Constant ionic strength

The pH will depend upon the ionic strength of the solution (which is, of course, related to the activity coefficient — see Section 2.5). Hence, when making a colour comparison for the determination of the pH of a solution, not only must the indicator concentration be the same in the two solutions but the ionic strength must also be equal or approximately equal. The equation incidentally provides an explanation of the so-called salt and solvent effects which are observed with indicators. The colour-change equilibrium at any particular ionic strength (constant activity-coefficient term) can be expressed by a condensed form of equation (4) ... 

The existence of the primary and secondary salt effects indicates the importance of maintaining control over ionic strength in kinetics studies. One may choose to keep the ionic strength low so as to minimize its effects, or one may make a series of measurements at various ionic strengths in order to permit extrapolation to the limit of infinitely dilute solution. Another useful alternative is to maintain the ionic strength constant at a value that is suffi-... 

These authors used activities instead of concentrations but at constant ionic strength (constant... 

For most academic investigations, reaction conditions are kept under as much control as possible. Solutions are thermostatted and buffered, and investigations are carried out in an excess of an inert salt. This is done to keep temperature, pH, and ionic strength constant. In industrial situations, it is often not possible, nor is it necessarily desirable, to control conditions. Temperature fluctuations within safe limits are not necessarily detrimental and the addition of external buffer or salt is out of the question. 

O Varying ionic strength Constant ionic strength... 

It should also be noted that in real foams as a result of reduced pressure in borders pa 1 kPa) films become relatively thin, while the electrostatic component (IX,/ - p ) strongly decreases. Furthermore, it has been shown [37] that with the increase in surfactant concentration while keeping the ionic strength constant, the transition from unstable to stable... 

In considering double-layer effects in electrode kinetics, care must be taken in defining the conditions under which R is measured and interpreted, for example, for conditions of constant ionic strength, constant overpotential, or, alternatively, constant electrode potential, as emphasized in Ref 143. Correspondingly, attention must be given to the role of chemisorption of the... 

These authors used activities instead of concentrations, but at constant ionic strength (constant ionic medium scale), the rate law can be written in terms of concentrations. 

Gradients formed by increasing ionic strength (constant pH) are preferable to gradients formed by changing the pH. Volatile buffers, such as formates and acetates of pyridine and ammonium are usually used because they can be easily removed from the eluted fractions. However, the use of non-ionic hydrophobic resins and the silica bonded alkyl phases offer a convenient alternative to remove also non-volatile buffers. 

In one series of the potentiometric measurements an undefined amount of Na N03 was added to maintain the ionic strength constant . The value of the ionic strength is not mentioned, but obviously the ionic environment varied substantially since a sodium and a potassium salt were mixed. Other experiments were carried out without the addition of a supporting ionic medium. 

This experiment, obtained by keeping the ionic strength constant, confirms that the contribution furnished by the liquid junction potential (vs ref. SCE) is decisive. 

The above discussion shows that the dependence of the reaction rate upon the pH contains very important information on the reaction mechanism. Each rate must be measured at constant pH, which usually involves measuring it in a buffer solution. In addition, usually an inert salt is added to maintain ionic strength constant to avoid the salt effects discussed in Chapter 9. In fact, experimentally, the rates are measured at different buffer concentrations, keeping the pH and the ionic strength constant. Under these conditions, and for a constant substrate concentration, there is a linear dependence between the rate and the buffer concentration, as illustrated in Figure 13.3. Extrapolating to zero buffer concentration, one obtains the rate for a constant pH. When general acid-base catalysis is present. 

Follow the same procedure as in Sec.7.6.1 replacing persulphate by hydrogen peroxide, remembering to add ammonium sulphate to the initial solution to maintain die ionic strength constant. After studying the effects of the concentrations of the reactants and temperature on the rate of reaction, investigate the effect of adding different [molybdate]. 

Indicators of pH, 5, 59 Ionic product of water, 103 Ionic strength constant, 14 defined, 28... 

Clearly, the uncertainties associated with both the stability constant and the relevant ion interaction coefficients are high. Nevertheless, the stability constant from the relationship is consistent with those accepted values for the zero ionic strength constant of Np020H(aq) given in Table 9.14. 

The zero ionic strength constant is also consistent with the available data accepted from nitrate media (figure not shown). 

From the relationship between ionic strength and the solubility constant, the derived zero ionic strength constant is log = -8.00 0.10. The associated ion interaction parameters are... 

Similarly, the data in nitrate media were used with the same zero ionic strength constant and the standard specific ion interaction theory to derive the ion interaction coefficient for the species with the nitrate ion. The relationship of the data from nitrate media is illustrated in Figure 13.24. The calculated ion... 

The datum from Cukrowski et al. (1995) at 50°C and in 0.10moll NaNOj (log =-6.91 0.10) is in good agreement with that determined from the relationship of the zero ionic strength constant with temperature (at 50 °C, the calculated stability constant is log = -6.73 0.12). 

Naturally, with this double standard addition procedure the same precautions (constant ionic strength, constant degree of complexation, AE = const.) must be observed... 

Keeping the ionic strength constant by means of an ionic strength adjustor (1—5 M solution of an inert electrolyte), so that the resulting constancy of the activity coefficients can allow direct concentration indication ... 

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