How does a polarimeter detect chiral substances?

A polarimeter detects chiral substances by measuring the rotation of plane-polarised light as it passes through a sample.

A polarimeter is an instrument used in laboratories to measure the optical activity of a substance. This optical activity is a characteristic of chiral substances, which are molecules that cannot be superimposed on their mirror images. Chiral substances have the ability to rotate the plane of polarisation of light, a phenomenon known as optical rotation.

The polarimeter works by passing plane-polarised light (light that vibrates in a single plane) through a sample of the substance being tested. If the substance is chiral, it will rotate the plane of polarisation of the light. The degree to which the light is rotated is measured by the polarimeter.

The polarimeter consists of a light source, a polariser (which produces the plane-polarised light), a sample tube (where the substance is placed), and an analyser (which measures the angle of rotation). The light from the source is first passed through the polariser, which ensures that it vibrates in a single plane. This light then passes through the sample tube. If the substance in the tube is chiral, it will rotate the plane of polarisation of the light. The rotated light then reaches the analyser, which measures the angle of rotation.

The degree of rotation depends on several factors, including the concentration of the chiral substance, the length of the sample tube, and the wavelength of the light used. By knowing these factors, it is possible to calculate the specific rotation of the substance, which is a characteristic property of each chiral substance. This allows scientists to identify unknown chiral substances by comparing their specific rotations with known values.

In summary, a polarimeter detects chiral substances by measuring the rotation of plane-polarised light as it passes through a sample. This rotation is a unique property of chiral substances and can be used to identify them.