数据来源: RSC

ISSN:0003-2654    

DOI:10.1039/AN95075FP061

出版商:RSC    

年代:1950

数据来源: RSC

摘要:

THE ANALYST viiT H E CIVIL SERVICE COJIMISSIOSEKS invite applica-tions for a permanent appointment as Senior Examiner(Male) in the Ministry of Fuel and Power. Candidates mustbe a t least 30 years of age on 1st January, 1950. They mustpossess an honours degree in physics with chenustry as asubsidiary subject, or an honours degree in chemistry withphysics as a subsidiary subject, and must have had a t leastfive years’ experience in industrial research. Inclusivesalary scale L750-f;1,000. Further particulars and applica-tion fornis from the Secretary, Civil Service Commission,Scientific Branch, 7th Floor, Trinidad House, Old BurlingtonStreet, London, W.l, quoting No. R126. Completed applica-tion forms must be returned by 29th September, 1950.CROWS AGESTS FOR THE COLOSIESTEMl’OR-4RY L.4BORATOKY SCPERISTESI>EST(MALE OR FEhftZLE) required by the Goveriiiiieiit ofSierra Leone for Agricultural Chemical Research for twotours, each of 1S to 24 months, in the first instance. Com-riiencing salary according to qualificatioiis and experiencei i i the scale ki74 rising to L912 a year (including allowances).Outfit allowance: L G O .Gratuity of L25 for each 3 monthsservice on satisfactorv coiiipletion of contract. Alternatively,employer’s contributions to F.S.S.1,’. niay bc paid if desiredin lieu of gratuity. 1;ree passages arid liberal leave on fullsalary. Candidates should have had sound laboratorytraining iii a recognised laboratory, arid should be Associatesof the Royal Institute of Chemistry, or hold a degree inchemistry or ;i certificate of the Institute of hledical Labora-tory Technologv.Applv at once by letter, stating age,full nanies in biock letters, whethcr inarried or single, andfull particulars of qualifications and esperience and iiien-tioning this journal to the Crown Agents for the Colonies,4, hlillbank, London, S.W.1, quoting M/N/26f;( t2/:;F on bothletter and envelope. The Crown Agents cannot undertaketo acknowledge all applications and will comniuiiicate onlywith applicants selected for further consideration.EXPERIESCED ASALYST, B.Sc., F.R.I.C. (Branch E jwishes to join Public Analyst with consulting practice inLondon, Home Counties, or South Coast. Write Bos No.:;7.53, THL A?.AI.YST, 47, Gresham Street, Loridon, E.C.2.LEVER BROTHERS & LSILEVEK LIlIITED havcvacancics for three Senior Analytical Chemists in theCVorks Laboratories of their Associated Cornpaiiies iuIVarrington and 011 Mersevside.Candidates for these posts,which carry responsibility and prospects, should be holdersof a 1’h.D. degree or the F.K.I.C., with several years analyticalesperience, and be not less than 28 years of age..%pplications should be addressed to Lever Brothers hLiiilever Lirnited. Personnel Division (FMD), Uiiilever House,Blackfriars, Loridon, E.C.l.THE BRITISH SOS-FERROUS m-r-us RESE.IRCH;\SSOCIATIOS has a vacancy for an analyst, preferablyexperirnced in the analysis of non-ferrous metals and alloys.The work involves analysis of a wide variety of materials andincludes development of iinproved techniques.Initial salaryup to L600 p.a. according to qualifications ,cnd experience.Apply to the Research SIanager, B.S.F.M.R..I., EustonStreet, London. X.W.I.COX,SFQUENT on the transfer of their food researchactivities to a new Food Research Department in Bedford-shire, LEVER BROTHERS & USILEVER LIMITEDhave vacancies on their permanent staff for research chemistshi their Research Department at Port Sunlight, Cheshire.Candidates, whose age should not be greater than 30, shouldhaire obtained not less than a 2nd Class Honours Degreein Chemistry, with or without post-graduate uni\-ersity oriiidustrial research experience.Salaries will be based on qualificatioiis aiid experience.Comprehensive superannuation scheme..4pplications should be addressed to Lever Brothers SrUiiilever Lirnited, Personnel Division (FJID-Sj, CnileverHouse, Blackfriars, London, E.C.4.NALYTICAL CHEMIST B Sc in Chemistry and/orA.4.K.1.C.u-ith Iilininiurii i f one ’year3 experience in ailiiidustrial analytical laboratory, required at once in the assavlaboratory of a large and developing Copper Mine in SortherhRhodesia. Work includes routine analysis of ore samples,concentrates and refined copper, niiscellaneous chemical worknot directly connected with copper refining, and interestingexperimental and development work. Ideal climate.Escellent facilities for all kinds of individual and collectivesport. Minimum salary L561 per annum (L624 after oneyear’s service)-plus cost of living allowance.A highercommencing salary may be offered for exceptional experience.Age limits 22-28 years-preferably but not essentiallysingle. Write, giving full details bf age, qualifications:esperience marital state and date available to Box KO 552Ioster Tu;ner Sr Everetts Ltd., 11, Old Jewry, London, E.C.2:SALYST REQUIRED for a Hayes factory. ExperienceThe work isof a general analytical nature, but specialises to some extenton the analysis of solid fuel. The analyst will be in chargeof sampling and will be given some training before com-mencing his duties. The post is permarlent, with a five-dayworking week; hours are 9.0 a.m. to 5.30 p.m.; pay 1saccording to experience and qualifications, but in generalfollows the A.Sc.W.rates. Write Box No. 3754, THEASALYST. 47. Gresham Street. London, E.C.2.Intermediate B.Sc. standard preferred.ARKE DAVIS & CO. have vacancies for AssistantPResearch Workers in Organic and PharmaceuticalChemistry. Applicants must be University Graduates.Apply to Superintendent, Parke Davis & Co., Staines Road,HE DISTILLERS COMPANY LIMITED has a vacancyan experienced analyst aged 27-30 years, to takecharge of an analytical t e a d conceriied with antibioticsresearch in the Central Research and Developmerit Depart-ment, Great Burgh, Epsom, Surrey. This post offers excep-tional opportunities for an analyst with esperience ofbiochemical analysis wishing to enter the antibiotics researchfield. Applicants must possess a 1st or 2nd class honoursBSc.degree, or equivalent. The coriiriiencing salary willdepend on oualifications and experience. Apply to theController of Research and Development.ASSISTANT CHEMIST reqliired by a niajor Oil Companyfor its London Laboratory handling a wide variety ofproblems affecting oils, greases and special petroleuniproducts. Good education and an Honours B.Sc. degree or.4.K.I.C. are essential qualifications. The 111an appointedwill be required to assist the Head of the Analytical Sectioiiin applying standard oil test methods and special analyticaltechniques to samples of new and used lubricants, engint.deposits, etc. The vacancy carries opportunities for progrcssfor a young, energetic man and in time will involve responsi-bility for the supervision of others.Salary will be coni-mensurate with qualifications aiid experience ; conditions ofemployment include pension and sickness benefit scherne3..4pplicants should write giving full particulars to .4.7, BoxSo. 3751, THE AKALYST, 4 i , Gresham Street, London, E.C.2.STAFFOKDSHI RE COUNTY COCKCILHEALTH DEPARTMEST.APPOISTMEST OF ASSISTAST AN.4LYST (&ISLE).APPLICATIOSS are invited for the above-inentioiledappointnient i n the County Chemical Laboratory,Stafford, from candidates having a university degree and/orthe Associateship of the Royal Institute of Cheniistry.Previous experience in the laboratory of a Public Analyst dlbe an advantage. The commencing salary will be at the rateof g520 per annum, and, subject to satisfactory servicq.willrise by annual increrrients of Ll5, L15 and c20 to a maxiniuniof L5ic) per anniini.The appohitnierit, which will be terminable by one nionth’snotice iri writing on either side, will also be subject to thyprovisions of the appropriate Superannuation Acts :inclRegulations. Confirnlatiou of the appointment will brsubject to the selected candidate passing a medical exaniiria-tion and producing his birth certificate.Applications, giving full particulars. including date ofbirth, qualifications, previous experience, etc., arid statingwhether or not the candidate is related to any member orsenior official of the County Council, accompanied by copies ot‘not more than three recent testimonials, should be forwardedto reach the Countv Medical Of!icer of Health, CountyHounslow, Niddlesex. -Buildings, Stafford, bv first post on 29th September, 1950.T. H. EVANS.Clrvk of f h e Cotinfi Couitcil.COUNTY BLILDI\C<,STAFFOKD.2 l S f J U l l . , 19511.HYDURO 808THE OUTSTANDING NEWASHLESS FILTER PAPERExtremely retentive with wonderfulwet strength. Retains the very finestprecipitates.Ash of I I cm. circle 0.0001 gramrn.Write for free sample and List MG to your dealeror to:-J. BARCHAM GREEN LTD.,MAlDSTONE E N G L A N D

ISSN:0003-2654    

DOI:10.1039/AN95075BP065

出版商:RSC    

年代:1950

数据来源: RSC

摘要:

SEPTEMBER 1950 Vol. 75, No. 894 THE ANALYST PROCEEDINGS OF THE SOCIETY OF PUBLIC ANALYSTS AND OTHER ANALYTICAL CHEMISTS BIOLOGICAL METHODS GROUP A MEETING of the Group was held at 2.15 p.m. on Tuesday, May 23rd, 1950, at the Medical Society of London, Chandos Street, Cavendish Square, London, W.l. The chair was taken by Dr. S. K. Kon. The subject of the meeting was “The Assay of Vitamin B1,”; after an introduction by the Chairman, the following papers were read and discussed: “Assays of Vitamin B,, in Man,” by C. C. Ungley; “Chick Assays,” by Miss M. E. Coates; “The Cup-Plate Assay of Vitamin B,,,” by W. F. J. Cuthbertson, J. T. Lloyd and H. F. Pegler; “Some Observations on the Cup-Plate Assay for Vitamin B,, using Lactobacillus lactis Dorner 10697,” by Frances E. Larkin and R.E. Stuckey; “The Assay of Vitamin B,, by the Turbidimetric Method using Lactobacillus Zeichmannii 313,” by W. B. Emery, K. A. Lees and J. P. R. Tootill; “A Com- parison of LnctobaciZZus Zactis Dorner and Lactobacillus leichmannii for the Assay of Vitamin B,, by a Test Tube Method,’’ by G. E. Shaw; “Experience with the Microbiological Assay of Vitamin B,, in an Analytical and Consulting Laboratory,” by H. Pritchard. About one hundred members and guests were present. PHYSICAL METHODS GROUP THE twenty-sixth Ordinary Meeting of the Group was held at 6.30 p.m. on Tuesday, May 23rd, 1950, in the Meeting Rooms of the Iron and Steel Institute, London. Mr. B. S. Cooper, the Chairman of the Group, was in the chair and about forty members and visitors were present. The following papers on Radio Chemistry in Analytical Chemistry were read and discussed : “Radiometric Assay in Tracer Research,” by F. P. W. Winteringham, A.R.I.C. ; “The Determination of Potash (in Fertiliser) by Measurement of its Radioactivity,” by D. S. Lees, B.A., A.Inst.P., W. Broomfield and H. N. Wilson, F.R.I.C. ; “Radioactivation Analysis-Some Glimpses of its Scope,” by A. A. Smales, B.Sc. DEATHS Eric Cecil Keeley Theodore Rendle. I’VE regret to record the deaths of 453

ISSN:0003-2654    

DOI:10.1039/AN9507500453

出版商:RSC    

年代:1950

数据来源: RSC

摘要:

454 ALBON AND GROSS : THE CHROMATOGRAPHIC, DETERMINATION [Vol. 75 The Chromatographic Determination of Raffinose in Raw Sugars BY N. ALBON AND D. GROSS SYNOPSIS-A paper chromatographic method is described for the separation of raffinose from otner sugars in a mixture. The raffinose is made visible by spraying with a suitable reagent. A quantitative estimation is possible by visual comparison of the developed spot with standard spots produced by known quantities of piire raffiriose. This technique is sufficiently sensitive to yield a positive result with as little as 0-05 per cent. The method has been applied to the determination of raffinose in a large number of raw beet sugars. EXISTING procedures for the determination of raffinose in sugar products are based mainly on the principle of the Clerget inversion or double polarisation method.Of these methods the two-enzyme procedure of Paine and Balchl is the most accurate and reliable, but the high cost of the enzymes, the skill and time required for this method make its adoption for routine use difficult. Owing to the limitations of the optical methods, especially when applied to technical products, the accuracy to be expected is not very high. According to Browne and Zerban2 the indication of a smaller amount of raffinose than 0.5 per cent. in commercial products is extremely doubtful. The object of the present investigation has been to devise a more rapid and accurate method for determining raffinose in commercial sugar products such as raw beet sugars. PartridgeS has shown the possibility of separating very small quantities of sugars by the use of paper chromatography. Preliminary experiments with known synthetic mixtures of sucrose and raffinose soon demonstrated the advantages of this method.The high sensitivity of the technique is a great advantage when dealing with products containing very small amounts of raffinose, mostly below 0-5 per cent., for which available methods are not suitable. A method has been developed which can be employed for the routine analysis of sugar products, particularly raw sugars, and which compares favourably with the available methods, both as regards accuracy and sensitivity. The paper chromatographic technique employed enables a large number of samples to be analysed simultaneously and the method is sufficiently rapid, simple and inexpensive to be adopted for routine use.The possibilities of this method were pointed out by H. C. S. de Whalle~.~ METHOD Accumulator vessels were modified for this purpose. Tanks of dimensions up to 9 x 32 x 30 inches were used according to number and size of chromatograms run. To minimise temperature changes, the tanks were lagged with several layers of cotton wool or other insulating material. The troughs holding the solvents were made from Pyrex glass or Polythene tubing of approxi- mately one and a quarter inches outer diameter. The troughs were supported inside the tanks. by suitably cut strips of glass. 'The drying cabinet was 64 x 26 x 30 inches, with a glass window 16 x 16 inches. Air, drawn in at the bottom over heating elements by means of a fan, flowed round the suspended chromatogram and was discharged from the top into a fume cupboard.The heating and air flow could be varied. Reagents were applied from glass spraying bottles connected to an air compressor so as to obtain an even and strong spray. REAGENTS- following solvent mixture was used- Apparatus-All-glass tanks were used for most of the experiments. Whatman's No. 1 paper of size 182 x 229 inches or 24 x 24 inches was used. The n-Butanol, lab. reagent . . . . 6 parts by volume Water .. .. .. .. 3 7. Pyridine (A.R.) . . .. .. 3 93 Benzene, lab. reagent . . .. 1 Y2Sept., 19501 OF RAFFINOSE IN RAW SUGARS 455 Mix gently in a separating funnel and allow to stand until the upper layer is transparent. Use the upper layer a s solvent in the trough, and the bottom layer for saturation of the atmosphere in the tank.Spraying reagent-Make up a 1 per cent. (w/v) solution of a-naphthol (A.R.) in ethanol and filter. Before use, mix 50 ml. with 5 ml. of phosphoric acid (A.R.). PROCEDURE Make up a 40 per cent. (w/v) solution of the raw sugar sample and prepare standards from a raffinose-free raw sugar solution (40 per cent. w/v) and pure raffinose hydrate (Kerfoot's Biochemical Reagent). Draw a pencil line parallel to, and 12 cm. from, the top of the paper sheet (Whatman's No. 1) and mark the points of application for each sample and standard 3 cm. apart. Apply from a micro-pipette 5 pl. of each solution to the appropriate mark. Allow a few minutes for drying and repeat the application, making a total amount of 10 p1. (4 mg.of sugar). Leave to dry for 30 minutes, fold the top of the sheet around a glass strip, 3 cm. wide, and place it in the trough containing the solvent. After 24 hours a t room temperature (15" to 25" C.), remove the chromatogram from the tank avoiding the spillage of surplus solvent down the sheet. Shorter runs of 16 hours often give sufficiently good separations. Hang in the drying cabinet and remove the solvent in a stream of air at 90" C. for 1 hour. Transfer to a fume cupboard and spray both sides rapidly and evenly with the a-naphthol reagent. The quantity of reagent required is approximately 1 ml. per 25 sq. cm. At no time during the spraying should the reagent be excessive or be allowed to flow over the surface of the paper.Replace the chromatogram in the drying cabinet and heat for 10 minutes at 90" C. to allow the colour of the spots to develop. Remove and immediately compare, by transmitted light, the raffinose spots in the samples with those in the standards. General remarks on the method-The method has proved to be applicable to a fairly wide range of sugar products, although it was primarily employed for the analysis of raw sugars. A preliminary run will indicate the approximate concentration of raffinose present and the standards required. With samples containing more than 0.4 per cent. of raffinose the quantity taken for the chromatogram may conveniently be reduced to 2 mg. (5 p1. of solution). This decreases the possible interference by salts in high concentration, such as are present in sugars with more than 1.5 per cent.of ash. These sugars can readily be de-ionised by passage through mixed beds of ion exchange resins prior to their application to the chromatogram. Examination of a large number of raw beet sugars showed that raffinose was always present, but none was detectable in a number of raw cane sugars. The standards were therefore prepared from a raw cane sugar in preference to a synthetic mixture of pure sugars. The chromatograms can conveniently be copied by exposing reflex copying paper through the chromatogram, with a yellow filter over the light source. Given even lighting and correct exposure, these copies may be used for the estimation of the raffinose. As the spots fade, copies should be made within half an hour.RESULTS Many samples of raw beet sugars have been examined by the method described above. Table I shows the accuracy of visual comparison of the separated raffinose spots with The standards were made up in steps of 0.1 per cent. and intermediate con- The figures represent the Each Estimations were made from the original chromato- TABLE I ESTIMATIONS OF A SERIES OF SAMPLES BY TWO ANALYSTS The results are summarised below in Tables I and 11. standards. centrations could be estimated to an accuracy of 0.05 per cent. differences between the estimations by two analysts from the same chromatograms. sample was run singly, not in duplicate. gram and also from the photographic copy. Difference in Original Photographic 0.2 2 1 0.15 13 2 0.1 33 40 0.05. 129 132 nil 175 170 Total 352 Total 345 estimation, chromatograms copies O f /O - -456 ALBON AKD GROSS THE CHROMATOGRAPHIC DETERMINATION [Vol.75 Table I1 shows the results obtained by running chromatograms of five samples at various times and dnder various conditions. Standards were made up from different raw sugars; tanks of various dimensions were used, the paper sheets were from different batches and of various sizes, and the solvents were mixed separately for each run. The room temperature varied within 10" C. Estimations were made from the original chromatograms by four analysts and the mean figures taken except in determination No. 1. TABLE I1 ESTIMATIONS OF FIVE SAMPLES, UNDER VARIOUS CONDITIONS, BY FOUR ANALYSTS Raffinose content, per cent. Determination r Maximum Sample No.1 No. 2 No. 3 so. 4 No. 5 difference A 0.10 0.10 0.11. 0.07 0.10 0.04 B 0.20 0.28 0.29 0.86 0.21 0.09 C 0.30 0.38 0.36 0.39 0.37 0.09 D 0.40 0.4 1 0.45 0.44 0.40 0.05 E 0.40 0.3'2 0.36 0.35 0.37 0.08 A DISCUSSION Raffinose is known to occur generally in beet sugar products, but accurate determination of the amounts present is difficult by hitherto available methods and is liable to interference from other constituents. The chromatographic method permits the raffinose to be separated from the other sugars present and enables the identity of this sugar, free from interfering substances, to be confirmed in various ways. The RF value* of the raffinose in beet raw sugars is the same as that of pure raffinose under the same conditions and differs from the RF values of any other sugar to our knowledge.In a number of different solvent mixtures, some of which were used for two-dimensional separations, the raffinose spot from the raw sugar behaved in the same manner as pure raffinose. The spraying reagent used reacts only with sugars yielding fructose on hydrolysis. A number of equal volumes of a raw sugar solution were chromatographed together on a paper sheet and the spots corresponding to raffinose extracted. After concentration and precipitation with acetone, characteristic needle-shaped crystals were obtained, indistin- guishable from those precipitated from pure raffinose. In other experiments a number of the spots separated from the raw beet sugar were extracted and concentrated to give a 2 per cent. solution of the supposed raffinose.On inversion with invertase (Wallerstein's) this gave fructose and melibiose, and, on inversion with invertase containing melibiase (Wallerstein's) , it yielded fructose, glucose and galactose. The presence of these sugars in the expected yields was demonstrated chromatographically, and so was the complete inversion of the supposed raffinose. A limit to the sensitivity of the method is set by the minimum quantity of raffinose that can be detected by the spray reagent and the maximum load that can be applied to the paper sheet. The a-naphthol reagent compares favourably with other spraying reagents tested in respect of sensitivity, 2 pg. of raffinose being detectable. Although other solvent mixtures that were tried gave good separations of the sugars, it was found that the mixture recommended has the best combination of high load, good separation and sharpness of spots for this particular application. An important feature of the method is that it enables a large number of samples to be examined at the same time.Thirteen samples and six standards can be analysed on one sheet of paper (24 x '24 inches) and several such sheets can be placed in one tank. movement of spot (band) movement of advancing front of liquid *RF = The RF value is characteristic of each compound in a given solvent under standard conditions and can be used for the identification of compounds.Sept., 19501 OF RAFFINOSE I N RAW SUGAKS 457 This procedure has made possible the routine determination of rafinose in hundreds of samples within a short time and with comparative ease. In view-of the small quantity of material required and the good agreement of results, the method has also proved of great convenience in exploratory and control work. We wish to thank Mr. H. C. S. de Whalley, Director of Research, for valuableadvice during the prosecution of this work, and the Directors of Tate gi Lyle Limited for permission to publish this paper. REFERENCES 1. 2. 3. 4. RESEARCH LABORATORY Paine, H. S., and Balch, R. T., Ind. Eftg. Chem., 1925, 17, 240; J . A ~ Z C Y . Chcni. Soc., 1027, 49, 1019. Browne, C. -4., and Zerban, F. W., “Physical and Chemical Methods of Sugar Analysis,” 3rd Ed., Partridge, S. M., Biochem. J . , 1948, 42, 238. de Whalley, H. C. S., 171t. Sug. J . , 1950, 52, 127. New York, 1941, p. 470. TATE & LYLE LTD. RAVENSBOURNE, ESTERHAM HAM ROAD KESTON, KENT N a y , 1950

ISSN:0003-2654    

DOI:10.1039/AN9507500454

出版商:RSC    

年代:1950

数据来源: RSC

摘要:

Sept., 19501 OF RAFFINOSE I N RAW SUGAKS 457 Determination of Theobromine in Cocoa Products BY K. E. HOLMES SYNoPsIs-The existing methods for determining theobromine in cocoa and its products are described and a new and improved procedure is proposed. In this, the alkaloid is completely extracted by means of boiling water and magnesia, clarified with lead acetate, filtered, concentrated and extracted with chloroform. The alkaloidal residue from the chloroform extract is then assayed by Boie’s method. By this procedure the results are equal to, and sometimes higher than those by Wadsworth’s method. A rapid continuous extractor, for use with chloroform, is described. THE determination of theobromine in cocoa products has always been difficult, and it has received considerable attention in recent years.The methods recently devised can be grouped under two headings. (a) Those using tetrachloroethane or chloroform after triturating with magnesia and a limited amount of water. ( b ) Those using aqueous solvent with added acid or alkali. The methods in group (a), which are the best known, were initiated by Wadsworth,l whose method was modified by Macdonald,2 Humphrie~,~ Kay and Haywood4 and Lowe.5 All these methods suffer from the defect that the amount of water to be left in the mixture of cocoa material and magnesia before the solvent is added is very critical. Insufficient water gives low results, too much yields an impure product, and there is always a doubt as to whether the extraction is complete or not. Jalade6 steeped cocoa in dilute caustic soda, and Pritzker and Jungkunz7 heated it under a reflux condenser with water and magnesia.Both took an aliquot part of the clean extract and extracted it with chloroform in a continuous extractor. Martin and Clergue8 refluxed cocoa with 10 per cent. acid and continuously extracted the mixture with chloroform. Moir and Hinksg extracted the alkaloid with aqueous ethanol and magnesia, evaporated the filtered extract to remove ethanol, extracted the clarified and filtered extract with successive amounts of chloroform, and assayed the final product by Kjeldahl’s method. Parkes and ParkeslO used a mixture of chloroform and phenol instead of pure chloroform. Moores and Campbell11 eluted cocoa material with boiling water, adsorbed the theobromine from an aliquot part of the clarified extract on a chromatographic column, eluted with.alkali and estimated the alkaloid by electrometric titration. The following is a rksurnk of recent work in the second group.458 HOLMES: DETERMINATION OF i-VOl. 75 Of these methods, those of Jalade and of Moir and Hinks are rather lengthy. In the author's experience, that of Pritzker and Jungkunz gives low results, and the same may be said of the method of Martin and Clergue. The Moores and Campbell method is laborious; it uses rather small quantities of sample and the extraction of theobromine is incomplete. EXPERIMENTAL In the present research a method was devised which obviated some of the faults of those mentioned above. Complete extraction of theobromine was effected, with reduced manipula- tion, in a working day of 8 to 10 hours.In this work the Wadsworthl method was used for the sake of comparison, and the Roie12 method for assaying the extracted theobromine. Despite the high solubility of theobromine in tetrachloroethane, this solvent was avoided and water was used instead. I t was shown firstly that all the alkaloid could be extracted from cocoa material of known theobromine content by refluxing 10 g., together with 5 g. Glass Wool- 1 I --!- 1 ; 11'5 1 1 I 1 I I25 I - - f - - I - - ; 50 I I - -I-- \ ( 80 Fig. 1. Rapid liquid - liquid extractor. Dimensions in millimetres. I I I I I I I I I . I I I I 1 350 1 I I I I I I I of magnesium oxide, three times with 250-ml. portioiis of water, followed by filtration and evaporation to dryness.The dried residue, when extracted with more magnesia and tetra- chloroethane by the Wadsworth method, gave a quantitative yield. It was found that if the mixture of cocoa, magnesia and water was clarified, better filtration resulted and that the impurities in the extract were reduced. For this purpose, basic lead acetate solution was found suitable and no loss was observed as a result of its use. An aliquot part of the filtrate was taken after only one extraction, but this led consistently to low results. The loss wasSept., 19501 THEOBROMINE I N COCOA PRODUCTS 459 traced to the adsorption of alkaloid by magnesia, for when pure theobromine done was carried through the aliquot part process, not more than 95 per cent. could be recovered. Complete washing, on the other hand, gave a quantitative yield.Having got all the theobromine into aqueous solution, the next problem was how best to isolate it. This could be done by evaporating the solution to dryness and applying the Wadsworth method to the residue. Moir and Hinksg extracted theobromine from aqueous solution by shaking with successive quantities of chloroform, and Parkes and ParkeslO added 5 per cent. of phenol to the chloroform. The method of continuous extraction with a liquid - liquid extractor seemed to be more promising, and the apparatus shown in Fig. 1 was devised for this purpose. It is a Soxhlet extraction apparatus with the syphon tube replaced by a continuous overflow pipe. With the dimensions shown, the volume of the aqueous layer is about 130ml.The special condenser adaptor has six nozzles with fine jets which ensure maximum contact of the condensed chloroform with the aqueous layer. If a sufficiently rapid reflux rate is maintained (about 50 ml. per minute), 0.3 g. of theobromine can be completely recovered from aqueous solution in less than 3& hours, although 5 hours were taken in practice. No advantage was gained by using a larger extractor; one of 400ml. capacity required 8 hours to complete the extraction of 0.3 g. theobromine, and one of 800 ml. capacity needed 16 hours to extract a similar amount. After extraction, the chloroform was distilled off leaving theobromine that was usually 90 to 92 per cent. pure. It was only slightly discoloured and contained caffeine and a trace of fat. Only the caffeine and fat could be removed by leaching with light petroleum; some of the other impurities could also be removed if tetrachloroethane and ether were used as at the end of the Wadsworth process, but no more than 98 per cent.purity could be attained by this means. All attempts to produce pure theobromine failed. The purity of the crude theobromine as extracted above was therefore determined by the Boie12 method; good sharp end-points have been consistently obtained with this quick and reliable method. It has been in continual use in these laboratories for two years. The following procedure is recommended. PREPARATION OF SAMPLE- petroleum. The defatted material is dried and powdered before use. defatted, dried and powdered. but cocoa powder requires no preparation.first be neutralised with 3 N hydrochloric acid. Cocoa beans or nib are ground to a mass and defatted with low-boiling (80" C.) light Mass or chocolate is Press cake, extrusion cake or shell are ground before use, Cocoa residues containing added lime should PROCEDURE Well mix 10 g. of material with 5 g. of magnesia that has been heated to 900" C. and kept for use in a stoppered bottle. Transfer the mixture to a flask of 500 to 1000 ml. capacity, add 30 ml. of boiling water and boil under a reflux condenser for 10 minutes. Take care at the commencement of the boiling to control the frothing, which may become excessive. Clarify the hot liquid with 10 to 15 ml. of basic lead acetate (the strong solution of lead sub- acetate of the B.P.) and then filter the liquid by suction through a Buchner funnel with a 9 cm.Whatman No. 1 filter-paper. Wash the precipitate, which up to this point must not be allowed to go completely dry, with 50 ml. of hot water, and then suck dry. Transfer the residue and filter-paper to the flask, add 200 ml. of hot water and boil under reflux for 10 minutes. Filter the liquid and wash the precipitate as before. Repeat the extracton procedure once more. Concentrate the combined filtrates to about 100 g. in a tared flask, or in a flask marked a t 100 ml. During the concentration of the filtrate, prepare the clean and dry extractor by the addition of a one inch layer of dry glass wool on the bottom of the extractor tube. Weigh a 250-ml. ground joint flask containing three small fragments of porcelain.Add 100 to 120 ml. of chloroform, connect the extractor flask and clamp the apparatus vertically. Add chloroform to the extractor to a depth of three inches and then, through a funnel, add the 100 ml. of concentrated solution at a temperature of 50" to 60" C. The liquid displaces some of the chloroform into the flask. Wash the flask used for concentrating the solution with a small volume of hot water and add the washings to the extractor tube, taking care that the layer of chloroform above the glass wool is not less than an inch deep. Fit the dispersal460 HOLMES DETERMINATION O F [Vol. 75 adaptor, the jets of which must be just below the surface of the aqueous phase. Add water, if necessary, until this condition is fulfilled. Connect the apparatus to an efficient double surface condenser with ground joint and support the flask on an asbestos sheet having a circular hole large enough to take the bottom quarter- to half-inch of the flask.Use an Amal burner practically full-on to give the necessary rapid rate of reflux. Make sure that, in addition to the jets of the adaptor being full, there is about a half-inch layer of chloroform in the body of the adaptor. If it is intended to extract overnight, use an electrically heated sand-bath. At the end of the extraction time, dismantle the apparatus and distil off the chloroform until a quarter-inch layer remains. Place the flask on a bath of boiling water and remove the remaining chloroform with a hand-blower, until there is no odour of chloroform. Dry the flask and weigh when cold.Determine the theobromine in the crude extract by the Boiel2 method as follows- Dissolve the crude extract in 100 ml. of water containing 1.5 ml. of N sulphuric acid by heating almost to boiling and maintaining at this temperature for 5 minutes. Cool the solution to 40" C. and add 1.5 ml. of phenol red indicator solution. Add N sodium hydroxide (approximately 1.5 ml.) until the solution turns red. Bring the colour back just to yellow by the careful addition of 0.10 N sulphuric acid. Add 0.10 N silver nitrate (which must be neutral to phenol red), taking care that the amount used in ml. is about 80 times the weight of crude theobromine in grams, e.g., for 0.2 g. use 16 ml. of 0.10 N silver nitrate. Titrate the solution with 0.10 N sodium hydroxide until the pink colour returns, The end-point is indicated by a marked change in colour with one drop of the alkali solution.Express the theobromine as a percentage of the original material; for beans, nib, mass or chocolate, the fat percentage is also required for the purpose of calculation. The results have been compared with those obtained by the Wadsworth method on various cocoa products. Some typical figures, expressed on moisture and fat-free material, are given in Table I. Do not use a gauze. For complete extraction, run the extraction for a t least 5 hours. 1 ml. of 0.10 N sodium hydroxide = 18.01 mg. of theobromine. TABLE I Material used Cocoa residue after solvent extraction of the fat .. .. .. .. .. (2) . . .. .. .. .. . . (1) * . (3) * - (4) - .( 5 ) * . (6) - - (7) * - (8) * * (9) * * (10) . . .. .. * . .. .. .. .. .. .. .. .. .. .. .. . . . . . . .. . * .. .. .. .. .. . . * . .. .. .. .. .. .. .. .. .. . . .. .. .. .. Cocoa shell- Unroasted . . .. .. .. .. Roasted . . .. .. .. .. Roasted . . * . .. .. .. Cocoa residues from theobromine extraction . . Cocoa expeller cake . . .. .. .. Milk chocolate . , .. .. .. .. Plain chocolate . . .. .. .. .. Cocoa powder . . .. .. .. .. Wadsworth method r 1 Theobromine, yo Crude Pure 3.14 3.05 3.12 3.30 3-24 3-25 3-23 3.28 2.65 2-s1 2-98 2.93 3.03 3.14 3.06 3-14 3.11 3.19 2-58 2.75 1.58 1.53 1-85 1.58 1-12 1-04 0.65 0-53 0-40 0.38 0.74 0.7 1 3.36 3.24 3.31 3.19 2-71 2.62 0.26 0.23 1-28 1-18 3.46 3.30 Proposed method Crude Pure 3.33 3.3 1 3-37 3.45 3-37 3.53 7 3.41 3.48 2.81 3.03 3.05 3.00 3-05 3.16 3.11 3-17 3.12 3.21 2.59 2.76 1:71 1-54 1-75 1-59 1-19 1-06 0.59 0.54 0-45 0.4 1 0.78 0.7 1 3.60 3.25 3.52 3.19 2.84 2.64 0.26 0.24 1.29 1.1s 3.65 3.32Sept., 19501 THEOBROMINE IN COCOA PRODUCTS 461 These figures show that the proposed method gives results that are as good as, and sometimes higher than, those obtained by the Wadsworth method.The validity of the method has been checked as follows- (1) Theobromine of known purity was carried through the process with and without cocoa material. Some of the results obtained are given in Table 11. TABLE I1 Pure theobromine added, g- 0.247 1 0.3014 0.0592 0.1026 0.1040 0.1327 Theobromine in cocoa material used, €5 nil nil 0.282 0.224 0.041 0.04 1 Theobromine found, g. Crude Pure h f $ 0.250 0.305 0.368 0.356 0.158 0.192 0-247 0.302 0.34 16 0.327 0.145 0-174 These figures show that no theobromine is lost in the course of the process.(2) Reproducible results have been obtained, as shown in Table 111. TABLE I11 Theobromine found in 10 g. Material used Cocoa residue from solvent plant-Sample X . . Ditto-Sample B . . .. .. .. Cocoa residue from theobromine extraction . . Crude, €5 0.312 0.311 0.311 0.313 0.326 0.325 0.324 0.045 0.04 7 Pure, g. 0.285 0.284 0.286 0.285 0.294 0.292 0-293 0.041 0.042 (3) Blank determinatiom-When blank determinations have been carried out, no weighable extract has been obtained, and less than 0.02 ml. of 0.10 N sodium hydroxide has been needed in the Boie assay. SUMMARY It is based upon the extraction of cocoa material with boiling water and magnesia, clarification, filtration and continuous extraction of the concentrated filtrate with chloroform for 5 hours.A design is given for a rapid liquid-liquid extractor for this purpose. The chloroform extract is assayed by the Boie method. The proposed method has been compared with the Wadsworth process. The author wishes to thank the Directors of Messrs. Cadbury Bros., Ltd. for permission to publish this paper, and he is indebted to Mr. S. B. Pliillips and to Dr. H. C. Lockwood for much helpful advice and guidance. 14 method is proposed for the determination of theobromine in cocoa products. REFERENCES 1. Wadsworth, R. V., ,4nalyst, 1921, 46, 32. 2. 3. 4. 5. Lowe, E. H., Ibid., 1948, 73, 679. 6. 7. 8. 9. 10. 11. 12. Boie, H., Pharm. Ztg., 1930, 75, 968. Macdonald, J. A., “6th Ann. Rep. Cocoa Res.,” Imp. Coll. Trop. -\griculture, 1936, p. 34. Humphries, E. C., “8th ,4nn. Rep. Cocoa Res.,” Imp. Coll. Trop. .4griculture, 1938, pp. 36-38. Kay, J., and Haywood, P. J. C., Analyst, 1946, 71, 162. Jalade, Ann. Falsif., 1929, 22, 396. Pritzker, J., and Jungkunz, R., Mitt. Lebensm. Hyg., 1943, 34, 185. Martin, F., and Clergue, H., Ann. Chinz. Analyt., 1942, 24, 202. Moir, 1). D., and Hinks, E., Analyst, 1935, 60, 439. Parkes, A. E., and Parkes, H. A., Ibid., 1937, 62, 791. Moores, R. G., and Campbell, H. A., Anal. Chew., 1948, 20, 40. CADBURY BROTHERS LIMITED BOURNVILLE March, 1950

ISSN:0003-2654    

DOI:10.1039/AN9507500457

出版商:RSC    

年代:1950

数据来源: RSC

摘要:

462 FRANT AND WESTENDORP: THE DETERMINATION OF [Vol. 75 The Determination of Trichloroacetic Acid in Urine Bx7 R. FRANT AND J. WESTENDORP SYNOPSIS-TWO methods are described for the estimation of trichloroacetic acid in urine, a semi-quantitative method with a possible error of about 25 per cent., and a more quantitative method with an average deviation of 5 per cent. Both methods are based on the well known Fujiwara reaction. Attention is drawn to the effect of carbon dioxide on the coloured solution. The final method proposed is suitable for samples having a minimum concentration of 10 pg. of trichloroacetic acid per ml. IN view of the widespread use of trichloroethylene, a siniple method for following its ingestion and excretion by industrial workers is of value. Studies of the metabolism of trichloroethylenel 7 2 have shown that it is converted into trichloroacetic acid.Any scheme of analysis must therefore determine trichloroethylene in air, blood and urine, and also trichloroacetic acid in blood and urine. Several excellent methods3 v4 j5 y 6 97 exist for the determination of trichloroethylene in the various media; we have, therefore, restricted our work to the determination of trichloroacetic acid in urine. EXPERIMENTAL Direct application of the well-known Fujiwara reaction3 9 4 gave orange colorations which were visually compared with a range of coloured papers. By this means standards were established equivalent to the range 1.5 mg. to 6 mg. of trichloroacetic acid per litre. This range proved insufficient to accommodate all the specimens received and it was found necessary to dilute the samples with urine free from trichloroethylene and trichloroacetic acid.It was noticed that, on standing, the pyridine layer changed colour from orange to yellow and this has been shown to be due to reaction with the carbon dioxide in the air; if this is excluded, the layer is orange. DIRECT METHOD- Pour the mixture on to 4.0 ml. of pyridine contained in a test tube. Close the mouth of the tube with a rubber bung with a tube leading to an absorption apparatus containing potassium hydroxide. Compare the red- orange coloration of the pyridine layer with a previously standardised series of coloured papers. Traces of trichloroacetic acid too small for determination by this means may be detected by passing a stream of carbon dioxide through the pyridine layer.A colour change from orange to yellow may then be perceived at a concentration as low as 1 mg. of trichloroacetic acid per litre. Table I shows the results obtained when this method was applied to a series of artificial standards. Apart from poor reproducibility this method suffers from the need for dilution with normal urine of samples containing large amounts of trichloroacetic acid. It was, therefore, decided to adapt the method to photometric determination. PHOTOMETRIC METHOD- The existence of two phases was considered a hindrance to successful photometric determination and attention was turned to the possibility of obtaining a single phase a5 in the process developed by Rogers and Kay7 for the estimation of carbon tetrachloride in air.The difficulty in applying this method to the determination of trichloroacetic acid in urine lay in the necessity of obtaining the chloro-compounds in acetone solution. Fortunately, acetone is immiscible with strongly alkaline aqueous solutions and we were thus able to obtain the required acetone solution by extracting alkaline urine with a measured volume of acetone. The use of ether instead of acetone was suggested, but although this would enable us to dispense with the addition of alkali, it was felt that the high vapour pressure of ether and its partial miscibility with water made acetone the more satisfactory solvent. The method finally adopted was as follows. Procedure-To 20 mi. of urine add 10 mi. of 12.5 K sodium hlvdroxide and mix.Heat to 70" C. for exactly 5 minutes, and cool rapidly.Sept., 19501 TRICHLOROACETIC ACID rx URINE 463 During the investigation into the suitability of this extraction for photometric work the following procedure was employed. TABLE 1 RESULTS OF DIRECT METHOD APPLIED TO KNOWN SOLUTIONS OF TRICHLOROACETIC ACID -%mount added in 20 ml., _\mount found, Error, Pba. PLg' '0 40 45 - 12.5 40 45 t 12.5 40 45 -y 12.5 50 45 - 10 50 45 - 10 70 60 - 14 90 90 0 90 90 0 100 90 - 11 110 60 - 45 120 120 0 120 90 - 25 190 90 - 25 140 120 - 14 oc Procedure-To 26ml. of urine add 10ml. of 12-5A7 sodium hydroxide and 10ml. of acetone. Shake vigorously for 5 minutes, and transfer 2 ml. of the acetone layer by means of a pipette into a test tube. Add 4 ml. of alkaline pyridine solution (100 ml. of pyridine, 40 ml.of water, 0.48 ml. of 15 per cent. sodium hydroxide) and heat the tube a t 70" C. for exactly 5 minutes, and then cool with running water. Immediately after cooling,- transfer the coloured solution to a 5-mm. cell and measure its extinction at 5400 A. with a Bleeker photo-electric colorirneter. Visual comparison of repeated acetone extracts from a single sample of urine led us to believe that almost complete separation of trichloroacetic acid and trichloroethylene was achieved with only a single extraction. The results obtained by single extraction are shown in Table 11; they suggest that Beer's law is followed by the coloured solution. TABLE I1 RESULTS OF SINGLE EXTRACTIONS OF SAMPLES OF URINE CONTAINING T7.-2RIOUS AMOUNTS OF TRICHLOROACETIC ACID Trichloroacetic acid, x 104 /*g-/ml.E I.: - Eo E - Eo (= C) c 0 0 g;;} Eo 10 0.122 0,076 '76 10 0.123 0.077 77 30 0.204 0.158 79 20 0.198 0.1qj2 76 A study was next made of the effect of variation in the time of heating the pyridine - It is clear from this table that the acetone mixture; the results are shown in Table 111. TABLE 111 EFFECT OF VARIATION IN TIME OF HEATING Trichloroacetic Heated for 5 minutes Heated for 10 minutes A acid, r 1 -- x 104 E - Eo E - E, (= C ) E C E E-Eo ____ E-EO ~ x 104 /*.g-/ml- 0 0.040 0.040 0 0.043 0.043 30 0.226 0.184 61 0.259 0.2 17 72 30 0.223 0.181 60 0.258 0.216 72 40 0.295 0-253 63 0.326 0-284 71 40 0.893 0.25 1 63 0.334 0.292 73464 FRANT AND WESTENDORPI THE DETERMINATIOS OF [Vol.75 time of heating is an important factor. The fact that the values (E - E,)/C x 1W are lower in Table I11 than in Table I1 is attributed to variation in the alkali content of the pyridine - sodium hydroxide mixture and we suggest that more consistent results might be obtained by mixing 100 ml. of pyridine with 40 ml. of 0.18 per cent. sodium hydroxide. The application of the method to urines containing large amounts of trichloroacetic acid with heating for 7 minutes gave the results shown in Table IV. RESULTS OBTAINED ON URINES CONTAINING LARGE AMOUNTS OF TRICHLORO*lCETIC ACID Heating time = 7 minutes Trichloroacetic acid, 0 0 30 30 60 60 90 90 120 150 150 PfT /ml- E 0.034 0.043 0.213 0.217 0-387 0.409 0.61 1 0-639 0,841 1.020 1.020 E - E, x 104 E - E, C 0.174 0-178 0.348 0.370 0.572 0.600 0.802 0.980 0.980 55 59 58 62 04 6 i 67 6.5 05 APPLICATION TO HIGH CONCENTRATIONS- An attempt to apply the method to higher concentrations by diluting the acetone before taking an aliquot proved unsatisfactory as the extract became turbid on the addition of pure acetone.This difficulty was overcome by diluting the acetone extract with the acetone extract from a trichloroacetic acid- and trichloroethylene-free urine. The results given in Table V were obtained with a urine containing lZOpg. of trichloroacetic acid per ml. TABLE V RESULTS OBTAINED AFTER DILUTION WITH ACETONE EXTRACT FROM TRICHLOROXCETIC ACID- AND TRICHLOROETHYLENE-FREE URINE Acetone extract, ml. 2 2 1.6 1-6 1.2 1.2 0.8 0.8 0.4 0 0 (= 4 Blank extract, ml. 0 0 0 4 0 4 0.8 0.8 1.2 1.2 1.6 2.0 2.0 E 1.010 1.020 0.716 0.773 0-60 1 0.608 0.423 0-420 0.238 0,030 0.036 E - E, 0.977 0.987 0.683 0.740 0.568 0.575 0.390 0-387 0*20.3 E - E, :: s 104 82 82 71 $7 79 so 81 81 85 120 a These results suggested that analysis of a urine sample containing an unknown amount of trichloroacetic acid might be successfully carried out by this method, provided that a urine containing a known amount of trichloroacetic acid were examined concurrently to serve as a standard.The method was found to be improved by closing the mouth of the test tube with a plug of cotton wool containing solid potassium hydroxide to remove carbon dioxide from the atmosphere during the colour development, and by adding 1 ml. of boiled water to prevent turbidity. We thus had two methods applicable to urine samples containing large amounts of trichloroacetic acid, viz., (a) to dilute the sample with trichloroacetic acid-free urine, and (b) to take an aliquot from the acetone extract.A sample was taken, which a semi-quantitative examination by the method already outlined had shown to contain about 600 pg. of trichloroacetic acid per ml., and was examined by both methods. The results are shown in Tables VI and VII.Sept., 19601 TRICHLOROACETIC ACID I N URINE TABLE VI 466 Sample, ml. 0 0 1 1 2 2 3 3 4 4 5 5 (= P ) RESULTS ON Trichloroacetic acid-free urine, rnl. E 25 0.009 25 0.009 24 0-152 24 0.155 23 0.261 23 0.261 22 0.383 22 0.386 21 0.533 21 0.500 20 0.627 20 0-6J 1 DILUTED SAMPLE E - E, x 103 P E - EO 0.143 0.146 0-252 0-252 0.374 0.377 0.524 0-49 1 0.618 0.642 Sample containing 60 pg./ml.. . .. .. 0.320 Acetone extract sample, ml. (= 4 ) 0 0 0.2 0.2 0.4 0.4 143 146 126 126 125 126 131 123 124 128 TABLE VII RESULTS ON ACETOSE EXTRACT OF SAME s h m x Blank extract, E - E, x 10' 4 ml. E E - Eo 2.0 0.009 2.0 0.009 1.8 0.29' 0.283 142 1.8 0.293 0.284 142 1-6 0-578 0.569 142 1.6 0.580 0.57 1 143 Sample containing 60 pg./ml. . . .. .. 0.330 pg./mf. 67 1 684 59 1 59 1 586 59 1 615 57 7 582 600 609 - pg./ml. 533 533 533 536 534 - A series of urines was then examined to determine the accuracy of the method; the results These results were considered satisfactory of this examination are shown in Table VIII. and the final method is given in detail below. Trichloroacetic acid added, pg. /ml. PO 40 80 80 133 133 120 120 173 173 211: 214 213 213 254 234 '267 267 Test 2 2 2 2 2 2 1 1 1 1 1 1 0.5 0.5 0.5 0.5 0.5 0 .5 T-4BLE VIII EXAMINATION OF ACCURACY Blank E 0 0 0 0 0 0 1 1 1 1 1 1 1.5 1.5 1.5 1.5 1.5 1.5 0.206 0.218 0.429 0.405 0.736 0.717 0.330 0-318 0.486 0.484 0-605 0.555 0.318 0-286 0.365 0.32 1 0.357 0.35." Blank 0 2-0 0.007 Blank 0 2.0 0.005 OF METHOD E - E, acid found, Trichloroacetic CLg. /ml. 0.200 364 0.212 38.5 0.423 76.6 0.399 724 0.730 132.5 0.71 1 129.0 0-324 117-5 0.312 113.1 0.480 174 0.478 173.5 0.599 217.5 0.549 199 0.312 216 0.280 203 0.359 260.2 0.3 15 228.5 0.35 1 254 0.346 25 1 Standard 2 Standard 2 0.336 0.326 ) 0 0.342 0 0.332 Average deviation . . .. Error, - 9.0 - 3.6 - 4.0 - 9-5 - 0-4 - 3-2 - 2.0 - 6.6 +- 0.6 + 0.3 + 1.7 - 7.0 +- 1.4 - 4.7 $.2.9 - 9.6 - 4.5 - 6-0 % - 4.3466 FRANT AND \YESTENDORP [Vol. 75 METHOD REAGENT- and 0-48 ml. of 15 per cent. sodium hydroxide. Alkaline pyridine reagent-A solution containing 100 ml. of pyridine, 40 ml. of water PROCEDURE- To 25 ml. of the specimen, add 10ml. of 12.5 N sodium hydroxide and 10-Oml. of acetone. Also carry out this procedure on a trichloroacetic acid-free urine and on 24ml. of trichloroacetic acid-free urine to which has been added 1 ml. of 0.15 per cent. trichloroacetic acid solution. Transfer 2-ml. portions of the acetone extract into test tubes by means of a pipette. If the amount of trichloroacetic acid is high, take a smaller quantity from the sample extract and dilute to 2ml. with acetone from the blank determination. Add 4 rnl.of alkaline pyridine reagent and close the tubes with a plug of cotton wool containing a little solid potassium hydroxide. Heat the tubes at 70” C. in a water-bath for 7 minutes, remove them from the bath and rapidly cool to room temperature under running water. Lift the cotton wool plugs, add 1.0 ml. of boiled water from a burette and replace the plugs. Finally transfer the coloured solutions to 5-mm. cells and record their extinction by means of a Bleeker photo-electric colorimeter and using a wavelength of 5400 A. The trichloroacetic acid in the sample (or samples) may be determined by comparison of the extinction of the sample with that of the standard. This method is suitable for samples having a minimum concentration of 1Opg. of tri- chloroacetic acid per ml. Agitate the whole vigorously for exactly 5 minutes. REFERENCES 1. 2 . 3. 4. 6. 6. 7. Barrett, H. M., and Johnston, J . H., J . Biol. Chent., 1939, 127, 765. Powell, J . F., Brit. J . I n d . Med., 1945, 2, 142. Forssman, S., Svenska Lakartidn., 1945, 29, 1. Fujiwara, K., S.B. Natuyf. Ges. Rostock., 1914, 6, 33. Jacobs, M. B. , “Analytical Chemistry of Industrial Poisons, Hazards and Solvents,” Interscience Burgen, A. S. V., Brit. Med. J . , 1948, 1, 1238. Rogers, G. W., and Kay, K. K., J . I n d . Hyg. Toxicol., 1947, 29, 229. Publishers, Inc, New York, 1941. THE MEDICAL DEPARTMENT AND ANALYTICAL LABORATORY N.V. PHILIPS GLOEILAMPENFABRIEKEN EINDHOVEN, HOLLAND December, 1949

ISSN:0003-2654    

DOI:10.1039/AN9507500462

出版商:RSC    

年代:1950

数据来源: RSC

摘要:

Sept., 19501 MILLER AND CURRIE 467 The Micro-Gravimetric Determination of Lead in White Metals, Fusible Alloys and Tin-Free Copper-Base Alloys BY CHRISTINA C. MILLER AND LESLIE R. CURRIE SYNoPsIs-Lead (0.2 to 6 mg.) is separated from other alloy components, except copper in excess of 1 mg., by precipitating it twice as the lead nitrate - thiourea complex. The latter is dissolved in water, lead sulphide precipitated, separated, and dissolved in hydrochloric acid, and, finally, lead nitrate formed, which is dissolved in an aqueous solution of ammonium acetate, and filtered. The solution is acidified with acetic acid and lead separated and weighed as lead chromate, which is dried at 140' C. The method is applied to 5 mg. amounts of white metals and a fusible alloy, which are dissolved in hydrochloric acid and nitric acid.When more than 1 mg. of copper is present with lead, it must first be removed by electro-deposition. The method is then applicable to tin-free copper-base alloys, 5 to 10 mg. quantities of which are dissolved in nitric acid. ON the micro-scale lead has been determined as lead sulphate, picrolonate, phosphate and dioxide,l but there have been few applications in alloy analysis. Strebinge? determined lead in lead - tin solders as lead sulphate. Lindsey3 and his collaborators are working out a comprehensive scheme of analysis for alloys, based on micro-electrolytic depositions, but so far no results for the determination of lead in alloys have been published. Our objective was to devise a procedure for lead that could be applied to the determination of 2 to 90 per cent.in 5 to 10 mg. quantities of some non-ferrous alloys. For white metals and a fusible alloy we have adapted to the micro-scale and suitably modified the procedure used by Mahr and O€~le,~ whereby lead is precipitated as the lead nitrate - thiourea complex and, after appropriate treatment, converted into lead chromate as the weighing form. In the analysis of copper-base alloys containing little or no tin, we have removed copper by electro-deposition before applying the method. EXPERIMENTAL Mahr and Ohle showed that 20 to 50 mg. of lead contained in about 100 ml. of 1 to 2 N nitric acid could be quantitatively separated from comparable amounts of nearly all the common metals if the solution were saturated with thiourea and maintained at a temperature of 0" C.In the presence of bismuth a second precipitation was necessary, and in the presence of copper and silver, an adequate excess of thiourea. Aqua regia could be used without objection for the solution of alloys containing antimony and tin. The precipitates were separated, washed and dissolved in hot water, and in the resultant, almost neutral solution, after the addition of ammonium acetate, lead was precipitated as lead chromate, They applied the method to a lead - tin solder, a fusible alloy, a brass and a bronze. The simplicity of the method was attractive for micro-analysis because it seemed that, by using centrifugal methods of separation and the filter-stick technique, one might be able to carry out all the operations in one vessel. In preliminary experiments many difficulties arose when we tried to apply the method microchemically.As solubility losses were easily incurred with lead nitrate - thiourea, the volume of the solution in which its precipitation was effected had to be made a minimum, partly in order to counterbalance the effect of having to use proportionately more wash solution on the micro-scale. In consequence of this the tendency for the precipitate to be contaminated with other metals was greater. Karaoglanov and Michov5 effected the separa- tion of lead from moderate amounts of other metals by precipitating lead chromate from solutions 0.06 N in nitric acid. When we applied the method to lead in the solutions obtained from contaminated lead nitrate - thiourea precipitates difficulties arose because, in adjusting the acidity by neutralising the solution with ammonium hydroxide and adding a calculated amount of nitric acid, a precipitate of metallic sulphides separated.When this precipitate was dissolved, the resulting solution was contaminated by sulphur, which made lead chromate468 MILLER AND CURRIE : MICRO-GRAVIMETRIC DETERMINATION OF LEAD I N [VOl. 75 difficult to filter. Reduction of excess chromate also occurred during the digestion that preceded filtration. Lastly, no satisfactory results could be obtained with small amounts of lead and the method was ultimately abandoned in favour of precipitation of lead chromate in solutions containing acetic acid and ammonium acetate, as was used by Mahr and Ohle.This meant that lead had first of all to be separated completely from other alloy components, as a rule by double precipitation with thiourea. Since partial decomposition of thiourea caused contamination of lead chromate, it was necessary to eliminate it before precipitating the latter. This was achieved by precipitating lead as sulphide, dissolving it in hydrochloric acid, expelling hydrogen sulphide and then converting lead chloride into lead nitrate by evaporation with nitric acid. As some lead sulphate was invariably produced it was essential to dissolve the residue obtained after evaporation in a concentrated solution of ammonium acetate. Filtration of its solution was still required in order to remove a slight turbidity, and, in alloy analysis, to remove a little hydrated stannic oxide.The filtrate was collected in the vessel in which the lead was to be determined as lead chromate. Guzelj6 has shown that lead sulphate dissolved in ammonium acetate solution can be satisfactorily determined as lead chromate. The reagents were as far as possible AnalaR. Thiourea was recrystallised from water. PROCEDURE FOR THE SEPARATION OF LEAD AS LEAD NITRATE - THIOUREA AND ITS DETERMINATION AS LEAD CHROMATE Place the solution containing 0.2 to 5 mg. of lead as lead nitrate in a test tube (75 mm. x 12 mm.) and make up to a volume of 1.5 ml. that is 2 N in nitric acid. Add 0-2 g. of thiourea, shake the tube until precipitation starts, then cool it for half an hour at -5" C., shaking vigorously at intervals. Centrifuge, while maintaining the temperature below 0" C.by placing the tube within a wider container of cooling liquid, that fits into the metal bucket of the centrifuge. Withdraw the centrifugate by suction through a sintered porcelain filter-stick and wash the precipitate once with 0-5 ml. of 2 N nitric acid that has been saturated with thiourea at room temperature and cooled to -5" C. Next dissolve the precipitate by placing the test tube containing it within a suitable suction apparatus and drawing 1.5ml. of hot water through the filter into the tube.' Add sufficient 7 N nitric acid to the solution to make it 1 to 2 N, add a small amount of thiourea, and repeat the above treatment, washing the precipitate thrice with 0-5-ml. portions of the prescribed solution before dissolving it.In order to precipitate lead sulphide, add to the solution 0.5 ml. of 15 N ammonium hydroxide, saturate with hydrogen sulphide, cool to room temperature and centrifuge and withdraw the supernatant liquid as before. Wash the precipitate twice with 0.25 ml. of a saturated solution of hydrogen sulphide, then dissolve it by drawing 1.5 ml. of boiling 5 AT hydrochloric acid through the filter-stick into the test tube. Heat to effect complete solution and evaporate the solution to dryness on the steam-bath, accelerating evaporation by means of a current of purified air. Dissolve the residue in 1 ml. of 7 N nitric acid, re-evaporate to dryness and dissolve again in 0.25 ml. of a hot, 20 per cent. ammonium acetate solution. Transfer the solution through a clean, porcelain filter-stick into a 5-ml.beaker that has been weighed together with a sintered glass (porosity No. 4) or asbestos-packed Emich type filter- stick. Wash the test tube, etc., twice with 0.25 ml. of hot water followed by 0.1 ml. of ammonium acetate solution, and finally twice with 0.25 ml. of hot water. If the amount of lead is small (0-2 mg.) reduce the volume to less than half by evaporation. Whatever the final volume, make the solution 0-25 to 0-7 N in acetic acid, heat to boiling and precipitate lead chromate by the dropwise addition of a 0.5 per cent. solution of potassium dichromate, in amount ranging from 1.5 ml. for 5 mg. of lead, to 0-1 ml. for 0.2 mg. Digets the precipitate at 90" C. for 10 minutes then let it cool for 1 hour and filter. Wash a large precipitate once with 0.5 ml.of hot water and five times with 0-25 ml., and a small precipitate once with 0.3 ml. and then three times with 0.25 inl. Dry the beaker and its contents at 140" C. in a Benedetti - Pichler drying block, cool and weigh, with strict adherence to the programme outlined in the next section. THE TREATMENT OF BEAKERS AND FILTER-STICKS BEFORE WEIGHING Trouble caused by the production of electrostatic charges on glassware that had been wiped, prior to weighing, in accordance with Pregl's well-known procedure, led us to abandon this in favour of another routine, which also compensated for changes in barometric pressure. The tare for each beaker and filter-stick was another beaker of the same glass, adjusted toSept., 19501 JVHITE METALS, FUSIBLE ALLOYS AND TXN-FREE COPPER-BASE ALLOYS 469 weight by the addition of short lengths of glass rod.The outside of the prepared beaker and its counterpoise were wiped once with a pair of very damp flannelette cloths, one held in each hand, a rotary motion being imparted to the wiping, and then similarly with a pair of slightly damp pieces of chamois leather. Each beaker, held by forceps, was next passed four times through a non-luminous flame in order to dispose of any electrostatic charge, and then placed in the drying apparatus at 140” C. for 10 minutes. After removal, the beakers were cooled for 5 minutes on metal blocks, and the tip of the filter, which had been connected by rubber tubing to the suction line, was wiped clean. Both beakers were then placed for 15 minutes in a desiccator containing a saturated solution of calcium nitrate, which allowed the glass surfaces to assume rapidly a reproducible film of moisture.8 Finally, they were transferred simultaneously to the balance case and weighed after 5 minutes.A reliable duplicate of the first weighing was obtained by merely reheating (no suction) and cooling both beakers as before. After use the beaker and filter-stick were submitted to the full treatment, but the counterpoise, which had been kept in the desiccator, was only given the heating at 140” C. All weighings must be duplicated. THE DETERMINATION OF LEAD IN ALLOYS Method 1. For white metals and fzfisible alloys-Dissolve 5 mg. of the alloy in 0.2 ml. of 5 N hydrochloric acid and the minimum amount of 7 N nitric acid, add 1 ml.of water and sufficient 7 N nitric acid to bring its concentration to about 2 K. Proceed with the deter- mination of lead as already indicated, noting, however, that with fusible alloys both precipitates of lead nitrate - thiourea must be washed thrice. For coeper-base alloys containing Little or no tin-Copper, which in excess of 1 mg. accompanied lead nitrate - thiourea, was removed initially by electro-deposition as follows. Dissolve 5 to 10mg. of the alloy, contained in the usual test tube, in 0.2 ml. of 7 N nitric acid, add 2.5 ml. of water and boil to expel nitrous acid. Next add 0.1 ml. of a 10 per cent. urea solution and electrolyse for 10 minutes at 2.5 volts, using as a cathode a cylinder (diameter 10 mm., height 15mm.) of platinum gauze (20 strands per cm.), and, as an anode, a platinum wire.Finally remove and rinse the electrodes, carefully protecting the anode on which some lead dioxide deposits. Evaporate the solution to dryness on a steam-bath and take up the residue in 0.2 ml. of 5 N hydrochloric acid. Dissolve the lead dioxide on the anode in dilute nitric acid containing hydrogen peroxide and collect the solution in the test tube, to make a final volume of 1-6 ml. at a concentration of about 2 N in nitric acid. RESULTS (i) The direct determinatiopi of lend as lead chromate-I!eighed portions of a standard solution of lead nitrate, prepared from “British Chemical Standards” lead, gave the following results, when the theoretical factor of 0.6411 was used for converting lead chromate into lead.TABLE I Method 2. Precipitate lead nitrate - thiourea once only and continue as described above. JVeight of Xumber of Maximum range lead taken, csperirnents in error, -4 \-erage error, mg. PF$ w 3 4 +6 t o - 5 - 1 1 9 +9 to -5 - 3 0.2 10 +5 t o -7 nil No significant difference was noted when the amount of ammonium acetate present was varied between 20 and 100mg., or when the amount of acetic acid ranged between 0.25 and 0.7 N , provided that the washing of the lead chromate was very carefully done. A negative error averaging 4 to 5 pg. was incurred when the smallest quantity of lead was precipitated with chromate in the same volume of solution as the larger quantities. (ii) The determination of lead as lead chromate aftev $first separating it as lead nitrate - thioztrea-Weighed portions of the above standard solution of lead nitrate were submitted to the full scheme of separations previously outlined and the following results obtained. In a “blank” run on the reagents no lead was found.Error on 5 mg. of lead, -22, -20, +13 and +7 pg. Error on 0-2 mg. of lead, -2, -19, +4 and -11 pg. Inadequate precautions for keeping the lead nitrate - thiourea precipitates cool during470 MILLER AND CURRIE : MICRO-GRAVIMETRIC DETERMINATION OF LEAD [Vol. 75 centrifugation caused the rather large negative errors in the first pair of results for 5 mg. of lead, and incomplete mastery of the technique, at this stage, the irregularity in the results for 0-2 mg. Influence of copfier aizd bismutk-The presence of 50 pg. of copper or bismuth did not affect the results for 5 mg.of lead. The maximum amount of copper that could be separated from 1 mg. of lead in 1.5 ml. of solution was 1 mg. Three mg. of bismuth could be separated from 1 mg. of lead if both precipitates of lead nitrate - thiourea were washed thrice with 0.5 ml. of the wash solution. (iii) Lead in alloys-In order to avoid errors due to lack of homogeneity in the samples of prepared alloys, we prepared small amounts for analysis by aliquot partition of a solution made up from 1 g. of the powdered alloy and treated in accordance with the prescribed micro- procedure. The metals used in preparing the synthetic alloy solutions were the purest available; the lead, copper and zinc having a purity in excess of 99.95 per cent.The bismuth and cadmium were examined for lead and none was found, All the results shown in Table I1 refer to 5 mg. of alloy, except for the high-speed brass where 10mg. was taken. In the analyses of the copper-base alloys, copper was removed by electro-deposition before precipitating lead nitrate - thiourea. I Sb 7cZ’hite metal “A”* . . 12.04 White metal “B”* . . 7.31 Lead-base bearing metal? 10.09 Cd Fusible alloy: . . . . 12.3 zn High-speed brasst . . 37.0 - Bearing metal: . . .. TABLE I1 Percentage composition Sn Bi c u 4.64 0.03 0.33 84.0 trace 4-08 10.91 0.06 0.05 25.00 30.00 3*0u * British Chemical Standards sample. -f U.S.A. Bureau of Standards sample. ’+ Synthetic. tj In all except the last experiment the lead nitrate - thiourea had k e n instead of the recommended half hour. Percentage of lead found 82-9, 82.9, 82.7, 82.9 78.7, 78.8 2500, 25-04, 24.79, 24.96 30.10, 30.02, 30.10, 30.00 3-03, 2-92, 2.94, 3.01 3.65, 3.67, 3.65, 3.88$ cooled for 15 niinutes only The results in Table I1 show that the micro-gravimetric determination of lead in the alloys cited is practicable provided that the conditions prescribed on p. 468 are closely followed. A negative error is rather easily incurred in dealing with quantities of lead of the order of quarter of a milligram. We gratefully acknowledge a maintenance grant to one of us (L. R. C.) from the Depart- ment of Scientific and Industrial Research and a grant from the Ritchie Fund for the purchase of apparatus. REFERENCES 1. 2 . 3. 4. 5. 6. 7. 8. Hecht, F., and Donau, J ., ‘‘ASnorganische Mikrogewichtsanalyse,” Edwards Brothers, Inc., Ann Strebinger, R., Chem. Zentr., 1918, 11, 471. Lindsey, A. J., Analyst, 1948, 73, 72. Mahr, C., and Ohle, H., 2’. anorg. Chem., 1937, 234, 224. Karaoglanov, Z., and Michov, M., 2. m a l . (;hem., 1935, 103, 113. Guzelj, L., Ibid., 1936, 104, 107. Hecht, F., and Donau, J ., “4norganische Mikrogefficl.ltsanalyse,” Edwards Brothers, Inc., -4nn Arbor, Michigan, 1940, p. 101. Thiers, R, E., and Beamish, F. E., .-lnal. Chaiir., 1947, 19, 434. Arbor, Michigan, 1940, p. 150. CHEMISTRY DEPARTMENT THE UNIVERSITY, EDINBUKGH May, 1950

ISSN:0003-2654    

DOI:10.1039/AN9507500467

出版商:RSC    

年代:1950

数据来源: RSC

摘要:

Sept., 195Oj MILLER AND CURRIE 47 1 The Micro-Gravimetric Determination of Lead in Bronzes BY CHRISTINA C. MILLER AND LESLIE R. CURRIE SyNoPsIs-Lead in amounts of 0.2 to 1 mg. is quantitatkrely separated under carefully controlled conditions from 5 to 10 mg. of copper and 1 mg. of tin, by depositing it electrolytically as lead dioxide from solutions containing hydrofluoric acid and nitric acid (cf. McCayl). Lead dioxide is dissolved and lead precipitated and weighed as lead chromate. The method is applied to 5 and 10mg. amounts of two bronzes con- taining, respectively, 20 and 1.83 per cent. of lead. WHEN the method described in the preceding paper, see p. 467, for the determination of lead in tin-free, copper-base alloys, was applied to those containing an appreciable amount of tin, no satisfactory results could be obtained.The average result for a bronze containing 20.00 per cent. of lead was 19.37, and for another containing 1.83 per cent., 1-69 per cent. In the final filtration that preceded the precipitation of lead chromate, hydrated stannic oxide was invariably withheld on the filter and it contained adsorbed lead. Although present in the original bronze solutions, stannic oxide dissolved in the hydrochloric acid subsequently added, but /I-stannic salts apparently contaminated the lead nitrate - thiourea complex and lead sulphide, and gave rise again to hydrated stannic oxide in the final treatment with nitric acid. Stannic oxide did not separate initially if tartaric or citric acid was included in the nitric acid solvent for the bronze, nevertheless it still appeared in the final solution.All efforts made to eliminate tin by volatilising its halides were unsuccessful. Attempts made to remove copper by depositing it electrolytically from hydrochloric acid solutions2 of the alloys, in which /?-stannic salts were not produced, unaccountably failed. The recovery of adsorbed lead from impure stannic oxide, by fusing the latter with sodium carbonate and sulphur and separating soluble sodium thiostannate from insoluble lead sulphide, was associated with complications of procedure that led to other negative errors of undetermined origin. The object of our investigation was therefore to devise an alternative method for the determination of lead in bronzes. McCayl showed that 60 to 400 mg.of lead could be separated from up to 500 mg. of tin by depositing it anodically as lead dioxide from a solution containing nitric and hydrofluoric acids. We have evolved comparable micro-procedures for the separa- tion of 1 and 0.2 mg. of lead from 1 mg. of tin and 5 mg. or more of copper, and applied them to the bronzes mentioned above, the lead being finally weighed as chromate. EXPERIMENTaL ELECTROLYTIC APPARATUS FOR THE ANODIC DEPOSITION OF LEAD DIOXIDE- Electrolyses were conducted in a platinum crucible (height 26 mm., diameter at the top, 20 mm., and at the bottom, 13 mm.) as the anode, with a platinum gauze (10 strands per cm.) cylinder (height 13 mm., diameter 10mm.) as the cathode. Several strands of platinum wire were placed across the bottom of the cylinder so that it formed a kind of basket.The effect of this was to induce the deposition of lead dioxide on the bottom of the crucible, which did not occur without this modification. To the upper edge of the cylinder was welded a platinum wire that was bent at right angles above the crucible and sealed into a glass support. The crucible rested in a triangle of platinum wire, which was attached to an insulated ring, and connected to the positive terminal of a 4-volt battery. The electrical circuit contained a %-ohm, variable resistance and a milliammeter, and across the electrodes was a &volt voltmeter of 50,000-ohm resistance. The electrically driven stirrer was a platinum wire, slightly flattened at the lower end, and sealed into a glass rod at the upper.Attached toit just above the crucible, and rotating with it, was a protective celluloid cover. A moderate rate of stirring only was required. Vigorous stirring caused depositions to be incomplete. In order to obtain adherent deposits of lead dioxide the crucible was etched by brief treatment with aqua regia.472 MILLER -4ND CURRIE THE MICRO-GRAVIMETRIC [Vol. 75 CONDITIONS FOR THE DEPOSITION OF LEAD DIOXIDE- Quantitative deposition, especially of a small amount of lead, occurred under carefully controlled conditions that were deduced from the results of a long series of experiments, in which electrolyses were done in the presence of 5 to 10 mg. of copper and 1 mg. of tin. The method is as follows: Evaporate the solution containing nitrates and fluorides of the metals to dryness in the crucible to be used for the electrolysis and, if 1 mg.of lead is present, take up the residue in 2.5 ml. of a solution that is 1-5 N in nitric acid and 1.0 IV in hydrofluoric acid. Add 5 mg. of urea, set up the electrolysis outfit and start the stirrer. Begin the electrolysis at room temperature, with the full resistance in the circuit, and, after 1 to 2 minutes, increase the voltage to 2.50 Maintain this value for 30 minutes, then remove the stirrer, withdraw the electrolyte by suction and rapidly replace it with water. Remove and replace the water until no current is registered. For 0.2 mg. of lead reduce the concentration of both acids to 0.5 AT. 0.05. THE DETERMINATION OF LEAD AS LEAD DIOXIDE OR AS LEAD CHROMATE- If the determination of lead as lead dioxide is required, proceed as follows: Dry the crucible containing lead dioxide for 10 minutes in the platinum-lined cavity of a metal drying block maintained at 230" C., then cool for 5 minutes on a metal block and 5 minutes in the balance case, and weigh against a similar crucible as a counterpoise.Next dissolve the lead dioxide in 0-5 ml. of hot 7 N nitric acid and 0.1 ml. of 3 per cent. hydrogen peroxide solution, and, if the further determination of lead as lead chromate is required, transfer the solution to a 5 ml. glass beaker that has been previously weighed together with a sintered glass filter- stick (porosity No. 4). Wash the crucible first with half the above volumes of nitric acid and hydrogen peroxide and then five times with 0.25ml.of hot water. Dry and weigh the crucible. For the determination of leadas lead chromate evaporate the solution in the beaker to dryness, dissolve the residue in 1.5 ml. of a 2 per cent. solution of ammonium acetate and 0.1 ml. of 7 N acetic acid, and proceed exactly as is described in the preceding paper. THE DETERMINATION OF LEAD IN BRONZE- In the crucible to be used for the electrolysis, dissolve 5 or 10 nig. of bronze in a mixture of 4 drops of water, 2 of hydrofluoric acid (40 per cent. w/w) and 3 of concentrated nitric acid. Evaporate the resulting solution to dryness and dissolve the residue in 2.5 ml. of a solution containing the amounts of nitric acid and hydrofluoric acid appropriate for the anticipated amount of lead. If a little antimony is present, oxidise it with a drop of a 0-5 per cent.solution of potassium dichromate (cf. McCay), then add 5 mg. of urea and proceed as already outlined. RESULTS The purity of the lead, tin and copper used in the following experiments exceeded 99.95 per cent. AnalaR reagents were employed throughout and examined when necessary for traces of lead. Stock solutions containing hydrofluoric acid were prepared and kept in platinum vessels. Lead as lead dioxide-All depositions were effected in the presence of 5 or 10 mg. of copper and the amounts of nitric acid and hydrofluoric acid appropriate for the weight of lead. Weighed portions of a standard solution prepared by dissolving lead in nitric acid were used. Error on 1 mg. of lead, (a) Tin absent: +B, +20, +17, +2l and +17 pg.(b) 1 mg. of tin present : +36 and +28 pg. (c) 5 mg. of tin present: +13 and +28 pg. Error on 0.2 mg. of lead, (a) Tin absent : +8, + 1, +9 and +6 pg. (b) 1 mg. of tin present: +4, +4, 0, +5 and + 2 pg. These errors, which are much in excess of what could be caused by our use of the theoretical factor for the conversion of lead dioxide to lead (cf. Sand3), are in accord with positive errors of about 2 per cent. found by McCay, who attributed them to fluoride contamination of the lead dioxide. He eliminated them by dissolving the lead dioxide and redepositing it in the absence of hydrofluoric acid. We inexplicably failed to improve our results by repeating the electrolysis in the presence of 5 mg. of copper and nitric acid only. The copper was added in the second electrolysis in order to prevent cathodic deposition of lead.v I.lzJEuence of other eZements-The addition of 50 pg.of phosphorus added as potassium dihydrogen phosphate had no significant adverse effect on the deposition of 0.2 or 1 mg. of lead as lead dioxide. If 200 pg. of manganese (11) or 100 pg. of antimony (V) were initially present with 1 mg. of lead, the amounts of manganese and antimony that were found in the lead dioxide were less than 1 and 3 pg. respectively. The influence of bismuth was con- siderable, 20 per cent. of that initially added accompanying the lead dioxide. Lead as lead chromate-The following results refer to experiments in which 5 or 10 mg. of copper and 1 mg. of tin were initially present with the lead, which was deposited as lead dioxide and then dissolved and converted into lead chromate.Error on 1 mg. of lead, -3, +2, +12 and -8 pg. Error on 0.2 mg. of lead, +9, - 1, -3, + 10, -4 and +S pg. The excessive positive errors were attributed to the failure to filter the nitric acid solution of the lead dioxide. In the following analyses of bronzes, filtration was carried out through a sintered glass filter-stick (porosity No. 4). Lead in bronzes as lead chromate-(1) High lead bronze. A synthetic solution was prepared by adding lead as lead nitrate to a nitro - hydrofluoric acid solution of British Chemical Standards Bronze “C,” so that the percentage composition of the corresponding alloy was : copper 69.82, tin 7-88, antimony 0.03, bismuth a trace, other (non-interfering) elements 2.27 and lead 20.00. The weighed portions of the solution taken for analysis corresponded to about 5 mg. of alloy. (2) British Chemical Standards Bronze “A.” A standard solution was prepared from 1 g. of bronze in accordance with the micro-procedure and weighed portions equivalent to 10 mg. of alloy were analysed. The percentage composition of the bronze was: copper 85.5, tin 9.7, antimony 0.24, other (non-interfering) elements 2.71 and lead 1.83. Lead found, 20.02, 20.02, 20-31 and 19.94 per cent. Lead found, 1-81, 1-80, 1.84 and 1-81 per cent. We gratefully acknowledge a maintenance grant to one of us (L. K. C.) from the Depart- ment of Scientific and Industrial Research and a grant from the Ritchie Fund for the purchase of apparatus. REFERENCES 1. 2. 3. McCay, L. W., .I. -4wiev. Cheriz. SOC., 1914, 36, 2376. Lindsey, A. J., Analyst, 1938, 63, 159. Sand, H. J . S., “Electrochemistry and Electrochemical Analysis,” Vol. 11, Blackie & Son Ltd., London, 1940, p. 74. CHEMXSTRY DEPARTMENT THE UNIVERSITY, EDIKBURGH May, 1950

ISSN:0003-2654    

DOI:10.1039/AN9507500471

出版商:RSC    

年代:1950

数据来源: RSC