NOVEMBER 15, 1936
ANALYTICAL EDITIOK
antimony), and evaporate to a thick paste to remove excess acid. Extract the residue with,water to remove soluble nitrates of possible interfering mktals. Then extract the hydro1 zed residue of stannic oxide and oxynitrate of antimony anc? bismuth with warm 3 N hydrochloric acid, in which antimony and bismuth will be dissolved and can be tested for as above. If very small amounts of antimony and bismuth are present, extract the residue from evaporation with nitric acid with concentrated nitric acid or dissolved in aqua regia; excess oxidizing agent may be removed bji evaporation, by potassium sulfite, or by stannous chloride before testing as above. all these procedures, microtechnic is to be employed, in refinement commensurate with the sensitivity required.
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Literature Cited (1) Condensed from Chamot and Mason, “Handbook of Chemical Microscopy,” Vol. 11, New York, John Wiley & Sons, 1931. (2) Feigl and Krumholz, Ber., 62, 1138 (1929). (3) Gutbier and Hausmann, 2.anorg. allgem. Chem., 128, 153 (1923). (4) Gutbier and Miiller, Ibid., 128, 137 (1923). (5) Petzold, W., Ibid.,215, 92 (1933). (6) Remy and Pellens, Ber., 61, 862 (1928). (7) Vournasos, A. C., 2. anorg. allgem. Chem., 150, 147 (1926). RECEIVED May 13, 1936. Presented before the Microchemical Section at the 91st Meeting of the dmerican Chemical Society, Kansas City, Mo., April 13 t o 17, 1936.
Electroanalysis of Silver-Copper Alloys WALTER L. MILLER Chemical Division of Material Laboratory, Navy Yard, Brooklyn, N. Y.
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H E increased application of silver solders in fabricating copper alloy products creates a demand for rapid methods of determining copper and silver. Gravimetric methods are preferable for greatest accuracy. The American Society for Testing Materials provides an accurate method for silver solders in which silver is precipitated and filtered as the chloride, the filtrate is evaporated to fumes with sulfuric acid, nitric acid is added, and the solution is electrolyzed for copper (1). The evaporation of the filtrate, which must be done slowly and carefully to prevent loss by spattering, is the most tedious part of this method. This step is eliminated in the method presented, by the substitution of electrodeposition for the chloride precipitation of silver. Experiments with electrolysis in hot nitric acid solutions gave fairly good separation of silver from copper, but the conditions were very strict and more than 0.2 gram of silver could not be weighed accurately because of poor adherence. Deposition of silver from ammoniacal solutions gave accurate results with a fair latitude in conditions. Complete separation from copper was obtained and the deposit was firmly adherent. The electrolysis of silver in ammoniacal solution requires continuous stirring. Silver is deposited a t the rate of about 0.0268 gram per minute, using 0.4 ampere, until deposition is practically complete. Hydrogen peroxide is then added to oxidize cuprous salts and to redissolve any particles of silver precipitated by inefficient stirring. Electrolysis is continued at 0.2 ampere until deposition is complete. Hydrogen peroxide is gradually destroyed during the latter stage of the electrolysis, leaving the electrolyte a nonsolvent for metallic silver and promoting complete deposition. Deposition of copper and less noble metals is prevented by the rapid circulation of cupric ions and oxygen from the anode. Cupric ions have a strong oxidation effect on metallic copper and less noble metals in ammoniacal solution, and by stirring sufficiently to prevent the formation of a protective layer of cuprous ions silver alone is deposited on the cathode. Stirring also helps to regenerate cupric ions a t the anode. Nitrates present from the nitric acid used in dissolving the sample play an important part in preventing deposition of base metals and also help to reduce the resistance of the electrolyte, thereby keeping the solution cool and preventing loss of the ammonia. Silver deposits completely from the ammoniacal solution and only unweighable traces have been found remaining in the electrolyte. Likewise only traces of copper are deposited with the silver, and if weighable amounts should be deposited because of inefficient stirring they are readily visible on the
surface of the silver. Silver has much less tendency to airoxidize than copper, and its firm adherence on the cathode prevents loss when handled with the care usually taken in copper electrolysis. The deposition of silver from ammoniacal nitrate solution is comparable with that from alkaline cyanide solution. The complex salt prevents the formation of excess metallic ions around the cathode. This gradual breaking down of the complex ions promotes the formation of finegrained deposits rather than coarse, loosely adherent crystals which are obtained from acid solutions. The electrolyte from the silver determination is acidified with nitric acid and copper is determined electrolytically. The results compare favorably with other electrolytic copper determinations. The presence of considerable amounts of ammonium nitrate apparently helps in obtaining complete deposition of copper. Anodic loss proved to be less than 0.00005 gram when both silver and copper were electrolyzed using the same anode.
Procedure Dissolve a 1-gram sample in a 300-cc. beaker, using 10 cc. of concentrated nitric acid and 20 cc. of water, and heat to expel lower oxides of nitrogen. Cool, and make the solution distinctly alkaline with ammonium hydroxide. Cool again to room temperature and add 10 cc. of concentrated ammonium hydroxide in excess. A cylindrical, platinum gauze cathode of about 100 sq. cm. and an anode of spiral-shaped platinum wire are required for electrolysis. The solution is diluted so that, when immersed, the top rim of the cathode is just above the solution level (about 150. to 200 cc.). Continuous stirring is required during the electrolysis and must be started before any current is used. (In the absence of a stirring device, efficient stirring may be obtained by passing a moderate stream of air bubbles from a glass tube with a capillary tip, adjusted to deliver at the bottom of the anode.) Cover the beaker with split watch glasses and electrolyze a t 0.4 ampere. Allow 10 minutes or longer over the time required for deposition of the silver at the rate of 0.027 gram per minute, then add cautiously from a pipet 10 cc. of a mixture of 1 part of U. S. P. hydrogen peroxide and 3 parts of distilled water. If the sample contains less than 10 per cent of copper, add only 4 cc. of the mixture. Do not direct the peroxide against the cathode. Reduce the current to 0.2 ampere and rinse the cover glasses. After 20 minutes, lower the beaker without interrupting the stirring and rinse the electrodes with a jet of distilled water, catching the rinsings in the beaker. Di the cathode in alcohol and dry at 110’ C. Any deposition o?copper is evidence of inefficient stirring. This copper may be removed by continuing the electrolysis with an increased rate of stirring. If a yellowish color is evident on the cathode, dip the cathode in dilute hydrochloric acid after weighing, rinse, dry, and weigh again, taking the difference in weight as copper. With proper stirring, however, only metallic silver will deposit. Acidify the electrolyte with concentrated nitric acid and add 10
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cc. in excess. If any cloudiness develops due t o cuprous oxide, add sufficient hydrogen peroxide to clear the solution. Cool the solution t o room temperature, transfer to a 500-cc. beaker, and electrolyze for copper at 2.5 amperes.
Precautions Interruption of the stirring during the early stages of the silver electrolysis will cause precipitation of considerable amounts of silver which remain undissolved on the bottom of the beaker. lnsufficient stirring near the end of the electrolysis will cause deposition of copper. With these precautions in mind, the analyst may even double the rate of deposition and obtain accurate results by using a sufficient stirring rate. TABLEI. PERCENTAGE OF SILVER IN REPRESENTATIVE SILVER SOLDERS Sample No.
1 2 3 4 5 6
7 8 9
10
Electrolytic Method 15.10 15.19 20.26 20.08 45.10 46.04 64.99 64.98 50.22 50.21
Silver Chloride Method
Lead and metals that are more noble than copper will deposit with the silver. Lead deposits partly on the anode and partly on the cathode. Nickel present in amounts over 5 per cent in the sample causes poor adherence of silver and loss by dusting. Cathodes should be carefully stripped of all silver before heating to high temperatures or silver will alloy with the platinum.
Discussion Table I shows results obtained by electrolysis as compared with the A. S. T. M. silver chloride precipitation method. The figures given in the silver chloride column were obtained by making several determinations until absolute checks were obtained. Five different types were selected as representative
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of the more common silver solders. In addition to the silver, the compositions of the samples were as follows: 1 and 2 contained 80 per cent of copper and 5 per cent of phosphorus; 3 and 4 contained 45 per cent of copper and 35 per cent of zinc; 5 and 6 contained 30 per cent of copper and 25 per cent of zinc; 7 and 8 contained 20 per cent of copper and 15 per cent of zinc; 9 and 10 contained 15 per cent of copper, 17 per cent of zinc, and 18 per cent of cadmium. TABLE11. WEIGHTOF SYNTHETIC SAMPLES Sample No.
11 12 13 14 15 16 17 18 19 20
Silver Added Grams 0,9998 1.9996 0,9998 0,9998 0,9998 0.3862 0.2536 0.3506 0.3057 0,3029
Silver Deposited Grams 0,9998 1.9995 0.9999
0,9997 0.9997 0.3861 0.2636 0.3606 0.3067 0.3028
Co er Ac&d Grams Nil Nil 0.1
0.5 1.0
1.0 1.0 0.8169 0.5075 1,0122
Copper Deposited Grams
.... .... .... .... .... .... ....
0.8171 0.5076 1.0124
Table I1 shows the wide range of applicability of the electrolytic method. The silver and copper used in preparing the samples were carefully assayed by A. S. T. M. methods. Both metals were 99.98 per cent pure and the weights used in the tables are based on this metallic content. Both tables indicate that the error for silver may be due only t o weighing. Slightly high results for copper may be due to air oxidation. The procedure apparently causes no abnormal error for copper.
Conclusion Electrodeposition of silver from an ammoniacal nitrate solution is rapid and accurate. Copper may be determined rapidly by electrolysis from the acidified electrolyte. The method may be used for silver solders and various other silver alloys.
Literature Cited (1) Am. SOC.Testing Materials, “A. S. T. M. Standarda,” Part I, pp. 831-3 (1933). RECEIVED June 16, 1936.
Determining the Evaporation Rate of Solvents at High Temperatures F. C. THORN AND C. BOWMAN, Garlock Packing Co., Palmyra, N. Y.
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HE recent technical literature has shown evidence of a considerable revival of interest in the subject of evaporation rate of solvents (1, 2, 5, 6, 7, 9). Most of the methods, however, have been designed to duplicate the conditions prevailing during the evaporation of solvents from varnish and lacquer films-i. e., the evaporation of thin layers of solvent into large volumes of air a t approximately room temperatures. There is an extensive field for solvents in factory processing wherein the solvent is expelled with the aid of heat, and for which the foregoing methods of solvent evaluation do not appear to be adequate. The authors thought that it might be of interest a t this time to report a method which has been used in this laboratory in substantially its present form for the past ten years for the purpose of evaluating petroleum solvents employed in the manufacture of rubber cements and compressed asbestos sheet doughs,
from which they are subsequently expelled with the aid of heat and air. The features that distinguish this method from any of the methods above referred to are: 1. Approximately complete saturation of the air stream, thereby eliminating the time element and making results available directly as liters (or cubic feet) of air per cubic centimeter (or gallon) of liquid solvent. 2. Provision for maintaining sample a t any desired temperature by vapor heating. Vapor heating is preferred to an air or water thermostat because of its high rate of heat input. 3. Provision for reading volume of sample a t all stages of evaporation, without moving any part of the equipment. 4. Use of a large sample, permitting accurate determination of the dry point.
