Potentiometric mass titration method

The potentiometric mass titration method [657,658] produces results equivalent to those of the drift method described above. The same amount of base is added to three dispersions with different solid-to-liquid ratios and a constant ionic strength. The dispersions are titrated with acid, and the pH is recorded as a function of the amount of acid added. The intersection point of the obtained curves is taken as the PZC. In other words, the PZC is identified with the pH at which solid addition does not induce a change in pH. The drift method and mass titration are based on the same principle, the difference being that in potentiometric mass titration, the reagents are added in a different order. Potentiometric mass titration is affected by the acid or base associated with the powder in the same way as in the drift method and mass titration. The advantage of potentiometric mass titration over the drift method is that in the former the pH is measured only in buffered systems. 

In the mass titration method, the PZC is determined as the natnral pH of a concentrated dispersion. A detailed description of the experimental procedure can be found in [667], Mass titration become popular in the late 1980s [668,669], but the same method was already known in the 1960s as the pH drift method [183], Usually, a series of natural pH values of dispersions with increasing solid loads is reported, but only the natural pH of the most concentrated dispersion is actually used. The only role of the data points obtained at lower solid loads is to confirm that a plateau was reached in pH as a function of solid load that is, a further increase in the solid load is unlikely to bring about a change in pH. The mass titration method is based on the assumption that the solid does not contain acid, base, or other surface-active impurities. This is seldom the case, thus mass titration often produces erroneous PZCs. In this respect mass titration is similar to the potentiometric titration without correction illustrated in Figure 2.7, only the solid-to-liquid ratio is different. The experimental conditions in mass titration (solid-to-liquid ratio, time of equilibration, nature and concentration of electrolyte, and initial pH) can vary, but little attention has been paid to the possible effects of experimental conditions on the apparent PZC. The effect of an acid or base associated with solid particles on the course of mass titration was studied in [670], To this end, a series of artificially contaminated samples was prepared by the addition of an acid or base to a commercial powder. The apparent PZC of silicon nitride obtained in [671] by mass titration varied from 4.2 (extrapolated to zero time of equilibration) to 8.2 for time of equilibration longer than 20 days. The method termed mass titration was used in [672], but it was different from the method discussed above. 

A completely different method is referred to as mass titration" in Ref. [64] Namely, a certain amount of powder is added to the solution of given pH and the dispersion is titrated with acid or base until the original pH value is reached. This procedure is very different from the mass titration discussed above, and it seems to be variation of potentiometric titration without correction for PZC = CIP, thus the results are reported as pH" in Table 3.1. 

Theoretical and experimental studies of the value and proton affinity of IBX solutions in aqueous media and DMSO have been published. In particular, the aqueous pATa value of 2.40 for IBX was obtained by using standard potentiometric titration methods [686]. The relatively high acidity of IBX should be taken into consideration while using this important reagent in the oxidation of complex organic molecules. The gas-phase proton affinities of the anions of IBX (1300 25 kJ moH) and 2-iodosylbenzoic acid (1390 10 kJ mor ) using mass spectrometry-based experiments were reported [687], The experimental results were supported by theoretical calculations, which yielded proton affinities of 1336 and 1392 kJ mor for the anions of IBX and 2-iodosylbenzoic acid, respectively, at the B3LYP/aug-cc-PVDZ level of theory [687]. 

Titrations can be carried out in cases in which the solubility relations are such that potentiometric or visual indicator methods are unsatisfactory for example, when the reaction product is markedly soluble (precipitation titration) or appreciably hydrolysed (acid-base titration). This is because the readings near the equivalence point have no special significance in amperometric titrations. Readings are recorded in regions where there is excess of titrant, or of reagent, at which points the solubility or hydrolysis is suppressed by the Mass Action effect the point of intersection of these lines gives the equivalence point. 

A method, free from Fe or Cr interference, (MacDonald and Savage 1979) involves a prior oxidation of plutonium to Pu(VI) with an excess of Ce(IV). Sulfamic acid is added to avoid side reactions with nitrites. Fluoride residues from the dissolution of solid samples are complexed hy additon of Al(in). The excess Ce(IV) is then reduced by a slight excess of arsenite in the presence of Os(Vni) as catalyst. Excess As(III) is oxidized by permanganate, also in a very quantity. Finally, oxalic acid is added to reduce the excess Mn(VII). Pu(VI) is then reduced quantitatively to Pu(IV) with Fe(II) and the excess Fe(II) backtitrated with Cr(VI). All steps are followed potentiometrically and carefully timed. The Pu mass fraction, Cpu, in g/g, in the sample aliquant submitted to titration is calculated according to the following equation ... 

UV-vis, fluorescence, and calorimetric titrations. It is probably safe to say that over 90% of all experimentally determined binding constants in snpramolecular chemistry are now determined nsing one of these four techniques. Other older or more speciahzed techniques such as solubility methods, potentiometric, and mass spectrometry have been discussed in earlier reviews and are not covered here. It should, however, be noted that the basic equations shown here can usually also be readily adapted to fit these older or more speciahzed techniques. 

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