Determining the viscosity of unknown liquid

Experiment 10: TO STUDY THE RATE OF FLOW OF LIQUID THROUGH OSTWALD’S VISCOMETER AND DETERMINING THE VISCOSITY OF UNKNOWN LIQUID

OBJECTIVE

TO STUDY THE RATE OF FLOW OF LIQUID THROUGH OSTWALD’S VISCOMETER AND DETERMINE THE VISCOSITY OF UNKNOWN LIQUID (ALCOHOL)

APPARATUS REQUIRED

1. Ostwald’s viscometer
2. Specific gravity bottle
3. Beaker
4. Balance
5. Pipette

CHEMICALS REQUIRED

Given liquid (ethyl alcohol)

THEORY

Viscosity is defined as resistance to the flow of liquid. That is the internal friction that one layer of liquid offers to another layer. The viscosity of liquid compared to water can be determined using Ostwald’s viscometer by using the following relation:

\begin{align*} 
\frac{\eta _{a}}{\eta _{w}} &= \frac{d_{a}t_{a}}{d_{w}t_{w}}\\
\eta _{a} &= \frac{d_{a}t_{a}}{d_{w}t_{w}}\times \eta _{w}
\end{align*}

Where,
η_a = viscosity of alcohol
η_w = viscosity of water
d_a = density of alcohol
d_w = density of water
t_a = time of flow of alcohol
t_w = time of flow of water

PROCEDURE

First of all, Ostwald’s viscometer was taken and filled with water up to the starting mark. The water was then allowed to fall until the final mark. The flow time was calculated thrice. After that, alcohol was filled up to the starting mark and the flow time was noted thrice. At last, the rate of flow of alcohol was calculated by using the formula.

CALCULATIONS

Lab temperature = 20 °C
Viscosity of water at lab temp. (η_w) = 1.005 centipoise
Weight of empty specific gravity bottle = 9.13 gm
Weight of specific gravity bottle + water = 36.25 gm
Weight of water (m_w) = 36.25 – 9.13 = 27.12 gm
Weight of specific gravity bottle + alcohol = 35.94 gm
Weight of alcohol (m_a) = 35.94 – 9.13 = 26.81 gm

\begin{align*} 
Viscosity\ of\ alcohol\ (\eta _{a}) &= \frac{m_{a}t_{a}}{m_{w}t_{w}}\times \eta _{w}\\
= \frac{26.81 \times 27}{27.12 \times 25.22} & \times 1.005\\
=1.06\ centipoise
\end{align*}

RESULT

The rate of flow of liquid (alcohol) is 27 sec. and the viscosity of alcohol is 1.06 centipoise.

CONCLUSION

The viscosity of a liquid can be obtained through Ostwald’s viscometer.

PRECAUTIONS

1. Ostwald’s viscometer should be clean.
2. The exact time should be determined.
3. Ostwald’s viscometer should be kept vertical.