Determination of Coefficient of Viscosity Using Ostwald Viscometer
Determination of Coefficient of Viscosity By Ostwald Viscometer
Determination of viscosity of a liquid using Ostwald's viscometer method is based on Hagen-Poiseulle's law. According to Hagen-Poiseulle's law, the volume (V) of homogeneous liquid flowing through a tube having radius 'r' and length 'l' under a presseue difference at the two ends 'p' for time 't' is given by-V = [pr4𝝅t] / 8lη
where η is coefficient of viscosity
or, η = [pr4𝝅t] / 8lV
With the help of above equation, absolute value of coefficient of viscosity can be determined but in practice, the relative viscosity of liquid with respect to water is determined.
Procedure: The liquid is taken in the wide limb and sucked up through the capillary into the right limb slightly above the mark A. The liquid is allowed to flow back and time taken for the meniscus to fall from A to B is noted. The above experiment is then repeated for water taking about the same volume. The same volume of liquid and water are then filled in a dried specific gravity bottle and their weights are determined.
If the weight of liquid and water be w1 and w2 respectively then-
d1/d2 = w1/w2
where, d2 is the density of water (taken to be unity) and d1 is that of liquid. Therefore, the time of flow of liquid-
t2 ∝ η/d2
or, η = k d2t2
or, η2 = k d2t2 ----- Equation-1
where, η2 is the viscosity of liquid.
Similarly, for water-
η1 = k d1t1 ----- Equation-2
From Equation-1 and Equation-2, we have-
η2/η1 = d2t2 / d1t1
where, η2/η1 is relative viscosity of liquid with respect to water, t1 and t2 are the time of flow of water and liquid respectively. Taing the viscosity of water under the experiment as unity.
Therefor, η2 = d2t2 / d1t1
Hence, η2 is the relative viscosity of liquid.
