Spectrophotometry Definition in Analytical Chemistry

Spectrophotometry

Definition and meaning of Spectrophotometry in chemistry.

Spectrophotometry is a laboratory method that measures how much light a sample absorbs at a chosen wavelength. A more concentrated solution absorbs more light, so the reading can be converted into a concentration. It is one of the most common ways chemists find out how much of a substance a solution contains.

In more detail

A spectrophotometer has four main parts. A lamp supplies the light, a monochromator selects a single wavelength, that light passes through the sample in a small clear container called a cuvette, and a detector measures what comes out the far side. The instrument compares the light going in with the light coming out.

The fraction that gets through is called the transmittance. Chemists usually convert this into absorbance instead, because absorbance rises in a straight line as concentration rises, which makes it far easier to work with. That straight-line behavior is Beer’s law, written A = εlc.

A is the absorbance, ε is the molar absorptivity of the substance, l is the distance the light travels through the sample, and c is the concentration. The path length is nearly always 1 centimeter, and molar absorptivity is a fixed property of the substance at that wavelength, so in practice absorbance depends on concentration alone.

Most of the time you do not use the equation directly. You measure several standards of known concentration, plot absorbance against concentration, and read the unknown off that calibration curve. One detail that catches students out is that absorbance is not a percentage.

It is a logarithmic value with no units. An absorbance of 1 means 10 percent of the light got through, and an absorbance of 2 means only 1 percent did. Beer’s law also breaks down in very concentrated solutions, so dark samples are diluted before measuring.

Key facts

What It Measures

How much light a sample absorbs at a set wavelength

Governing Equation

Beer's law, A = εlc

Instrument Parts

Lamp, monochromator, cuvette, detector

Typical Path Length

1 centimeter

Absorbance Scale

Logarithmic and unitless; A of 1 means 10 percent transmitted

Main Use

Finding concentration from a calibration curve

Field

Analytical Chemistry

Example

To measure the concentration of a copper sulfate solution, a chemist sets the instrument near 630 nanometers, in the red part of the spectrum, because a blue solution absorbs red light most strongly. Standards at 0.02, 0.04, 0.06, and 0.08 molar are read first, and their absorbances fall on a straight line. The unknown gives an absorbance of 0.45, which sits on that line at roughly 0.05 molar.

Frequently asked questions

What is the difference between spectrophotometry and spectroscopy?

Spectroscopy is the broad study of how matter interacts with light, and it is often used to work out what a substance is. Spectrophotometry is the narrower job of measuring how much light gets absorbed, usually so you can calculate a concentration.

Why do chemists use absorbance instead of transmittance?

Absorbance changes in a straight line as concentration changes, so a graph of absorbance against concentration is easy to read. Transmittance changes as a curve, which makes it much harder to use for calculations.

Why do dark samples need to be diluted first?

Beer's law only holds while the solution is reasonably dilute. In concentrated solutions the particles start to interact and very little light reaches the detector, so readings become unreliable. Diluting brings the sample back into the range where the straight-line relationship works.