EQUIPMENT--pH System of Hydrolab 4041 Units In Reply Refer To: October 7, 1983 EGS-Mail Stop 412 QUALITY OF WATER BRANCH TECHNICAL MEMORANDUM NO. 84.01 Subject: EQUIPMENT--pH System of Hydrolab 4041 Units The results of detailed comparisons of pH measurements using 11 Hydrolab model 4041 units with 4 Ross electrodes as controls in a variety of solutions and natural waters indicate that the pH systems on Hydrolab units equipped with the glass/porous-Teflon sleeve reference electrodes are not performing satisfactorily. Of 319 comparisons, only 46 percent were within 0.1 pH unit and only 70 percent were within 0.2 pH units of the controls. The problem appears to be that the porous-Teflon sleeve of the reference electrode is a static, rather than a flowing, junction and is subject to contamination from sample materials producing variable junction potentials in solutions of different ionic strengths and dominant anions. There is a definite bias to low pH measurements in low ionic strength solutions and a tendancy toward poorer performance in chloride-dominated waters than in sulfate or carbonate-dominated waters. Unfortunately, there is too much scatter in the data to allow reasonable quantification of the bias. Moreover, tests of the units with only pH 7 buffer as the filling solution in reference electrodes, the condition prior to issuance of Quality of Water Branch Technical Memorandum 82.17 advising that 3M KCL should be added to the filling solution, showed performance to be much worse--only 17-31 percent of comparisons were within 0.1 pH units of controls. Based on these results, District and Project Offices are advised to (1) either discontinue use of or provide back-up measurements for the Hydrolab 4041 pH system until further notice, (2) critically review historic pH data obtained using these units and consider purging files if pH values recorded do not agree with confirmatory measurements from a back-up instrument, and (3) qualify all data obtained with these units in published reports of hydrologic investigations. The approach to correcting the problem with these units is clear; the current reference electrode must be replaced. Tentatively it appears that a 3M KCL gel-filled Ag/AgCl reference will work well, but pressure tests must be conducted for confirmation. The Quality of Water Branch is working with the Hydrolab Corporation and the Hydrologic Instrumentation Facility to resolve questions about the feasibility of retrofitting all units presently in the field and to develop a plan for carrying out the retrofitting. Instructions will be communicated by memorandum or Multics when these issues have been resolved satisfactorily. R. J. Pickering Distribution: A, B, PO, FO Key Words: Water quality, instrumentation, pH meter This memorandum does not supersede any previous memorandum.