Studies on polyelectrolytes. VII. Gum tragacanth

Abstract

In two previous communications (1, 2) it has been shown tha t the free acids and their sodium salts from both arabic and agar gums behave as polyelectrolytes. In the present paper the results of measurements on the free acid and its sodium salt of gum tragacanth are reported. The method of preparation of free acid and its sodium salt was exactly the same as described in the case of gum arabic (1). Removal of calcium from the gum was effected in a way described for gum agar (2). The equivalent weight of free t ragacanth acid was 550. The molecular weight of the free acid and its sodium salt was in the range of 80,000 to 100,000, as determined osmometrically in the presence of HC1 and NaC1, respectively. On an average there are about 180 dissociable groups per molecule. As will be evident from Fig. 1, the viscosity measurements with solutions of free acid in water showed a rapid rise in reduced viscosity with dilution. In the presence of HC1, ~p/c vs. c curve showed a maximum which shifted to higher concentrations of gum acid with increase in the free HC1 concentration and ultimately became linear at sufficiently high concentration of free acid. All these clearly indicate the polyelectrolyte nature of the free acid and the linear character of the molecule. When, however, viscosity measurements were extended to the sodium salt of the gum acid, certain peculiarities were noted (Fig. 2). The reduced viscosity diminished with diminishing concentration of sodium salt, cont ra ry to what has been noticed in the case of gum acid. The ~p/C vs. c curve was not linear as with neutral polymers. Addition of sodium chloride caused an over-all lowering of the viscosity of the solution at all concentrations measured and the ~p/c vs. c curves became linear. At any particular concentration, however, the reduced viscosity of the sodium salt is higher than that of the free acid. Since reduced viscosity is a measure of the hydrodynamic volume unit of the dissolved molecules, the sodium salt must have a more extended structure than the free acid, or in other words, the COOH groups are less dissociated than the COONa groups. Measurements of pH of the solution of sodium salt showed that the