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Salinity determination in seawater has been carried out for almost 30 years using the Practical Salinity Scale 1978. However, the numerical value of so-called practical salinity, computed from electrical conductivity, differs slightly from the true or absolute salinity, defined as the mass of dissolved solids per unit mass of seawater. The difference arises because more recent knowledge about the composition of seawater is not reflected in the definition of practical salinity, which was chosen to maintain historical continuity with previous measures, and because of spatial and temporal variations in the relative composition of seawater. Accounting for these spatial variations in density calculations requires the calculation of a correction factor δ<i>S</i><sub>A</sub>, which is known to range from 0 to 0.03 g kg<sup>−1</sup> in the world oceans. Here a mathematical model relating compositional perturbations to δ<i>S</i><sub>A</sub> is developed, by combining a chemical model for the composition of seawater with a mathematical model for predicting the conductivity of multi-component aqueous solutions. Model calculations for this estimate of δ<i>S</i><sub>A</sub>, denoted δ<i>S</i><sub>R</sub><sup>soln</sup>, generally agree with estimates of δ<i>S</i><sub>A</sub> based on fits to direct density measurements, denoted δ<i>S</i><sub>R</sub><sup>dens</sup>, and show that biogeochemical perturbations affect conductivity only weakly. However, small systematic differences between model and density-based estimates remain. These may arise for several reasons, including uncertainty about the biogeochemical processes involved in the increase in Total Alkalinity in the North Pacific, uncertainty in the carbon content of IAPSO standard seawater, and uncertainty about the haline contraction coefficient for the constituents involved in biogeochemical processes. This model may then be important in constraining these processes, as well as in future efforts to improve parameterizations for δ<i>S</i><sub>A</sub>.