Molybdenum colour reactions

The most commonly used reducing agent is SnC. With this reductant, the presence of Fe(III) in the sample solution is indispensable. In the absence of iron the results obtained are low and of poor reproducibility. Ascorbic acid, hydrazine, thiourea, and potassium iodide in the presence of Cu(I) are also used, besides Sn(II). With these weaker reducing agents, more intensely coloured solutions are obtained. Stannous chloride is a strong reducing agent and is believed to reduce some of the Mo(VI) to an oxidation state lower than Mo(V). This molybdenum does not participate in the colour reaction [48,49]. 

Europium(III) has been selectively reduced in the Jones column to Eu(II) which can reduce Methylene Blue to the colourless leuco- form [126]. Europium(II) also reduces phosphomolybdic acid to molybdenum blue [127] and gives a colour reaction in solutions containing 1,10-phenanthroline and Fe(III). 2,2 -Diquinoxalyl has also been used for determining Eu [128]. 

As some other sugars are also present in blood sample, and besides the above reaction not being absolutely stoichiometric, it has become necessary in actual practice to establish an emperical calibration curve using known concentrations of glucose. The above reaction is allowed to proceed for exactly 8 minutes at 100°C. To the resulting solution phosphomolybdic acid is added, which is subsequently reduced by Cu20 to give rise to an intensely coloured molybdenum blue that is measured at 420 nm accurately. 

Molybdenum and Tungsten Peroxide Compounds. Pour ammonium molybdate and sodium tungstate solutions into separate test tubes, acidify them with sulphuric acid, and add several drops of a 1 % hydrogen peroxide solution to each tube. How does the colour of the solutions change Write the equations of the reactions. 

CARCINOGENIC.) In this test use is made of the fact that benzidine, which is unaffected by normal molybdates and by free molybdic acid, is oxidized in acetic acid solution by phosphomolybdic acid or by its insoluble ammonium salt (see reaction 4 above). This reaction is extremely sensitive two coloured products are formed, viz. the blue reduction product of molybdenum compounds ( molybdenum blue ) and the blue oxidation product of benzidine ( benzidine blue ). Moreover, solutions of phosphates which are too dilute to show a visible precipitate with the ammonium molybdate reagent will react with the molybdate reagent and benzidine to give a blue colouration. 

Reducing agents (sulphides, thiosulphates, etc.) interfere since they yield molybdenum blue hexacyanoferrate(II) ions give a red colouration. Arsenates (warming is usually required), arsenites, chromates, oxalates, tartrates, and silicates give a similar reaction with some variation in the colour of the precipitate. All should be removed before applying the test. 

Molybdenum gives a similar reaction. If, however, a thiocyanate is added, the red complex ion [Mo(SCN)6]3 is formed, and upon the addition of concentrated hydrochloric acid the red colour disappears and the blue colour due to tungsten remains. 

Many methods for sulphide and H2S are based on the reducing properties of S(-II). Hydrogen sulphide reduces molybdate in acid medium to molybdenum blue, and the molybdophosphate to phosphomolybdenum blue [52]. Iron(III) reduced by H2S in the presence of 1,10-phenanthroline gives the orange Fe(phen)3 complex [2,53], Hydrogen sulphide may be determined after conversion into thiocyanate by the reaction with Fe(III) [54]. Sulphide has been determined also by a colour redox reaction with nitroprusside [55-57], In another sensitive reaction the sulphide ions decompose the Ag complex with Cadion 2B and Triton X-100 (e = 2.5-10 ) [58], In another indirect method sulphide releases the chloranilate ion from the Hg(Il) chloranilate [59]. Sulphide has also been determined by a method based on its reaction with bromate, followed by bromination of 2 ,7 -dichlorofluorescein by the bromine released [60]. 

Blue ring test when a solution of thiosulphate mixed with ammonium molybdate solution is poured slowly down the side of a test tube which contains concentrated sulphuric acid, a blue ring is formed temporarily at the contact zone. The colour is caused by the reduction of molybdate to molybdenum blue (see Section 6.6, reaction 3). 

Reducing agents e.g. zinc, tin(II) chloride solution, colour a molybdate solution acidified with dilute hydrochloric acid blue (due to molybdenum blue M02O5), then green and finally brown (see also section 3.15, reaction 8). 

Neutralization reactions have been followed by various techniques. A molybdenum electrode was found useful for potentiometric titrations in anhydrous thio-nyl chloride , but the mechanism of the electrode reaction has not been established. Likewise reactions between sulphur trioxide and various bases have been followed by conductometric and potentiometric titrations in solutions of acetyl chloride . Reactions in thionyl chloride can also be followed by the use of various colour indicators, such as methyl orange, methyl red, phenolphthalein, p-nitro-phenol, thymolphthalein or bromine thymolblue, which appear to be indicative of the chloride ion activities of the solutions. 

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