Determination of Food Oxalates Using Silica–Titania Xerogel Modified with Eriochrome Cyanine R

The interaction of silica–titania xerogel with triphenylmethane dyes(pyrocatechol violet, chrome azurol S, eriochrome cyanineR) has been investigated to create a new sensor material for solid phase spectrophotometric determination of food oxalates. The complex forming reaction between xerogel incorporated titanium(IV) and triphenylmethane dyeshas been studied; half-reactionperiods, complex composition, equilibrium constants, and xerogel sorption capacity have been calculated for each dye. Eriochrome cyanineR (ECR) is characterized by the shortest half-reactionperiod, the smallest equilibrium constant, and the greatest capacity; it has been chosen for the sensor material construction because titanium(IV)-ECR complex is formed faster and can be destroyed easier than other studied complexes. The interaction of this sensor material with oxalateshas been described: the presence of oxalatescauses sensor material discoloration and the absorbance is used as analytical signal. The analytical range is 35–900 mg/L (LOD 10.5 mg/L, n = 7). High concentrations of interfering inorganic anions, organic acids, and sucrose did not affect oxalatedetermination. Proposed solid phase spectrophotometric procedure has been successfully applied for the determination of oxalatesin food samples (sorrel, spinach, parsley, ginger, and black pepper) and the results are in good agreement with HPLC oxalatedetermination.