Use of colorimeter for the determination of the concentration of a solution

Absorbance is important in determining concentration of a substance in a sample through colorimeter analysis. Colorimeter measures the intensity of colour and light transmittance by the sample to achieve the concentration. When a beam of light passes through a coloured solution, the amount of light absorbed depends on the nature of the molecules absorbing the light, their concentration and thickness (path length) of the solution. The ratio of transmitted intensity to original intensity s known as the “transmittance”, T.

Transmittance (T) = I/I₀

I = intensity of the transmitted light

I₀ = intensity of incident light

The Beer- Lambert law states that there is a logarithmic dependence between the transmittance and the absorbance. Therefore the transmittance is expressed in terms of absorbance;

Absorbance (A) = -log₁₀ T

                             = -log₁₀ (I/I₀)

According to this, the absorbance becomes linear with concentration considering;

A = ℰ ℓ C

ℰ = Molar absorbance coefficient

ℓ = path length

C = concentration of the solution

Therefore in dilute solution,

A = -log₁₀ (I/I₀) = ℰ ℓ C

Molar absorbance coefficient indicates the absorbance under a standard set of conditions, i.e. the light travelling 1cm through a solution of 1moldm^-3. In a material with a low absorption coefficient, light is poorly absorbed and vise versa. This depends on the material and on the wave length of the light.

When using the colorimeter the path length i.e. the width of the glass cell is constant. Also the concentration of one solution used at one specific wave length. Therefore ℰ is also constant through the measurements. This shows out clear relationship between the absorbance and the concentration.

A ∝ C as ℓ and ℰ are constant

The glass cell/ container with plane parallel faces are transverse by monochromatic radiation in the colorimeter. If the glass cell is filled with non absorbing solution, there is 100% transmittance; therefore the absorption would be zero.

Colorimeter applies only in relation to the visible region. Also Beer- Lambert law is applicable for 0.800-0.200 absorbances.

In the experiment, firstly the absorbance reading of the colorimeter should be zeroed using distilled water as distilled water is used to prepare the solutions. 

Also before taking the measurements of the absorbance value in each solution, the glass cell should be washed with distilled water in order to prevent interferences to the reading. It is important not to touch the two smooth surfaces of the glass cell and wipe out the additional drops remain on the surfaces of the glass cell, using a tissue. Otherwise the beam of the radiation would be scattered incorrectly and interfere the accuracy of the reading.

When filling the cell, air bubbles should not be remained inside the cell as it would decrease the absorbance value.

When refilling a glass cell with a different solution, small amount of the new solution should be used to rinse the cell before filing as it would give more accurate results.

Spectrophotometer also uses a monochromatic light to pass though a solution and measure its absorbance. The principle of spectrophotometer and colorimeter is same but a colorimeter can only use one wavelength at a time and have a fixed number of wavelengths that can be used. Also they have to be in visible range only.

A spectrophotometer on the other hand can not function like a colorimeter but take a spectrum of a solution across the entire wave spectrum especially in UV – IR. Therefore use of spectrophotometer is beneficial than a colorimeter and useful to determine concentration of unknown solutions.