February IS, 1942
121
ANALYTICAL EDITION
SENSITIVITY. A difference of 0.25’ in the spectrophotometric readings corresponds t o about 0.25 mg. of sulfate in 7.5 ml. of urine. I n the case of the neutral wedge photometer, a difference of 1 unit (1 mm.) corresponds to about 0.33 mg. of sulfate in 7.5 ml. of urine. Both instruments are calibrated for slightly finer divisions, but the readings given above may be duplicated without difficulty. Although determinations may be made over a greater range with the Leitz instrument, the sensitivity is little more than 1 mg. of sulfate in 7.5 ml. of urine for a difference of 1 unit. PRACTICAL APPLICATION. The method works well in practice. The urine sulfates of rabbits treated with cyclic compounds were determined by the photometric method and the proportion of the inorganic sulfates was found to have fallen to 10 per cent or less. Upon cessation of the treatment, the inorganic sulfates returned to a normal of 85 to 95 per cent.
The presence of proteins does not interfere measurably with the method, since a solution containing 15 mg. of sulfate and 0.125 ml. of monkey blood serum yielded 15 mg. of sulfate.
Acknowledgment The writers are indebted to W. Deichmann for carrying out the gravimetric sulfate determinations.
Literature Cited (1) Cholak, J., IND. ENQ.CHEM., ANAL. ED., 7, 287 (1935). (2) Medes, Grace, and Stavers, Elizabeth, J. Lab. Clin. Med., 25, 624 (1940). (3) Society of D y e r s and Colourists, “Colour Index”, 1 s t ed., p. 99, Bradford, England, 1924. (4) T r e o n , J. F.,and Crutchfield, W. E., Jr., in preparation.
Ceric Sulfate in the Determination of Iron Using the Molybdisilicic (Silicomolybdic) Acid Methog ALBERT C. TITUS
T
AND
CLAUDE W. SILL, University of Utah, Salt Lake City, Utah
H E authors have varied their method for the determination of iron (3) by substituting ceric sulfate for potassium dichromate, and have found that o-phenanthroline ferrous ion indicator is satisfactory in place of N-phenylanthranilic acid in the ceric sulfate runs. The method and general procedure for runs, blanks, and standardizations follow the ideas presented in the earlier paper. The “stock ferric chloride” and “stock dichromate” were those used in the previous paper. The normality of 0.09733 for the former was taken from that paper. For the normality of the dichromate 0.10058 was used in place of 0.10073 being obtained from pure dichromate from volume ratios of the respective solutions used in titrating the stock ferric chloride by the molybdisilicic acid method. This method of substitution allows cancellation of errors in the procedure and blanks, giving an “absolute” normality. From the dichromate the ceric sulfate Kas standardized by another substitution procedure, cancelling out any slight errors (1, 4). The oxidizing solutions were in turn run into potassium iodide and acidified with sulfuric acid, and the iodine liberated was titrated with a sodium thiosulfate solution. From the relative volumes of the latter the normality of the ceric sulfate was obtained, after making a suitable correction for acidity differences. The ceric sulfate was also standardized against sublimed arsenic trioxide, using osmium tetroxide catalyst and ophenanthroline ferrous indicator. This method gave a normality of 0.09664 for the ceric sulfate, which is somewhat lower than the values of 0.09704 and 0.09697 obtained by the molybdisilicic (suggested by ill. B. Ptlellon as a name preferable to silicomolybdic) acid method a t the end and beginning of the work, using the stock ferric chloride whose stated normality was shown in the preceding paper to be absolute and so independent of the method used there. However, the thiosulfate method gave 0.09682 for the cerio sulfate normality. It appears that after application of the blanks the molybdisilicic acid method with ceric sulfate gives slightly too low results for iron, since the volume of ceric sulfate used seemed t o be as much as 3 parts in a thousand too low, giving too high a normality above. In Table I are comparisons of the volumes of ceric sulfate used for the titration of iron, with different indicator combinations. Unless otherwise noted, each figure represents the
average of four runs from which no run varies by over 0.02 ml. I n untabulated cases the blanks were positive but not over 0.02 ml. It will be noted that nonapplication of the blanks resulted in much better agreement than did application. The blanks for the molybdisilicic acid method and the volumes tabulated are those used in calculating the normalities given above. Had the blanks not been applied for the runs a t the beginning of the work the normality would have been 0.09684 in place of 0.09697, a better agreement with the other methods. There is some question as to the real significance of the blanks, especially since their application in the earlier paper resulted in slightly low values for iron as compared with the value by weight. The use of o-phenanthroline ferrous indicator appears to be most satisfactory (Table I). TABLEI. TITRATION OF STOCK FERRIC CHLORIDE WITH CERIC SULFATEO Method Molybdisilicic acid, Xphenylanthranilic acid (first runs) Same (at end of work)
Ceric Sulfate per M1.
Applied Experimental Blank M1.
Corrected Volume ME.
45.23 45.23
-0.06 -0.07
45.17 45.16
45.22 45.21 45.23 45.22 45.23C 45.20
-0.08 -0.08
45.14 45.13
45.00 M1. of FeCh
Mercuric chlorides, 0Less than .... .. phenanthroline, cold -0.02 hlolybdisilicic acid, o-0.02 45.21 phenanthroline (same -0.02 45.18 directions as with N phenylanthranilic acid) a 32 ml. of 1 t o 1 H&OI added in all runs, in addition t o equivalent of 8 ml. present in added ceric sulfate volume. b N o trouble (8) experienced with adsorption of indicator on calomel with small excess of stannous chloride used. C Three runs only.
Literature Cited (1) F u r m a n and Wallace, J . Am. Chem. SOC.,53, 1283 (1931). (2) L a n g (private communication cited), Oesper, “Newer Methods of Volumetric Analysis”, p. 191, N e w York, V a n Nostrand Co., 1938. (3) T i t u s a n d Sill, IND. ENQ.CHEM., ANAL.ED., 13,416 (1941). (4) Vosburgh, J. A m . Chem. SOC.,44, 2130 (1922).
CONTRIBUTION 68 from the Chemical Laboratories of the University of Utah.
