DETERMINATION OF POLYVINYL-ALCOHOL) VIA ITS COMPLEX WITH BORIC-ACID AND IODINE

The maximum absorbance of the complex is proportional to initial polymer concentration over the range 0-100 mg dm-3 for PVA polymers with degrees of hydrolysis in the range 75-100 mole-%, in the presence of an excess of boric acid, iodine and potassium iodide. Within these limits 1-mg dm-3 solutions of several commercial and laboratory-prepared polymers have maximum absorbance 0.035 within ± ca. 3% in 1-cm cells. Sources of error in the determination of PVA are discussed, and the reduction in the absorbance at low iodine concentration is examined with respect to the mole-% hydrolysis. Absorbance measurements on two solutions with, respectively, high and low reagent concentrations allow the determination of both the concentration of polymer and its % hydrolysis. The equilibrium for complex formation by pure PVA at room temperature with low iodine concentration yields the values-150 kJ mol-1 and -498 J mol-1 K-1 for the enthalpy and entropy of reaction; for a large excess of iodine the reaction is complete. Variations in the wavelength of maximum absorbance of the complexes correlate approximately with their stability. Complex formation occurs through the alcohol groups of the partly hydrolysed poly(vinyl acetate) polymers and also the residual acetate groups of these polymers at the higher iodine concentrations. © 1979.