AbstractThe role of a detergent builder in fabric washing formulations is complex because the builder is multifunctional. The primary function of a builder is to counteract the detrimental effects of hardness ions (Ca2+ and Mg2+) which arise in the wash solution not only from the tap water but also from the washload. The need to control the free hardness ion level to 0.1 °FH is well established and is mainly required to release calcium bound soil and prevent further calcium soil interaction during the wash process. The most elegant way of doing this is with a sequestrant. Sodium triphosphate is the preferred material. Measurement of the equilibrium constants of sequestrants with Ca2+/Mg2+/Na+/H+ as a function of temperature and ionic strength allows prediction of free hardness ion levels over a wide range of conditions such as temperature, pH, water hardness, and dosage. Secondary functions such as specific adsorption effects are important, and a true builder will aid not only soil removal but alleviate soil redeposition.