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Determination of Viscosity by Ostwald Viscometer
The viscosity of liquid is a resistance to flow of a liquid. All liquids appear resistance to flow change from liquid to another, the water faster flow than glycerin, subsequently the viscosity of water less than glycerin at same temperature. Viscosity occurs as a result of contact liquid layers with each other. The viscosity is measuring by Ostwald viscometer. Relative Viscosity is the ratio of the absolute viscosity of the fluid on the viscosity of water at a certain temperature. Ostwald viscometer is a commonly used viscometer, which consists of a U-shaped glass tube held vertically. For more accurate measurements it is held in a controlled temperature bath. It is also known as a glass capillary viscometer. The result is typically expressed in centipoise (cP), which is the equivalent of 1 mPa s (millipascal second). In one arm of the U is a vertical section of precise narrow bore (the capillary). Above there is a bulb, with it is another bulb lower down on the other arm. In use, liquid is drawn into the upper bulb by suction, and then allowed to flow down through the capillary into the lower bulb. Two marks (one above and one below the upper bulb) indicate a known volume. The method of determining viscosity with this instrument consists of measuring the time for a known volume of the liquid (the volume contained between the marks A and B) to flow through the capillary under the influence of gravity. The time taken for the level of the liquid to pass between these marks is proportional to the kinematic viscosity. The time required for the test liquid to flow through a capillary of a known diameter of a certain factor between two marked points is measured.
The instrument must first be calibrated with materials of known viscosity such as pure (deionized) water. Knowing the value of viscosity of one liquid, one can calculate the viscosity of other liquid.
η1 = ρ1t1/ρ2t2 ×η2
where η1 and η2 are viscosity coefficients of the liquid and water, and ρ1 and ρ2 are the densities of liquid and water, respectively.
Chemicals and materials:
1. Distilled water
2. Beaker
3. Ostwald viscometer
4. Pipette
5. Weighing machine
6. Stop watch
7. Sample liquid
Procedure:
1. Clean the viscometer by the water and ethanol and dry it in oven for 10-15 min at 50o C.
2. Fill the bulb 1 completely with fixed amount of water with the help of pipette.
3. Through the rubber tube, water is sucked to the level above the upper mark A.
4. Then water is allowed to flow down. When water meniscus reaches the mark A, the stopwatch is started.
5. When the meniscus reaches the mark B, the stopwatch is stopped.
6. This time to flow the liquid from mark A to mark B is recorded for water. Averages of three determinations were taken to measure the accurate value. The time of flow of sample liquid represents t1.
7. Similarly the above procedure is repeated with the liquid sample. Averages of three determinations were taken to measure the accurate value. The time of flow of sample liquid represents t2.
8. The densities of water and sample liquid are determined by using specific gravity bottle.
9. The viscosity of sample liquid is determined by substituting the value in the equation given.
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