The invention provides chromoionophore compounds comprising a triazacryptand (TAC) K+ ionophore conjugated to at least a first chromophoric moiety (e.g., xanthylium dyes and derivatives thereof). In related embodiments, the chromoionophore compounds further comprise a second chromophoric moiety which is insensitive to potassium binding by the TAC ionophore, thus providing for dual wavelength detection and absolute determination of K+ concentration. The invention further provides methods and kits for the determination of K+ concentrations in biological systems, either in vitro or in vivo, using embodiments of inventive chromoionophores.