Determination of Electrolytic Copper A Microcerimetric Method L. H. BRADFORD AND PAUL L. KIRK, University of California Medical School, Berkeley, Calif.
T
performed until the current \vas essentially zero, as indicated by an ammeter in the electric circuit.
HE classical Pregl method (2) of determining copper by electrolysis from a sulfuric acid bath has been used successfully with certain limitations. I n the Pregl method, a
Procedure Samples of known copper content were placed in the electrolysis cell and the solution was made 2 N in sulfuric acid and 1 N in nitric acid. The electrodes were inserted, the condenser was put in place, and the solution was heated to boiling. The circuit was closed and electrolysis continued for 8 minutes. The condenser and flame were removed, a small funnel and siphon were put in place, and distilled water was circulated through the cell until the ammeter reading dropped to zero. The gauze electrode was removed, given a quick rinse in distilled water, and immersed in 10 ml. of the standard ceric sulfate solution made 0.5 N in chloride ion. After 2 minutes of stirring with the electrode, this stripping solution was titrated back with the standard Mohr salt solution. With the proper setup and using two gauze cathodes (one solution being electrolyzed while a titration was being carried out) complete determinations could be made in not more than 20 minutes. Two equivalents of ceric ion were reduced per mole of copper, which was oxidized from the metallic state to cupric ion.
platinum electrode is weighed on a microbalance before and after deposition of the copper. The time factor imposed by microgravimetric procedures is objectionable, even when the microbalance is readily available. Hence, a volumetric method utilizing ceric sulfate oxidation of the copper has been developed in order t o facilitate rapid, accurate determinations of the electrolytic copper. During the course of investigation, the Pregl electrolysis technique was modified somewhat in order t o ensure complete deposition and provide for a minimum of manipulation. EFFECT ON COPPERTITRATION TABLE I. CHLORIDE Chloride Concentration ,V 0.025
0.5
(1.234 mg. of copper taken) Chloride ConcenError tration Mo. % N 1.215 -1.74 1.0 1.170 -5.2 1.239 -I-0.38 1.233 -0.08 1.22s -0.56 2.0 1.235 $0.08
Copper Found
Copper Found
MO
.
1.232 1.246 1.245 1.226 1.242 1.237
Error
% -0.18 4-1.10 +1.01 -0.74 i-0.74 +0.28
Results
It had been observed that the direct oxidation of electrolytic copper by ceric sulfate alone was not satisfactory, apparently because of a slow rate of oxidation of the metallic copper to cuprous ion, giving a n opportunity for air to produce a portion of the oxidation in the acid solution. The addition of chloride ion markedly improved recovery, coinciding with a large increase in rate of the first oxidative step. Since chloride is not rapidly oxidized in the cold by ceric sulfate, this procedure was tested. The most favorable concentration of chloride was studied, the results being given in Table I. Dilute chloride tended to give slightly low and variable r e covery, while 1 and 2 N concentrations showed the formation of a yellowish-green color, tending to obscure the end point. The most favorable concentration was found to be 0.5 N .
The method described is based on the finding t h a t electrolytic copper can be stoichiometrically oxidized with excess ceric sulfate in 2 N sulfuric acid when the proper amount of chloride ion is present. The excess ceric sulfate can then be titrated with Mohr salt, using o-phenanthroline ferrous complex as an indicator. A fine-grained adherent copper deposit may be obtained by electrolysis at 2.0 volts from a 2 N sulfuric acid bath when the bath is 0.8 N to 1.0 N in nitric acid. This concentration of nitric acid prevents interference of other metals when electrolysis is conducted at this voltage.
TABLE 11. COPPERDETERMINATION
Reagents Ceric sulfate solution, approximately 0.015 N , was prepared according to the instructions of Willard and Young (5) modified by Kirk and Tompkins ( I ) , and carefully standardized against pure sodium oxalate by the procedure of Smith and Getz (5). Ferrous ammonium sulfate (Mohr salt) solution, approximately 0.015 N , was prepared in 0.1 N sulfuric acid, and accurately standardized against pure sodium oxalate through ceric sulfate solution. Before standardization, 0.025 N ferrous o-phenanthroline was added in the pro ortion of 2 drops to each 5 ml. of solution. Copper sulfate soktion containing approximately 1 mg. of copper per ml. of solution was prepared from selected crystals of .copper sulfate. The solution was analyzed by the method of Swift (4). Apparatus The electrolysis apparatus was essentially that of Pregl (8), the most important modification being in the condenser, which was constructed as shown in Figure 1. Experience has shown that if the solution bumps or foams, the Pregl condenser does not have enough condensing surface to prevent some loss of liquid. Introducing the bulb within the electrolysis cell and inserting a capillary, a, of 1-mm. bore and 50-mm. length to assist in building up the convection circuit of the condenser water increased the efficiency markedly, as could be readily demonstrated by adding dye solution. A small funnel and siphon were provided for washing out the *electrolyzed solution at the end of the electrolysis. This W=
Copper Taken
Copper Recovered
Error
Copper Taken
Copper Recovered
Mg.
Mg.
%
Me.
Mo.
%
1.234
1.233 1.228 1.235 1.233 0.490 0.497 0.487
-0.0s
+0.82
0.243
0.496 0.490 0.493 0.242 0.243 0.252 0.242
0.492
-0.56 f0.08 -0.08
-0.41 +1.02 -1.02
Error
+0.41 +0.24 -0.41 0.00 +3.7 -0.41
Table I1 shows the data obtained in determining various quantities of copper from about 0.2 to 1.2 mg., using the method outlined. I n all but one case the errors were not greater than 1 per cent, a n accuracy which compared satisfactorily with the gravimetric procedure. The time and technical difficulties were definitely reduced over those of the standard micromethod. Literature Cited (1) Kirk, P. L., and Tompkins, P. C. (unpublished). (2) Pregl, F., “Quantitative Organic Microanalysis”, Philadelphia, P. Blakiston’s Son & Co., 1937. (3) Smith, G. F., and Getz, C. 8..IND.ENQ.CHEM.,Anal. Ed., 10, 104 (1938). (4) Swift, E. H., “A System of Chemical Analysis”, New York, Prentice-Hall, 1939. ( 5 ) Willard, H. H., and Young, P., J . Am. C h m . Soc., 51, 149 (1929). AIDEDby a grant from the Research Board of the University of California.
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