Chemical Properties of Tin

Metallic tin remains bright in moist air at ordinary temperatures, but is oxidised when sufficiently heated in the air. Thus molten tin becomes covered with a grey film containing stannous oxide, which is gradually oxidised to cream-coloured stannic oxide. Tin also burns to dioxide when sufficiently heated in the air, and likewise decomposes steam at a red heat with formation of the same oxide. The following thermal values have been established by Mixter:

Sn + O₂ = SnO₂ (cryst) + 137,200 calories
Sn + 2Na₂O₂ = Na₂SnO₃ + Na₂O + 133,800 calories.

In the electropotential series of the metals tin stands just before hydrogen; consequently tin possesses little power of displacing hydrogen from dilute acids. Cold dilute hydrochloric and sulphuric acids have very little action on tin, but concentrated hydrochloric acid dissolves the metal with a fairly brisk evolution of hydrogen and formation of stannous chloride in solution. The rate of evolution of hydrogen is increased by contact of the tin with copper, silver, or platinum, owing to electric action. Hot concentrated sulphuric acid dissolves tin with evolution of sulphur dioxide and formation of stannic sulphate, the stannous sulphate first formed being oxidised at the expense of the sulphuric acid. Very dilute nitric acid gradually dissolves tin without evolution of gas, with the formation of stannous and ammonium nitrates; acid of density about 1.3 converts tin to a white powder, with evolution of oxides of nitrogen. This powder is hydrated metastannic acid (q.v.), probably formed by the decomposition of unstable stannic nitrate. Tin resembles antimony in its behaviour towards nitric acid, and this behaviour signifies the position which these metals occupy amongst the elements; that is, it shows them to be metalloids, intermediate between non-metals and metals. For representatives of these three classes of elements behave thus towards nitric acid:

Metal.	Metalloid.	Non-metal.
Soluble nitrate.	Insoluble hydrated oxide.	Soluble oxyacid.

Absolute nitric acid has no action on tin. Aqua regia dissolves tin, forming stannic chloride, provided too much nitric acid is not present; otherwise metastannic acid separates. Diluted organic acids act very slowly on tin in the presence of air. The metal dissolves in warm concentrated alkalis with formation of alkali stannate and evolution of hydrogen. Stannate is formed rather than stannite owing to the superior acidity of stannic tin; indeed a solution of stannite decomposes on concentration into stannate and metallic tin.

Tin forms two series of compounds in which the metal is bi- and quadri-valent respectively. No hydride is known to exist, but various stannic alkyls have been obtained, as well as other compounds containing hydrocarbon radicles, which elucidate the analogy between tin and carbon. Stannous oxide is sufficiently basic to form a sulphate and an unstable nitrate; stannic oxide forms a sulphate, but scarcely a nitrate, and it is characterised by the power to polymerise, forming β-metastannic acid and its derivatives.

Tin is tested for qualitatively by reactions in the dry way and in solution.

Tin compounds are reduced to the metal when heated on a carbonised match in the inner blowpipe flame, or when mixed with potassium cyanide and heated on charcoal before the blowpipe. The bead of metal thus obtained is white and malleable, but quickly becomes covered, when hot, with a film of white oxide. The metal may be identified by its behaviour with nitric acid; by fusing a particle of it into a borax bead coloured blue with a copper salt, when the bead becomes ruby-red; or by dissolving it in hydrochloric acid and adding mercuric chloride to the solution of stannous chloride thus formed, when a white precipitate of mercurous chloride, or a grey one of mercury, will be obtained. Tin compounds colour the Bunsen flame greyish blue, but give no spectrum in this way.

Stannous and stannic compounds are capable of numerous reactions in solution, which have been given in detail in the previous pages; therefore only those of analytical importance will here be noticed.

If a tin compound cannot be dissolved in hydrochloric acid, as, for instance, stannic oxide, it may be fused with sodium or potassium hydroxide; or, better, with a mixture of sodium carbonate and sulphur, which produces sodium thiostannate. After the excess of sulphur has been vaporised there remains a dark brown mass which will dissolve completely in water, yielding a yellow solution. From this solution dilute hydrochloric acid precipitates thiostannic acid, H₂SnS₃, which may then be dissolved in concentrated hydrochloric acid.

Stannous and stannic sulphides are precipitated from slightly acid solution by hydrogen sulphide, and so are brought down with the other metals of the second analytical group. Stannous sulphide, however, is rather easily dissolved by hydrochloric acid; and, therefore, unless the solution is well diluted, will be incompletely precipitated. Both sulphides, together with those of arsenic and antimony, are separated from the other sulphides of the group by reason of their solubility in alkali solutions. Stannous sulphide is, however, imperfectly dissolved by sodium hydroxide solution; but dissolves more readily in presence of a polysulphide, such as may be formed by heating the alkali solution with sulphur, or adding to it yellow ammonium sulphide. This is because stannous is thereby converted into stannic sulphide, which produces thiostannate.

If arsenic and antimony were originally present with tin their sulphides will all be present in the alkaline solution as thio-salts, whence they will be precipitated together by dilute acid. Various means are available for the separation and identification of these three metals. Arsenic may be separated from antimony and tin by either of two methods -

1. Boiling concentrated hydrochloric acid dissolves the sulphides of antimony and tin, but has no action on sulphide of arsenic.
2. Digestion with water and solid ammonium carbonate dissolves sulphide of arsenic, but not the sulphides of antimony and tin, which may be dissolved in hydrochloric acid after the removal of the arsenic.

The following methods are available for separating antimony and tin present in hydrochloric acid solution -

1. The acid solution is poured upon a piece of zinc resting on platinum foil in a dish; when by electrolytic action metallic antimony is deposited on the platinum, and tin upon the zinc. The deposited tin may then be dissolved in concentrated hydrochloric acid, and tested for, after diluting the solution, by means of mercuric chloride.
2. Antimony may be removed and tin left in solution by passing hydrogen sulphide gas after addition of oxalic acid, since the stanni-oxalic acid thus formed contains no tin ions to be precipitated by hydrogen sulphide. Tin may then be precipitated from the filtrate from the antimony sulphide by metallic zinc, or the oxalic acid may be destroyed by permanganate, and the tin then precipitated by hydrogen sulphide.
3. Sodium hydroxide is added to the solution containing tin and antimony until the precipitated hydroxides are redissolved; bromine water is then added to convert stannite and antimonite into stannate and antimonate, and this is followed by solid ammonium chloride. After the evolution of nitrogen by the action of hypobromite on ammonia, a precipitate of stannic hydroxide separates owing to the hydrolysis of ammonium stannate; and this, after boiling the liquid, may be filtered off, leaving antimonate in solution. The stannic hydroxide is then dissolved in hydrochloric acid, the solution reduced to the stannous state by iron wire, and the tin tested for by mercuric chloride.
4. Another method of detecting antimony and tin, which is very simple and satisfactory, consists in causing iron wire to react with the hydrochloric acid solution of the mixed chlorides. By this means metallic antimony is separated as a black powder, and tin is reduced to the stannous state, and may be detected, after the solution has been filtered, by the mercuric chloride test.

Tin may be estimated - (a) volumetrically, (b) gravimetrically, (c) electrolytically.

1. Tin when present in the stannous state is estimated volumetrically by titration with standard iodine solution. It is usual to add Rochelle salt to the solution, and then excess of sodium bicarbonate. The latter combines with the hydriodic acid formed so as to prevent the reversal of the reaction, and the Rochelle salt serves to retain the tin in solution as a complex tartrate. The results are apt to be low owing to atmospheric oxidation of the stannous salt, and it is, therefore, preferable at once to add excess of iodine and titrate back with thiosulphate. Tin may be estimated in acid solution by adding iodine in excess, and titrating back with dilute stannous solution of known strength; and also by adding excess of ferric chloride, which converts stannous into stannic chloride, and titrating with dichromate the ferrous iron produced. When tin is present in solution in the stannic state, the metal may be precipitated by zinc, then dissolved in hydrochloric acid, and the solution titrated with iodine.
2. Tin is estimated gravimetrically as dioxide, into which the metal is converted by the action upon it of nitric acid, followed by the ignition of the β-stannic acid so formed. When the tin is present in an alloy, the stannic oxide so obtained will not be pure. If copper and iron are present they may be separated by fusing the impure stannic oxide with sodium carbonate and sulphur, dissolving the fused mass in water, and reducing the solution with sodium sulphite, when the sulphides of iron and copper will be precipitated and may be filtered off and weighed. Instead of weighing the iron and copper sulphides it is permissible to reprecipitate the stannic sulphide from the filtrate, convert it into stannic oxide by ignition and weigh it as such. When the alloy contains antimony, this metal will be present as a thio- salt together with tin after fusion of the oxides of these metals with sodium carbonate and sulphur. The solution containing the mixed thio-salts is treated with caustic potash, hydrogen peroxide, and tartaric acid1 to convert thio- into oxy-salts and retain the latter in solution. Oxalic acid is then added, followed by hydrogen sulphide. By this means tin is kept in solution, whilst the antimony is precipitated as the pentasulphide. It is then usual to estimate the tin in the filtrate electrolytically.
3. Tin, when present in solution as stannous chloride, may be estimated electrolytically by the method of Engels, by adding to the solution hydroxylamine to prevent oxidation, together with tartaric acid and ammonium oxalate, and passing a current of electricity through the warm solution. It is difficult, however, to remove all the tin.

Classen separates antimony electrolytically from a solution of the sulphides in concentrated sodium sulphide, tin not coming down if the current is weak.

Sand has carried out the rapid electrolytic estimation of tin, and its separation from antimony. The pure metal was dissolved in concentrated sulphuric acid, and the solution was diluted and nearly neutralised by ammonia, oxalic acid being added to keep the stannic tin in solution. From this solution, by the use of rotating platinum electrodes, about 0.3 gram of tin was completely separated in thirty minutes or less at a temperature of 70°-100° C., with a current of 3 to 5 amperes and 2 to 4 volts. In the solution was old, and therefore contained some β-stannic compound, the separation was more difficult. The separation of antimony from tin in an alloy such as type-metal has been carried out successfully by the use of a graded potential. The alloy was dissolved in a mixture of nitric and sulphuric acids; the greater part of the nitric acid was expelled by heating, and the remainder destroyed; and then the antimony was reduced to the antimonious state by hydrazine sulphate. Finally, the antimony was separated at a limited potential from the sulphuric acid solution, leaving the tin behind.

Tin may also be estimated electrolytically after precipitation as sulphide by dissolving the precipitate in ammonium sulphide solution, adding sodium sulphite, and electrolysing the solution.

The electrochemical behaviour of tin has been studied by Foerster and Yamasaki.