Self-ionisation of water

Metals in practice are usually coated with an oxide film that affects the potential, and metals such as Sb, Bi, As, W and Te behave as reversible A//A/,Oy/OH electrodes whose potentials are pH dependent electrodes of this type may be used to determine the solution s pH in the same way as the reversible hydrogen electrode. According to Ives and Janz these electrodes may be regarded as a particular case of electrodes of the second kind, since the oxygen in the metal oxide participates in the self-ionisation of water. 

Pure water has a detectable conductance, indicating the presence of ions. These result from the self ionisation of water ... 

Whether it does, or does not, depends critically on the relative contributions to the concentration of H30" (aq) coming from each of the equilibria. If the weak acid is sufficiently ionised so that its contribution to the actual concentration of H30 (aq) is large compared with that from the self ionisation of water then, to a first approximation, the self ionisation of water can be ignored. The weak acid can then be taken to be the only effective source of H30 (aq). Provided the total concentration of the weak acid is not too small, this generally holds for acids with TpK < 10.0. 

Again both the weak base and the solvent water are sources of OH (aq), and the relative contributions to the actual [OH (actual from both have to assessed and an active decision has to be made as to whether the contribution from the self ionisation of water can be ignored. These points are dealt with in detail in Chapter 4. 

This approximation holds for acids with pA a of around 10.0 or less, unless the concentration is very low. If the concentration of the weak acid is very low, then even if all the acid were ionised, the concentration of H30+(aq) would still approach that from the self ionisation of water. For acids which have >10 and are thus very weak acids the extent of ionisation will be so low that the contribution to [H3O+] from the self ionisation of water must not be ignored (see Section 4.1.4). 

Where there is fairly extensive ionisation, the contribution to the actual concentration of HaO" " (aq) from the ionisation of HA(aq) 3> than that from the self ionisation of H20(l). The weak acid is then effectively the only source of both A (aq) and H30+(aq) and the self ionisation of water can be ignored ... 

The weak add is suffidently weak so that the self ionisation of water cannot be ignored and approximation 2 is invalid... 

For acids with pALa greater than around 10.0 the acid is so weak that the fraction ionised is very low. The contribution to the actual concentration of H3O from the weak acid is then so small that the contribution from the self ionisation of water is comparable to, or even greater than, that from the weak acid. There are now two sources of H30 (aq). 

The self ionisation of water can be ignored, the ionisation of the acid will be sufficiently extensive for the acid to be the main source of H30+(aq). 

There will be extensive ionisation. The amoimt of produced by the acid will be considerably greater than that from the self ionisation of water, which can be ignored. 

H30 ]3 ( = 1.95 X 10 moldm . Comparing this with [H3O+] = 10 moldm in pure water at 25°C shows that there is a significant contribution to the actual [H3O+] from the water. It would certainly not be justified to assume the self ionisation of water can be ignored. 

If the contribution to OH (aq) from the self ionisation of water is small compared with the contribution from the protonation of B(aq), then to a first approximation it can be ignored. 

OH ]actual = 5.96 X 10 moldm which is sufficiently high for the contribution of [OH ] from H20(l) to be insignificant. The approximation of ignoring the self ionisation of water is fully justified. 

This concentration is just sufficiently large for the approximation of ignoring the self ionisation of water to be just valid. 

A more rigorous calculation assuming that it is valid to ignore the self ionisation of water, but acknowledging that there is significant protonation gives ... 

Urea is a very weak base and the amount of OH (aq) resulting from the self ionisation of water will be significant in comparison to the amount from the protonation of urea, and ... 

The [OH is 1.01 x lO- moldm- which is very close to the value of [OH ] resulting from the ionisation of pure water. The self ionisation of water must not be ignored for this base. 

The fraction protonated must be calculated taking note that the self ionisation of water must be included. Electrical neutrality gives ... 

These results do not include any approximations regarding the fraction ionised or the self ionisation of water. The magnitude for the limiting value of or of... 

If the stoichiometric concentration of a weak acid becomes too small then, even with an acid with a p ta within the normal limits of the approximations, it may become necessary to consider both extensive ionisation and the self ionisation of water, i.e. 

The approximations of no extensive ionisation and ignoring the self ionisation of water apply equally to weak bases with piib values in the range 4.0 to 10.0, with the aU-important proviso provided the soiution is not too dilute again being necessary. 

The quadratic equation has to be solved for fairly strong weak bases, and for weak bases at low concentration. The self ionisation of water can likewise be ignored for a large number of weak bases, but becomes progressively less able to be ignored as the base becomes weaker, or as the concentration decreases for ordinary weak bases. 

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