Magnesium electrode potentials

In view of the ionisation energies the electrode potentials for lithium and beryllium might be expected to be higher than for sodium and magnesium. In fact... 

The standard electrode potential of magnesium is given, along with the potentials of other metals, in Table 4.17 and the steady-state potentials of magnesium in various solutions are listed in Table 4.18. ... 

When metals are arranged in the order of their standard electrode potentials, the so-called electrochemical series of the metals is obtained. The greater the negative value of the potential, the greater is the tendency of the metal to pass into the ionic state. A metal will normally displace any other metal below it in the series from solutions of its salts. Thus magnesium, aluminium, zinc, or iron will displace copper from solutions of its salts lead will displace copper, mercury, or silver copper will displace silver. 

Other commonly employed redox electrodes are metals such as copper, cobalt, silver, zinc, nickel, and other transition metals. Some p-block metals such as tin, lead and indium can also function as redox electrodes. However, s-block metals such as magnesium do not make good redox electrodes since the elemental metal is reactive and forms a layer of oxide coating, which leads to poor reproducibility, poor electronic conductivity and electrode potentials that are difficult to interpret, (see Section 3.3.1). 

Sacrificial anode — is a piece of metal used as an anode in electrochemical processes where it is intended to be dissolved during the process. In -+ corrosion protection it is a piece of a non-noble metal or metal alloy (e.g., magnesium, aluminum, zinc) attached to the metal to be protected. Because of their relative -+ electrode potentials the latter is established as the -+ cathode und thus immune to corrosion. In -+ electroplating the metal used as anode may serve as a source for replenishing the electrolyte which is consumed by cathodic deposition. The sodium-lead alloy anode used in the electrochemical production of tetraethyl lead may also be considered as a sacrificial anode. 

Beryllium has a standard electrode potential less negative than magnesium the anomalous behaviour of lithium in this respect is not repeated by beryllium, even though Be + hydrates strongly. But, unlike K, Rb+ and Cs+, all these bipositive cations are hydrated. 

Though the first and second ionisation potentials of Mn are similar to those of Mg, the standard electrode potential Mn /Mn is much less negative than that for magnesium because of the much greater heat of sublimation. 

Metals differ greatly in how easily they are oxidized. The most reactive metals are those with a very negative standard electrode potential. Of the commonly used metals, the alkali metals lithium, sodium, and potassium are highly reactive the next most reactive is magnesium, then zinc. 

Sacrificial Anodes Incontrastto the impressed current technique, the use of sacrificial anodes does not depend on the creation of driven electrochemical cell. Rather, a galvanic cell is formed between the structure and the sacrificial anode in which electrons pass spontaneously from the latter to the former (Fig. 9). Thus, the source of the electrons (the sacrificial anode) must have a more negative electrode potential than the structure. It was for this reason that Humphrey Davy chose zinc or iron to protect copper, and it also explains why magnesium, aluminum and zinc alloys are used to protect steel today. 

Galvanized steel is a common example of galvanic coupling where steel (Fe), with a standard electrode potential of —0.440 V vs. SHE, is cathodicaUy protected by zinc, which has a more active standard electrode potential of —0.763 V. Obviously, zinc is not a corrosion-resistant metal and cannot be classified as a barrier coating. It protects steel from corrosion through its sacrificial properties. Because zinc is less noble than iron in terms of the standard electrode potentials, it acts as an anode. The sacrificial anode (zinc) is continuously consumed by anodic dissolution reaction and protects the underlying metal (iron in steel) from corrosion. In practice, sacrificial anodes are comprised of zinc, magnesium alloys, or aluminum. 

The raw salt can be rock, solar or vacuum salt, where the latter has been purified by vacuum crystallization. Impurities as calcitun and magnesium etc. in the salt may harm the electrolysis operation by precipitating on the electrodes and result in high electrode potentials. Therefore purification of the salt is needed, and the quality of the salt set requirements on necessary purification steps. In Fig. 1 the incoming salt is first dissolved and then subject to ion exchange for removal of divalent cations as Ca " and Mg The evaporator illustrates re-crystallization of the... 

Magnesium is a divalent metal and is silvery white in appearance. It is the eighth most abundant element and sixth most abundant metal. The atomic weight is 24-32 and the specific gravity of the pure metal 1 -738 at 20°C. The structure is close packed hexagonal. The melting point is 6S0°C and the boiling point 1 I07°C. The sp ific heat at 20 C is 1 -030 kJ/kg "C and the thermal conductivity at 20°C is 157- 5 W/m C the electrochemical equivalent is 0-126 mg/C. The standard electrode potential = -2-37 V, but in... 

The magnesium half-cell has the more negative electrode potential, hence the equation and the sign of the electrode potential are reversed ... 

Magnesium has the lowest standard potential of all the engineering metals, as is illustrated in Fig. 4-3. At 25 °C magnesium (Mg VMg) has a standard electrode potential of-2.37 Vnhe (Shreir, 1965 Chapter XX... 

As can be seen in Table 19.6, aluminum and its alloys become the anode in galvanic cells with most metals and corrode sacrificially to protect them. Only magnesium and zinc are more anodic and corrode to protect aluminum. Neither aluminum nor cadmium corrode sacrificially in a galvanic cell because they have nearly the same electrode potential. 

---