Determination of Methacrylic Acid by Coulometric Titration D. H. Grant* and V. A. McPhee Department of Chemlstry, Mount Allison Unlversity, Sackvllle, N.B., Canada EOA 3CO
For a projected study of the pyrolysis of poly(methacry1ic acid), we required a method of analyzing sub-milliliter samples of aqueous poly(methacrylic acid) solutions to determine their monomer concentration, expected to be about F. Although gas chromatography can be used to determine monomer during poly(methy1 methacrylate) pyrolysis in solution ( I ) ,we rejected this method on the following grounds: polymer build-up at the injection port is always a nuisance, preliminary esterification would probably be necessary, and solvent-solute peak separation would be much more difficult in the present situation. On a macro-scale, “bromine titration” is a routine method for determining residual monomer in polymers and is applicable to methacrylates ( 2 ) . More often than not the term is misleading, as the actual titration is an iodinehhiosulfate determination of the excess bromine remaining after it has been allowed to react with the unsaturated species. Coulometric titration using electrogenerated bromine seemed to offer a suitable micro-scale adaptation, particularly as there is an appropriate bi-amperometric method of endpoint detection (3). It seemed likely that up to a 1000-fold dilution of sample between reaction vessel and titration cell could be tolerated. Such monomers as styrene, a-methyl styrene, and vinyl acetate ( 4 ) and methyl vinyl ketone ( 5 ) have been determined satisfactorily by coulometric titration by bromine.
EXPERIMENTAL The constant current power supply used was constructed substantially to Stock’s design (6),I t provides currents of either 9.65 or 0.965 mA. The bi-amperometric polarizing voltage was 150 mV. Methacrylic acid (Matheson, Coleman and Bell) was re-distilled before use, Standard dilute solutions in electrolyte/solvent were prepared on a weight basis. BDH Reagent Grade glacial acetic acid was used. Potassium hydrogen phthalate and all inorganic reagents were of Fisher Certified Reagent quality. The conventional technique of coulometric titration was followed. The electrolyte/solvent was pretitrated until a specific galvanometer reading was reached, significantly above fluctuations about zero. Then the sample was added and the titration continued till the same galvanometer reading was reached. Further samples can be added to the same batch of electrolyte and the titrations repeated.
RESULTS AND DISCUSSION Crucial t o the development of a satisfactory coulometric titration procedure is the selection of an electrolyte/solvent system. As a starting point, we used the typical system of 0.2 F KBr in 50% aqueous acetic acid (7), but found it unsatisfactory because the rate of bromine addition is too slow. No significant improvement was observed in the presence of A1C13, AlBr3, HgCl1, FeC13, NiC12, LiC1, or LiBr, all of which are reported to catalyze bromine additions. More promising, but still unsatisfactory, results were obtained in 1 F HCl in 80% acetic acid where the electrogenerated species is presumably chlorine (8).
Relatively slow bromine addition is less of a problem in those macro-scale methods where the actual titration involves iodine/thiosulfate. While comparable “back titration” pro-
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cedures can be developed for coulometry, they can result in a loss of some of its advantages. It seemed preferable to try to increase the rate of bromination. According to Ingold (9),halogen addition to the double bond in methacrylic acid is slowed by the electron-withdrawing character of the carboxylic -COOH group. Since the carboxylate -COO- group is electron-releasing, addition to the methacrylate anion is much more rapid. Only Critchfield (IO)seems to have modified macro-scale analytical procedures to take advantage of this. Even his method still has a final iodinehhiosulfate step. The ~ K ofA methacrylic acid is 4.66 (11). Methacrylate anions will therefore be the predominant species at a pH over 4.7. However, solutions that are much more alkaline must also be avoided. Bromine reacts with hydroxide to form hypobromide. One result that has been reported is that the biamperometric detection method becomes unsatisfactory (12). A pH in the range 5 to 6 therefore seemed the best compromise. The electrolyte that we found to be satisfactory was 0.05 F in potassium hydrogen phthalate, 0.035 F in sodium hydroxide and 0.2 F in potassium bromide. It has a pH of 5.5. No added catalysts are necessary; the bromine addition is sufficiently fast that either generator current can be used. Blanks are low. Eleven samples of methacrylic acid between about 0.2 and 3.0 mg in 150 ml of electrolyte were titrated with an average error of +0.029 f 0.013 mg at a current of 9.65 mA. This calculation assumes 100%efficiency of bromine generation and addition. Similarly, at 0.965 mA, with 20 samples between about 0.02 and 0.70 mg, the average error was -0,009 f 0.015 mgAdditions of pure poly(methacry1ic acid) do not interfere with the analyses. It cannot be assumed that the method will be easily applicable to all a,@unsaturated acids. We found that acrylic acid does not brominate fast enough, and that with cinnamic acid, although the titration is possible, the precision is substantially less.
LITERATURE CITED (1) S. Bywater and D. H. Grant, Trans. Faraday SOC., 59, 2105 (1963). (2) L. S. Luskln, “High Polymers”, Vol. XXIV, Part 1, Wlley-lntersclence,New York, 1970, p 161. (3) D. H. Evans, J. Chem. Educ., 45, 88 (1968). (4) D. B. Gurvlch, V. A. Balandlna, and L. M. Palklna, Zavod. Lab., 30(3), 276-81 (1964); Chem. Absh., 80, 13890e (1964). (5) A. P. Zozulya and E. V. Novlkova, Zavod. Lab., 29(5) 543-5 (1963); Chem. Absb., 59, 4548e (1963). (6) J. T. Stock, J. Chem. Educ.,48, 856 (1969). (7) D. 0. Marsh, 6.L. Jacobs, and H. Veenlng, J. Chem. Educ., 50, 626 (1973). (8) F . Cuta and 2 . Kucera, Chem. Llsty, 47, 1166-72 (1953); Chem. Abstr., 48, 3850b (1954). (9) C. K. Ingold, “Structure and Mechanism In Organlc Chemlstry”, Bell, London, 1953, p 665. (10) F. E. Crltchfleld, Anal. Chem., 31, 1406 (1959). (11) E. Larsson, 2.Phys, Chem. A, 159, 316 )1932). (12) G. M. Arcand and E. H. Smith, Anal. Chem., 28, 440 (1956).
RECEIVEDfor review April 15, 1976. Accepted June 11, 1976.
ANALYTICAL CHEMISTRY, VOL. 48, NO. 12, OCTOBER 1978
