50, 100 ml
Dissolve the residue in a few drops of dil HCl and add 2-3ml water.
Take 1-2 drops of test solution in a spot plate and add 2-3 drops of magneson I reagent (4-(4-Nitrophenylazo)-resorcinol) and add 1 drop of 2M sodium hydroxide NaOH to make it alkaline. Blue colouration or blue precipitate is formed depending on the concentration of magnesium.
It is also commonly used as a violet dye pH indicator, appearing yellow below pH 11, and violet above pH 13.