The Viscosity of Liquids

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  • Topic: Viscosity, Surface tension, Liquid
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39. The Viscosity of Liquids

After studying the present lecture, you will be able to

Define viscosity and viscosity coefficient
Outline the method to measure viscosity using Ostwald viscometer Determine the average molecular weight of a polymer

Determine the surface concentration of 1-butanol in aqueous solution

Measure the distribution coefficient of a solute betweenn two solvents

39.1 Introduction

Viscosity, one of the transport properties, arises because of intermolecular attractive and relatively long-range forces. Viscosity coefficient ([pic]), a specific constant characteristic of a liquid could be expressed by the following equation of Poiseuille.

[pic] (39.1)

where V is the volume of liquid delivered in time t, through a capillary of radius r and length L, with a hydrostatic pressure P.

In an apparatus designed so that equal volumes of liquids can flow through the same capillary of length L and radius r, ( may be written as

[pic] (39.2)

If g remains constant in any given location and h, the height through which the liquid falls is kept constant, above equation becomes,

[pic] (39.3)

where k is the dimensional constant of the apparatus.

If two liquids are compared using the same apparatus, it follows that

[pic] (39.4)

If [pic], the coefficient of viscosity of one of the substance, is known from a previous measurements, then[pic], the viscosity of the other liquid, can be calculated from the measured quantities, d1, d2, t1 and t2.

30.2 Viscosity measurement of a liquid:

The viscosity of two liquids can be compared by making use of an Ostwald Viscometer which consists of two bulbs, one attached with a capillary tube and U-tube below the capillary tube while other bulb is attached to the other arm of u-tube at a level lower than the other bulb. The liquids of known densities are allowed to flow through the capillary maintaining the same differences of levels in the limbs and the time equation which governs the flow lead to the relation:

[pic] (39.5) where (1 and (2 are viscosity coefficients of the liquid and water, respectively. d1 and d2 are the densities of liquid and water, respectively. Knowing the value of viscosity of one liquid, one can calculate the viscosity of other liquid.

The procedure for measurement of viscosity is as follows. The viscometer is fixed vertically on the stand and 10 mL or 20mL of water is pipetted into the lower bulb. The volume of water (10 mL or 20 mL) is chosen so that the liquid can be conveniently sucked into the upper bulb leaving some in the lower bulb. It is sucked up into the other bulb to a point about the mark above the bulb. Now it is released and stop clock is started when the meniscus crosses the mark. The clock is stopped when the mark below the bulb is passed. The time is recorded at the moment. The same procedure is repeated twice or thrice and their average is used in calculations.

Similarly, the experiment is repeated with the given liquid. Using the specific gravity bottle, one can determine the specific gravity of the liquid and calculate the viscosity. The viscosity of water at room temperature is used from the tables.

39.3 Experiment: to determine the average molecular weight of the polymer

Viscosity of a polymer solution could be studied to determine average molecular weight of the polymer. An average molecular weight is calculated because the polymer molecules do not all have the same mass.

Empirically it has been found that the intrinsic viscosity is sensitive both to the shape and molecular weight of the macromolecular...
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