During the setting of alginate impression materials:

 # During the setting of alginate impression materials:

A. Trisodium phosphate reacts preferentially with calcium sulfate

B. Trisodium phosphate reacts preferentially with sodium alginate

C. The colloidal gel state changes to a sol

D. The material in contact with the soft tissues set last

The correct answer is. A. Trisodium phosphate reacts preferentially with calcium sulfate

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, Anusavice, Shen, Rawls, 12th Edition, Page No: 172

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