The Liquid-phase polymer-based retention (LPR) technique allows calculating the formation constant of polymer-metal ion complexes (Kf). The dependence of the relative error in Kf (eK) on the error in the total retention coefficient a (e a) has been searched and it was found that its influence is higher as a increases, so that its measurement must be performed with higher precision. In addition, e a is higher for high a values due to the stronger influence of errors in the measurements of the different magnitudes that allow its calculation. In order to achieve a measurement of Kf with a relative error lower than 7 %, the experimental a found should not exceed the value 0.4 when relative errors for the independent variables ranging between 2 % and 5 % are considered