THE ACTION OF STANNOUS CHLORIDE ON SILVER MIRROR FORMATION. By DR. O. MACCHIA. It is well known that the reduction of Ag,(NH)(OH), the hydrate of silver diamine (ammoniacal silver hydrate), and the reduction of nitrate of silver diamine (ammonjacal silver nitrate), Ag(NH3)2 (NO), into metallic silver (looking-glass mirrors), and the adherence of the latter to a glass plate is very much facilitated by previous immersion of the glass in a solution containing stannous chloride, SnCl, even when in the smallest quantities (down to a dilution of 1 in 2,000. Namias, in his valuable work3, admits the formation of a tin silicate (produced by the reaction of the tin chloride and the silicates of which the glass is composed), which, as it deposits tenaciously on the surface of the object, facilitates the reduction and the adherence of the silver. I started this study in order to see if there should be any possibility to prove the hypothesis of Namias, but when I had finished the whole experimental part which I had intended to go through, I came to the following conclusions: (a) That the hypothesis of Namias does by me (and corroborated by experi- Apart from the main object of the re search it was my intention to carry out investigations to solve the following problems. 1. At first to see if the tin silicate really facilitates the reduction of the silver and the adherence, since these two effects would disappear if the object to be silvered did not contain silicates. 2. By using a solution of sodium stannite, the aforesaid facts (i.e., increasing reduction and adherence) should also cease as the silicate of tin should be dissolved in the slight excess of free sodium hydroxide existing in the alkaline tin solution when freshly prepared. The solution for silvering used in all experiments was prepared according to the method of Miethe as Namias has indicated, i.e. : 20 gr. 900 c.c. B: pure hydrate of potash To 750 c.c. solution A, ammonia is added drop by drop until the brown precipitate of Ag2O is redissolved. Then the 900 c.c. of Solution B is added with agitation. Afterwards there is to be added more NH ̧OH until the precipitate of Ag2O is re-dissolved. Drop by drop, we add what remains of solution A until the solution has assumed a slight colouring of yellow-brown, i.e., a little AgO remains suspended. The reducing solution used was the one indicated from Namias. This solution thus prepared has to be boiled for about ten minutes. composed of It is after having cooled it we add 50 c.c. of ethyl alcohol 95 per cent., to maintain the solution, and then we add distilled water until the volume has reached 250 c.c. The boiling is necessary in order to transform quicker the saccharate CH,O,, into d-glucose CH,OH (CHOH),CHO and in d-frutose CH,OH (CHOH),COCH2OH. 12 22 For every silvering I took 20 c.c. of the silvering solution, together with three c.c. of the reducing solution and 3 c.c. of water. For the solution of tin chloride (in all my experiments I used solutions of about 1 part of SnCl, in 1,500 of water), I prepared beginning with the purest and previously tested tin, pure tin chloride (absolutely free from the stannic compound) and I kept it (to avoid that during the different trials it might be altered) suspended in absolute alcohol. The solution of about 1 in 1,500 I prepared just at the moment I had to use it. After having well agitated the vessel to stir up the suspended tin chloride. I took a known quantity of alcohol containing suspended SnCl, and added afterwards the calculated quantity of distilled water to get the 1 in 1,500 solution. To prove experimentally the first question I had intended to answer (i.e., if really the tin silicate was the cause that silver was reduced and adhered to an object contain20 minutes. I next prepared the solution prevent silvering with things not containing silicates. Using the same procedure, I chose for silvering three objects which seemed to me the most adapted for the purpose to solve the question I intended to answer. They were celluloid, wood and galatite. Of course, two trials were necessary for every silvering to be accurately observed; the first one without previous immersion of the object in the solution of tin chloride and the second one with immersion of the object in the solution of tin salt for about ing silicates) these two suppositions would (1 in 1,500) in a small basin, of the same substance which I intended to silver, in order to avoid the silicates having any influence on the solution of tin chloride. Also the silvering proceeded in a basin of the same substance (as wood, for wooden plates, and celluloid for celluloid plates, etc.). Silvering Celluloid. First operation.After having cleaned carefully the surface. of the plate with alcohol and washing I immersed the plate for 20 seconds in the solution of tin chloride. After washing I immersed the plate in the silvering bath, observing a slight deposit of silver (mirror) after 18 to 20 seconds. The silver mirror formed by the silver deposited is very adherent, Second operation.-After having cleaned in the above way the surface of the plate, I immersed the plate directly in the usual silvering bath. I observed a slight deposit of silver after one minute 45 seconds to one minute 55 seconds. The silver deposited, although it had a sufficient consistency, was much less adherent than that obtained by the first operation. Silvering Wood.-First Operation.--The wood chosen was taken from a fir tree and was well preserved. After having smoothed the surface to be silvered (having polished it with glass paper up to the number 00000) I polished it by alcohol and washed it again with abundant water. I then immersed the plate in the usual solution of SnCl, for 20 seconds. After washing with water I immersed it in the usual silvering bath. I was thus able to observe a slight silver deposit after 30 to 35 seconds. The silver deposited is sufficiently adherent. sur Second operation.-I prepared the face of the wood to be silvered in the usual manner and immersed it directly into the silvering bath. I observed a slight deposit of silver after two minutes and fifty seconds to three minutes and ten seconds. The silver was scarcely adherent. Silvering of Galatite.-First Operation.Having cleaned very carefully with alcohol the surface of the plate, I immersed the plate in the solution of SnCl,; I washed it again and then silvered. I observed here a slight deposit of silver after 25 to 30 seconds. The cover of deposited silver is very adherent. Second Operation. Without the previous immersion in the solution of Snc, I observed a slight deposit of silver after one minute and thirty to thirty-five seconds. The cover of deposited silver was little adherent. The adherence has been proved in all the operations when the plate was perfectly dry by means of a kind of sclerometer here illustrated. Treating now the second question we shall conclude thus: As the characteristic qualities of the composed of tin chloride adherent to the surface to silver show themselves, using also extremely diluted solutions, even 1 in 2,000), it is logical to conclude that the compound of SnCl, remains adherent at the surface in only very small quantities. It follows also that, using a solution of sodium stannite, the phenomenon should not appear, as always admitting the possibility of formation of tin silicate, which by and by deposits on the surface of the plate of glass and dissolves slowly. It is true, experimentally, that if, after having immersed the plate in the solution of SnCl2, we immerse the same plate in a solution of 10 to 12 per cent. sodium hydrate (or potash) we do not again meet these phenomena caused by the presence of tin salt, during the silvering. This could, in a certain manner, prove the hypothesis of the formation on tin silicate. This is however due to the slight corroding done on the glass plate by the solution of NaOH. 2 and in such a way to permit the plate being washed by water without the cover of Na,SnO, disappearing. In Table II. the quickness of the formation and adherence of the plates previously immersed for 20 seconds in a solution of 5 per cent. of Na, SnO is indicated. I prepared the sodium stannite, starting from a known quantity of tin chloride SnCl, pure, and adding the reckoned out quantity of NaOH. Each time I prepared the Na SnO, and added the calculated quantity of water (to get a solution of 5 per cent.) Thus the quantity of stannite that oxydated (by the oxygen of the air) into metastannate, Na,SnO,, was a very small one. The silvering of the different plates has been executed using the same quantities of silvering and reducing solution used in the preceding silverings. SnCl, used are always very much diluted, and using concentrated solutions the silvering does not become perfect owing to phenomena still unexplained. Tin chloride, as the salt generated from a strong acid HCl and a weak base Sn(OH)2, is undergoing in aqueous solution the hydrolysis which is increased with the growing dilution. The hydrolysis as is known is proceeding in two stages; the first part is formation of basic stannous chloride Sn(OH)CI, and this last one from its part reacting with another molecule of water is transformed into stannous hydroxide Sn(OH)2. In our case, as the solution is very much diluted, we can suppose that hydrolysis is complete i.e., the equilibrium Sn(OH)2 SnCl ← Sn(OH)Cl is completely directed to the right. By the dilution most of the Sn(OH), remains dissolved in the colloidal state as hydrosol. With time, however, the hydrosol of Sn(OH), coming into contact with the surface of the object immersed in the solution assumes the form of hydrogel fixing energetically on the surface'. Thus it is enerplained why the solution of SnCl2 does not keep for long. Indeed (supposing of course as always that the solution of SnCl, comes into contact with a surface perfectly clean when the hydrosol of Sn(OH), conmes into contact with the surface of the object, it becomes transformed into the hydrogel of Sn(OH)2, i.e., the equilibrium : This transformation proceeds with great rapidity. Indeed I foud that in order to observe the properties of the Sn(OH), deposited on the surface of a plate to be silvered, it is quite sufficient that this remains in contact with the solution for about 10 seconds. A first cause that gives the short durability to the solution of SnCl, is the continuous going towards the right in the above equilibrium, therefore after a certain time all the hydrosol of Sn(OH)2 transformed into hydrogel. The more a small quantity of SnCl, reacts with the Oxygen of the air, the more it is transformed into stannic chloride SnCl, and into basic tin chloride Sn(OH)Cl, i.e., 6 SnCl, + 2H2O → 2SnCl, 4Sn (OH)Cl. latter, on the other hand, partly hydrolysing is transformed into Sn(OH)2, and the is The we can explain how the immersion of the object to be silvered in the solution of SnCl cannot keep except for a short time. Indeed, the excess of hydrogel of Sn(OH), deposited on the surface of the object to be silvered, produces a too rapid reduction, and the mirror appears to assume a slightly yellow colour. This is produced by tracks of AgO (not more reduced by the bath of metallic silver) and by silver not precipitated under the reduced form of finely subdivided silver and black. Similarly, using a too concentrated solution we do not get a silvering really perfect, as on the surface of the object to be silvered an irregular deposit of hydrogel of Sn(OH), is formed. Concluding, the more a silvering shall be perfect, the lighter the cover is, and the more regular the Sn(OH), precipitated. To obtain this we need to use a very dilute solution of SnCl, and it has to be immersed for only a short time. INTERNATIONAL CONFERENCE ON IMPORT & EXPORT RESTRICTIONS. An International Conference of representatives of the Governments of Members and non-Members of the League of Nations has been summoned to meet at Geneva on October 17 with a view to framing an international agreement for the abolition of import and export prohibitions and restrictions. His Majesty's Government will be represented at the Conference by Sir Sydney Chapman, K.C.B., C.B.E., Economic Adviser to His Majesty's Government, assisted by Mr. H. V. Reade, C.B., an Assistant Secretary to the Commissioners of Customs and Excise,as deputy delegate and Customs adviser, and by Mr. Gilbert C. Vyle, President of the Association of British Chambers of Commerce, and Colonel the Hon. F. Vernon Willey, ex-President of the Federation of British Industries, as deputy delegates and commercial advisers. GERMAN ELECTRIC POWER The British Commercial Secretary at Berlin has forwarded the following brief summary of some of the published results of the industrial census for 1925 in regard to electric power stations in Germany :The census included 7,492 generating plants. Of these 5,751 were small stations with a capacity of up to 500 kilowatt hours; 1,124 plants had a capacity of from 500 to 5,000 kilowatt hours, and can, therefore, be called medium-sized; and only 617 plants existed producing more than 5,000 kilowatt hours, of which 434 plants had a capacity of more than 10,000 kilowatt hours. The small plants supplied 4.7 per cent., the medium-sized plants 12.7 per cent., and the large ones 82.6 per cent. of the total electric current. It will thus be seen that, although the small and medium-sized are large in number, the highest percentage of electric current is produced by the few large plants. ASSOCIATION OF SPECIAL LIBRARIES AND INFORMATION BUREAUX CONFERENCE. It is expected that some 200 organisations will be represented at the Fourth Conference of the Association of Special Libraries and Information Bureaux, which meets' at Trinity College, Cambridge, during the week-end, September 23-26. Following a reception by Sir J. J. Thomson, O.M., Master of Trinity, Sir Geoffrey Butler, K.B.E., Senior M.P. for Cambridge University, will deliver the Presidential address, while among the many well-known experts giving papers are Sir Henry Lyons, Director of the Science Museum, Mr. A .E. Overton, of the Board of Trade, and Sir Richard Gregory, Editor of Nature. SCARCITY OF PAPER IN The subject of paper and the possibilities of utilising bamboo for its manufacture, was dealt with by Mr. William Railt, the cellulose expert to the Government of India. He pointed out the practicability of using bamboo to meet the situation and it would cost several pounds per ton less than wood. The The world's consumption for paper and the cellulose industries was now in the neighbourhood of 15,000,000 tons a year, nearly the whole of this being from coniferous wood. The fact that this figure represented nearly 40,000,000 tons of raw wood indicated something of the strain put upon our resources by these industries. normal expansion of paper consumption was about 25 per cent. in ten years; consequently the figure just quoted would be about 50,000,000 in 1937, and over 60,000,000 in 1947. A contributory cause of this expansion was the wonderful development during the past twenty years in uses for cellulose other than paper. For the moment the position was merely one of scarcity and higher cost, which might be represented by £7 as the cost of the wood required to produce a ton of pulp, as against £2 10s. forty years ago. RICH GOLD DEPOSITS FOUND IN Following closely on the announcement that a rich gold deposit had been discovered in Inverness, N.S., comes another announcement backed up by a report made on ore submitted to the Nova Scotia Technical College, that a rich gold deposit has been discovered at Renfrew, Hants County. The lead from which the samples were taken is said to be from eight inches to three feet wide, and is in a district which is easily accessible. "BRITISH-RUSSIAN GAZETTE AND TRADE OUTLOOK." The object of this publication is apparently to foster trade between Great Britain and Russia. It is printed partly in English and partly in Russian. The September issue |