WATER QUALITY--Interim procedures for measuring pH in low-conductivity waters.


                                                June 23, 1980


QUALITY OF WATER BRANCH TECHNICAL MEMORANDUM NO. 80.19

Subject: WATER QUALITY--Interim procedures for measuring pH in 
                        low-conductivity waters.

Districts are being asked with increasing frequency to measure pH 
in samples of precipitation and other waters of low conductivity. 
At least two effects are being observed in such samples that do 
not occur in most others; l) a delay of several minutes in 
reaching a stable reading in the sample and 2) large differences 
(up to 0.6 units) in the apparent pH between a stirred and a 
quiescent sample. Both of these effects have been discussed in a 
recent paper by Galloway and others (Limnology and Oceanography, 
1979, vol 24, no 6, pp 1161-65). The first effect is probably 
caused by incomplete washing of the buffer from the glass 
electrode or by storing electrodes in buffer solutions. The second 
effect may be due to distortions of the potential field in the 
sample as a result of stirring, an effect which has been referred 
to (perhaps incorrectly) as a streaming potential. Both effects 
are thought to become insignificant at conductivities above 50 
umhos, although this limit is not well established.

The Quality of Water Branch is continuing to investigate the 
effects described above and to develop standard procedures for pH 
measurement under these conditions. Until such procedures are 
developed and tested, we advise all districts to: l) where 
possible use probes designed for low conductivity waters; and 2) 
make pH measurements according to the following interim procedure:

1. Calibrate using pH 7 and 4 or 9 buffers to bracket the 
   expected pH.

2. Rinse the probes copiously with distilled water; do not wipe 
   the probes.

3. Place probes in distilled water (or sample if plenty is 
   available) with stirring for 10 to 20 minutes, replacing the 
   water at least 3 times. During this step, periodically stop the 
   stir and note the pH in quiescent water. When readings in 
   quiescent water drift less than 0.1 units/minute, the system is 
   ready to use.

4. Remove the probes, shake off excess water, gently pour the 
   sample into a clean beaker, insert probes and gently swirl the 
   sample to achieve good contact of sample to probes.

5. Make readings in a quiescent sample. Take the first reading 
   after drift is less than 0.1 units/minute.

6. Remove the probes and rinse with distilled water.

Measurements on several samples may be made from one calibration, 
skipping steps 1 through 3. If the meter must be moved, place the 
probes in distilled water and leave the meter in a measurement 
mode. The stability of calibration will vary with instrument and 
handling. The decision to recalibrate must be left to the 
judgement of the person making the measurements. Probes should be 
stored in distilled water, not buffer.

Measurements with in situ probes, which will often be in flowing 
streams, should be checked against measurements in quiescent 
samples following the above procedure. When differences exceed 
0.2 units, accept the measurements in quiescent samples.



                                R.J. Pickering

Memoranda superseded: None

Keywords: Water quality, analytical methods, instrumentation, pH.

WRD Distribution: A, B, S, FO, PO