CONCENTRATION NONUNIFORMITIES AND AVERAGES IN FLOW-THROUGH SPECTROPHOTOMETRIC MEASUREMENTS

The use of a spectrophotometer in a flow-through configuration to meas ure an 'averaged' solute concentration, under conditions where local n on-uniformities in concentration are present, is investigated in a tub ular flow system. Various definitions of an 'averaged' concentration a re possible, and which of these, if any, most closely approximates the one that is actually measured is studied. Numerical solutions of the convection-diffusion equation without reaction are compared to experim ental results using different analytes under a range of steady laminar -flow conditions. The cross-sectionally averaged concentration from nu merical simulations is found to underestimate measurements consistentl y, whereas a flux-weighted averaged concentration yields surprisingly good agreement with measured values. (C) 1998 Elsevier Science B.V.