Ca²⁺ is a major signaling molecule in both prokaryotes and eukaryotes. Its cytoplasmic concentration is tightly regulated by transporters in the plasma membrane and in the membranes of various organelles. For this purpose, it is either extruded from the cell through exchangers and pumps or stored in organelles such as the endoplasmic reticulum and the mitochondria. Changes in the concentration of the cation regulate numerous enzymes including many involved in ATP utilizing and in ATP generating pathways and thus ultimately control metabolic activity of mitochondria and the entire cell. Measuring changes in Ca²⁺ levels is thus of considerable interest in the context of high-resolution respirometry.

Ca²⁺ concentration can either be measured in the NextGen-O2k Fluo and O2k-Fluo fluorometrically by using the fluorophore Calcium Green or potentiometrically with the O2k-TPP+ ISE-Module, which can be applied with any O2k configuration (Series B or higher), by applying Ca²⁺ membranes.