Table of Contents
Determination of Arsenic and Antimony Together: Instead of separating arsenic from antimony in the hydrochloric-tartaric acid solution which was finally obtained, dilute the solution to four times its volume; pass sulphuretted hydrogen to saturation; filter the combined sulphides on a 4 cm. filter (weighed and dried in weigh-bottle at 110° C.); wash with water, absolute alcohol, carbon bisulphide, and finally absolute alcohol again, very carefully; then dry at 110° C., cool and weigh. More about Electrolytic Assay.
By allowing to dried filters in weigh-bottles the same time for cooling in the desiccator when getting weight of bottle and filter-paper only, as afterwards when filtered sulphides are dry and ready for weighing, very uniform results are obtained.
Arsenic being precipitated as As2S3 and antimony as Sb2S5, their respective factors being 0.609 and 0.600, the factor 0.6 is used in calculating, from the weight of the combined sulphides, the percentage of arsenic plus antimony.
Determination of Arsenic as Trisulphide
Re-dissolve the sulphides in a solution containing hydrochloric acid, water and chlorate of potash. Dilute to at least twice the volume. Pass through the warm solution sulphuretted hydrogen to saturation, to insure the thorough reduction of the solution. Arsenic will be precipitated as trisulphide. After 5 hours’ standing, filter. Wash carefully with water, absolute alcohol and carbon disulphide, in succession. Finish the washing with alcohol. Dry at 110° C. Weigh on filter as trisulphide.
Usually the trisulphide method is employed. For a check upon it, we use the pyro-arseniate method.
Treatment of Filtrate Containing Antimony
Dilute the filtrate containing antimony to at least four times the original volume, and pass sulphuretted hydrogen through it to saturation. The antimonic sulphide (Sb2S5) is filtered off and converted into Sb2O4 by treating filter and sulphide in a weighed porcelain crucible with fuming nitric acid. Heat gradually to bright red heat. Cool and weigh as Sb2O4.
Treatment of Insoluble Residue from the Electrolyte
Treat any residue from filtering by destroying the filter with fuming nitric acid. Fuse the residue with a mixture of sulphur and carbonate of soda. Dissolve in water and filter. Acidulate the filtrate with dilute sulphuric acid. Allow the precipitate to settle, then filter, and dissolve in hydrochloric acid, water and chlorate of potash (KClO3). Add this solution to the filtrate from separation of antimony from arsenic.
This method of determining arsenic and antimony has been repeatedly checked by the sulphocyanide method, and found to be equally accurate and more convenient, as a great many determinations can be run through at the same time.
For very impure anode-copper, we use 25 grammes for determination, and obtain good results by cutting the electrolysis short when about 90 per cent, of the copper has been deposited on the platinum cylinder.
The copper should have the light rose-color and luster of copper deoxydized by hydrogen, and should show a very fine texture.
It seems that some statements in my original paper have started the ball of discussion rolling, so that the original topic has merged into a very interesting discussion by practical men on the highest amount of antimony and arsenic allowable in refined electrolytic copper, the influence of impurities on the conductivity of wire-bars and on the properties of brass, and, finally, methods for the rapid estimation of these noxious elements.
Mr. Sperry has agitated an important question with reference to the limitations of the battery-assay by discussing the influence of antimony in solution, and emphasizing the fact that copper apparently deposits in a cleaner, brighter plate when a few drops of nitric acid are added to the sulphuric acid solution of copper.
It will be noted that my preceding paper recommends a carefully proportioned mixture of the two acids in preference to either alone; the proportion of sulphuric to be increased with the increase of impurities in the material analyzed.
There is some disagreement among experimenters concerning antimony.
Alex. Classen (on p. 93 of the book Mr. Sperry quotes) states that “ Copper can be separated (from Sb and As) only when the quantity of arsenic or antimony is small, and when the current is allowed to act only long enough to reduce the copper.”
Hampe, according to his article cited by Mr. Sperry, found a trace of antimony in electro-deposited copper. He used the current of six Meidinger cells (or probably three-tenths ampere, if the cells were in full working order), which may have been a little too strong, or too long continued after the deposition of copper was complete.
I have just finished the following experiment, which illustrates the point:
Laboratory Experiment No. 8
0.1 gramme of antimony was dissolved in 5 c.c. strong sulphuric acid, diluted to twice its bulk, and poured into a solution of 1 gramme of pure copper dissolved in 5 c.c. of strong nitric acid.
The solution was diluted and electrolyzed, without filtering off antimonic acid, with the use of a current which measured only 0.05 ampere with the Bunsen voltameter in circuit.
Under these conditions the plate remained perfectly bright until the liquid was colorless, and all but a trace of copper was deposited, when the plate quickly began to discolor, and the result was a little high. This corroborates Classen’s statement.
I cannot agree with the inference that the battery-assay is not suited to any refined electrolytic copper, since I have, by Hampe’s methods, proved much electrolytic copper to contain only traces of arsenic or antimony, and have obtained as good rose-red plates of copper by the electrolysis of such material as with native metal.
In case a plate should be slightly darkened, it may be ignited to drive off a trace of impurity (as recommended by Classen); and the oxide may sometimes be removed from the deposit in a neat manner by placing the electrode in a beaker of distilled water, containing a few c.c. of acid, and reversing the current a short time, then reversing the current again. A positive plate, with considerable depositing-surface, is to be used.
I have been accustomed to remove antimony and arsenic from alloys, casting-metals, etc., in the usual manner, by appropriate treatment of the copper solution with hydrogen sulphide and alkaline sulphides, but this is a tedious process.
When arsenic only is present, Mr. A. H. Low’s method of removal has appeared to be the most satisfactory.
He prepares a solution of 2 grammes of sulphur in 10 c.c. of bromine, and adds 2 c.c. of this mixture to a hydrochloric acid solution of the copper, from which most of the free acid has been first removed by evaporation. The liquid is boiled one- half-minute ; then 10 c.c. of strong sulphuric acid is added, and the flask is heated until the acid boils freely.
Mr. Low, however, says : “ There appears to be no simple way to remove antimony.”
Appreciating the value of a method to accomplish this, I have made a few experiments since receiving the foregoing discussion, and have found a method which, as far as present practice goes, proves very satisfactory for the removal of large quantities of antimony.
The process depends on the following statement of Watts :
“ Bromide of antimony (SbBr3) is purified by distillation. It forms, on cooling, a mass of colorless needles, deliquescent, melting at 90° Cent., volatile at 270°. Water decomposes it, forming an oxy-bromide.
“ This compound may be prepared by distilling a mixture of antimony sulphate and potassium bromide.”
As arsenic would generally accompany antimony in copper, the tests were made as follows:
Laboratory Experiment No. 9
0.1 gramme of metallic antimony was placed in a porcelain casserole (No. 3A; 3¾ inches, or 9.5 cms. in diameter) and dissolved in aqua regia, the nitric acid removed by evaporation and the solution treated with 10 c.c. of bromine containing 2 grammes of sulphur in solution.
The liquid was then evaporated on an asbestos plate, or sand-bath, to a syrupy condition; 20 c.c. of pure liquid bromine was then added, and the mixture carefully evaporated until the mass was pasty and the bromide (or sulphobromide ?) of antimony volatilized slowly in white fumes.
Care was taken not to over-heat any portion of the dish; and in an hour or more nothing was left but a little carbonaceous film containing only traces of antimony.
Laboratory Experiment No. 10
0.1 gramme of antimony was dissolved in 5 c.c. of strong sulphuric acid and the bromine distillation conducted as in Experiment No. 9, until the sulphuric acid was also driven off. The removal of antimony was nearly complete, but not quite so perfect as in No. 9. Possibly, if the sulphuric acid solution had been neutralized with an excess of potassium bromide (Watts, loc. cit.), the action would have been complete.
Laboratory Experiment No. 11
A hydrochloric acid solution of 0.05 gramme of antimony was added to a hydrochloric acid solution of 1 gramme of pure copper, contained in a (No. 3A) casserole, and the solution was evaporated nearly to dryness.
It was then evaporated again, as in Experiment No. 9, with 2 grammes of sulphur dissolved in 10 c.c. of bromine until the mass was pasty. The syrupy residue was then treated with 20 c.c. of pure liquid bromine, and the evaporation was continued on the sand-bath until whitish fumes of antimony bromide were no longer evolved, and the copper salt was quite dry and light gray in color.
The casserole was kept covered a short time at first, to guard against loss by spattering.
The hydrochloric acid was, of course, partly removed, but it would possibly be better to use hydrobromic acid in the first place, instead. The distillate on the under-side of the glass cover, which was at first employed, was dissolved and tested, but contained no copper. There ought not to be any loss of cuprous bromide at this temperature, if no part of the dish is too strongly overheated. Chloride of copper, if very strongly heated in contact with air, would be liable to loss.
The residue from Experiment No. 11 was found to contain only a trace of antimony; hence this method for the removal of the obnoxious elements, antimony and arsenic, has proved, so far, to be very satisfactory, and is recommended to chemists for trial. If it proves as successful with others as with the writer, it will enlarge the scope of the battery-assay, and make possible its direct application to the analysis of crude metal.
Methods have been published by well-known authorities for the subsequent estimation of traces of bismuth, if present in the deposited copper plates, or for the electrolysis under special conditions which will partially, at least, prevent the deposition of bismuth.
I will give a short account of other practical methods which I have successfully used for the estimation of arsenic and antimony in refined copper. It may be noted that bismuth—and tellurium—are more detrimental to the refined electrolytic copper than even arsenic and antimony. As bismuth has a greater tendency to deposit on the cathode than the latter metals, a trace of it at least is generally found in the refined metal.
The scheme of Mr. Heberlein, communicated by Mr. Klepetko, is evidently a very good commercial method. In a recent article Mr. Herzig attributes a similar method to Mr. Thofehrn when in charge at the Anaconda works.
I have used Hampe’s methods for the separation of traces of impurities from copper for years, and have been accustomed to separate arsenic and antimony from copper and from each other by one of two or three methods, according to circumstances.
Mr. Heberlein’s plan of fractional precipitation of arsenic in very strong acid solution, in presence of antimony, must give a very good separation, except possibly (as stated by Mr. Herzig) when a little antimonic acid is present in the solution.
The employment by Mr. Heberlein of a large cylindrical platinum cathode with many perforations is a commendable plan for the rapid removal of copper from solution, and worthy of adoption by all laboratories.
After arsenic and antimony sulphides have, in the course of analysis, been separated from precipitated copper sulphide by extraction with pure sodium sulphide, and that solution acidified with dilute sulphuric acid and filtered, the residual sulphides of arsenic and antimony may be separated as follows:
- By fractional precipitation; Mr. Heberlein’s method.
- By electrolysis; Classen’s method, modified by Lecrenier as follows :
The sulphides are dissolved in fuming nitric acid, the acid removed by evaporation, the residue dissolved in pure sodium mono-sulphide (60 c.c.), diluted to 200 c.c. after heating with sodium sulphite, as directed by Lecrenier, and the antimony removed by electrolysis.
One can obtain very good results in this way, but it requires care. A better method for the separation of arsenic from traces of antimony is the well-known precipitation of arsenic acid with magnesia-mixture. The usual method given by Fresenius gives low results, unless a correction is made for the solubility of the ammonium magnesium arsenate.
I have modified this process so that the loss is so small as to be practically negligible.
This is accomplished by keeping the total volume of the solution, after the arsenic is precipitated, at 15 c.c. only.
3. By a modified method described below. By this method the following results were obtained with quantities of pure arsenious oxide as large as would ordinarily be present in a refined electrolytic copper:
Details.—After the purified sulphides of arsenic and a little antimony have been treated with fuming nitric acid or aqua-regia, and the sulphides and sulphur dissolved, the solution is diluted somewhat and filtered from asbestos (if an asbestos filter had been used) and the clear solution evaporated to dryness on the water-bath.
Then dissolve in 1 c.c. of strong sulphuric acid 0.1 gramme of solid tartaric acid; add 5 c.c. of water, filter again (if not perfectly clear) through a very small filter into a No. 000 beaker; wash twice; neutralize with a slight excess of ammonia, and bring the solution either by dilution or evaporation, as necessary, to a total volume of 10 c.c., as shown by a marked paper pasted on the beaker. If much copper was dissolved in the first treatment with sodium sulphide, a second separation of soluble sulphides from that element may be necessary before this part of the operation.
Cool the solution (Vol. = 10 c.c.); add 1 c.c. magnesia-mixture (= 0.04 gramme MgSO4) and make up to the 15 c.c. mark with concentrated ammonia.
The magnesia is made as follows (Fresenius’s formula): By weight, 1 part magnesium sulphate, 2 parts ammonium chloride, 4 parts strong ammonia, and 8 parts water.
Filter after twelve hours; transfer ammonium magnesium arsenate to the 3 cm. filter with the aid of filtrate and wash finally with 10 c.c. of dilute ammonia (1 part to 3 of water).
Place the moist filter in a weighed porcelain crucible, moisten well with saturated solution of ammonium nitrate, ignite cautiously until the paper is charred, moisten with the same reagent again and ignite finally at red heat.
Separate the trace of antimony from acidulated solution as sulphide.
4. By the following method, when arsenic alone is present in copper. Much of the refined product on the market contains no antimony. To detect and accurately determine a slight trace of arsenic in such metal, I devised a very delicate modification of the old, well-known Marsh test, which has been improved (in accordance with the suggestion of Schmidt) so as to yield strictly quantitative results in three hours from the time the residue of arsenious sulphide has been obtained by acidification of its solution in sodium sulphide by the usual method of separation from copper.
This residue is attacked with fuming nitric acid, 5 c.c. of concentrated sulphuric acid added, and the nitric removed by heating. The remaining acid is diluted with 2 parts of water and the solution slowly passed through a Marsh evolution apparatus containing 25 grammes of powdered zinc and a drop of platinic chloride, and enough water to seal the tube. Then 0.5 gramme of pure tin foil is dissolved in 5 c.c. hydrochloric acid and added to the arsenic solution, before passing through the generator. This makes the evolution of arsenic as arseniuretted hydrogen strictly quantitative. The details of the method are fully described in the Engineering and Mining Journal, with the proofs of its accuracy.
The evolution of the gas and the cooling and weighing of a section of the glass tube with metallic film do not require constant attention. The operation maybe completed in two hours, and is very useful for the testing of copper containing no antimony, and only minute quantities of arsenic. It does away with any further purification of the first precipitate of sulphur and arsenious sulphide by reprecipitation and washing with various reagents on tared filters.
I hope to be able to communicate, in a short time, a perfected method, on the lines suggested in my original paper, for the direct electrolysis of copper containing both arsenic and antimony, thus widening the applications of the “ battery assay ” of copper.
Determination of Arsenic & Antimony Separately in Wire Bar & Cathode Copper
Weigh out 100 grammes in portions of 25 grammes each. Dissolve each portion in 95 c.c. of concentrated nitric acid (HNO3) and 75 c.c. of water. Should the sample consist of borings, add the acid slowly. Treat each portion as follows: evaporate to one-third the original volume; take up with solution containing 800 c.c. of water, 22 c.c. of concentrated sulphuric acid (H2SO4), and 2.5 grammes of ammonium nitrate (NH4, NO3) ; electrolyze in a tall No. 6 Griffin beaker with a two-ampere current.
Use a platinum cylinder, 4 inches high and 3 inches in diameter, having 30 rectangular 1¼ by 1/8-inch perforations in two parallel rows ½ inch apart, for cathode. For anode, use a cork-screw spiral of No. 16 platinum wire, running at a distance of 7/8 inch parallel to the inside of the cylinder for the full length of the latter. The shape of the cylinder, and that of the spiral, insure a splendid circulation and a uniform deposit.
Fifteen hours should be sufficient for the deposition of 98 per cent, of the copper on the cylinder, and the deposition should not be carried further than this. Wash off spiral and cylinder. Combine the contents of the four beakers and evaporate till strong fumes of sulphuric acid indicate the absence of nitric acid. Allow the contents of the evaporating-dish to cool. After taking up with 300 c.c. of water, transfer to a No. 6 beaker. Wash out the dish with 20 c.c. of concentrated hydrochloric acid to remove any trace of antimony adhering to the inside of the dish. After combining the two solutions, dissolve 1 gramme of tartaric acid (C4H6O6) in the warm solution. If any insoluble residue be present, filter off and treat according to directions given below.
Through the warm solution pass sulphuretted hydrogen till saturated. After 12 hours’ standing, filter and digest the sulphides in warm sodium sulphide solution, 1.1 specific gravity, in a stoppered flask for five hours, which dissolves completely the sulphides of arsenic and antimony. Filter and precipitate in the filtrate the above-mentioned sulphides by the addition of dilute sulphuric acid. When the sulphides (mixed with sulphur) are settled, filter, and dissolve the contents of the filter in 1 part concentrated hydrochloric acid, 1 part water and chlorate of potash (KClO3) sufficient for oxidation. Drive off the chlorine. Filter off the sulphur. Add hydrochloric acid until the filtrate contains two parts of hydrochloric acid (HCl) to one part of water. Add also 300 milligrammes of tartaric acid. Pass through sulphuretted hydrogen to saturation. All the arsenic will be precipitated as a mixture of As2S3 and S, while antimony will remain in solution. Allow to stand 10 hours; treat the filtrate according to directions later given. Filter off the sulphides of arsenic, re-dissolve, and weigh ultimately as pyro-arseniate or as trisulphide.
Determination of Arsenic as Pyroarseniate
Re-dissolve the sulphides in a solution containing hydrochloric acid, water and chlorate of potash. Drive off chlorine by heat. Add ammonia to alkaline reaction. Add an additional quantity of ammonia equal in volume to the volume of the solution. Cool, and add a few drops of magnesia mixture, containing one part magnesium sulphate, two parts ammonium chloride, eight parts water and four parts ammonia. Allow to stand 12 hours. The precipitate consists of magnesium-ammonium arseniate. Dry, heat gradually as prescribed by Fresenius, until the filter is consumed. Finish at bright red heat. Cool and weigh as pyro-arseniate of magnesia.