A chemical whose molecular structure can envelop and hold a certain type of ion in a stable and soluble complex. Divalent cations, such as hardness ions, form stable and soluble complex structures with several types of sequestering chemicals. When held inside the complex, the ions have a limited ability to react with other ions, clays or polymers. Ethylenediamine tetraacetic acid (EDTA) is a well-known sequestering agent for the hardness ions, such as Ca⁺², and is the reagent solution used in the hardness test protocol published by API. Polyphosphates can also sequester hardness ions. The addition of sodium acid pyrophosphate (SAPP) to a cement-contaminated mud renders the calcium ions essentially nonreactive with clays in the mud. As a side benefit, SAPP also lowers mud pH. Sequestering is not the same as precipitation because sequestering does not form a solid.