A controlled release, encapsulated acid composition and a method of preparation are disclosed. The composition comprises (1) a porous matrix having an acid absorbed therein, the matrix being substantially inert to the absorbed acid; (2) a midlayer surrounding the matrix, this midlayer consisting essentially of at least one salt of the absorbed acid; and (3) a membrane surrounding the matrix, the membrane being exterior of the midlayer, the membrane further being substantially inert to the midlayer and permeable to a selected environment. The membrane preferably also includes particulate material which acts as an anticoalescent.
The midlayer can be applied as a coating to the acid-bearing matrix, or it can be evolved by exposing the acid-bearing matrix to a base such as ammonia gas or a solid metal oxide for a time sufficient to allow reaction of a proportion of the acid at or near the surface of the matrix with the base or oxide to form a layer of the corresponding salt.