A solid composition in the form of a tablet that generates and releases a biocidal solution comprising at least chlorine dioxide with an enhanced weight percent yield is presented. The composition comprises reactants capable of in-situ generation of chlorine dioxide comprising a chlorite donor that is coated with a gel-forming material that slows the rate of dissolution of the high solubility chlorite donor, a free halogen donor, and an acid source, resulting in an unexpectedly high weight percent yield and providing a controlled release of biocidal solution. The compositions of the invention show improved environmental stability which can reduce the cost of packaging and significantly increase the utility of the composition. The controlled release allows the use in multi-tablet chemical dispensers which may otherwise induce potentially explosive conditions or allow rapid release of the biocidal solution thereby inducing a spike in chemical concentration rather than a controlled and sustained release.