One part of peroxide in 25 million parts of water may be detected by the liberation of iodine from potassium iodide, giving a blue colour with starch. Other substances, such as ozone and nitrites, give this reaction. The liberation of iodine occurs somewhat slowly, but is rapid in presence of ferrous sulphate: 2KI + H2O2 = 2KOH + I2.
The most delicate reaction for hydrogen peroxide is the formation of a yellow colour, due to titanium peroxide, TiO3, with a solution of titanium dioxide in dilute sulphuric acid. This solution is prepared by heating TiO2 with twice its volume of concentrated sulphuric acid, cooling, and diluting with ice-water.
A delicate test is f the formation of a deep blue perchromic acid with chromic acid, CrO3.
Expt. 6. - To very dilute hydrogen peroxide is added a dilute solution of potassium dichromate acidified with sulphuric acid. The solution is rapidly shaken with ether, which floats to the surface with a beautiful blue colour. An unstable perchromic acid is formed, which dissolves in ether to form the blue liquid: this decomposes after a time, with evolution of oxygen, and a green, aqueous solution of chromic sulphate is formed in the lower layer.Other tests are: (1) guaiacol solution acidified with sulphuric acid gives a blue colour; (2) guaiacum tincture, with a little blood, or malt extract, gives a blue colour (this is also a delicate test for blood, and can be used in identifying blood-stains); (3) a mixture of aniline and potassium chlorate, dissolved in dilute sulphuric acid, gives a violet colour; (4) filter-paper soaked in a solution of cobalt naphthenate, and dried, changes from rose to olive-green with hydrogen peroxide.