[′ham·ət ə′sid·əd·ē ‚fəŋk·shən] (chemistry) An expression for the acidity of a medium, defined as h0= KBH+[BH+]/[B ] , where KBH+is the dissociation constant of the acid form of the indicator, and [BH+] and [B ] are the concentrations of the protonated base and the unprotonated base respectively.