Molybdate reagent

Phosphorus. The presence of phosphorus may be indicated by a smell of phosphine during the sodium fusion. Treat 1 ml. of the fusion solution with 3 ml. of eoneentrated nitric acid and boil for one minute. Cool and add an equal volume of ammonium molybdate reagent. Warm the mixture to 40-50°, and allow to stand. If phosphorus is present, a yellow erystalline precipitate of ammonium phosphomolybdate wUl separate. 

It is usually preferable to oxidise the compound directly as follows. Intimately mix 0 02-0 05 g. of the eompound with 3 g. of sodium peroxide and 2 g. of anhydrous sodium carbonate in a niekel erucible. Heat the crueible and its eontents with a small flame, gently at first, afterwards more strongly until the eontents are fused, and eontinue heating for a further 10 minutes. Allow to stand, extract the contents of the crucible with water, and filter. Add exeess of eoneentrated nitrie acid to the filtrate and test with ammonium molybdate reagent as above. A yellow preeipitate indicates the presenee of phosphorus. It must be borne in mind that the above treatment 1 eonvert any arsenie present into arsenate. 

Note. Prepare the ammonium molybdate reagent as follows. Dissolve 125 g ammonium nitrate in 125 mL water in a flask and add 175 mL nitric acid, sp. gr. 1.42. Dissolve 12.5 g ammonium molybdate in 75 mL of water and add this slowly and with constant shaking to the nitrate solution. Dilute to 500 mL with water, heat the flask in a water bath at... 

Fig. 12.4 Effect of temperature of heating water bath on the apparent concentration of phosphorus. The 1.1 N H2S04 refers to the concentration of sulphuric acid in the ammonium molybdate reagent solution...

Eluent for TLC was 8 4 1 AcOEt/MeOH/H20. Substrates and products were visualized at 254 nm and with the molybdic reagent ((NH4)eMo7024 4H2O, 42 g Ce(S04)2, 2 g H2SO4 concentrated, 62 mL made up to 1 L with deionized water). 

The reaction was gently shaken at 30 °C for 36 h, monitoring the conversion by TLC (AcOEt/MeOH/H20, 8 4 1, molybdic reagent) and checking the residual enzymatic activity in time using the method shown in Procedure 1 (Section 7.4.1.2) (22 U after 26 h). 

Similarly pipette 5 ml of the soil extract into a plastic vending cup, followed by 5 ml sulphuric acid, 1.5 M. If the soil extract solution is highly coloured after addition of the acid, pipette a duplicate sample and add the 5 ml of sulphuric acid, 1.5 M. Add the other reagents, as detailed above, to the first sample, but only add the ammonium molybdate reagent to the duplicate. Measure the absorbance at 880 nm. For coloured extracts, subtract the absorbance of the duplicate without ascorbic acid, which will not develop the blue colour, from the absorbance of the sample extract with ascorbic acid. 

Ammonium molybdate reagent - dissolve 4 g powdered ammonium molybdate in 1 I of water. Slowly with stirring add 80 ml sulphuric acid, 98% m/m H2SO4, and dilute to 2 I. 

Vanadium molybdate reagent - Note wear PPE to prevent injury from concentrated nitric acid. Separately dissolve 20 g of ammonium molybdate and 1 g of ammonium vanadate in water, transfer to a 250-ml beaker and swirl to mix. Slowly add 140 ml nitric acid (approximately 70% m/m HNO3), carefully transfer to a 1-1 volumetric flask, make up to the mark with water and mix. 

Vanadate-molybdate reagent - mix the ammonium molybdate solution with the ammonium vanadate solution in the ratio 2 3 molybdate vana-date. For optimum reproducibility, make up fresh each day just before use. 

The fact that twelve molybdate ions are bound to each phosphate unit in molybdophosphate has been exploited as an amplifying factor. The molybdophosphoric acid is extracted from the original solution into a mixture of diethyl ether or chloroform with butanol, in order to eliminate the excess of molybdate reagent it is then re-extracted into an aqueous phase and broken down with alkali. The molybdate ions are reacted with ammonium rhodanide90 or aminochlorobenzenethiol76 to give coloured products with absorption maxima at 470 and 710 nm, respectively. One phosphate ion thus yields twelve molybdate units, the optical absorbance is amplified with respect to that of the heteropoly phosphate complex and its measurement provides a sensitive determination procedure. 

The acid liquid is evaporated to about 10 c.c. and, when its temperature lias fallen to 50°, treated with 50 c.c. of the ammonium molybdate reagent at 45-50°-... 

Ammonium molybdate reagent may be prepared by dissolving 45 g of pure ammonium molybdate in a mixture of 40 ml of aqueous ammonia (d 0.88) and 60 ml of water and then adding 120 g of ammonium nitrate and diluting the solution to 1 litre with water. 

Place 50 mL sample in a Nessler tube. Add 1 mL of 1 1 HC1 and 2 mL ammonium molybdate reagent. Shake well. Let the solution stand for 5 min. Add 2 mL oxalic acid solution, shake thoroughly, and allow the solution to stand for another 5 min. Measure the absorbance of the yellow color developed at 410 nm. Run a blank using 50 mL distilled water following the above procedure. Compare the absorbance of the sample solution with the standards and determine the concentration of silica from the calibration curve. 

As with some of the other methods, one of the reagents, i.e., amino-naphtholsulfonic acid, acts as recipient for the material dialyzed from the sample. The molybdate reagent is added to the acceptor stream after the dialyzer, mixed, and heated, and the developed color is read and recorded in the usual manner. 

A nephelometric method using a strychnine molybdate reagent has also been devised.4... 

Acid molybdate reagent—Cautiously add 680 ml concentrated H2S04 to 1.8 liters of distilled water and allow to cool. Dissolve 125 g (NH4)6Mo7024 4 H20 in 2.5 liters of distilled water and add to the above sulfuric acid solution. Make up to 5 liters with distilled water. 

Mix the solid test sample with a little pure silica powder in the tube of the apparatus shown in Fig. 11.53 and moisten the silica with 1-2 drops concentrated sulphuric acid. Place a drop of water on the glass knob of the stopper, insert it in position and heat the apparatus gently for about 1 minute. Remove the source of heat and allow to stand for 5 minutes. Wash the drop of water into a micro crucible, add 1-2 drops ammonium molybdate reagent and warm the mixture until bubbling just commences. Allow to cool, introduce a drop of a 005 per cent solution of benzidine in 2m acetic acid and a few drops of saturated sodium acetate solution. A blue colour is obtained. 

Place a drop of the test solution and of the molybdate reagent upon drop-reaction paper, and warm gently over a wire gauze. Add a drop of 0 05 per cent benzidine acetate and hold the paper over ammonia vapour. A blue colouration results. 

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