October 15, 1931
INDUSTRIAL AND ENGINEERING CHEMISTRY
361
Volumetric Barium Chromate Method for Sulfates' J. R. Andrews 604 CHEWST,ALLENTOWN, PA.
HE barium chromate Using the ferrous sulfate-permanganate titration for sodium Acetate: Dissolve 250 method for the deterestimating the liberated chromate, the method of grams of sodium acetate in 830 ml. of water. mination of s u l f a t e s Hinman may be used in determining from 2 to 200 mg. Standard Ferrous Ammonium of sulfate. From 0.5 to 2 mg. is determined by fortihas been a d o p t e d in some water laboratories with suefying the water with 2 mg. of sulfate and deducting f [ ~ f ~ & s t ~ ~ ~~ $ o l ~ g cess. The results listed here this from the amount recovered. containing 50 ml. of sulfuric Hydrogen sulfide and other reducing matter present acid. This solution, about half i n d i c a t e t h a t this method in the water are oxidized by potassium permanganate again as strong as the standard may be applicable also to oilwell brines whose sulfate conPrior to the addition of the barium chromate reagent. $ ~ , ' , a , ~ ~ i ~ g nate each day a relation is retents vary considerably with different samples. The method described by Hinman (4) in quired, and is kept in an amber-colored bottle in a cool place. Standard Potassium Permanganate. Heat 1100 ml. of distilled the fact that barium chromate is 'lightly 1877 depends water to boiling, add 1 ml. of perianganate reagent, and boil more soluble in a neutral solution than barium sulfate; that several minutes. ~ i 1 . ~ 7 5grams ~ of~ potassium ~ permanl ~ when barium chromate is added to a soluble sulfate, the more ganate in 1000 ml. of the cooled water prepared above. Standinsoluble barium sulfate is precipitated and a soluble chromate ardize the solution against a sulfate water whose sulfate content known, and adjust SO that 1 ml. is equivalent t o 2 mg. of is formed. The soluble chromate is then determined in the is SO4--. The sulfate waters used for standardizktion should filtrate by titration. be selected from those received for analysis in the regular course Schmidt (3) discussed the procedures recommended by of the laboratory work, and are evaluated by the barium chlorideBruhns, Nachtigall and Raeder, and Koszegi. Schmidt, using gravimetric method. a 0.2 N hydrochloric acid solution of barium chromate, recomProcedure mends that the precipitations be carried out in the cold on the sulfate water as received, and that the final determination of Using methyl orange as indicator, neutralize 100 ml. of soluble chromate be made by the iodometric method. Germ muth (1) notes that the method of Wildenstein, using 0.02 N the boiling water, containing between 2 and 200 mg. of Sod--, of 0.02 N potassium in a 300-ml. conical flask, with concentrated hydrochloric acid, barium &loride followed by a slight adding 2 or 3 drops in excess. Oxidize any reducing matter chromate, is accurate and rapid. The method recommended by Schmidt would not be satis- with Permanganate reagent, avoiding an excess of more than factory for oil-well waters, inasmuch as the precipitations are 1 drop and continue to boil for a minute. Then destroy the carried out on the water as received and many oil-well waters Permanganate color, 01 cause the precipitated manganese contain hydrogen sulfide, ferrous iron, and other reducing dioxide to dissolve, with a drop or two of 3 per cent hydrogen matter which would reduce the chromate and give low results. peroxide, and to the excess completely. Add lo It appears that the method of Wildenstein would give very ml. of sodium acetate solution, and for each 15 mg. of so4-satisfactory results, since an excessive amount of chromate present add lo ml* Of barium chromate reagent* In no case could be avoided regardless of the amount of sulfate present. add less than lo ml*Of the barium chromate reagent* is formed with add 3 drops of concentrated ammonia water for each 10 ml. Although an excessive amountof high sulfate waters,H ~method ~appears ~to be simpler ~ of ~ bariumJ chromate ~ reagent and continue to boil 5 minutes. Allow the solution to cool several minutes on the bench and in that the standard so~utionsof barium &loride and potasthen in tap water to 15" C. Occasional agitation while the sium chromate are eliminated. The following so~utionsare used in the suggested procedure: solution is cooling assists the precipitation of that barium chromate which is soluble in the hot solution. Filter the Per?naWanate Reagent: Dissolve 0.5 D a m of Potassium Per- supernatant liquid through a retentive paper into a 400-d. manganate in 100 ml. of water. ~~~i~~Chromate Reagent: ~ i 5 grams ~ of ~ barium ~ beakerl and agitate ~ the ~ precipitate remaining in the flask with 20 ml. of cold water. Wash the flask and filter with cold chromate in a solution of 26 ml. of hydrochloric acid and 975 ml. of water. (10 ml. of this reagent should be used for precipiwater, using 80 ml. altogether. Note that the precipitate is tating each 15 mg. of SOr--, and a minimum Of 10 ml should tinted yellow, indicating that an excess of barium chromate be used for less than 15 mg. of so4--.) The barium chromate should not contain sufficient soluble chromate or barium salts reagent was used* to give a titration difference greater than 0.05 ml. of standard For each 100 ml. of filtrate add 10 ml. of cold (1 to 1) SUIpermanganate in the following tests: Prepare two solutions, 1 furic acid. Titrate with standard ferrous ammonium sulfate and 2, each containing 2 mg. of chromium as sodium chromate until the pale blue color of reduced chromium is evident, in distilled water. Carry solution 1 through the procedure for and add 2 ml. excess. After 1 O r 2 minutes, back titrate with the determination of sulfates as outlined in this paper, omitting the oxidation and reduction by the permanganate and peroxide standard permanganate to the faint purple end point. The reagents, and adding 30 ml. of the barium chromate reagent. end point is very clearly seen against a white background when To Solution 2 add 2 drops Of concentrated hydrochloric acid, light from an electric lamp is used, but direct sunlight hides 10 ml. of sodium acetate, and dilute to the same volume as occupied by solution 1 after the filtration. Add the same amount the end point- Look down through the solution from such of sulfuric acid required by solution 1, and titrate the chromate. an angle that a beam of light reflects from the bottom edge If the titrations on solutions 1 and 2 do not check within 0.05 of the beaker. ml. of standard Permanganate, a soluble chromate Or barium Nachtigall and Raeder ( 2 ) recommend the use of hydrogen salt is indicated to be present in sufficient amount t o cause an but permanganate appears to peroxide as an Oxidizing error in a sulfate determination. This deleterious salt is removed from the barium chromate by washing the powder with dilute be better in that an excess can be avoided. An acid solution acetic acid or by precipitating the barium chropate from dilute of peroxide reduces chromate so that the excess must be rehydrochloric acid solution with ammonia water moved completely before the barium chromate reagent is added. 1 Received May 5, 1931.
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ANALYTICAL EDITION
3 62
Rather than add a large excess of sodium acetate, the hydrochloric acid, added with the barium chromate reagent, is neutralized partially with ammonia water, leaving the solution slightly acid with acetic acid. Table I-Analyses
of Water with S o d i u m Sulfate Added ERROR( M A X . ) Mg. %
SO&--PRESENT Sod-- FOUND(Av.) Mg. Mg. 0 00 0 49 0 07 1 46 1 95 2 44 2 02 3.41 3 89 4 38 4 86 9 73 12 18 24 35 48 710 73 1 07 4
4 9 11 24 48 73 97
60 64 95 25 71 6 6
-0 -0 -0 -0 $0
25 09 23 10 39
1zi:o 40.0 18.0 6.7 3.7 1.0 4.4 5.9 1.1 5.1 0.0 1.9 0 4 0.8 0 7 0 2
Reference point at which solutions were standardized. 2 mg. of chromium as sodium chromate added. 170 mg. of magnesium chloride, 1700 mg. of sodium chloride, 220 mg. of calcium chloride added. a
b 0
The results in the tables indicate that amounts of sulfates ranging from 2 mg. to 200 mg. may be determined with reasonable accuracy, and down to 0.5 mg. may be determined by adding 2 mg. of sulfate and deducting this from the amount recovered. Since 2 ml. of N/29.2 sulfuric acid contain 3.29 mg. of sulfate, this could be used as the source of sulfate. When 2 mg. of chromium as sodium chromate were added t o distilled water and the procedure for the determination of sulfates followed, omitting the oxidation and reduction by the pwmanganate and peroxide reagents and adding 30 ml. of
5’01. 3, No. 4
barium chromate reagent, only 2 mg. of chromium were recovered. When distilled water alone was carried through the above procedure, a blank of 0.85 mg. of sulfate was obtained. These tests indicate that it is incorrect to deduct a blank from the titration figures, since the blank is obtained only on amounts of sulfate less than 2 mg. (TabIe 1)-that is, when there are sufficient chromate ions in solution, the solubility of the barium chromate in weak acetic acid solution becomes negligible. A study of the results in Table I indicates that the permanganate solution may be standardized against a solution containing 48.71 mg. of sod-- with a maximum error of 0.39 mg. This would give the standard permanganate an SO4-value with a maximum error of 0.8 per cent. However, by averaging several values at 48.71, the standard permanganate is made accurate within 0.2 per cent, as shown in the table where that accuracy was attained on 97.4, 121.8, and 194.8 mg. of Sod--. Since, on smaller amounts of Sod-- than 24.35 mg., a small error in milligrams means a large percentage error in the evaluation of the solution, it is recommended that the permanganate be standardized against a minimum of 25 mg. of Sod--. The third column indicates the greatest error recorded in two or more determinations. There are, of course, several other methods for the determination of chromates, such as the iodometric method and several electrometric methods. It appears that almost any of these methods would be suitable for this determination, although the permanganate end point in the ferrous sulfatepermanganate method is much more easily distinguished than is the starch end point in the iodometric method. Literature Cited (1’) (2) (3) (4)
Germuth, J . A m Wulialev Works Assocn., 19, 607 (1928) Nachtigall and Raeder, Arch. Hyg., 100, 31 (1928). Schmidt, 2. anal. Chem., 82, 353 (1030). Treadwell-Hall, “Analytical Chemistry,” Vol 11, p. 610, Wiley, 1928.
Comparison of Solubilities of Calcium and Strontium p-Bromobenzoates in Acetone-Water Mixtures’ John C. Bailar, Jr. UNIVERSITY OF ILLINOIS, URBANA, ILK.
HE similarity between the compounds of calcium and those of strontium is so great that difficulty is experienced in effecting a separation of the two metallic ions. According to Hillebrand and Lundell (6),‘‘KOreally
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good methods for separating calcium, strontium, and barium from each other are known.” The ratios of the solubilities of calcium and strontium salts in water are too small to offer satisfactory methods of separation. Fresenius (1) developed a method of separation based upon the difference in the solubilities of the nitrates in a mixture of equal amounts of absolute alcohol and absolute ether. Although the solubility ratio in this case is very large, Fresenius reported that the results for strontium were high, whereas those for caIcium were correspondingly low. The method of Rawson (3) depends upon the fact that calcium nitrate is soluble in concen.trated nitric acid, whereas strontium nitrate is not appreciably soluble. This method also gives high results for strontium. Another instance is reported here in which there is a rather large solubility ratio, and which, it is hoped, may ultimately be developed into a good method of separation. Water dissolves the p-bromobenzoates of calcium and stron1
Received April 29, 1981.
tium to the extent of 2 per cent at room temperature. The addition of acetone to the water solution decreases the solubilities of both salts, that of strontium p-bromobenzoate much more than that of the calcium salt. Under optimum conditions, the ratio of the solubilities in a water-acetone mixture is about 45 to 1. Practically all of the metal p-bromobenzoates are insoluble in pure acetone, but acetone which contains a little water dismany of them. The alkali and ammonium p-bromotes are soluble to the extent of I per cent in acetone which contains 6 per cent water. The reagent used in experiments in this laboratory was a saturated solution of ammonium p-bromobenzoate in 94 per cent acetone. This reagent was mixed with water solutions of calcium and strontium nitrates. Fifteen cubic centimeters of acetone were then added for each cubic centimeter of the water solution that was used, bringing the acetone concentration again to 94 per cent. Under these conditions, a solution of strontium nitrate containing 1 mg. of the salt per cubic centimeter gives a distinct crystalline precipitate, and a solution of calcium nitrate fifty times as concentrated remains perfectly clear.
