14 Faraday Mercury

Faraday s law (p. 496) galvanostat (p. 464) glass electrode (p. 477) hanging mercury drop electrode (p. 509) hydrodynamic voltammetry (p. 513) indicator electrode (p. 462) ionophore (p. 482) ion-selective electrode (p. 475) liquid-based ion-selective electrode (p. 482) liquid junction potential (p. 470) mass transport (p. 511) mediator (p. 500) membrane potential (p. 475) migration (p. 512) nonfaradaic current (p. 512)... 

Purified via the mercury salt [see Kern J Am Chem Soc 75 1865 1953], which was crystd from benzene as needles (m 121°), and then dissolved in CHCI3. Passage of H2S gas regenerated the mercaptan. The HgS ppte was filtered off, and washed thoroughly with CHCI3. The filtrate and washings were evaporated to remove CHCI3, then residue was fractionally distd under reduced pressure [Mackle and McClean, Trans Faraday Soc 58 895 1962]. 

On the other hand, this same amount of electricity will deposit exactly twice as much mercury, 2 x (6.03) = 12.1 grams, from a solution of mercurous perchlorate, Hg2(ClCh)2. If we restate Faraday s experimental finding in terms of the atomic theory, we see that the number of atoms of mercury deposited by a certain quantity of electricity is a constant or a simple multiple of this constant. Apparently this certain quantity of electricity can count atoms. A simple interpretation is that there are packages of electricity. During electrolysis, these packages are parcelled out, one to an atom, or two to an atom, or three. 

In coulometry, one must define exactly the amount of charge that was consumed at the electrode up to the moment when the endpoint signal appeared. In galvanosta-tic experiments (at constant current), the charge is defined as the product of current and the exactly measured time. However, in experiments with currents changing continuously in time, it is more convenient to use special coulometers, which are counters for the quantity of charge passed. Electrochemical coulometers are based on the laws of Faraday with them the volume of gas or mercury liberated, which is proportional to charge, is measured. Electromechanical coulometers are also available. 

The application of this technique (even in its various modes such as cyclic voltammetry) to other electrodes has already been mentioned in the description of LSV at the dme [Section 3.3.1.2.1(5)]. Especially with stationary electrodes LSV becomes fairly simple, under the conditions of sufficient solubility of ox and red, because of the constant and undisturbed electrode surface at an inert electrode the residual faraday current can be adequately eliminated by means of "J compensation (cf., Fig. 3.23) or by subtractive [cf., Section 3.3.1.2.1(3)] and derivative59 [cf., Section 3.3.1.2.1(4)] voltammetry at a stationary mercury electrode (e.g., HMDE), in addition to the residual faradaic current,... 

Balson, E.W. Studies in vapour pressure measurement. Part III. - An effusion manometer sensitive to 5 x 10 millimetres of mercury vapour pressure of D.D.T. and other slightly volatile substances, Trans. Faraday Soc., 43 54-60, 1947. 

The concentration of the metal in the mercury electrode after a given preconcentration period is given by Faraday s law ... 

Electroreduction of trans- and ds-o a -difluorostilbene at a mercury cathode in DMF at -1.95 V (see) consumes two Faradays of current and gives diphenylacetylene in high yield (measured spectrophotometrically) by what is proposed to be a concerted electrochemical process (equation 88)142. 

Because of a higher theoretical decomposition voltage mercury cathode electrolyzers have also a higher theoretical energy consumption. If for the preparation of 100 kg of sodium hydroxide 67 kA-hr. are required according to Faraday s law, then the theoretical energy consumption equals... 

In the cases referred to above, the anode consists of a metal which is not attacked during the passage of current, but if an attackable metal, e.g., zinc, silver, copper or mercury, is used as the anode, the latter dissolves in amounts exactly equal to that which would be deposited on the cathode by the same quantity of electricity. The results obtained by Bovard and Hulett for the loss in weight of a silver anode and for the amount of silver deposited on the cathode by the same current are given in Table VI the agreement between the values in the eight experiments shows that Faraday s laws are applicable to the anode as well as to the cathode. 

Commentary on Experimental Study of the Transition from van der Waals, over Covalent to Metallic Bonding in Mercury Clusters, H. Haberland, H. Kommeier, H. Langosch, M. Oschwald and G. Tanner, J. Chem. Soc., Faraday Trans., 1990, 86, 2473. 

---