Spectrophotometric determination of the degree of acetylation of cellulose acetate via ferric hydroxamate complexation: method development and validation

Abstract

A rapid, accurate, and cost-effective spectrophotometric method has been developed for determining the degree of acetylation (DA) of cellulose acetate (CA). The method is based on the ferric hydroxamate principle. A mixed solvent system of dichloromethane and methanol (7 : 3, v/v) was optimized via Hansen solubility parameter theory to ensure complete dissolution of CA across a wide DA range (1.83–2.95). The reaction of CA with hydroxylamine to form the corresponding hydroxamic acid was systematically optimized, achieving >98.8% conversion within 20 minutes at 25 °C. Subsequent complexation with Fe(III) ions produced a stable, purple-colored chelate with a maximum absorbance at 520 nm. The method obeys Beer–Lambert law over a concentration range of 0.05–2.5 mg mL⁻¹, with a molar absorptivity of 1.09 × 10³ L mol⁻¹ cm⁻¹ and a correlation coefficient (r) of 0.999. The limits of detection and quantification were 0.012 mg mL⁻¹ and 0.036 mg mL⁻¹, respectively. Validation studies confirmed excellent precision (RSD < 2%), high accuracy (recovery 98.5–101.2%), and no significant statistical difference compared to the standard titration method (t-test and F-test, p > 0.05). This method offers a simple, reliable, and inexpensive alternative to conventional techniques for the routine analysis of CA acetylation.