How to Determine the Viscosity of a liquid at room temperature by viscometer?

 

Experiment:

Determine the Viscosity of a liquid at room temperature by viscometer.


 

Theory:

 

Viscosity is a characteristic property of all fluids. The internal resistance of a liquid to its flow is called viscosity. Viscosity can be explained if a liquid is considered to consist of concentric layers which slide past one another when a liquid flows. Resistance to flow is regarded as an internal friction between the different layers of a liquid. So viscosity is a property of liquids by virtue of which it tends to oppose the relative motion between different layers of the liquid.

Viscosity of a liquid is expressed in terms of Coefficient of Viscosity.

Coefficient of Viscosity is defined as the force (in dynes) per unit area (one square centimeter) required to maintain unit difference of velocity between two parallel layers in the liquid, unit distance (one centimeter) apart. It is denoted by h (eta).

 

Units of Viscosity (Coefficient of Viscosity)

 

It is expressed in dynes. cm-2 or g.cm-1.s-1, also called Poise. Mostly its sub-multiples centipoises and millipoise are used.

 

Centipoise = 10-2 poise

Millipoise = 10-3 poise

 

Apparatus:

 

Ostwald’s viscometer, stop-watch, specific gravity bottle, rubber tube, screw type pinch cock, iron stand etc.

 

Procedure:

 

·         Take an Ostwald’s viscometer and fix it in a stand in upright position.

·         Fix a small rubber tube to the limb with capillary tube to suck the liquid in.

·         Use the screw type pinch cock with rubber tube to control flow rate of the liquid.

·         Fill the viscometer with water to a level a bit lower than the mark B at the upper end of the capillary tube.

·         Suck the water through the rubber tube and allow the water to come above the mark A.

·         Allow the liquid to flow (slowly) and note the time to flow of water from mark A to mark B using a stop watch.

·         Repeat the experiment with the given liquid in the same way as for water.

·         Find the density of given liquid by using a specific gravity bottle.

 

It is advisable to take three readings of flow time for each liquid and then taking mean time to flow.

 

Observations:

 

Room temperature =   oC

 

Liquid

Time of flow (s)

Density (g.cm-3)

Water

 

 

Organic Liquid

 

 

 

Weight of empty specific gravity bottle = W1 g

Weight of empty specific gravity bottle + water =W2 g

Weight of water = W2 – W1 = W3 g

Weight of specific gravity bottle + liquid 1 = W4 g

Weight of liquid = W4 – W1 = W5 g

 

Calculations:

 

Viscosity of given organic liquid can be calculated using following relationship:

h/hw  = (t1 / tw) x (dl / dw)

 

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