Question

Explain the molecular mechanism by which phosphate interferes with calcium determination.

What is the nature of the compound formed and what two actions can be performed to eliminate this type of interference?

Answer 1

We use flame photometry mechanism by which we can detect phosphate interfere with calcium determination. Basically, interference is the change in intensity of the analyte signal in spectroscopy. When phosphate interfer in calcium determination then it is known as chemical interference. Calcium when reacts with phosphate then it forms calcium phosphate which is stable compound in air and insoluble in water. Phosphate combines with calcium in condensed phase formes less volatile compound. This compound decrease the concetration of free calcium ions in spectrometer light beam because of which analyte signal decreases.

Two actions which can be performed to eliminate phosphate interference in calcium.

1) The measurment of calcium is difficult due to lower emission energy of calcium. This can be overcome by high flame temperature using nitrous oxide- acetylene flame instead of air-acetylene flame. This provide higher dissociation energy, means higher calcium dissociation which is unaffected by phosphate interference to 100 fold.

2) If in the presence of air-acetylene flame we add another element such as lanthanum which binds with phosphate leaving free calcium ions, due to which analyte concentration increases and also increases its signal. Lanthanum must be added in the ratio of lanthanum to phosphate in molar ration of 5:1 so that maximum calcium ion in free state.