Volume 11, Number 8
Dec 1987

pH is Hard to Measure

Since Elaine Haas's letter about the frequent inaccuracy of pH measurements taken with distilled water (July issue), several people have mentioned to the Editor that they often got measurements they knew were too low even when all precautions were taken. Among then was Robert Espinosa, Conservator at Brigham Young University's Lee Library, who resolved to get to the bottom of the problem. He invited a representative of Orion Research, Inc., to the lab to give the staff of three what amounted to a tutorial in accurate pH determination using a meter. The rep explained the choice, use and maintenance of electrodes and buffer solutions, and how to calibrate and get a good reading from a meter. Temperature has a significant effect on pH, so some units are made with automatic temperature compensation. Staff member Jane Dalrymple-Hollo took 11 pages of notes.

When you measure pH with a meter and distilled water, the Orion rep said, you are using the water to complete a circuit. The purer the water is, the poorer the conduction. Potassium chloride (KCl) can be added to speed the readings without affecting their accuracy.

For dealing with the peculiarities of distilled water, he recommended putting a CO₂-absorbent canister on the bottle and only keeping it for a limited period of time. He said boiling also gets rid of CO₂ absorbed from the air. He mentioned a "low ionic strength reagent kit" which has a special buffer, and a "high purity water reagent kit" which includes KCl. One of the pieces of literature he left was an article, "A Simple Method to Measure pH Accurately in Acid Rain Samples," by Peter F. Boyle et al., Impact of Acid Rain and Deposition on Aquatic Biological Systems, ASTM STP 928, B. G. Isom, S. D. Dennis, and J. M. Bates, Eds., American Society for Testing and Materials, Philadelphia, 1986, pp. 98-106. The abstract reads:

Conventional pH electrodes, which are designed to function in high-conductivity solutions, show slow and erratic response in pure water samples such as acid precipitation. Increasing conductivity without shifting pH solves the measurement problem. This method correctly measures pH in low conductivity solutions, using a Ross combination pH electrode, after the addition of a small amount of potassium chloride (KCl) solution to the sample. The Ross electrode, with its unique internal redox system, is chosen since it eliminates problems associated with temperature and because the precision and accuracy of data obtained in preliminary testing with it were comparable to those obtained with the standard hydrogen electrode. KCl addition to samples does not alter the pH significantly.

The telephone number for technical information from Orion Research, Inc., is 800/225-1480. They are at 529 Main St., Boston, MA 02129. They have a booklet, "A Concise Guide to pH Measurement," which appears quite useful and is probably free.