Quantification of aspirin as sodium salicylate using ImageJ digital image analysis validated by UV–Visible spectroscopy: A CIE L*a*b* colour space study

This study arises from a supervised practical investigation conducted by four Post-Graduate Diploma in Analytical Chemistry students at Durban University of Technology (Josephs, Buthelezi, Sibali and Mundembe), whose experimental datasets form the basis of the reported findings. A validated colorimetric method for the quantification of aspirin (acetylsalicylic acid, ASA) as sodium salicylate is reported, using digital image analysis with ImageJ software (v1.8.0), benchmarked against UV-Visible (UV-Vis) spectrophotometry and comprehensively evaluated using the CIE L*a*b* colour space. Aspirin standards were converted to sodium salicylate and reacted with 1% ferric(III) nitrate to form a violet-coloured complex whose intensity is proportional to concentration. Digital images of six working standards (0-100 mg/L) were captured using a Samsung Galaxy A52 smartphone and processed in ImageJ to extract Intensity (grayscale) and RGB (Red-Green-Blue) colour coordinates, which were transformed to CIE XYZ and subsequently to CIE L*a*b* values. Colour difference (ΔE), hue angle (h°) and chroma (C*) were computed as scalar CIE L*a*b* descriptors and all showed monotonic, concentration-dependent increases across the working range, confirming their suitability as quantitative colorimetric signals. UV-Vis spectroscopy at λ max = 515 nm yielded excellent linearity (r² = 0.99934) with LOD and LOQ of 26.97 mg/L and 89.89 mg/L, respectively. The ImageJ method demonstrated acceptable linearity (r² = 0.99938), LOD of 7.84 mg/L and LOQ of 26.12 mg/L, with repeatability (RSD% < 5%) statistically equivalent to UV-Vis by F-test in three of four student datasets. Paired t -tests identified a statistically significant systematic bias in ImageJ-derived concentrations relative to UV-Vis, attributable to standard Red-Green-Blue (sRGB) gamma compression, non-linear pixel response at elevated concentrations, and indeterminate optical path length. Percentage recovery for commercial aspirin tablets ranged from approximately 63 to 88% relative to post-dilution nominal concentrations, reflecting partial hydrolysis under mild alkaline conditions. Despite this systematic bias, the multi-metric CIE L*a*b* characterisation encompassing ΔE, h° and C* alongside individual L*, a* and b* parameters demonstrated the breadth of analytical information accessible from a single smartphone image, establishing a robust methodological framework for accessible pharmaceutical colorimetric analysis. The ImageJ/CIE L*a*b* approach holds considerable promise as a low-cost, field-deployable screening tool for pharmaceutical quality control in resource-limited settings.