The methods for rejection of salts interference on colloid titration are studied. The results are summarized asfollows.
1) The end point became appreciably clear by addition of chelating reagents such as EDTA and citric acid. But the practical value of this method was very small since it is difficult to decide the appropriate amount of chelating reagent.
2) The accuracy of colloid titration was remarkably improved by the removal of Ca2+ and SO42- which was done by centrifugation of the precipitate of CaF2 and BaSO4 formed by adding NaF and BaCl2. But the amounts of NaF and BaCl2 required for accurate analyses were restricted within the narrow range around the equivalent values of Ca2+ and SO42-.
3) The accuracy of colloid titration was improved very much by desalting by use of anion and cation exchange resins. Although a certain amount of some polyelectrolytes such as PAA and PEI were absorbed by the ion exchange resins, this problem was solved by use of the resins pretreated with the desired sample.