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QUANTITATIVE AGRICULTURAL ANALYSIS

(b) Iodide Method.—Sodium thiosulphate solution is prepared and
standardized as follows. Dissolve 19 gm of pure crystals of sodium thio-
sulphate and dilute to 1000 cc with recently boiled and cooled distilled
water. Mix well. One cubic centimeter should then be equivalent to about
0.005 gm of copper. Weigh in duplicate about 0.2 gm of pure copper foil,
place in 250-cc Erlenmeyer flasks and dissolve the copper by adding 10 cc of
a mixture of equal parts of water and concentrated nitric acid. Boil until
red fumes have been expelled, add 40 cc of water and 5 cc of saturated
bromine water, mix and boil until the bromine vapor has disappeared. Add
7 cc of ammonium hydroxide and boil again to expel excess of ammonia,
but not far enough to cause a precipitate. Add 4 cc of glacial acetic acid
(or 40 cc of 10-per cent acid), cool to room temperature and add 10 cc of
30-per cent potassium iodide solution. Immediately titrate with sodium
thiosulphate until the solution bearing the white precipitate shows only a
faint yellow tinge and then add 1 cc of starch indicator. (The starch
indicator is made by mixing 1 gm of starch with 1 cc of cold water, pouring
into this 100 cc of boiling water and boiling for a minute. This solution
should be made fresh each day, as required.) Continue the titration with
sodium thiosulphate until the blue color is discharged. Calculate the weight
of copper equivalent to 1 cc of the solution.

Drop into the Gooch crucible containing the cuprous oxide, 5 cc of warm
nitric acid (1:1) and cover the crucible. Collect the filtrate in a 250-cc
flask. Wash the crucible once or twice with hot water. Pour 5 cc of
bromine water into the crucible, then wash with 50 cc of hot water. Boil
filtrate and washings to expel bromine, then proceed from this point as
directed for standardizing sodium thiosulphate solution. Calculate tho
weight of copper present, from which the corresponding weight of dextrose
can be obtained by reference to table VII. Multiply by 6.025 (see page
159) and calculate the per cent of reducing sugars in the feed sample.

Approximate Volumetric Method.—Prepare:

Standard Invert Sugar Solution.1—Dissolve 4.75 gm of pure sucrose in
75 cc of water, add 5 cc of concentrated hydrochloric acid and let stand
at a temperature not below 20° for 24 hours, or for 10 hours if the tempera-
ture is above 25°. The solution should not be heated. Neutralize the
acid with 5-per cent sodium hydroxide solution (using methyl orange),
dilute to 1000 cc in a volumetric flask and mix well. Ten cubic centimeters
of this solution contain 0.050 gm of invert sugar and it should reduce
about 5 cc of the Fehling's copper solution. Standardize as follows:

Pipette 5 cc of each of Fehling's solutions (a) and (6) (page 158) into a
small casserole or beaker and add 10 cc of water. Heat to boiling and add,
from a burette, 9 cc of the standard invert sugar solution and boil for 2
minutes. This should reduce nearly all of the copper to cuprous oxide,
removing all but a faint blue color. Continue to add small portions of the
invert sugar solution, boiling after each addition. When the end is nearly
reached and the amount of sugar solution to be added can no longer be

1 For a discussion of the common sugars and of the process of "inversion''
of sucrose, see page 131, Part II.

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