I N D UXTRIAL A N D ENGINEERING CHEMISTRY
October 15, 1929
231
Solubility of Benzidine Sulfate and Benzidine Hydrochloride in Hydrochloric Acid Solutions' Wm. B. Meldrum and Ira G. Newlin HAVERFORD COLLEGE,HAVERFORD, PA.
Solubility of Benzidine Sulfate
N THE course of an investigation requiring the quantitative determination of sulfate it was found that the usual
I
barium sulfate method was not entirely satisfactory owing to the formation of mix-crystals with other constituents present. Accordingly an attempt was made to adapt the well-known benzidine sulfate method. As the precipitation of the sulfate had to be made from solutions of decided acidity, it was necessary to know to what extent the solubility of the benzidine sulfate is affected by changes in acid concentration. No solubility data were found in the literature, however, apart from those given by Bisson and Christie (1) on the solubility of benzidine sulfate in pure water a t various temperatures. Accordingly determinations were made of the solubility in hydrochloric acid solutions of various concentrations. The solubilities were determined directly by agitating a known weight of the salt with a known weight of the acid solution (about 50 grams) in a glass-stoppered flask immersed in a thermostat a t 25" C. until equilibrium had been attained.
creasing acidity. It passes through a maximum a t a concentration of about 3.5 normal and then decreases. It was thought that this effect might indicate that the solubility limit of the benzidine hydrochloride presumably formed by interaction of the acid with the sulfate had been reached. On this point also data were lacking and determinations were made of the solubility of benzidine hydrochloride in hydrochloric acid solutions. The results are given in Table I1 and Figure 1. It will be noted that the solubility of benzidine hydrochloride in 3.5 N hydrochloric acid is actually less than the solubility of the sulfate. of Benzidine Sulfate in Hydrochloric Acid Solutions CONCNHCIIN FILTRATE SOLUBILITY FILTRATE SOLUBILITY N G. per 1000 g. s o h . N G.per 1000 g. s o h . 0.098 (1) 0.000 0.537 4.139 1.885 0.239 0,548 1.889 Av. 1.887 Av. 0.542 0.943 5.622 1.771 0.530 0,941 1.778 Av. 0.942 Av. 1.774 1,252 1,551 1,009 7.601
Table I-Solubility COHCX.HCI
IN
2.074 2.825
Av.
1.254 1.253
Av.
1.790
1.789 1,792
9,942
1.921 1,939
11,099
Av. 1.930 Table 11-Solubility COJTCN.HCl
IN
Av.
1.536 1.644
1,496 1.501 Av. 1.499 1.488
1.493
Av. 1,490
of Benzidine Hydrochloride in Hydrochloric Acid Solutions
FILTRATE
SOLUBILITY
N
G.per 1000 g. s o h .
0.000 2.884
2.114
5.346 2,225 Av. 2.172 1.244 1.248 Av. 1 . 2 4 6 1,226 1.221 Av. 1 , 2 2 4
5.607 9.870
Conclusions
Concentration HCI Normality
Figure 1
Preliminary experiments showed that attainment of equilibrium required about 36 hours; twice this time was allowed. The residue was then filtered off on a sintered glass crucible, washed with acid solution of the same concentration as that used, dried, and weighed. A portion of the filtrate was titrated with standard sodium hydroxide solution. The benzidine sulfate used was prepared by adding a slight excess of sulfuric acid solution to a solution of Kahlbaum's benzidine "for analysis," filtering, washing free from excess acid, and drying to constant weight a t 110" C. The results are shown in Table I and Figure 1. Solubility of Benzidine Hydrochloride
As these data show, the solubility of benzidine sulfate in hydrochloric acid solution increases very rapidly with in1
Received June 20, 1929.
From these results it is obvious that in applying the benzidine sulfate method in the determination of sulfate, accuracy can be attained only if the acid concentration is kept very low, and it should be pointed out that even under these conditions there may be a considerable error due to solubility of the precipitate. Literature Cited (1) Bisson and Christie, J. IND.END. CHEX.,12, 486 (1920).
Large Metal Soxhlet Extractor Editor of Industrial & Engineering Chemistry: I am reminded of the large metal Soxhlet extractor that I had made about thirty years ago which was essentially like that shown on page 140 of the Analytical Edition for July 15, with the exception that the siphon tube was made of block tin. The advantage of this feature was that it could be bent down to accommodate larger or smaller quantities of materials to be extracted and hence lesser amounts of solvents were needed. B. 111. PILHASHY SANFRANCISCO, CALIF. July 31, 1929
