Raising sensitivity to acidity trends.

Extensive monitoring programs are now in place in the United States, Canada and Europe to determine the relationship between the acidity of precipitation and the acidity of fresh water. This requires precise measurements over long time periods of the water's hydrogen ion concentration, or pH. However, conventional electrochemical methods for measuring pH are sometimes unreliable because dissolved organic material interferes with the instrument.

In the May 9 NATURE, Andrew L. Herczeg and his colleagues at the Lamont-Doherty Geological Observatory in Palisades, N.Y., and David W. Schindler of the Freshwater Institute in Winnipeg, Manitoba, contend that "the present strategies for monitoring temporal trends in the acidity of fresh waters will not yield results sensitive enough to detect trends in acidity even on a 10-year timescale." In their paper, the researchers suggest an alternative scheme for tracking acidity trends, particularly in lakes not yet acidified.

Their method is based on calculations using careful measurements of the amount of dissolved inorganic carbon in a water sample and the partial pressure of carbon dioxide in the air above a lake. "This scheme elminates biases inherent in pH electrode determinations," the researchers report, "and minimizes the perturbations in acidity caused by changes in the [carbon dioxide partial pressure] of lake waters associated with biological cycles."