Detection and Estimation of hydrogen peroxide

Many of the reactions described in the preceding pages can easily be applied to the detection of hydrogen peroxide. The blue coloration with chromic acid, especially with subsequent extraction of the coloured compound by ether. The oxidation of potassium iodide in a solution containing acetic acid and starch, especially in the presence of a catalyst such as ferrous sulphate or molybdic acid; with the latter catalyst, there is the additional advantage that the oxidation of the iodide by atmospheric oxygen is not accelerated, so that atmospheric oxidation has less likelihood of introducing a serious disturbing factor. The formation of Prussian blue on addition to a solution of ferric chloride and potassium ferricyanide. Of these three tests, which are all of considerable sensitiveness, the last possesses the additional advantage of being unaffected by nitrogen dioxide.

Numerous colour reactions depending on the oxidation of various organic compounds have been suggested; paper impregnated with cobalt naphthenate changes in colour from rose to green; a dilute solution of aniline or dimethylaniline containing potassium dichromate and a little oxalic acid yields a red coloration; m-phenylenediamine in hot ammoniacal solution gives a blue coloration and p-phenylene- diamine has also been used. These reactions are all delicate for hydrogen peroxide.

Certain inorganic reagents possess the additional advantage of being specific for hydrogen peroxide. Titanium sulphate in neutral or acid solution gives a deep yellow coloration due to the formation of a pertitanic acid, TiO₃.xH₂O. Acid solutions of ammonium molybdate give an intense yellow coloration of permolybdic acid, H₂MoO₅. Both these supply very sensitive tests.

For the purpose of distinguishing hydrogen peroxide vapour from nitrogen dioxide or ozone, potassium permanganate and manganese dioxide are useful reagents, the former leaving only ozone unaffected whilst the latter fails only to decompose nitrogen dioxide.

The liberation of iodine from an acidified solution of an iodide provides a convenient method for estimating hydrogen peroxide quantitatively, and by first allowing the solution or vapour to react with a slightly acid solution of potassium bromide, any ozone present may be previously determined quantitatively and removed in one process.

The reaction of potassium permanganate or hypochlorite with hydrogen peroxide in acidified solution also can be conveniently made the basis of a volumetric determination, the volume of liberated oxygen being twice that expected from the decomposition of the peroxide into an equimolecular proportion of water. If the solution of hydrogen peroxide is sufficiently dilute, direct titration with the permanganate is possible in the presence of sulphuric acid, a molecule of hydrogen peroxide decolorising two-fifths of a molecule of potassium permanganate.

In those cases where there exists a possibility of organic substances interfering with the estimation of hydrogen peroxide by treatment with a solution of potassium permanganate or potassium iodide, the use of a standard solution of stannous chloride has been proposed; the reaction follows the course

SnCl₂ + 2HCl + H₂O₂ = SnCl₄ + 2H₂O

and may be applied by direct titration with methylene blue as indicator, or by the addition of an excess followed by titration of the superfluous stannous salt with iodine.