Determination of quercetin in pharmaceutical formulations via its reaction with potassium-titanyloxalate: Determination of the stability constants of the quercetin-titanyloxalato complex

Abstract

Asimple, rapid and accurate procedure for the quantitative determination of quercetin in its pure form and in formulations has been developed. The method is based on the spectrophotometric determination of a complex formed between quercetin and potassium titanyloxalate in 50 % ethanolic solutions. To characterize the quercetin titanyloxalato complex, the stability constants of the complex were determinated potentiometrically and spectrophotometrically at different temperatures (T = 26.0oC, 34oC and 39.0oC), as well as at different ionic strengths (I = 5.0x10-4 mol dm-3, 3.0x10-2 mol dm-3 and 6.0x10-2 mol dm-3) and the thermodynamic parameters were calculated. As quercetin is usually conjugated to vitamin C in pharmaceutical formulations, two procedures for the quantitative determination of quercetin by this complexing reaction were tested - both in the absence and presence of ascorbic acid. In both procedures, the Beer law was obeyed over the same concentration range of quercetin, i.e., 0.85 μg mL-1 - 16.9 μg mL-1. In the first procedure in the absence of ascorbic acid the molar absorptivity coefficient of the quercetin-titanyloxalate complex is a=2.49 x 104 mol-1 dm3 cm-1, Sandells sensitivity of the method is S = 1.35 x 10-2 μg cm-2 and the detection limit is d = 0.67 μg mL-1. Whereas, in the presence of ascorbic acid (second procedure) a = 3.04 x 104 mol-1 dm3 cm-1, S = 1.11 x 10-2 μg mL-1. The proposed method was verified for the determination of quercetin in pharmaceutical dosage forms.

Predložen je brz, jednostavan i tačan metod za određivanje kvercetina u farmaceutskim sirovinama i doziranim oblicima.Metod je baziran na spektrofotometrijskom određivanju kompleksa formiranog između kvercetina i kalijum-titaniloksalata u 50 % etanolu. Određene su konstante stabilnosti kvercetin titaniloksalatnog kompleksa potenciometrijski i spektrofotometrijski na različitim temperaturama (T = 26.0oC, 34oC and 39.0oC) i jonskim jačinama (I = 5.0x10-4 mol dm-3, 3.0x10-2 mol dm-3 and 6.0x10-2 mol dm-3) i izračunati su termodinamički parametri. Kako se kvercetin uobičajeno nalazi zajedno sa vitaminom C u doziranim oblicima, predložene su dve procedure za određivanje kvercetina: bez i u prisustvu askorbinske kiseline.U obe predložene procedure, Beer-ov zakon važi u oblasti 00.85 μg mL-1 - 16.9 μg mL-1 kvercetina. Prema prvoj proceduri, molarni apsorpcioni koeficijent kvercetin–titaniloksalatnog kompleksa je a=2.49 x 104 mol-1 dm3 cm-1, Sandelova osetljivost metode je S = 1.35 x 10-2 μg cm-2, a limit detekcije je d = 0.67 μg mL-1. Prema drugoj proceduri, a = 3.04 x 104 mol-1 dm3 cm-1, S = 1.11 x 10-2 μg mL-1. Predloženi metod je primenjen za određivanje kvercetina u doziranim oblicima.