Determination of calcium in water, urine and pharmaceutical samples by sequential injection analysis

A sequential injection analysis system for the determination of calcium in water, urine and pharmaceutical samples is proposed. The method is based on the fast complexation reaction between cresolphthalein complexone and calcium with spectrophotometric detection. Baseline shift due to a background originating from coloured free reagent species, was eliminated by an optimised sequence of samples and reagents. The proposed system is fully computerised and is able to monitor calcium in samples at a frequency of 43 samples per hour with a relative standard deviation of < 1.4%. The calibration graph is linear between 0 and 20 mg l−1. The detection limit is 0.05 mg l−1.