2. Methods


Method 1
USING LASERS


Equipment list:

  • 1x laser
  • 2x A4 paper
  • 1x vanguard sheet
  • 1x acrylic hollow prism
  • 1x masking tape
  • 1x pen


We have found a graph regarding the calibration,from (Hanson. J, 2003).It shows the graph of percentage sucrose against refractive index.


Place laser beam on flat table with elevation. Its beam should be perpendicular to the wall

Place paper in front of laser pointer. Secure to table with tape. The paper will note where the laser beam enters and exits the prism.

Place the prism on the paper, a few centimetres in front of the laser pointer.A face of the prism should be resting on the paper.  Trace the prism base out. If we move the prism, always return it to this location before rotating it, if needed. 

Add elevation to laser pointer till beam points to the middle of the prism.

Tape the cardboard to the table, and then tape the laser pointer to the cardboard. Make sure that neither the cardboard nor the laser pointer can move. If the laser pointer's position changes, our measurements will not be accurate. 

Note: If you have two or more sheets of elevation stacked together, you may need to tape the pieces of elevation together so that they do not slide e.g. paper, cardboard .etc

Tape a big piece of paper to the wall in front of the laser pointer. We will use this paper to note the position laser beam hits the wall.



Figure B is what the setup would look like if we were looking down on it from above. (Note that the diagram is not to scale.)



To measure the angle of minimum deviation, θmd, which we will use to calculate the index of refraction of the tested liquids, we need to mark several points and measure the distances between some of these points. Figure 9, is view of the set up. It illustrates the points we need to mark in order to obtain the angle of minimum deviation, θmd. The steps below explains how to position these points and obtain the angle of minimum deviation, θmd.


















When the prism is rotated correctly, mark the position where the laser beam hits the paper attached to the wall (point a in Figure 9). Label it point a .

On the paper on the table, mark the point where the beam emerges from the prism (point f in Figure 9). Label it point f.

Now we can place the prism to one side. Leave the papers taped in place.

Use a ruler to draw a line from point d to point e. This marks the path of the undiverted beam.

Next, extend a line from point a (on the wall) through point f (on the table). To do this, stretch a string from point a so that it passes over point f. Note the point where the string passes the line between d and e. Mark is point as c.

Measure the distance between points a and b, and record it down. This is distance x (see Figure 9).

Measure the distance between points b and c, and record it down. This is distance L (see Figure 9).

The distances we have measured define the angle of minimum deviation, θmd. The ratio x/L is the tangent of the angle of minimum deviation, θmd. To obtain/calculate the angle, use when the prism is empty (filled only with air), placing it in the laser's path should not divert the beam. Switch on the laser on, and mark the spot where the beam hits the paper taped to the wall. Mark this as point b (point b in Figure 9).    Note: Before testing a new solution, turn on the laser and shine it through an empty prism to make sure that the laser beam still hits point b. If the laser beam no longer hits point b, your measurements will not be accurate. Adjust the laser's position, if necessary, until the undiverted beam hits point b.

With the prism empty, mark where the beam enters the prism on the paper the prism is sitting on (point d in Figure 9). Label it point d.

With the prism still empty, mark where the laser beam exits the prism on the paper the prism is sitting (point e in Figure 9). Label it point e.

Switch off the laser. Fill the prism with plain water. If you moved the prism to fill it with water, return it to the outline you made on the piece of paper. Turn the laser back on.

Rotate the prism so that the path of the refracted beam within the prism (solid blue line from d to f in Figure 9) is parallel with the base of the prism, the side of the prism that has no laser beam hitting it.    Note: A pinch of non-dairy creamer in the liquid can help you see the beam in the prism, and should not have a significant effect on the index of refraction of the liquid. Or, if you do not have non-dairy creamer, take a straight edge and line it up with the laser beam's entrance and exit points (as seen from the top of the setup). Rotate the prism until the straight edge connecting those two points is parallel to the side of the prism that the laser beam does not hit.

Use your calculator (or ecalc.com) to find the arctangent of x/L. (The arctangent of x/L means "the angle whose tangent is equal tox/L.") Record the angle and its units (radians or degrees) down.

Calculation of Index of Refraction:

Use the angle of minimum deviation to calculate index of refraction as shown in Equation 5.
23.  Equation 5 : 

n = 2.00056 × sin[0.5(θmd + 60°)]

n = index of refraction of solution (unit-less, since it is a ratio)
θmd = angle of minimum deviation (degrees)



Method 2
USING REFRACTOMETER


Equipment list:

-1x refractometer 

A refractometer is a laboratory or field device for the measurement of an index of refraction (refractometry).

Begin the calibration of our refractometer by lifting up the daylight plate and placing 2-3 drops of distilled water on top of the prism assembly. 
Close the daylight plate so the water spreads across the entire surface of the prism without any air bubbles or dry spots. 
Allow the test sample to sit on the prism for approximately 15 seconds before we attempt calibration in the next step. This allows the sample to adjust to the ambient temperature of the refractometer.
Hold the refractometer in the direction of a natural light source and look into the eyepiece. We should see a circular field with graduations down the center. We may have to focus the eyepiece to clearly see the graduations.
Turn the calibration screw until the boundary between the upper blue field and the lower white field meet exactly at ZERO on the scale.


















Method 3
USING BRIX HYDROMETER

Equipment list:

-1x measuring cylinder 
-1x BRIX hydrometer

A hydrometer is an instrument used to measure the specific gravity (or relative density) of liquids; that is, the ratio of the density of the liquid to the density of water. A hydrometer is usually made of glass and consists of a cylindrical stem and a bulb weighted with mercury or lead shot to make it float upright.

Steps:
1.Fill the glass cylinder with sugar drink 
2.Put the hydrometer with the bulb end down. It will bob up and down in the sample. Note that the liquid  may overflow from the cylinder.
3.Assure that the hydrometer is not in contact with the sides of the cylinder and take the reading.

Once the hydrometer has stopped bouncing up and down and the hydrometer is not touching the walls of the cylinder, a reading can be made. Note that a meniscus forms on the neck of the hydrometer. Just as reading the meniscus in a graduated cylinder, the user must take the reading where the plane of water is and not where the water clings up the neck of the  hydrometer. 

Temperature Adjustment: for hydrometers calibrated for 20 degrees C or 68 degrees F

Add or subtract factors in chart from observed readings for given temps.
Temperature adjustment factors are usually given on most hydrometer instruction inserts.  Hydrometers are calibrated for different temperatures.  Check the hydrometer, if it is not calibrated for 20 degrees C or 68 degrees F, do not use this chart.   

Method 4
POLARISER

Equipment list:

  • 1x mooncake box
  • 1x lamp
  • 2x polariser lens
  • 1x PVC pipe
  • 1x cylinder 
  • 1x glue
  • 1x lamp 
In order to understand more about the Polariser and the purpose of using it, we have copied the following from Robert Carey(2011)

  Most physical properties of enantiomers i.e., melting point, boiling point, refractive index, etc. are identical. However, they differ in a property called optical activity, in which a sample rotates the plane of polarization of a polarized light beam passing through. This effect was first discovered in 1808 by E.L. Malus, who passed light through reflective glass surfaces. Four years later, J.B. Biot found that the extent of rotation of the light depends on the thickness of the quartz plates that he used. He also discovered that other compounds i.e., sucrose solutions were capable to rotate the light. He attributed this "optical activity" to the certain features in their molecular structure (asymmetry). Because of his research, he designed one of the first polariscopes, and formulated the basic quantitative laws of polarimetry. In 1850, Wilhelmy used polarimetry to study the reaction rate of the hydrolysis of sucrose. In 1874, van't Hoff proposed that a tetrahedral environment of the carbon atom could explain the phenomenon of optical activity. Today, polarimetry is used routinely in quality and process control in the pharmaceutical industry, flavor, fragrance and essential oil industry, food industry, and chemical industry. The optical purity of the product can be determined by measuring the specific rotation of compounds like amino acids, antibiotics, steroids, vitamins, lemon oil, various sugars, and polymers and comparing them with the reference value (if the specific rotation of the pure enantiomer is known).

  How does it work? Normal monochromatic light contains light that possesses oscillations of the electrical field in all possible planes perpendicular to the direction of propagation. When light is passed through a polarizer (i.e., Nicol prism, Polaroid film) only light oscillating in one plane will leave the polarizer ("picket fence model"). This linear polarized light can be described as a superposition of two counter-rotating components, which propagate with different velocities in an optical active medium. If one component interacts stronger than the other with a chiral molecule, it will slow down and therefore arrive later at the observer. The result is that the plane of the light appears to be rotated.

  In a polarimeter (figure 2), plane-polarized light is introduced to a tube (typically 10 cm in length, figure 3) containing a solution with the substance to be measured. If the substance is optical inactive, the plane of the polarized light will not change in orientation and the observer will read an angle of [a]= 0o. If the compound in the polarimetry cell was optical active, the plane of the light would be rotated on its way through the tube. The observed rotation is a result of the different components of the plane polarized light interacting differently with the chiral center. In order to observe the maximum brightness, the observer (person or instrument) will have to rotate the axis of the analyzer back, either clockwise or counterclockwise direction depending on the nature of the compound. For clockwise direction, the rotation (in degrees) is defined as positive ("+") and called dextrorotatory (from the Latin: dexter=right). In contrast, the counterclockwise direction is defined as negative ("-") and called levorotatory (from the Latin laevus=left).       Unfortunately, there is no direct correlation between the configuration [(D/L) in Rosanoff, (R/S) in Cahn-Ingold-Prelog nomenclature] of an enantiomer and the direction [(+) or (-)] in which they rotate plane-polarized light. This means that the R-enantiomer can exhibit a positive or negative value for the optical rotation depending on the compound. In some cases, the solvent has an impact on the magnitude and the sign as well i.e.,(S)-lactic acid exhibits an optical rotation of [a]= +3.9o in water and [a]= +13.7o using 2 M sodium hydroxide solution as solvent because the observer looks at a different species (lactate).


  The specific rotation [a] depends on the length of the tube, the wavelength that it is used for the acquisition, the concentration of the optical active compound (enantiomer), and to a certain degree on the temperature as well. However, the temperature effect is very difficult to specify since it differs for each compound. For instance, the [a]-value for a-pinene only slightly increases in the range from 0 oC to 100 oC (at l=589.3 nm), while it is almost cut in half for b-pinene. These two compounds only differ by the position of the alkene function. In order to see the effects more clearly, we will be using a bright torch light.
This is a pictorial representations of what a polariser does and its effects on light itself.








We have constructed our own polariser using the equipments as written above.



Diagram of polariser 

Calibration:
  1. Prepare a solution with 5% sugar and 95% water.
  2. Pour the solution into the glass cylinder.
  3. Turn on the lamp 
  4. Turn the top polariser lens until there is completely no light passing through.
  5. Mark it with a marker on the PVC piper with respect to the 0 degree mark on the protractor.
  6. Turn the top polariser lens until the next darkest spot 
  7. Measure the angle between the two brightness point and divide by 2.
  8. That will be the angle between the darkest and the brightest.

Repeat the calibration with a solution that have 5 % more sugar each time until it reaches 20% sugar.




We plotted a graph using the results we got from our calibration results.

percentage of sugar in solution Degree of rotation
2
87.5
4
88.5
6
89.5
8
90.5
10
91.5
12
92.5
14
93.5
16
94.5
18
95.5
20
96.5




Steps :

  1. We pour 50 ml of a drink into the glass cylinder, and put it into the polariser.
  2. Turn on the lamp 
  3. Turn the top polariser lens until there is completely no light passing through.
  4. Mark it with a marker on the PVC piper with respect to the 0 degree mark on the protractor.
  5. Turn the top polariser lens until the next darkest spot 
  6. Measure the angle between the two brightness point and divide by 2.
  7. That will be the angle between the darkest and the brightest.
  8. Using the degree of rotation and the calibration graph, we will be able to find out how much sugar content the drinks has.


These are some of the pictures we took on our experiment...




Laser method 
BRIX hydrometer  
Polariser 




Refractometer




























No comments:

Post a Comment