INDUSTRIAL AND ENGINEERING CHEMISTRY
428
and enables the stopper to be withdrawn or firmly seated while the pig is in the drier. The effectiveness of this apparatus was tested on cupric sulfate pentahydrate. A few clear crystals were selected and crushed; 9.783 mg. weighed in a previously dried boat and pig were heated in the apparatus 0.5 hour a t 100” C. and pumped during only half this time. The loss in weight was 3.102 mg., corresponding to 4.4 moles of water per mole of sulfate. This may have been all the water originally present in the sulfate; for the present purpose it does not matter. The efficiency of the apparatus and procedure is shown by the fact that two more treatments for the same time gave no further gain or loss, in spite of the fact that the vapor pressure of this substance is approximately 2.7 mm. (9) and the average room temperature was 26.67’ C. (80” F.), with relative humidity of 65 to 80 per cent. In the analysis of organic compounds the following procedure is used. It eliminates much of the work in the method proposed by Hayman (3). The method has been generally applied in this laboratory for the last several years in organic
VOL. 8, NO. 6
microanalytical determinations on many hundreds of compounds. Before the sample is weighed out, the pig and empty boat aSe dried and capped in the apparatus. The sample is weighed and dried and the result recorded. Then the pig is opened and the sample allowed to equilibrate with the moisture in the room. Usually after 30 t o 60 minutes equilibrium is so nearly approached that any further gain in weight while transferring the sam le to the combustion tube is negligible. The pig is then closet! and reweighed. The gain in weight upon exposure to air is subtracted from the weight of water found before the percentage of hydrogen is calculated.
Literature Cited (1) Booth and McIntyre, IND. ENQ.CHEN,,Anal. Ed., 8, 148 (1936). (2) Bower, Bur. Standapds J . Reseawh, 12, 241 (1934). (3) Hayman, IND.ENG.CHEM., Anal. Ed., 4, 256 (1932). (4) Hilbert and Jansen, J . Am. Chem. Soc., 58, 60 (1936). ( 5 ) Pregl, “Quantitative Organic Microanalysis,” 2nd ed., London, J. and A. Churohill, 1930. (6) Sando, Milner, and Sherman, J. Bid. Chem., 109, 203 (1935). RE~CEIVED July 18, 1936. Preaented before the Microchemical Section a t the 92nd Meeting of the American Chemical Society, Pittsburgh, Pa., September 7 to 11, 1936.
Microscopical Qualitative Analysis of Antimony and Bismuth Tetraethyl Ammonium Iodide as a Reagent FRANCIS T. JONES’ AND CLYDE W. MASON, Cornel1 University, Ithaoa, N. Y.
0
F T H E existing microscopical tests for antimony and bismuth, none is sufficiently free from interferences to be satisfactory. Since these two elements often occur together, especially in alloy samples, it is desirable that characteristic reactions should be developed rather than that separation methods should be elaborated, for in rapid analysis a series of preliminary treatments or the precise control of conditions is usually impractical. A brief review (I) of the best microscopical reactions for antimony and bismuth is in order, since in an analysis it is often desirable to employ several tests as checks, or conditions may necessitate the use of one which is not of the highest sensitivity. Cesium chloride gives double salts, 3CsCl.2SbC13, colorless hexagonal plates or rosettes, and 3CsCl.BiCl8, colorless rhombshaped plates, with pure salts. However, 3CsC1.2BiCl8, isomorphous with the antimony double salt, will form if cesium chloride is not in excess, or if antimony is resent. Ag, Pb, Hg.+, Cd, Sn, Tlf, and Cu may also yield crysta?line precipitates of distinctive appearance, but in complicated mixtures or for the detection of relatively small amounts of the elements sought cesium chloride is not specific or sensitive enough. Cesium sulfate yields, with bismuth as sulfate, hexagonal plates; no reaction is given with antimony. Sodium sulfate gives rods or short prismatic crystals. Insoluble sulfates and alum-forming elements interfere. Potassium binoxalate gives tiny tetragonal “octahedra” with bismuth, and trichites with antimony, but so many other metals yield insoluble oxalates that this reaction is useful only for relatively pure salts. Antimonyl tartrates of potassium or barium, possessing characteristic crystal forms, are subject to this same objection. Stibine, generated on a micro scale, may be used as a means of identifying antimony, since bismuth forms no analogous hydride. 1
Present address:
est Grove, Ore.
Department of Chemistry, Pacific University, For-
Metallic bismuth, obtained by the reduction of bismuth salts by sodium stannite, is usefuI confirmatory evidence, since antimony is not reduced under similar treatment. Bismuth cobalticyanide gives a crystalline precipitate, but is not suitable for mixtures containing much antimony. Numerous organic reagents (alkaloids, etc.) have been suggested, most of which yield “amorphous” precipitates, and are subject to interferences.
Reactions with Tetraethylammonium Iodide When a solid fragment of tetraethylammonium chloride is added to a fairly concentrated hydrochloric acid solution of antimony trichloride, colorless hexagonal plates or short prisms are formed. If an excess of potassium bromide is present, colorless hexagonal plates and tablets or short hexagonal prisms will be formed. Both compounds are fairly soluble, and no precipitate will be obtained from dilute solutions of antimony. Both compounds give a positive, uniaxial interference figure. Pentavalent antimony yields a colored and much less soluble precipitate of purple hexagonal plates (Figure 1, left) if iodides are present in a test drop containing even a very dilute solution of antimony. When the concentration of antimony is high, the crystals are likely to be imperfect (fragments, ribbed plates, skeletal stars), and so thick as to appear black. Attempts to prepare this compound in sufficient quantity for analysis failed because of its instability. It may be analogous to the bromo compound reported by Petzold (5) which forms “red pyramids” of the formula [ ( C B W I [SbBrsI. Trivalent antimony gives yellow anisotropic crystals which are usually too small to be reliable as an indentifying form. These crystals generally appear in clusters of three or four, but some individuals may become large enough to be identi-
NOVEMBER 15, 1936
ANALYTICAL EDITION
fiable as rhombohedra growing on a pinacoid face. In concentrated solutions many trigonal and hexagonal plates and also a few hexagonal prisms may grow. I n nearly all cases some purple hexagonal plates of the pentavalent antimony compound will appear with the yellow crystals from trivalent antimony, either immediately or near the edges as the drop evaporates. If the test drop is allowed to stand in the air for 4 or 5 minutes before making the test, the hexagonal plates will appear in large numbers, indicating that antimony is oxidized by free oxygen. Attempts to analyze the yellow compound formed by trivalent antimony have been unsuccessful because it decomposes on standing. T r i v a l e n t bismuth precipitates as small dark amber triangular p l a t e s , t a b l e t s , o r clusters of the same habit as those given by trivalent antimony, but generally larger (Figure 1, right), When mixtures of trivalent antimony and bismuth are precipitated, the color of the crystals obtained varies from yellow to dark amber as the proportion of bismuth is increased, and there is only one crystal habit. This is evidence that the antimony and bismuth compounds are isomorphous. Because of this isomorphism, trivalent bismuth might be interpreted as antimony or vice versa. Oxidation can be utilized to accentuate the difference between these elements, for antimony can readily be rendered pentavalent by evaporation to dryness with nitric acid, while bismuth is not easily oxidized under similar conditions. The purple hexagonal plates of tetraethylammonium antimony iodide then constitute a distinctive test for antimony, and cannot be confused with the amber rhombohedral grains due to bismuth. OF THE (C2H&NI TESTFOR ANTIMOKY TABLE I. SENSITIVITY IN THE PRESENCE OF INTERFERENCE$
Interference. M Bismuth Cadmium Cop er (011s) Leal Mercury Tin (ous) Tin (ic) Oxidizing agents: Arsenjc (AsOs-9 Arsenic ( A s O ~ - ~ ) Copper ( i d Iron (io)
Direct Test Smallest Ratio quantity deSb/M tectable, Sb +5 Gram % 1.0 3 . X 10-8 0.05 0.2 X 10-8 1.0 1.5 x 10-8 3.0 2 5 . 0 X 10-6 2.0 2 0 . 0 X 10-6 0.2 3 . 0 x 100.02
4.0 7.0 1.0 1.0
0.2
x
Volatilization in Presence of SnClz Smallest Ratio quantity deS b / M tectable, Sbf3 % Gram 0.04 0 . 2 X 10-6 o'.o3 0.05
0.05 0.02
10-6
0.02
2 5 . 0 X 10-6
0.03 0.03 0.03
25.0 X 10-8 1 . 5 X 10-6 1.0 X 1 0 - 6
0.07
0.2'Xio-6 0.4 X 10-6 0.4 X 10-6 0 . 2 x 10-1
x
10-6
0.2 X 0.2 X 0.2 X 0.2 X
10-6
0.2
10-6 10-6 10-6
I n the absence of interferences the sensitivity is: concentration limit, 2 X 10-6 gram of Sbf6 per 00. or 1 to 500,000. Smallest quantity detectable, 0.05 X 10-6 gram of Sb c5. a
Bismuth alone, after the above treatment, may yield with the reagent a few light red to pale pink very thin hexagonal plates along with the amber grains. These are evidently isomorphous with the purple hexagonal plates from pentavalent antimony for, when a solution of the latter is added to a test drop containing them, they become much darker and more purplish. Also, when crystals prepared by precipitating the pentavalent antimony compound in the presence of bismuth are treated with sodium stannite, they are blackened, owing to the bismuth in solid solution, whereas if antimony alone is present the crystals are completely dissolved. However, it is apparently not possible to complete the oxidation to pentavalent bismuth by the use of nitric acid and the amber grains are the chief constituent of the precipitate. The pentavalent antimony tetraethylammonium iodide is apparently not isomorphous with the well-known isomorphous
429
FIGURE 1. CRYSTALS OF ANTIMONYAND BISMUTH Left Sb'5 with (CzH6)rNI Rigit, B I ' ~ with(CzHa)rNi
100 X Sb+d gives similar crystals. 100 X
compounds, 3CsI,2BIa and 3CsI.2SbCl3,which form orangered hexagonal plates. Numerous substituted ammonium complexes with antimony and bismuth are described in the literature (3, 4,6, 7). It seems probable that the formula of the precipitate with trivalent antimony or bismuth is 3(C2H6).!X1.2SbI3or 3(C&& NI.2Bi13,and that pentavalent antimony yields (C2HJ4NI+3bIs. TABLE11. SENSITIVITY OF (C2Hj)qNI TESTFOR BISMUTHIN THE PRESENCE OF INTERFERENCES"
Interference, M
Direct Test Smallest Ratio quantity deBi/M tectable, Bifa Gram % 10 25 X 10-6 0.05 0.2 x 10-6 1.0 1.5 X 10-6 3.0 2 5 . 0 X 10-8 12.0 X 10-8 1.7 12.0 x 10-8 1.0 0.2 x 10-6 0.02
NaSnOz on Dry Residue after P t n by (CZH3)4NE1 Plus K I Smallest Ratio quantity deBi/M tectable, Bi+a Gram % 0.08 0.2 X 10-8
Antimony Cadmium .. .. Copper ( o w ) Lead 0:05 0.2 x 10-1 Mercury 0 . 2 b X 10-6 0.05 Tin (ous) .. .. Tin (ic) .. Oxidizing agents: Arsenic (AsOr-8) 2.0 6 . 0 X 10-6 0.1 0 . 2 X 10-6 Arsenio (AsOs-a) 1.0 6.0 X 10-6 0.1 0.1 X 10-6 Copper (10) 1.0 1.5 x 10-6 ., .. Iron (io) 2.0 3 . 0 x 10-6 .. .. With KzSOa (ous) 0.1 0.2 X 10-6 .. .. In the absence of interferences the sensitivity is: concentration limit, 2 X 10-6 gram of Bi per 00. or 1 to 500,000. Smallest quantity detectable, 0.05 X 1 0 - 6 gram of Bi. b Removal of mercury with SnClz, and oxidation of excess of SnCh to SnClr with HNOa before testing.
..
The acidity of the solution to be tested need not be controlled precisely; it is necessary to have enough hydrochloric acid present to prevent hydrolysis, but excess acid tends t o dissolve the precipitates and lower the sensitivity of the test. It is possible to recrystallize the precipitates by warming with hydrochloric acid and allowing to cool; this yields larger crystals and may improve the sensitivity of the test when interfering substances are present. Oxidizing agents, including cupric, ferric, and arsenate ions, interfere by liberating iodine, which may give yellow crystals with tetraethylammonium chloride, or may appear as black to brown rhombs and prisms of the element. Potassium sulfite will reduce the free iodine without reducing the antimony or bismuth, unless the solution is heated.
Mixtures and Possible Interferences Antimony in the presence of bismuth will be recognized most readily by the purple plates of the Sb+5compound, after oxidation with nitric acid but, since bismuth may in dilute solutions yield some red to pink hexagonal plates, the distinction is not conclusive if the ratio of antimony ta bismuth is less than about 1 per cent.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
The volatility of antimony trichloride (b. p. 220') may be utilized to separate it from interfering substances, with a corresponding increase in the sensitivity of the reaction. The acidified test drop is heated nearly to boiling above a microburner, while just above its surface a slide carrying a small hanging drop of tetraethylammonium chloride solution is held. The upper slide is cooled, a drop of water is added to the reagent, and then a fragment of potassium iodide is introduced into this solution. As the drop concentrates by evaporation, crystals of the antimony compound will form. (Crystals of a compound between the reagent and potassium iodide may appear next to the potassium iodide fragment, but these are easily disregarded.) The separation just described is most satisfactory if a drop of stannous chloride has been added to the unknown just before volatilization. Stannous chloride reduces the oxidizing agents, acts as a carrier for antimony, and provides an acid atmosphere. The amount of tin carried over does not reduce the sensitivity of the test. will give large, Arsenic in the form of arsenites or as yellow, hexagonal or trigonal plates of arsenic triiodide when potassium iodide is added to the test drop, but this compound is too soluble to cause trouble unless the concentration of arsenic is high, and its formation can be avoided by diluting the test drop. Addition of the reagent then causes no change. The chief difficulty is caused by the fact that arsenites are readily oxidized by the air to arsenates which liberate iodine. The limit of sensitivityin the presence of arsenates is 4 per cent of antimony, or 2 per cent of bismuth. If potassium sulfite is used to minimize the liberation of iodine, the limit of sensitivity can be somewhat extended. The use of stannous chloride and separation by volatilization of antimony trichloride is desirable in the presence of arsenic. The limit of sensitivity is 0.03 per cent of antimony. The use of stannite solution as described below permits a limiting ratio of 0.1 per cent of bismuth to arsenic. Bismuth in the presence of antimony is not recognizable with absolute reliability by means of tetraethylammonium iodide. The most useful confirmatory reaction is based upon reduction to metallic bismuth by stannite solutions. If there is any question as to whether the colored (pink to purple) hexagonal plates obtained after oxidation and treatment with tetraethylammonium chloride and potassium iodide are the bismuth compound, or contain bismuth in isomorphous mixture with the antimony compound, flooding the dry residue from the test made in the usual way with a moderately concentrated solution of sodium or potassium stannite will result in a blackening of crystals containing bismuth. Pseudomorphs are formed if the ratio of bismuth to antimony is greater than 2 per cent; below this concentration black grains and skeletal masses of bismuth are formed (examine by reflected light and at high magnification, to be sure this residue is black). Crystals which do not contain bismuth will become white from the formation of hydrated antimony trioxide, and slowly dissolve. Trivalent antimony may be recognized in this solution, as yellow rhombohedral grains, by acidification with hydrochloric acid. The stannite solution must be clean, and contain no black specks of tin. The reduction of the bismuth may not be immediate,- and tin may separate in the crust a t the edges of the test drop. Lead compounds also may be reduced and blackened by the stannite reagent, but this occurs slowly in the cold. A very small amount of bismuth catalyzes the reduction (3) and may thus be recognized in smaller quantity than if lead were absent. Cadmium may yield colorless rectangular plates, not to be confused with bismuth or antimony compounds. Copper liberates iodine and forms fine granular (actually
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isotropic tetrahedra) white cuprous iodide, soluble in excess of potassium iodide. Iron (ferric) liberates iodine. If reduced by sulfite, there is no interference. Lead is not completely precipitated as the chloride; the characteristic yellow hexagonal plates of its iodide are soluble in excess potassium iodide. When tetraethylammonium chloride is added to the solution containing Pb++ and potassium iodide, a precipitate of colorless needles and prisms may be formed; lead iodide is also likely to appear. Neither of these should be confused with the antimony or bismuth reactions. In the presence of much lead the crystals of the bismuth compound tend to become imperfect and leafy; tiny black needles are also likely to appear; the red bismuth compound may overgrow the edges of the plates of lead iodide. The ratio of antimony or bismuth to lead must be above 3 per cent for a positive test. The stannite reduction affords an excellent means of avoiding the interference of lead with bismuth, but since the presence of bismuth will hasten the reduction (ordinarily very slow) of lead to the metal, it is desirable to compare the test with a blank made on lead alone. Mercuric ion yields red mercuric iodide, soluble in excess potassium iodide, from which solution the reagent may cause yellow tapering prismatic crystals to form. After reduction with stannous chloride, metallic mercury may be separated from the solution to be tested, by decantation or filtration, and the liquid then evaporated with nitric acid and tested with tetraethylammonium chloride and potassium iodide. The volatility of mercuric chloride interferes with the separation of antimony as antimony trichloride, unless any mercury present has first been reduced in this manner. Tin (stannic) may yield large colorless octahedra of potassium chlorostannate if present in large amount, and a red crust may form a t the edge of the drop on evaporation, but these crystals are readily soluble, and not to be confused with those from antimony or bismuth. Stannous ion gives a precipitate of colorless to pale yellow rods, needles, or hexagonal prisms, and also reduces antimony to the trivalent condition. Unless some reoxidation by the air occurs, the characteristic purple plates from pentavalent antimony, or the pink ones obtained from bismuth, will not be produced. The amber rhombohedral grains of the trivalent bismuth compound will still appear. The needles and prisms of the stannous compound are amber to brownish red in the presence of bismuth, and are pleochroic; this coloring is a useful indication of bismuth in very small proportion (< 0.1 per cent). The interference of stannous tin is readily eliminated by evaporation with nitric acid.
Analytical Procedure If the unknown is a solution, evaporate to dryness with nitric acid; then dissolve in about 3 N hydrochloric acid (Ag, Hg+, and Pb may precipitate); add a fragment of solid potassium iodide to the test drop (note any significant precipitate-Ag, Hg, Pb, Cu, Se, Te, I-or color in solution-Bi, Sb, As). Then add a small drop of a moderately concentrated solution of tetraethylammonium chloride. Examine, and note any subsequent preci itation. If the product is too fine-grained, recrystallize by aiding a little hydrochloric acid, warming gently and cooling. If numerous purple hexagonal plates are formed, antimony is resent; if only a few pink ones, together with yellow granules, {ismuth may be present. Confirm antimony by volatilization as antimony trichloride in the presence of stannous chloride. Confirm bismuth by the stannite reaction on the precipitate with tetraethylammonium chloride. If the unknown is a solid, dissolve in water if ossible, noting any indications of hydrolysis t o oxy compounds (Eb, Bi). Wash any residue, and extract with 3 N hydrochloric acid, or with aqua regia. Evaporate almost t o dryness by warming gently, dissolve in 3 N hydrochloric acid, and test as above. If the unknown is an alloy, treat with concentrated nitric acid in a microcrucible (note any formation of white oxides of tin or
'
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.
431
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.
T
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
