68 QUANTITATIVE AGRICULTURAL ANALYSIS
the equation. However, it must be considered in calculating the
equivalent weight of the salt, since it is weighed along with the
ferrous sulphate. The same is true of water of crystallization.)
For standardizing by calcium carbonate the latter is weighed
and treated according to the principles discussed on page 65,
the value of the permanganate solution being calculated from
the volume of solution found to be equivalent to this weight.
The equivalent weights of all of these substances may be
calculated by the methods illustrated in the preceding discussion.
Preparation of Solution.—Prepare 1200 cc of a solution, either tenth-
normal or of such concentration that 1 cc is equivalent to 0.002 gm of
calcium, as the instructor may direct, as follows: Weigh to centigrams on
counterpoised glasses, 1 per cent more than the calculated amount of the
best grade of potassium permanganate obtainable, brush this into a glass
stoppered bottle and add 1200 cc of water. Agitate until the salt is
thoroughly dissolved and the solution is well mixed. Place the bottle out
of bright light for 24 hours then decant through a Gooch filter or an alundum
crucible into a cleaned flask or bottle, using a pump. The solution must not
be allowed to come into contact with rubber. Rubber stoppers used during
the filtration process should first be well washed to free them from loose
material. Do not attempt to recover the last portions of solution remaining
in the bottle.
Standardization.—Use one of the following methods.
(a) With Sodium Oxalate.—Weigh two or three portions of about 0.2 gm
each of sodium oxalate of known purity (a Bureau of Standards sample
if this is available) into 250-cc Erlenmeyer flasks. Dissolve in 50 cc of
recently boiled and cooled distilled water and add 10 cc of dilute sul-
phuric acid. Place a thermometer in the flask, warm to 90° and titrate with
permanganate solution, stirring vigorously and continuously. The per-
manganate must not be added more rapidly than 15 cc per minute and the
last cubic centimeter must be added drop-wise, with particular care to allow
the color from each drop to disappear before the next drop is added. When
a final permanent pink is obtained observe the volume of solution required
and calculate (a) the normality of the permanganate solution and (6)
the weight of calcium equivalent to each cubic centimeter, referring to the
discussion on page 67.
(6) With Ferrous Ammonium Sulphate.—Use accurately weighed portions
of about 1 gm of the pure crystallized salt. Titrate as for sodium oxalate
except that the solution is not to be heated and the titration may be carried
out more rapidly, the reaction being nearly instantaneous. The experiment
must be completed immediately after dissolving the iron salt, as otherwise
oxidation by air will vitiate the results.
(c) With Calcium Carbonate.—Use a dried sample in which calcium has
been determined gravimetrically as directed on page 64. The method to be