DECEMBER 15, 1937
ANALYTICAL EDITION
Literature Cited (1) Benedetti-Pichler, A. A., and Rachele, J. R., Mikrochemie, 19, 1 (1935). (2) Benedetti-Pichler, A. A,, and Spikes, W. F., “Introduction to the Microtechnique of Inorganic Qualitative Analysis,” Douglaston, N. Y., Microchemical Service, 1935. (3) Benedetti-Pichler, A. A,, and Spikes, W. F., Mikrochemie, 19, 239 (1936) : Mikrochemie, Molisch-Festschrift, 3 (1936). (4) Emich, F., and Schneider, F., “Microchemical Laboratory Manual,” pp. 12,31, New York, John Wiley & Sons, 1932. (5) Hillebrand, W. F., and Lundell, G. E. F., “Applied Inorganic Analysis,” p. 211, New York, John Wiley & Sons, 1929.
591
(6) Mellor, J. W., “‘Comprehensive Treatise on Inorganic and Theoretical Chemistry,” Vol. X, p. 752, New York, Longmans, Green & Co., 1930. (7) Noyes, A. A., and Bray, W. C., “A System of Qualitative Analysis for the Rare Elements,” New York, Macmillan Co., 1927. RECEIVEDAugust 5, 1937. Presented in preliminary form before i,he Microchemical Section at the 93rd Meeting of the American Chemical Society, Pittsburgh, Pa., September 7 t o 11, 1936. Abstracted from a thesis t o be submitted by J. R . Raohele to the faculty of the Graduate School of New York University in partial fulfillment of the requirements for the degree of doctor of philosophy.
An Improved Method for the Determination of Iodates VICTOR J. ANHORN
AND
H. HUNT, Purdue University, West Lafayette, Ind.
S
TARCH used as a n indicator in the determination of iodine does not give a reproducible end point which warrants the determination of a fraction of a milligram of potassium iodate. The physical condition of starch solutions or their absorptive power varies and this causes the sensitivity of the end point to fluctuate. Extraction methods do not place all the iodine in a nonaqueous solution free from optically interfering aqueous layers; besides, there are extra steps involved in an extraction process. Therefore, t h e authors found it expedient to develop a more convenient and accurate procedure for the determination of iodine than they could find in the literature. This paper describes inexpensive apparatus and an accurate method for determining as little as 0.3 mg. of potassium iodate. The method involves titration, with weight burets, using the color of free iodine as the indicator.
Apparatus and Procedure A weight buret of 1-ml. capacity and weighing about 6 grams was constructed as shown in Figure 1. It was possible to regulate the flow to 0.1 mg. of solution delivered. Delivery was made below the surface of the liquid being titrated. The titrating vessel, shown in Figure 1, had a capacity of about 15 ml. The observation tube was 40 cm. in length, its sides were painted black, and the bottom was illuminated with a 150-watt blue Mazda bulb. As little as 0.0000003 gram of iodate gave a pronounced color. The solution was stirred with compressed nitrogen, using the gas lift principle. (About 15 to 30 minutes were used for a determination.) A blank was run with each determination. The blank contained the same concentration of acid and iodide, and was stirred rl
6
at the same rate as the sample. The titration was stopped when the colors were identical in the two tubes. The analytical balance had a sensitivity of three divisions per 0.1 mg The weights were calibrated with a Bureau of Standards set
Preparation and Standardization of Solutions Analytical reagent sodium thiosulfate, potassium iodate (purity checked with electrolytic copper), and potassium iodide (with no free iodine) were recrystallized and carefully dried. c. P. acids and freshly distilled water (4) were used. All solutions were prepared by weighing the calculated amount of reagents and water. The normality of the solutions was approximately 0.015. A 2 per cent solution of potassium iodide (6) was found most satisfactory; 0.01 N hydrochloric or sulfuric acid (2) gave optimum hydrogen-ion concentration for rea,ctions IOa516HC + 312 3Hz0
+
2SOa‘ 4-
+
18 +
SaOe-
+ 21-
+
Results Typical data obtained by this method are given in Table I. TABLEI. DETERMINATION OF IODATE, USING 2 PIR CENT IODIDE SOLUTIONS AND 0.01 N ACID Sample No.
KIOa Found Gram 0.0002866 0.0003654 0,0003144 0.0003862 0.0003474 0.0003464 0.0003393 0.0001954 0.0003100
KIOs Present
Psrcentags Eiror
Cram 0.0002853 0.0003650 0.0003143 0,0003854 0.0003488 0.0003460 0 0003393 0.0001947 0.0003128
$0.5 $0.1 031 $0 2 -0 5
+o
A,!+
$0.4 -0 9
This method has proved more accurate and more convenient than the starch indicator (3) method or the iodinecyanide (1) procedure.
Literature Cited (1) Kolthoff, I. M., Mikrochemie, 3, 75 (1925). (2) Kolthoff and Furman, Volumetric Analysis, Vol. I, p 233; Vol, 11,p. 354, New York, John Wiley & Sons, 1929. (3) Liebhafsky, H. A., J. A m . Chem. Soc., 53, 165 ( 931). (4) Rice, F. O., and Kilpatrick, M., Jr., Ibid., 44, d1 (1922). (5) Washburn, E. W., Ibid., 30, 40 (1908). RECEIVED July 22, 1937.
FIGURE 1. WEIGHTBURET (LEFT) AND T~TRATION CELL (RIGHT) The observation tube at right of titration cell had an approximately optically plane bottom and was open a t the top.
CORRECTION.In the article on “Quantitative Determination of Arsenic in Small Amounts in Biological Materials” [IN’D. ENG. CHEM.,Anal. Ed., 9, 448 (1937)] the figure “6100” in line 4, paragraph 2, of “Development of Color” should have been “610.” HERBERT 0. CALVERP
