Abstract: An apparatus for measuring ion concentration of a solution includes an ion sensitive field effect transistor (ISFET), a reference device, an ISFET control circuit, a memory, a measurement circuit and a diagnostic circuit. The ISFET has a drain, a source, an ion sensitive gate region and a plurality of device characteristics. The reference device is separated from the gate region by a sample solution. The ISFET control circuit is coupled to the ISFET and operates the ISFET at a drain-source voltage V.sub.DS and at n successive drain currents I.sub.Di and corresponding gate-source voltages V.sub.GSi, wherein i is an integer from 1 to n and n is an integer greater than 1. The memory stores the plurality of device characteristics and the n successive drain currents I.sub.Di and gate-source voltages V.sub.GSi. The measurement circuit measures ion concentration of the solution as a function of at least one of the n successive drain currents I.sub.Di and gate-source voltages V.sub.