2CuCl₂ + 4KI = Cu₂I₂ + 4KCl + I₂.
The analytical process is carried out as follows: In a 100 cubic
centimeter flask are boiled fifty cubic centimeters of the copper
solution, ten cubic centimeters of about one-tenth per cent reducing
sugar solution are added, the boiling continued for five minutes,
the flask filled to the mark with boiling water and its contents
filtered. Fifty cubic centimeters of the hot filtrate are cooled,
slightly acidified, potassium iodid solution added in slight excess;
and the iodin set free determined by titration with sodium thiosulfate.
The quantity of iodin obtained corresponds to the unreduced copper
remaining after treatment with the reducing sugar. The number of cubic
centimeters of thiosulfate used subtracted from twenty-five will give
the number of cubic centimeters of the copper solution which would be
reduced by five cubic centimeters of the sugar solution used.
_Example._—In the proportions given above it was found that eleven
cubic centimeters of thiosulfate were required to saturate the iodin
set free. Then 25 - 11 = 14 cubic centimeters of copper solution
reduced by five cubic centimeters of the sugar solution. Since one
cubic centimeter of the copper solution is reduced by 0.0036 gram of
dextrose the total dextrose in the five cubic centimeters = 0.0036 × 5
= 0.0180 gram.
The above method does not seem to have any practical advantage over
those based on noting the disappearance of the copper and is given only
to illustrate the principle of the process. While the titration of the
iodin by sodium thiosulfate is easily accomplished in the absence of
organic matter, it becomes difficult, as shown by Ewell, when organic
matters are present, as they always are in the oxidation of a sugar
solution. Ewell has therefore proposed to determine the residual copper
by a standard solution of potassium cyanid, but the method has not yet
been developed.[104]