The slope of a combination pH sensor is defined as the quotient of the potential voltage difference developed per pH unit:
In theory a pH sensor should develop a potential difference of +59.16 mV per pH unit between pH 7 and pH 0, and correspondingly –59.16 mV between pH 7 and pH 14. In practice however, a new and well hydrated pH sensor reaches at best 99.8% of the theoretical value. With time the slope decreases, initially slowly and later more rapidly. It is essential that a slope compensation be carried out during the calibration procedure, using the slope calculation of the pH meter/transmitter. As with the zero point adjustment, the slope adjustment has to be performed at regular intervals to maintain best accuracy.
The slope of a pH sensor is temperature dependent in accordance with the Nernst equation. The slope increases with the rise in temperature of the measured liquid solution, as can be seen from the diagram to the right. In theory, all temperature dependent slope lines intersect the theoretical zero point (pH 7).
In order to produce a response as near as possible to the Nernst equation a pH sensor must fulfill certain criteria:
- The inner and outer gel layers of the glass membrane must produce potentials having identical slopes.
- The internal buffer solution must maintain a constant pH value.
- The asymmetry potential should be as small and as constant as possible.
- The electrode assembly must be symmetrical, i.e. measuring and reference electrodes must have identical conducting systems in order to neutralise their galvanic potentials.
- The diffusion potential of the diaphragm should be as small and as constant as possible.