July, 1931
INDUSTRIAL AND ENGINEERING CHEMISTRY
831
Preparation of Stannous Sulfate' F. C. Mathers and H. S. Rothrock INDIANAUNIVERSITY, BLOOMINGTON. IND.
The aim of this research was to prepare stannous TAKNOUS sulfate baths Use of Tin Foil sulfate for use in electroplating and refining baths. are best for the electroOrdinary metallic or feathered tin dissolves only in Tin foil and hot, rather conplating and refining of hot concentrated sulfuric acid under which conditions centrated sulfuric acid formed tin (2). S t a n n o u s fluoride, too much stannic sulfate is formed for plating baths. stannous sulfate, a reaction perchlorate, fluosilicate, and Fifteen parts of tin foil or of tin, electrodeposited in impossible w i t h o r d i n a r y fluoborate can be used for feathered tin. Many experia spongy or loose condition, dissolves readily in 40 these electrolytic baths, but ments were made with varyparts of 75 per cent sulfuric acid with formation of these salts are as difficult to ing p r o p o r t i o n s of tin foil, stannous sulfate free from stannic sulfate. A final prepare as the sulfate and are temperature of 140' C. is necessary. sulfuric acid, and water and m o r e expensive. Stannous v a r y i n g temperatures and Feathered tin tumbled out of 50 per cent sulfuric salts cannot be prepared by periods of heating. Fifteen acid into air and then back into acid is changed rapidly the treatment of ordinary tin grams of tin foil, 30 grams of to stannous sulfate at room temperature. The tumwith any of the above acids. bling prevents the adherence of insoluble stannous sulsulfuric acid, and 10 grams of Stannous chloride baths, alfate to the surface of the tin and the exposure to the water h e a t e d slowly for 2 though the least expensive, air depolarizes the hydrogen. hours to a final temperature of cannot be used for electroplatThe stannous sulfate produced by these processes is 140" C. gave a thick paste of ing, as they yield only loosely stannous sulfate mixed with mixed with su5cient sulfuric acid for the preparation crystalline cathode deposits. the excess of sulfuric acid. of tin-plating baths. Stannous sulfate has never This paste dissolved comThe method to be chosen for making stannous sulfate been marketed in commercial pletely and quickly in cold depends upon the materials and apparatus available. quantities; hence electroplatwater and g a v e a s l i g h t l y The use of tin foil or of the finely divided electrolytic ers and refiners of tin have tin is the most convenient for small quantities. The black b u t clear s o l u t i o n . been forced to devise special The black color was due probtumbling in sulfuric acid is probably best if large schemes for making and mainquantities of stannous sulfate are wanted. ably to particles of organic taining their baths. A comm a t t e r , s u c h as candy, tomon method (5) is the electrolysis of tin anodes in a solution of sulfuric or other acids using bacco, etc., which had adhered to the foil. Such a paste, when a diaphragm or a gravity separation of the anode liquid, con- dissolved in water, gave a satisfactory ratio of stannous sulfate taining the stannous salt, from the cathode liquid. Stannous and sulfuric acid for making the tin-plating and refining baths. sulfate may also be prepared by the action of granulated or Any foil of lead or of tin-lead alloy mixed with the tin foil always feathered tin on copper sulfate solution. This is uneconomi- remained undissolved, and hence did not interfere with the tincal because the precipitated copper is so mixed with the plating baths. Aluminum foil dissolved readily in the sulfuric excess of metallic tin that it cannot be recovered success- acid. As aluminum sulfate has no noticeable effect in the tinfully. Probably the most satisfactory method of making plating baths, the presence of some aluminum foil makes little dilute solutions of stannous sulfate depends upon the fact difference. Plenty of this tin foil mixed with more or less that metallic tin dissolves slowly in dilute sulfuric acid if lead and aluminum foil was available from old metal comthe tin is depolarized by exposure to the air a t frequent panies a t low prices compared with virgin tin. An exintervals. Only dilute solutions of stannous salts can be perienced chemist could easily pick the real tin foil from the prepared by these processes. Concentrated solution or others, but this was too slow and expensive except for smallcrystalline solids, suitable for sale and shipment, cannot be scale experiments. obtained by evaporating such dilute solutions because of All samples of this second-hand tin foil from various oxidation by the air. sources and obtained at various times dissolved in the sulStannic sulfate can be prepared easily by the action of furic acid. However, one sample of tin foil, obtained diordinary tin with hot concentrated sulfuric acid, but stannic rectly from a manufacturer in order to avoid contamination salts are unsuited for electrolytic baths. It is impossible to with the lead and aluminum, did not dissolve in the sulfuric make tin sulfate by direct heating of sulfuric acid and ordi- acid any better than the granulated or feathered tin. Also nary feathered, granulated, or massive tin. metallic tin, rolled very thin in the laboratory, did not disThe following experiments were therefore undertaken with solve. Analyses of these tin foils showed that the secondthe object of preparing stannous sulfate for use in electro- hand tin foil from tobacco contained less than 0.67 per cent plating and refining baths. of metallic impurities and the new tin foil contained less than 0.23 per cent. These total impurities were determined by Use of Stannous Hydroxide and of Stannous Sulfide evaporating the filtrate from the nitric acid treatment of the Tin sulfate made by the action of sulfuric acid upon foil, drying a t 200" C., and weighing. These residues of stannous hydroxide was only partly soluble in water. Oxi- nitrates and oxides gave a black color with ammonium suldation of the stannous hydroxide during filtration and wash- fide. A mixture of second-hand and new tin foil was treated ing gave insoluble stannic salts. with the sulfuric acid. The second-hand foil dissolved while Hot sulfuric acid acting upon stannous sulfide produced the new foil did not. This showed that the second-hand large quantities of insoluble stannic salts. foil did not contain metallic impurities that aided the solubility of other tin in sulfuric acid. The new tin foil did not Received April 10, 1931. Presented before the Division of Industrial dissolve when mixed with pieces of lead foil or aluminum foil and Engineering Chemistry at the 81st Meeting of the American Chemical Society, Indianapolis, Ind., March 30 to April 3, 1931. or both. Some of the second-hand tin foil w&s melted and
S
832
INDUSTAIAL A N D ENGINEERING CHEMISTRY
poured into water. This tin dissolved no better than that made from ordinary Straits or electrolytic tin ingots. Therefore, the solubility of the tin foil depends upon its physical or crystalline condition rather than on its chemical composition. Experiments along this line were suggested by the work on intercrystalline corrosion ( 1 , 3, 4). Pure tin does not corrode intercrystally, but very small quantities of aluminum or zinc greatly increase such corrosion. As little as 0.22 per cent of aluminum causes tin to become brittle. It seemed probable that such alIoys, which were brittle as a result of this intercrystalline corrosion, would dissolve more readily in the sulfuric acid than would pure tin. Many such alloys containing small quantities of zinc and of aluminum were tested in the sulfuric acid, but none dissolved satisfactorily. These alloys did become very brittle and could be crumbled almost to dust by a little rubbing. Use of Amalgamated Tin
An alloy of tin with mercury dissolved in the warm sulfuric acid as readily as did tin foil. The mercury, perhaps, caused a fresh surface of the tin to be exposed constantly to the action of the acid. These experiments were discontinued because the mercury could not be recovered easily from the tin sulfate paste. Use of Finely Divided Electrodeposited Tin
Tin was electrodeposited from stannous sulfate solution in the form of sponge-, fern-, or tree-like crystals by the use of high current densities. Such finely divided tin dissolved in the sulfuric acid as readily as the tin foil. Tall beakers, 9 em. in diameter, were filled about threefourths full with 1.75 N sulfuric, which was found to be the best concentration. The anode was a cone-shaped piece of cast Straits tin, the small end of which was immersed in the bath of sulfuric acid. A circular sheet of lead, covering the bottom of the beaker, served as cathode. Eight amperes of current were used. Hydrogen gas was evolved a t the cathode for a time, but this gradually decreased as the concentration of stannous sulfate increased. After 3 hours the concentration was constant a t 18.2 grams of stannous sulfate per liter and gas was no longer evolved a t the cathode. The bath was mixed automatically by the downward movement of the concentrated solution of stannous sulfate, which was formed a t the anode in the top of the beaker. The tin trees, extending upwards from the cathode, were pushed down occasionally with a rod. It was not possible to determine the cathode current yield by direct weighing, owing to the impossibility of drying the feathery loose tin deposit without oxidation. However, it is probable that the yields were practically 100 per cent. This spongy tin was removed from the cell, squeezed tightly to expel as much electrolyte as possible, and, in its moist condition, put into and heated with the calculated weight of concentrated sulfuric acid (2 parts of acid to 1 of tin), thereby forming stannous sulfate. The weight of cathode tin was considered equal to the weight of tin dissolved from the anode; which was true, of course, only after the concentration of the solution was constant. The water in the mass of spongy tin was sufficient for the proper dilution of the concentrated sulfuric acid. Agitation of Tin in Sulfuric Acid and Air
Tin dissolves slowly in sulfuric acid if it is exposed to the air a t intervals. The following experiments were performed with the hope of hastening this reaction. The feathered tin was placed in an Erlenmeyer flask, which was attached to a mechanical gear in such a way that the flask could be rotated in a nearly horizontal position. The flask was filled
Vol. 23, No. 7
one-third full of feathered tin and then sulfuric acid was added until the flask was half full. Two bent glass rods were held inside against the walls of the flask by a tightly fitting rubber stopper which was perforated to admit air. I n the process of revolving, the rods, moving with the flask, served to drag the tin out of the acid into the air for depolarization and then to drop it back into the acid. Cleats or other irregularities on the sides of the flask would have brought about the same results. The tin dissolved in almost any concentration of sulfuric acid. The strength of the acid could be regulated to give either a dilute solution of stannous sulfate in dilute sulfuric acid or a thick paste of solid stannous sulfate in strong sulfuric acid. It was necessary that the tin be carried into the air and then dropped back into the acid. Agitation of the tin in sulfuric acid without the exposure to the air produced little action. Fifty per cent of sulfuric acid is recommended, although 40 to 80 per cent gives very good results. It is probable that the coating of insoluble stannous sulfate, which is only slightly soluble in strong sulfuric acid, is detached or broken offfrom the metallic tin by the tumbling, whereby a fresh surface of clean tin is exposed to the acid and to the air. With 50 per cent of sulfuric acid 375 grams of stannous sulfate were formed in each liter of the acid in 18 hours. The rate of rotation was 3 to 5 times per minute. The rate of formation of the stannous sulfate was increased 34 per cent by increasing the speed of rotation from 0.5 to 3.5 times per minute. An increase in rotation from 3.5 to 17 times per minute increased the resction only 18 per cent. This shows that extremely rapid rotation is not necessary. The yields a t 50" C. were very little larger than a t room temperature. Concentrated sulfuric acid reacted to give sulfur dioxide and other reduction products; hence somewhat dilute acid should be used. -4stannous sulfate paste in sulfuric acid was made easily by this method. At suitable intervals the acid liquid containing the solid stannous sulfate was poured from the residue of metallic tin. Solid tin sulfate very quickly formed in the solution of 50 per cent sulfuric acid, because tin sulfate is only slightly soluble in strong sulfuric acid. After the stannous sulfate had settled, the clear acid liquid was poured back upon the tin and the process continued. This solid tin sulfate, after decantation of the excess of acid, contained sufficient sulfuric acid for dilution with water to form the tin-plating baths, Afew small flakes of metallic tin, which had broken loose from the larger pieces, were present in the paste. I n one experiment the tin was placed in a trough containing the 50 per cent sulfuric acid. A mechanical arrangement slowly moved one end of the trough up and down in such a way that the tin was alternately in the air and in the acid. The rate of movement was six times per minute. The total stannous sulfate never exceeded 5 grams per liter. This shows that the mechanical abrasion of the tin surfaces is needed to give large quantities of tin sulfate. The tinaluminum alloys which were brittle, dissolved in this trough apparatus more slowly than pure tin. A somewhat similar experiment was made by bubbling air through the feathered tin immersed in the 50 per cent acid. The concentration of stannous sulfate was only 15 grams per liter after several days. Literature Cited (1) Burgess and Merica, Bur. Standards, Tech Paper I S (1915). (2) Mathers, U. S Patents, 1,397,222 and 1,540,354, The process described in these patents was used by the American Smelting and Refining Co., at Maurer, N. J., beginning in 1918. (3) Rawdon, IND. ENG.CHEX..19, 613 (1927). (4) Rawdon, Krynitskand, and Berliner, Chem. Met. Eng., 36, 109, 154, 212 (1922). (5) Whitehead, U. S. Patents 1,157,830 (1915) and 1,287,156 (1919).
