Mercury intermetallic compounds

The major types of interferences in ASV procedures are overlapping stripping peaks caused by a similarity in the oxidation potentials (e.g., of the Pb, Tl, Cd, Sn or Bi, Cu, Sb groups), the presence of surface-active organic compounds that adsorb on tlie mercury electrode and inhibit the metal deposition, and the formation of intermetallic compounds (e.g., Cu-Zn) which affects the peak size and position. Knowledge of these interferences can allow prevention through adequate attention to key operations. 

Incorporation of Metal In certain cases, metal atoms, after their discharge, can penetrate into the substrate metal, forming alloys or intermetallic compounds in the surface layer and down to a certain depth. This effect has been known for a long time in the discharge of metals at liquid mercury, where liquid or solid amalgams are formed. In 1968 B. Kabanov showed that an analogous effect is present in metal ion discharge at many solid metals. 

Although the properties of metal amalgams and intermetallic compounds in mercury have been investigating for many years and are already well recognized, the nature of ammonium amalgam, as well as quaternary ammonium amalgams, still remains a subject of controversy. 

G. Gumihski, Z. Galus, in Solubility Data Series, Vol. 51- Intermetallic Compounds in Mercury (Eds. J. G. Osteryoung, M. M. Schreiner, C. Gumihski et al.,) Pergamon Press, Oxford, New York, Seoul, Tokyo,... 

The systems obtained when gallium is in the presence of bismuth, cadmium, germanium, mercury, lead, silicon, or thallium present miscibility gaps. No intermetallic compounds are formed. 

The preparation of pure mercury is not difficult. Any metal with a standard potential more negative than that of mercury may be oxidized easily (with the exception of nickel, which forms a mercury intermetallic compound) by dispersing mercury into a solution of its salts acidified with HN03 and saturated with oxygen. Metals insoluble in mercury may be also removed this way, although the process may be slow. More effective in this respect is the separation of metal microcrystals by filtration. The elimination from mercury of metals more noble than itself (as well as less noble metals) is accomplished by distillation under reduced pressure. Usually such distillations are repeated several times. Triple-distilled mercury is commonly used for electrochemical purposes. 

Electrodes of this sort have many different chemical properties from pure mercury electrodes, because of the formation of a gold or platinum amalgam [18]. Normally, a drop is suspended just prior to an experiment, so this problem will be of no serious consequence. Nevertheless, since the solubility of these noble metals in mercury is about 0.05 M at room temperature [19], the concentration of gold or platinum in mercury may be quite significant on a longer time scale. In such cases, gold or platinum may form intermetallic compounds with several metals that are electrodeposited into the mercury [18]. 

For instance, the stripping peaks of metals deposited in a contaminated mercury film may be changed by formation of intermetallic compounds. Compounds may form between the metals and the metal of a support that has dissolved in the film, or between the metals and the surface atoms of the metal... 

Fig. 29. Synergetic effects with intermetallic compounds. Hydrogen evolution in 1 M KOH at 30 °C on (1) Ni, (2) La, and (3) LaNis. (MOE = mercury oxide electrode). Adapted from ref. 226, by permission of Elsevier Sequoia.

The fourth method finds limited application, although amalgamated metallic surfaces are readily prepared by dipping the metal into a solution of a mercuric salt. An illustration of the action of mercury on the solution of a salt of a more noble metal is to be found in the preparation of the silver mercuride, Ag3Hg4.10 The addition of drops of mercury to a solution of silver nitrate yields crystals of this intermetallic compound. 

There is much evidence that mercury forms intermetallic compounds with platinum at the mercury-platinum interface. Apparently these do not diffuse to... 

Use of mercury as electrode material resolves the problem of the negative potential zone being too small but brings others, as it is a liquid. When a metal ion is reduced on the mercury surface to the metal, this can diffuse to within the mercury film, forming Hg-M bonds, or, if there is more than one dissolved metal, intermetallic compounds can be formed within the mercury, as is the case of Cu-Zn. Reoxidation of intermetallic... 

The electrode substrates typically considered for potentiometric stripping analysis are -> glassy carbon and wax-impregnated graphite. Application of metal substrates is usually limited due to the possibility of contamination of mercury by amalgamation. Also other intermetallic compounds can be formed. Very careful polishing is essential to obtain good results. 

Copper, nickel, and cobalt were found by Seitz (5) to diminish the height of the zinc current peak by broadening it. Although the concentration of cobalt in seawater was deemed too low to cause serious problems, the eflFect of copper and nickel required further study. The interference by copper in the stripping determination of zinc was extensively investigated by Bradford (8). He concluded that in the mercury film, copper and zinc formed a 1 1 intermetallic compound that dissociated to release zinc during the oxidation. Thus zinc peak areas remained proportional to the zinc concentration even in the presence of copper, and the analysis of zinc by standard addition was not affected. The interference from nickel was found to be similar to that from copper although the stoichiometry of the intermetallic compoimd could not be determined. 

The preparation and electrical properties of magnesium mercury, MgjHgj, have been reported. The intermetallic compound was prepared by heating the elements together above 562 °C, at which temperature the compound melts, in an evacuated silica tube. Electrical resistivity and Hall-effect measurements were made from 2.4 to 297 K. No phase transition was observed over this range. The Hall constant was low and the ideal resistivity (the difference between total and residual resistivity) had the form p a T , where a = 1.9 and 1.0 at 11.50K and... 

Important interferences that sometimes occur with mercury electrodes involve (a) reactions of the metals with the substrate material (e.g., Pt or Au) or with the mercury (e.g., Ni-Hg), or (b) formation of an intermetallic compound between two metals deposited into the mercury at the same time (e.g., Cu-Cd or Cu-Ni). These effects are much... 

However, formation of intermetallic compounds can cause problems. When metals such as copper and zinc are present in solution there is a tendency to form a Zn/Cu intermetallic compound when larger amounts are deposited at a mercury electrode. When an intermetallic compound is formed the stripping peaks for the constituent metals may be shifted, severely depressed, or even be absent altogether. When an alloy is formed at a solid electrode its dissolution potentials, in the stripping step, may be quite different to those of the constituent metals. 

These interfering effects can be minimised or avoided by reducing the deposition time and the total amount of metal deposited. This would of course mean a loss of sensitivity, but use of differential pulse stripping voltammetry can offset this. The use of a hanging mercury drop electrode with its larger mercury volume offers less intermetallic interference than does the mercury film electrode. Careful choice of a deposition potential can also sometimes prevent the codeposition of metals forming intermetallic compounds. 

When nickel is deposited electrolytically on a mercury cathode, it appears to form an intermetallic compound containing about 24% nickel, but when the mercury is distilled off, finely divided, strongly pyrophoric nickel is formed. Iron and cobalt are deposited by electro-... 

Me Me + -I- ze . When no metal compounds are formed with mercury, the value of Eg is close to or equals zero [1]. Compounds of the alkali metals and the alkaline earths with mercury have the best-defined composition, while copper and zinc do not form any intermetallic compounds with mercury. The compositions of many intermetallic compounds are variable. In diluted amalgams, the compounds are dissociated to various degrees. 

When several metals are simultaneously electrodeposited in mercury, intermetallic compounds between them may be formed. In anodic stripping voltammetry the following compounds of copper, zinc, and antimony may influence the measurements CuZn, CuSn, CuGa, SbZn, SbCd, and Sbln. The values of their solubility products are between 2 x 10 (SbZn) and 4 x 10 (CuZn) [1]. 

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