Indian Journal of Chemistry Vol. 23A, December 1984, pp. 1057-1058 Iodometric Determination of Milligram Amounts of Some Carboxylic Acids by Potentiometric Titration Method (Miss) A WADHWA & R M VERMA· Department of Post-graduate Studies and Research in Chemistry, University of Jabalpur, Jabalpur 482001 Received 20 March 1984; revised and accepted 18 July 1984 An iodometric method is described for the microdetermination of some carboxylic acids by potentiometric titration method using platinum-calomel electrode assembly. The proposed method can be applied to the quantitative analysis of sample solutions containing 0.01-0.1 meq. of the acids. The titration peaks are quite sharp even with 0.001 N of the titrand and the titrant solutions. An indirect method has also been worked out for determining certain acids to which the direct method is not applicable. Some of the difficulties associated with alkali metric titration of a dilute solution of a w.eak acid 1.2 are removed when weak acids are potentiometrically titrated with alkali using glass-calomel electrode combination. But, due to the formation of a buffer 3 , the potential jump near the equivalence point is small which further decreases with increasing dilution of the titrant, rendering the location of the end-point difficult. An alternative method, based on iodometric titration, was developed by Bruhns and Kolthoff4 for the determination of weak acids. However, this method also was only partially successful and was not applicable for analysis on micro scale. A visual micro- titrimetric procedure using solid potassium iodate and iodide has been reported 5 -7. The procedure is not applicable for samples containing less than 0.025 meq of certain weak organic acids. We report here a potentiometric method for the iodometric microdetermination of some weakly dissociated aliphatic acids. The method gives accurate results for 0.01-0.1 meq of the tested acids. With higher fatty acids, which were dissolved in ethanol or DMF, the liberation of iodine on treatment with iodide- iodate reagent was too sluggish to be of any practical application. In such cases an indirect method was used. The following solutions were prepared using A R grade chemicals and conductivity water. Potassium iodate solution (0.1 N) was prepared by dissolving calculated amount of the dried salt. A 5% (m/v) solution was also prepared. Sodium thiosulphate (0.1 N), oxalic acid (0.1 N) and sodium hydroxide (0.1 N) solutions were prepared; these were diluted to obtain 0.01, 0.0025, 0.0012 anti 0.001 N solutions. Acid sample solutions (0.1 N) were prepared and standardized with sodium hydroxide and then suitably diluted to prepare the test solutions. Owing to the poor solubilities of heptanoic, octanoic, nonanoic and decanoic acids in water, their solutions were prepared in 1:9, 1:2, I: 1 and 1:1 ethanol-water mixtures, respectively. The solution of dodecanoic acid was prepared in ethanol and that of palmitic acid in DMF. Procedure (I) A known aliquot (5-10 ml) of the test solution containing 0.01-0.1 meq of the acid was treated with 1g of potassium iodide and 5 ml of 5% potassium iodate solution and contents were stirred by means of a magnetic stirrer. Platinum and calomel electrodes were dipped into the solution and thiosulphate solution (0.001-0.01 N) was gradually added through a microburette. After each addition, the solution was stirred and the potentiometric reading was recorded. In the case of nonanoic and decanoic acids, the rate of reaction was found to be slow and a period of 2 min had to be allowed after each addition for the completion of reaction. Later, the end-point was located graphically. The thiosulphate solution was standardized potentiometrically with a standard solution of oxalic acid. I ml of 0.001 Nthiosulphate =0.045 mg of carboxylic function. The potential of- the indicator electrode (platinum) depends on iodide-iodine ratio, and because this changes during the course of the titration, the platinum electrode shows a potential change. Procedure (II) A sample solution (5-10 ml) containing 0.025-0.1 meq of the acid was taken in a 100-ml Erlenmeyer flask and treated with 20 ml (0.005-0.01 N) potassium iodate solution and I g of potassium iodide. The flask was immersed into a boiling water-bath for 15-60 min (15 min for dodecanoic acid and 60 min for palmitic acid). The flask was then cooled to room temperature and one ml of starch solution was added. Thiosulphate solution was gradually added until the blue colour disappeared. An excess of 2 N sulphuric acid was then added and the liberated iodine titrated against 0.005- 0.01 N thiosulphate. The difference from the blank gave the amount of iodate used up by the acid. 1 ml of 0.01 N thiosulphate =0.45 mg of carboxyl function. The proposed method has been applied for the determination of some carboxylic acids at four 1057