Compounds that selectively bind sodium.
A compound having the property of selectively binding sodium ions in the presence of potassium ions and having the formula: where X is- O - or and where any aromatic ring may be substituted and/or may form part of a fused aromatic ring system.At least one of the aromatic rings may be substituted by a spectroscopic reporter group, a group which permits the compound to enter and remain in a cell and/or an electron donating or withdrawing group.A method of determining cytoplasmic sodium concentration by using this compound as a probe is also described.