Precision in chelatometric titrations of barium (II)

The photometric back-titration of barium with o-cresolphthalein complexone (PC), methylthymol blue (MTB), thymolphthalein complexone (TPC) or eriochro me black T (BT) as an indicator and EDTA as a chelate reagent was studied. The intersection of both the tangent at an inflection point and phi (appare nt indicator transition) = 1 was adopted as an end point of titration. The back-titration of barium with magnesium as a titrant and PC as an indicator showed the most sharp titration curve. Ammonium salt as a buffer did not i nterfere with the color transitions of PC and TPC until 0.5 mol dm(-3). The color transition and end point of titration were, however, influenced by a large amount of neutral salt. The relative standard deviations of titratio n with PC were 0.034% (n = 15) and 0.028% (n = 10) for the barium and magne sium titrants, respectively. These two titration systems are recommended fo r barium titration. In spite that the titration with the barium titrant was less sharp than that with the magnesium titrant, the relative standard dev iation of the former was close to that of the latter. This fact resulted be cause the shape of the titration curve with the barium titrant was advantag eous for the precision of determining an end point.