158 QUANTITATIVE AGRICULTURAL ANALYSIS
only in the case of solutions of pure sugars. Molasses and
syrups usually contain colloidal organic matter which cannot be
washed out of the precipitate. It is then necessary to ignite in
air, when cuprous oxide is oxidized to cupric oxide and organic
matter is destroyed by oxidation.
Method (6) is an approximate volumetric one, differing from
method (a) in that a standard copper sulphate solution is used,
whose sugar equivalent is known.
In method (c) the cuprous oxide is removed and redissolved
and the copper is determined volumetrically by the " iodide"
method. Potassium iodide and acetic acid are added, cuprous
oxide being precipitated and iodine liberated:
2Cu(C2H302)2 + 4KI -»2CuI + 4KC2H3O2 + I2.
The free iodine is titrated with standard sodium thiosulphate
and the copper equivalent to it calculated.
It is also practicable to dissolve the cuprous oxide and to
determine the copper by electrolysis or by any other standard
gravimetric or volumetric method.
Asbestos.—Since this is to be used as a filtering medium for
strongly basic solutions it must be prepared with special reference
to removing base-soluble materials. The amphibole variety is
required, as serpentine asbestos is too easily soluble.
Determination of Sucrose and Reducing Sugars.—Prepare the following
materials:
(a) Fehling's Solution (1).—Dissolve 34.639 gm of pure crystals of copper
sulphate in distilled water and dilute to 500 cc. Filter, if not clear, through
asbestos.
(6) Fehling's Solution (2).—Dissolve 173 gm of sodium potassium tar-
trate ("Rochelle salts7') and 50 gm of sodium hydroxide in water and
dilute to 500 cc. Allow the solution to stand for two days and filter through
asbestos, if not clear.
(c) Neutral Lead Acetate.—Prepare a saturated solution of lead acetate
(the normal salt). This is made by warming 50 gm of lead acetate with
100 cc of water until the salt is dissolved, then cooling to room temperature.
(d) Asbestos.—Digest the fiber for three days with dilute hydrochloric acid.
Wash free from acid in a large funnel fitted with a perforated porcelain
plate, then digest for a similar period with 10-per cent sodium hydroxide
solution. Drain away this solution and then treat for two or three hours
with alkaline tartrate solution similar to solution (b), above described.
Wash practically free from base and then digest for several hours with dilute
nitric acid. Finally wash free from acid and shake the material to a pulp