NOVEMBER 15, 1937
ANALYTICAL EDITION
cal procedure, have the final acidity of their colored solutions adjusted to be only 1 N , and in a similar colorimetric method Zinzadze (6) uses a final acidity of 0.35 N . The Zinzadze method employs molybdenum trioxide in place of the ammonium heptamolybdate used here.
TABLE11. EXPERIMENTAL DATA Sample Blank Blank Blank Blank Flank
Scale Reading
amounts of ferric iron in the sample, taken from Zinzadze’s work, is as follows: Immediately following the addition of the 10 hrsulfuric acid to the sample and before the addition of the molybdate reagent, 5 cc. of 16 per cent sodium metabisulfite solution are added and the flask is heated a t about 100” C. for 1 hour. This procedure will take care of at least 0.2 mg. of iron.
Acknowledgment
Acid Normality
76.6 26.0 9.3 1.2 0.2
0.5 0.75 1.0 1.25
1.6
529
The author wishes to acknowledge the cooperation of B.
W. Story and C. W. Brown of this laboratory in the development of this method.
Literature Cited Interfering Substances The interference of other substances in the development of the color in the methods based on the reduction of phosphomolybdate have been discussed by Zinzadze (6). The means employed here for overcoming the influence of small
(1) Eddy and DeEds, IND.ENQ.CHEM., Anal. Ed., 9, 12 (1937). (2) Farber and Youngburg, Ibid., 4, 107 (1932). (3) Howk and DeTurk, Ibid., 4, 111 (1932). (4) Roepke, Ibid., 7, 78 (1935). (5) Story and Kalichevsky, I b i d . , 5, 214 (1933). (6) Zinzadze, I b i d . , 7, 227(1935).
RECEIVED Auguat 16, 1937.
Determination of Chlorides and Bromides L. A. REBER AND WALLACE M. MCNABB Department of Chemistry and Chemical Engineering, University of Pennsylvania, Philadelphia, Pa.
IT
HAS been shown (2) that silver nitrate may be titrated with potassium iodide, using ceric ammonium sulfate and starch as indicators. As this method has never been applied to the determination of chlorides and bromides, it was considered desirable to learn its degree of accuracy as compared with the thiocyanate and the gravimetric methods. Pure potassium chloride and bromide were used for the analyses. The procedures employed were essentially those given by Kolthoff and Sandell (1) as the Volhard method for chlorides. The directions were made more specific wherever it was considered advisable.
Analytical Procedure PROCEDURE 1. Add 5 ml. of 6 N nitric acid t o 25 ml. of 0.03658 N potassium chloride solution, and then 25 ml. of 0.1 N silver nitrate. Stir to coagulate the precipitate, filter, and wash the precipitate of silver chloride with 60 ml. of 1 N nitric acid. To the combined filtrate and washings add 3 ml. of 0.5 per cent starch solution and 0.1 ml. of 0.1 N ceric ammonium sulfate solution. Titrate the silver with 0.1 N potassium iodide solution to the appearance of a permanent blue-green color. Subtract 0.1 ml. from the buret reading as an indicator blank. TABLEI. EXPERIMENTAL RESULTS C1 in KC1,
Br in KBr,
47.67 47.67 47,67 47.64 47.40 47.51 47.40 47.44 47.68 47.68 47.68 47.68 47.62 47.52 47.42 47.62 47.56 47.50 47.52 47.53
67.46 67.43 67.43 67.44 67.61 67.47 67.54 67.64 67.48 67 48 67.48 67.48 67,40 67.46 67.46 67.44 67 42 67.46 67 44 67.44
%
Method Procedure 1 Av. Procedure 2 Av. Volhard 1 Av. Volhard 2
Av. Gravimetric Av.
%
Av.
Av.
Av.
Av. ’
Av.
The same procedure was followed in analyzing a 0.08N potassium bromide solution. PROCEDURE 2. Add 15 ml. of 6 N nitric acid to 25 ml. of 0.03658 N potassium chloride solution, and then 25 ml. of 0.1 N silver nitrate. Dilute to the mark in a 100-ml. volumetric flask, mix thoroughly, filter through a dry filter, and reject the first portion of filtrate. Titrate 50 ml. of the subsequent filtrate with 0.1 N potassium iodide solution as previously described. The same procedure was followed in analyzing a 0.08 N potassium bromide solution. Volhard methods 1 and 2 are similar to procedures 1 and 2, respectively, except that the silver is titrated with a standard solution of potassium thiocyanate in the presence of ferric alum as indicator. A 0.03658 N potassium chloride solution and a 0.08 N potassium bromide solution were analyzed by these methods. Finally, the solutions of potassium chloride and of potassium bromide were analyzed by a gravimetric method, precipitating and weighing silver chloride and silver bromide, respectively.
Summary A method for determining chlorides and bromides is described, which compares favorably with the standard Volhard method and the gravimetric method. Iodides may be determined similarly, it being unnecessary to remove the precipitated silver iodide prior to titration. In the reverse titration, running silver nitrate into the iodide solution, the end point appeared too soon, introducing an error of about 1 per cent. This has been ascribed to adsorption of iodide ions by the silver iodide. The advantages of the new method lie in the ease of preparation of standard solutions and in their stability.
Literature Cited (1) Kolthoff and Sandell, “Textbook of Quantitative Inorganlc Analysis,” p. 544, New York, Macmillan Go., 1936. (2) McNabb and Bloom, ISD. ENG.CHEM., Anal. Ed., 8, 167 (1936).
RECEIVED July 8, 1937.
