Density calculation and concentration scale conversions for natural waters

Abstract

Abstract A program is presented that calculates the density of a natural water from its composition. It is based on the recent modeling of the volumetric properties of natural waters at 25°C and 1 bar to high concentrations using Pitzer's equations. The required input is the concentrations of the major solutes of natural waters, that is Na, K, Ca, Mg, Cl, SO 4 , HCO 3 , and CO 3 , expressed in any of the most widely used scales: molinity (grams or moles per kg of solution), molality (moles per kg of water), or molarity (moles or grams per liter). The output consists of the density of the solution, the partial molal volumes of the solutes, and a table in which the input concentrations are converted into all the scales as cited plus the solute mole fractions. The error in t he calculated seawater density is 0.00002 g/cm 3 , but it increases to about 0.0004 g/cm 3 for dense brines. The program then is used to calculate the consequences of the approximation that natural waters behave as pure sodium chloride solutions, both for the computed density and for the concentration conversions.