General aspects of Ion Selective Electrodes

 

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General aspects of Ion Selective Electrodes

The Ion Selective Electrodes are the potenciometric sensors that selective respond on the ion activity in the solution according to Nikolski-Eisenmann equation:

(1)

where E_i is the potencial of ion selective electrode, E₀ is so called standart potential of the electrode, a_i is the activity of the indicating ion, K_ij^Pot is the selectivity coefficient of the electrode toward j-th ion regarding to indicating ion, a_j is the activity j-th interfering  ion, z_i is the charge of the indicating ion and z_j is the charge j interfering ion, n in the sum means n types of interfering ions, R is universal gas constant, T is absolute temperature and F is Faraday constant. For the practical reason is advantegous use expression with decadic logarithm:

(2)

From the practical wiev is advantigous so that the contributions of the interfering ions were neglibile so that was be valid:

(3)

So then we can eqation  rewrite:

(4)

For the series measuring is advantige , when the activity  coefficient is constant. Betwen ion activity  and his concetration is the folowing relation:

a _i = c_i.γ_i. (5)

Then we can write:

(6)

When we keep the activity coefficient constant, we can write:

(7)

If the measuring pass in the same temperature the eqation can be written:

(8)

For this purpose are used  some indiferent electrolytes which we add to solution. They are not   interfere the determination, but they ensure constant ionic strenght during of the measuring. Some special  mixtures of the electrolytes (TISAB,SAOB) not only fix ionic strenght, but as well mask for example ions, that can interfere determination.

First, known ion selective electrode was glass pH electrode on the base of the sodium calcium silicate glasses. When that pH was measured in alkaline region in sodium hydroxide, potassium hydroxide or lithium hydroxide the pH electrode shown so called alkaline error. This alkaline error is typical example of the interfering sodium, potassium and lithium ion on the function of the pH glass electrodes.The influence of these ions on the potential of the glass pH electrode  for sodium ,potassium and lithium ions is diferent for the same electrode and depends on the value of the selectivity coefficients toward these ions regarding hydrogen ion for this electrode. When the composition of the sodium calcium silicate glasses is changed so that calcium ion is replaced by aluminium ion the electrode begin be sensitive to sodium ion. On this base are produced sodium ion selective electrodes.

 

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