The association constant Kass, of R1 with fluoride and acetate was calculated in accordance to the Benesi–Hildebrand equation [37,38], Fig. 5(a) & (b) which is given as follows: where A0 is the absorbance of free R1, A is the absorbance with a specific anion concentration, A∞ is the absorbance with excess amount of anions F− & AcO−, K is the association constant (M−1), and [X−]0 is the concentration of F− & AcO− anions respectively, added (M). The plot of 1= A−A0 ð Þagainst 1/[X−]0 shows a good linear relationship [F−(R = 0.993) & AcO−(R = 0.996)], indicating that R1 associates with fluoride and acetate in a 1:1 stoichiometry