Pure liquids and true solutions have a constant viscosity at a given temperature and pressure.  Such fluids are known as Newtonian fluids.  The viscosity of these fluids decrease with increased temperature.  This suggests the existence of molecular clustering. or association in liquids.  An increase in temperature or kinetic energy disrupts these associations with a resulting decrease in viscosity.  Solutions of liquids which associate to a great deal or have a high dipole interaction may vary greatly in viscosity.  Such solutions may exhibit a maximum in their viscosity greater than either component.  An ideal solution, however, would exhibit a viscosity that is a direct sum of the contribution of each component on a mole fraction bases.  This is an inverse relationship and can be expressed mathematically as  1/ viscosity of the solution = (1/viscosity of component one) times the mole fraction of component one + (1/viscosity of component two) times the mole fraction of component two.

In liquid polyphasic pharmaceuticals (dispersions of gums or other polymers) the rate of shear is not directly proportional to the shearing stress and the viscosity is not constant.  Such fluids are called non-Newtonian fluids.  Because of the difficulty in measuring these mixtures we will only evaluate Newtonian fluids in this laboratory.  However, the nature of non-Newtonian mixtures will be discussed in lecture.

Clinically we know that changes in the viscosity of the blood can indicate a disease state.  The erythrocyte sedimentation rate is an indication of the viscosity of the plasma.

PROCEDURE:

Absolute viscosity can be measured directly by measuring the time required for a measured volume of liquid to flow through a capillary tube under an applied pressure.  The experimental determination of absolute viscosity is a difficult task.  The measurement of relative viscosity is a easy task.  We will, therefore, measure relative viscosity by using reference fluids with known viscosities. We will use an Ostwald viscometer.  You will have the opportunity to view a video of the correct use of the viscometer.  You will also need to be able to use a Westphal balance to measure the density of solution you prepare.  This will also be demonstrated on the video.

With the Ostwald viscometer the pressure driving the liquid through the capillary depends on the difference in the liquid level (h) and the density of the liquid(rho) and the acceleration due to gravity (g).  This is often called hydrostatic pressure. This pressure represents the shearing stress.  The rate of flow of the liquid through the capillary represents the rate of shear.  Therefore, the viscosity of a liquid is proportional to the hydrostatic pressure divided by the volume of fluid that flows through the capillary per unit time.  If you use the same visometer to make the measurements than the terms h & g and Volume are constant and you can establish that the ratio of the viscosity of one liquid to the viscosity of the second is equal to the ratio of the time it take to pass through the viscometer times the density of the liquid for the two liquids.

Viscosity one/ viscosity two = (Rho one times Time one)/(Rho two times times two)

To obtain good results the apparatus must be clean and dry.  Clamp the viscometer vertically so it can be easily viewed and add an exact quantity of the reference liquid (water).  [The viscosity of water is 1 cps.]  Measure the time is takes for a fixed volume of water to pass through the capillary.  Repeat this measurement at least twice.  Replace the liquid with the test solution and repeat the measurement.

We will perform three procedures (A,B, & C). One student should work on section A, two students should work together on section B, and one student should work on section C.

Part A Using the Oswalt viscometer with a narrow diameter capillary determine the viscosity of acetone.  Record the viscosity.  The density of acetone is 0.789.  The density of water is 1.

Part B Prepare approximately 100 ml of a 0.2, 0.4, 0.6, and 0.8 mole fraction solution of acetone and water.  Using the Westphal balance determine the density of each solution.  Determine the viscosity of each solution and tabulate it together with the results of part A and the known viscosity of water. Plot the relative viscosity against mole fraction with the mole fraction as the abscissa.

The viscosity of an ideal solution is linear function of the concentration; however, few solutions used in pharmacy are ideal.  Various carbohydrate gums and derivatives are employed as suspending or dispersing agents.  Methylcellulose is available in several types designated according to the viscosity of a 2 % aqueous solution.  Methylcellulose 1500 for example is that methylcellulose which when dissolved in water at a 2% concentration yields a solution with a viscosity of 1500 cps.

Part C - Prepare solutions of  methycellulose in the concentrations of 1, 0.5, 0.1 and 0.05 % using the stock solution of 2% methycellulose .  Using the large bore Viscometer and the water as a standard determine the relative viscosities of the solutions.  Tabulate the data and plot with the % concentrations on the abscissa.  We will assume that the density of all the solutions is 1. Check the number of the methycellulose that was prepared as the stock solution.  Did its viscosity as you measured it match its nominal viscosity??