## University of the Pacific Theses and Dissertations

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1992

#### Document Type

Dissertation - Pacific Access Restricted

#### Degree Name

Doctor of Philosophy (Ph.D.)

Chemistry

#### Abstract

Two different systems involving the dye rhodamine 6G (R6G) were studied using absorption spectrophotometric techniques. The first is a compound that forms when R6G, HgCl$\sb2,$ and KI are combined in aqueous solution at concentrations near 10$\sp{-5}$ m. The second involves the formation of an addition complex between 10$\sp{-5}$m R6G and 10$\sp{-3}$ to 10$\sp{-2}$m $\beta$-cyclodextrin ($\beta$-CD) in aqueous solution. When R6G, HgCl$\sb2,$ and KI are combined, the absorption spectrum shows a shoulder around 571 nm that is attributed to the formation of a complex. The formula for this compound was determined by two methods: (1) the mole ratio method, which results in a 1:1 ratio between Hg(II) and R6G, and (2) the method of continuous variations, which this work has extended for a three reactant system, resulting in a ratio of 3:1 between iodide and R6G. Therefore, the formula for the complex is proposed to be R6GHgI$\sb3$ in the range of concentrations under study. Thermodynamic constants for the formation of the R6GHgI$\sb3$ compound are determined. Mole ratio plots at temperatures ranging from 15$\sp\circ$C to 30$\sp\circ$C are used to determine the mole fraction equilibrium constants. The values for the constants are 6.74 $\times$ 10$\sp{26}$, 5.53 $\times$ 10$\sp{26},$ 1.67 $\times$ 10$\sp{26},$ and 1.31 $\times$ 10$\sp{26}$ at 15.0, 18.0, 25.0, and 30.0$\sp\circ$C, respectively. The changes in enthalpy, $\Delta$H$\sb{\rm x}$, and free energy, $\Delta$G$\sb{\rm x}$, are then determined from the mole fraction equilibrium constant. $\Delta$H$\sb{\rm x}$ is $-$80.8 kJ/mol, $\Delta$G$\sb{\rm x}$ is 150 $\pm$ 2 kJ/mol, and $\Delta$S$\sb{\rm x}$ is 233 $\pm$ 2 J/mol. The addition of R6G to $\beta$-CD results in a red shift of the absorption peak of R6G from 526 nm to 529 nm. When the concentration of $\beta$-CD is varied, an isosbestic point is observed at 528 nm. Two different methods were used to calculate the equilibrium constant of the R6G - $\beta$-CD complex. Both methods use the same absorbance data, which is obtained by varying the concentration of $\beta$-CD and keeping the R6G concentration constant. One involves a double reciprocal plot using the Benesi-Hildebrand equation. The second uses a non-linear least squares fit of the absorbance data to the hypothetical equilibrium equation to calculate the equilibrium constant. The equilibrium constant determined by the Benesi-Hildebrand equation is 36 $\pm$ 44. The non-linear least squares equilibrium constant is (1.2 $\pm$ 0.4) $\times$ 10$\sp2.$

#### Pages

263

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