NOVEMBER 15, 1936
ANALYTICAL EDITION
The pronounced effect of variations in acidity is shown for solutions such as cupric chloride and bromide and cobalt chloride. Presumably this depends upon the extent to which complex ions are formed. While certain other complexes were included, such as tetrammino cupric sulfate and chloropentammino cobaltic chloride, no attempt was made to exhaust the possibilities among the large number of complex compounds of cobalt or chromium. While a solution containing trivalent chromium is a desirable green, the system becomes somewhat dichromatic and the hue changes toward the violet on standing, with the ultimate attainment of equilibrium. At the higher acidities, the change is retarded. As any data obtained for ferric chloride merely corroborated bhose presented in a n earlier report, they are not included here.
465
Summary Spectral transmission curves are presented for the visual region for aqueous solutions of ceric sulfate and of various salts of elements from atomic number 23 to 29. These include a considerable range of concentrations and acidities,
Literature Cited (1) ilrny, Am. J. Pharm., 104, 2 7 2 (1932). (2) Hardy, J. Optical Soc. Am., 25, 305 (1935). ENG.CHEM.,Anal. Ed., 7, 187 (1935). (3) Kasline and Mellon, IND. (4) Mellon, Proc. Indiana Acad. Sci., 32, 164 (1922). (5) Snell and Snell, “Colorimetric Methods of Analysis,” New York, D. Van Nostrand Co., 1936. (6) Yoe, “Photometric Analysis,” Vol. 1, New York, John Wiley & Sons, 1928. RECEIVED July 23, 1936. Abstracted from 8 portion of a dissertation submitted by C. T. Kasline to the graduate school of Purdue University in partial fulfillment of the requirements for the degree of doctor of philosophy.
Determination of Iodine and Bromine in the Presence of Each Other LADISLAUS SPITZER, Peterdi ucca 11, Budapest, Hungary
I
T I S WELL known that bromine is a more powerful oxidizing agent than iodine. Formic acid, for example, is not oxidized by iodine, as is apparent from the nature of Romijn’s method for determining formaldehyde (1); bromine, on the other hand, oxidizes not only the acid but also the formates to form carbon dioxide ( 2 ) as is indicated in the equations HCOOH Brz = COz 2 HBr (1) HCOONa Brz = COz HBr NaBr (2)
+
+
+
+
+
On the basis of these reactions and the well-known ability of bromine to displace iodine from a n iodide, it appeared possible to establish methods (1) for the determination of iodine and bromine in the presence of each other, and (2) for the determination of iodide in the presence of bromide. 1. Sodium formate is added to one aliquot portion of the solution of bromine and iodine; after the bromine has been reduced to bromide, the iodine is titrated with standard thiosulfate eolution. Potassium iodide is added to a second aliquot portion of the solution and the total iodine is titrated with thiosulfate solution. The quantity of bromine may be calculated from the difference between the two titrations. 2. The iodide in a mixture of bromides and iodides is liberated by adding bromine; the excess of bromine is reduced by adding sodium formate solution and the iodine is then titrated with thiosulfate.
Experimental IODIKE SOLUTION. Iodine (1.542 grams) was dissolved in 25 per cent potassium bromide solution and diluted to 1 liter with the potassium bromide solution. Ten milliliters of the solution were equivalent to 12.1 ml. of 0.01 1V sodium thiosulfate. BROMIKE SOLUTION.Bromine (1.052 grams) was made up to 1 liter with 25 per cent potassium bromide solution. Ten milliliters of the solution were equivalent to 13.1 ml. of 0.01 N sodium thiosulfate. POTASSIUM IODIDE.Potassium iodide (1.82 grams) was made up to 1 liter with potassium bromide solution (25 per cent). SODIVMFORMATE. A 5 per cent solution was prepared from formic acid that had been purified by distillation and from sodium hydroxide of reagent grade. The sodium formate was purified by recrystallization.
Procedure for Bromine in the Presence of Iodine From 0.25 to 0.35 gram of the substance to be tested is weighed and made up to 250 ml. in a 25 per cent potassium bromide solution. Two 25-ml. portions of this solution are placed in flasks and each portion is diluted to 100 ml. To one of the aliquots is added 1 to 1.5 ml. of the 5 per cent sodium formate solution. The flask is shaken vigorously and then allowed to stand for 10 minutes. The solution is then titrated with sodium thiosulfate solution, using starch indicator. The second aliquot is treated with 5 ml. of 10 per cent potassium iodide solution and then the amount of thiosulfate that is equivalent to the sum of the bromine and the iodine, is determined by titration.
The procedure was tested by preparing mixtures containing from 1 to 20 ml. of each of the halogen solutions. The amount of the sodium formate solution ranged from 0.5 to 2 ml. and the excess of sodium formate from 24 to 82 mg. Each mixture of the bromine and iodine solutions was diluted to 100 ml. A period of 10 minutes was allowed for the interaction of the bromine and the sodium formate in each case. From 2 to 4 ml. of 10 per cent potassium iodide were used prior to titrations for the sum of bromine and iodine. The results are summarized in Table I. TABLEI. DETERMINATION OF BROMINE AND IODIXE IN PRESEKCE O F E A C H OTHER Iodine Present Mg 15.35 23.02 15.35 23.02 7.67 7.67 1.53
.
Bromine Present
0.01 N Thiosulfate for Iz
Mg.
MZ .
10.47 5.23 5.23 1.04 15.70 20.94 10.47
12.10 18.10 12.05 18.15 6.05 6.00 1.20
THE
0.01
N Thiosulfate for Br2
Iodine Found
MI. 25.15 24.65 18.60 19.45 25.65 32.25 14.25
15.35 22.97 15.29 23.03 7.67 7.61 1.52
Mg.
Bromine Found
MMg 10.43 5.23 5.23 1.04 15.70 20.98 10.43
The p H ranged from 3.0 to 6.9, depending upon the amount of bromine present. This variation does not appear to have affected the accuracy of the results. The presence of sodium formate, even if in tenfold excess over the theoretical amount, does not affect the accuracy of the determination of iodine. The sodium formate must be pure.
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.
Me.
13.91
10.95
13.81 6.91 27.79
Mn .
5.45
6.95 27.82
21.85
Iodine Found
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.
