The oxide of tin is placed in a porcelain boat (fig. 58), which is then introduced into a piece of combustion tube. The latter, wrapped in a piece of wire gauze, is supported on a couple of iron rings, and heated by one or two Bunsen burners in a furnace fitted up with loose fire-brick tiles, as shown in fig. 59.
When the reduction is complete the tube is allowed to cool; the boat is removed and the tin dissolved. Add a rod of zinc to the freely-acid hot solution, and in a few minutes decant through a filter and wash with water, after having removed the zinc. Wash the precipitated metal back into the beaker, and dissolve in 10 c.c. of dilute nitric acid, evaporate off the excess of acid; dilute, boil, and filter. Wash, dry, ignite strongly in a porcelain crucible, and weigh.
In the absence of antimony the above separation works very well, but if this metal is present in quantity the metals precipitated on the zinc must be covered with hydrochloric acid and treated with a few drops of nitric. It is then warmed with iron wire until no more of the latter dissolves. The antimony is precipitated as metal, and the tin remains in solution as stannous chloride. The antimony is filtered off, and may be washed with alcohol, and weighed, whilst the tin in the filtrate is precipitated with zinc, and treated as already described.
GRAVIMETRIC METHOD.
If the tin is not already in the metallic state it is reduced to this condition by the method given (precipitation by zinc). Treat the finely-divided metal (washed free from chlorides) in a four-inch evaporating dish with 10 c.c. of dilute nitric acid, cover with a clock-glass, and apply a gentle heat until the precipitate appears of a white colour and the metal is completely attacked. Evaporate nearly to dryness on a water-bath; then add 50 c.c. of water, heat to boiling, and filter. Wash with hot water, dry, transfer to a weighed porcelain crucible, add the filter-ash, ignite strongly, and weigh. The precipitate after ignition is stannic oxide (SnO2). It is a yellowish-white powder (darker whilst hot), insoluble in acids, and contains 78.67 per cent. of tin. Cold dilute nitric acid dissolves tin to a clear solution, which becomes a white enamel-like jelly on heating; this (filtered off, washed, and dried) forms an opal-like substance, which is converted on ignition into stannic oxide with evolution of nitrous fumes. Stannic oxide when ignited with chlorides is more or less completely converted into stannic chloride, which volatilises. The presence of chlorides during the evaporation with nitric acid causes a similar loss.
Determination of Tin in an Alloy.—(Bronze.)—Take 2 grams, and attack with 20 c.c. of dilute nitric acid in a covered beaker with the aid of heat. Boil till the bulk is reduced by one-half, dilute with 50 c.c. of water, allow to settle for a few minutes, and filter; wash well first with water acidulated with a little nitric acid, and afterwards with water; dry, ignite, and weigh as stannic oxide.
Determination of Tin in Tin Ore.—Treat 5 grams of the dried and finely-powdered ore with 30 c.c. of hydrochloric acid in a four-inch evaporating dish. After the soluble oxides have been dissolved add 1 or 2 c.c. of nitric acid, boil off nitrous fumes, dilute, and filter. Dry the filter, transfer the cleaned ore to a piece of combustion tube ten or twelve inches long and narrowed at one end. Pass a current of hydrogen through the tube and heat to redness for 30 minutes; cool whilst the gas is still passing. Dissolve in 20 c.c. of dilute hydrochloric acid and keep the solution tinted with permanganate of potassium. When the colour of the permanganate becomes permanent dilute to a bulk of 50 c.c. with water, filter, and wash. Heat; add a rod of zinc weighing about 3 grams; allow to stand for a few minutes; decant through a filter; and wash, removing the remaining zinc and returning the tin to the beaker. Treat with 5 c.c. of dilute nitric acid, boil for some time, take up with water, filter, wash, dry, ignite, and weigh as stannic oxide.
VOLUMETRIC METHOD.
Titration with Solution of Permanganate of Potassium.—This titration may be made either directly on the solution of stannous chloride (prepared by dissolving the precipitated metal in hydrochloric acid), or indirectly, on a solution of ferrous chloride (produced by the reducing action of the precipitated metal on ferric chloride). The standard solution of permanganate of potassium is made by dissolving 5.356 grams of the salt in water and diluting to one litre. 100 c.c. are equivalent to 1.00 gram of tin.