Aqueous Sodium Borohydride Chemistry The Coinage Metals, Copper, Silver, and Gold SIR: A 1)revious publication describes procedures for the quantitative separation of lead and barium, of cadmium and mercury, of lead and zinc, and the semiquantitative separation of cadmium and zinc ( 3 ) . Since this technique is simple and rapid and does not require elaborate equipment, the investigation was continued to develop procedures for the reduction of the coinage metals. EXPERIMENTAL
Apparatus and Reagents. T h e p H of t h e solutions was determined with a 13eckman Zeroniatic p H meter. SODIUM BOROHYDRIDE. Approximately 1% aqueous solution was prepared just before use from 98+% sodium borohydride purchased from Metal Hydrides, Inc. Solution concentrations were approximated on a weight basis-for example, 1% Na13H4was made by dissolving 1 gram of I%aBH4in 100 ml. of solution. GoLD(III) CHLORIDE.Stock solutions of gold chloride, approximately 0.065, were prepared by dissolving pure metallic gold in chlorine water and diluting t o 1 liter. The 0.1,V solutions of comer chloride, copper nitrate, copper sulfate, and silver nitrate were standardized using established urocedures described in the literatwe ( 2 ) Procedure. Twenty-five-milliliter aliquots of copper chloride, copper nitrate, copper sulfate, and silver nitrate were adjusted t o initial p H values of 2, 5 , 7 , 9, and 11. Fifty milliliters of aqueous sodium borohydride were added dropwise to each. T h e reduction medium was mixed with a magnetic stirrer and the reductions were carried out in a well ventilated hood. T h e metallic precipitates which formed were collected on No. 2001 Selas crucibles, washed with distilled water and acetone, vacuum dried, and dissolved in nitric acid. The solutions were boiled to expel dissolved oxides of nitrogen and were transferred to Erlenmeyer flasks. The copper present was determined by the iodine method (1). The silver was determined volumetrically with a standard thiocyanate solution using ferric alum as the indicator for one set of samples, and gravimetrically as the chloride for another set. A third set of samples, after the reduction of the silver by sodium borohydride, was collected on crucibles, washed well with distilled water, and finally with acetone, and then dried in a vacuum desiccator. For the study of the reduction of gold(II1) by sodium borohjdride, a set of test solutions containing ilu(II1) ion were made 1.0, 0.5, and 0.1N in HC1, 1.0, 0.5, and 0.1N in NaOH, 0.1N in H N 0 3 , and 0.1N in KH3. For these
experiments the sodium borohydride was dissolved in 0.LV NaOH and added dropwise with constant stirring. The reaction mixture was left standing overnight or digested on a hot plate for 1 to 2 hours. The resulting brown spongy precipitate was filtered and washed thoroughly with water. The precipitate was transferred to a porcelain crucible, the filter paper was charred off, and then the residue was brought to a red heat in a Bunsen burner flame. The crucibles were cooled and weighed. The precipitates were washed with water, dissolved in a 1:1 hydrochloric acid solution and 30% hydrogen peroxide, and tested for boron. RESULTS AND DISCUSSION
Reduction of Copper(I1). When sodium borohydride is added t o a copper(I1) solution there is a marked change from a blue colored solution or a blue colored precipitate to a green then t o a brown colored solution or precipitate and finally t o a reddish brown solid or t o a black finely divided velvety solid depending on conditions used. Both forms contain shiny pieces of, apparently, free copper. These observations and the differences in the solubilities of the products in
Table 1.
various solvent systems suggest that possibly copper(I1) is reduced to copper (I) then to copper(0) or t o a mixture of copper metal and small percentages of some other copper species. A study was made of the effect of anions present and of the sodium borohydride solvent system. I n all instances the copper was completely recovered as determined by quantitative tests on the precipitate and by qualitative tests on the filtrate. The results are given in Table I. The anion present and the solvent system for the reducing agent affect the nature of the precipitate, and under some conditions a product of varying and undetermined composition is obtained. However, the direct weighing of the product can be made the basis for the quantitative determination of copper in the absence of interfering species if the copper(I1) salt is a nitrate, or if it is a sulfate and the reduction is carried out in the presence of ammonia. Reduction of Silver(1). When sodium borohydride is added to the silver nitrate aliquots, there is a rapid evolution of hydrogen and the solution turns brownish black. With samples adjusted t o initial p H 11 with
Effect of Anion Present and Solvent System for NaBH4 on Reduction of Cu(II) Salts
Solvent for NaBHa HzO 6 N NH, 15N NH, Cu in Cu in Cu in precipitate, Rel. std. precipitate, Rel. std. precipitate, Rel. std. %a dev., % %a dev., 97, 97," dev., 97,
Copper species used to prepare Cu(I1) solution CUClZ 96.23 Cu(N03)z 99.88 CUSOA 92.15 Cu(NH3)&12 100 1 CU(NHs)r(N03)~ 100 5 Cu(NHa)aSO, 100 9 a Average of three values.
Table II.
Initial PH 2 5
7 11 a
0 2 0 2 0.03 0 7 1 3 0 7
94 88 99 72 100 0
0 05 0 5 0 6
93 18 99 79 99 96
0 2 0 5 0 06
Percentage of Silver Recovered after NaBH4 Reduction of Ag(l) Determination of Ag
Volumetricallv Gravimetricallv with SCN-" as AgCl - Directly as Ag Ag in Ag in Ag in precipitate, Rel. std. precipitate, Rel. std. precipitate, Rel. std. 7Oa dev., % %" dev., 9% %a dev., yc 99,89 99.86 99.97 100.04
0.09 0.08 0.09 0.00
99.81 99,96 99.82 99.68
0.11 0.20
100,06
0.15
100.04 100.77
0.15
100.00
0.07 0.13 0.07 0.18
Average of four values.
VOL. 37, NO. 9, AUGUST 1965
1163
Table 111.
Gravimetric Analysis of Reduction Products from Known Amount of Gold(ll1)
Procedure Oxalic acid reduction Sodium borohydride reductions Au(II1) in 0 . 1 N HC1 Au(II1) in 0 . 1 N “ 0 % AuiIIIj in neutral solution Au(1IIj in neutral solution A~i(I11)in 0 5N NaOH a
Au taken, mg. 107.1
Au found, mg. 106.1 f 0.4O
107.1 100.0 100.0 107 1 107 1
107.9 f 0 . 3 100.7 f 0 . 2 100.4 f 0 . 3 107 4 f 0 2 108 1 f 0 2
Av. 70AU recovered (95% Confidence Limits) 99.10 f 0 . 6 100.8 f 0 . 2 100.7 f.0 . 3 100.4 st 0 . 3 100 3 f 0 3 100 8 f 0 2
f, the standard deviation from the mean for the 95% Confidence Limits.
NaOH, the silver ion is precipitated as the insoluble Ag20. T h e reduction of this species takes place slowly but completely. The results, summarized in Table I1 show that under all conditions employed, Ag(1) is converted quantitatively to Ag(O), and that in weakly acidic and neutral solutions, the contamination is a t a level so low as to have no significant analytical effect. A consideration of the data in Table I1 suggests a gravimetric procedure for the determination of silver in silver salt solutions in the absence of interfering ions. This procedure would consist of the addition of aqueous sodium borohydride to a very weakly acidic or neutral solution. (The statistical “t” test indicates no reason for believing that there is any significant difference between the sets in the determination of silver a t pH 2 , 5 , and 7 ) . The reduced silver would be collected on Selas crucibles, washed with distilled water, then with acetone. That the product from such a procedure can be weighed directly as silver to give analytical results comparable to standard methods is apparent from the data in Table 11. By this procedure, sodium borohydride could be used for the preparation of
pure silver and simultaneously for the analysis of the amount of silver present. Reduction of Gold(II1). T h e Au (111) reduction product digests in 1 hour to give spongy brown or black lumps (containing yellow specks), which form is readily filterable. When ignited in a porcelain crucible in a Bunsen flame, the precipitate turns to the dull yellow color of pure gold. The dissolved precipitates give a negative flame test for boron. The comparative study of the standard oxalic acid method with the sodium borohydride method is given in Table 111. The sodium borohydride procedure is much simpler and more rapid than other standard methods for determination of gold and allows wider limits in the acidity of the reaction medium. An important factor of the borohydride procedure is the ease with which the gold precipitate is digested; it requires less than one hour as compared to at least four hours by the oxalic acid procedure. The oxalic acid precipitate is very fine, almost colloidal, and offers difficulty in filtration. One disadvantage of the borohydride reduction is its nonselectiveness, as contrasted to the high selectivity of the oxalic acid as a reducing agent for Au(II1).
Sodium borohydride thus offers a new, simple, rapid method for the determination of silver and gold from solutions of 4g(I) and Au(II1) ions in the absence of interfering species -namely, the reduction of the element by the reagent in dilute acid or neutral solution and the weighing of the dried reduction products. The coinage metals caiinot be separated from one another using NaBH4 according to our procedure described in the previous publication (3) because they are all reduced within the limits of the p H range studied. Also, when our procedures are employed, cobalt, nickel, palladium, platinum, cadmium, mercury, tin, lead, arsenic, antimony, and bismuth when present in solutions of the coinage metals interfere with the purity of the reduction products. However NaBH4 reduction under appropriate conditions leads to near quantitative separation of the coinage metals from the metals listed. LITERATURE CITED
(1) Furnam, N. H., “Standard Methods of Chemical Analysis,” pp. 404, 472, Van Nostrand, Princeton, N. J., 1962. ( 2 ) Hildebrand, W. F., Lundell, G. E. F., “Applied Inorganic Analysis,” pp. 207, 248, Wiley, New York, 1953. (3) Schaffer, G. W., Waller, Mary Concetta, Hohnstedt, L. F., ANAL. CHEM. 33, 1719 (1961).
L. F. HOHNSTEDT B. 0. MISIATAS~ WALLER~ SISTERM.CONCETTA
Department of Chemistry Saint Louis University Saint Louis 19, MO. WORK supported in part by ru’ational Science Foundation Grant G5102. I n part: Division of Inorganic Chemistry, 140th Meeting, ACS, Chicago, Ill., September 1961. Present address. DeDartment of Chemistry, Illinois Insiituce of Technology, Chicago, Ill. 2 Present address, Ursuline College, Louisville 6, Ky.
Addendum. Simultaneous Use of Beta-Particle Transmission and Backscatter Gauges for Determining Hydrogen, Carbon, and Oxygen Percentages in Liquids
SIR: I t has come to our attention that in our recent paper (1) we failed to acknowledge the prior work of Gray, Clarey, and Beamer (2) on a similar topic. the instruments and techniques ( 2 ) are capable of higher 1 164
ANALYTICAL CHEMISTRY
accuracy than ours, but are more expensive and tedious. LITERATURE CITED
(1) Gardner, R. P., Dunn, J. W. 111, ANAL.CHEM.37, 528 (1965).
(2) Gray, P. R., Clarey, D. H., Beamer, W. H., Zbid., 32, 582 (1960).
R. P. GARDNER
J. W. D U N NI11
Measurement and Controls Laboratory Research Triangle Institute Durham, N. C.
