Compare Eqs. 4,5, and 6 to Eqs. 1 and 3. The reaction represented by Eq. 4 is a reversible equilibrium reaction between cellulose and alkali, to generate the fiber nucleophile with which the dye can react, Cell-0-. This equilibrium is established almost immediately when the alkali is added to the system. The reaction represented by Eq. 5 is the key to the formation of the covalent dye-fiber bond and is the reaction, within the fiber, of the dye with the fiber nucleo- phile, Cell-0-. This fixation reaction pro- ceeds at a rate determined by the product of the concentrations of the reactants and their intrinsic reactivity, expressed by the rate constant kR. Squared brackets indi- cate concentrations; subscripts f and s mean fiber and solution respectively: Fixation Rate =