Complexation of polyvinyl alcohol with iodine Analytical precision and mechanism.

Polyvinyl alcohol (PVA), boric acid and tri-iodide form a characteristic blue complex. For a number of PVA samples, prescribed conditions were used to examine the precision of the formation and spectrophotometry of the complex. The precision of calibration curves was 1-2 % over the range 0-4 mg of PVA per 50 ml of final solution, over which Beer's law holds. Greater deviations can be caused by faulty preparation and aging of individual PVA solutions. The absorbance is independent of the content of residual acetate groups in the PVA for the range 0-15 %. The limit of detection is about 0.01 mg of PVA in 25 ml of sample. A pink colour in the system is due to association of iodine with acetate groups in the PVA. A blue or green colour is due to helical envelopment of iodine molecules by PVA chains stiffened by scattered cyclic groups. The mechanisms of these effects are discussed.