h1

Experiment: Relative and Absolute Viscosities of Distilled Water, Methanol, Ethanol and Propanol

December 21, 2008

EXPERIMENT

VISCOSITY

Abstract

To measure the relative and absolute viscosities of distilled water, methanol, ethanol and propanol , Ostwald – Cannon- Fenske viscometer was standardized and used.  The group recorded the time required for the upper meniscus of the liquid samples to subsequently pass the two calibration marks of the viscometer at a specified temperatures (40, 50 and 60˚C ) and observed that as the temperature of the sample increases, the time necessary from one point to another decreases.  A pycnometer was then used to measure the density of the samples at 40, 50, 60 ˚C through ascertaining the volume of the apparatus and mass of the liquid.  Through the data we had gathered, relative viscosity can be computed using the formula (μ1 2 ) = (ρ1/ ρ2) * (t1/t2) and found out that as the temperature of a liquid increases, its relative viscosity decreases…

I. Introduction

When two solid bodies in contact move relative to each other, a friction force develops at the contact surface in the direction opposite to motion.  To move a table on the floor, for example, we have to apply a force to the table in the horizontal direction large enough to overcome the friction force.  The magnitude of the fore needed to move the table depends on the friction of coefficient between the table and the floor.

The situation is similar when a fluid moves relative to a solid or when two fluids move relative to each other.  It appears that there is a property that represents the internal resistance of a fluid to motion or the “fluidity”, and that property is viscosity.

Viscosity is a measure of the resistance of a fluid to being deformed by either shear stress or extensional stress. It is commonly perceived as “thickness”, or resistance to flow. Viscosity describes a fluid’s internal resistance to flow and may be thought of as a measure of fluid friction.  There are actually two quantities that are called viscosity. The quantity defined above is sometimes called dynamic viscosity, absolute viscosity, or simple viscosity to distinguish it from the other quantity, but is usually just called viscosity. The other quantity called kinematic viscosity (represented by the symbol ν “nu”) is the ratio of the viscosity of a fluid to its density (Mott, R. L. 2006).

II. Review of Related Literature

Viscometer

An instrument used for measuring the viscosity and flow properties of fluids. A commonly used type (Brookfield) measures the force required to rotate a disc or hollow cup immersed in the specimen fluid at a predetermined speed.

Of the many types, some employ rising bubbles, falling or rolling balls, and cups with orifices through which the fluid flows by gravity. Instruments for measuring flow properties of highly viscous fluids and molten polymers are more often called plastometers or rheometers (J.M, C. a. 2006).

Units of Dynamic viscosity

Poise (symbol: P)

Named after the French physician Jean Louis Marie Poiseuille (1799 – 1869), this is the cgs unit of viscosity, equivalent to dyne-second per square centimetre. It is the viscosity of a fluid in which a tangential force of 1 dyne per square centimetre maintains a difference in velocity of 1 centimetre per second between two parallel planes 1 centimetre apart.

Even in relation to high-viscosity fluids, this unit is most usually encountered as the centipoise (cP), which is 0.01 poise. Many everyday fluids have viscosities between 0.5 and 1000 cP.

Some typical viscosities (cP at 20°C)
air 0.02 motor oil SAE 20 125
acetone 0.3 motor oil SAE 50 540
methanol 0.6 castor oil 986
water 1.0 glycerin 1490
ethanol 1.2 pancake syrup 2500
mercury 1.5 maple syrup 3200
linseed oil (raw) 28 treacle 20,000
corn oil 72 peanut butter 250,000
olive oil 84 window putty 100,000,000

Pascal-second (symbol: Pa·s)

This is the SI unit of viscosity, equivalent to newton-second per square metre (N·s m-2). It is sometimes referred to as the poiseuille (symbol Pl).

One poise is exactly 0.1 Pa·s. One poiseuille is 10 poise or 1000 cP, while 1 cP = 1 mPa·s (one millipascal-second).

Table of equivalents

Dynamic viscosity

symbol

centipoise equivalent

1 kilogram-force second per square metre

kgf·s m-2

9 806.6501248

1 poundal second per square foot

pdl·s ft-2

1 488.164435

1 pound per foot hour

lb (ft·h)-1

0.4133789

1 pound per foot second

lb (ft·s)-1

1 488.1639328

1 pound-force second per square foot

lbf·s ft-2

47 880.2595148

1 pound-force second per square inch (reyn)

lbf·s in-2

6 894 757

1 slug per foot second

slug (ft·s)-1

47 880.25898

(White, F. M. 2008)

III. Methodology

APPARATUS/ MATERIALS

Ostwald – Cannon – Fenske viscometer

distilled water, methanol, ethanol, and propanol

Thermometer

Pycnometer

suction bulb

600 mL beaker

Brookfield viscometer

stopwatch

analytical balance

hot plate with stirrer

stirring rod

Iron stand

iron clamp

starch

PROCEDURE

A. Standardization of the Viscometer

1.        Before the viscometer is used, it must be carefully and thoroughly cleansed.  To ensure this, the tube should be filled some hours previous to the experiment with a detergent solution and then rinsed with distilled water several times by means of suction bulb, and then dried.  After cleaning, the viscometer is clamped vertically into a thermostat bath with all the bulbs immersed.  Enough water is then introduced into the viscometer to fill the large bulb.  The temperature of the water bath must be kept constant.  Constant stirring ensures the temperature uniformly set at room conditions.  Record this temperature.

2.       When the conditions are satisfactory, the liquids in the tube are then drawn through the capillary by gentle suction, to a point somewhat above the upper boundary mark above the upper bulb.  Allow it to run back to its own accord, recording the time required for the upper meniscus to successively pass the two calibration marks.

B. Determination of Relative Viscosities

1.       Prepare a sample of water and the three other liquids samples.  Determine the outflow time for each of these liquids, as directed under Part A and record the time of efflux.  Repeat at temperatures 40, 50 and 60 ˚C.  For every liquid, make three readings at each temperature.

2.       Determine the density of each liquid at 40, 50 and 60 ˚C using a pycnometer.  To do this, weigh the empty pycnometer and then, fill it to the brim with sample and gently cover with the lid.  The volume of the liquid is the volume marked on the pycnometer.  Make sure that there are no bubbles present inside.  Wipe the pycnometer dry on the outside.  Weigh the pycnometer containing the sample.

C. Determination of the Absolute Viscosities (optional)

1.       Prepare and heat to 95˚C about 400 mL of 5% starch – water (SW) mixture and continuously stir the mixture until homogenous.

2.       Transfer the starch – water sample to a smaller beaker and use the Brookfield viscometer for viscosity measurement (use the smallest spindle first). Make sure that spindle is submerged in the solution.  Record the temperature and at which the viscosity reading was taken.  Do this every 5 minutes for 30 minutes.  Repeat the procedure using another 2 spindles.  (WARNING:  Be careful, in removing and placing spindles. Ask the help of your lab instructor)

3.       Repeat steps 1 and 2 for 10 % and 15 %

IV. Data and Discussion

A. Standardization of the Viscometer

Table I: Average time of a liquid sample to pass the two calibration marks at a given temperature (40˚C, 50˚C & 60˚C)

Liquid Sample

AVERAGE TIME REQUIRED TO PASS  FROM THE CALIBRATION MARKS  AT A GIVEN TEMPERATURE

@ 40˚C (s) @ 50˚C (s) @ 60˚C (s)
Distilled Water (H2O) 3.05 2.90 2.61
Methanol (CH3OH ) 2.92 2.86 2.74
Ethanol (CH3CH2OH ) 4.16 3.84 3.55
Propanol (CH3CH2CH2OH ) 5.60 5.09 4.87

12

  • The data in the graph shows the relationship of temperature and time. Notice that as the temperature of liquid sample increases, the time necessary to flow decreases.
  • The liquid sample that has fastest time to flow was methanol (purple line) (2.92 to 2.74), while propanol ( blue green line) said to be the slowest (from 5.60 secs to 4.87 secs)
Graph 1: Time vs Temperature of the Liquid samples

B. Determination of Relative Viscosities of Liquid Samples

Volume of Pycnometer = 24. 755 cm3
mass of pycnometer w/o liquid sample = 42.7 g
mass of pycnometer w/o liquid sample & thermometer = 31.4 g

Table II-A: Determination of Relative Viscosities of Distilled Water (H2O) at a given temperature (40˚C, 50˚C & 60˚C)

Temperature Mass of pycnometer w/ liq (g) Density ( g / cm3 ) Relative Viscosities ( μ ) (cP)
Initial Temperature 55.9 -
@ 40˚C 55.6 0.9776 0.6529
@ 50˚C 55.7 0.9816 0.5468
@ 60˚C 55.7 0.9816 0.4665

21

  • The data in the graph shows the relationship of viscosity and temperature of a distilled water at 40, 50 and 60˚C
  • The graph tells us that as the temperature of the distilled water increases, its viscosity decreases (from 0.6529 @ 40˚C to 0.4665 @ 60 ˚C)

Table II-B: Determination of Relative Viscosities of Methanol (CH3OH) at a given temperature (40˚C, 50˚C & 60˚C)

Temperature Mass of pycnometer w/ liq (g) Density ( g / cm3 ) Relative Viscosities ( μ ) (cP)
Initial Temperature 62.1 -
@ 40˚C 61.9 0.7756 0.4959
@ 50˚C 61.5 0.7594 0.4172
@ 60˚C 61.0 0.7392 0.3688
  • The data in the graph shows the relationship of viscosity and temperature of methanol at 40, 50 and 60˚C
  • The graph tells us that as the temperature of the methanol increases, its viscosity decreases (from 0.4959 cP @ 40 ˚C to 0.3688 cP@ 60˚C)
  • Methanol gathered the smallest value of viscosity compared to the other liquids.

31

Table II-C: Determination of Relative Viscosities of Ethanol (CH3CH2OH) at a given temperature (40˚C, 50˚C & 60˚C)

Temperature Mass of pycnometer w/ liq (g) Density ( g / cm3 ) Relative Viscosities( μ ) (cP)
Initial Temperature 62.8 -
@ 40˚C 62.4 0.7958 0.7249
@ 50˚C 62.0 0.7796 0.5750
@ 60˚C 61.8 0.7716 0.4988
  • The data in the graph shows the relationship of viscosity and temperature of ethane alcohol at 40, 50 and 60˚C
  • The graph tells us that as the temperature of the methanol increases, its viscosity decreases (from 0.7249cP @ 40 ˚C to 0.4988 cP @ 60˚C)

.

41

Table II-D: Determination of Relative Viscosities of Propanol (CH3CH2CH2OH) at a given temperature (40˚C, 50˚C & 60˚C)

Temperature Mass of pycnometer w/ liq (g) Density ( g / cm3 ) Relative Viscosities ( μ ) (cP)
Initial Temperature 61.9 -
@ 40˚C 61.6 0.7635 0.9362
@ 50˚C 61.3 0.7514 0.7347
@ 60˚C 61.0 0.7392 0.6554
  • The data in the graph shows the relationship of viscosity and temperature of propanol at 40, 50 and 60˚C
  • The graph tells us that as the temperature of the distilled water increases, its viscosity decreases from 0.9362cP @ 40 ˚C to 0.0.6554 cP @ 60˚C)
  • Propanol gathered the highest value of viscosity compared to the other liquids

51

ANSWER TO THE FOLLOWING QUESTIONS

1.    What is the effect of trapped bubbles in the viscometer during the run on a) the measured time?  b) the relative viscosity? Explain your answer.

  • Trapped bubbles in the viscometer cause great difficulty and large error in the experiment. Based on what we had encountered in the experiment, the effect of trapped bubbles is that its measured time is faster compare to time of a viscometer without bubbles. On the other hand, its effect on the relative viscosity is that it can make result smaller since time is directly proportional to viscosity, which means, the smaller the value of time is, the smaller the value of viscosity will be.

2.    How would you explain the difference in the viscosities of methanol, ethanol, and propanol relative to that of water?  What factors bring about such differences?

  • Based on the experiment we performed, it shows that the viscosity of water is greater than the viscosities of the other three liquid samples. Density is the main factor that brings about such differences. Through the data we gathered, it is apparent that the density of water is greater than these three liquid by about 0 .2000 g/cm3. Since density is directly proportional to the viscosity, therefore, the smaller the density of a liquid is, its viscosity will also be small.

3.    Why is it necessary to measure the viscosity of a liquid (or a gas)?  Give practical examples where such physical property is needed.

  • Measuring the viscosity of a liquid made a vital role to our daily lives. Suppose our blood, if its too thick then it can clot and cause a heart attack or stroke, or if it’s too thin can bleed from a small cut for hours. Doctors have to know the viscosity of our blood when performing operations. On the other hand as future Chemical Engineers, pretending that we are designing the distribution system of water from a water plant for a town. Given the average demand of water for the town for any given time, and we know the viscosity of water, what will be the flow? What pressure will the pipes be under? What size of pipes will be needed? Can the pipe withstand the pressure? Will the water flow smoothly? All these are influenced by the viscosity of water. It gets even more complicated in designing chemical plants, where a lot of different fluids other than water, with different viscosities, have to be considered.

4.    Why do you think does the Brookfield viscometer have different spindles of different sizes?  How does the size of the spindle relate to the viscosity measurement?  With the aide of illustrations, explain your answer.

  • Brookfield Viscometer is used in determining the absolute viscosities (see part C of Methodology). Since the experiment is said to be optional, then we must perform the experiment first before answering the question.

V. Conclusion and Recommendation

After performing this experiment, I therefore conclude that viscosity varies with temperature. In general, the viscosity of a simple liquid decreases with increasing temperature (and vice versa). As temperature increases, the average speed of the molecules in a liquid increases and the amount of time they spend “in contact” with their nearest neighbors decreases (see graph 1). Thus, as temperature increases, the average intermolecular forces decrease. The exact manner in which the two quantities vary is nonlinear and changes suddenly when the liquid changes phase.  Viscosity is normally independent of pressure, this do not varies the value of viscosity. Since liquids are normally incompressible, an increase in pressure doesn’t really bring the molecules significantly closer together.

Since the group experiences unexpected hindrances while performing, I recommend that:

1.       Ostwald viscometer should be cleansed thoroughly before and after conducting the experiment.  Contamination may occur if the apparatus was not cleansed and this may greatly affect the values of the viscosity of the liquid sample.

To ensure that viscometer is clean, the tube should be filled some hours before the experiment with a detergent solution and rinsed with distilled water several times by means of suction bulb. Be sure that it is totally dried.

2.       The viscometer should vertically clamped to the iron stand. Tilted viscometer may affect the time spent of the liquid as well as the reading at which it passes the calibration marks.

3.       Water bath eats lots of time. Prior to set -up, group should start to warm the water. If the water is exceeds the required temperature, it can be cooled by adding enough tap water.  Constant temperature is a must in the experiment. Incorrect temperature reading will make large error in the results.

4.       The member who reads the liquid passes the calibration marks should be the one who should record the time, because he / she is the one who controls the release of air in the tube. It is easier that he / she record the time to avoid errors.

References:

J.M, C. a. (2006). Fluid Mechanics Fundamentals and Application. United Kingdom: Mc Graw – Hill. Viscosity. pg 46

Mott, R. L. (2006). Applied Fluid Mechanics 6th ed. Singapore: Pearson Prentice Hall. Viscosity of Fluids. pg 23-24

Serway, R.A. (1996). Physics for Scientists & Engineers, 4th Edition, Saunders College Publishing.

White, F. M. (2008). Fluid Mechanic 6th ed. New York: Mc Graw – Hill.Viscosity. pg 14

About these ads

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

Follow

Get every new post delivered to your Inbox.

%d bloggers like this: