# 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 = [pr

^{4}𝝅t] / 8lη

where η is coefficient of viscosity

or, η = [pr

^{4}𝝅t] / 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 w

_{1}and w

_{2}respectively then-

d

_{1}/d

_{2}= w

_{1}/w

_{2}

where, d

_{2}is the density of water (taken to be unity) and d

_{1}is that of liquid. Therefore, the time of flow of liquid-

t

_{2}∝ η/d

_{2}

or, η = k d

_{2}t

_{2}

or, η

_{2}= k d

_{2}t

_{2}----- Equation-1

where, η

_{2}is the viscosity of liquid.

Similarly, for water-

η

_{1}= k d

_{1}t

_{1}----- Equation-2

From Equation-1 and Equation-2, we have-

η

_{2}/η

_{1}= d

_{2}t

_{2}/ d

_{1}t

_{1}

where, η

_{2}/η

_{1}is relative viscosity of liquid with respect to water, t

_{1}and t

_{2}are the time of flow of water and liquid respectively. Taing the viscosity of water under the experiment as unity.

Therefor, η

_{2}= d

_{2}t

_{2}/ d

_{1}t

_{1}

Hence, η

_{2}is the relative viscosity of liquid.