In chemically reacting systems at equilibrium, forward and backward reactions proceed at statistically equal rates, so that the average properties of the system are time independent. However, a series of instantaneous microscopic analyses of the system is expected to reveal recurring random concentration fluctuations about the equilibrium state. Until recently no one had directly measured the rates of fluctuation relaxation by the mechanism of chemical reaction, but now Douglas Magde, Elliott Elson and Watt Webb of Cornell University have seen the thermodynamic concentration fluctuations that reflect the dynamic balance of chemical reactions at equilibrium. They have studied the binding between DNA and the dye ethidium bromide (EtBr), which form a strongly fluorescing complex, and they were able to determine the chemical rate constants, the diffusion coefficients and the coupling between the two by observing fluctuations of the fluorescent intensity.
Marian S. Rothenberg; Fluctuations at chemical equilibrium. Physics Today 1 December 1972; 25 (12): 17–18. https://doi.org/10.1063/1.3071135
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