Titration hysteresis

There is also evidence for stable 3,4-adducts from the X-ray analysis of 2-amino-4-ethoxy-3,4-dihydropteridinium bromide, the nucleophilic addition product of 2-aminopteridine hydrobromide and ethanol (69JCS(B)489). The pH values obtained by potentiometric titration of (16) with acid and back-titration with alkali produces a hysteresis loop, indicating an equilibrium between various molecular species such as the anhydrous neutral form and the predominantly hydrated cation. Table 1 illustrates more aspects of this anomaly. 2-Aminop-teridine, paradoxically, is a stronger base than any of its methyl derivatives each dimethyl derivative is a weaker base than either of its parent monomethyl derivatives. Thus the base strengths decrease in the order in which they are expected to increase, with only the 2-amino-4,6,7-trimethylpteridine out of order, being more basic than the 4,7-dimethyl derivative. 

Fig. 1. Hysteresis loop produced when 6-hydroxypteridine is titrated with acid and alkali.

Fig. 2. Hysteresis loop in rapid titration of O.OOlM 2-hydroxy-6-methyl-pteridine with 0.01-M potassium hydroxide and back-titration with hydrochloric acid, and the equilibrium titration curve.

Zinc increases the rate of dimerization 21, 65, 66) however, Apple-bury and Coleman 48) showed that zinc is not necessary for dimerization to occur. Starting at a high pH and slowly lowering the pH they found that all of the zinc is lost by the time the pH reaches 4.0, yet the molecule though inactive is still dimeric. However, upon increasing the pH of a solution of monomers, the dimer reforms by pH 5.0, yet the zinc does not bind completely until pH 6.0. Also, the optical rotatory dispersion (ORD) spectrum is the same for dimer at pH 8.0 and monomer at pH 4.0, but a spectral change occurs for the monomer at pH 2.0 48, 67). Applebury and Coleman 48) reasoned that there must be a kinetic barrier which prevents any rapid equilibration of the dimer monomer system at intermediate pH values and that the same barrier exists in the hysteresis loop involved in the titration of carboxyl groups on the enzyme 21, 67). 

Once formed, the protonated amine groups remain in the aqueous phase until the pH is increased to the point that the proton is removed. At this point the microdomains of amine reform. This accounts for the hysteresis in the titration curves that is shown in Figure 5. 

In Fe(II)-dichromate titrations, Winter and Moyer observed a time dependence of the potential after the end point. When potential readings were taken soon after each addition, an asymmetrical titration curve was observed, but when a time interval of 10 to 15 min was allowed after each addition, the curve approached the theoretical shape. We have noted that automatically recorded titration curves for the Fe(II)-dichromate titration show a considerably smaller potential jump than manually observed curves, the difference being due to lower potentials after the end point. But curves plotted with 15 s of waiting for each point differed only slightly from curves plotted with 150 s of waiting. Ross and Shain also studied the drift in potential of platinum electrodes with time and noted hysteresis effects in recorded potentiometric titration curves. These effects, due to oxidation and reduction of the platinum surface, are discussed below. 

FIGURE 1.9 Real situation hysteresis. The loop narrows as the titration rate decreases, but it is often difficult to avoid, even with very slow titration. 

Electroacoustic measurements are usually carried out in titration mode, and several papers report results of both acid and base titration. Substantial hysteresis (Figure 1.9) was found in [242,427,491-493]. Other studies in similar systems report negligible hysteresis [444,449,494,495]. Four cycles of titration in ESA measurements reported in [496] produced different values of the potential in the acidic range but a common IEP. Similar lEPs were observed in titrations of alumina from pH 2 to 12 and back, but in titrations from pH 1 to 12, the IEP was shifted to high pH [497]. Generally, more pronounced hysteresis is expected in titrations over a wider pH range. 

The IEP (Rank Brothers, 0.001 M KNO3) shifts to low pH as the silica content increases from 0.62% (negligible shift) to 5% (shift from pH 5.5 to 3). The electrokinetic curves of particles with 1-2% silica show a hysteresis (base titration produces a higher IEP). The IEP is also sensitive to storage for 3 weeks at pH 2, but is insensitive to storage for 3 weeks at pH T [2101]. 

It seems from product distribution studies that even after dissociation of the MMOB from the diferrous MMOH, the active site in MMOH remembers that it had just been in complex with MMOB. This is presumably because MMOB induces a metastable (on the time scale of catalysis) conformation of the MMOH (129). The effect is observed as a hysteresis effect upon titration with MMOB to the diferrous MMOH (see Fig. 13 for a model), where a ratio 0.1 of MMOB/(MMOH active site) could induce a complete metastable conformation of MMOH. This could be observed by a drastic change in product distribution, and no further change in product distribution was observed in the presence of higher ratios of MMOB/MMOH. The model was corroborated by the low ratio of MMOB/MMOH needed to induce the MMOB-dependent changes of the diferrous MMOH EPR signal (129). The affinity between MMOB and MMOH depends on the redox state of MMOH. This explains... 

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