This term applies broadly to variations of @Q04991@ of @P04647@ (e.g. @F02453@ or @P04569@) or @P04585@ (usually reaction @Q04991@) with the concentration of a given @R05190@ which may be a substrate or a @Q05006@. In the simplest case, a plot of \(\frac{\mathit{\Phi }^{0}}{\mathit{\Phi }}\) (or \(\frac{M^{0}}{M}\) for emission) vs. concentration of @Q05006@, \(\text{[Q]}\), is linear obeying the equation: \[\frac{\mathit{\Phi }^{0}}{\mathit{\Phi }}\quad \text{or}\quad \frac{M^{0}}{M}=1+K_{\text{sv}}\ [\text{Q}]\] In equation (1) \(K_{\text{sv}}\) is referred to as the Stern–Volmer constant. Equation (1) applies when a @Q05006@ inhibits either a @P04585@ or a photophysical process by a single reaction. \(\mathit{\Phi} ^{0}\) and \(M^{0}\) are the @Q04991@ and emission intensity @R05041@, respectively, in the absence of the @Q05006@ Q, while \(\mathit{\Phi}\) and \(M\) are the same quantities in the presence of the different concentrations of Q. In the case of @Q05007@ the constant \(K_{\text{sv}}\) is the product of the true @Q05008@ \(k_{\text{q}}\) and the @E02257@ @L03515@, \(\tau ^{0}\), in the absence of @Q05006@. \(k_{\text{q}}\) is the @M03989@ reaction @O04322@ for the @E02035@ of the @E02257@ with the particular @Q05006@ Q. Equation (1) can therefore be replaced by the expression (2): \[\frac{\mathit{\Phi }^{0}}{\mathit{\Phi }}\quad \text{or}\quad \frac{M^{0}}{M}=1+k_{\text{q}}\ \tau ^{0}\ \left[\text{Q}\right]\] When an @E02257@ undergoes a @M03989@ reaction with @O04322@ \(k_{\text{r}}\) to form a product, a double-reciprocal relationship is observed according to the equation: \[\frac{1}{\mathit{\Phi }_{\text{p}}} = (1+\frac{1}{k_{\text{r}}\ \tau ^{0}\ \text{[S]}})\ \frac{1}{A\cdot B}\] where \(\mathit{\Phi} _{\text{p}}\) is the @Q04988@ of product formation, \(A\) the efficiency of forming the reactive @E02257@, \(B\) the fraction of reactions of the @E02257@ with substrate S which leads to product, and \(\text{[S]}\) is the concentration of reactive ground-state substrate. The intercept/slope ratio gives \(k_{\text{r}}\ \tau ^{0}\). If \(\text{[S]}=\text{[Q]}\), and if a photophysical process is monitored, plots of equations (2) and (3) should provide independent determinations of the product-forming @O04322@ \(k_{\text{r}}\). When the @L03515@ of an @E02257@ is observed as a function of the concentration of S or Q, a linear relationship should be observed according to the equation: \[\frac{\tau ^{0}}{\tau} = 1+k_{\text{q}}\ \tau ^{0}\ [\text{Q}]\] where \(\tau ^{0}\) is the @L03515@ of the @E02257@ in the absence of the @Q05006@ Q.