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Sequential Titrations of Mixtures, Selectivity, Masking, and Demasking

6 Sequential Titrations of Mixtures, Selectivity, Masking, and Demasking [Pg.535]

As for other sorts of titrations, the question of of carrying out sequential titrations with EDTA arises. It is admitted that the ratio miy miiy about [Pg.535]

10 for the sequential titration of the metal cations Mi and Mu to be satisfactory. Of course, the stability constants Kmiy and Kmhy of the complexes of Mi and Mu ions with EDTA that pertain here must correspond to the experimental conditions, that is, must be the conditional ones. If the above value of the ratio is reached, the Mi ion is titrated first since it is designated conventionally as giving the more stable complex. As in the other sorts of titrations, the obtained precision in determining the first endpoint—in other words, the magnitude of the pMi change—depends on the value of the ratio of both stability constants. This is not the case for the determination of Mil. In this case, the precision depends only on the AT nY value and, of course, on the titrant s and titrand s concentrations. [Pg.535]

If the ratio is less than 10, only the sum of the concentrations of both ions can be determined satisfactorily by titration with EDTA. Then, of course, we must use an indicator of the second ion Mp. [Pg.535]

The influence of the presence of the Mu ion on the titration of Mi may also be taken into account by introducing an appropriate conditional constant. As usual in the definition of conditional constants, we begin with the recognition of the principal and parasitic reactions. Here we consider the parasitic reaction is that of the Mu ion with EDTA. If, in order to simplify, we suppose that Mi reacts only with EDTA to give MiY and that EDTA reacts only with Mi and Mu (it gives rise to no further reaction, such as, for example, a protonation), we can then write [Pg.535]




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