If the equilibrium mixture of a @C01033@ is disturbed by a sudden change, especially of some external parameter (such as temperature, pressure or electrical field strength), the system will readjust itself to a new position of the @C01023@ or return to the original position, if the perturbation is temporary. The readjustment is known as chemical @R05285@. In many cases, and in particular when the displacement from equilibrium is slight, the progress of the system towards equilibrium can be expressed as a first-order law: \[C_{t}- \left(C_{\text{eq}}\right)_{2} = [\left(C_{\text{eq}}\right)_{1}- \left(C_{\text{eq}}\right)_{2}]\ \text{e}^{\frac{-t}{\tau }}\] where \(\left(C_{\text{eq}}\right)_{1}\) and \(\left(C_{\text{eq}}\right)_{2}\) are the equilibrium concentrations of one of the chemical species involved in the reaction before and after the change in the external parameter, and \(C_{t}\) is its concentration at time \(t\). The time parameter \(\tau \), named @R05287@, is related to the rate constants of the chemical reaction involved. Measurements of the @R05285@ times by @R05285@ methods [involving a @T06265@ (T-jump), @P04823@, electric field jump or a periodic disturbance of an external parameter, as in ultrasonic techniques] are commonly used to follow the kinetics of very fast reactions.