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.