anuary 15, 1933
A N A L Y T I C AL E D I T I O N
nitrate which had been recrystallized twice and dried at 70” C. in a vacuum desiccator over sulfuric acid. POTASSIUM THIOCYANATE. An approximately 0.05 N solution was prepared and standardized against the silver nitrate solution. SODIUMCHLORIDE, This was prepared by precipitating a sample of reagent quality sodium chloride three times from aqueous solution with concentrated hydrochloric acid. It was dried at dull redness for 30 minutes. NITROBENZENE.A reagent grade gave no precipitate with alcoholic silver nitrate and was used without further purification. FERRICALUM INDICATOR. Concentrated, freshly boiled nitric acid was added to a saturated solution of ferric alum until the solution became greenish yellow. Titrations were made in 250-ml. glass-stoppered bottles. A solution (25 to 50 ml.) containing from 0.0483 to 0.2606 gram of sodium chloride, free from the usual interfering ions, was acidified with 8 to 10 drops of concentrated nitric acid and 1 ml. of nitrobenzene was added for each 0.05 gram of chloride. Standard silver nitrate was added until an excess of 1 to 4 ml. of 0.1 N solution was present. The bottle was then tightly stoppered and shaken vigorously until the silver chloride settled out in large spongy flakes. Usually 30 to 40 seconds’ agitation was required. It was found that a perfectly clear supernatant solution mas not necessary. Fine droplets of nitrobenzene were often left in suspension. However, nearly all the nitrobenzene seemed so closely attached to the silver chloride that there was little evidence of it as a separate phase. One milliliter of ferric alum indicator was added and the titration completed with standard thiocyanate solution. The ferric alum acted as an effective flocculating agent and coagulated any suspended matter which was present. Standard potassium thiocyanate solution was added slowly with gentle swirling until a pink color was produced. Usually a false end point appeared one drop before the true end point; it faded in about 30 seconds and may have been due to the desorption of the la& traces of silver nitrate from the preci itate. The next drop of thiocyanate produced a decided copor change which persisted 10 to 15 minutes. Titrations should be made a t temperatures below 25” C., as is customary in other titrations with thiocyanate. Samples of pure sodium chloride were dissolved and titrations were made according to the method just described. The results are shown in Table I. TABLEI. RESULTSOF TESTANALYSES SODIUM CHLORIDE FOUND
Gram
Qram
0.0483 Q . 0486 0.0491 0.0492 0.1005 0.1011 0.1011 0.1132 0.1210 0.2006 0.2211 0.2508 0.2518 0.2605 0.2606
0.0484 0.0487 0.0489 0.0491 0.1003 0.1013 0.1010 0.1132 0.1209 0.2004 0.2207 0.2506 0.2520 0.2605 0.2607
liquids. A very small volume is required, and since i t is heavier than water it does not form a troublesome layer over the aqueous solution. Most of it attaches itself t o the silver chloride, so that its presence is hardly noticed in the subsequent thiocyanate titration. Divalent ions, such as calcium and barium, have no apparent effect on the functioning of the nitrobenzene. The method was used with success b y 0. C. Dermer in this laboratory for the titration of solutions containing piperidine hydrochloride.
LITERATURE CITED
PROCEDURE
SODIUM CHLORIDE TAKEN
39
DIFFERBNCE Gram
+o .0001
+0.0001 -0.0002 -0.0001 -0.0002 +o ,0002
-0.0001 0.0000
-n nnni
-0.0002 -0.0004 -0.0002 +o ,0002 0.0000
+0.0001
The method was checked by another operator who used a standard silver nitrate solution prepared with specially purified silver supplied by C. W. Foulk. The silver was prepared by Foulk and Pappenhagen (2) in a study of silver as a n ultimate standard in acidimetry. It was dissolved in nitric acid a n d the solution diluted to a definite volume, so that the final strength was approximately 0.1 N . Several titrations were made with this solution with results equally as good as those shown in Table I.
DISCUSSION Nitrobenzene, as used in this determination, exhibits the interesting property of inhibiting the darkening of silver chloride in light and this also improves the end point. In several other respects i t is superior to other proposed organic
(1) Caldwell, J. R., accepted for publication in J. Am. Chem. Soc.
(2) Foulk, C. W., and Pappenhagen, L. A,, IND. ENQ.CHEM.,Anal. Ed., 6, 430 (1934). (3) Kolthoff, I. M , “Volumetric Analysis,” Vol. 11, p. 227, New York, John Wiley Sons, 1929; 2. anal. Chern., 56,568 (1917). (4) Rothmund, V., and Burgstaller, A., 2. anorg. Chem., 63, 330 (1909). ( 5 ) Schoorl, N., Pharm. Weekblad, 42, 233 (1905). (6) Stschigol, M. B.. 2. anal. Chem., 91, 182 (1932). RECEIVEDOctober 11, 1934. Presented before the Division of Physical and Inorganic Chemistry at the 88th Meeting of the American Chemical Society, Cleveland, Ohio, September 10 to 14, 1934.
Determination of Perchlorates M. L. NICHOLS, Cornel1 University, Ithaca, N. Y. ELLOR states (2) that “Williams (4) found t h a t the perchlorates are reduced by titanium trichloride, while the chlorates are not affected, and based a process for the volumetric determination of perchlorates-in the presence of chlorates and chlorides-on this reaction.” However, he also states (3) that “chlorates and perchlorates are reduced to chlorides’’ by titanium trichloride. Knecht and Hibbert (1) give directions for the determination of both chlorates and perchlorates with titanous chloride. T h e chlorates are quantitatively reduced in the cold while the perchlorates are not appreciably reduced in dilute aqueous solution, even on prolonged boiling. The perchlorates are completely reduced, however, b y a strong solution of titanous chloride in a fairly strong sulfuric acid solution. The two statements of Mellor and t h a t of Knecht and Hibbert do not agree, and a closer investigation of the work of Williams shows that he boiled weighed quantities of potassium chlorate and ammonium perchlorate with an excess of titanous chloride in the presence of sulfuric acid and then titrated the excess of titanous chloride with ferric alum. This should reduce both the perchlorate and chlorate and he states in his results that “the table below gives results (for perchlorate) after the chlorate had been allowed for.” He, therefore, undoubtedly did not find that the chlorates were unaffected by titanous chloride, as stated by Mellor, but corrected his results for the known amount of chlorate present. LITERATURECITED (1) Knecht and Hibbert, “New Reduction Methods in Volumetric Analysis,” pp. 5 , 25, London, Longmans, Green & Co., 1925. (2) Mellor, “Comprehensive Treatise on Inorganic and Theoretical Chemistry,” Vol. 11, p. 381, London, Longmans, Green & Co., 1922. (3) Mellor,Ibid., Vol. VII, p. 78, 1927. (4) Williams, J. G., Chem. News, 119,s (1919). RECEIVED December 4, 1934.
