a procedure for the determination of the concentration of a particular species in a sample by adding known amounts of that species to the sample solution and recording the change of potential of an ion-selective electrode versus a suitable reference electrode, a procedure in which test sample is divided in two (or more) test portions. One portion is analysed as such and known amounts of the standard analyte are added to the other test portions before analysis. The amount of the standard analyte added has to be between two and five times the estimated amount of the analyte in the sample. This procedure is designed to determine the content of an analyte in a sample, taking account of the recovery of the analytical procedure.