Improved Methods for Measuring Residual Effective Alkali in Kraft Black Liquors - Part 2: Comparison of Titration Methods, 2007 Engineering, Pulping & Environmental Conference

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Although digesters can now be equipped with automatic or online analyzers which should provide reliable and rapid measurements of alkali concentration at different points in the process, these devices need either to use, or be calibrated with, an accurate method of measuring the residual effective alkali (EAr) content in black liquor. The most frequently used methods of measurement are based on acidimetric titrations. These methods, however, overestimate the true concentration due to interference from dissolved organic acids. Recently, we developed a chromatographic technique (capillary electrophoresis) which measures the absolute or true residual EA. In this work, we measured the relative accuracies of four titration methods and found that all four require a correction factor (the SCAN-N 33:94 Standard Method already includes a prescribed correction factor). Of the four methods tested, we recommend, based on the highest correlation coefficient and simplicity of the methodology, a modified Milanova-Dorris method. In it, the liquor is titrated at a dilution factor of 4, and a correction factor is applied to the measured EAr at an inflection point between 11.0 and 11.5. The correction factor equation is: true EAr = 0.921 (measured EAr) + 0.51, in g/L as Na₂O.