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The Publisher of the CHEMICAL NEWS has obtained the required permission of the War Office, and he will forward copies direct from the Office to any neutral country on receipt of instructions. NEWS THE CHEMICAL NEWS VOL. CXVII., No. 3043. quantity (1 grm. copper requires 3'904 grms. crystallised thiosulphate, or about 158 cc. of "N/10 thiosulphate "), and (immediately) a sufficient excess of potassium sulphocyanide (equal to, say, 1 grms. per grm. of copper). The white precipitate of CuSCN has then to be filtered off, roughly washed, and rejected. Since it goes through every known variety of filter-paper, and since the solution A NEW METHOD OF DETERMINING COPPER. cannot be heated (else the trace of acetic acid would act By JAMES MOIR. THE object of the author in inventing this process was to find a method of doing without the large quantities of the expensive potassium iodide required in the ordinary accurate process for copper. For example, if high accuracy is required in determining the true percentage of copper in a "pure" copper, about 4 gr. of the metal must be taken, involving the use of about 30 grms. of KI and 16 grms. of crystallised thiosulphate. Now, if the equations concerned in the ordinary process be studied, it will be seen that the net result of the addition of iodide and thiosulphate to cupric acetate is to produce an insoluble cuprous salt along with a soluble tetrathionate. 1. CuÃ2+2KI= CuI+2KÃ+{I2. 2. 12+ Na2S2O3 = NaI + Na2S406. The net result is as if the equations were3. CuA2+KI+Na2S2O3 = CuI+KA+NA+Na2S406. It therefore struck me that the simple direct reaction between cupric salts and sodium thiosulphate would be worth reinvestigating. Now, a number of double copper sodium thiosulphates were described by German authors thirty or forty years ago, but it is probable that the whole of this work is wrong, and that the substances obtained were really tetrathionates and not thiosulphates. When equimolecular solutions of copper acetate (or copper sulphate in presence of acetic acid and sodium acetate) and sodium thiosulphate are mixed, the mixture is greenish yellow, and, in my opinion, consists mainly of soluble cuprous tetrathionate, but also contains a propor tion of unchanged cupric thiosulphate 4. CuÃ2+ Na2S2O3 = CuS2O3"2NaA. Equation 5 is perhaps more intelligible with separate ions. 6. Cu+Cu+S2O3′′ +S2O3′′ = Cu + Cu+SO6′′. The action being a balanced one is not suitable per se for an analytical process; nevertheless, it was obvious that the success of the old iodide method depended on the almost complete insolubility of the cuprous salt produced, else the above balanced reaction would occur. This was confirmed on trying to replace iodide with bromide or chloride (the cuprous salts of which are fairly insoluble), when it was found that the reaction was incomplete. However, on trying sulphocyanide (thiocyanate) as a sub. stitute for iodide success was obtained. The above balanced reaction then goes entirely in the direction of cuprous tetrathionate because the cuprous ion is removed as insoluble CuSCN as fast as it is formed, and on filtration a colourless solution containing only Na2S406 and excess KSCN and Na2S2O3 is obtained, in which under special conditions the latter can be estimated with iodine even in the presence of sulphocyanide. The analytical process suggested is therefore as follows: -Dissolve the weighed specimen in HNO3, and get it into the condition of very faintly acid cupric acetate, either by the oid methods or by the quick process which I describe farther on. Add a single excess of thiosulphate in known on the excess thiosulphate) it is necessary to add a quantity of pulped filter-paper before attempting to filter; (this is made by boiling a loose-texture filter-paper in dilute caustic soda until disintegrated and neutralising with acetic acid); on filtering, about 5 cc. will come through turbid before the pulp chokes the pores of the paper, after which a clear filtrate is obtained. As there is very little excess thiosul phate in this filtrate, any elaborate washing of the CuSCN precipitate is a waste of time. To finish the analysis, the filtrate is first diluted to about a litre, and then treated with 5 to 10 cc. concentrated pure H2SO4 (do not use HCl which, in this country, owing to bright sunlight, always contains free chlorine) and a little starch solution, and titrated back with N/10 iodine, only a small quantity being required if the directions about thiosulphate have been followed. The calculation is merelyCc. true N/10 thiosulphate at beginning-cc. N/10 iodine 157'3 =copper present (in grms.), assuming 63.6 for its atomic weight. The filtrate must not be titrated with iodine without the high dilution and the mineral acid, otherwise iodine is absorbed by the excess KSCN present. Iodine does not apparently attack dilute HSCN. If the approximate percentage of copper in the specimen is totally unknown, the experiment must be done twice-(1) on a small specimen (o'i grm.) using 16 cc. of "N/10 thiosulphate "; (2) repeating using thiosulphate equal to ("thiosulphate " minus iodine) of the first experiment. Thus if experiment (1) used 2.5 cc. iodine at the end, the correct "thiosulphate" for experiment (2) is at the rate of 135 cc. per grm. of material. This means that the method gives high results for copper (102 to 105 Fer cent of the truth) when a large excess of "thiosulphate has been used, but is sharply accurate when the "thiosulphate" is just in excess. 39 If great accuracy is required, 2 or 3 grms. of specimen must be taken, in which case it is better to weigh out the thiosulphate required (see above). This is because 400 cc. of thiosulphate solution would have to be taken, and the factor of any thiosulphate solution is not generally known within 1 part in 500, besides being variable with temperature. The following is a description of some of my results with the method: 1. Analysis of CuSO4.5H2O.-0 2500 grm. in water treated with sodium acetate and two drops HA, then with 12.19 cc. "N/10 thiosulphate" of factor o'985 = 12.01 CC. true N/10 thiosulphate. At the end the diluted acid filtrate required 199 cc. N/10 iodine; the quantity of copper is— |