Simultaneous determination of total and free calcium in milk by flow injection

A fast and reliable procedure for the determination of total and free calcium in milk is described. The method is based on the flow injection (Fl) technique. Total calcium is determined by atomic absorption spectrometry (AAS) (422.7 nm) and free calcium by spectrophotometry (580 nm). Interference in the determination of free calcium is eliminated by using a dialyser, which also separates the total and free calcium. Interference from phosphates in milk in the determination of total calcium by AAS is overcome by using a dinitrogen oxide-acetylene flame with the necessary suppression with K+. With 30-microliters samples the Fl system covers a standard working range of 100-1500 mg dm-3 of Ca2+. The system is suitable for the simultaneous determination of total calcium (relative standard deviation less than 1.30% for 1300-1500 mg dm-3 of total calcium) and free calcium (relative standard deviation less than 0.85% for 120-170 mg dm-3 of free calcium) in milk at a sampling frequency of about 60 samples h-1. The results obtained agree reasonably well with results from the AAS method.