We describe a new method for the spectroscopic determination of high calcium concentration using a fluorescent probe Rhod-5N. This method was investigated in order to be utilized in high ionic strength solution, such as seawater. The probe is fluorescent when bound to calcium, LM, but not as the free form L. The dissociation constant of the equilibrium (0.14mM) was determined at several ionic strengths, i.e. in the absence and in the presence of additional ions (0.7M NaCl). The influence of pH was studied. In order to correctly model the experimental data, we included a new fluorescent compound: LHM (calcium bound protonated probe). The first acidity constant (0.02muM) and the second dissociation constant (4.5mM) were calculated. A useful range for the determination of calcium concentration is provided. Such a method is fast and easy to carry out.