Experiment No. 4 Measurement of Viscosity of Liquids by Capillary Flow Method Group No.3:
Date Performed: May 7,2012
Dayrit, Elaine Cezarra De Luna, Johnson Dicuangco, Sarah Jane A. Dioquino, Colleen Samantha
Professor: Engr. A.R.D. Marquez
ABSTRACT The experiment is about the determination of the viscosity of a number of normal saturated alcohols by means of an Ostwald viscometer. It studies the effects of salt concentration on the viscosities of aqueous solutions and of temperature on the viscosity of water. The reagents used are distilled water, ethanol, ethylene glycol, 1-butanol, NaCl, and chromic acid. Several materials are used I the determination of viscosity such as Ostwald-Fenske viscometer, constant temperature bath, stopwatch, 10-mL pipettes, and 250-mL beakers. The viscosity of pure liquids is determined with the use of the Ostwald viscometer. The constant A of the viscometer is determined using water which has known density and viscosity. With this constant, other viscosities are computed. The experiment studies some factors affecting the viscosity of fluids. Salt concentrations increase the viscosities of aqueous solutions. Viscosity of liquids is greatly affected by the temperature. Viscosity is inversely proportional to temperature. As the temperature of a fluid increases the viscosity of that fluid decreases. When the liquid is heated, the cohesive forces between the molecules reduce thus the forces of attraction between them reduce, which eventually reduces the viscosity of the liquids.
INTRODUCTION The objective of the experiment is to determine the viscosity of a number of normal saturated alcohols by means of an Ostwald viscometer. It also studies the effects of salt concentration on the viscosities of aqueous solutions and of temperature on the viscosity of water. Viscosity is a measure of the resistance of a fluid which is being deformed by either
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shear stress or tensile stress. In everyday terms, viscosity is thickness or internal friction. (see Figure 1)
When two layers of fluids move past one another with a certain relative velocity, both layers experience a force which tends to oppose their relative motions. The area of the plane of contact between the layers and the velocity gradient normal to it affect the
Experiment No. 4 Measurement of Viscosity of Liquids by Capillary Flow Method magnitude of the force. The effect is expressed as:
where u is the velocity (m/s), du is the difference in velocity between two layers (m/s), dx is the difference in position normal to the flow between two fluid layers (m), and A is the area of contact (m2). The ratio F/A is termed the viscous shear stress ( ). Introducing of a proportionality constant makes equation 1 to:
where μ is the dynamic viscosity or simply the viscosity of fluid (kg/m-s).
Newton's law of viscosity (equation 2), is a constitutive equation. It is not a fundamental law of nature but an approximation that holds in some materials and fails in others. Non-Newtonian fluids exhibit a more complicated relationship between shear stress and velocity gradient than simple linearity. Thus there exist a number of forms of viscosity:
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1. Newtonian: fluids, such as water and most gases which have a constant viscosity. 2. Shear thickening: viscosity increases with the rate of shear. 3. Shear thinning: viscosity decreases with the rate of shear. Shear thinning liquids are very commonly, but misleadingly, described as thixotropic. 4. Thixotropic: materials which become less viscous over time when shaken, agitated, or otherwise stressed. 5. Rheopectic: materials which become more viscous over time when shaken, agitated, or otherwise stressed. 6. A Bingham plastic is a material that behaves as a solid at low stresses but flows as a viscous fluid at high stresses. 7. A magnetorheological fluid is a type of "smart fluid" which, when subjected to a magnetic field, greatly increases its apparent viscosity, to the point of becoming a viscoelastic solid. There are several experimental methods for the determination of the viscosity of fluids such as flow through the capillary tubes, fall of solid spheres through fluids, torque or viscous drag on a rotating disk or cylinder immersed in the fluid, and flow of fluids through an aperture in a plate. The experiment used the Ostwald viscometer which is based on the principles of flow through capillary tubes.
U-tube viscometers are known as glass capillary viscometers or Ostwald viscometers (see Figure 2), named after Wilhelm Ostwald. Ostwald viscometers measure the viscosity of a fluid with a
Experiment No. 4 Measurement of Viscosity of Liquids by Capillary Flow Method known density. It uses a small-diameter tube (capillary). The fluid’s time of flow of a specific volume through the capillary is measured. There is a constant pressure drop across the capillary. Ostwald viscometers must be calibrated with a known material.
The Ostwald-Fenske method makes use of the Hagen-Poiseuille equation for the determination of liquid viscosities by measuring the time of flow of a given volume liquid through a vertical capillary tube under the influence of gravity. For a virtually incompressible fluid, this flow is governed by the Poiseuille’s law in the form
viscometer to fall from the upper to the lower fiducial mark.
Equation 4 can be reduced having ΔP is equal to ρgh:
where A is a constant for a given viscometer which can be determined through calibration with a liquid of known viscosity and density.
METHODOLOGY
Materials where dV/dt is the rate of liquid flow through a cylindrical tube of radius r and length L, and ΔP is the pressure difference between two ends of the tube.
When ΔP is constant, equation 3 becomes
The reagents used in the experiment are distilled water, ethanol, ethylene glycol, 1butanol, NaCl (table salt), and chromic acid. The following equipment and glassware are used namely Ostwald-Fenske viscometer, constant temperature bath, stopwatch, 10mL pipettes, and 250-mL beakers.
Methods
where t is the time required for the upper meniscus of the liquid in the left arm of the
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The experiment is about the determination of the viscosity of a number of normal saturated alcohols by means of Ostwald viscometer. It also tends to determine the effect of salt concentration on viscosities of
Experiment No. 4 Measurement of Viscosity of Liquids by Capillary Flow Method aqueous solutions and of temperature on viscosity of water.
First, the Ostwald viscometer is needed to be set up. The apparatus is cleaned with chromic acid and rinsed with distilled water. Before starting the measurements of the viscosities, the viscometer has been calibrated.
The viscometer is filled with 10 mL of liquid sample in the water bath set at a preferred temperature. The sample must first reach thermal equilibrium before starting the measurement of viscosity.
Using one end of the viscometer which has a larger diameter, the rubber bulb is used to push the liquid sample to the capillary arm. The pushing of the liquid sample is continued until the sample fills the feed bulb and the meniscus which is above the upper fiducial mark.
After the pushing of the sample, the liquid is released and is allowed to flow by its own weight. The time required for the upper meniscus of the liquid in the feed bulb in order for it to pass two calibration marks is recorded.
The same procedure is done with the other liquid samples (saturated alcohol samples).
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Also, the viscosities of aqueous solutions are determined. This part will determine the effect of salt concentration on the viscosities of aqueous solutions. Three salt solutions are prepared with different concentrations. Same procedure is done to each sample for the use of the viscometer.
The last part of the experiment determines the effect of temperature on the viscosity of water. Same procedure is done with the use of the viscometer but each water sample has different temperature. The temperature is increased by 5°C from the room temperature until three water samples are done.
RESULTS AND DISCUSSION Viscosity of Pure Liquids
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear stress or tensile stress. In everyday terms (and for fluids only), viscosity is thickness or internal friction.
The experiment made use of the Ostwald viscometer which is based on the principles of flow through capillary tubes. It used three different pure liquids namely ethanol, ethylene glycol and 1-butanol.
Experiment No. 4 Measurement of Viscosity of Liquids by Capillary Flow Method To determine the viscosity of each liquid, this equation is used:
where μ is the viscosity, ρ is the density of the liquid, t is the time required for the upper meniscus of the liquid in the left arm of the viscometer to fall from the upper to the lower fiducial mark, and A is a constant for a given viscometer which can be determined through calibration with a liquid of known viscosity and density.
The constant A is determined using water with viscosity of 9.8 x 10 -4 kg/m-s and density of 1000 kg/m3. Using the equation above, the constant A can be determined. The time required by the water to fall from the upper and lower fiducial mark is 15 seconds. Thus, the constant A is computed as 5.7 x 10 -8 m2.
The computed A is used to determine the viscosity of the other three samples of alcohol. Table 1 shows the results of the determination of viscosity of pure liquids.
On the three alcohols, ethanol has the shortest time while ethylene glycol has the longest time. The densities of ethanol, ethylene glycol and 1-butanol are 790 kg/m3, 1113.2 kg/m3, and 810 kg/m3
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respectively. Using these values, the viscosities of the liquids are computed and determined.
The computed viscosity and the literature value of viscosity have small percentage of error. One possible cause of error is the recording of time. It is very difficult to start and stop the timer on how the sample liquid would pass the markers. Another source of error is the temperature. The viscosity of a fluid is greatly affected by the temperature. The effects of temperature to the viscosity will be discussed on the later part of the discussion.
Effect of Salt Concentration on Viscosities of Aqueous Solutions
The experiment tried to investigate the effect of salt concentration on viscosities of aqueous solutions. To do so, three different concentrations (0.20 M, 0.50 M and 1.00 M) of salt solutions are prepared.
The viscosity of pure water serves as the basis for the study of the effect of salt concentration since the salt solutions use water as the solvent.
When the first concentration is tested in the Ostwald viscometer, the time it takes for the solution to pass the upper and lower marks
Experiment No. 4 Measurement of Viscosity of Liquids by Capillary Flow Method increases compared to the time of pure water. With this, the viscosity of water increases when salt is added.
As the salt concentration of the solutions increases, the time it takes for the solution to pass the marks increases based on the results of the second and third solutions. This means that salt increases the viscosity of water.
Temperature Effects on Viscosity of Water
In the discussion of the viscosity of pure liquids, temperature is a factor that affects viscosity. In the experiment, the viscosity of water at different temperatures is determined.
This means that viscosity is inversely proportional to temperature. As the temperature of a fluid increases the viscosity of that fluid decreases.
In the liquids, the cohesive forces between the molecules predominates the molecular momentum transfer between the molecules mainly because the molecules are closely packed. It is this reason that liquids have lesser volume than gases. The cohesive forces are in maximum in solids so the molecules are even more closely packed in them. When the liquid is heated the cohesive forces between the molecules reduce thus the forces of attraction between them reduce, which eventually reduces the viscosity of the liquids.
REFERENCES: [1] Atkins, P., & De Paula, J. (2006). Atkins' From the room temperature, the water temperature is increased by 5°C in every trial. This part studied the relationship of viscosity and temperature.
At room temperature, the time taken by the water to pass the upper and lower fiducial marks of the viscometer is 15seconds. When the temperature is increased by 5°C, the time taken by the water to pass the marks is 12 seconds only. As the temperature keeps on increasing the time taken by the water to pass the marks becomes shorter.
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Physical Chemistry 8th Edition. W. H. Freeman and Company [2] Caparanga, A., Baluyut, J. Y., & Soriano, A. Physical Chemistry Laboratory Manual, Part 1 [3] www.udel.edu/pchem/C444/Lectures Lecture3.pdf [4] http:// library.thinkquest.org/C0112681 /Eng/Normal /Magmas/factors.htm [5] http://www.brighthub.com/engineering/ mechanical/articles/10873.aspx
Experiment No. 4 Measurement of Viscosity of Liquids by Capillary Flow Method
APPENDICES Figure 1. Flow of Some Liquids
Figure 2. Ostwald Viscometer
Table 1. Viscosity of Pure Liquids
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Experiment No. 4 Measurement of Viscosity of Liquids by Capillary Flow Method
Flow time through Ostwald viscometer, s 15 22 223 53
Sample Water Ethanol Ethylene glycol 1-Butanol
Calculated viscosity .85 cp .99 cp 14.14 cp 2.44 cp
Literature value of viscosity .85 cp .95 cp 15 cp 2.3 cp
Room Temperature: 32°C
Table 2. Effect of Salt Concentration on Viscosities of Aqueous Solutions
NaCl concentration 0 0.20 M 0.50 M 1.00 M
Flow time through Ostwald viscometer, s 15 16 17 18
Calculated viscosity .85 cp .91 cp .96 cp 1.02 cp
Table 3. Temperature Effects on Viscosity of Water
Temperature Room Room Room Room
temp. temp. temp. temp.
(29°C) + 5°C + 10°C + 15°C
Flow time through Ostwald viscometer, s 14 12 11 10
Sample Computation:
Water at 32°C:
(
)( (
) )(
)
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Calculated viscosity .85 cp .72 cp .66 cp .60 cp
Experiment No. 4 Measurement of Viscosity of Liquids by Capillary Flow Method Ethanol:
Salt Concentration (0.20 M):
Temperature (at 29°C + 15°C):
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