i : 64 QUANTITATIVE AGRICULTURAL ANALYSIS
1
Solubility.—The solubility of calcium oxalate in water at
I ordinary temperatures is about 0.0050 gm per liter, expressed
1 I , ' as the anhydrous salt, this containing 0.0016 gm of calcium.
I j A slight excess of ammonium oxalate diminishes the solubility, as
I explained on page 24, so that the recovery is very good. It
! , is necessary to precipitate from hot solutions in order to avoid the
f ; , formation of very fine crystals.
I j Purity of Precipitate.—Examination of physical data will
? ; show that oxalates of all of the alkaline earth metals and of the
1 * heavy metals have comparatively small solubilities in water.
I If any of these metals are present it will therefore be necessary
i to effect a preliminary separation before calcium can be pre-
t ', < cipitated and recovered as pure oxalate. This will be given due
1 attention later in the work but in the following exercises calcium
I is assumed to be the only metal present, with possible exceptions
of the alkali metals.
!
! Determination of Calcium: Gravimetric Method.—From a closed weighing
I { bottle or on counterpoised glasses (according to the nature of the sample)
f weigh accurately two portions of about 0.2 to 0.4 gm of the prepared cal-
| cium compound, placing in 200-cc Pyrex beakers. Add 75 cc of water and
I 5 cc of a 10-per cent solution of ammonium chloride, the latter to prevent
j the precipitation of possible traces of magnesium.
I If the calcium salt contains carbonate it will not be completely dissolved
| in water. In this case do not add ammonium chloride but provide cover
f ' glasses for the beakers and add 10 cc of dilute hydrochloric acid. Calcium
§ carbonate will dissolve with effervescence. The covered solution is then
| boiled for a few minutes to expel carbon dioxide. Now remove the covers
| , and rinse them and the upper portions of the beakers with a jet of distilled
water, allowing all of the rinsings to run back into the beakers. Dilute to
about 75 cc.
Having obtained a solution by either method add 15 cc of ammonium
hydroxide (5-per cent ammonia). A distinct odor of ammonia should be
perceptible after blowing away the vapors above the liquid. Heat nearly
to boiling and add, from a pipette, a recently prepared saturated solution
of ammonium oxalate, drop by drop and with constant stirring. Ten to
15 cc of solution may be required. Digest on the steam bath until the
precipitate settles and test the solution above by adding another drop of
oxalate solution.
When precipitation has been completed, filter on a paper of medium
density and wash precipitate and paper with hot water until the washings
test free from chlorides, as determined by allowing the washings to fall into
a test tube containing silver nitrate acidified with nitric acid. Finally drain
as well as possible, remove the paper from the funnel and fold as shown in