Spectrophotometric Estimation of Molybdenum at 475nm

ESTIMATION OF MOLYBDENUM

Complex thiocyanate  of Mo(V)

The controlled reduction of Mo(VI) in the presence of thiocyanate ions leads to the formation of a complex orange –red Mo(V) thiocyanate. The reduction should  not be too vigorous and the acidity should be fixed at a definite level, since several alternative reductions may occur. Thus for example , Mo(III) may be formed or, in a weakly acidic solution ‘molybdenum blue’ may appear.

Sensitivity:

Interfering Ions:

Colored ions may be interfere if the molybdenum is not first isolated by extraction . large amounts of Cr(III) should be separated in the form of CrO₂Cl₂

Certain other ions give rise to extractable colored thiocyanate complexes; moderate quantities of Fe(III) are reduced and to do interfere. Pt(IV) interferes. Co(II) interfere if its content exceeds half that of the Mo(IV). Cu(II precipitates in the form of cuprous thiocyanate and may be separated in this way. W(IV) should be complexed by citrate or tartrate ions, and Ti(IV) by F ^–

Large amounts of Bi(III), V(V) and P(V) interfere. Re(VII) gives the same reaction. U(VI) interferes

Reagents:

Potassium thiocyanate, 10%

Stannous chloride: 10g of SnCL₂. 2 H₂O are dissolved in 10ml of concentrated HCL and the solution is made up to 100ml with water.

Solutions of ferrous ion:  1 g of Mohr’s salt is dissolved in 100ml of 0.2N( I/80) sulphuric acid.

Isoamyl alcohol.

Standard solution of molybdenum: 750 mg of guaranteed purity MoO₃ are dissolved in a few ml of dilute caustic soda. The solution is made slightly acid with HCl and the volume adjusted to 500ml. The solution id diluted to the point where 1 ml contains 1 µg of Mo(VI).

Operating Procedure:

2.0 ml of concentrated HCL . 1ml of ferrous solution, 3.ml of thiocyanate, and 3ml of stannous chloride are added to 15ml of solution containing from 1 to50µg of molybdenum. The solution is diluted to 25ml and an accurately measured 10ml portion of iso-amyl alcohol is added. After shaking vigorously for 1 minute and allowing to settle , the colorimetry is performed at 475nm