Abstract As yet, investigations on mixing time have been focused on small and large scale stirred tanks. The aim of this study was to develop a new method for characterizing the mixing process and quantitatively measuring the mixing time in shake flasks by introducing a rotating camera for online observation of the fluid flow in combination with a typical colorimetric method using a pH indicator. In this presented scheme, only one drop of sulfuric acid was injected with a peristaltic pump into a bulk solution consisting of either deionized water or a viscous polyvinylpyrrolidone (PVP) solution. In this study, a novel interpretation of mixing time for the colorimetric method was introduced. The macroscopic mixing time is independent of the concentration of added acid. Interestingly, the recorded mixing time was similar irrespective of the shaking diameters (25, 50 mm). In addition, the dimensionless mixing number stayed constant in the regime of high Reynolds number. Our results showed that mixing number of the fluid in the regime of high Reynolds number is independent of the given geometries of flasks.