APRIL, 1906. VoI. XXXI., No. 361. THE ANALYST. PROCEEDINGS OF THE SOCIETY OF PUBLIC ANALYSTS. THE PURIFICATION OF ZINC AND HYDROCHLORIC ACID FROM ARSENIC. BY L. T. THORNE, PH.D., AND E. H. JEFFERS. (Read at the Meeting, February 7, 1906.) 1. Zinc.-Hehner first suggested (ANALYST, 1902, 261) the treatment of zinc with sodium for the purpose of removing traces of arsenic, but the method was some- what uncertain, owing to the exact conditions necessary not being ascertained. Later we described before this Society conditions of this treatment under which we obtained reliable results; but as the manipulation we then used was complicated, we delayed printing the details, in order, if possible, to simplify it. After long trials and use we now lay before the Society the simplest conditions which we find reliable.Commercially pure zinc is melted in a, crucible in a gas furnace, and when at or only just above its melting-point sodium is mixed with it in the proportion of about 1 gram to each pound of zinc. This is best effected by adding the sodium in pieces of 1 to 2 grams weight which have not been dried from the adhering petroleum in which the sodium is stored. If the zinc be at the right temperature (ie., not too hot) the sodium is dissolved without either it or the petroleum vapour becoming ignited. After all the sodium has been added, it is preferable to add a small piece of zinc to insure the temperature being reduced to about the melting-point. The crucible is heated for a few minutes till the zinc is completely fluid, and the zinc is then poured into a second previously heated crucible, and back again once or twice to insure thorough melting.If the temperature is not allowed to get too high practically no oxidation takes place during this transference. The crucible, with the lid on, is then put back into the furnace and heated to a dull red heat, when the furnace and crucible lids are both removed, and the heating is continued for at least half an hour (preferably for an hour), care being taken that the temperature should not rise much. A scum (or alloy) gradually rises to the top and forms a hard, dark gray crust on the surface about & inch thick. This crust contains practically all the arsenic and the greater part of the sodium in combination with zinc. The crucible is allowed to cool slightly, the crust is pierced at one side, and the molten mass is poured into a second previously heated crucible and skimmed if necessary.This crucible is then heated to bright redness, any slight scum removed, then allowed to cool, and the zinc granulated just before the solidifying-point is reached.102 THE ANALYST. The essential points for success appear to be to get as thorough mixing as possible at a low temperature, and to avoid material rise in temperature whilst the scum or alloy is forming and rising. The arsenic sodium and zinc compound seems to be most stable at somewhere near the melting-point of zinc, and to be partially broken up at high temperatures. The temperature, before the scum is removed, should not be hot enough for the zinc or scum to take fire when the crucible lid is removed.In this way zinc absolutely free from arsenic is obtained, which gives in the Marsh-Berzelius apparatus a perfect blank with the solution of 15 to 20 grams of zinc, and is yet so sensitive as to give a definite and reliable result with 0.000002 grams of arsenious oxide, and the sensitiveness is always the same. The yield is about 95 per cent. of the zinc taken. 2. Hyd~ochloric Acid.-In 1902 we described (Proc. Chem. SOC., 1902, 118) a method for the freeing of hydrochloric acid from arsenic by a modification of the Reinsch test. I n use we found the presence of a little stannous chloride (i.e., of a strong reducing agent) greatly facilitated the deposition of the arsenic on the copper gauze, lessened the amount of copper dissolved, and shortened the process, but it still remained somewhat tedious.Noticing the great effect of the tin, we then tried the employment of a copper-tin couple with complete success. Redistilled hydrochloric acid is diluted to rather under a specific gravity of 1,100 (water = l,OOO), and is then poured on to a copper-tin couple, prepared by reducing with zinc a mixture of copper and tin chlorides in hydrochloric acid and washing the precipitated spongy couple. The metals are used in about the propor- tion of 1 part by weight of tin to 4 to 8 parts by weight of copper. A convenient mode of preparing the couple is as follows : Cuprous chloride is dissolved in excess of hydrochloric acid, and a small quantity of granulated tin added. The tin rapidly dissolves, at the same time reducing some copper in a spongy form.Zinc dust, or the smaller portions of granulated zinc, is then added, and this reduces the remainder of the copper together with the tin in the form of a dark gray spongy couple, which is washed by decantation. Ordinarily pure cuprous chloride and tin may be used, as traces of arsenic which may be present are eliminated, and do not affect the fins1 result. The couple may also be readily obtained by reducing with zinc the residual liquor in the distilling flask, which consists of the mixed chlorides. The couple should be gray in colour; if it is nearly black the copper is in too great excess. About 2 or 3 grammes of the couple are used to each litre of the diluted acid. The acid containing the couple is gradually heated, a strong reaction taking place at about 70" C., and kept on the boil for about a quarter to half an hour.During this time most of the tin and a part of the copper is dissolved, and practically the whole of the arsenic is evolved. The acid may then be distilled direct, or be cooled and kept under a layer of petroleum (to prevent absorption of oxygen) ; but in the latter case it must be distilled within twenty-four hours, or partial oxidation will occur. In the distilling-flask a small quantity of the couple should be placed, and also a small piece of 100-mesh copper gauze, This latter determines the pre- cipitation of any minute traces of arsenic which may have escaped elimination by the preceding treatment, and also prevents bumping.The distilled acid is perfectly free from arsenic.THE ANALYST. 103 Good commercial muriatic acid may also be used, but in this case the first treat- ment must be repeated twice, as, owing to the traces of iron chloride and other impurities contained in the acid, the first lot of couple is generally completely dissolved, and does not fully eliminate the arsenic, The residue in the distilling-flask serves conveniently for the preparation of fresh copper-tin couple, as the proportion in which these two metals are present in the couple is not important. DISCUSSION. The PRESIDENT (Mr. Bevan) having invited discussion, Mr. HEHNER desired personally to thank the authors for laying down the conditions under which the sodium treatment was fully successful. As he had explained a few years ago, he had often succeeded in purifying satisfactorily very bad zinc by treatment with sodium, but he had often failed to improve materially even .comparatively pure zinc.He had not, however, had so much patience as the authors, and so had never ascertained the exact conditions that were necessary. He was very glad now to find that the authors had brought what was formerly an entirely rule-of- khumb process into something like law and order. Mr. CHAPMAN said that his own experience was that it was now rarely necessary t o purify zinc from arsenic, the great difficulty at the present time being to get zinc fhat would give out the full amount of arsenic that was put in. Consequently, the problem was not quite SO urgent a one as it was when Mr.Hehner took the matter up a few years ago. There could, he thought, be no doubt whatever that the sodium method, if properly applied, was capable of removing the arsenic. He himself had gometimes been unsuccessful with it, but that was probably because he had not known the precise conditions necessary for success. The many experiments he had made, however, went to show that, unless the sodium mas entirely removed, the zinc obtained was distinctly ‘‘ insensitive.” He thought that that was proved by the mirrors which the authors had shown. For example, their mirror for the mgm. .of arsenic was not more than half that which he was in the habit of getting, and which he had every reason to believe to be correct. The same applied to the other mirrors. Through the kindness of Dr.Thorne, he had had the opportunity of examining some of this sodium-treated zinc with his own materials, and the conclusion he came to was that it was distinctly insensitive. If, therefore, the sodium treatment was to be employed, it must be with very great care, and the operator must recognise the possibility, or even probability, of the resulting zinc being more or less insensitive. The method for the purification of hydrochloric acid recommended by the joint committee of the Society of Chemical Industry and the Society of Public Analysts (treatment with bromine and sulphurous acid and distillation) in some cases worked well, but in others did not. He had, however, for some time been invariably successful in purifying the most impure acid by a slight modification of that method.The only modification was that the strong acid was distilled, instead of being first diluted down to constant boiling-point strength. The pure acid obtained amounted to about three-fifths of the total quantity treated. The only failure he had had was due to the mechanical carrying over of a little iron into the distillate from a very104 THE ANALYST, impure acid. This, as had been recently shown, caused diminished Sensitiveness, but it was easily obviated by a suitable arrangement of the distilling apparatus. Dr. DYER said that Mr. Chapman had anticipated a, remark that he had been about to make regarding the faintness of the authors’ mirrors. Their mirror from mgm. appeared to be less than the mirror obtained in his laboratory from half that quantity of arsenic, working with acid which gave a good blank, and using tubes of approximately the same size as those shown.That seemed to point to slight insensitiveness of the authors’ zinc. Mr. H. D. LAW said that, from the results of the investigation recently corn-, municated to the Society by Mr. Chapman and himself, it was evident that sodium was one of those metals which seriously interfered with the sensitiveness of zinc. He had since made some experiments to find out the particular reducing efficiency of hydrogen liberated by sodium and potassium. By passing an electric current through a solution of sodium hydroxide or potassium hydroxide, a deposit of sodium or potassium was obtained. This dissolved in the water, with liberation of hydrogen at the potential of the metal.The reducing power of this hydrogen had been compared with that of the hydrogen liberated by various metals of different potentials. Camphor wa,s very readily reduced with lead electrodes, but when he tried to reduce camphor with the hydrogen liberated by sodium or potassium, he found that he was quite unable to do so. This showed that the potential of the sodium or potassium was very much lower than that of zinc or lead. He then took different materials which were capable of being readily reduced, such as benzaldehyde, in order to find out, by lowering the reducing potential, at what point a degree of reduction was obtained corresponding with that produced by sodium hydrogen. With lead elec- trodes benzaldehyde yielded a very sticky mass quite characteristic of high-potential reduction, while with metals of lower potential the sticky masses produced were very much less in quantity.Finally, he took refined copper electrodes, which yielded no sticky substance at all, but a crystalline, brittle hydrobenzoin corresponding to that obtained by reduction in sodium and potassium solutions, This proved conclusively, he thought, that the potential of sodium and potassium was about the same as that of copper, which was extremely low. Dr. THORNE said that he was inclined to think that, if there was any diminution a t all in the sensitiveness of the zinc, it was but very slight. Personally, he was strongly of opinion that in many cases where zinc was said to be highly sensitive either it or the acid was not absolutely free from arsenic.He had certainly not had the same success as Mr. Chapman in obtaining zinc that would give an absolutely clear blank without repurification, though there had undoubtedly been a great improvement in commercial zinc during recent years. As a test of the absolute purity of the zinc, they ran blank determinations for an hour and a half. A twenty- minutes’ blank might to a certain extent be satisfactory, but they had found after a large experience that the very small traces of arsenic which were present in most commercial zincs tended to cause irregular results, giving undue prominence to small quantities as compared with larger quantities of arsenic in the material under examination. They found the sensitiveness of each successive batch of zinc purified to be practically the same, and although he was quite prepared to admit that theTHE ANALYST.105 mirrors they obtained might be slightly less intense than the small-quantity mirrors obtained with other zincs, he thought that the continuous concordance of the mirrors after repeated treatments with sodium showed that they were truly representative, and that the danger of loss of sensitiveness by the treatment was practically nil. The more strongly marked character of the low-quantity mirrors in other cases was probably due, in part at any rate, to traces of arsenic in the other materials. With reference to Mr. Hehner’s remarks, he should like to say that a good deal of the patience expended on this matter had been that of his collaborator, Mr.Seffers. With regard to hydrochloric acid, they also had found that the method first proposed by the joint committee did not answer in all cases. They did not say that the method they had described was a better one than the committee’s method as now modified, but it had been found reliable with ordinary impure acid as well as with acid of good quality, and was a simple and convenient method. In answer to a question put by Dr. DYER, Dr. THORNE said that they had not tried Mr. Chapman’s device of introducing cadmium sulphate into the flask. Mr. CHAPMAN said that he could put forward in support of his own contention exactly the same argument which Dr. Thorne had used. The mirrors he obtained were perfectly concordant, whatever the method or materials were. Sometimes he put cane-sugar, soluble starch, etc., into the flask, sometimes he left it out ; but the mirrors in either case were always of the same intensity within the very narrow limits of experimental error. He asked what evidence there was that the appearance of a bright surface on the zinc coincided with the disappearance of the whole of the sodium. Mr. JEFFERS said that he did not think it could exist alongside the zinc at that temperature. Mr. CHAPMAN said that it might quite well exist in very small quantity as an alloy dissolved in the zinc. Mr. JEFFERS said there was no room for doubt as to the complete removal of the sodium. If the sodium were not entirely removed, it was impossible to get a bright surface on the zinc at a red heat.