Permolybdic acid

The presence of benzo[6]thiophene in commercial naphthalene, its possible contamination with isomeric thienothiophenes 1 and 2, and their ability to poison aromatic hydrogenation catalysts led Maxted and Walker to develop detoxification by a preliminary short hydrogenation, in which thienothiophenes 1 and 2, and benzo[6]-thiophene are adsorbed on the catalyst. This is followed by their hydrogenation products that can easUy be oxidized with hydrogen peroxide or permolybdic acid to nontoxic sulfones subsequent hydrogenation of the aromatic hydrocarbons is then performed as usual. 

If a solution of a molybdate be treated in acid solution with hydrogen peroxide, oxidation to permolybdic acid takes place, the solution becoming yellowA The colour remains on warming,- and cannot be extracted by ether. Moreover, when molybdic anhydride, M0O3, is dissolved in hydrogen peroxide, a yellow insoluble compound is deposited from the stable, dark yellow solution. ... 

Hydrogen peroxide reacts with acidified molybdate solutions. Chaveau, Souchay and Tridot (1955) have shown the presence of the acid H2M02O44 in solutions of molybdate in concentrated perchloric acid to which HgOg has been added. The so-called permolybdic acid is regarded as a salt of this acid with the cation [HMo20g]+, not itself peroxidic. 

The low result for the orthophosphate standard was probably caused by the presence of unreacted peroxide which during the subsequent colorimetric procedure reacted with molybdic acid to form permolybdic acid (Table VI). 

This procedure, including decomposition of any iodine compound by the addition of excess permolybdic acid, will reveal as little as 0.5 y iodine and/or 0.5 y bromine (as halide) in a drop of saturated solution of sodium chloride. 

Permolybdic acid does not affect chloride ions bromide ions are converted to elemental bromine iodine is converted via free iodine into iodic acid. Thus, the conditions are provided under which the color reaction with fluorescein becomes unequivocal. 

Procedure. A small amount of the solid test material or a drop of its solution is treated in a micro test tube with several drops of chromic-sulfuric acid (preparation see Vol. II page 65). The open end of the test tube is covered with a disk of filter paper moistened with 2 N sodium hydroxide. The test tube is kept for 5 minutes in boiling water. The disk of reagent paper is then placed on a glass plate, spotted with several drops of permolybdic acid solution (for preparation see Vol. II page 68) and then dried with a current of cold air. If iodine is present, a brown fleck will appear but it fades within a few minutes. The stain is then treated with a drop of 0.1 % alcoholic solution of fluorescein. A positive response is indicated by the development of a red spot. 

Reagent Solution of permolybdic acid 5 ml of 30% hydrogen peroxide and 5 ml of concentrated acetic acid plus 2-3 drops of 0.5 M ammonium molybdate. 

Mehkov PG, Pisai vskij L (1898) Salts of permngstic and permolybdic acids. Bta- Dtsch Chem Gesell 31 632-636... 

---