28 
BULLETIN 1460, U. S. DEPABIMENT OP AGHICULTTJKE 
Gram dis- 
Pressure 
Per cent 
HCL 
tillate for 1 
mol of 
HCL 
770 
20. 218 
180. 39 
760 
20.242 
180. 17 
750 
20. 266 
179. 96 
740 
20. 290 
179. 74 
730 
20. 314 
179. 53 
720 
20. 338 
179. 31 
For correcting weights to vacuum (brass weights), add 0.17 gram for each mol of HCL. 
If no barometer is available, the barometric pressure may be ob- 
tained by calling up the local weather office. It is to be noted, 
however, that an error of several millimeters would effect no very 
appreciable error in the final solution. 
Only pure C0 2 -free distilled water should be used for diluting 
the constant boiling acid to the desired strength. The water need 
not be boiled, but may be satisfactorily purified by aspirating it with 
air which first passes through soda-lime, until 1 drop of N/10 NaOH 
added to 200 cubic centimeters produces a distinct pink color in the 
presence of a few drops of phenolphthalein. 
If these directions are all carefully followed one can not fail to 
obtain exact solutions, providing, of course, that the weights and 
measuring apparatus are accurate. The method is not only ex- 
tremely accurate, but requires relatively little time and attention 
and does away with the necessity of relying upon purity of reagents. 
(c) Standard Sulphuric Acid. — Determine the absolute strength 
of the acid by precipitation with barium chloride solution as fol- 
lows : Dilute a measured quantity of the acid to be standardized to 
approximately 100 cubic centimeters, heat to boiling and add drop 
by drop a 10 per cent solution of barium chloride until no further 
precipitation occurs. Continue the boiling for about five minutes, 
allow to stand for five hours or longer in a warm place, pour the 
supernatant liquid on a tared Gooch crucible or an ashless filter, 
treat the precipitate with 25 to 30 cubic centimeters of boiling water, 
transfer to the filter and wash with boiling water until the filtrate 
is free from chloride. Dry, ignite over a Bunsen burner, and weigh 
as barium sulphate. 
(d) Standardization by direct titration. — Laboratories which in- 
terpret protein analyses in terms of the standard alkali, will find that 
standard alkali solutions can be accurately made up by the use of 
potassium acid phthalate or benzoic acid as has been shown by 
Hendrixson. 21 
(e) Standard alkali solution, — Accurately determine the strength 
of the standard alkali solution by titration against standard acid, or 
by the use of potassium acid phthalate or benzoic acid. Standard 
alkali solutions should not be greater than N/8 in strength. 
(/) Sulphuric acid. — The specific gravity of the acid should be 
1.84 and should contain 95 per cent or more sulphuric acid. The acid 
should be free from nitrates and ammonium salts. 
(g) Mercury. — Red oxide of mercury is preferable, and should be 
free from nitrates. 
21 See footnote 18. 
