In this work, a comparison of borax concentrations measured by potentiometric and refractometric methods during the batch cooling crystallization of this salt was made. The potentiometric and refractometric methods do not directly show the concentration of the solution, but the calibration curves need to be constructed for their use. In potentiometric measurements, the calibration curve represents the relationship between ion-selective electrode potential and the logarithm of the concentration of the analyte, while in the refractomeric measurement it represents the relationship of the refractive index and the concentration of the substance. In both cases the calibration curves need to be constructed at a constant temperature. Since both methods will be applied during batch cooling crystallization, calibration has also been performed under the conditions of the crystallization. In six borax solutions of known concentrations (c = 0.05-0.187 mol dm -3 ), cooled by rate of 6 °C h -1 , the potential and the refractive index were simultaneously measured. In order to ensure a uniform heat transfer rate in the system, during calibration, the impeller speed (N = 300 rpm) as the volume of the solutions (V = 2.66 dm 3 ) were kept constant. From the measured potential of the sodium ion-selective electrode and the refractive index, calibration curves were constructed for each integer value of the temperature in the tested temperature range from 30 to 10 ° C. Batch crystallization of borax was then carried out one week and four weeks after calibration, the mother liquor concentrations were determined by both methods and compared with the results of the volumetric method. It was found that the refractometric method, immediately after calibration, shows a much larger deviation of the results than the potentiometric method compared to the volumetric one. However, within a few weeks after calibration, the deviations of the results obtained by refractometry remained unchanged, whereas due to the large deviation of results, the sodium ion-selective electrode had to be calibrated again.