Nov., 1941
THE
REACTTVITY OF AMALGAMS
31YS
[CONTRIBUTION FROM THE RESEARCH LABORATORY, GENERALELECTRICCOMPANY ]
The Reactivity of Amalgams BY HERMAN A. LIEBHAFSKY AND ARTHURF.WINSLOW The oxidation of zinc amalgams proceeds very b. Lead amalgams shaken with 1 N HC104; electrolyte simply1 when the experimental conditions are solution for e. m. f . measurements: 1 M Pb(N0&-0.02 M "03-0.001 M HgAc properly chosen. Provided agitation is suffi2.9 6 0.1 ciently violent, the rate of oxidation is virtually 3.1 .O1 independent of the zinc concentration and is 3.8 8 ,001 limited mainly by the rate a t which fresh amalOver-all average rate 3.3 gam surface is exposed; when the latter rate is c. Zinc amalgams shaken with 1 A; H,SO,; electrolyte constant, the rate of oxidation is roughly pro- solution for e. m. f. measurements: 1 M %nS04-0.05M portional to the concentration of oxidizing agent HiSOi and almost independent of temperature. These 9 0.1 3 3 10 .Ol 3.1 results led to the important conclusion that the 11 .005 3.7 two elementary processes involved in the reaction,001 3.2 12 electron capture by the oxidizing agent, and ex.001 2.5 13 pulsion of the resultant positive charges as zinc Over-all average rate 3.2 ions-occurred separately and (practically) simuld. Cadmium amalgams shaken with 1 A' HzS04; electrotaneously. This conclusion has now been lyte solution for e. m. f . measurements: 1 M Cd(N0& strengthened by an investigation of other amal0.01 M HzS04-0.001 M HgAc gams, which was undertaken because of its bear3.2 14 0.1 ing on the cleaning of mercury: obviously, if it is 2.0 15 .01 3.0 16 ,001 true that all base metals dissolved in mercury can Over-all average rate 2.7 be oxidized a t rates nearly independent of their concentrations, the removal of these impurities e. Tin amalgams shaken with 1 N H2S01; electrolyte ought to become progressively easier as their con- solution for e. m. f . measurements: 0.1 M SnC12-0.15 M HClb centration decreases. 17 0.1 1.7 Air as the Oxidizing Agent.-When zinc amal18 .1 1.5 gams are shaken with 1 N sulfuric acid in contact .01 2.4 19 with air, oxygen-not hydrogen ion-is the prin.01 1.4 20 cipal oxidizing agent.' The separatory funnel 21 ,001 2.3 method of ref. 1 (pp. 452-453) was used to measOver-all average rate 1.9 ure the rates a t which five other representative f. Copper amalgams' shaken with 1 N HzSOc; electrolyte base metals reasonably soluble in mercury were solution for e. m. f. measurements: 1 JI CuSO4-0.01 M HzS04-0.001 M HgAc oxidized by air in contact with acid solutions. 0.29 22 0.00148 The results for these amalgams are abstracted in 23 ,00112 .25 Table I, which summarizes also the results from Over-all average rate 0.27 ref. 1, Table I, for zinc amalgams; by and large,
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TABLE I OXIDATION OF AMALGAMS BY AIR No.
Initial concentration, yo
IN
CONTACT WITH ACID 104 X average rate
a. Thallium amalgams shaken with ,V HzSO4; electrolyte solution for e. m. f . measurements: 0.1 M TlNO3-0.02 M "08-0.001 M HgAca 1 1.0 10 2 0.1 14 3 .1 21 4 .Ol 9 5 .Ol 13 Over-ail average rate 13 (1) Liebhdsky, Tma JOWBNAL, DO, 462 (18871,
a Mercurous acetate was added to stabilize cell reading for exhausted amalgam; this salt could not be used in the tin experiments owing to its reaction with stannous chloride. 0.15 M hydrochloric acid necessary to repress hydrolysis. Copper amalgams prepared by electrolysis; others by dissolving metals in hot mercury in the absence of oxygen. Electrolysis is resorted to because the solubility of copper in mercury near room temperature is only 0.0020/,, according t o unpublished results from this Laboratory agreeing with earlier measurements.
a detailed picture of the new experiments can be constructed from the latter table. All amalgam concentrations are percentages by weight; rates
3138
HERMAN A. LIEBHAPSKY AND ARTHURF. WINSLOW
are the decreases each second in the percentage of base metal for 25 cc. of amalgam. Table I leads to these conclusions. (1) For each solute metal listed, the rate of oxidation is virtually independent of the amalgam concentration. (2) These rates vary over only a 50-fold range. (3) At least a rough parallelism exists between the character of the metal and the rate of oxidation; copper, the most noble of these metals, is oxidized least rapidly. These conclusions show that ridding mercury of dissolved base metals is a relatively simple matter, for the removal of such metals becomes progressively easier as their concentration decreases.
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-0.5 Standard electrode potential, volts. Fig. 1.-Equivalent rate of oxidation for base-metal amalgams as a function of the standard electrode potential for the metal.
If the elementary process of electron capture by the oxidizing agent is involved in the oxidation of base-metal amalgams, there might be a correlation between the rates of oxidation and the electrode potentials of the various metals. The standard electrode potentials naturally do not apply for the conditions existing at the acid-amalgam interface, but they can serve as a measuring stick for the potentials that make the electron capture possible. The rates of Table I have been divided by the equivalent weights of the respective metals, since the faraday is the rational unit for measuring the amount of electricity transferred in unit time. The resulting quotients, or equievllent rates, have been plotted on semilogarithmic paper in Fig. 1against the standard electrode potentials2 of the metals. Within the experimental error, the logarithm of the equivalent rate is proportional to (2) All standard electrode potentials are taken from Latimer, “Oddstion Potentials,” Prentice-Hall, Inc., New York, N. Y., 1938.
Vol. 63
the electrode potentid, a decrease of one volt causing a decrease of about one power of ten in this rate. The functional relationship in Fig. 1 is of the kind to be expected when the rate of a process is proportional to the free energy decrease that accompanies it. Such correlations between kinetics and thermodynamics usually occur only for very simple processes; the logarithm of the rate of evaporation into high vacuum, for example, is often proportional to the free energy decrease for the vaporization. In other words, that such a simple relationship should be even approximately valid for the oxidation of base-metal amalgams is supporting evidence for the idea that electron capture as an elementary process is involved in these reactions. Furthermore, Fig. 1 can be used to explain why the rates of oxidation are virtually independent of the amalgam concentrations: according to the Nernst law, changes in these concentrations produce relatively slight changes in the electrode potentials, thus leaving the rates practically unaffected. A general, thermodynamic picture like the foregoing cannot, of course, be a close-up of the oxidation process. The following detailed mechanism is consistent with the experimental results. The violent shaking continually re-forms the amalgamsolution interface so that a typical element of interface has in effect a very limited lifetime (
