10.2.5.2 Range and precision
The molar absorptivity of the partly reduced heteropoly acid is 22700 at 880 nm, which means that the net absorbance for a 1.0 µmol/L phosphate sample is 0.227 measured in a 10 cm cuvette. With a good spectrophotometer an absorbance of 0.002 can be read, and the smallest concentration of phosphate that can be determined directly thus is 0.01 µmol/L. The absorbance are a linear function of the concentration up to 28 µmol/L, equal to a net absorbance of approximately 0.630 in a 1 cm cell