DECEMBER 1947
1041
from other plastics, such &s methyl methacrylate, which would have less tendency to darken or warp on aging.
trophotometric analyses of the standard color solutions, of dyes for matching, and of the dyed cellophane strips.
ACKNOWLEDGMENTS
The authors are indebted to P. Giesecke and A. H. Struck of the Visible Light Spectroscopy Laboratory of this company for spec-
LITERATURE CITED
(1) Patty and Potty, J . I n d . Hug. Toxicol., 25, 361-5 (1943).
RECEIVED January
15, 1947.
Electrometric Titration of Nitric Acid in Oleum C. DANA M C K I N N E Y , JR., WILLIAM H. ROGERS, AND WALLACE M. AICNABB Department of Chemistry and Chemical Engineering, University of Pennsylvania, Philadelphia, Pa. Y ELECTROblETRIC method is described for the deterA h n a t i o n of the amount of nitric acid added to oleum as an antifreeze. Bowman and Scott ( I ) recommend the titration of nitric acid with a standard solution of ferrous sulfate in which the first appearance of the red-brown color of the ferrous nitrosyl sulfate is taken as the end point of the titration. The color of the end point is easily obscured if the oleum is dark in color. This is frequently the case when reclaimed sulfuric acid is used as “drip acid,” the sulfuric acid in which sulfur trioxide is absorbed in the manufacture of oleum. A previous publication (3) on the electrometric titration of nitric acid requires the use of a potentiometer and a specially prepared electrode and titration cell. In the present work the authors used the Serfass electron ray titration assembly ( 2 ) fitted with a platinum-tungsten electrode combination. The advantages of this setup we the convenience, elimination of the interference of the end point by the dark color of the oleum, and elimination of the empirical blank correction of about 0.20 ml. Although the authors used the Serfass assembly in this work, any similar assembly should pwform equally well. ANALYTICAL PROCEDURE
The ferrous sulfate solution is prepared by dissolving 176.5 grams of ferrous sulfate heptahydrate in about 400 ml. of water and adding 500 ml. of 60% sulfuric acid while stirring vigorously. The solution is diluted to 1 liter and is approximately 0.6 N . This solution is standardized against 0.5 N potassium dichromate in the presence of 10 ml. of 18 N sulfuric acid in the manner described below for the determination of nitric acid. A sample of oleum containing about 0.4 gram of nitric acid is weighed from a Lunge pipet into a 250-ml. beaker containing 100 ml. of concentrated (95%) nitric-free sulfuric acid. The sensitivity control of the titration unit is turned to position eight and the polarization control is set in the off position. The electrodes and mechanical stirrer are placed in the beaker and the solution is mixed thoroughly. The cathode ray tube is adjusted to the closed position by means of the “eye control.” The sample is titrated with the ferrous sulfate solution; the first few milliliters cause the eye to open momentarily. Further slow addition brings the eye back to the closed position, and the titration is continued until the eye remains open for 30 seconds. Thenitrosyl sulfuric acid is determined by titration with standard potassium permanganate. A 10-gram sample of the oleum is weighed into a 100-ml. volumetric flask containing about 20 ml. of 9570 sulfuric acid. The sample is diluted with sulfuric acid to the mark and a 25-ml. aliquot is transferred to a 250-ml. beaker c o n t a h n g distilled water and a large piece of ice. The solution is titrated with standard potassium permanganate to an end point which persists for one minute. DISCUSSION OF RESULTS
This procedure was applied to two samples of oleum which had been analyzed by the familiar nitrometer method. It was noted that the electrometric end point preceded the visual end point by 0.12 ml., which was assumed to be due to the excess ferrous sulfate that is added to obtain a detectable color change. No correction is necessary in calculating the results by the electrometric method.
The ferrous sulfate titration of nitric acid in oleum measures only the actual nitric acid content of the sample, while the nitrometer method measures the nitrosyl sulfuric acid in addition to the nitric acid. The nitrosyl sulfuric acid present is due to the oxides of nitrogen in the nitric acid reacting with the oleum. Therefore, to place the two methods on the same basis, it was necessary to determine the nitrosyl sulfuric acid content of the oleum samples’ and to correct the results of the ferrous sulfate titration.
Table I. Determination of Nitric Acid in Oleum Sample S O .
Sample Weight
Volume of FeS0.r
Actual ISSO:
HKOI
Total HNOi
Sitrometer
%
%
Grams
M1.
%
102-70
2.766 6 048 6.950 8.517 7.272 7.158
4.57 9.99 11.53 14.14 12.06 11.89
3.52 3.52 3.54 3.54 3.54 3.54
3.53 3.53 3.55 3.55 3.55 3.55 Av. 3 . 5 4
3 57
102-71
4.837 3.476 6 071 5.777 8.145 5.027
15.36 11.10 19.41 18.49 26.03 16.13
6.73 6.78 6.77 6.77 6.77 6.80
6.74 6.79 6.78 6.78 6.78 6.81 Ar.678
678
The data in Table I show that concordant results are obtained by this method over a fairly wide range of sample sizes, and the values agree well with the values by the nitrometer method. The actual nitric acid values are those calculated directly from the titration; the total nitric acid is the sum of the actual nitric acid and the nitric acid equivalent to the nitrosyl sulfuric acid. The nitric acid equivalent to the nitrosyl sulfuric acid in both samples is 0.01%. Although the nitrosyl sulfuric acid content of these samples is not significant, samples are encountered frequently in which it is as high as 0.1 to 0.2%. Although this method has been applied only to oleum in this investigation, it may be applied equally well to inorganic and organic nitrates ( 1 , 3 ) . ACKNOWLEDGiMENT
The authors wish to express their appreciation to Hercules Powder Company of Wilmington, Del., which supplied the oleum samples and the nitrometer analyses used in this investigation. LITERATURE CITED
(1) Bowman, F. C., and Scott, W. W., J . Ind. Eng. Chem., 7, 766-9 (1915). (2) Serfass, E.J., IND. ENG.CHEM.,ANAL.ED.,12,636-9 (1940). (3) Treadwell, W.D., and Vontobel, H., Helv. Chim. Acta, 20,573-89 (1937). RECEIVED January 18, 1947.
