Polarimeter
Encyclopedia
A polarimeter is a scientific instrument used to measure the angle of rotation
Angle of rotation
In mathematics, the angle of rotation is a measurement of the amount, the angle, that a figure is rotated about a fixed point, often the center of a circle....

caused by passing polarized light through an optically active substance.

Some chemical substances are optically active, and polarized (aka 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.

## Polarimeter History

Polarization by reflection was discovered in 1808 by Etienne Malus (1775-1812).

## Construction

The polarimeter is made up of two Nicol prisms
Nicol prism
A Nicol prism is a type of polarizer, an optical device used to produce a polarized beam of light from an unpolarized beam. See polarized light. It was the first type of polarizing prism to be invented, in 1828 by William Nicol of Edinburgh...

(the polarizer and analyzer). The polarizer
Polarizer
A polarizer is an optical filter that passes light of a specific polarization and blocks waves of other polarizations. It can convert a beam of light of undefined or mixed polarization into a beam with well-defined polarization. The common types of polarizers are linear polarizers and circular...

is fixed and the analyzer can be rotated.
The prisms may be compared to as slits S1 and S2. The light waves may be considered to correspond to waves in the string.
The polarizer S1 allows only those light waves which move in a single plane. This causes the light to become plane polarized.
When the analyzer is also placed in a similar position it allows the light waves coming from the polarizer to pass through it.
When it is rotated through the right angle no waves can pass through the right angle and the field appears to be dark.
If now a glass tube containing an optically active solution is placed between the polarizer and analyzer the light now rotates through the
plane of polarization through a certain angle, the analyzer will have to be rotated in same angle.

## Operation

Polarimeters measure this by passing monochromatic light through the first of two polarizing plates, creating a polarized beam. This first plate is known as the polarizer. This beam is then rotated as it passes through the sample. The sample is usually prepared as a tube where the optically active substance is dissolved in an optically inactive chemical such as distilled water
Distilled water
Distilled water is water that has many of its impurities removed through distillation. Distillation involves boiling the water and then condensing the steam into a clean container.-History:...

, ethanol
Ethanol
Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...

, methanol
Methanol
Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...

. Some polarimeters can be fitted with tubes that allow for sample to flow through continuously.

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 detector.

### Manual

The earliest polarimeters, which date back to the 1830s, required the user to physically rotate the analyzer, and the detector was the user’s eye judging when the most light shone through. The angle was marked on a scale that encircles the analyzer. This basic design is still used in the simplest polarimeters.

### Semi-automatic

Today there are also semi-automatic polarimeters, which require visual detection but use push-buttons to rotate the analyzer and offer digital displays.

### Fully automatic

The most modern polarimeters are fully automatic, and simply require the user to press a button and wait for a digital readout.

## Sources of error

The angle of rotation of an optically active substance can be affected by:
• Concentration of the sample
• Wavelength 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)

Most modern polarimeters have methods of compensating for or controlling these.

## Calibration

Polarimeters can be calibrated – or at least verified – by measuring a quartz plate, which is constructed to always read at a certain angle of 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 solutions.

## Applications

Because many optically active chemicals such as sucrose
Sucrose
Sucrose is the organic compound commonly known as table sugar and sometimes called saccharose. A white, odorless, crystalline powder with a sweet taste, it is best known for its role in human nutrition. The molecule is a disaccharide composed of glucose and fructose with the molecular formula...

, are stereoisomers, a 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 optical activity of racemic mixtures.

### Chemical industry

Many chemicals exhibit a specific rotation as a unique property (like refractive index in many cases) 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.

### Food, beverage and pharmaceutical industries

Concentration 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:
• Steroid
Steroid
A steroid is a type of organic compound that contains a characteristic arrangement of four cycloalkane rings that are joined to each other. Examples of steroids include the dietary fat cholesterol, the sex hormones estradiol and testosterone, and the anti-inflammatory drug dexamethasone.The core...

s
• Diuretic
Diuretic
A diuretic provides a means of forced diuresis which elevates the rate of urination. There are several categories of diuretics. All diuretics increase the excretion of water from bodies, although each class does so in a distinct way.- Medical uses :...

s
• Antibiotic
Antibiotic
An antibacterial is a compound or substance that kills or slows down the growth of bacteria.The term is often used synonymously with the term antibiotic; today, however, with increased knowledge of the causative agents of various infectious diseases, antibiotic has come to denote a broader range of...

s
• Narcotic
Narcotic
The term narcotic originally referred medically to any psychoactive compound with any sleep-inducing properties. In the United States of America it has since become associated with opioids, commonly morphine and heroin and their derivatives, such as hydrocodone. The term is, today, imprecisely...

s
• Vitamin
Vitamin
A vitamin is an organic compound required as a nutrient in tiny amounts by an organism. In other words, an organic chemical compound is called a vitamin when it cannot be synthesized in sufficient quantities by an organism, and must be obtained from the diet. Thus, the term is conditional both on...

s
• Analgesic
Analgesic
An analgesic is any member of the group of drugs used to relieve pain . The word analgesic derives from Greek an- and algos ....

s
• Amino Acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

s
• Essential Oils
• Polymers
• Starch
Starch
Starch or amylum is a carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds. This polysaccharide is produced by all green plants as an energy store...

es
• Sugar
Sugar
Sugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...

s

Polarimeters are used in the sugar industry for determining quality of both juice from sugar cane and the refined sucrose. Often, the sugar refineries use a modified polarimeter with a flow cell called a saccharimeter
Saccharimeter
A saccharimeter is an instrument for measuring the concentration of sugar solutions.This is commonly achieved using a measurement of refractive index or the angle of rotation of polarization of optically active sugars ....

. These instruments use the International Sugar Scale (as defined by the International Commission for Uniform Methods of Sugar Analysis
International Commission for Uniform Methods of Sugar Analysis
The International Commission for Uniform Methods of Sugar Analysis is an international standards body, founded in 1897, that publishes detailed laboratory procedures for the analysis of sugar....

(ICUMSA).

## See also

• Optical rotation
Optical rotation
Optical rotation is the turning of the plane of linearly polarized light about the direction of motion as the light travels through certain materials. It occurs in solutions of chiral molecules such as sucrose , solids with rotated crystal planes such as quartz, and spin-polarized gases of atoms...

• Polarimetry
Polarimetry
Polarimetry is the measurement and interpretation of the polarization of transverse waves, most notably electromagnetic waves, such as radio or light waves...

• Polarization
Polarization (disambiguation)
Polarization or polarisation can refer to:-In the physical sciences:*Polarization , the orientation of oscillations in the plane perpendicular to a transverse wave's direction of travel which may include:...

• Chirality
Chirality (chemistry)
A chiral molecule is a type of molecule that lacks an internal plane of symmetry and thus has a non-superimposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom....

• Enantiomers
• Louis Pasteur
Louis Pasteur
Louis Pasteur was a French chemist and microbiologist born in Dole. He is remembered for his remarkable breakthroughs in the causes and preventions of diseases. His discoveries reduced mortality from puerperal fever, and he created the first vaccine for rabies and anthrax. His experiments...

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