Ion sensitive field effect transistors applied to the measurement of the pH of brines

Abstract

The ability to measure the pH (the negative logarithm of the hydrogen ion activity) of harsh fluids such as geothermal oil field brines is important, since pH is a fundamental property; as one chemist stated: very often pH is a critical test because its accuracy lays the foundation for other measurements''. In our research, we focus on the analysis of brines similar to those found in underground geothermal reservoirs. Since the brines are deep under the ground, the values of the pressure and the temperature are high (up to 14 Mpa and 150[degrees]C); therefore the usual methods of pH measurement, e.g., glass electrode, are not applicable. The hydrogen ion sensitive ISFET (Ion Selective Field Effect Transistor) was studied as a pH sensor in this research. An ISFET can detect the electrochemical potential difference between the solution and the semiconductor due to the concentration of H[sup +] ions in the solution. Because of its solid state construction, an ISFET should work properly under high pressure and high temperature conditions. Earlier results, have indicated that it is possible to use ISFETs under the harsh conditions presented by geothermal brines.