52 QUANTITATIVE AGRICULTURAL ANALYSIS

Volumetric, by Titration with a Standard Solution of Silver
Nitrate. — Chlorine of inorganic chlorides may be titrated very
accurately by a standard solution of silver nitrate, potassium
chromate serving as indicator. The solubility of silver chloride
is so much less than that of silver chromate (1000 cc of water at
20° dissolves 0.0015 gm of silver chloride and 0.024 gm of silver
chromate) that the latter exists permanently only after the
chlorine has been practically completely precipitated. Its
intensely purple color then serves as an indicator of the end
point of the reaction with chlorides:

AgN03 + MCI -> AgCl + MN03; (1)

(2)

This method may be employed to determine the chlorine of
chloride solutions but the latter must be neutral before the titra-
tion can be made.
' Equation (1), above, shows that the hydrogen equivalent of
silver nitrate is 1 and its equivalent weight is therefore the same
as its molecular weight. A tenth-normal solution (see page 7)
; will then contain 16.989 gm of the salt in each liter. Instead of
this the solution may be made in the decimal system (page 8),
1 each cubic centimeter being equivalent to some simple weight of
chlorine. If n is used to indicate this required weight of chlorine,
> u r, * i *• , * • 1000 X 169.89 n
* each liter 01 solution must contain - — -— - cm Of silver
I 35.4o
\ nitrate.
\
(
* Volumetric Determination: Silver Nitrate Method. — Prepare the follow-
I ' ing solutions:
I (a) Silver Nitrate. — Calculate the weight of silver nitrate necessary to
make 1200 cc of a solution, either tenth-normal or of such concentration
that 1 cc is equivalent to 0.005 gm of chlorine, adding 1 per cent for
possible impurities. Weigh to centigrams on counterpoised glasses and dis-
solve in chloride-free distilled water. Dilute to 1200 cc, mixing the diluted
solution very thoroughly.
(6) Sodium Chloride. — Prepare 500 cc of a solution of pure sodium
chloride which has been powdered and dried at 105°. The concentration
should be equivalent to that desired for the silver nitrate solution. The
equivalent weight of sodium chloride being its molecular weight (58.46),
a tenth-normal solution must contain 5.846 gm of the salt in each liter.