A Dilute Standard for pH, Conductivity, and Acid Neutralizing Capacity Measurement

Abstract

ABSTRACT A quality control standard suitable for assessing measurement errors of pH, conductivity, and acid neutralizing capacity (ANC, also known as titration alkalinity) of dilute, neutral pH waters has been developed and tested for field and laboratory use in ecological studies. A dilute phosphate standard is prepared as a 100-fold dilution (by mass) of 0.025 mol kg−1 (of solvent) KH2PO4 and Na2HPO4 standard buffer solution. At 25°C and in equilibrium with a standard atmosphere containing 340 μL L−1 CO2, the standard has a theoretical pH of 6.98, a calculated conductivity of 75.3 × 106 S cm−1, and an ANC of 250 μEquiv Kg−1 (249.28 μEquiv L−1 or 12.5 mg L−1 as CaCO3). Long-term systematic and random errors are reduced for pH, compared to other circumneutral standards, because of reduced sensitivity to atmospheric CO2 variations.