ACID POTASSIUM AND ACID SODIUM PHTHALATES 223 
and conditions of crystalliziation. The titrations showed a varia- 
tion of about 1 part in 700 too low to 1 part in 700 too high. 
The hydrous salt belongs to the same class as other hydrous 
substances used as standards such as oxalic acid and potassium 
tetroxalate. In this connection it may be stated that in the 
course of this study the writer prepared with great care samples 
cf oxalic acid and potassium tetroxalate and made many titra- 
tions. The results were always too low. In the case of the tet- 
roxalate the difference between the acidity found and the the- 
ory was so marked as to suggest some other reason than hygro- 
scopic water. The potassium contained in the salt was therefor 
determined. Two determinations as sulfate and one as carbon- 
ate gave the per cents, 15.74, 15.77 and 15.73, while the theory 
requires 15.38. While not a perfect standard, hydrous acid 
isodium phthalate will give much more nearly correct results than 
either oxalic acid or potassium tetroxalate according to my ex- 
perience. For this purpose, however, the salt should be de- 
hydrated. 
Dehydrated Acid Sodium Phthalate. Before the acid sodium 
phthalate was dehydrated for use in titration it was subjected to 
tests to ascertain its degree of stability when heated. A portion 
of it in a boat was heated in a tube through which passed a 
current of pure air, and the air then passed through, a bottle 
of clear baryta water. The heat was gradually raised to 225°, 
the time occupying about two hours. No trace of carbon dioxide 
could be detected but a small amount of phthalic anhydride sub- 
limed above 200°. The salt may, therefore, safely be heated 
much higher than the necessary temperature to dehydrate it 
within reasonable time, which is 120°. It was heated at that 
temperature and attained constant weight in about three hours, 
though the heating continued much longer. It was then used in 
the following titrations: 
Weight of NaOH 
Weighed acid 
sodium phthalate. 
Calculated HCl in 
1 gram of solution 
of HCl. 
1. 
42.754 
.9548 
.0036524 
2. 
43.610 
.9737 
.0036515 
8 . 
52.897 
1.1824 
,0036557 
4. 
51.337 
1.1470 
.0036540 
5. 
61.626 
1.3758 
.0036511 
6 
77.619 
1.7354 
.0036565 
7. 
51.479 
1.1494 
.0036516 
Average of seven titrations 0036533 
