The precipitated tin is transferred to a flask; and dissolved in 10 c.c.
hydrochloric acid, with the aid of heat and in an atmosphere of carbonic
acid. The acid and metal are placed in the flask; which is then filled
with the gas, and stopped with a cork provided with a rubber valve. When
solution is complete the flask is again filled with carbonic acid. Fifty
c.c. of water freed from air and saturated with carbonic acid are then
added. This water is made by adding a gram of bicarbonate of soda and 2
c.c. of hydrochloric acid to 100 c.c. of water: the effervescence sweeps
out the dissolved oxygen. The permanganate of potassium solution is then
run in from a stop-cock burette in the usual way until a faint pink
tinge is obtained.
The following experiments show the effect of variations in the
conditions of the assay. A solution of stannous chloride equivalent in
strength to the "permanganate" was made by dissolving 19.06 grams of the
crystallised salt (SnCl_{2}.2H_{2}O.) in 50 c.c. of water and 10 c.c. of
hydrochloric acid and diluting to 1 litre with water freed from
dissolved oxygen. 100 c.c. contain 1 gram of tin. In the first
experiments tap water was used and no precautions were taken for
excluding air. Except when otherwise stated, 20 c.c. of the stannous
chloride were used in each experiment with 10 c.
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