Collection of Czechoslovak Chemical Communications - digital archive 

Abstract

Using a two-step oxidation of pectic acid (85% of D-galacturonan in the preparation) a series of 2,3-dicarboxy derivatives (Na⁺ form) was prepared containing 4.2 to 8.5 mmol of carboxyl groups per gram (COOH g^-1). After the centrifugation of the diluted suspensions of the Ca-salts of these polyacids the activity of the calcium counterions (a_Ca²⁺) was determined using the metallochromic indicator (tetramethylmurexide) and the activity coefficient γ_Ca²⁺ was calculated. Using potentiometric titrations of polyacids with potassium and calcium hydroxides the decrease in electrostatic free enthalpy, Δ(G_el/N)_K^Ca, of the cation exchange Ca²⁺ → 2 K⁺ was determined. In the course of the oxidation, degradation of the macromolecules took place, characterized by the limit viscosity number [η]. In spite of the considerable degradation, preparations with higher oxidation degrees display a high selectivity in the exchange of cations Ca²⁺ → 2 K⁺, similar to that determined recently by us in 2,3-dicarboxy derivatives of starch and amylose. The strong binding of Ca²⁺ ions to the investigated substances is also documented by very low activity coefficient values, γ_Ca²⁺, ranging from 0.057 to 0.037.