ISO 6295 Petroleum products - Mineral oils - Determination of interfacial tension of oil against water - Ring method
9 Procedure
9.1 Determine the density of the test portion (clause 8) to the nearest 0.001 g/ml at 25 °C.
9.2 Introduce 50 to 75 ml of distilled water at a temperature of 25 +/- 1 °C to a freshly cleaned test portion container (4.3) and place it on the adjustable platform of the tensiometer. Clean the platinum ring (4.2) and suspend it from the tensiometer (4.1). Raise the adjusting platform until the ring is immersed in the water to a depth not exceeding 6 mm in the centre of the test portion container as determined by visual observation.
9.3 Slowly lower the platform, maintain the torsion arm in the zero position by increasing the torque of the ring system. As the film of water adhering to the ring approaches the breaking point, proceed slowly with adjustment to ensure that the moving system will be in the zero position when rupture occurs. Using the scale reading at which this occurs, calculate the tension of the water test portion as specified in 10.1 using the value 0.997 g/ml for the difference of density of water and air (δ0 - δ1); a value of 71 to 72 mN/m should be obtained. If low values are found, they may be due to improper adjustment of the tensiometer or improperly cleaned apparatus. In this case, make the following readjustments: clean the test portion container with hot chromic acid cleaning solution, rinse, and repeat the measurement. If a low value is still obtained, further purify the distilled water (for example, by redistilling from an alkaline solution of potassium permanganate).
9.4 Return the tensiometer scale to zero and raise the adjustable platform until the ring is immersed in the distilled water to a depth of about 5 mm. Pour the filtered oil, previously brought to a temperature of 25 +/- 1 °C, on the water to a depth of about 10 mm. Take care that the ring does not touch the oil-water interface. If a ring with a short stirrup is used, keep the oil level below the top of the stirrup to prevent bridging. If this is not possible, break the bridge with a suitable clean, sharp instrument as soon as possible after withdrawing the stirrup from the oil.
9.5 Allow the oil-water interface to age for 30 +/- 1 s, then slowly lower the platform, maintaining the torsion arm in the zero position by increasing the torque of the ring system. As the water adhering to the ring approaches the breaking point, proceed slowly with adjustment to ensure that the moving parts will be in zero position when rupture occurs. Time these operations so that, as nearly as possible, 30 s are required to draw the ring through the interface. Proceed very slowly as the breaking point is approached, since the break is usually sluggish and too rapid movement may result in a high reading. Complete the entire operation, from the time of pouring the oil into the test portion container until the film ruptures, in about 1 min. Record the scale reading at which the ring breaks free from the interface.
