Spectrophotometric Estimation of Calcium

ESTIMATION OF CALCIUM

INTRODUCTION:

Although direct colorimetry can be performed in several ways, the available methods are very unselective. . To estimate small amounts of Ca (II), it is thus often preferable to use micro volumetric estimations, spectrography, or flame photometry.

With ammonium purpurate. (Muroxide)

Under suitable pH conditions, muroxide gives a yellow –red coloration with Ca (II). The absorption maximum is located at 506 nm the pure reagent shows an absorption maximum at 537 nm

The coloration disappears slowly.

Sensitivity: Molar Extinction co-efficient e=10,000 at 506 nm

Interfering ions:

Mg (II) gives a yellow coloration, but the Mg (II) muroxide compound has, at 500 nm, approximately the same extinction coefficient as the pure reagent. Consequently, Mg (II) does not interfere, unless present in an amount exceeding 10ppm when precipitation takes place..

The majority of the ions interferes and gives colorations or precipitates with the reagent.

Reagents:

Saturated solution of muroxide in water (about 0.5%) to which 2 ½ times its own volume of alcohol has been added. The reagent is stable for one day.

Caustic soda. 0.1N (1/100) or 4 g/l.

Standard solution of Ca (II) at 10ppm, prepared from pure calcium carbonate.

Operating procedure:

            The solution is neutralized if necessary. For the best results, it should contain 30mg of Ca (II). 1ml of reagent, accurately measured by means of a micro burette is added and the volume is made up to about 45ml. After a further addition of 2ml of 0.1N caustic soda, the solution is measured calorimetrically at 506 nm. The calibration curve should be checked daily.