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mercuric iodide. Percentage yield: mercuric iodide, 95.7; iodine, 92.2. PREP.4RATION O F NESSLER-FOLIN REAGEXT.The NesslerFolin reagent is prepared according to the method of Koch (5) except that 40.3 grams of the recovered mercuric iodide (instead of 30 grams of mercury and 22.5 grams of iodine) are added to 30 grams of potassium iodide in 30 cc. of water. The resulting solution is filtered, adjusted with a solution of iodine in potassium iodide, and diluted to 200 cc. This stock solution is then added to 975 cc. of 2.5 molar sodium hydroxide to give the
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Nessler-Folin reagent. No difference has been observed in the behavior of the solutions prepared by the two methods. The recovered iodine has been used successfully in the preparation of Kessler-Folin reagent according to the method of Koch ( 3 ) .
Literature Cited (1) Clifford, W., J . Soe. Chem. Ind., 37, 179T (1918).
(2) Koch, F. C., “Practical Methods in Biocheniistry”. 2nd ed., pp. 119-21, Baltimore, Wm. Wood & Co., 1937. (3)Ibid., pp. 261-2. (4) Pullman, D.,Analyst, 44, 1 2 P 5 (1919).
Determination of Iodate Ion in the Presence of Cupric Ion P. L. KAPUR AND M. R. VERMA University Chemical Laboratories, Lahore, India
F
OR a number of routine determinations a method for estimating iodate ion in the presence of copper salts was required. Of the methods suggested for the estimation of iodates of alkali metals or iodic acid alone, the most important are: reducing iodates with oxalic acid and backtitrating the excess of oxalic acid ( I ) , reducing iodates with titanous chloride (S),and allowing the iodate and potassium iodide to react in the presence of a mineral acid and titrating the liberated iodine against standard sodium thiosulfate solution ( 5 ) . The last reaction is sufficiently rapid and accurate for all purposes. The velocity of the reaction 6H+
+ IOP + 51- +312 + 3Hz0
in acetic acid solution has been shown to be proportional to the square of the concentration of hydrogen and iodide ions and directly pFoportiona1 to the concentration of iodate ion (2). This rcaction has also been utilized for estimation of iodate ion in the presence of bromate and chlorate ions. If, however, estimation of iodate ion be attempted in the presence of copper ions, a complication is likely to arise on account of the liberation of iodine through the simultaneous reaction of cupric ions with potassium iodide according to the equation 2cu++
+ 21- *2 c u + I2
Kolthoff and Cremer (4, 7) showed that trivalent arsenic can be estimated volumetrically in the presence of copper ions by adding sodium pyrophosphate to a neutral solution of the mixture, when copper ions form a blue complex and no longer react with potassium iodide. It was found, however, that the complex of copper does not decompose in acidic solution up to pH 5.0, but decomposes a t greater acidity to react with potassium iodide, The authors made use of this fact for the estimation of iodate ion in the presence of copper salts. Mixtures containing known amounts of potassium iodate and copper sulfate were prepared, An excess of sodium pyrophosphate (free from reducing agent), acetic acid, and potassium iodide solution were added, in this order, to each of the solutions to be titrated. The iodine was liberated slowly and was titrated against standard sodium thiosulfate solution, using starch as indicator. After iodine had ceased to separate, the solution was set aside and kept for 24 hours in the dark. Under these conditions no more iodine was evolved, showing that the blue copper complex did not decompose to react with iodide.
In Table I are given typical results obtained in the course of this investigation. In the fifth column are given the exact quantities of various solutions that were found to give the most consistent results. The results given in the last column show that the amount of iodate added to the solution in terms of its equivalent iodine can be estimated with accuracy even in the presence of very large amounts of copper salts. The only difficulty encountered in the present case was the extreme slowness with which the reaction proceeded, each titration taking several minutes, sometimes even half an hour, for completion. A number of catalysts, including ammonium molybdate (6), which has been mentioned by Kolthoff as suitable for catalyzing the liberation of bromine from a mixture of bromide and bromate, were tried without success. The kinetics of the reaction are being studied and will be published shortly. TABLE I. TYPICAL RESULTS A. Sodium pyrophosphate solution 10%. B. Acetic acid solution, 10% C. Potassium iodide solution, 10%. Volume Volume of 0.1 N NazSzOa Volume Concenof of tration 0.2N TheoNo. KiOa N CuSO, Conditions retical Found cc. cc , cc. cc Cc . 1 25 0.1 5 B 25.0 25.0 10 c 2 25 0.1 .. 75 A 12 B 25.0 25.0 10 c 3 25 0.1 10 10 c 25.0 25.0 4 25 0.1 25 10 c 25.0 25.0 5 50 0.1 50 150 A 25 B 50.0 50.05 20 c Volume of 0.01 N NazSzOs 5.0 5.0 0.01 Sameasin4 6 5 10.0 sameasin4 10.0 0.01 7 10 25.05 25 Sameasin4 25.0 0.01 8 25
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E
Literature Cited DQbourdeaux,L., Compt. rend., 138, 147 (1904). Dushman, S.,J . Phys. Chem., 8,453 (1904). Kikuchi, S.,J . Chem. Sac. Japan, 43,173 (1922). Kolthoff, I. M.,and Cremer, C. J., Pharm. Weekblad, 58, 1620-4 (1921). Kolthoff, I. M., and Furman, N. H., ”Volumetric Analysis”, Vol. 11, p. 385,New York, John U’iley & Sons, 1929. Ibid., p. 387. Ibid., p. 431.
