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Dropping zinc amalgam electrodes in polarography Coghlan, William Richard Easton

Abstract

The general principles of polarography are mentioned briefly and the equations for the polarographic diffusion current are compared. The several derivations are traced and the points of departure leading to correction terms are noted. It is known that the assumptions upon which the derived equations for the diffusion current are based are not in accord with reality. It is also known that the departures from ideality tend to oppose so that, finally, the basic assumptions, even if imperfectly fulfilled, may be considered valid. These factors are discussed during development of the equations. A review of amalgam polarography is essentially complete. Publications dealing with the subject are reported almost in entirety. The preparation of dilute amalgams is discussed in the light of results obtained in preliminary work. The ideal nature of these amalgams is shown in a review of several papers dealing with E. M. F. measurements. The evidence for the ionization and polymerization of zinc metal in solution in mercury is discussed briefly along with the hypothetical reactions which explain the instability of very dilute amalgams. In addition, it is noted that a few workers have investigated the behaviour of dilute amalgams during passage of direct current. The evidence indicates that a dilute amalgam has properties analogous to a salt solution. Particularly, the phenomena of transport during passage of the electrical current, has been reported. Further investigation of the phenomena is suggested since transport of the metal in amalgam during passage of the electrical current violates the conditions required by the polarographic theory. The physical properties of zinc and zinc amalgams pertinent to the polarographic method are tabulated. It is noted that the diffusion coefficient of zinc in mercury has not been satisfactorily established. A method for stabilizing dilute zinc amalgams by impressing a voltage across the amalgam reservoir and a platinum electrode in a water layer over the amalgam is described. The utilization of the Sargent Polarograph Model XXII in conjunction with a Leeds and Northrup Student Type potentiometer for manually recording polarograms is described. Polarograms of standard zinc solutions in O.IN potassium chloride plus a trace of gelatin were recorded and measured under the same conditions intended for recording and measuring the polarograms of zinc amalgams. The data so obtained agreed to within 5% of the values previously reported for the diffusion current constant and to within 2% of the reported half-wave potential for zinc salts in O. IN potassium chloride plus traces of maximum suppressors. It was noted that the values of the diffusion current constant increased with decreasing concentration of zinc in agreement with other published reports but contrary to the findings of Kalthoff and Lingane. The deviation is believed to be due in part to the presence of traces of oxygen. Further investigation is suggested. The behaviour of dilute zinc amalgam at the dropping electrode is shown to correspond to that of mercury except for an inflection in the drop time vs potential curve. The inflection is found in the range of potential over which the 'wave' for zinc amalgam develops. It is also shown that the amalgam stabilizing circuit has a negligible effect on the recorded polarograms. In short, accuracy of the same order obtained in ordinary polarography can apparently be attained. Reproducible polarograms of 0.000288% zinc amalgam have been obtained. Calculations fail to reveal gross error. The diffusion current constant calculated from the original Ilkovic equation is found to be 5.16 ±. 0.07. The mean value of the diffusion coefficient of zinc in mercury is calculated 2.4 X 10ˉ⁵cm² secˉ¹ at 25.0°C from values reported in the literature. For this value of the diffusion coefficient the numerical constant 'B' in the second term of the Strehlow-Stackelberg equation,⁽ᵃ⁾ id =607nD½Cm⅔ԏ⅙ (1-B D½mˉ⅓ԏ⅙) has been calculated 23.6 ± 2.3. This value is within range of the values predicted for B, from theoretical approximations. Choice of the larger values of the diffusion coefficient would give larger values for B. Here, the result is taken as evidence that streaming at the drop surface is near minimum. From the data obtained in polarography of standard zinc solutions it is observed that the Lingane-Love- ridge equation gave the best agreement with accepted values. Accordingly, the value for the diffusion coefficient of zinc in meroury at 25.0°C has been calculated according to equation ⁽ᵇ⁾ as 2.58 cm² secˉ¹±0.09. id = 607 n D½m⅔ԏ⅙ (1-39 D½mˉ⅓ԏ⅙) A value in agreement with the larger of the several reported values. Consistent results with amalgams less than 0.005% solute metal have not previously been reported in the polarographic literature. In this work an amalgam 0.000288% has provided not only consistent results, but also results from which remarkable agreement with published data has been obtained. This is taken as evidence that the method is suitable for work of the precision and accuracy obtainable in ordinary polarography. The conditions for work of greater precision and accuracy are set forth with a brief discussion of the difficulties and sources of error involved. The practical applications of amalgam polarography are suggested.

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