Ions at various pH values

The influence of metal ions on the aggregation of Cl Reactive Red 2 (3.14) in aqueous solution at various pH values has been examined in detail recently [18]. Sodium ions have a profound effect on these disulphonated dye anions, so that the enhancement of substantivity during the dyeing process is accompanied by aggregation attributable to hydrophobic interaction between the phenyl and s-triazine ring systems. Aggregation is much greater,... 

The presence of residual unbound transition-metal ions on a dyed substrate is a potential health hazard. Various eco standards quote maximum permissible residual metal levels. These values are a measure of the amount of free metal ions extracted by a perspiration solution [53]. Histidine (5.67) is an essential amino acid that is naturally present as a component of perspiration. It is recognised to play a part in the desorption of metal-complex dyes in perspiration fastness problems and in the fading of such chromogens by the combined effects of perspiration and sunlight. The absorption of histidine by cellophane film from aqueous solution was measured as a function of time of immersion at various pH values. On addition of histidine to an aqueous solution of a copper-complex azo reactive dye, copper-histidine coordination bonds were formed and the stability constants of the species present were determined [54]. Variations of absorption spectra with pH that accompanied coordination of histidine with copper-complex azo dyes in solution were attributable to replacement of the dihydroxyazo dye molecule by the histidine ligand [55]. 

Figure 1. Nmr spectra of the methylammonium ion (p/ta = 10 5) at various pH-values, showing the effects of exchange (recorded at 60 MHz and S3°C). After Grunwald et at., 1957.

Figure 1. Isotherms for the adsorption of potassium ion on manganese (II) manganite at 25°C. at various pH values...

This discussion should not be seen as explaining the acid/base character of oxides, i.e. their solubilities in water at various pH values. We are emphasising the close relationship between the acid/base behaviour of oxides and the nature of aqueous species. The dissolution of an oxide (other than a neutral oxide) in water, or in acids/alkalies, is an acid-base process, a chemical reaction rather than a mere separation of ions. The relative acid/base strengths of oxides are further discussed in Section 9.2. 

When a biochemical half-reaction involves the production or consumption of hydrogen ions, the electrode potential depends on the pH. When reactants are weak acids or bases, the pH dependence may be complicated, but this dependence can be calculated if the pKs of both the oxidized and reduced reactants are known. Standard apparent reduction potentials E ° have been determined for a number of oxidation-reduction reactions of biochemical interest at various pH values, but the E ° values for many more biochemical reactions can be calculated from ArG ° values of reactants from the measured apparent equilibrium constants K. Some biochemical redox reactions can be studied potentiometrically, but often reversibility cannot be obtained. Therefore a great deal of the information on reduction potentials in this chapter has come from measurements of apparent equilibrium constants. 

Ion exchange with different exchange groups of polyfiinctional ion exchangers takes place at various pH values, selectivity of the different exchange groups toward the same ions being different as a rule. 

Figure 11-5 shows plots of calculated titration curves of 10 M nickel(II), iron(III), and calcium(II) at various pH values in buffers of ammonia-ammonium ion at a total buffer concentration of 0.1 M. Before the end point, the nickel curves at... 

FIGURE 11-5 Titration curves of 10" Mmetal ions with EDTA in a solution withENHa] + [NH +] = 0.1 M at various pH values as indicated. Left, Ni(II) right, calcium (solid lines) and iron (dotted lines). 

The fccat values thus measured at various pH values are illustrated in Fig. 6. If ionization of myoglobin or BU is disregarded, the pATa values (5.50, 8.68) estimated from analysis of the bell-shaped pH profile may be assigned to the ionization of water molecules coordinated to the Co(III) ion of the Co BU-myoglobin complex. Note that the catalyst is most active at the physiological pH. The fccat measured at pH 7.5 and 37°C corresponds to a half-life of 30 h. 

There are several types of compounds that one might attempt typically to isolate from a crude extract using an ion-exchange method strong and weak acids and bases, as well as amphoteric compounds. As both adsorption and elution processes will generally involve pH adjustments, it is wise first to determine the stability of the compound of interest at various pH values, as mentioned earlier. With information on the stability of the compound of interest at hand, one can then choose an appropriate ion-exchange process. 

Shown in Fig. 2 is a plot of the concentrations of the various entities in equilibrium calculated using these values. At arbitrarily chosen concentrations of 0.1 M ketone and 0.01 M amine, the concentration vs. pH profiles are as shown. Only a very tiny fraction of either the amine or ketone is tied up a imine or iminium ion at any pH value. The concentration of imine is greatest at high pH values and decreases with decreasing amine concentration. The concentration of iminium ion is linked to the concentration of ammonium ion. Thus the concentration of iminium ion does not decrease above the pA for the iminium ion, since it is dependent upon the concentration of protonated amine. This is a critical point for understanding the kinetics of reactions involving the iminium ion since the reasonable initial expectation is that the concentration of this species should decrease above the pA for the... 

We can use pC-pH diagrams that include heterogeneous equilibria for the rapid evaluation of both the total concentration of all species present as well as to provide a graphic representation of the concentrations of individual species present at various pH values. From Fig. 6-7, for example, we can deduce that in the pH range from 4.5 to 8 (which covers virtually all natural waters), FefOHla is the predominant soluble ferric iron species. The polymer (dimer) Fe2(OH)2 + does not predominate at any pH value, but it is a significant species at pH values below about 2.5. It is important to note that Fe , ferric ion, controls the solubility of Fe(OH)3tsj only below about pH 2.5. Conversely stated, at typically encountered natural water pH values in the presence of Fe(OH)3 s), Fe + is a minor component of the ferric iron species. 

Table 20.3. Values of Masking Factor (- log aufrom Eqn. 7Q. 6)for Representative Metal Ions and Masking Agents at Various pH Values... 

This method of plotting ionic data was developed in Oxford by Simon (1950), who found that the majority of those substances that are most active when least ionized nevertheless have ions which exert some of the activity. For another example, the anti-fungal action of 2,4-dinitrophenol (p. Ta 4.0), at various pH values, on the fungus Trichoderma viride, see Simon and Beevers (1952). 

A similar effect of chloride ions was also observed in an oxidation-type reaction of positronium with Fe(C104)3 in aqueous solutions at various pH values [Go 72]. As the pH was increased from 0 to above 2, the rate constant of the Ps reaction markedly decreased, but on the addition of chloride ions it became higher again. 

Purified receptor together with ATPase was reconstituted in BLM by Thompson et al. (25) for electrochemical experiments as outlined above. Figure 7.9 shows the electrochemical response of a BLM to fractions of receptor protein, ATP and naphthalene-1-acetic acid in various time sequences under an applied dc voltage of 15 mV. Only when all three components are present (in any order) is a substantial increase in current observed. Adjustment of hydronium ion concentration in solution resulted in a correlation of current at various pH values with the known receptor biochemical activity at the same values. This behaviour indicates that previous suggestions that an ATPase pump acts to translocate protons are indeed correct. From the sensor point of view, it is interesting to note that the signals exhibited a limit of detection of about 10 M NAA and were quite selective for the hormone. 

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