Slide 1

QC II POST LABPOLARIMETER

AS AN INSTRUMENTis a scientific instrument used to measure the angle of rotation caused by passing polarized light through an optically active substance.

Some chemical substances are optically active, and polarized (unidirectional) light will rotate either to the left (counter-clockwise) or right (clockwise) when passed through these substances. The amount by which the light is rotated is known as the angle of rotation.

The specific rotation is a physical property and defined as the optical rotation at a path length l of 1 dm, a concentration c of 1g/100 mL, a temperature T (usually 20C) and a light wavelength (usually sodium D line at 589.3nm):

2

PARTS OF

Polarization by reflection was discovered in 1808 by tienne-Louis Malus (17751812).

Polarimeters measure this by passing monochromatic light through the first of two polarising plates, creating a polarized beam. This first plate is known as the polarizer.[6] This beam is then rotated as it passes through the sample. After passing through the sample, a second polarizer, known as the analyzer, rotates either via manual rotation or automatic detection of the angle. When the analyzer is rotated to the proper angle, the maximum amount of light will pass through and shine onto a detector3

Polarimeter tube

Parts of the polarimeterFRONT SIDErear sideoperation panelEye piecepower switchzero set switchDisplay channelpower input connectorzero set ready lampSample chamber coverfuse holderrotate switchThermosensorrating labelrotate (left handed rotation) switchSample stageshift temp switchIndication selector switch5

Operation Panel

ZERO SET

ROTATERLTEMP

PRINCIPLE

The ratio, the purity, and the concentration of two enantiomers can be measured via polarimetry. Enantiomers are characterized by their property to rotate the plane of linear polarized light. Therefore, those compounds are called optically active and their property is referred to as optical rotation. Light sources such as a light bulb, a light-emitting diode (LED), or the sun emit electromagnetic light waves. Their electric field oscillates in all possible planes relative to their direction of propagation. In contrast to that, the waves of linear-polarized light oscillate in parallel planes.[3]If light encounters a polarizer, only the part of the light that oscillates in the defined plane of the polarizer may pass through. That plane is called the plane of polarization. The plane of polarization is turned by optically active compounds. According to the direction in which the light is rotated, the enantiomer is referred to as dextrorotatory or levorotatory.The optical activity of enantiomers is additive. If different enantiomers exist together in one solution, their optical activity adds up. That is why racemates are optically inactive, as they nullify their clockwise and counter clockwise optical activities. The optical rotation is proportional to the concentration of the optically active substances in solution. Polarimeters may therefore be applied for concentration measurements of enantiomer-pure samples. With a known concentration of a sample, polarimeters may also be applied to determine the specific rotation ( physical property) when characterizing a new substance. 7

RESULTS IN POLARIMETERWhen the left semicircular field is the brighter (left handed rotating sample) continuously press the left handed ROTATE switch and the translucent semi-circular fields gradually change as below:

RESULTS IN POLARIMETERWhen the right semicircular field is the brighter (right handed rotating sample) continuously press the right handed ROTATE switch and the translucent semi-circular fields gradually change as below:

Laurent's half-shade polarimeter[edit]When plane polarised light passes through some crystals,the velocity of left polarised light is different from that of the right polarised light thus the crystals are said to have two refractive indices i.e. double refractingBiquartz polarimeter[edit]In biquartz polarimeters, a biquartz plate is used. Biquartz plate consists of two semi circular plates of quartz each of thickness 3.75mm. One half consists of right-handed optically active quartz,while the other is left-handed optically active quartz.Lippich polarimeter[edit]Quartz-Wedge polarimeter[edit]Manual[edit]The earliest polarimeters, which date back to the 1830s, required the user to physically rotate one polarizing element (the analyzer) whilst viewing through another static element (the detector). The detector was positioned at the opposite end of a tube containing the optically active sample, and the user used his/her eye to judge the "alignment" when least light was observed. The angle of rotation was then read from a simple protractor fixed to the moving polariser to within a degree or so.Although most manual polarimeters produced today still adopt this basic principle, the many developments applied to the original opto-mechanical design over the years have significantly improved measurement performance. The introduction of a half-wave plate increased "distinction sensitivity", whilst a precision glass scale with vernier drum facilitated the final reading to within ca. 0.05. Most modern manual polarimeters also incorporate a long-life yellow LED in place of the more costly sodium arc lamp as a light source.Semi-automatic[edit]Today, semi-automatic polarimeters are available. The operator views the image via a digital display adjusts the analyzer angle with electronic controls.Fully automatic[edit]Fully automatic polarimeters are now available and simply require the user to press a button and wait for a digital readout. Fast automatic digital polarimeters yield an accurate result within a second, regardless of the rotation angle of the sample. In addition, they provide continuous measurement, facilitating High-performance liquid chromatography and other kinetic investigations.Another feature of modern polarimeters is the Faraday modulator. The Faraday modulator creates an alternating current magnetic field. It oscillates the plane of polarization to enhance the detection accuracy by allowing the point of maximal darkness to be passed through again and again and thus be determined with even more accuracy.As the temperature of the sample has a significant influence on the optical rotation of the sample, modern polarimeters have already included Peltier Elements to actively control the temperature. Special techniques like a temperature controlled sample tube reduce measuring errors and ease operation. Results can directly be transferred to computers or networks for automatic processing.[7] Traditionally, accurate filling of the sample cell had to be checked outside the instrument, as an appropriate control from within the device was not possible. Nowadays a camera system allows accurate monitoring of the sample and filling conditions in the sample cell from inside the instrument. A telecentric camera gives a sharp image over the complete length of any sample cell placed within modern instruments. The online monitoring of the filling process ensures that no bubbles or particles obstruct the measurement. A picture can be saved together with the recorded data. Any temperature gradients, inhomogeneous sample distributions or air bubbles can immediately be recognized before measurement, so that potential errors caused by bubbles or particles are no longer an issue.

10

TYPES OF POLARIMETERLaurent's half-shade polarimeterBiquartz polarimeterLippich polarimeterQuartz-Wedge polarimeterManualSemi-automaticFully automatic

Laurent's half-shade polarimeter[edit]When plane polarised light passes through some crystals,the velocity of left polarised light is different from that of the right polarised light thus the crystals are said to have two refractive indices i.e. double refractingBiquartz polarimeter[edit]In biquartz polarimeters, a biquartz plate is used. Biquartz plate consists of two semi circular plates of quartz each of thickness 3.75mm. One half consists of right-handed optically active quartz,while the other is left-handed optically active quartz.Lippich polarimeter[edit]Quartz-Wedge polarimeter[edit]Manual[edit]The earliest polarimeters, which date back to the 1830s, required the user to physically rotate one polarizing element (the analyzer) whilst viewing through another static element (the detector). The detector was positioned at the opposite end of a tube containing the optically active sample, and the user used his/her eye to judge the "alignment" when least light was observed. The angle of rotation was then read from a simple protractor fixed to the moving polariser to within a degree or so.Although most manual polarimeters produced today still adopt this basic principle, the many developments applied to the original opto-mechanical design over the years have significantly improved measurement performance. The introduction of a half-wave plate increased "distinction sensitivity", whilst a precision glass scale with vernier drum facilitated the final reading to within ca. 0.05. Most modern manual polarimeters also incorporate a long-life yellow LED in place of the more costly sodium arc lamp as a light source.Semi-automatic[edit]Today, semi-automatic polarimeters are available. The operator views the image via a digital display adjusts the analyzer angle with electronic controls.Fully automatic[edit]Fully automatic polarimeters are now available and simply require the user to press a button and wait for a digital readout. Fast automatic digital polarimeters yield an accurate result within a second, regardless of the rotation angle of the sample. In addition, they provide continuous measurement, facilitating High-performance liquid chromatography and other kinetic investigations.Another feature of modern polarimeters is the Faraday modulator. The Faraday modulator creates an alternating current magnetic field. It oscillates the plane of polarization to enhance the detection accuracy by allowing the point of maximal darkness to be passed through again and again and thus be determined with even more accuracy.As the temperature of the sample has a significant influence on the optical rotation of the sample, modern polarimeters have already included Peltier Elements to actively control the temperature. Special techniques like a temperature controlled sample tube reduce measuring errors and ease operation. Results can directly be transferred to computers or networks for automatic processing.[7] Traditionally, accurate filling of the sample cell had to be checked outside the instrument, as an appropriate control from within the device was not possible. Nowadays a camera system allows accurate monitoring of the sample and filling conditions in the sample cell from inside the instrument. A telecentric camera gives a sharp image over the complete length of any sample cell placed within modern instruments. The online monitoring of the filling process ensures that no bubbles or particles obstruct the measurement. A picture can be saved together with the recorded data. Any temperature gradients, inhomogeneous sample distributions or air bubbles can immediately be recognized before measurement, so that potential errors caused by bubbles or particles are no longer an issue.

11

CALCULATION(a) t=100aDl X c

= al Xc (g/ mL)

t/d=specific rotation of the substance determined at 25Cusing the D line of NaA=relative optical rotationL=length of the observation tube (dm)100cm=1 m

1m=10 dmC=conc of the optically active matters in sampleT=temperatureD=symbol to show the wavelength of measuring light D ray= 589 nm.12

PROBLEM SOLVINGGDATAconc of sample-300 gramsLength of the observation tube-1 dmTemperature- 25CWavelength of measuring light used-589nmReading of the rotation of the sample-113Specific rotation of sample-?

PROBLEM SOLVINGG2. DATAconc of sample-?Length of the observation tube-10 cmTemperature- 25CWavelength of measuring light used-589nmReading of the rotation of the sample-125Specific rotation of sample-145

PROBLEM SOLVINGG3. DATAConc of sample-10 gLength of the observation tube-10 cmTemperature- 25CWavelength of measuring light used-589nmReading of the rotation of the sample-?Specific rotation of sample-110

SOURCE OF ERRORSThe angle of rotation of an optically active substance can be affected by:Concentration of the sampleWavelength of light passing through the sample (generally, angle of rotation and wavelength tend to be inversely proportional)Temperature of the sample (generally the two are directly proportional)Length of the sample cell (input by the user into most automatic polarimeters to ensure better accuracy)Filling conditions (bubbles, temperature and concentration gradients)Most modern polarimeters have methods for compensating or/and controlling these errors

CALIBRATIONPolarimeters can be calibrated or at least verified by measuring a quartz plate, which is constructed to always read at a certain angle of optical rotation (usually +34, but +17 and +8.5 are also popular depending on the sample). Quartz plates are preferred by many users because solid samples are much less affected by variations in temperature, and do not need to be mixed on-demand like sucrose solutions16

APPLICATIONChemical industryMany chemicals exhibit a specific rotation as a unique property (an intensive property like refractive index or Specific gravity) which can be used to distinguish it. Polarimeters can identify unknown samples based on this if other variables such as concentration and length of sample cell length are controlled or at least known. This is used in the chemical industry.By the same token, if the specific rotation of a sample is already known, then the concentration and/or purity of a solution containing it can be calculated.Most automatic polarimeters make this calculation automatically, given input on variables from the user

polarimeter can be used to identify which isomer is present in a sample if it rotates polarized light to the left, it is a levo-isomer, and to the right, a dextro-isomer. It can also be used to measure the ratio of enantiomers in solutions.The optical rotation is proportional to the concentration of the optically active substances in solution. Polarimetry may therefore be applied for concentration measurements of enantiomer-pure samples. With a known concentration of a sample, polarimetry may also be applied to determine the specific rotation (a physical property) when characterizing a new substance

17

APPLICATIONFood, beverage and pharmaceutical industriesConcentration and purity measurements are especially important to determine product or ingredient quality in the food & beverage and pharmaceutical industries. Samples that display specific rotations that can be calculated for purity with a polarimeter include:SteroidsDiureticsAntibioticsNarcoticsVitaminsAnalgesicsAmino acidsEssential oilsPolymersStarches are the most abundant substances in nature and used in various sectors of the food and pharmaceutical industry as well as the building sector. Polarimetric quality control of starch therefore is important in various industries.Sugars

SEATWORK1.A sample of pure 2-butanol was placed in a 10cm polarimeter tube. Using the D-line of a sodium lamp, the observed rotation at 20C was a= +104. The conc of the compound is 0.805 g/ mL. What is the specific rotation of 2-butanol?

ANSWER TO SEATWORKT/D=A/ Lx CT/D=?A=104l=10 cm or 1 dmc=0.805 g/ mL=104/ 1 dm X 0.805 g/ mL= + 129

SEATWORK:2. Calculate the observed rotation of a solution of 5.245 g of 1-ammonium-1-phenylethane diluted to a volume of 100 mL w/ a methanol at 20C using the D-line of a sodium lamp and a 1 dm tube.

Specific rotation of this material=(-30)Sample concentration is 5.245 g in 100mL

ANSWER TO SEATWORK2. T/D=A/ Lx CA=?T/D=-3OL= 1.00 dmC=5.245 g in 100 mL-30=X1001 X5.245 g/ 100 mL-30 (5.245)=x100 100x=-157.35

SEATWORK:3. Calculate the specific rotation of 2,3-tartaric acid based on the ff observation:

A 0.856 g sample of pure acid was diluted to 10 mL w/ water and observed in a 1.00 dm polarimeter tube. The observed rotation using the 589 nm line of a sodium lamp at 20C was a=+1.06.

ANSWER TO SEATWORK3. T/D=A/ Lx CT/D=?A=+1.06 L= 1 dmC= 0.856 g in 10 mLT/D=1.06/ 1 X 0.856 g/ 10mL=1.06/ 0.0856= 12.38