This method is used for the determination of Calcium in feed, fertilizer, plants and soils. The standard range of this method is 2 to 200 mg/L as Ca. However, this method is also applicable to other ranges.
Samples are digested by Kjeldahl or hot acid digestion followed by colorimetric analysis. (1-3) Calcium reacts with cresolphthalein at a pH of 10.2 to form a reddish-purple complex measured at 570 nm.
Magnesium interference is eliminated by the use of 8-hydroxyquinoline. Turbid samples must be filtered prior to analysis. Samples with background absorbance at the analytical wavelength may interfere.