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Comparison of potentiometric and refractometric methods for borax concentration measurement during batch cooling crystallization

In order to define the kinetics of nucleation and crystal growth, supersaturation as a driving force of crystallization needs to be determined over process time since it is the driving force of crystallization. Before selecting of an appropriate analytical method for bulk supersaturation measurement, it is necessary to examine its sensitivity [1]. In this work, application of ion-selective electrode (ISE) and refractometer during batch cooling crystallization was analysed. Their calibration, unlike as is commonly done at a constant temperature, was performed here under crystallization process conditions in terms of cooling rate. The refractive index, n, and electrode potential, E, (Na-ISE - Ag/AgCl) of borax solutions of known concentrations, c, cooled at a rate of 6 °C h-1, were measured at certain temperatures, T, in the range from 30 °C to 5 °C. The parameters of the calibration equation for the refractometric (n = K1·ca·Tb) and the potentiometric method (E = K2·cd·Tg) for examined temperatures and solution concentrations were derived. For the blind probe concentration measurement, refractometry was found to be more accurate while potentiometry showed greater precision. However, when deciding on a particular method, it should be borne in mind that the refractometer is calibrated only once, while ISE calibration is required frequently [2, 3]. [1] M. Barrett, M. McNamaraa, H. X. Hao, P. Barrett, B. Glennon, Chem. Eng. Res. Des. 88 (2010) 1108-1119. [2] A. Čelan, I. Milanović, M. Ćosić, N. Kuzmanić, Chem. Eng. Technol. 44 (2021) 11. [3] M. Ćosić, A. Čelan, I. Pehnec, N. Kuzmanić, Chem. Eng. Comm. (207) 6 (2019) 847-860.

Potentiometric method ; refractometric method ; concentration ; crystallization

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