Development of a high temperature pH electrode for geothermal fluids. Final report, Task III and year end summary

This report summarizes work done to demonstrate the applicability of a stabilized zirconia pH sensor to high temperature measurements on brines containing as much as 20 weight percent NaCl and 100 ppM hydrogen sulfide. Throughout the program stable operation was achieved, and measured pH values were in good agreement with calculated values. Differences were generally less than 0.5 pH unit at 285/sup 0/C, and it is not yet certain whether the discrepancies are associated with the measured or calculated values of the pH. While some sensors failed through cracking or because of uperfected seals, no signs of chemical degradation of the ceramic were detected during tests at 285/sup 0/C covering a range of pH between 3 and 9. Two sensors were operated at 285/sup 0/C for periods of 11 days, and one was employed in successive tests for a total of 37 days. At the end of this period the sensor was still satisfactory, and it was forwarded to the sponsoring laboratory, PNL, for further tests. Although most of the work was performed at 285/sup 0/C a limited amount of testing was done at lower temperatures: 95, 150, and 225/sup 0/C. Sensors prepared from in-house tubes and from tubes obtained from a new supplier performed well at 95/sup 0/C for extended periods, in spite of earlier difficulties with the standard ceramic at this temperature. There is still, however, some uncertainty concerning the adequacy of our seals particularly in cycling between 285/sup 0/C and lower temperatures

Development of a high temperature pH electrode for geothermal fluids. Final report, Task 1

There has been no serious application of pH measurements above 150/sup 0/C stemming from the fact that there have been no practical probes available that are capable of operation under the required conditions of temperature and pressure. Recently, a new approach to such measurements was developed in which an oxygen ion conducting ceramic membrane (e.g., stabilized zirconia) is being employed in a sensor somewhat analogous to the glass electrode. The new sensor retains the specificity of the glass electrode, is equally insensitive to interference from redox active species, and possesses markedly superior resistance to attack by aqueous media at high temperatures. The status of the new sense at the inception of the contract is summarized and plans for the first year's investigation are outlined