Abstract: The present invention provides an improvement to the two, inaccurate, traditional approaches relating concentration to density. The first of these assumes that the solute is completely insoluble in the solvent; the second is based on a soluble solute. The present improved relationship is referred to as the CONCENTRATION-DENSITY MODEL. This model allows for a theoretical determination of the concentration-density relationship for a multi-component solid/liquid or liquid/liquid mixture. Included in the Concentration-Density Model is a new concept referred to as ADDITIVE VOLUME COEFFICIENT (AVC). This concept compensates for the fact that the net volume of a mixture does not always equal the sum of the volumes of each component. This improved Concentration-Density Model provides fluid-handling manufacturers with a method for accurately determining a mixture's concentration on-line with the aid of current density measurement instrumentation.