REAL-TIME MEASUREMENT AND INTERPRETATION OF THE CONDUCTIVITY AND PH OF PRECIPITATION SAMPLES

A microprocessor-based acid rain monitor was used to make real-time me asurements of conductivity and pH of rainwater within individual storm s. The automated measurements were compared with laboratory analyses o f a subset of the samples taken. The laboratory measurements tended to overestimate the pH because of temperature induced changes in dissoci ation and Henry's Law constants affecting ionic compounds in the rainw ater. The measurement artefact due to these effects may result in aver age hydrogen ion concentrations being underestimated by approximately 10 to 15% at UK sites. The greatest systematic discrepancies would be anticipated at highly polluted sites and during low temperature acidic episodes. The concept of a rainwater acid fraction was investigated a nd found to be useful for quality control and interpretative purposes. The field measurement of conductivity of low ionic strength samples w as slightly lower than the corresponding laboratory measurement, possi bly caused by limited resolution of the conductivity probe or dissolut ion of fine particulate material.