The objective of manual fitting is to reduce the systematic trends in the residuals by adjusting the stability constant values. A residual is the difference between an observed and calculated data value. In this example there are two buffer regions separated by an end-point. A systematic trend is shown by the residuals gradually becoming more negative in the left-hand buffer region and by there being all large positive residuals in the other buffer region. As there is a systematic trend in the residuals in both regions the species the constant for (stg)H, which is present in both buffer regions. should be adjusted.
As can be seen in the illustration, there is 100% formation of one species at the end-point, there is no equilibrium and the solution is not buffered. This has two consequences: i) because the solution is unbuffered electrode readings tend to have large errors and ii) the points do not help to characterize the equilibrium constant. In Hyperquad the second point is accommodated to some extent by the automatic assignment of lower weights to points where the slope of the titration curve is greater. In any case, large residuals in the region of an end-points need not be a matter for concern.
With polyprotic ligands it has been observed that the calculated position of an end-pont is very sensitive to the amount of mineral acids added initially to the titration vessel. In these circumstances it is permissible to refine the total mmoles of added acid.
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