Calmagite is the common name for 1-(1-hydroxy-4-methyl-2-phenyl­azo)-2-naphtol-4-sulfonic acid.

Calgamite structure

It was first described in:

F. Lindstrom and H. Diehl (1960) Indicator for the titration of calcium plus magnesium with (ethyl­ene­dinitrilo)­tetra­acetate, Anal. Chem., vol.32, pp.1123-1127.

Calmagite is used in analytical chemistry as a metal ion indicator. Calmagite changes colour when it is bound to an ion: wine red when it is bound to a metal ion and blue (depending on the pH) when it is not bound to a metal ion.

Calmagite is normally used in Ca or Mg titrations with edta by adding 5 drops (≈0.25 mL) of a 0.1% solution (≈2.8 mM). This means that the indicator concentration in the solution being titrated is somewhere between 3 and 14 × 10−6 M.

A spectrophotometric procedure is described in:

Gordon et al. (2001) A micro­scale spectro­photo­metric determination of water hardness, J. Chem. Edu. vol.78, pp.1089-1090.

back  Back to titration of Mg with edta