Alexandr I. Chernomorskii
Scientific Resources Co
e-mail:sciresources@aol.com
Both the ideal polarizable and non-polarizable mercury electrodes are, in principle, the reversible electrodes with the different current exchange of potential determining ions. Indeed, potential of zero charge of mercury is measured in “zero solution”. Electrocapillary curves (ECC) of non-polarizable mercury electrode Hg22+/Hg with the maximum at the equilibrium potential EHg22+/Hg ~ +0.47 V SHE (Billiter potential) were measured by a drop-weight method in [1-3]. These curves were not acknowledged in [4-7] comparing with the significant electrocapillary effects at the potential of zero charge [5, 7].
The results [1-3] were verified in our measurements by a drop-weight method [8, 9]. There were used 1012 ÷10-5 N solutions of Hg2(NO3)2 previously prepared by an exact successive dilution of 10-2 N solution Hg2(NO3)2 acidified by HNO3. The solutions were successively introduced (beginning from the more diluted 10-12 N) in the cell (Fig.1a). Nitrogen was passed through solution before all measurements. Mercury drops P (period of dropping > 10 c) falling into solutions were collected in a glass receiver, dried and weighed. The maximum of the weight P falls on 10-10 ÷ 10-11 N Hg2(NO3)2 (Fig.1, b - curve 1). The corresponded calculated EHg22+/Hg ,maximum is close to Billiter potential since the standard potential EoHg22+/Hg = + 789 m V SHE and 789 mV SHE – 11 x 29 mV ~ + 470 mV SHE.
Measurements were also carried varying of EoHg22+/Hg using complex agents [3] which were carefully added to acidified 10-2 N Hg2(NO3)2 [8,9]. A thin glass capillary tube wetted by solutions of complex agents was immersed in the solution Hg2(NO3)2 in the cell. This allows to change measured equilibrium potentials EHg22+/Hg of mercury drops and bottom on units mV.
The small electrocapillary maximums in the narrow interval ~ 0.2 V (Fig.1,b – curves 2,3; c – curves 1,2) were obtained at EHg22+/Hg = ~ +0.18 ÷ +0.23 V SCE ( ~ +0.43 ÷ +0.48 V SHE). The potentials of maximums on the curves 1 and 2 (Fig. 1, c) are differed on units mV from ~+0.23 V SCE (~+0.475 V SHE). Such a small maximum was also obtained using other complex agents [8].
Simultaneously registered by galvanometer and recorder (Fig.1a) a positive current from mercury bottom to dropping electrode at EHg22+/Hg > EHg22+/Hg,maximum is changed on a negative at EHg22+/Hg < EHg22+/Hg,maximum. [8-10].
The rising branch of ECC of the ideal polarized electrode (positive charge corresponds to this branch) is obtained at the following change of EHg22+/Hg to the negative side after passing the maximum at ~ + 0.23 V SCE (~ +0.47 V SHE) and its short descending branch (negative charge should correspond to this part). Indeed, at this transfer, the second change of the current direction (opposite to the registered one in maximums) was recorded [ 10].
Using the strong complex agent C2H5)2NCS=SNa we obtained both maximums at EHg22+/Hg = ~ +0.23 V SCE (~ +0.47 V SHE) and EHg22+/Hg = ~ - 0.53 V SCE (~ - 0.28 V SHE), curve 2 (Fig.1 c). This displacement of maximum to - 0.28 V SHE ( ~ -0.61 V vs 0.1 N CE) was also found in “zero solution” in the old measurements with complex agents [4]. The decrease of the ECC (curve 2, Fig.1 c) in the interval ~ +005 ÷ -0.3 V SCE was explained by the appearance of the electrode Hg/HgS/S-2 with the standard potential ~ -0.3 V SCE [8].
According to [11], the sum ∆μHg22+ + ∆μe = 0 corresponds to the equilibrium at +0.475 V SHE and the sum ∆μHg22+ + ∆μe in which ∆μHg22 = 0 and ∆μe ≠ 0 corresponds to equilibrium at - 0.19 V SHE where ∆μHg22+ and ∆μe are the changes of chemical potentials of mercury ions and electrons in half-reactions as interactions. An imposed adsorption of ions, complex agents displaces the potentials of these equilibriums (maximums).
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