It is a technique based on the measurement of the optical rotation produced on a beam of polarized light passing through an optimally active substance. The rotary optical activity of a substance has its origin in the structural asymmetry of the molecules.
The polarimeter allows to measure the concentrations of solutions of optically active substances, that is of substances that produce rotation of the plane of oscillation of the light that passes through it.
Since natural light is composed of electromagnetic oscillations that oscillate in all planes, we must single out only one of those planes and then analyze the direction of oscillation after the light has passed through the sample.
Functioning
The white light coming from the lamp must first be filtered as each wavelength of light has a different rotation. The color chosen internationally is yellow, which corresponds to the light emission of sodium. To emulate the color, a 12 mm thick prismatic cuvette containing a 10% P/V potassium dichromate solution is used.
The specific rotation depends on the substance to be analyzed and corresponds to the rotation caused by a solution of one milligram of sample per milliliter of solution when it passes through a cell of one decimeter (10 cm). The length L is measured from the bottom of the tube (excluding wall thickness) to the free surface of the beta solution is the angle measured. It is evident that knowing 3 of the parameters, the fourth can be calculated.
If the substance is unknown, a solution of known concentration is taken and the specific angle is measured and compared with known substances.
The problem arises when there are mixtures of optically active substances where each one contributes a different rotation. It is suggested as an experience to measure the concentration of a sucrose solution (common sugar) and compare it with the real one (weight/volume).
As technical data it is known that sucrose rotates in the light clockwise at an angle of 66.5 degrees. It will be appreciated that the rotation tends towards the counter-clockwise direction. This is because the
sucrose splits, in reaction with water (hydrolysis) into glucose (clockwise rotation of 52.5 degrees) and fructose (counterclockwise rotation of 93 degrees). As each molecule of sucrose reacts with one of water to give one of glucose or another of fructose, the rotation will be increasingly dominated by the latter, which is left-handed.
Components.
The first polarimeters were designed in the forties of the last century, thanks to the use of prisms devised in 1828 by William Nicol, built with two sheets of Iceland spar. This instrument is used to measure the rotation of polarized light caused by optical isomers.
The basic components of the polarimeter are:
- A source of monochromatic radiation
- A prism that acts as a polarizer of the radiation used
- A tube for the sample
- An analyzing prism
- A detector (which can be the eye
- or a photoelectric detector)
Applications
They are widely used in the chemical and pharmaceutical industries for quality control. There are more than 60 varieties of listed chemicals, of which can be measured with a polarimeter. These include: ascorbic acid, testosterone, and cocaine.
Measurements are applied with polarimeters for food additives, essences and perfumes.
in sugar analysis, being the standard form of measurement using the International Standard unit of sugar scale.
They are used for educational purposes to understand the optical activity capacity of substances, polarized light and much more.
