For the general @[email protected]: \[a\,\text{A}+b\,\text{B}\rightarrow p\,\text{P}+q\,\text{Q}+...\] occurring under constant-volume conditions, without an appreciable build-up of reaction @[email protected], the rate of reaction \(\nu\) is defined as: \[\nu = -\frac{1}{a} \frac{\mathrm{d}\text{[A]}}{\mathrm{d}t}=- \frac{1}{b} \frac{\mathrm{d}\text{[B]}}{\mathrm{d}t}=\frac{1}{p} \frac{\mathrm{d}\text{[P]}}{\mathrm{d}t}=\frac{1}{q} \frac{\mathrm{d}\text{[Q]}}{\mathrm{d}t}\] where symbols placed inside square brackets denote amount (or @[email protected]) concentrations (conventionally expressed in units of \(\text{mol dm}^{-3}\)). The symbols \(R\) and \(r\) are also commonly used in place of \(\nu\). It is recommended that the unit of time should always be the second. In such a case the rate of reaction differs from the rate of increase of concentration of a product P by a constant factor (the reciprocal of its @[email protected] in the @[email protected] equation, \(p\)) and from the rate of decrease of concentration of the reactant A by \(\alpha ^{-1}\). The quantity: \[\overset{\text{.}}{\xi }=\frac{\mathrm{d}\xi }{\mathrm{d}t}\] defined by the equation: \[\overset{\text{.}}{\xi } = -\frac{1}{a} \frac{\mathrm{d}n_{\text{A}}}{\mathrm{d}t}=- \frac{1}{b} \frac{\mathrm{d}n_{\text{B}}}{\mathrm{d}t}=\frac{1}{p} \frac{\mathrm{d}n_{\text{P}}}{\mathrm{d}t}=\frac{1}{q} \frac{\mathrm{d}n_{\text{Q}}}{\mathrm{d}t}\] (where \(n_{\text{A}}\) designates the @[email protected] A, conventionally expressed in units of @[email protected]) may be called the '@[email protected]' and is appropriate when the use of concentrations is inconvenient, e.g. under conditions of varying volume. In a system of constant volume, the rate of reaction is equal to the @[email protected] per unit volume throughout the reaction. For a @[email protected] this definition of 'rate of reaction' (and '@[email protected]', \(\xi \)) will apply only if there is no accumulation of intermediate or formation of side products. It is therefore recommended that the term 'rate of reaction' be used only in cases where it is experimentally established that these conditions apply. More generally, it is recommended that, instead, the terms '@[email protected]' or 'rate of consumption' of A (i.e. \(-\frac{\text{d}\text{[A]}}{\text{d}t}\), the rate of decrease of concentration of A) or 'rate of appearance' of P (i.e. \(\frac{\text{d}\text{[P]}}{\text{d}t}\), the rate of increase of concentration of product P) be used, depending on the concentration change of the particular @[email protected] that is actually observed. In some cases reference to the @[email protected] observed may be more appropriate. The symbol \(\nu\) (without lettered subscript) should be used only for rate of reaction; \(\nu\) with a lettered subscript (e.g. \(\nu_{\text{A}}\)) refers to a rate of appearance or @[email protected] (e.g. of the chemical species A).