This thesis describes the principles of the most commonly used methods for determining the surface tension of liquids. The most direct method is the tensiometric method, which measures the force required to detach a geometrically well-defined test element (du Noüy ring, Wilhelmy plate or platinum rod) from a liquid-air phase boundary. The instruments using this method are distinguished by the degree of automation of the measurement (manual instruments or automatic instruments) and by the criteria for when the measured force is relevant to the measurement. In this method, measurements on a manual tensiometer K6, an older automatic tensiometer K10 ST and a modern automatic tensiometer Tensíío were compared with each other. The second and third instruments differ by the speed and accuracy of the measurements made and by the criteria for when the measured force is relevant to the measurement, with the newer instrument giving more accurate and precise results. Surface tension was measured on distilled water, 2 M aqueous mixtures of NaBr and KF salt solutions and some hydrophobic deep eutectic solvents. We found that the measured surface tension is affected by various factors such as temperature (the higher the temperature, the lower the surface tension), mechanical effects (walking or vibrations around the instrument), mechanical deformation and other damage of the test elements, calibration efficiency and accuracy of the instruments, the volume of liquid used, contamination of the test elements and finally the properties of the liquid being measured.
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