The first potassium optode based on a protic chromoionophore immobilized in a hydrogel matrix is presented. The highly selective protic chromoionophore consists of a cryptohemispherand moiety and a trinitroanilino chromophore part. The acidifying power of potassium ions over sodium ions is 0.6 pH units. This correlates with the findings in solution. In contrast to several crown and ate-crown based chromophores the highly preorganized moiety allows ion detection even in aqueous environment. The detection limit for potassium ions at pH 7.7 is 5 mu M.