Solution pH is the most important and most difficult parameter measured in the steam/water cycle. There two methods of monitoring pH in this environment, direct measure and calculated pH by differential conductivity.
The term pH is a measure of the acidity (or alkalinity) of an aqueous fluid. There is much debate regarding the origin of the term pH. Some say it stands for the potential of hydrogen. Others say it is routed in Latin, pondus hydrogenai, which means hydrogen exponent.
The factor that most influences the level of acidity is the activity of the hydrogen (H+) ions in the fluid. By definition, pH is the logarithm of the reciprocal of the H+ activity:
pH = log [1/H] = - log [H+]
In weak solutions typical for power plants on-line instrumentation applications, H+ ion activity is approximately equal to H+ ion concentration in moles/liter. The pH is obtained by measuring the potential difference between a pH-sensitive electrode and a reference electrode, both of which are immersed in the solution of interest.