Direct spectrophotometric determination of calcium in clinical samples with carboxyazo–p–CH3 Original Research Article

A highly sensitive and relatively interference-free spectrophotometric method for determination of calcium is described. The method is based on the reaction between calcium ions and carboxyazo–p–CH3 in aqueous citrate medium of pH 7, to form a blue complex with maximum absorption at 716 nm. The calibration is linear up to 0.12 μg ml−1 calcium with a repeatability (R.S.D.) of 1.0% at a concentration of 0.04 μg ml−1 (n=5). The molar absorptivity of the complex and Sandell’s sensitivity are 3.5×105 l mol−1 cm−1 and 0.11 ng cm−2, its 10σ limit of quantification and the 3σ limit of detection were found to be 0.3 ng ml−1 and 0.09 ng ml−1 respectively. The influence of reaction variables and the effect of interfering ions are studied; no interference was observed in clinical samples. The proposed method has been applied directly to the determination of calcium in clinical samples without the need for pre-concentration, masking metal ions and digesting samples.