Alkali ions

The critical requirements for the ion source are that the ions have a small energy spread, there are no fast neutrals in the beam and the available energy is 1-10 keV. Both noble gas and alkali ion sources are conunon. Por TOP experunents, it is necessary to pulse the ion beam by deflecting it past an aperture. A beam line for such experiments is shown in figure B1.23.5 it is capable of producing ion pulse widths of 15 ns. 

Niehus H and Comsa G 1984 Determination of surface reconstruction with impact-collision alkali ion scattering Surf. Sc/. 140 18-30... 

Electrical conductivity is of interest in corrosion processes in cell formation (see Section 2.2.4.2), in stray currents, and in electrochemical protection methods. Conductivity is increased by dissolved salts even though they do not take part in the corrosion process. Similarly, the corrosion rate of carbon steels in brine, which is influenced by oxygen content according to Eq. (2-9), is not affected by the salt concentration [4]. Nevertheless, dissolved salts have a strong indirect influence on many local corrosion processes. For instance, chloride ions that accumulate at local anodes can stimulate dissolution of iron and prevent the formation of a film. Alkali ions are usually regarded as completely harmless, but as counterions to OH ions in cathodic regions, they result in very high pH values and aid formation of films (see Section 2.2.4.2 and Chapter 4). 

The system aluminum/water belongs to group II where represents the pitting potential and lies between -0.8 and -1.0 V according to the material and the medium [22,23,36,39,42]. Since alkali ions are necessary as opposite ions to the OH ions in alkalization, the resistance increases with a decrease in alkali ion concentration (see Fig. 2-11). In principle, however, active aluminum cannot be protected cathodically [see the explanation of Eq. (2-56)]. 

The production of OH ions according to Eq. (2-17) or (2-19) in pores or damaged areas is responsible for cathodic disbonding [9,10], where the necessary high concentration of OH ions is only possible if counter-ions are present. These include alkali ions, NH and Disbonding due to the presence of Ca ions is... 

In this context it is important to note that the detection of this land of alkali cation impurity in ionic liquids is not easy with traditional methods for reaction monitoring in ionic liquid synthesis (such as conventional NMR spectroscopy). More specialized procedures are required to quantify the amount of alkali ions in the ionic liquid or the quantitative ratio of organic cation to anion. Quantitative ion chromatography is probably the most powerful tool for this kind of quality analysis. 

The network-modifying ions (commonly alkali and alkaline-earth ions) are ionically bound to the network although the field strength and diameter of the alkali ions allow them some mobility. 

The attack of most glasses in water and acid is diffusion controlled and the thickness of the porous layer formed on the glass surface consequently depends on the square root of the time. There is ample evidence that the diffusion of alkali ions and basic oxides is thermally activated, suggesting that diffusion occurs either through small pores or through a compact body. The reacted zone is porous and can be further modified by attack and dissolution, if alkali is still present, or by further polymerisation. Consolidation of the structure generally requires thermal treatment. 

Initially there is the replacement of an alkali ion (/ ) in the glass by a proton (H) from the water. 

With the migration of alkali ions (R ) into the leachant the attack is no longer by water. If is sodium the attack on the glass is ... 

Alkali ions in solution increase the pH, with two consequences. The rate of silica extraction increases with the rise in pH value above 9 and the rate of alkali exchange is reduced. However, since the higher alkalinity favours solution (of silica) further alkali is released by the lattice. The quantity of alkali extracted can be used as a measure of the resistance of glass to attack... 

Studies on hot water tank enamelsin media of varying pH demonstrate a minimum corrosion rate at pH value of 4. In citric acid (pH 2), IR measurements indicate that ion exchange is the principal mode of corrosion. Distilled water (pH 7) showed evidence of a bulk dissolution mechanism with no silica enrichment of the surface layer. In neutral solutions, the first stage of attack is leaching of alkali ions, raising the pH of solution, which subsequently breaks down the glass network of the acidic oxides. 

The theory of the structure of ice and water, proposed by Bernal and Fowler, has already been mentioned. They also discussed the solvation of atomic ions, comparing theoretical values of the heats of solvation with the observed values. As a result of these studies they came to the conclusion that at room temperature the situation of any alkali ion or any halide ion in water was very similar to that of a water molecule itself— namely, that the number of water molecules in contact with such an ion was usually four. At any rate the observed energies were consistent with this conclusion. This would mean that each atomic ion in solution occupies a position which, in pure water, would be occupied by a water moldfcule. In other words, each solute particle occupies a position normally occupied by a solvent particle as already mentioned, a solution of this kind is said to be formed by the process of one-for-one substitution (see also Sec. 39). 

At the same time we recall that, in an external field, the electrical mobility of the Li+ ion is unusually small and in this connection we should certainly discuss the passage just quoted. Although the Li+ ion has the smallest radius of all the alkali ions, it has the smallest electrical mobility. According to the traditional explanation, described in Chapter... 

Kinetics and mechanism of alkali ion complex formation in solution. R. Winkler, Struct. Bonding (Berlin), 1972,10,1-24 (38). 

On the basis of the dipole moment, Paik, values computed from the Helmholtz equation (2.21) and the alkali ion radius one can estimate the effective positive charge, q, on the alkali adatom, provided its coordination on the surface is known. Such calculations give q values between 0.4 and 0.9 e (e.g. 0.86e for K on Pt(lll) at low coverages) which indicate that even at very low coverages the alkali adatoms are not fully ionized.6 This is confirmed by rigorous quantum mechanical calculations.27,28... 

This can be considered to be the case when using alkali ion conductors. But classical promotion by species like O2 or H+ does not appear to be experimentally feasible, due to the experimental difficulty of introducing them under controlled conditions from the gas phase. Also their short lifetime under reaction conditions essentially limits their usefulness only to situations where they can be continuously replenished on the catalyst surface, i.e. only to electrochemical promotion. 

Other alkali ions (except lithium) also probably have the coordination Number 8 as a rule, and should similarly have a tendency to a 1 1 ratio with aluminum this is shown in NaAlSi3Os, albite, H2Na2Al2Si30i2, natro-lite, H2Cs4Al4Si9027, pollucite, etc. 

It is further shown that the theory requires that no stable basic silicates of divalent metals exist, and that in aluminum silicates of alkali metals there should be at least one aluminum ion for every alkali ion. 

Winkler R (1972) Kinetics and Mechanism of Alkali Ion Complex Formation in Solution. W 1-24... 

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