Principle of substoichiometric:

Since masses of portions counted are identical, the specific activities SA₁ and SA₂ in Eq. (13.3) can be replaced with the measured activities A₁ and A₂ of the two portions. This eliminates the quantitative determination step for the samples. Having known the amount of reagent exactly (as in the composition of reaction product) the amount of substance (mcal) isolated by performing the isolation step can be determined. Unknown amount of component (m) in the analyte may be calculated by using the eqn.

m  =   m_cal × (A₁/A₂)  - m_o

where, m_o is the amount of activity added. A necessary aspect of the technique is the availability of a appropriate reagent that is added in equal amounts in the standard and sample solution.

An important benefit of substoichiometric method is the enhanced selectivity of the procedure caused through competing ability of the unreacted substance towards the impurities that are kept away from reacting along with the reagent and passing into the similar phase as the substance of interest. The method has been extensively used for the accurate and specific determination of a large number of elements in a variety of complex matrices. Various other variations of IDA have been developed e.g. sub and super equivalence IDA. Those techniques have lost popularity since of complex chemical procedures.