Used in mass transfer calculations, it is the mass flux of one fluid to another across an interface divided by the difference between the concentrations of the diffusing component in the bulk of the two fluids. The subscripts refer to the overall mass transfer coefficients in the gas and liquid phases, respectively. The overall mass transfer coefficient is defined as either the liquid- or gas-side since the interfacial composition cannot be determined. The bulk driving force is used. (p. 269) That is, the composition in the gas phase is assumed to be in equilibrium with the bulk concentration in the liquid phase, or the liquid composition is in equilibrium with the composition of the bulk gas phase. The SI units are m2 s−1. The mass transfer coefficients are dependent on the geometry of the contacting equipment as well as fluid behaviour. Many correlations have been established to determine the coefficients. An example is the use of the Sherwood number for describing the gas phase in terms of the Reynolds number and Schmidt number.