Measurement of pH in concentrated brines

Accurate measurement of pH in concentrated brines is a critical aspect of materials testing in the oil and gas industry. Concentrated sodium chloride solutions, typically up to 4 M and saturated with carbon dioxide, are often used to simulate production water during testing programmes. The pH of the solution may be adjusted to the desired value by the addition of sodium bicarbonate (to raise pH) or sodium acetate and hydrochloric acid (to lower pH). Recently, concerns have been raised over reports of discrepancy between pH values of concentrated NaCl solutions measured in the laboratory and those predicted by thermodynamic models. Measurements carried out by Bodycote (UK) and IFE (Norway) are compared with predictions of the BP and Shell models in Figure 1. Here the concentration of NaCl is defined in terms of mass of solute per mass of solution.