During setting of alginate impression materials:

Gelation Process The typical sol-gel reaction can be described simply as a reaction of soluble alginate with calcium ions from calcium sulfate and the formation of an insoluble calcium alginate. Structurally, calcium ions replace the sodium or potassium ions of two adjacent molecules to produce a cross-linked complex or polymer network. The production of the calcium alginate is so rapid that it does not allow sufficient working time. A water-soluble phosphate salt (e.g., trisodium phosphate) is added to the composition as a retarder to extend the working time. The strategy is that the calcium ions will react preferentially with the phosphate ions in the solution. Thus, the rapid reaction between calcium ions and the soluble alginate is deferred until phosphate ions from trisodium phosphate are exhausted. The amount of retarder is adjusted to provide the proper setting time. A similar gelation process is expected with the modified alginate product. Reference: PHILLIPS’ SCIENCE OF DENTAL MATERIALS, 12th ed page no 172