Principle:

pH Measurements;
The pH of a solution is defined as: pH = -logaH ISE Measurements; Ion-selective electrodes (ISE's) respond, more or less exclusively,to a specific type of ion in solution. The particular ion to which an ISE responds depends upon the chemical makeup of its sensing membrane.

All ISE's also have atleast some unintended response to additional ions. These are referred to as 'interferences', and if present at high levels, they may 'interfere' with ISE measurements. Placed in a solution containing the ion type to which it responds, but no significant interfering ions, the ISE develops an output voltage proportional to the concentration of the ion.