Direct, selective flow injection spectrophotometric determination of calcium in wines using methylthymol blue and an on-line cascade dilution system Original Research Article

A flow-injection (FI) method is reported for the determination of calcium in wines using both off- and on-line ‘cascade’ dilution FI systems. Methylthymol blue (MTB) was used as a colour agent and the absorbance of the coloured Ca–MTB complex produced was monitored at 610 nm. For both FI systems, the various chemical and physical variables were optimized and a study of interfering ions was also carried out. Linear calibration graphs were obtained from 0.0 to 10.0 mg l−1 Ca (90 injections per hour) and 0.0 to 200.0 mg l−1 Ca (60 injections per hour) for the off- and on-line dilution FI systems, respectively. The precisions were very good (relative standard deviations 0.45% and 0.54%, respectively) and the 3σ limits of detection were 0.032 and 1.7 mg l−1, respectively. The average accuracies evaluated by comparison of the results with those obtained by flame atomic absorption spectrometry (FAAS) were also very good (0.67% and 0.97%, respectively). The method was found to be adequately selective, considering the ions that the wine contains, and has been successfully applied to the determination of calcium in red and white Greek dry wines.