Polarographic Determination of Sulfate A. D. HORTON AND P. F. THOMASON Analytical Chemistry Diuision, Oak Ridge National Laboratory, Oak Ridge, Tenn. In metathesizing lead sulfate precipitates with potassium carbonate, the residue was analyzed for microgram amounts of sulfate to ascertain the completeness of the metathesis. Such amounts are difficult to estimate, especially when the material is only slightly soluble. A nephelometric measurement of precipitated barium sulfate, normally used for water-clear solutions, is obviously difficult with the samples mentioned, while indirect colorimetric methods depend on precipitation of benzidine sulfate followed by diazotization to a dye, which re-
S
ULFATES have been determined turbidimetrically (4) with the Jackson candle turbidimeter in the range 1 to 60 mg. of sulfate. Excess hydrochloric acid interferes and sample solutions must be clear. A nephelometric method (3) for water and biological materials will determine as little as 20 micrograms of sulfate; hom-ever,even low concentrations of sodium, magnesium, zinc, cadmium, mercury, aluminum, and nitrates interfere. The precipitation of sulfate as benzidine sulfate ( 2 ) and diazotization to a dye require the precipitation to be carried out in organic solvents at low temperature. The solubility of benzidine sulfate is rather high, and the results Cor low concentrations of sulfates are uncertain. Precipitation of sulfate with barium chromate ( 5 ) ,removal of excess barium chromate with lime, and determination of equivalent chromate with diphenyl carbazide is usable for as little as 10 micrograms of sulfate. Phosphate and iron interfere. The method discussed here was developed primarily to determine microgram amounts of residual sulfate in lead sulfate metathesized with potassium carbonate. This method depends
quires all of the sulfate to be in solution. The precipitation time for benzidine sulfate isslow,requiring an hour or more near 0" C. The indirect polarographic method estimates sulfate in the range of 15 to 150 micrograms with an over-all accuracy of &5",',. This method, adapted to polarographic analysis from a volumetric macromethod should have wide application for estimation of small amounts of sulfate; i t is rapid and simple, and reasonable accuracy can be obtained by inexperienced analysts. The sample need not be water-soluble.
on the reduction of the sulfate to hydrogen sulfide, which is precipitated as cadmium sulfide. This precipitate is dissolved in hydrochloric acid, and the cadmium is estimated polarographically. Sulfate is calculated from the total cadmium. Substances, with the exception of nitric acid, that interfere with most of the other methods are not known to interfere in this procedure. . All inorganic sulfates and sulfides may be determined by this method. PROCEDURE
If the sample contains nitric acid, the volume of the sample solution is reduced to 2 ml. in the distillation flask, 2 ml. of 90% formic acid are added, and the solution is evaporated to about 0.5 ml. If the sample does not contain nitric acid, no pretreatment is necessary.
TO VACUUM LINE SEMI BALL JOINT
40/25
Figure 2.
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5
125ml.ERLENMEYER FLASK
SCALE-
lii: I"
Figure 1. Distillation Apparatus
Vacuum Filter Flask
The combination distillate receiver and filter (Figure 1) is filled with 10 ml. of the ammoniacal cadmium chloride solution. The capillary condenser tip is placed near the bottom of the funnel. An aliquot of the sample solution containing 15 to 150 micrograms of sulfate (5 to 50 micrograms of sulfide) is placed in the distillation flask (Figure 1) with a few glass beads, together with 20 ml. of the reducing mixture, and the flask is attached to the column by means of a semiball joint. The mixture in the flask is boiled gently until the evolution of hydrogen sulfide has ceased, as indicated by cessation of bubbling in the receiver. Sitrogen gas is then introduced into the flask to carry the hydrogen sulfide through the column, and boiling is continued for a total of 10 minutes. The receiver which contains the cadmium sulfide precipitate is removed from the distillation apparatus together with the capillary condenser tip and the solution is filtered in a specially designed vacuum filter flask (Figure 2). The cadmium sulfide is washed with 10 ml. of cadmium sulfide wash solution delivered by a pipet. The funnel is removed from the filter flask and the stem is 1859
ANALYTICAL CHEMISTRY
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in the range 15 to150 micrograms of sulfate. Sulfate Found Principal sources of error were Volume of Contains Known Sulfate Corrected Deviations solubilityof the cadmium sulfide, Sample “01 Concentration Total Total Blank” Value dmount Ul. Y Y Y Y Y % gain or loss of the cadmium chloride solution in transfers, coprecipitation of basic cadmium sulfides, and the presence of extraneous sulfates in reagents and glassware. Nitric acid reacts with both the reducing mixture and the hydrogen sulfide; therefore, it must be removed quantitatively from the clianing solution. samples. Formic acid was found to be the most satisfactory rewashed out thoroughly with distilled water to remove traces of agent for the removal of nitric acid. According to Mellor (8): cadmium chloride solution. 2HNOd 4HCOOH +4C02 5H20 f Nz0 The bottom part of a polarographic cell (Figure 2), calibrated to 4 ml., is placed on the glass indentations in the filter flask 4 moles of formic acid will reduce 2 moles of nitric acid. The and the funnel is re laced with the stem inside the cell. The procedure specifies 2 ml. of 90% formic acid for removal of nitric cadmium sulfide in t i e funnel and the condenser tip is dissolved acid; therefore, 2 ml. (47 me.) of formic acid will remove 23.5 with 500 microliters of 2 M hydrochloric acid, washed into the cell with 1 hi! potassium chloride, and diluted to volume with the me. of nitric acid. The maximum nitric acid encountered was same solution, The resulting cadmium chloride solution is 2 me.; however, for very high concentrations, proportionately analyzed polarographically. The sulfate equivalent is 0.854 of more formic acid must be used. Perchloric acid, 70% (Y), the total cadmium found. and 0.1 N titanous chloride (9) were also tested for nitric acid removal, but perchloric acid volatilized some of the sulfate during REAGENTS AYD APPARATUS evaporation procedure; titanous chloride prcduced a high blank. The most satisfactory results were obtained when the following Several cadmium chloride solutions were tested as collecting chemically pure reagents were used in the investigations: solutions for hydrogen sulfide. The concentration of cadmium Reducing Mixture (6). A mixture of 160 ml. of 47% hydriodic chloride in the ammoniacal cadmium chloride solution was varied acid, 160 ml. of 13 M hydrochloric acid, and 50 ml. of 30% hypobetween 1and 10 mg. per ml. without affecting the results appreciphosphorous acid is placed in a beaker with a few glass beads ably. The volume of the reagent is not critical, but must be and boiled for 20 minutes. Although a satisfactory reducing agent can be obtained with a boiling period of 5 minutes, a total sufficient to retain all the hydrogen sulfide. of 20 minutes is recommended because a lower blank is thus obAn excess of ammonia is necessary in the cadmium chloride tained. solution, because of the large amount of noneondensable acids Ammoniacal Cadmium Chloride Solution. This solution discharged from the still. However, an excess of ammonia contains 1 gram of cadmium chloride, 300 ml. of 28% ammonium hydroxide, and 1 mole of ammonium chloride per liter of solution. produced high results due to coprecipitation of basic cadmium Cadmium Sulfide Wash Solution. Fifty milliliters of 28% sulfides which were retained on the filter with the desired cadammonium hydroxide are diluted to 1 liter vith distilled water. mium sulfide. A solution of cadmium chloride in Eimer and Distillation Apparatus. This equipment is adapted from a reflux Amend pH 4 buffer was tried with low results due to the solucolumn designed by Boaz et al. ( 1 ) for semimicro volumetric bility of cadmium sulfide at this pH. Coprecipitation was fidetermination of sulfate sulfur. This column is an improvement nally prevented by making the cadmium chloride solution 1 M of an orginal by Luke ( 6 ) , and was further altered for microin ammonium chloride. chemical polarographic analysis as noted. The cadmium sulfide precipitate was washed initially with an unmeasured quantity of distilled water. The solubility of A combination distillate receiver and filter (Figure 2) was cadmium sulfide in xater is great enough to cause low results fabricated by sealing a funnel cone to a 12-mm. borosilicate lass tube containing a medium-porosity fritted glass disk. The within the range of this procedure. It is almost completely isk is h e enough to retain the ammoniacal cadmium chloride insoluble in dilute ammonia water, and varying amounts of solution at atmospheric pressure, and the cadmium sulfide precadmium sulfide wash solution were used for washing the precipitate under vacuum. This replaces an Erlenmeyer flask used as a receiver in volumetric analysis. To prevent spattering in cipitate. The optimum amount for washing it free of the amthe small receiver it was necessary to reduce the condenser tip moniacal cadmium chloride solution is 10 ml. to a small opening (
