Pharmaceutical Analysis

Chapter 3

Viscosity Determination

INTRODUCTION

Viscosity is defined as the resistance in the flow of liquid. It is also defined as the internal friction between two layers of liquid which resists the flow of liquid.

George Gabriel Stokes derived an expression in 1851 known as Stokes’ law.

 

F_d = −6Π_μRV_s

 

where F_d = frictional force; μ = dynamic viscosity; R = radius of the spherical object; V_s = viscosity.

Then he defined the viscosity as the resistance in the flow of liquid at applied stress.

FACTORS AFFECTING VISCOSITY

• Size of molecules: High particle size solutions show high viscosity. That is, the size of the molecules is directly proportional to the viscosity.
• Shape of the molecules: Spherical molecules show low viscosity.
• Inter-molecular ...