216 MICROSCOPIC AND CULTURAL STUDY OF BACTERIA 
Preparation of Standards from pn 7.8 to ph 10.0. 
Cc. M H3BO4, 
Water 
• to make a total 
m'kCI. 
Cc. M NaOH. 
volume of 100 cc, cc. 
PH. 
' 25 
1.30 
73.70 
7.8 
25 
1.99 
73.01 
8.0 
25 
2.95 
72.05 
8.2 
25 
4.25 
70.25 
8.4 
25 
6.00 
69.00 
8.6 
25 
8.15 
66.85 
8.8 
25 
10.65 
64.35 
9.0 
25 
13.35 
61.65 
9.2 
25 
16.00 
61.00 
9.4 
25 
18.43 
56.57 
9.6 
25 
20.40 
54.60 
9.8 
25 
21.95 
53.05 
10.0 
2. Pure, disodium hydrogen phosphate containing two molecules of 
water of crystallization. The C. P. crystalline salt (Xa2HP04. 12 HoO) 
is exposed to the dry air of a warm room for about two weeks in a 
thin layer protected from dust. Every day the salt is turned to 
expose it uniformly to the dehydrating process.^ 11.876 gm. of the 
salt so prepared are dissolved in 1000 cc. of conductivity water. 
f Boric acid. Potassium chloride. 
12.405 gm. of recrystallized C. P. boric acid, and 14.912 gm. C. P. 
recrystalliz&d potassium chloride are dissolved in 1000 cc. of conduc- 
tivity water. 
§ NaOH solution, free from CO2 (see Clark-), kept in a paraffined 
bottle, shielded from the air by a soda-lime tube. 
Apparatus.— A series of glass test-tubes of uniform bore (1 cm.) and 
thickness of. wall, free from color, are carefully cleaned and set apart 
for hydrogen-ion determinations. Standard sets of pH solutions, 
each containing the proper amount of indicator (and 10 cc. of solu- 
tion) are set up and protected from evaporation by paraffined cork 
stoppers. When not in use they should be kept in a dark place. 
Some of the indicators are somewhat unstable; the series should be 
checked every few days, and fresh standards prepared whenever 
necessary. 
A Comparison BIock.^—A block of wood or a rack, containing places 
for the standard tubes in the front row and a corresponding space 
for tubes in the back row, so arranged that the color of the standards 
and of the solution under investigation may be viewed by a light 
passing through correspondingly colored solutions of media is of 
great advantage. The holes and entire block or rack should be 
painted dead black. 
The Test.— The medium under examination, containing the proper 
amount of indicator, is placed in one of the front holes, with a tube of 
distilled water in the hole immediately back of it. Light passes 
^ The loss in weight should be checked; the process is complete when two weighings 
on successive days agree within 0.1 per cent. 
' Loc. cit., p. 71, for details of preparation. 
■■' Walpole: Biochem. Jour., 1911, 5, 207; 1914, 8, 628. 
