VISCOSITY

VISCOSITY

Viscosity is the resistance to flowing of a liquid, and like other properties of liquids, result from the intermolecular forces of attraction. As temperature is increased, the liquid’s viscosity decreases. The viscosity of a liquid can be determined by measuring the time required for a specified volume of liquid to empty through a capillary tube in a simple laboratory apparatus called a viscometer. Liquids with moderate to low viscosities may be readily measured in this manner. Using the simple viscometer, a set of simultaneous equations can be derived for determining the viscosity, n, of any liquid by measuring the flow time in seconds for the liquid. In equation (l), A is a constant, dependent on the volume capacity of the viscometer, B is a constant, dependent upon the length and radius of the capillary, D is the density of the liquid, and t is the flow time of the liquid in seconds. So that constants A and B may be conveniently determined for your viscometer, hexane and water must have their flow times measured.

By obtaining their respective viscosities and densities from the CRC Handbook of Chemistry and Physics, the following two simultaneous equations may be solved for A and B by using determinants or substitution.

In equation ( 2 ) , the viscosity

nz = ADztz - BDz/tz ( 3 )

of water, ni , and the density of water, Di , at laboratory temperature are combined with water’s flow time at the same i.=?m:.erature resulting in only A and B as unknowns in the eqiistin .

Likewise, in equation ( 3 ) , the viscosity of hexane, n2, and the density of hexane, Dz, at laboratory temperature are combined with hexane’s flow time at the same temperature. Since viscosity is a temperature dependent property, the viscosity value for hexane may not be found in the CRC Handbook for the specific laboratory temperature at which the flow times were measured. If this is the case, use linear interpolation for two viscosity values closest to the laboratory temperature. Once the constants have been determined for the viscometer, the viscosity of any liquid may be determined by measuring the flow times in the viscometer and the density of the liquid. For highly viscous materials, such as molasses and motor oils, a Brookfield viscometer is used to mechanically measure the viscosity by the drag of the liquid on a moving metal cylinder at -a fixed speed. Both methods will be used in this experiment.

Procedure

Section A

Fill your viscometer with hexane so that the liquid level at the bottom does not reach the capillary tube in the viscometer. Using a pipet bulb draw the hexane up to fill the calibrated volume area, ensuring that the liquid is drawn above the top volume line. Start the flow time measurement as the liquid passes the top line and stop timing when the liquid passes the bottom line of the volume area. All flow times should be measured at a constant temperature. If the viscometer is submerged in a beaker of room temperature water, a fairly constant temperature can be maintained. Measure the flow time of hexane several times. Pour the used hexane into the appropriate container in the fume hood. Use a pipet bulb to force air through the viscometer’s capillary to remove all traces of hexane. The viscometer can also be placed in the drying oven but must be allowed to cool before measuring the flow time for water. After the flow time for water at room temperature is measured several times, placed the viscometer in an ice-water bath for ten minutes. Record the temperature of the ice-water bath and then measure and record the flow time for water at this temperature. Calculate the constants A and B.

Measure the flow times for the following compounds: methanol,propanol, isopropanol, acetone, reagent alcohol and heptane. Dispose of the liquids each time in the container in the fume hood. Be sure that the viscometer is free of all traces of residual liquid before starting the measurement of subsequent flow times. The density of reagent alcohol will have to be determined since it is a mixture of alcohols. See the appropriate sections in the Chemical Technician’s Ready Reference Handbook, CTRRH, for the pycnometer or hygrometer method.

Section B

The instructor will demonstrate proper use of the Brookfield viscometer. You will be supplied with a group of commercial products such as shampoo, creme rinse, olive oil, corn oil, and glycerin. The accuracy of the Brookfield can be checked by comparing values for the oils with those found in the CHC Handbook of Chemistry and Physics.

Requirement for Report

items: name of compound, compound’s formula weight, compound’s density, compound’s structure, compound’s polarity, compound’s flow time average, compound’s calculated viscosity, compound’s handbook viscosity, relative error for compound’s calculated viscosity for Section A. Show the calculations for the constants A and B. For Section B construct a table containing the following data: sample, spindle number, speed, dial reading, multiplying factor, viscosity, handbook viscosity if applicable.

Questions

1. Analyze the data and results from Section A and answer the

following questions:

a) Why are the viscosities of propanol and isopropanol different?

b) Why are the viscosities of isopropanol and acetone different?

c) Does the viscosity of reagent alcohol occur in the sequence of alcohols where you would expect to find its value?

d) Reagent alcohol is a mixture of the alcohols you have measured and ethanol. Using the percent composition of reagent alcohol from its label and the handbook values for the pure alcohcls, calculate an expected viscosity and compare this to the measured value.

2. How do the measured viscosities from the Brookfield viscometer

compare to the handbook values?

3. Use the CTRRH or chemical supplier catalog to describe another

device for measuring viscosity. Take care to describe the method used and how the device operates. Cite your reference source.

WILS BEHGSTHOM, M.S. and MCIRLYS HOWELLS, Ph.D.

St. Paul Technical Institute