Potential series

The difference in rest potentials (see the practical potential series in Table 2-4) determines mostly the direction of the current and less of the level for these the resistances are significant. In particular can be neglected in the external corrosion of extended objects. In addition, the IJJJ) curve is usually steeper than the I U) curve (i.e., < R. By introducing the surface areas of anode and cathode... 

Table 7.7 The electrode potential series (against the SHE). The electrode potential series is an arrangement of reduction systems in ascending order of their standard electrode potential...

It will be noted that with the exception of the organic cations and anions the more mobile hydrogen and hydroxyl ions are most readily adsorbed, whilst in the case of the metallic ions the influence primarily of the valency of the ion and both the position of the metal in the eleotrolytic potential series as well as the ionic mobility is most marked, the higher the valency and the more noble the element the more readily it is adsorbed. 

The degree to which the reaction can proceed is the greater the further apart both substances are in the standard potential series. Thus e. g. from the potential values of the elements Zn Zn++ and Pt I Co++, Co+++ it follows that the equilibrium constant of the reaction % Zn -f- Co+++ = Co++ -f- % Zn++ at 25 °C is approximately K = 1044, which means that the salt of the trivalent cobalt is reduced by zinc almost completely. The equilibrium constant K — 6.4 will belong on the other hand to the reaction Fe + Cd++ = Cd + Fe++ this comparatively small value shows a considerable amount of cadmium ions remaining in a state of equilibrium and not reduced. 

It is natural that the possibility of ionic reaction can be reliably predicted from the standard potential series only when the activities of all components taking part in the reaction equal unity. At other activities the mutual relations of substances in the potential series can be changed. There is a second limitation, namely, no retardation of the reactions by various foreign phenomena (e. g. by overvoltage or mechanical pasivity of the surface, due to the existence of oxide films). 

A standard electrode potential series consists of equilibrium potentials, not corrosion potentials as you have been measuring in this laboratory. These potentials are usually referenced to NHE but could be referenced to any reference electrode system. How would you convert a standard electrode potential series from Vnhe to Vsce ... 

The standard potential series obtained is given in Table 1.5. 

The standard potential series can be used as only a rough guide with respect to the ability of a metal to resist corrosion. In most of the corrosion reactions, the potential values shown in the table are not applicable because of the presence of a film on the metal surface, and the change in potential because the activity of metal ions is less than unity. 

The potential series and the Pourbaix diagrams involving equilibrium conditions discussed thus far led to determine the feasibility of the corrosion process based on thermodynamics. These concepts do not give any information on the rates of corrosion processes. In order to ascertain the corrosion rates it is imperative to understand the intimate dynamical processes occurring at the metal exposed to an electrolyte solution. 

Standard electrode potential series - potential, and subentries -> standard potential, and - tabulated standard potentials... 

Argention.—Silver forms a colourless, univalent ion. Its electrochemical equivalent in milligrams per coulomb is given as 1-1172,10 1-1180,11 1-11827,12 1-11829,18 1-1188,14 and 1-1198.18 In the potential series the metal occupies a position intermediate between mercury and platinum. In correspondence with its low electroaffinity it exhibits a strong tendency to form complex ions. The ionic conductivity of silver at 18° C. is 54-0, and at 25° C. 63-4. 

It needs to be noted that a trefoil knot is only the first member of a potential series of related molecular knots.If the number of metal centres employed in generating the associated helix is even, then the number of crossing points will be odd, and a single, closed knotted structure will be formed (trefoil, pentafoil, heptafoil, etc.). On the other hand, if the number of metal centres is odd, then an even number of crossing points will result, giving rise to multiply-interlocked [2]-catenanes. 

The greater the solution pressure P of a metal, the more completely will it throw out of solution the ions of a metal having a lower solution pressure The precipitation will tend to become less complete the nearer the solution pressure values are to one another Since the values for P for various metals follow the order of electrolytic potential series, it is clear that two metals in close proximity in this senes can only give rise to the phenomenon of incomplete precipitation when the one with the higher value of P is added to a solution of the ions of the one with a somewhat smaller P value... 

In addition to the standard reversible potential series we have praetieal galvanie series that show corrosion potentials of various metals in a given environment. 

A relevant series of this type is a more adequate basis for prediction of galvanic corrosion risks than is the standard reversible potential series. However, the practical galvanic series provide only qualitative and incomplete information. To explain what conditions determine the rate of galvanic corrosion, and to predict the rate, we must study the overvoltage curves of the actual reactions that take place (see Figure 7.7). 

The examples dealt wifli above show for one thing that use of practical galvanic series (Ecorr) and even more standard equilibrium potential series (Eo ) is more or less... 

L. Schmid, with an apparatus like Wild s, found that solutions of alkali-metal chlorides and nitrates obey Volta s law. Solutions of sulphates, nitrates, and chlorides of silver and bivalent metals followed the potential series of the metals. This was used to find the order of several metals in the potential series. Schmid concluded that the electrical difference of two different salts of the same metal is equal to the electrical difference of the acids. He also used solid salts pressed into discs which were brought in contact. 

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