4-NITRO-5-METHYL-2-SULPHOBENZOIC ACID 
177 
The separation was accomplished finally by systematic frac- 
tional crystallization, the crystals being filtered off first from 
the fairly concentrated solution at 50°, and then from the cold, 
somewhat diluted filtrate after standing several hours. The prod- 
ucts thus obtained were recrystallized with due attention to 
temperature and concentration, until a complete separation 
was effected. Two criteria were used in determining purity.' 
First, the salt of the original xylene sulphonic acid crystallized 
in needles, the complete absence of which under the micro- 
scope indicated purity; second, it was found that while the un- 
oxidized nitro-xylene sulphonic acid salt has a very intense 
bitter sweet taste, the pure oxidized substance has practically 
none, what little can be perceived being only that characteristic 
of the potassium ion. This latter test is more delicate than the 
first and w^as used largely in determining the purity of a given 
fraction. About one hundred separate crystallizations were 
made in the purifying of the 235 grams of the potassium salt 
used as a basis for this research. The infiuence of the impurity 
has very definite effects, and its removal was a necessity, as 
will be seen especially in the preparation of the acid chlorides. 
Analyses of the purified potassium salt, while slightly low, are 
much better than before. 
.1678 g. potassium salt gave .0859 g. K 2 SO 4 . 
.1560 g. potassium salt gave .0800 g. K 2 'S 04 . 
Theory for C 8 H 5 O 7 NSK 2 Found 
Potassium, 23.21 per cent. 22.95 - 23.01 per cent. 
The neutral barium salt, was prepared and analyzed pre- 
viously, as was also the neutral silver salt. Neither contain 
water of crystallization. The acid barium salt crystallizes with 
two molecules of water. In addition to the foregoing, we have 
prepared the acid potassium salt and the lead salt. 
' Neutral Lead Salt. 
2-1 
4 5 SO3 
NO2 • CH3 • C6H2<^ ^Pb • 3 HsO 
COO 
This salt was made by adding slightly more than the calcu- 
lated amount of Pb(N 03)2 to ttie water solution of the potas- 
sium salt. It crystallizes quickly from moderately concentrated 
solution in fine crystals, which in some cases form leaf-like 
12 
