The invention relates to methods of determining potassium ions in a sample, wherein the ions are contacted with a compound having chromophoric moiety and an ionophoric moiety, where the ionophoric moiety interacts with the potassium ions present in the sample, resulting in the chromophoric moiety changing its radiation absorption properties in the ultraviolet and visible regions of the spectrum. For example, a change in an intensity of an absorption maximum is measured and the ion concentration is determined accordingly.