peptization
/ pep-tih-ZAY-shun /
Imagine carefully gathering a pile of fine dust, then rinsing it with plain water — and watching it scatter into a haze that drifts away rather than staying put. Peptization is that frustrating moment when washing a precipitate makes it fall apart into a colloid and slip through the filter.
Peptization is the reversion of a coagulated precipitate back into dispersed colloidal particles, typically caused by washing it with pure water. Removing the dissolved ions that held the tiny particles clumped together lets them repel one another and float off again.
It matters because a peptized precipitate passes straight through the filter as cloudy filtrate, so the analyte is lost and the result comes out too low. The fix is to wash not with pure water but with a dilute solution of a volatile electrolyte that keeps the particles coagulated and then burns off during ignition.
Washing freshly coagulated silver chloride with pure water can peptize it back into a milky colloid; washing instead with dilute nitric acid keeps it clumped, and the acid simply evaporates on drying.
Pure-water washing can scatter a precipitate back into a colloid.
Peptization is the reverse of coagulation. That is why wash liquids for colloidal precipitates always carry an electrolyte that volatilizes — it preserves the clumping yet leaves nothing behind on the weighed solid.