| Abstract |
The kinetics of hydrolysis of methyl paraben (methyl 4-hydroxybenzoate, MP) by NaOH has been proposed as a reference system to calibrate the effective volume (V-eff) of flow-calorimetry vessels that are often used in studies of living cells. This parameter is essential to relate the observed calorimetric signal with a specific cell number or biomass and its determination relies on a rate constant, k, obtained under the assumption of pseudo-first-order kinetics. The significant discrepancies noted between published k values led us to test the reliability of this approximation for different NaOH/MP concentration ratios and at temperatures in a range typically used for living cell studies (292-310 K). Kinetic experiments carried out by spectrophotometry, isothermal microcalorimetry, and flow microcalorimetry indicated that the 9/1 NaOH/MP molar ratio typically used for V-eff determinations does not ensure the pseudo-first-order kinetic conditions assumed in the V-eff calculation model. It is recommended that a NaOH excess of at least 20/1 be used to warrant a < 5\% error in k, which translates into an identical error in V-eff. |
