Analyst October 1983 Vol. 108 pp. 1227-1234 1227 Differential Electrolytic Potentiometry with Periodic Polarisation Part XXVII." Polarisation in Substitution Addition and Oxidation Titrimetry with Dibromine in Anhydrous Acetic Acid Direct and Mark-space Biased Periodic Adballa M. S. Abdennabit and E. Bishop University of Exeter Chemistry Department Stocker Road Exeter EX4 4QD Applications of direct current and mark-space biased square wave differential electrolytic potentiometry to bromination and oxidation reactions in anhydrous acetic acid have been examined. A few oxidation reactions e.g., of iodine and ascorbic acid are fast enough to permit direct titration. Other reactions such as nuclear bromination of aromatic hydroxy and amino com-pounds addition to unsaturated aliphatic moieties and oxidation of covalent group 5B compounds require double excess back-titration.Of the three electrolytes examined perchloric acid lithium chloride and sodium acetate, the last gives the best results for aromatic substitution being alkaline enough to allow the phenoxide ion intermediate to form. Certain oxidation indicators of the diphenylamine and benzidine classes and aminotriphenylmethanes are not oxidised but react by substitution by nuclear bromination and are cleanly determined without interference from unstable oxidation products. Addition to double bonds is slow but complete. Keywords ; Difleerential electrolytic potentiometry ; non-aqueous bromination ; oxidation titrimetry ; anhydrous acetic acid solvent Direct current differential electrolytic potentiometry (d.c.DEP) has been applied to oxidation -reduction titrations in aqueous media,l and gives two types of curve (Figs.2-6) a peak when the electrode processes are of comparable speed (type I) or a 2-shaped curve when one of the electrode processes is slow rising if the slow process originates from the titrant falling if from the titrand [types II(a) and II(b) respectively]. Symmetrical periodic current (~.c.DEP)~ and mark-space biased (rn.s.b.DEP)39* polarisation have also been applied to aqueous oxida-tion - reduction titrimetry and have been found to lead to rapid equilibration of potentials and sustained electrode activity due to the high current densities required and the continued polarity reversal that prevents accumulation of films on the electrodes.Little use has hitherto been made of polarised electrodes in non-aqueous titrimetry; although studies of DEP in non-aqueous acid - base titrimetry have been r e p ~ r t e d . ~ ~ ~ This paper presents an examination of the application of DEP to substitution addition and oxidation reactions in anhydrous acetic acid with dibromine. Experimental Apparatus and titration processes have been des~ribed.~,~ Zero-current potentiometry, d.c.DEP and m.s.b.DEP measurements were conducted simultaneously to afford compar-ability. Platinum electrodes were used. Reagents The calculated amount of acetic anhydride' was added to glacial acetic acid allowed to stand for 24 h refluxed for 8 h and finally distilled. In a fume cupboard approximately 0.1 mol of Aristar grade dibromine was transferred into a 1-1 flask containing anhydrous acetic acid mixed and Anhydrous acetic acid.Dibromine solution 0.1 moll-l. * For Part XXVI of this series see reference list p. 1234. t Present address University of Petroleum and Minerals P.O. Box 144 Dhahran International Airport, Dhahran Saudi Arabia 1228 ABDENNABI AND BISHOP DIFFERENTIAL ELECTROLYTIC Analyst Vol. 108 diluted to volume. The solution was standardised each day against the 0.2 moll-1 arsenic(II1) solution by DEP lithium bromide being added as electrolyte. AnalaR grade arsenic(II1) oxide was dried at 120 "C for 3 h and 9.981 g were dissolved in 100 ml of warm 2.5 moll-1 sodium hydroxide solution, 100 ml of 2.5 mol 1-1 hydrochloric acid were added and the solution was cooled and diluted to 1 1 with water.4-Hydroxybenzonitrile phenol quinol 4-aminopheno1 thymol 4-methoxyphenol 1-naphthyl-amine aniline 1,3-phenylenediamine and 2-nitrophenol solutions 0.1 moll-1. These solutions were prepared by dissolving 0.01 mol of the pure amine or phenol in anhydrous acetic acid and diluting to 100 ml. Dipkenylamine benxidine N-phenylanthranilic acid rosaniline and 3,3'-dimethoxybenxidine solutions 0.01 mol 1-l. The indicator solutions were prepared by dissolving 1 mmol of pure indicator in anhydrous acetic acid and diluting to 100 ml. Limonene cyclohexene oleic acid and pentane-2,4-dione solutions 0.1 moll-l. The solutions of unsaturated compounds were prepared by dissolving 0.01 mol of the pure substance in anhydrous acetic acid and diluting to 100 ml.Diiodine and L-ascorbic acid soltxtions 0.01 moll-l. A 1-mmol amount of the AnalaR grade compound was dissolved in anhydrous acetic acid and diluted to 100 ml. The limited soh-abilities precluded the use of stronger solutions. Bis(pentane-2,4-dionato)oxovanadium(IV) (vanadyl acetylacetonate) soltxtion 0.002 moll-1. Ammonium metavanadate(V) (4.6 g) was dissolved in 60 ml of 2.0 mol 1-1 sodium hydroxide solution 170 ml of 1.0 mol 1-1 sulphuric acid were added and sulphur dioxide was passed through until no further change in the blue colour occurred. The excess of sulphur dioxide was expelled by boiling and the solution was allowed to cool and filtered. To the stirred filtrate 10ml of pentane-2,4-dione were added followed by a solution containing 20g of anhydrous sodium carbonate in 120 ml of water.The precipitate was filtered off and recrystal-lised from chloroform. The low solubility in anhydrous acetic acid permitted dissolution of 2 mmol in the solvent and dilution to 100 ml. Electrolyte solutions 0.1 moll-l. The required volume of 72% m/m Aristar grade perchloric acid was diluted with acetic acid the amount of acetic anhydride required to react with the water present was added the solution was diluted to volume with acetic acid and stored until the reaction was complete. For lithium chloride and sodium acetate 0.1 mol of the anhydrous compound was dissolved in anhydrous acetic acid and diluted to 1 1. The standard 1 mg ml-l arsenic solution (BDH Chemicals Ltd.) in hydrochloric acid was standardised by the blank titration with the standardised dibromine solution, Arsenic(III) solution 0.2 moll-l.Arsenic(1II) chloride solution 0.0133 moll-1. Procedures Direct titration The method with the simultaneous use of zero-current potentiometry d.c.DEP and m.s.b.DEP by means of five platinum wire indicator electrodes and the double bridge filled with the appropriate non-aqueous electrolyte solution connecting the S.C.E. to the titration cell has been de~cribed.~ The current density was 3 x 10-6 A for d.c.DEP and the bias for m.s.b.DEP was 23% at a frequency of 60 Hz. Double excess back-titration.s To take account of the volatility of dibromine and its slow reaction with the electrolyte the method was modified by conducting the extended reaction alongside the blank in two stoppered 100-ml Quickfit flasks.To each 20 ml of the appropriate non-aqueous electrolyte solution is added. To one an aliquot of 0.1 mmol of sample is added followed by 0.5 mmol of dibromine (an excess of 40-400~o); to the other the blank 0.5 mmol of dibromine is added. After the selected time of reaction the same amount of arsenic(II1) chloride in an excess of that needed to react with the unconsumed dibromine is added to each vessel. The contents are transferred into the titration cell washed with similar amounts of electrolyte solution and the excess of arsenic(II1) is back-titrated with dibromine as in the direct method. The difference between the two back-titrations gives the amount of dibromine consumed by the sample October 1983 POTENTIOMETRY WITH PERIODIC POLARISATION.PART XXVII 1229 Results and Discussion Glacial acetic acid has proved to be a suitable solvent for oxidation - reduction titrations.799-12 Phenols have been directly titrated potentiometrically by dibromine in glacial acetic acid with sodium acetate as the ele~trolyte.~ IngbermanlO has catalysed the reaction with pyridine, which permits direct tit rat ion using constant - curren t pot ien t iomet ry . l1 Glacial acetic acid has also been used as the solvent for iodosobenzene diacetate titrations.12 Initially the direct titration of phenol thymol 4-hydroxybenzonitrile and quinol in glacial acetic acid with dibromine was attempted. Although the potential of the zero-current electrode was reasonably steady the potentials of the polarised electrodes fluctuated because the water present affected the electrode processes.On substituting anhydrous acetic acid in the same titrations all potentials became steady but the bromination reactions became excessively slow ; direct titration proved to be impossible even in the presence of sodium acetate or pyridine. The difference from earlier observation^^^^^ is due to the presence in glacial acetic acid of water which acts as a base assisting the formation of the phenoxide ion which is easily attacked by the bromonium ion. Brominations in glacial acetic acid are accelerated by small amounts of added water.13 Brominations in anhydrous acetic acid had perforce to be studied by the tedious and time-consuming double excess back-titration method as the only alternative in anhydrous systems.Reaction with Solvent and/or Electrolyte To 20 ml of anhydrous solvent or a solution of the electrolyte therein 0.628 mmol of dibromine was added and after 24 h the residual dibromine was determined by back-titration with arsenic(II1). For the solvent alone the loss was 4.00%. In increasingly alkaline solution as the sodium acetate concentration was increased the loss increased more or less linearly approaching lo%, and levelling at 1.0 mol 1-1 sodium acetate as shown in Fig. 1. This is due to the base enhanced solvolysis High blanks suggested significant reaction with the solvent and/or electrolyte. 0 0 II II CH3COH + Br + CH,CO-Br+ + HBr which is retarded by the accumulation of hydrogen bromide. Acyl hypobromite has been is01ated.l~ In the slightly acidic 0.1 mol 1-1 lithium chloride the loss fell to 2.90y0 while in the strongly acidic 0.1 mol 1-1 perchloric acid solvolysis was suppressed to 0.96Y0.4 ’ 1 I I I I 0.2 0.4 0.6 0.8 1.0 Concentration of CH3COONa/M Fig. 1. Loss of dibromine in 24 h from a solution initially 0.1 mol 1-1 in anhydrous acetic acid as a function of the concentration of sodium acetate electrolyte. Substitution Reactions of Aromatic Amines and Phenols The bromination of a selection of amines and phenols in basic (0.1 mol 1-1 sodium acetate), slightly acidic (0.1 moll-1 lithium chloride) and strongly acidic (0.1 mol 1-1 perchloric acid) electrolyte solutions in anhydrous acetic acid was examined by double excess back-titration. It is immediately evident from the results in Table I that all the reactions are very slow and o 1230 ABDENNABI AND BISHOP DIFFERENTIAL ELECTROLYTIC Analyst VoZ.108 small analytical utility. They are fastest and most complete in the basic electrolyte appreci-ably incomplete in lithium chloride and grossly incomplete even for the amines in the strongly acidic electrolyte. The low dielectric constant of t.he solvent favours ion pairing hindering the dissociation of Br+-Br- and slowing down the bromination reaction. The acidity of the solvent discourages the formation of the active phenoxide ion and the protonation ion pair e.g., C6H50H,+ -OCOCH, a much less effective intermediate is stabilised. The sodium acetate electrolyte is sufficiently basic to allow the phenoxide mechanism to operate but at a slow rate.The reactions are mainly substitution phenol thymol and (surprisingly) 4-hydroxybenzo-nitrile are tribrominated ; 1-naphthylamine is 2,4-dibrominated with 50-60% of a tribromo compound ; 4-methoxyphenol is partially demethoxylated as well as 2,6-dibrominated. Other reactions are incomplete even in sodium acetate. Possibly after very prolonged reaction further brominations might reach completion but the mounting blank causes uncertainty. In addition to dibromination to the 2,4-derivative quinol and 4-aminophenol are oxidised to the anion radical consuming 2.5 mol of dibromine. A mixture of 1 mol each of these two com-pounds consumes a total of 7 mol of dibromine. This synergic effect as demonstrated by the superadditivity effect in photographic development by metol - quinol mixtures most probably leads to the 2,3,5,6-tetrabromo-l,4-benzoquinone and the 2,5-dibromo-4-aminophenol.Amino oxidation - reduction indicators dication radicals by a two-electron step.15 Colourless indicators of the diphenylamine - benzidine class are oxidised to deeply coloured Direct titrations of diphenylamine N-phenylan-TABLE I BROMINATION OF PHENOLS AND AMINES IN VARIOUS 0.1 moll-1 ELECTROLYTE SOLUTION'S IN ANHYDROUS ACETIC ACID Sample* 4-Hydroxybenzonitrile . . . . 4-Aminophenol . . . . . . . . Quinol +'4-ami,ophenbit . . . . Phenol . . . . . . . . . . 4-MethoxGhendl' . . . . . . 2-N i trophenol . . . . . . . . 1-Naphthylamine . . . . . . Aniline . . . . . . . . 1,3-Phenylenedi&ne . . . . Quinol . . . . Thymol .. . . . . Sodium acetate Time of Amount of Br, standing/h consumed/mmol 24 0.304 2 0.242 8 0.244 6 0.247 24 0.705 24 0.304 16 0.302 16 0.236 16 0.123 16 0.256 L I 7 Lithium chloride Perchloric acid Time of Amount of Br Time of Amount of Br, standing/h consumed/mmol standing/h consumed/mmol A L I r > 24 0.281 24 0.075 24 0.246 72 0.054 24 0.228 48 0.048 24 0.281 48 0.218 16 0.278 16 0.197 16 0.087 16 0.156 48 0.056 96 0.161 * A 0.1-mmol amount of sample was taken. t 0.1 mmol of each. thranilic acid benzidine 3,3'-dimethoxybenzidine and the bromine indicator rosaniline were attempted. The reactions were very slow gave a white precipitate and showed no colour changes. Double excess back-titration was adopted with the results shown in Table 11.In TABLE I1 DOUBLE EXCESS BACK-TITRATION OF 0.1 mmol OF VARIOUS SUBSTANCES IN ANHYDROUS ACETIC ACID Compound Diphenylamine N-Phenylanthranilic'&id : Benzidine 3,3'-Dimetho~~benzidine : Rosaniline . . . . . . r Bromination of indicators Time of s t anding/h 6 6 24 8 2 4 5 24 72 24 Amount of Br, consumed/mmol 0.406 0.306 0.308 0.204 0.085 0.097 0.101 0.148 0.183 0.184 Addition to double bonds Time of Amount of Br, Compound standing/h consumed/mmol Limonene 6 0.097 Oleic acid 48 0.102 Pentane-2,4-dione . . . . 48 0.204 . . . . . . Cyclohexene . . . . . . 48 0.103 . . . . . . Oxidation of covalent inorganics r 1 Time of Amount of Br , standing/h consumed/mmol Antimony(II1) chloride .. 48 0.086 Phosphorus(II1) chloride . . 24 0.102 Sulphur dioxide . . . . 48 0.09 October 1983 POTENTIOMETRY WITH PERIODIC POLARISATION. PART XXVII 1231 no instance was a two-electron mole per mole ratio obtained despite the formal potentials favouring direct oxidation. Instead the ratios corresponded to or approached full nuclear bromination. The method is therefore useful for the determination of these substances and it avoids the formation of unstable products.15 Analysis of the product from diphenylamine confirmed tetrabromination. The products from the above-named compounds were di(2,4-dibromophenyl)amine N-2,4-dibromophenyl-4-bromoanthranilic acid 3,3’-dibromobenzidine, 5,5’-dibromo-3,3’-dimethoxybenzidine and 2,2’-dibromo-2’’-rnethyl-4,4’,4’’-triaminotriphenyl-methane respectively.The example of 3,3’-dimethoxybenzidine is of interest as the first bromine enters fairly quickly but the second approaches completion exponentially the plot of moles of dibromine consumed veYsus the logarithm of time being a straight line. 100 a 60 20 800 4 e E a 0 uj 700 > 600 - - - _ _ - - -\ --8.4 8.6 8.8 9.0 9.2 Volume of titrantlml Fig. 2. Titration of 10 ml of 0.1 mol 1-1 limonene with 0.109 moll-’ dibromine. Conditions as follows electrolyte lithium chloride A. anode - S.C.E. Dotential 2. 6oo t 0.6 0.8 1.0 1.2 1.4 Volume of tit rantlml zero-current electrode - S.C.&. potential Titration of 10 ml of 0.002 mol 1-1 bis-C cathode - S.C.E. potential; EA differ- (pentane-2,4-dionato)oxovanadium(IV) with 0.081 ential potential.Solid line d.c. DEP; and Conditions and symbols as in broken line m.s.b. DEP. Fig. 3. moll-’ dibromine. Fig. 2. Addition to Double Bonds The direct titration of various unsaturated compounds was attempted but the reactions were too slow although limonene gave interpretable curves (Fig. 2) in a titration lasting 6 h; extrapolation of the lines on either side of the first break of the differential curves to inter-section gives the end-point. Double excess back-titration reveals complete saturation of the side-chain methylene bond (Table 11). Clearly the charge-separation process producing the bromonium ion intermediate or the bridging species,ls is strongly inhibited by the solvent an indication of the necessity for trace amounts of water in Wijs-type reagents1’ An attempt was made to solubilise an oxidisable ion by formation of a neutral complex that might be titratable in the solvent.Bis(pentane-2,4-dionato)oxovanadium(IV) (vanadyl acetylacetonate) proved to be usable although the reaction was slow taking 2 h. The curves in Fig. 3 are like those for limonene (Fig. 2) and are similarly interpreted but oxidation of vanadium(1V) would require 2 mole of vanadium(1V) per mole of dibromine whereas th 1232 ABDENNABI AND BISHOP DIFFERENTIAL ELECTROLYTIC Analyst VoZ. 108 experimental ratio was 1 mol of vanadium(1V) to 4 mol of dibromine indicating enolisation of the ligand and addition of two dibromines to the double bonds of each ligand as shown in Table 11.Oxidation Reactions The end-point of the back titrations reported above is of course the oxidation of arsenic(II1) by dibromine in anhydrous acetic acid but the arsenic(II1) solution used is aqueous as there is no point in employing a non-aqueous solution. The medium therefore contains 10% or more of water and the titration curves shown in Fig. 4 are similar to those found in bromate titra-tions of arsenic(II1) in aqueous media,l and are of excellent quality. Diiodine in anhydrous acetic acid titrates cleanly and rapidly duration 10 min in a two-electron 1 1 reaction with dibromine to give iodine monobromide. The electrode reactions are fast and type I peaks are produced as shown in Fig. 5. The two-electron dibromine also oxidises L-ascorbic acid cleanly and directly into dehydroascorbic acid the m.s.b.(Fig. 6 ) is a sharp falling-Z of type II(b) but the solvent and the inactive product inhibit any fast response by the other electrodes. Other covalent p-block compounds were examined by double excess back-titration and showed slow reaction with consumption of one dibromine per molecule as in Table 11. 700 600 4 500 s !i E a 4 0 a > 30C 20( 3.0 3.2 3.4 3.2 3.4 Volume of titranvml Fig. 4. Titration of 20 ml of 0.013 mol 1-l arsenic(II1) chloride with 0.081 mol 1-1 dibromine. Conditions and symbols as in Fig. 2. Anhydrous Acetic Acid - Methanol Solvent In an attempt to encourage formation of bromonium and phenoxide ions by increasing the dielectric constant of the solvent the use of a 1 + 1 V/V mixture of anhydrous acetic acid an October 1983 POTENTIOMETRY WITH PERIODIC POLARISATION.PART XXVII 1233 methanol was examined with lithium chloride as the supporting electrolyte. Direct titration of phenols was unsuccessful while back-titration led to unacceptably excessive blanks arising from the reaction of bromine with methanol. 600 I I I I I 0.4 0.6 0.8 1.0 1.2 1.4 Volume of titrantlml Fig. 5. Titration of 10 ml of 0.01 moll-' diiodine with 0.090 moll-' dibromine. Conditions and symbols as in Fig. 2. 700 600 > E . Lu" 500 400 30C / I *' I -- I- ' I I I I I I I I I I I I I I I I I I 0.6 0.8 1.0 1.2 1.4 Volume of titrant/mI Fig. 6. Titration of 10 ml of 0.01 mol 1-' ascorbic acid with 0.081 moll-' dibromine.Conditions and symbols as in Fig. 2. Conclusions Although successful in solubilising organic and covalent inorganic compounds without solvolysis anhydrous as opposed to glacial acetic acid is not a good solvent for bromine reactions and oxidations. The reagent has poor stability and reaction is inhibited by the ion-pairing effect of the solvent which additionally inhibits phenoxide ion formation even with the strongly basic sodium acetate electrolyte. Although the double excess back-titration method gives reproducible and reasonable results and excellent end-points it is tedious and time-consuming and the blanks are high. Aromatic substitution and addition to double bonds are slow in this solvent and few oxidations are fast enough for direct titration.Clean bromi-nation of certain oxidation indicators may be of advantage despite the long reaction times. The reaction mechanisms and kinetics are at fault in this solvent rather than the titration procedures but where the titrations are feasible DEP offers the advantages of enhanced response speed and sharpness of the end-point. In direct titrations of limonene and acetyl-acetone DEP permits location of the end-point where the classical method fails and in direct titration of ascorbic acid the m.s.b. method alone gives a good end-point by virtue of its electode activation effect 1234 ABDENNABI AND BISHOP 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. References Bishop E. Analyst 1958 83 212. Bishop E. and Webber T. J. N. Analyst 1973 98 712. Bishop E. and Webber T. J. N. Analyst 1973 98 769. Hartshorn L. G. PhD Thesis University of Exeter 1972. Abdennabi A. M. S. and Bishop E. Analyst 1982 107 1032. Abdennabi A. M. S. and Bishop E. Analyst 1983 108 71. Kucharskf T. and Safaiik L. “Titrations in Non-aqueous Solvents,” Elsevier Amsterdam 1965. Bishop E. and Jennings V. J. Talanta 1962 8 679. TomiEek O. and Heyrovsky J. Chem. Listy 1950 49 169. Ingberman A. K. Anal. Chem. 1958 30 1003. Huber I. O. and Gilbert J. M. Anal. Chem. 1962 34 247. Sivasankarapillia V. N. and Nair C. G. R. Talanta 1975 22 57. Keefer R. M. and Andrews L. J. J. J . Am. Chem. SOC. 1956 78 3637. Hundiecker C. PhD Thesis Koln (Wintgen) ; privat labor. Koln (Braunsfeld). Bishop E. and Hartshorn L. G. Analyst 1971 96 26. Gold E. S. “Mechanism and Structure in Organic Chemistry,” Holt and Reinhart Winston New Wijs J. J. A. J . SOC. Chem. Ind. 1898 17 698. York 1959. NOTE-References 1 2 3 5 and 6 are to Parts 11 XXII XXIII XXV and XXVI of this series respec-Received March 7th 1983 Accepted May 9th. 1983 tively