Alkalinity is needed in many water treatment calculations, scale, corrosion, precipitation, oxidation, etc., yet the concept is often misunderstood. In natural waters, alkalinity often is not equal to bicarbonate concentration since natural waters contain base contributing anions that can significantly affect alkalinity. However, alkalinity is commonly assumed to be equal to the bicarbonate concentration in many scale and corrosion prediction algorithms. When other anions, e.g. carboxylates, are present, bicarbonate concentration in production tubing is not a conservative quantity; it varies with CO2 partial pressure, temperature and carboxylate concentrations in a complicated manner, up and down a well.

Reliable methods to accurately measure true alkalinity are scarce, especially when multiple weak acids are present and the effects of TDS on electrode and color end point are significant. Oil field brines contain aliphatic carboxylic acids of one to six carbons, e.g. acetate, up to 5000 mg/L. The highest concentrations of carboxylates tend to be in waters from reservoirs at temperatures of 80 to 100 °C. In this paper, a new analytical procedure and computation routine to determine alkalinity and carboxylic acids simultaneously will be discussed. The procedure was recently debugged and simplified by the Rice University Brine Chemistry Consortium (Rice BCC). The new titration method is based upon simultaneous analysis of the titration curve determined at fixed PCO2 and emphasizes the titration shape (profile) instead of the endpoint inflection as is done presently.

A wide range of natural and synthetic waters has been tested. Excellent agreement was observed between the true and calculated carboxylic acid concentration with a correlation coefficient squared of 0.9986. Once the total alkalinity and acetic acid concentrations are determined, the theoretically correct bicarbonate concentration and/or pH at any given operation temperature, pressure can be calculated. The intricate inter-relationship of total alkalinity, carboxylic acids, and pH on scale and corrosion will be discussed by using case studies.

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