8.4.2 The method of standard additions requires preparing
calibration standards in the sample matrix rather than
reagent water or blank solution. It requires taking four
identical aliquots of the solution and adding known
amounts of standard to three of these aliquots. The forth
aliquot is the unknown. Preferably, the first addition
should be prepared so that the resulting concentration is
approximately 50% of the expected concentration of the
sample. The second and third additions should be prepared so that the concentrations are approximately 100% and 150%
of the expected concentration of the sample. All four
aliquots are maintained at the same final volume by adding
reagent water or a blank solution, and may need dilution
adjustment to maintain the signals in the linear range of
the instrument technique. All four aliquots are analyzed.