Colorimetry, as the term suggests, involves the quantification of colour. Chemical analysis specifically refers to the measurement of the concentration of a specific compound (the solute) in a coloured solution (the solvent). During scientific investigations, researchers often need to measure the quantities of a specific compound mixture or the concentration of the solution. The goal is to distinguish all the colours in various mixtures and determine their absolute values. This provides more scientifically useful information than subjective judgments such as solutions being dark or light in color. This process requires specialized equipment like a colorimeter.
So, what is a colorimeter?
A colorimeter, as the name implies, is a measuring device used to quantify the absorbance and transmittance of light as it traverses a sample liquid. It can detect different colours that our human eyes cannot perceive.
The human eye has evolved to allow us to see a large proportion of the visible light spectrum (wavelengths of 400-700 nm) and as result, the wider world. Using the three types of receptors (or cones) in our eyes, we can identify different wavelengths of light. The cones, blue (short range), green (medium), and red (long range), activate to different degrees depending on the light received and they allow us to get an accurate (enough) view of the world.
Where the human eye starts to let the scientific community down is in its lack of precision. There is a threshold beyond which the human eyes are not sensitive enough to distinguish small changes in the colours of a solution. Furthermore, the brain adds its own layer of subjectivity, making objective comparisons even more difficult. This can make certain findings difficult to distinguish. Therefore, a more sensitive measuring instrument is needed to provide reliable and consistent results. This instrument is known as a Colorimeter.
Components of a Colorimeter
A colorimeter consists of the following components:
Illuminant: A specific, fixed light source, usually composed of blue, green, and red light that passes through the object.
Filters: For the red, blue, and green wavelengths of light.
A slit: This focuses the beam of light.
Condenser Lens: This focuses the beam of light into parallel rays.
Cuvette: A sample holder where the liquid is placed. This is usually made from glass, quartz, or plastic.
Standard observer: A two-degree standard observer, which is a small and specific field of view.
Photocell: A vacuum-filled cell used to transmit light and convert it into electrical output. These are made from light-sensitive materials such as selenium.
Tristimulus absorption filter: A filter that isolates specific wavelengths to be applied to the sample.
Analog/Digital meter: Displays output as transmittance or absorbance.
There are two types of colorimeters:
Colour densitometers: Measure the density of primary colours.
Colour photometers: Measure colour transmission and reflection.
How Does a Colorimeter Work?
A colorimeter operates using the Beer-Lambert law. It’s essential for the solution to be homogeneous. When a ray of light passes through the solution, a part of the light radiation is absorbed by the solution. The amount of light absorbed and transmitted is defined by the Beer-Lambert law.
This law is a combination of two laws, Beer’s law and Lambert’s. Beer’s law states that when a parallel beam of monochromatic light (only one wavelength) passes through a solution, the amount of light absorbed by the solution is directly proportional to the solute in the solution. Lambert’s law states that the absorbance of light by a coloured solution depends on the length of the column and the volume of liquid through which the light passes.
So, the two laws establish a link between the light absorbed by the sample, the path it travels across the sample, and the concentration of the sample itself.
For instance, suppose a ray of light of a particular wavelength has an intensity of “I0”, and after it has passed through the solution, the intensity is now “Ia”, while the solution has absorbed a portion of the intensity “Ib”. The following equation explains this phenomenon:
I0=Ia+Ib
The amount of light absorbed by the solution is directly proportional to the concentration of solute in the solution and the path length of the cuvette through which the light travels. This relationship between the absorption of light and the concentration of the solution is defined in
Beer-Lambert’s law:
A=edC
Where,
A= Absorbance
e= Molar absorptivity (a measure that shows how well a solution absorbs light of a particular wavelength) (in mol L-1 cm-1 )
d= Path length of the cuvette (in cm)
C Concentration of the solute in solution (in mol L-1)
Once the light has passed through the solution, it is then collected into a detector. The relationship between the light that is passed through the solution (I) and the original light applied on the sample (I0) is shown by the formula:
T=I/I0
Where,
T= Transmittance (amount of light that passes through a solution)
The above two equations are represented in the form of Absorbance (A) as follows:
A=−log10(I/Io)=–log10T
Absorbance is more commonly used than transmittance when determining the concentration of a solute in a solution.
Applications of Colorimeters
Colorimeters are used to compare the results of a new sample to an existing one. Common applications include monitoring the growth of yeast or bacterial culture, assessing beverage colour, and measuring ink colours used in printers and scanners, particularly during the reproduction and inspection stages of manufacturing.
Colorimetry is a very quick and efficient way of analyzing coloured solutions or any coloured substance. Decades of research have enabled the development of high-end colorimeters, such as those available at Delta Scientific. We offer a range of colorimeters all from the reputable brand Lovibond.
Get in touch with a member of our team to find the right colorimeter for you.