The relationship between sodium diethyldithiophosphate collector concentration and pH for conditions of incipient flotability of pyrite, galena, and chalcopyrite. If one considers that hydroxide ions compete with collector ions for the adsorption sites on the surface of these minerals, that the amount of collector adsorbed is constant under conditions of incipient flotation, and that the standard free energy of adsorption is constant, then each line in Fig. 2 must be characterized by a constant ratio of [X-]/[OH-]. Flotation should occur under conditions to the left of each curve but does not occur to the right of the curve. These diagrams were a major contribution to early research on the development of flotation reagents, since they clearly indicate under what conditions a reagent functions as a collector and show the deleterious effect of depressants.