WATER QUALITY--Electrodes for pH measurement in low-conductivity waters In Reply Refer To: February 10, 1981 EGS-Mail Stop 412 QUALITY OF WATER BRANCH TECHNICAL MEMORANDUM NO. 81.08 Subject: WATER QUALITY--Electrodes for pH measurement in low-conductivity waters As pointed out in Quality of Water Branch Technical Memorandum 80.19, the procedures and equipment needed to obtain an accurate pH value are more exacting for samples with very low specific conductances than for samples with higher specific conductances. At specific conductances below about 70 mhos, many instrument- electrode systems give erroneous readings. Recent tests with several different pH electrodes and instruments, and evaluations of their precision when used to measure pH in low-conductivity waters, have suggested that the electrode is the critical component. Many manufacturers are now supplying electrodes designed for low conductivity waters. Some electrodes not so designed seem to work well also. Following is a list of electrodes that have been either claimed by the manufacturer to work well in low conductivity waters or tested by personnel in the Division and found to work well. Corning Glass Works 476182 * Plastic barrel 476223 Semi-micro, combination glass Orion Research 91-62 * combination glass with KCl/KN03 solution Sargent-Welch S-30072-15 * General purpose electrode Leeds ~ Northrup 117493 Meredian, KCl gel combination * Tested by Division personnel. Any instrument used must have slope compensation and temperature compensation, and be accurate to at least 0.05 pH units. Although most general-purpose portable pH meters are suitable, the better grade meters usually provide a more stable measurement. Electrodes designed for low-conductlvity waters are not as resistant to physical shock or exposure to strong chemicals as are the electrodes customarily used in the field and can be easily damaged by improper treatment. To protect the electrodes follow these guidelines in addition to the procedures given in Memo 80.19: l) store the electrodes in deionized water or covered with a cap containing deionized water; do not use KCl solution or buffer for storage; 2) do not expose the electrodes to strongly acidic or alkaline solution, or to very high-conductivity waters; 3) restrict use of the electrodes to waters with conductivities less than 200 umhos. You will recognize that the buffers used for calibration are high- conductivity solutions. Use of buffers for calibration is the single exception to the guidelines. Efforts are continuing to develop suitable standards in dilute acids for use in the field. We advise offices that are or may be making pH measurements in low-conductivity waters such as precipitation to select their equipment carefully, then dedicate it solely for this purpose. By so doing, much time will be saved and more reliable results will be obtained. R. J. Pickering Chief, Quality of Water Branch WRD Distribution: A, 8, S, FO, PO Key words: water quality, equipment, field measurements, pH, precipitation This memorandum supersedes no previous memorandum. This memorandum supplements Quality of Water Branch Technical Memorandum 80.19.