Metastannic acid

Anhydrous tin(IV) oxide ( Superlite grade) and 6-stannic acid ( Metastannic acid ) were supplied by Keeling Walker Ltd., Stoke-on-Trent. 6-stannic acid paste was prepared at Chinghall Ltd., Milton Keynes, by dispersing Keeling Walker s Metastannic acid, at a level of 73% in a phthalate plasticiser. Colloidal tin oxide, a 25% aqueous dispersion of Sn02> was supplied by Nyacol Products Inc., Ashland, Mass., U.S.A. 

Anon. Tin Oxide and Metastannic Acid. Keeling Walker Ltd. Data Sheets, Stoke-on-Trent, U.K., 1987. 

WET—Sn sample diss. in HN03 + metastannic acid pptd. 

Alloys of copper and tin such as the phosphor bronzes are dissolved in nitric acid of density 1-5, and the metastannic acid, which contains all the phosphoric oxide, after ignition and weighing is fused with KCN. The aqueous solution of the melt is freed from tin globules by filtration and from traces of soluble copper and tin by H2S, then containing all the phosphorus as potassium phosphate, which is determined as described below. 

With concentrated nitric acid a vigorous reaction occurs, a white solid, usually formulated as hydrated tin(IV) oxide Sn02. xH20 and sometimes known as metastannic acid, being produced ... 

In the presence of antimony and tartaric acid tin dissolves readily in nitric acid (induced dissolution) because of complex formation. If larger amounts of iron are present, the formation of metastannic acid is again prevented. 

Stannous chloride has the formula SnCl2, but the formula of metastannic acid varies with the temperature at which it is dried. 

Stannic acid readily condenses with itself to form large molecules, in which the octahedra of oxygen atoms surrounding tin atoms are linked together, by having oxygen atoms serve as the corners of two octahedra. The condensed acid H SnOg, called metastannic acid, is a white substance insoluble in water and dilute alkalis, but slowly soluble in hot concentrated sodium hydroxide solution. 

Properties White to light-tan crystals. Soluble in water insoluble in alcohol decomposes in air aqueous solution slightly alkaline loses 311,0 at 140C. Derivation (1) By fusion of metastannic acid and sodium hydroxide, (2) by boiling tin scrap and sodium plumbate solution. 

Tin (Sn, at. mass 118.69) occurs in its compounds in the II and IV oxidation states. Tin(II) is unstable. The hydroxide, Sn(OH>2, precipitates at pH 2 but is amphoteric, redissolving in NaOH (pH 13) to form stannite. Tin(II) forms oxalate and chloride complexes. Tin(fV) hydrous oxide (metastannic acid) precipitates even at pH 0.5 and redissolves at pH 9 as stannate. Tin(IV) gives stable halide-, oxalate-, and tartrate eomplexes. 

Fropebties.—A BO malleable, bluish-white metal but slightly tenacious emits n peculiar sound, tbe tin-cry, when bent A good conductor of heat and electricity. Air affects it but little, except when it is heated more rapidly if Sn be alloyed with Pb. It oxidizes slowly in H,0, more rapidly in the presence of sodium chloride. Its presence vrith Pb accelerates action of H,0 upon the latter. It dissolves in HCI as SnCl,. In presence of a small quantity of H,0, HNO, converts it into metastannic acid. Alkaline solutions dissolve it as metastannates. It combines directly with Cl, Br, I, S, P, and Aa... 

Metastannic Acid.— HjSnsOn—766.5—is a white, insoluble pow-[Pg.172]

Ferric iron will oxidise tin to Sn", which will precipitate in the leaching stage as metastannic acid (1148004) and is hence separated with the leach residue. It is therefore possible to process smelter bulhon directly without softening, although decopperising will still be beneficial to provide a separate copper-rich stream, to reduce the quantity of mixed leach residue for further processing and to minimise interference with subsequent silver recovery. 

This flame color is specific for tin. The test may be carried out on a single drop of the solution, in which case the reducing power of the gases of the flame is sufficient. The presence of arsenic, if more than equivalent to the amount of tin, causes this very characteristic reaction to fail. Hydrated metastannic acid, prepared in the wet way and ignited, responds decisively to this test, but native tinstone (cassiterite) does not. 

The tinned surface to be tested is washed free of grease with alcohol and ether. Then a drop of concentrated nitric or acetic acid is placed on it, and the specimen carefully heated until the acid has evaporated. The gray fleck that remains may contain lead nitrate or acetate as well as metastannic acid. It is treated with a drop of 5 % potassium iodide solution. In the presence of lead (down to 1 %) yellow needles of lead iodide are formed. A slight yellow-green tint indicates the presence of not more than the small amounts of lead which are always present in commercial tin. 

Two sorts of stannic acid are known in experimental, and also in analytical chemistry. They have been called a and p, or ordinary and metastannic acid. The first modification is prepared according to the well-known method of treating a solution of stannic chloride with a limited amount of alkali the second is made from the first by different processes. The P form is the more stable because the a form always turns to the 3, or to a product intermediate between a and p. The latter may be made directly by the action of nitric acid on tin. In the water content the two modifications do not differ materially, as shown in van Bemmelen s article. ... 

Not only are the reactions of metastannic acid partially hidden by the mixture, but also those of iron oxide. If sufiKcient stannic acid is in the precipitate, ammonia will cause peptisation. This is contrary to the behavior of iron oxide, of course, because the latter is thrown out of solution by ammonia. 

Whatever the mechanism of formation, however, it is essential to recognize the existence of two forms of stannic acid, and to realize that if the acid -has been precipitated under conditions where metastannic acid is the predominant form, subsequent dissolution and treatment of the tin sample might be fairly difficult. 

It should be emphasized once again, however, that conditions under which metastannic acid is formed are usually avoided like the plague in radiochemical separation procedures. - If it is formed in a carrier-free solution. before tin carrier is added, the isotopic exchange problem can be formidable. 

If it is used for quantitative precipitation of tin, care must be taken that all complexing and interfering ions are absent, or precipitation will not be complete. If its precipitation is intended as a separation step, Sb, Te. Nb, and the other acid-insoluble elements must be expected to coprecipitate with the tin. If it is once precipitated, it is di icult to wash because of its marked tendency to peptize. And if it is washed effectively without peptizing, it is difficult to redissolve. It is possible to dissolve metastannic acid by boiling... 

The chief occurrence of tin in ores is as cassiterite, SnO, and less of-ten in combination with sulfur and sulfides of other metals. Dissolution of these ores, or any other tin sample, for that matter, is complicated by the fact that tin hydrolyzes to metastannic acid in neutral and most acid solutions, and that tin halide compounds are appreciably volatile. 

There are two common oxidation states, 11 and IV, in tin, and Sn is of more importance in the sol-gel ceramic field. Sn + ions are easily hydrolyzed and stannic acid forms. By low temperature and slow hydrolysis of Sn salts, an amphoteric of-starmic acid is formed, which is soluble in aqueous solutions of acids and bases. The high temperature and rapid hydrolysis of 80 + salts, as well as aging of a-stannic acid , results in -stannic acid or metastannic acid , which is insoluble in solutions of ordinary acids and bases. Both a- and j3-form are rutile-type crystal structure with absorbed water. And it is easy to be dehydrated by heating around 100°C to form crystalline SnOi, cassiterite. The difference between a- and 8-form was attributed to the particle size, though the discussion is open (Bailar et al., 1973). The aqueous solutions were mainly ai lied for fabrication of thin films, such as electrically conductive and optically transparent films doped with Sb + or Nb +, and gas sensing films. 

Concentrated nitric acid produces the hydrated tin(IV) oxide sometimes called metastannic acid. Tin evolves hydro n with hot alkali solutions to form alkali stannates. 

Tin(IV) hydroxide is not known. Hydrolysis of tin(TV) salt solutions produces a white voluminous precipitate, which is not detectably crystalline, and may contain bonded water in a tin oxide gel. The freshly prepared material is called a-stannic acid, and is easily soluble in acid. Ageing produces -stannic acid (or metastannic acid), which is more inert in acids, and has also developed an X-ray diifraction pattern characteristic of tin(IV) oxide. This metastannic acid may be obtained directly from tin and concentrated nitric acid. It remains open whether the differences between a- and /3-stannic acids are solely due to differences in particle size. 

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