A commonly used preservative for starch, xanthan gum, guar gum and other natural polymers that are prone to attack by bacteria. It is as a trimer of formaldehyde and has the formula O-CH₂-O-CH₂-O-CH₂. Paraformaldehyde is a white, water soluble powder. When added to a mud in advance of a bacterial inoculation and maintained, paraformaldehyde can effectively control many strains of bacteria. The amount or paraformaldehyde in a mud can be estimated by oxidizing it with sulfite into formic acid and performing an alkalinity titration, according to a procedure published by API.