The analysis of refined copper is a subject of great importance, and has not received the attention it deserves. Copper metallurgists, therefore, will welcome the paper of Mr. Heath with satisfaction. The state of the art of the chemical analysis of copper has been such that consumers, if they could not afford to run any risk, have been obliged to buy copper from the smelter having the best reputation, and a specification for copper to meet certain requirements could not be drawn up by the mere giving of the chemical constituents. Mr. Heath has brought the analysis of the copper to such a state that when a specification calls for copper “ equal to Lake copper ” one will know immediately what percentage of the pure metal to expect.
It seems to be a pretty well established fact that the copper of Lake Superior is practically free from antimony and bismuth ; and, with the exception of arsenic, the other elements are easily separated from copper by electro-deposition in an acid solution. The authorities differ in regard to the question whether antimony is deposited with the copper in an acid solution. Classen recommends the employment of a solution containing 10 per cent, of free nitric acid, sp. gr. 1.21, and the strength of the current from 0.3 to 0.4 ampere, in order to prevent the other metals from being deposited with the copper; and for this reason, I presume, the method has been generally used. I, myself, have been accustomed to use such a solution for the analysis of copper-alloys, but have found that for such alloys only a small quantity of nitric acid is necessary, if sulphuric acid is present in sufficient amount to give the solution the required conductivity.
E. F. Smith says that arsenic can be previously removed from a copper-solution by evaporation with bromine. By analogy, antimony would probably be likewise volatilized; but no experiments or references to bear out this point are given. If such is the case under all conditions, it is an important fact, as an assay can be made of copper containing a large amount of antimony or arsenic by first removing them by bromine, and then electrolyzing. A searching investigation upon this point is of importance. I should be glad to hear whether Mr. Heath has made any experiments along this line.
The fact that antimony is deposited with the copper was noticed by Hampe. He found that electrolytic copper contained from 0.007 per cent, to 0.02 per cent, of antimony, and this led him to believe that antimony is carried down with the copper. He accordingly carried out the following experiments :
A sample of copper sulphate containing 25.09 per cent, of copper and 0.0083 of antimony was electrolyzed by a current from six Meidinger-Pincus cells. The deposited copper contained 0.007 per cent, of antimony. Next, an alloy of 99.44 per cent, of copper and 0.529 per cent, of antimony was dissolved in nitric acid, evaporated with sulphuric acid until fumes were given off, the residue of antimonic acid filtered off, and the solution then electrolyzed after adding 10 per cent, of dilute nitric acid (sp. gr., 1.21). The deposited copper contained 0.0198 per cent, of antimony. For this experiment he used a large amount of alloy, 50 grammes.
From these experiments it would appear that, if antimony is present in the copper solution, it is impossible to prevent its simultaneous deposition with the copper. Bismuth is apparently deposited in the same manner. The copper with which Mr. Heath has to deal can be accurately assayed by the electrolytic process; but if antimony or bismuth are present, the determination of these elements as well as that of the percentage of copper is of vital importance; the assay by the electrolytic method, therefore, is not suited for such copper.
The injurious effect of antimony and bismuth on brass is not generally appreciated, but the difference between good and poor brass often lies in the presence or absence of 0.02 per cent, of antimony; so that it can readily be seen how important the determination of these elements is. The freedom of Lake copper from antimony and bismuth, and their presence in electrolytic material, easily shows that the reputation of Lake brands as the most suitable material for the manufacture of high-grade brass and German silver has been justly earned.
For the determination of the arsenic, antimony and bismuth I have found the method of Rivot to be the most satisfactory; in fact, it is the only method known which is really suitable for copper containing very small amounts of these elements. By it one is enabled to employ a large amount of copper for analysis, precipitate it by potassium sulphocyanide, and determine the impurities in the filtrate. The copper in such material is best determined by difference.
I agree with Mr. Heath that copper can be deposited in a sulphuric acid solution, and have repeatedly carried out such a deposition; but the solution should contain enough acid to give it the proper conductivity. The deposited copper is of a dark red color, and has not as clean and pure an appearance as when a little nitric acid is present. Even a few drops of nitric acid is all that is necessary to accomplish this result. As Hampe has shown that nitric acid does not prevent antimony from coming down, and as bismuth is likewise deposited, the question occurred to me whether nitric acid was necessary in a large amount to prevent the co-precipitation of such metals as zinc. To investigate this point the following experiments were carried out:
A sample of brass made from Lake copper and pure spelter was rolled to No. 30 gauge in order to render it homogeneous, and three determinations of the copper were made. One gramme was used in each case.
A. —Dissolved in nitric acid; added enough sulphuric acid to convert the zinc and copper into sulphates, but to leave no excess. Evaporated until all the nitric acid was removed, and then added 8 c.c. of nitric acid of sp. gr. 1.21. Diluted until the volume was 80 c.c. and electrolyzed with a current of 0.3 ampere. Allowed the deposition to take place until all the copper was deposited, as shown by testing with hydrogen sulphide. Washed with water, and then with alcohol. Dried by igniting the alcohol.
B. —Dissolved in nitric acid and evaporated with 5 c.c. of concentrated sulphuric acid, sp. gr. 1.8, until the nitric acid was removed. Diluted to 80 c.c., added 5 drops of nitric acid, and then electrolyzed as before.
C. —Dissolved in nitric acid, evaporated with 2 c.c. of concentrated sulphuric acid until the nitric acid was removed. Diluted to 500 c.c.; added 50 c.c. of hydrochloric acid of sp. gr. 1.1, and precipitated the copper by hydrogen sulphide. Filtered, with the usual precautions, and then dissolved in nitric acid, evaporated with 2 c.c. of concentrated sulphuric acid, added 4 c.c. of nitric acid, sp. gr. 1.21, and electrolyzed. Treated the copper as before.
The results obtained were as follows:
These results show that a large amount of nitric acid is unnecessary in the analysis of the alloys of copper and zinc, and that a sulphuric acid solution is satisfactory. There is one point to be taken into consideration, however, namely, the disadvantage of considerable sulphuric acid in the solution remaining after the deposition of the copper. If such a solution is to be evaporated to dryness, a nitric acid solution such as was employed in experiment A above is the most suitable. In the assay of copper, as described by Mr. Heath, the sulphuric acid solution of the strength mentioned by him certainly offers more advantages than the nitric acid solution.
The following assays of Lake copper made on 1 gramme of material closely agree with those of Mr. Heath, and may be of interest. The assays were made upon borings from the same ingot. The following results were obtained:
The mention by Mr. Heath of the fact that ammonia is formed during the deposition of the copper is important, and should be taken into consideration, if other metals are to be determined in the solution. For instance, in the analysis of brass by depositing the copper in a solution containing considerable nitric acid, enough ammonia is often formed to prevent the complete precipitation of the zinc by sodium carbonate. I have repeatedly found results to come too low, if in such a case the ammonia is not removed before precipitation with sodium carbonate.
I would call attention to a method of charging storage-cells which, though rarely used, is particularly adapted to work of the kind here described. Owing to the large extent of ground covered, the alternating current is frequently used for incandescent lighting on mine- locations, while the outside lighting is done by arc-lamps. Because of the high first cost and trouble of looking after a rectifier to charge storage-cells from alternating-current mains, I believe it is in every way best, in such cases, to charge from the arc-circuit. The only reason why this is not more generally done is because of the element of danger existing when defective switching-apparatus is employed. Several years ago I had to handle a problem like this at the Michigan College of Mines, and designed automatic apparatus which has since been in continuous use and has given perfect satisfaction. The battery consists of over 60 cells. When charging, they are all arranged in series. Charging is done at night, when no one has occasion to use the battery. Beyond looking after the physical condition of the cells nothing needs to be done except to throw the main switch at any time during the day, if the cells are to be cut in or out that night. Even when charging with a 6.8- ampere current, sufficient energy can be stored during the night to supply the needs of the departments of physics and chemistry for the following day. Anyone interested can see the plant in full operation at the college.
The switching-apparatus is fully described in the Electrical Engineer, November 11 and December 16, 1896. They are free to anyone who cares to make them. The Automatic Circuit Breaker Company, Newaygo, Mich., made from my sketches the apparatus used at the college, and no doubt would be glad to supply anyone who needed similar apparatus.
Mr. Erwin S. Sperry mentions the fact that Hampe found from 0.007 to 0.02 per cent, of antimony in electrolytic copper. Mr. Hampe, at the time he made his experiment, may indeed have found this amount of antimony in electrolytically deposited copper, for the science of depositing copper electrolytically was then in its infancy. Such copper as was then produced by this method could not now be sold in competition with the metal deposited to-day in modern electrolytic plants.
At the Great Falls works we sample and analyze each shipment-lot of cathodes, also each shipment-lot of wire-bars. We invariably find that the total quantity of arsenic and antimony is below 0.005 per cent., and we do not think that this quantity of arsenic and antimony would have any deleterious effects on the qualities of brass. We find that this quantity of arsenic and antimony is not sufficient to prevent the copper from being used for high-conductivity wire. We do not find it necessary to estimate the arsenic separately from the antimony, separate determinations having shown that about two-thirds of the combined quantity is antimony and one-third arsenic. Our usual analysis, therefore, determines only the total of both.
As a further check on the chemist, we have a wire-drawing apparatus, and the wire is tested for conductivity. Our results check very closely with those obtained on samples of the same copper sent East for conductivity-determinations.
Mr. Sperry may have some information showing at what point or at what percentage antimony and arsenic as impurities of copper begin to affect injuriously the qualities of brass. I think the Institute would be pleased to hear from him on this point.
In this connection I give a description, written by Mr. G. A. Heberlein, our chemist, of our method, of determination of combined arsenic and antimony in cathodes and wire-bars, and also for the separate determination of these elements, as practiced in our laboratory.