A glass Electrode is never in complete chemical equilibrium with the surrounding solution. Aqueous solutions slowly but continuously attack the glass membranes, while forming a gel layer. The ageing of a glass electrode depends on the ageing of the gel layer, which becomes thicker with time. Also, as the gel layer thickens, its internal structure undergoes alteration. The ageing symptoms of a glass electrode are
1) Sluggish response
2) Increasing electrical resistance
3) Smaller slope
4) Zero point drift
Detailed investigations have shown that the slope only changes slightly in acid region. In the alkaline region, on the other hand, the ageing symptoms are more pronounced, especially with a narrow pH Range. With modern pH amplifiers, the increase in electrical resistance is of no great resistance, while the zero point shift is compensated every time the electrode is calibrated. What is far more important is the exponential increase of ageing rate with temperature.
The ageing of glass electrode is a continual process without any abrupt change in the electrode characteristics, although it is highly dependent on the measuring conditions. For this reason it is difficult to give any hard and fast data concerning the life expectancy of glass electrodes. The following will give a rough idea of the order of magnitude of the times involved:
Ambient Temperature – Life expectancy of 1 to 3 years
90°C – Life expectancy of a few months
120° – Life expectancy of a few weeks.
Reference show no inherent ageing phenomena. On the other hand, their life can be considerably shorten by incorrect maintenance and use. This indirect ‘ageing’ of reference electrodes is very important and is often the main cause of inaccurate or spurious pH measurements.
Gel filled reference electrodes
Unlike normal reference electrodes with refill apertures, gel filled electrodes show definite ageing phenomena:
1. KCI diffuses continually outwards through the junction into the sample solution. This causes zero point drift of the reference electrode. This phenomenon is not critical since the drift is compensated on recalibration.
2. After long periods of use, particularly under fluctuating pressure, the electrolyte concentration within the junction falls off, leading to higher junction potentials and hence to large measuring errors.
3. The indirect ‘ageing’ phenomena of all reference electrodes are more marked in the case of gel filed electrodes:
– infiltrated sample solution cannot be removed;
– the chemical cleaning of contaminated junction is more difficult.