# What is the scientific principle of colorimetry?

## Document ID

Document ID TE1598

## Published Date

Published Date 01/15/2018
Question
What is the scientific principle of colorimetry?
Summary
Colorimetry: Beer-Lambert Law
Colorimetric reactions can be measured on either a spectrophotometer or a colorimeter. Both of these instruments measure the intensity of light passing through the colored sample and convert this light intensity to a concentration based on a stored calibration curve. The instruments follow the principles of the Beer-Lambert Law. The Beer-Lambert law (referred to as Beer’s law) is the linear relationship between absorbance and concentration. Generally Beer’s law is written as:

A = Ɛ x b x c

A is the measured absorbance
Ɛ is a wavelength-dependent absorptivity coefficient
b is the cell-path length
c is the analyte concentration
There is also a known relationship between absorbance and transmittance where:

A = -log T

A is absorbance
T is transmittance

Transmittance is defined as:

T = I / I0

I is the light intensity after it passes through the sample
I0 is the initial light intensity.

Combining the two equations above:

A = -log( I / I0 )

Instruments use the above equation to calculate the absorbance and display concentration based on the stored calibration curve. In short, the colorimetry reading is the light absorbed when passing through the sample.

In a spectrophotometer a lamp produces a spectrum of light and a single wavelength of light is selected using a monochromator (prism or diffraction grating) and a colored filter. The specific wavelength of light is focused through a lens and directed through the sample. The detector measures the amount of light absorbed (or transmitted) by the sample.

A basic colorimeter has a light emitting diode (LED) instead of a white light lamp and does not utilize a diffraction grating. A colorimeter is limited to performing analysis on parameters that fall within the wavelength of the individual filter, while spectrophotometers can analyze samples is a broad range of wavelengths determined by the diffraction grating.  