Activity coefficients inaccuracy

This is a key feature of the system for anyone who wants to understand and rationalize the effects of the microenvironment of a biocatalyst on its activity, its stability, or its specificity. Since for many years the use of thermodynamic activity was recommended for quantifying substrate availability in non-conventional media [17, 18], the replacement of concentrations of species by their thermodynamic activities in liquid non-conventional media requires a knowledge of their activity coefficients (y values). And this point is still far from being straightforward, as (a) values depend on molar ratios of other species present in the medium, and (b) methods used to estimate these values, such as UNI FAC group contribution method [19], are often called into question, and claimed to be sources of inaccuracy [20, 21]. 

Why 5% Because we are ignoring activity coefficients, the equilibrium constants are only approximate, and we are judging 5% to be the typical level of inaccuracy. 

The LCM also predicted activity coefficients of bi-univalent electrolytes (e.g., MgC an< CadO, which exhibit the experimentally observed reversal of slope and dramatic increase in the activity coefficient at higher ionic strengths. For bi-univalent electrolytes regressing binary parameters over a 0-6 molal range causes some inaccuracy in the low concentration region. However, regressing the LCM binary parameters over a smaller concentration range (0-2.5 molal) improves the LCM accuracy. 

Calculate pH versus -log[H+]e at 298 K for 0 < Ief < 0.5 mol dm 3 with the two derived relationships and estimate the inaccuracy of pH that could result from activity coefficient corrections. ... 

The analysis presented so far shows that the least-squares procedure cannot distinguish between the inaccuracy of data and the model capacity to describe a particular system. We need a deeper insight. Firstly, we apply the test of activity coefficients found from experimental data by means of the equation (6.82). Figure 6.8 presents the plot of ln(f2 /yO against x,. Two areas are formed around x-axis. Data are consistent if the following condition is satisfied (Sandler, 1999) ... 

If the relevant activity coefficients can be evaluated, it is possible to establish how the different supersaturations differ from one another and, more importantly, from the fundamental supersaturation, Sa- The decisive factor is the activity coefficient ratio, A. The more it deviates from unity, the greater is the incurred inaccuracy. In general, when Am> I, m-based concentration units are preferred, but when < 1, or c-based units are better than m-based. The choice between x- and c-based units in this case again depends on the activity coefficient ratio if Ax > Ac, the x-based units are preferred and vice versa. 

Although we wrote activity coefficients in the equilibrium equations, we are not so masochistic as to use them in the rest of the problem. At this point, we are going to ignore the activity coefficients, which is equivalent to saying that they are unity. There will be some inaccuracy in the results, but you could go back after the calculation and compute the ionic strength and the activity coefficients and find a better approximation for the solution if you had to. 

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