THE USE OF TEXTILE FIBERS IN MICROSCOPIC. 
QUALITATIVE CHEMICAL ANALYSIS 
Vv. THE DETECTION OF GOLD BY MEANS OF STANNOUS 
CHLORIDE-PYROGALLOL VISCOSE-SILK FIBERS * 
By Howarp Irvine Cote 
Chemist, Bureau of Science, Manila 
The reduction of a solution of gold chloride by a mixture of 
stannic and stannous chlorides leads to the formation of a red 
or violet color in the gold solution. This color is due to the 
precipitation of finely divided gold on the stannic hydroxide. 
It is called purple of Cassius because A. Cassius wrote a pam- 
phlet entitled De Auro, describing its preparation, in 1685. 
In 1904 Donau ’ applied this color reaction as a test for gold. 
He prepared silk fibers in the following manner: Raw-silk 
fibers were purified by treatment with 10 per cent sodium or 
potassium hydroxide, for from three to four hours, and then 
thoroughly washed. The fibers were treated with a dilute 
solution of stannous chloride and pyrogallol, washed several 
times with water, and dried between filter paper. Donau 
states that silk fibers thus prepared last two or three days, but 
if exposed longer to the air they become dark colored in the 
gold solution. 
He made the test for gold as follows: The gold was dissolved 
in aqua regia, and the solution was evaporated to dryness and 
taken up in water. A drop of this solution was placed on a 
slide and a fiber, treated as described above, was dipped vertic- 
ally into the drop from a tiny wax cone. The fiber was then 
examined under the microscope. Gold turned the fiber red or 
red-violet. 
In continuing the study of the use of textile fibers in micro- 
scopic qualitative analysis I have endeavored to make a test fiber 
for gold that would remain stable indefinitely and still be sen- 
sitive to the presence of a minute amount of gold. The prep- 
*For Parts I to IV see Journ. Ind. and Eng. Chem. 9 (1917) 969; 10 
(1918) 48. : 
*Donau, J., Monatsh, 25 (1904) 545. 
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