Experiment No. 1 Determination Determination of Molar Refraction of a Given Sample Refractometry
A Research Study Presented to the Faculty of the Department of the Chemical Engineering School of Engineering and Architecture Saint Louis University
In Partial Fulfilment of the Requirements for the Degree Bachelor of Science in Chemical Engineering
By: Bullago, Joshua G. Domalanta, Marcel Roy B. Caw-is, Imee A.
February 2017
ACKNOWLEDGEMENT
The researchers would like to extend their warmes t gratitude to everyone who contributed for the success of this experiment: To Engr. Genevieve De Vera, for providing deeper understanding of the topic to the researchers by explaining the theory and principles related to the experiment, as well as the different procedures that must be carried out. To the laboratory custodians, for supervising and assisting the researchers in using the instruments needed for the experiment. To God, for giving the researchers the strength and patience to perform the experiment as best they could.
TABLE OF CONTENTS
TITLE PAGE
i
ACKNOWLEDGEMENT
ii
TABLE OF CONTENTS
iii
LIST OF TABLES
iv
LIST OF FIGURES
v
ABSTRACT
vi
Chapter 1:
INTRODUCTION
1
Chapter 2:
DESIGN AND METHODOLOGY
5
Chapter 3:
RESULTS AND DISCUSSION
6
Chapter 4:
CONCLUSION AND RECOMMENDATION
10
REFERENCES
11
APPENDICES
12
LIST OF TABLES
Table 3.1: Molar refraction of samples
LIST OF FIGURES
Figure 2.1: Schematic diagram of procedures Figure D.1: The refractometer used for the experiment Figure D.2: Some reagents used for the experiment Figure D.3: Looking through the refractometer Figure D.4: Displaying the index of refraction reading
ABSTRACT
Chapter 1 INTRODUCTION
The speed of light in a vacuum is always the same, but when light moves through any other medium it travels more slowly since it is constantly being absorbed and reemitted by the atoms in the material. The ratio of the speed of light in a vacuum to the speed of light in another substance is defined as the index of refraction (aka refractive index or n) for the substance.
Whenever light changes speed as it crosses a boundary from one medium into another its direction of travel also changes, i.e., it is refracted (Figure 1). (In the special case of the light traveling perpendicular to the boundary there is no change in direction upon entering the new medium.) The relationship between light's speed in the two mediums (vA and vB), the angles of incidence (qA) and refraction (qB) and the refractive indexes of the two mediums (nA and nB) is shown below:
Thus, it is not necessary to measure the speed of light in a sample in order to determine its index of refraction. Instead, by measuring the angle of refraction, and knowing the index of refraction of the layer that is in contact with the sample, it is possible to determine the refractive index of the sample quite accurately. The refractive index of different substrates measures with refractometers. There are four main types of refractometers:
traditional
handheld
refractometers,
digital
handheld
refractometers,
laboratory or Abbe refractometers, and inline process refractometers. There is also the
Rayleigh Refractometer used (typically) for measuring the refractive indices of gases.A sodium lamp may be used to provide the light source at a known wave-length (589.6 nm) although many instruments are corrected for daylight use. Nearly all refractometers utilize this principle, but may differ in their optical design. In the Abbe' refractometer the liquid sample is sandwiched into a thin layer between an illuminating prism and a refracting prism (Figure 2). The refracting prism is made of a glass with a high refractive index (e.g., 1.75) and the refractometer is designed to be used with samples having a refractive index smaller than that of the refracting prism. A light source is projected through the illuminating prism, the bottom surface of which is ground (i.e., roughened like a ground-glass joint), so each point on this surface can be thought of as generating light rays traveling in all directions. Inspection of Figure 2 shows that light traveling from point A to point B will have the largest angle of incidence (qi) and hence the largest possible angle of refraction (qr) for that sample. All other rays of light entering the refracting prism will have smaller qr and hence lie to the left of point C. Thus, a detector placed on the back side of the refracting prism would show a light
region
to
the
left
and
a
dark
region
to
the
right.
Samples with different refractive indexes will produce different angles of refraction (see Equation 2 above and recall that the angle of incidence and the refractive index of the prism are fixed) and this will be reflected in a change in the position of the borderline between the light and dark regions. By appropriately calibrating the scale, the position of the borderline can be used to determine the refractive index of any sample. In an actual Abbe' refractometer there is not a detector on the back of the refracting prism, and there are additional optics, but this is the essential principle. (It is also possible to design a refractometer based on the reflection of light from the boundary between the prism and the sample. These types of refractometers are often used for continuous monitoring of industrial processes.)
In most liquids and solids the speed of light, and hence the index of refraction, varies significantly with wavelength. (This variation is referred to as dispersion, and it is what causes white light moving through a prism to be refracted into a rainbow. Shorter wavelengths are normally refracted more than longer ones.) Thus, for the most accurate measurements it is necessary to use monochromatic light. The most widely used wavelength of light for refractometry is the sodium D line at 589 nm. If white light were used in the simple Abbe' refractometer optics shown in Figure 2, dispersion would result in the light and dark borderline being in different places for different wavelengths of light. The resulting "fuzziness" of the borderline would make precise work impossible. However, many Abbe' refractometers are able to operate satisfactorily with white light by introducing a set of "compensating prisms" into the optical path after the refracting prism. These compensating prisms are designed so that they can be adjusted to correct (i.e., compensate for) the dispersion of the sample in such a way that they reproduce the refractive index that would be obtained
with
monochromatic
light
of
589
nm,
the
sodium
D
line.
As mentioned earlier, the speed of light in a substance is slower than in a vacuum since the light is being absorbed and reemitted by the atoms in the sample. Since the density of a liquid usually decreases with temperature, it is not surprising that the speed of light in a liquid will normally increase as the temperature increases. Thus, the index of refraction normally decreases as the temperature increases for a liquid (Table 1). For many organic liquids the index of refraction decreases by approximately 0.0005 for every 1 °C increase in temperature.
However
for
water
the
variation
is
only
about
-0.0001/°C.
Many refractometers are equipped with a thermometer and a means of circulating water through the refractometer to maintain a given temperature. Most of the refractive index measurements reported in the literature are determined at 20 or 25 °C.
The study aims to identify the different refractive indices of each of the following substance at particular temperature: chloroform, ethyl acetate, butanol, benzene, sugar solution and carbon tetrachloride. Refractive index has the large number of applications. It is mostly applied to identify a particular substance, confirm its purity, or measure its concentration. Generally it is used to measure the concentration of a solute in an aqueous solution. For a solution of sugar, the refractive index can be used to determine the sugar content (Brix degree). It can be used also in determination of drug concentration in pharmaceutical industry. It is used to calculate the focusing power of lenses, and the dispersive power of prisms. Also it is applied for estimation of thermophysical properties of hydrocarbons and petroleum mixtures. As can be seen in the above rows, the study on refractive index of substrates can be useful in various fields (both industry and academic).
Chapter 2 DESIGN AND METHODOLOGY
The following sets of apparatus were used in the experiment: Abbe refractometer, medicine droppers, small beaker. On the other hand, the following reagents and materials were used: cotton, ethyl acetate, benzene, chloroform, butanol, sugar solution, ethyl
alcohol
and
carbon
tetrachloride.
Calibration with distilled water was first done by the laboratory supervisor. Distilled water using glass dropper was placed on the prism surface of the refractometer. It remained to stand for at least three to five minutes for temperature stability. The turn mode selector was shifted to refractive index option. The eyepiece is then focused until crosshair is clear. The adjustment control was rotated counterclockwise so that the shadow line an crosshair meet. The dispersion correction wheel is also rotated until a possible color free shadow line is produced. Then the shadowline is adjusted to meet the crosshair. The red button and temperature button were pressed. To calibrate further, the mode selector is turned to BX-TC position. The “read” display button was pressed. An LED reading of 000.0 or 0.1 is normal. A negative sign and decimal point however will indicate no reading. If this persists, the adjustment control unit must bte turned until a normal reading can be displayed. Only slight adjustment is required. Reading through the reticle crosshair to shadowline, should measure 000.0. The process is repeated until persistent normal readings occur. Determining the refractive index of the substance has now been made possible. The prism assemble cover is then opened and the protective lens tissue removed. The measuring prism surface is cleaned with cotton and alcohol before any sample has been placed. The mode selector is put to desired position. Adjustment control was turned counterclockwise to
position the shadowline at bottom of field of view. The shadowline is in center and is in center to crosshair for accurate reading. The eyepiece is also adjusted to focused on crosshair. The shadowline is moved to crosshair reticle through coarse adjustment knob. The dispersion correction wheel has been rotated to eliminate any red or green color at edge of shadownline. After
such
conditions
are
met,
the
molar
refraction
was
calculated.
Proper laboratory guidelines were followed on disposing some excess samples and on handling the excess unknown liquid. The sets of apparatus were cleaned thoroughly with water and detergent.
FIGURE 2.1 Schematic diagram of procedures
Chapter 3 RESULTS AND DISCUSSION
A. RESULTS Table 3.1. Molar refraction of samples Sample
n = Index of refraction
Chloroform Ethyl Acetate Butanol Benzene Sugar Solution Carbon Tetrachloride
1.4439 1.3718 1.3980 1.4991 1.3367 1.4581
M= Molecular weight (g/mol) 119.378 88.105 74.122 78.112 342 153.82
ρ = Density (g/cm3)
T= Temperature (0C)
Rm = Molar refraction (cm3/mol)
1.4968 0.8999 0.8089 0.8768 1.02 1.59
20.3 20.3 21.1 21.9 23.0 22.7
21.813 22.2411 22.1158 26.1622 69.6632 26.4022
B. DISCUSSION The Abbe Refractometer used was very precise, accurate and fast in acquiring the respective indexes of refraction of the solutions to be analyzed. Using the equipment, the values needed for the computation for the molar refraction was easily taken. When adjusting the eyepiece, there is a dependence on the clarity and focus of the sample with the eyes of the person manipulating the refractometer because what is clear to one person may not on the other. When adjusting the dispersion correction wheel, there is a great deal to make sure the crosshair is adjusted properly. The refractive index of a liquid varies with temperature that is why there is a must for the researchers to make use of the proper density of the liquid to be analyzed in the proper temperature scale. These values were calculated given the constants given in the Perry’s Chemical Engineers’ Handbook. The molar mass values were also taken in the handbook. The molar refraction was then computed given the formula and the following solution and calculations can be seen in the Appendix Section C.
Chapter 4 CONCLUSION AND RECOMMENDATION
Refractometry is a technique that measures how light is refracted when it passes through a given substance. The amount by which the light is refracted determines the refractive index. Refractive index can be used to identify an unknown liquid compound. Refractive index is defined as the ratio of the velocity of light in air to the velocity of light in the medium being measured. In the experiment the refractometer used was able to determine the index of refraction of Chloroform, Benzene, Ethyl acetate, Sugar solution, Butanol, and Carbon tetrachloride as well as its temperature. The mode in the refractometer was already settled and was ready to use. The sample solution was placed to the prism using a dropper and the prism was closed. Adjustments in both the dispersion correction wheel and the eyepiece was needed primary to the control knob before reading the output value. Since the index of refraction depends on the temperature and the pressure the density at a given temperature was used to correct and achieve the molar refraction of the samples. Combining refractive index and substance density it is possible to define a quantity that is temperature independent. It is called specific refraction and multiplying it with the molar mass gave the molar refraction results in the experiment. The experiment enabled the researchers to successfully manipulate and operate the refractometer as well as determining the molar refraction of different samples while critically capturing the underlying principles of refractometry. To limit the errors in determining the molar refraction by means of the refractometer, the following should be observed; the prism must be wi ped properly and cleaned with alcohol followed by distilled water to avoid contaminants in the sample, do not wipe the prism dry to avoid any residue in the prism that can affect the accuracy of the reading, when rotating the
eyepiece to focus the crosshair, one person can see the crosshair more clearly than another because of human error and visible clarity and it is suggested that only one should operate per liquid, and the shadow line must also be made sure to be below the field of view. When proper care and correct procedure is followed and exercised using the refractometer, this equipment can be used for highly accurate and precise determination of the refractive index of materials that can help to determine the specific refraction and molar refraction of a sample.
REFERENCES
Hanson, J. (2006). Refractometry. Koohyar F (2013) Refractive Index and Its Applications. Journal of Thermodynamics and Catalysis
APPENDIX A List and Uses of Apparatus
Abbe Refractometer - is a highly reliable instrument with high precision that is used to measure the refractive index of liquid samples. Medicine Dropper- a laboratory apparatus used to transfer samples through suction of the liquids in and out of the dropper. Small Beaker- a laboratory apparatus used mainly as containers for the liquid samples and cleaning agent
APPENDIX B Definition of Terms
Refraction- is the bending of a wave when it enters a medium where its speed is different. Refractometry- is a technique that measures how light is refracted when it passes through a given substance. Refractometer- is an instrument that measures the extent to which light is bent, or refracted, when it moves from air into a sample and is typically used to determine the index of refraction of a sample. Refractive index- also known as the index of refraction, this is defined as the speed of light in vacuum divided by the speed of light in the medium. Specific Refraction- is a parameter characterizing the electronic polarizability of a unit mass of a substance in the high-frequency electromagnetic field of a light wave. Molar Refraction- indicates the manner in which a molecule interacts with light. This is a function of the density of the medium.
APPENDIX C Computations
Molar refraction, R m, is calculated by the formula:
=
( − 1) ( + 2)
Where: M = molecular weight; ρ = density of the substance; n = index of refraction For Chloroform:
) = = 21.8130 (1.4439 + 2)(1.4968 )
) = = 22.2411 (1.3718 + 2)(0.8999 )
) = = 22.1158 (1.3980 + 2)(0.8089 )
) = = 26.1622 (1.4991 + 2)(0.8768 )
) = = 69.6632 (1.3367 + 2)(1.0200 )
(1.4439 − 1)(119.378
For Ethyl Acetate:
(1.3718 − 1 )(88.105
For Butanol:
(1.3980 − 1 )(74.122
For Benzene:
(1.4991 − 1 )(78.112
For Sugar Solution:
(1.3367 − 1 )(342
For Carbon Tetrachloride:
) = = 26.4022 (1.4581 + 2)(1.59 ) (1.4581 − 1 )(153.82
APPENDIX D Documentation
Figure D.1: The refractometer used for the experiment
Figure D.2: Some reagents used for the experiment
Figure D.3: Looking through the refractometer
Figure D.4: Displaying the index of refraction reading
