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 (SnCl2.2H2O.) 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.c. of hydrochloric acid, and were diluted to 100 c.c. with water before titration.
Effect of Varying Hydrochloric Acid.
| Acid added | 1.0 | c.c. | 10.0 | c.c. | 20.0 | c.c. | 50.0 | c.c. |
| "Permanganate" required | 18.8 | " | 18.9 | " | 18.8 | " | 18.8 | " |
The only effect of the increase in quantity of acid was to give the brown of perchloride of manganese instead of the pink of permanganic acid to mark the finishing point.
Effect of Varying Temperature.
| Temperature | 15° C. | 50° C. | 70° C. | 100° C. |
| "Permanganate" required | 18.8 c.c. | 18.7 c.c. | 18.6 c.c. | 18.4 c.c. |
Rate of Atmospheric Oxidation.—Solutions ready for titration were exposed to air at the ordinary temperature for varying lengths of time and then titrated.
| Time exposed | 0 min. | 5 min. | 10 min. | 20 min. | 60 min. |
| "Permanganate" required | 18.8 c.c. | 18.8 c.c. | 18.8 c.c. | 18.8 c.c. | 18.6 c.c. |
It is best to titrate at once, although the loss by oxidation is only small after one hour's exposure.