• In recent years the use of Acid Neutralising Capacity (ANC) as a measure of environmental change, especially in the field of surface water acidification, has focused attention on the efficacy of titration procedures used for ANC measurement. Historical titration data have usually been reported as Alkalinity (ALK) or ANC although, in theory, these terms are quite different and, in practice, require different titration conditions for their measurement.

• In this report a conceptual approach is taken to provide clear definitions of both parameters with special attention given to the role of dissolved organic carbon (DOC) in ANC measurements.

• Three titration procedures were applied to samples taken from upland streams covering a wide range of pH and DOC concentrations. Data collected from a survey of Scottish lochs were then used to validate the methods.

• The recommended methods provide relatively simple and reliable titration procedures giving a fast throughput of samples. Historical ALK data can also be converted to comparable ANC concentrations.

• The use of ANC in Critical Load assessment and SEPA classification schemes is briefly discussed.

KEY WORDS: ACID NEUTRALISING CAPACITY, ALKALINITY, TITRATION METHODS,

DISSOLVED ORGANIC CARBON, SURFACE WATERS, CRITICAL LOADS.

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