Volumetric Determination of Sulfate in Water The Barium Chromate Method MARGARET D. FOSTER, U. S . Geological Survey, Washington, D. C.
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a blank determined as described, has been found to give results consistent with those obtained gravimetrically.
HE barium chromate method for the volumetric determination of sulfate was proposed in 1889 by Andrews who stated "The process depends upon the following series of reactions: First, to the solution of a sulfate is added a n excess of a solution of barium chromate in hydrochloric acid; second, the solution is neutralized with ammonia or calcium carbonate and filtered; third, the' filtrate is acidified with hydrochloric acid, potassium iodide added, and the free iodine titrated with decinormal sodium thiosulfate solution." This description implies exact equivalence between the chromate determined by titration and the sulfate originally present in the sample. Under the conditions of the determination, however, barium chromate is slightly soluble. Several investigators (2, 3) have recommended the use of a blank obtained by carrying a definite portion of the reagent solution through the procedure used by them, to correct for the barium chromate dissolved under the conditions of the determination and, therefore, once determined, applicable in all subsequent determinations of sulfate by the method. This is true if the reagent is free of impurities. Most of the barium chromate available on the market, however, contains either soluble barium salts or soluble chromates, usually the latter. In order to eliminate error from these sources it is necessary to determine the thiosulfate requirement of each new reagent solution made up, or first to free the barium chromate of impurities and then apply a correction for the barium chromate dissolved under the conditions of the determination. As the use of a blank is necessary in either case, it has been found more practical in this laboratory to purify the barium chromate partially before the preparation of the reagent solution by a simple process, in order to reduce the quantity of soluble chromate or soluble barium salts present, and then to determine the blank on each new reagent solution made up. The blank represents in this case barium chromate soluble under the conditions of the determination plus soluble chromate in the reagent or minus an amount of soluble chromate equivalent to soluble barium salts in the reagent.
Procedure Measure 100 ml. of the water sample into a 250-ml. Erlenmeyer flask and make just acid to methyl orange (1 drop) with N hydrochloric acid, added dropwise. Add, at room temperature, 10 ml. of barium chromate solution (7.5 grams of barium chromate in 1 liter of 0.25 N hydrochloric acid). Allow the sample to stand 10 minutes, swirling the flask several times during this period. Precipitate the excess barium chromate with ammonium hydroxide, adding 3 drops in excess of the amount necessary to change the color from orange yellow to lemon yellow (a total of 6 drops is usually sufficient when 10 ml. of the chromate reagent are used for the precipitation), and again allow the sample t o stand 10 minutes, swirling and shaking several times as before. The swirling accelerates coagulation, so that the subsequent filtration is rapid and clean. Filter, catching the filtrate in another 250-ml. Erlenmeyer flask, and wash the precipitate several times with a fine stream of distilled water. Add to the filtrate 10 ml. of 10 per cent potassium iodide solution and 2 ml. of hydrochloric acid (sp. gr. 1.18 to 1.19) and mix carefully with a glass stirrer. After 10 minutes titrate, while rotating the flask P t l Y , yTith 0.05 N sodium thiosulfate (1 ml. = 1.64 mg. of 01), using starch solution as indicator. From the milliliters of thiosulfate required for the titration subtract the blank, previously determined, and mult'iply the remainder by 16.4 (if a 100-ml. sample has been used) to get parts per million of SO4. One hundred water samples on which the sulfate had been determined gravimetrically in the course of regular complete analysis were tested by the procedure and the use of a blank determined as described in this paper. The samples tested contained from 2 to 1786 parts per million of SO4,but for each test a volume of the sample containing not more than 25 mg. of SO.,was used. The results obtained, compared with the gravimetric results on the same sample, are shown in Figure 1. The value (in mg. of sod) used for plotting was the SO, content of the sample taken for the test. With few exceptions the results obtained by the barium chromate method check those obtained gravimetrically within *0.2 mg. The interfering effects of aluminum, zinc, nickel, and iron have been mentioned by various investigators. Natural waters, however, rarely contain more than a few tenths of a part per million of aluminum, zinc, or nickel, and an examination of more than 650 analyses of waters from all parts of the United States shows that in practically all alkaline waters iron in excess of 0.1 part per million is precipitated within a few hours after the waters come from the ground. These constituents might, however, be present in other substances tested in quantities sufficient to affect the results. Calcium,
Purification and Preparation of Reagent Dissolve 25 grams of barium chromate in N hydrochloric acid, make 1,he volume up to approximately 2 liters with distilled water, precipitate the barium chromate with ammonium hydroxide, wash three or four times by decantation with distilled water, and redissolve in as little N hydrochloric acid as possible. Determine the strength of the solution iodometrically (using 5 ml.) and dilute so that 1 liter contains approximately 7.5 grams of barium chromate. A dilution of four times is usually necessary to obtain a solution of this strength, making the acidity 0 25 N .
Determination of the Blank Carry a series of samples containing known amounts of sulfate (5, 10, 15, and 20 mg.) through the procedure for the determination of sulfate. The difference between the thiosulfate titration for the 5-mg. SO1 sample and that for the 10-mg. sample represents the amount of thiosulfate required by chromate equivalent to 5 mg. of Sod. This volume, subtracted from the titration obtained on the 5-mg. sample, gives the volume of thiosulfate solution required for dissolved barium chromate and soluble chromate in the reagent. Similar calculations for the titrations on the other samples give a series of results from which to calculate the average blank. The following slightly modified procedure, with the use of
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FIGURE1. DIFFERENCE BETWEEN RESULTS FOR SO, Difference in results obtained by chromate method and those obtained gravinietricnlly i n the course of regular roinplete analyses oi uater samples.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
if present in large amounts as the sulfate, is occluded in the barium sulfate precipitate in sufficient quantity to cause an appreciable error. Because of the high 804 content of such waters, however, small volumes of the sample are usually taken for the volumetric determination and diluted to 100 ml., thus minimizing the chance of occlusion. The result then obtained is comparable to that obtained gravimetrically on a similarly diluted sample. High results are obtained if phosphate is present in excess of 5 mg. in the sample tested. Boiler waters are almost the only waters that contain phosphate in quantities approaching this magnitude. They may contain 50 parts of phosphate to more than 1000 parts of sulfate and consequently the amount of phosphate present in the
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sample used for the sulfate test would be negligible. Of the other acidic constituents, nitrite is the only one that might have disturbing effects, and this is seldom present in natural waters in quantities sufficient to affect the results, because it is so readily oxidized to nitrate after exposure of the sample to air.
Literature Cited W-., Am. Chem. J.,11, 567 (1889). (2) Komarowsky, A., Chem.-Ztg., 31, 498 (1907). (3) Schmidt, R., 2.anal. Chem., 82, 353-61 (1930). (1) Andrews, L.
RECEIVEDApril 3, 1936. Published by permission of the Director, U. S. Geological Survey.
Determination of Selenium in Steel W. C. COLEMAN
AND C. R. MCCROSKY Syracuse University, Syracuse, N. Y.
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FIGURE 1. SOLUTION APPARATUS A . 500-ml. Erlenmever Erlenmeyer flask containing 50 ml. of 1.18 hvdrochloric hydrochloric acid and 1 ml. of O0.1 r 1 N iodine in potassium iodide B. 250-ml. Erlenmeyer flask containing one-fourth filter paper (Whatm a n No. 40, ? om.) finely macerated, and 6 ml. of 0.1 N iodine in potassium iodide diluted t o 100 ml. C. 250-ml. Erlenmeyer flask containing 200 ml. of water.
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ITH the advent of the use of selenium in steel, the desirability of an accurate, inexpensive, and rapid method for its determination becomes increasingly important. As the result of work in this laboratory, the authors developed a method for the determination of selenuim in steel and submitted it to the Carpenter Steel Company in the spring of 1935. Marvin and Schumb (2) have recently published a volumetric method in which they use perchloric acid as a solvent. The present method makes use of hydrochloric acid as a solvent and a modification of the Norris and Pay (3) volumetric procedure for selenious acid. Briefly outlined, the method consists in the solution of the steel sample in hydrochloric acid, trapping the hydrogen selenide evolved in a potassium iodide solution of iodine, combining the selenium precipitated in this solution with the bulk of the selenium in the hydrochloric acid, and filtering through macerated filter paper. The precipitate is then dissolved by a solution of bromine in hydrochloric acid, refluxed, the excess bromine discharged with alcoholic acetanilide solution, ferric ion suppressed by the addition of sodium fluoride, and the selenious acid titrated by the Norris and Fay method.
The apparatus used for the solution of the sample is shown in Figure 1. A 5-gram sample of steel is placed in flask A , containing hydrochloric acid and a potassium iodide solution of iodine. The iodine solution prevents the loss of hydrogen selenide while connecting this flask, and iodide insures complete reduction of any selenite. The flask is heated with a microburner, at first gently, and finally with boiling until solution is complete and the volume is reduced to 25 to 30 ml. Flask B, containing a potassium iodide solution of iodine for oxidizing hydrogen selenide, is kept cooled in a water bath. Flask C, containing water, maintains a desirable back pressure. (The water bath and flask C, as well a9 the refluxing operation following, are refinements that may be omitted where percentages are reported to only two decimal places.) The contents of flask A are carefully transferred to flask B, rinsing the connecting tube into this flask, removing any selenium from the end with moist filter paper. The solution, containing the selenium precipitate and macerated filter paper, is digested on a hot plate for 15 minutes, then filtered through a Gooch containing a circle of filter paper over which is a matte made from one-quarter of a macerated filter paper (Whatman No. 40, 9 cm.). A qualitative analysis showed that this precipitate contains nickel, silicon, and iron as impurities. The precipitate is washed with warm water and then transferred to flask B and 5 ml. of bromine in hydrochloric acid (1 ml. of bromine in 100 ml. of 1.12 acid) are added. If, after shaking, the bromine color disappears and black particles still remain, more bromine solution is added, dropwise, until the bromine color is permanent. The mixture is refluxed in a 40-cm. (16-inch) condenser for 5 minutes, with a connecting tube from the top of the condenser dipping into a test tube containing 5 ml. of water to trap bromine. The tube is disconnected a t the top of the condenser, the burner removed, and through the condenser tube are poured 50 ml. of water t o which has been added 1 ml. of a saturated solution of acetanilide in alcohol. Excess bromine not removed by refluxing is discharged by the acetanilide. (Of several reducing agents tried, acetanilide was found most satisfactory.) After removing the flask, 20 ml. of 2.5 per cent sodium fluoride and starch solution are added, and the solution is cooled to Z O O , diluted to 150 ml., and titrated by the Norris and Fa method, using approximately 0.02 N solutions of sodium thiosul&te and iodine.
As a means of evaluating this work, a gravimetric method was developed which depends on separating the selenium from the steel by distillation as selenium tetrabromide, its reduction to elemental selenium, and weighing as such. The selenium tetrabromide distillation method was first used by Gooch and Pierce (1) and its reliability as an accurate method ably demonstrated by the work of Robinson, Dudley, Williams, and Byers (4). A 5-gram sample of steel is treated with 50 ml. of 1.18 hydrochloric acid in a special distilling flask fitted with a ground-glass funnel tube (Figure 2). A low flame is used until the steel is
