The effect of citrate species on fluorescence quenching of RF solutions at pH 4.0–7.0 is reported in Table 4. The values indicate that fluorescence quenching of RF decreases from 39.2–19.6% at pH 4.0–7.0. This is in accordance with the decrease in the concentration of divalent citrate ion concentration from 99.6% to 20.0% in this pH range. It suggests that these ions are involved in the quenching of RF excited singlet state [1RF⁎], thereby inhibiting the reaction and thus the stabilization of RF solutions. This is evident from the decrease in the rate of photolysis of RF with

an increase in the citrate ion concentration at all pH values (Table 2). The fluorescence quenching and its effect on kinetic results show that the divalent citrate ions deactivate the RF excited species and in this manner stabilize RF solutions. The effect of trivalent Cobimetinib molecular weight citrate ions on the rate of photolysis INK1197 nmr is discussed in the next

section. The catalytic effect of phosphate and inhibitory effect of borate buffers on the photolysis of RF have been studied and the kinetics of these reactions has been evaluated [4], [5], [6] and [9]. Citric acid (pKa 4.78, 6.40) forms divalent and trivalent species in the pH range of 4.0–7.0. The divalent species have been found to inhibit the rate of reaction in the pH range of 4.0–7.0 as observed by fluorescence loss and a linear relationship between the rate constants (kobs) and the buffer concentration (0.2–1.0 M). However, the role of trivalent citrate ions in the reaction remains to be determined. It has been shown ( Section 3.6) that the trivalent

citrate ions have a greater inhibitory effect on the rate of photolysis compared to that of the divalent citrate ions. The concentration of trivalent citrate ions in total citrate buffer increases with pH (0% pH 4.0, 80% pH 7.0) and its greater inhibitory effect may be explained on the basis of the quenching of excited triplet state [3RF⁎] by these ions. However, trivalent citrate ions may also be involved in the quenching of excited 6-phosphogluconolactonase singlet state [1RF⁎], as indicated by its greater inhibitory effect on the reaction ( Section 3.6). The quenching of [3RF⁎] by different substrates has been reported by Ahmad and Tollin [2]. The stabilizing effect of divalent and trivalent citrate ions on the photolysis of RF could be considered as a consequence of the deactivation of both [1RF⁎] and [3RF⁎] by the two ions. Citrate buffer has been reported to stabilize penicillin solutions [31]. The modes of RF photolysis have been discussed in detail by various workers [22], [23] and [3]. The stabilizing effect of citrate species on the photolysis of RF in the pH range of 4.0–7.0 has been studied. These species cause an inhibitory effect on the rate of photolysis in this pH range. The log k–pH profiles indicate a gradual decrease in rate with an increase in citrate concentration. The value of kobs at pH 5.