Question

Optical activity is measured by (a) refractometer (b) tracer technique (c) spectrograph (d) polarimeter

Short answer

Optical activity is measured by a polarimeter.

Step-by-step solution

Understand Optical Activity

Optical activity refers to the ability of some substances to rotate the plane of polarized light. This property is characteristic of chiral molecules, which lack an internal plane of symmetry.

Review Measurement Instruments

Refractometer: It measures the extent of light bending, or refraction, as light passes through a substance. It does not measure optical activity. Tracer Technique: It involves tracking the movement of substances through a system, typically using radioactive or fluorescent labeling. It is not related to measuring optical activity. Spectrograph: This instrument is used to separate and record spectra of light, which is used in analyzing the composition of substances, but not for measuring optical activity. Polarimeter: It is specifically designed to measure the angle of rotation caused by passing polarized light through an optically active substance.

Identify the Correct Instrument

Since optical activity is the rotation of polarized light, the instrument used to measure this property is the polarimeter. A polarimeter can determine the specific rotation of chiral substances by measuring the angle of rotation of polarized light.

Key concepts

Chiral Molecules

Chiral molecules are fascinating due to their unique three-dimensional structure. They possess a characteristic known as 'chirality,' which signifies that they lack an internal plane of symmetry. This means that chiral molecules are like left and right hands—non-superimposable mirror images—imagine trying to place your right hand over your left in a way that aligns all fingers perfectly. You can't, and this is how chiral molecules behave.
Chiral molecules are key players in many biological and chemical processes. Their unique structures allow them to interact with specific enzymes or other chiral molecules in a way that their mirror-image counterpart cannot. This specificity is crucial in biological systems and pharmaceuticals, as one enantiomer of a chiral drug might be therapeutically beneficial, while the other could be harmful.

• The presence of an asymmetric carbon atom, usually with four different substituents, often marks the existence of chirality in a molecule.
• Chirality is a property encountered in optical activity, where these molecules can rotate the plane of polarized light.

Polarized Light

Light is typically an electromagnetic wave that vibrates in all directions. However, when light is passed through a polarizing filter, it becomes polarized, and the waves vibrate in only one direction. This filtered light is essential when measuring optical activity, as it provides a more controlled environment to observe how substances interact with light.
Polarized light forms the crux of understanding optical activity. When polarized light passes through a chiral substance, the plane of light's vibration is rotated. This rotation is what we measure to gauge the optical activity of the substance. The degree of rotation can tell us a lot about the molecule's chiral properties.

• Polarized light is crucial in differentiating between two forms of chiral molecules, known as enantiomers.
• The rotation of light's plane is measured in degrees, which helps in determining the specific rotation.

Polarimeter

A polarimeter is an instrumental jewel in the study of optical activity. This device accurately measures the angle by which a substance rotates the plane of polarized light. It provides a direct connection between light and the unique three-dimensional structure of chiral molecules.
To use a polarimeter, a sample of the optically active compound is placed in a tube, which is then inserted into the device. Polarized light is passed through the sample, and the polarimeter gauges the angle of rotation. This reading is directly related to the concentration of the chiral substance and its specific rotation, a property intrinsic to the substance.

• The greater the concentration of an optically active substance, the larger the angle of rotation.
• The polarimeter helps chemists and biologists explore the identity and purity of chiral compounds, especially in pharmaceuticals.

Instrumental Analysis

Instrumental analysis refers to the techniques and tools used to quantify chemical compounds and their properties. In the context of optical activity, the main instrument used is the polarimeter. However, several other instruments are available for various analytical purposes.
When comparing instruments:

• A refractometer measures how much light bends passing through a substance.
• A tracer technique involves tracking movements, often using labeling, not suitable for optical activity.
• A spectrograph examines light spectra for compound analysis but isn't used for measuring optical activity.
This demonstrates how specialized instruments cater to different aspects of how substances interact with light and other energies. Instrumental analysis allows for precision and accuracy in researching and developing new compounds, especially in industries like pharmaceuticals, where understanding the properties of chiral molecules is vital.