THE

SAMPLING AND ASSAY OF CHINESE TIN.

By FRANK BROWNE, F.I.C., formerly Government Analyst, Hongkong.

TIN is sent to Hongkong from the Yunnan mining district to be further refined in the native foundries. After purification the metal is cast into I cwt. ingots, stamped with the quality of the tin, namely, No. 1 or 3, together with the name of the smelter. The No. 1 quality is usually from 99'0 to 99.5 and the No. 3 from 96 0 to 96.5 per cent purity. When tin is bought the native firm is approached for a lot of 5 to 25 or more tons. The Chinese vendor sets aside the appropriate number of ingots, tells the pur chaser he may test them how he pleases, and that when the Hongkong Government has passed the tin as being of the stated quality, the metal must be paid for, and further, that after such payment no question arising from inferior quality will be considered. It was at the unanimous request of those concerned that the Hongkong Government undertook the certificating of the metal.

The Sampling.

The Government Analyst is notified by the prospective purchaser that some tin is to be examined, and samples are thereupon drawn by the former from one-tenth of the number of slabs in the lot submitted. With a twist drill these slabs are bored, top and bottom, alternately. The drilling machine is a very strong portable kind, to which a heavy steel plate has been added to form a base, the ordinary support for metal having been removed. When thus fitted the drill is placed on the ground, and the slabs carried one by one to the plate are dealt with rapidly by the driller sitting alongside. The borings are collected and taken to the laboratory for the assay. The mark HAK is then punched on to every slab as a guarantee of quality and to prevent substitution. In the event of the assay showing that the tin is under the quality vouched for, the punch marks are at once removed by a Government officer.

The Fusion.

The borings are melted together under palm oil at a low temperature in an iron ladle. The resulting ingot is washed in water containing washing soda, scraped to brightness, and filed with a fine file. The melting together of the borings has been shown by analysis to eliminate no impurity. A sample of No. 3 quality melted four times as described, and assayed after each fusion, gave identical figures, and the No. 1 quality after melting has given no higher percentage of metal. Good tallow seems to answer just as well as the vegetable fat.

The Assay.

An exact quantity of 1 grm. of the filings is put into each of two flasks, each fitted with a rubber cork and a short glass tube, and the two estimations are then performed in accordance with the method of assay described by L. thousands of these analyses have been done by the ferric Parry-"The Assay of Tin and Antimony." As several chloride solution recommended by him, considerable experience has been obtained of this process. Some details may be useful to those seeking a trustworthy and rapid method for tin estimation, which has been found to be accurate to o'r per cent of metal. The writer can uphold every word that Parry says for the ferric chloride titration. quality, free from sulphurous acid. Any sample showing The hydrochloric acid employed should be of good rejected or set aside till free. When performed with care this impurity by Girardin's stannous chloride test should be and attention the titrations do not require an atmosphere of carbon dioxide. Of course the standardisation of the ferric chloride solution, and the subsequent assays performed with it, must be carried out in exactly the same way. The metal used for standardising was of 99.76 per cent purity, determined by ascertaining the total of the impurities in a piece of nearly pure metal. It was found that the titration figure was the same before and after the standard had been melted under palm oil. No estimations of tin the ferrie chloride solution. As in a hot climate the room were done in any burette other than that used for valuing temperature may vary between 15° and 30° C., endeavours were made always to titrate both the standard tin and ordinary samples at the same temperature. This could not always be done, but it was found that a correction at the rate of or cc. for 10° C. below or above the temperature at which the volumetric solution was set, gave a true reading when 50 cc. were used. tion was marked always so as to show the final tint The soluobtained, thus 49'64 cc. = 1 grm. 99'76 tin titrated to a pale colour.

Probably the absence of any appreciable oxidising effect by the air on the titrations was due to its being possible with these tins of fixed quality to run in almost the whole of the volumetric solution at once. The upper part of the liquid in the assay flask would thus be highly coloured, and forms a protective covering to any unattacked stannous solution below. Then on again raising the flask contents to boiling the two layers mix, and repeated experiments have shown there is not sufficient exposure to lower the true content of tin. Of course the operation from start to finish must be conducted with no unnecessary delay. When the quality of the tin submitted was found to be between 99'0 and 99.1 for the No. I. and between 96'0 and 96.1 for the No. 3 quality, the smelter

was notified that he was running the quality too near the rejection limit of "anything under 99'0 or 960 respec. tively," and improvement invariably followed.

With the precautions mentioned, it was possible to export tin of reliable and uniform quality. Much to the satisfaction of those interested in this valuable export trade the London Metal Exchange decided that the results as found in the Hongkong Government certificate should be binding on all purchasers, thus placing the tin sales on a firm and workable basis.

Usually Chinese tin dissolves completely in hydrochloric acid, but occasionally there is a little black powder which obstinately resists attack. After investigation it was found that although the ferric chloride solution when run in dissolved everything up, the percentage of tin was not above that found when the impurity had been eliminated by the usual methods. The amount of black powder was never very large, and was mainly antimony.

Finally, possibly the most likely source of error In tin examinations is due to imperfect solution ef the metal. Particularly is this the case when the filings have been placed in a water or air-oven for several hours. Superficial oxidation may then hinder the action of the hydrochloric acid, so that minute, very slowly dissolving, particles escape observation. The nascent hydrogen evolved by

these vitiates the estimation.

The above description represents the Hongkong methods of tin examination in use tn 1915, since when no information has been received that the processes have in any way been altered.

THE CONSTITUTION AND FUNDAMENTAL PROPERTIES OF SOLIDS AND LIQUIDS.* PART II.-LIQUIDS.

By IRVING LANGMUIR (Continued from vol. cxvi., p. 317).

THE iso-compounds gave practically the same results as the normal, No reason is given by Szyszkowski for the constancy of B or the changes in A. If we deal with rather concentrated solutions so that c is large compared to A then in Equatlon 3 we may neglect unity in comparison with c/A. Under these conditions y is a linear function of the logarithm of c. This is the relation found for acetic acid by Milner, and according to Gibbs' equation this must mean that the amount of fatty acid adsorbed in the surface is independent of the concentration. By differentiating (3) we obtain

dc C+A

0'434 Bo C RT

C + A

(4).

(5).

Substituting the value B 0'411 found by Szyszkowski value comes reasonably close to the value a。 = 215 × we obtain as = 31 X 10-16 sq. cm. per molecule. This films on water. 10-16 found for the higher fatty acids by the study of the The fact that as has the same value for

each of the acids from propionic to caproic shows that in the more concentrated solutions the molecules in the adsorbed surface layer are packed tightly side by side with the hydrocarbon chains arranged vertically. Equation 6 that the different acids are adsorbed in the In the case of very dilute solutions we see from surface layer in different amounts, these being inversely proportional to the values of A (Table II.). The longer the hydrocarbon chain the greater the adsorption from a the length of the chain is without influence. dilute solution, although with more concentrated solutions

surface tension. Let us consider what must be the These facts are readily explainable by the new theory of mechanism of these phenomena. In the surface of a sufficiently dilute solution the molecules of the fatty acid will be so far apart that they do not influence one another. There must be a kinetic equilibrium between the molecules arriving at the surface from the interior and those passing from the surface to the interior. The rate at which the molecules arrive at the surface is proportional to the concentration. The rate at which they pass into the interior is proportional to the number in the surface. For equilibrium in dilute solutions it is evident that the number in the surface must be proportional to the number in the interior. This explains the form of Equation 6, which merely states that q is proportioal to c and serves as a definition of the constant A. From the above kinetic considerations, together with the conclusion that a saturated surface layer must consist of a layer one molecule deep, we thus obtain from Gibbs' equation a rational derivation of Szyszkowski's empirical equation.

The potential energy of a molecule of a fatty acid is lower when the molecule is on the surface than when it is in the interior. The rate at which molecules pass from the interior to the surface depends primarily on the concentration of the solution, but not on the difference of potential energy between the molecules in the interior and on the surface. The rate at which the molecules pass from the surface back to the interior depends on the number of molecules in the surface, but is also dependent to a very great degree on the difference in the potential energy of The phenomena is quite the molecule in the two states. analogous to evaporation. The manner in which m, the rate of evaporation" of the molecules from the surface into the interior of the liquid, varies with the difference of potential energy may be calculated from Maxwell's distribution law. (For further details see Equation 5 and the references given in the footnote on page 2254 of Part I. of this paper-Journ. Am. Chem. Soc., 1916, xxxviii., 2221).

Taking into account the kinetic equilibrium between the surface layer and the interior of the solution it may be readily shown that the following relation should hold between the amount adsorbed in the surface and the concentration in the solution

Here is the decrease in potential energy which occurs when a grm. molecule of the dissolved substance passe