POTENTIOMETRIC TITRATION STUDIES ON XANTHAN SOLUTIONS

The potentiometric behavior of xanthan solutions was analyzed as a fun ction of ionic strength, polymer concentration and temperature. For a given value of the polymer dissociation constant, alpha, pK(a) was lin early correlated with C-s(1/3) (salt concentration). At polymer concen trations below 7.5 g liter(-1), pK(a) was a function of polymer concen tration. However, when the polymer concentration was higher than this value, pK(a) was independent of polymer concentration. Potentiometric titration confirmed the significance of the hydrogen bond and the carb oxylate group on the secondary structure of xanthan which suggested th at the ordered xanthan conformation is stabilized mainly by hydrogen b onds. These can be disrupted by increasing the electrostatic potential of the molecule or by increasing the solution temperature. Neither th e ionic strength nor the polymer concentration affected the secondary structure of xanthan at 25 degrees C. However, ionic strength should r educe the repulsive force between charged groups in xanthan; this woul d explain the salt stabilization of the secondary structure observed a t the higher test temperatures (35-70 degrees C).