Swelling titration curves

Figure 1. Swelling titration curves for 5 mol% crosslinked polyacids.

Fig. 2.14. (A) Potentiometric titration curves of a polymer imprinted with L-PA (PLPA), a polymer imprinted with benzylamine (PBA), a blank non-imprinted polymer (PBL) and acetic acid in MeCN/O.lM NaCl 70/30 (v/v). The NaOH equivalents (x-axis) are calculated based on the theoretical amount of carboxylic acid groups present in the polymer. In (B) is seen the calculated distribution as a function of the degree of ionization (a ). The polymer swelling (ml/ml) in this solvent system was constant in the pH interval 3-12 and was for PLPA 1.32 and for PBL 1.26. From Sellergren and Shea [67].

The previous states of both the gel ionization and the swelling influence the shape of the titration curve Cgj=/(pH). This means that high signal reproducibility can be ensured only by performing a conditioning procedure before every measurement in order to achieve a certain reference sensor signal. 

The titration curve of 3-lactoglobulin at various ionic strengths (Fig. 1.34) shows that the isoelectric point of this protein, at pH 5.18, is independent of the salts present. The titration curves are, however, steeper with increasing ionic strength, which indicates greater suppression of the electrostatic interaction between protein molecules. At its isoelectric point a protein is the least soluble and the most likely to precipitate ( isoelectric precipitation ) and is at its maximal crystallization capacity. The viscosity of solubilized proteins and the swelling power of insoluble proteins are at a minimum at the isoelectric point. 

Methods are also available for rapidly estimating polymer molecular weight distribution. These methods include swelling, in which the amount of polymer precipitated vs nonsolvent added gives a cumulative distribution curve (i.e., higher molecular weights first precipitate out with nonsolvent addition) and tur-bidimetric titration, which uses turbidity induced by nonsolvent addition as a measure of molecular weight distribution. These methods are described in detail in Ref. 28. 

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