26 
white crystalline precipitate. From three to five crystallizations 
are carried out in precisely this manner, and in this way the phenol- 
phthalin is finally obtained in the form of a white crystalline com- 
pound entirely free from all traces of phenolphthalein. It may 
then be dried in the air at ordinary temperature or on the hot plate 
or in the oven at temperatures ranging from 50° to 80° C. In han- 
dling and drying the compound it is necessary to keep it out of con- 
tact with all metallic surfaces and also to prevent access of dust and 
impurities from the laboratory. When first prepared, the com- 
pound is perfectly white. It dissolves in alkali to form solutions 
which are perfectly colorless and which remain colorless for some 
time even after the addition of small amounts of hydrogen peroxide. 
When preserved in glass-stoppered bottles, however, or even in 
sealed tubes, for some time, phenolphthalin gradually acquires a slight 
yellowish-pink color, and on dissolving in alkali, such specimens show 
the characteristic color of phenolphthalein in alkali, in greater or 
less intensity, depending on the extent of the oxidation of the com- 
pound by atmospheric air. If preserved in a glass-stoppered bottle 
in a dark closet the compound may be kept in good condition for a 
month or even longer; indeed I have prepared satisfactory reagents 
for blood testing from specimens of phenolphthalin which had been 
preserved in the dark in cork-stoppered specimen tubes for nine 
months. Such reagents, however, were never entirely colorless, 
although the}^ were not sufficiently colored to interfere with their 
use in hematologic work. One of the great advantages of phenol- 
phthalin in all work of this kind is that we know we are dealing with 
a perfectly definite compound the purity of which can be deter- 
mined very simply and easily whenever desired, and which on oxi- 
dation passes into another equally definite compound the smallest 
amount of which can be readil}^ detected by means of alkali, and 
which may be determined very accurately by colorimetric methods. 
(1) Alkaline j^henolphthalin . — This reagent is prepared by dissolv- 
ing 0.032 gram of phenolphthalin in 21 c. c. of N/10 sodium hydrox- 
ide and adding sufficient water to make up the volume of the solu- 
tion to 100 c. c. This reagent may also be prepared by bringing 
together 1 c. c. of N/10 sodium hydroxide with somewhat more 
phenolphthalin than will dissolve in this quantity of alkali, diluting 
with 10 to 20 c. c. of water, filtering, and adding to the filtrate 20 c. c. 
of N/10 sodium hydroxide and sufficient water to make up to 100 
c. c. Since phenolphthalin itself is practically insoluble in water 
and since 1 c. c. of N/10 sodium hydroxide exactly neutralizes 0.032 
gram of the compound, we obtain a solution by the latter mode of 
procedure of the same concentration as that prepared by the first 
method, no weighing being required. 
