VOL. 8, NO. 6
INDUSTRIAL AND ENGINEERING CHEMISTRY
466
Determination of Iodide in the Presence of Bromide
TABLE11. DETERMINATION_ OF IODIDE IN THE PRESENCE OF
PROCEDURE. From 0.35 t o 0.5 gram of the substance to be tested is made up to 250 ml. with water; 25 ml. are pipetted out and diluted to 100 ml.; 10 ml. of 1 per cent bromine dissolved in potassium bromide solution are added; then 2 ml. of 5 per cent sodium formate solution are run in. The mixture is shaken well and allowed to stand for 10 minutes. Finally the iodine is titrated with thiosulfate solution in the usual manner.
The procedure was tested on measured portions of the potassium iodide solution. The iodide content was found from the amount of dried Merck pro analysi grade potassium iodide, which had been examined gravimetrically. The excess of bromine added ranged from 91 to 232 mg. and the final excess of sodium formate was from 22.4 to 144.5 mg. The results are given in Table 11.
BROMIDE 0.01 N
Iodine Present
Sodium Thiosulfate MI. 10.95
Mn .
13.91
5.45
6.95 27.82
21.85
Iodine Found
Me. 13.81 6.91 27.79
Literature Cited (1) (2)
Romijn, 2. anal. Chem., 36, 18 (1897). Spitzer, L., Chem,-Ztg.,5,, 224 (1933),
RECEIVED March 18, 1936.
Determination of Arsenic in Silver Arsenate AUSTIN POMERANTZ AND WALLACE M. McNABB The John Harrison Laboratory of Chemistry, University of Pennsylvania, Philadelphia, Pa.
A
METHOD of determining silver by titration with standard potassium iodide in the presence of starch and ceric ions has recently been described ( I ) . This procedure can be extended t o the determination of arsenic by means of the above titration after precipitation as silver arsenate and its subsequent solution in nitric acid. This method gives excellent results but cannot be applied in the presence of interfering elements such as phosphorus, vanadium, molybdenum, tungsten, and sexivalent chromium. The precipitation of silver arsenate and the determination of its silver content by the Volhard method is a procedure frequently employed in the analysis of arsenic compounds, particularly arsenic ores (3). The use of the potassium iodide method instead of the Volhard titration eliminates the necessity of frequent standardizations. Standard potassium iodide can be easily prepared by a direct weighing, while the thiocyanate solution must be standardized against pure silver or silver nitrate. Various methods have been described for the precipitation of silver arsenate from solutions of alkali arsenates. pH control is the important factor; t h e precipitation will not be complete if the p H is too low, and if too high there will be a co-precipitation of silver oxide. The procedure recommended is that given by Hillebrand and Lundell (2). Because of t h e solubility of the precipitate in cold water, the procedure was modified b y washing with a saturated solution of silver arsenate. When this precaution was not taken the results were consistently low.
Analytical Procedure From 0.04- to 0.23-gram samples of pure dry potassium dihydrogen arsenate were transferred into 4OOi-cc. beakers. The salt was dissolved in 100 cc. of water and the solution acidulated with nitric acid. To this was added enough 0.1 N silver nitrate to give an excess of approximately 10 cc., and then just enough of a 10 per cent solution of sodium hydroxide to produce a turbidity. Dilute nitric acid was added drop by drop until the solution became clear, and then the silver arsenate was precipitated by the dropwise addition of 10 cc. of a saturated solution of sodium acetate. The solution was heated to boiling to coagulate the precipitate.
After cooIing, the solution was filtered and the precipitate washed by decantation with a saturated solution of silver arsenate until a portion of the filtrate gave only a faint opalescence with hydrochloric acid. This opalescence was to be expected because of the silver ions already present in the wash solution. The precipitate was then dissolved from the filter with approximately 30 cc. of warm 2 N nitric acid followed by several washings with hot water. This solution was caught in the beaker in which the precipitation was made. Sufficient 6 N sulfuric acid was added to make the solution about 1 to 2 N with respect to this acid. To this were added 3 cc. of a 0.5 per cent starch solution and 3 drops of an approximately 0.1 N ceric ammonium sulfate solution. The volume before the titration was made ranged from 120 to 150 cc. The solution was titrated with 0.1 N potassium iodide solution to a permanent blue-green end point. A blank titration was made under the same conditions, omitting the silver nitrate. TABLEI. DETERMINATION OF ARSENIC KHs.4~04
AszOs Calculated
AmOs Found
Gram
Qram
Uram
Uram
0.0448 0.0899 0.1352
0.0286 0.0574
0.0285 0.0573 0.0864
-0.0001
0.1803 0.2251
0.0863 0.1151 0.1437
0.1154 0.1436
Difference
-0.0001
$0.0001 + O s 0003 -0.0001
Summary A method is described for the determination of arsenic in silver arsenate b y titration of the silver with potassium iodide, using ceric ammonium sulfate and starch as internal indicators. When the silver arsenate is washed free of silver nitrate with cold water, a n appreciable error is introduced due t o the solubility of the precipitate. This may be corrected by using a saturated solution of silver arsenate for the washing,
Literature Cited (1) Bloom and McNabb, IND.ENG.CHEM.,Anal. Ed., 8, 167 (1936). (2) Hillebrand and Lundell, “Applied Inorganic Analysis,” p. 216, New York, John Wiley & Sons,1929. (3) Treadwell and Hall, “Analytical Chemistry,” 7th ed., Vol. 11, p. 622,New York, John Wiley & Sons, 1930. RECEIVED July 7, 1936.
