What is the primary difference between Absorbance and Transmittance in colorimetry?

Absorbance measures the amount of light blocked by a sample and increases linearly with concentration, while Transmittance measures the light that passes through and decreases exponentially as concentration increases.

When comparing a 'blank' solution to a 'sample' solution, what is the purpose of the blank?

The blank is used to 'zero' the colorimeter, accounting for any light absorbed by the solvent or the cuvette itself so that the final reading reflects only the absorbance of the solute of interest.

How does the relationship between concentration and absorbance differ from the relationship between concentration and transmittance?

The relationship with absorbance is linear ($A \propto c$), making it easy to plot and analyze, whereas the relationship with transmittance is non-linear (logarithmic), which is more difficult to use for direct quantification.

What error occurs if a student handles the clear sides of a cuvette with their bare fingers?

Fingerprints leave oils and smudges that scatter light, which the detector interprets as light being absorbed by the solute, resulting in an inaccurately high absorbance reading.

What happens to the accuracy of a colorimeter reading if the wrong color filter is selected?

The sensitivity of the measurement decreases significantly because the sample will not absorb the chosen wavelength of light effectively, leading to small, unreliable changes in absorbance as concentration varies.

Why is it a mistake to ignore the 'zeroing' step before measuring a series of samples?

Without zeroing, the device does not have a baseline for $100\%$ transmission, meaning the absorbance of the solvent and cuvette will be added to the sample's absorbance, causing a systematic overestimation of concentration.

Define the Beer-Lambert Law in the context of colorimetry.

It is the principle stating that the absorbance of a solution is directly proportional to the concentration of the absorbing solute and the path length of the light through the solution ($A = \epsilon c l$).

What is a 'Calibration Curve'?

A graph showing the relationship between known concentrations of a substance and their corresponding absorbance values, used to determine the concentration of unknown samples.

What is 'Serial Dilution'?

A step-by-step dilution of a substance in solution, where the concentration decreases by the same factor in each step, used to create a range of standards for a calibration curve.

Why must the filter color be complementary to the solution color?

A solution appears a certain color because it reflects that color and absorbs its complement; using the complementary filter ensures the light is at the wavelength the solution absorbs most strongly, maximizing sensitivity.

Colorimetry | Cambridge International Examinations AS-Level Biology

1. Definition & Core Concepts

Colorimetry is the science of measuring the intensity of color in a solution to quantify the concentration of a specific substance. It relies on the principle that the more concentrated a colored solution is, the more light it will absorb and the less light it will transmit.
A colorimeter is the primary instrument used in this field, designed to pass a beam of light through a sample and measure the resulting light intensity. The device typically outputs data as either Absorbance (A), which is the amount of light trapped by the sample, or Percentage Transmission (%T), which is the amount of light that passes through successfully.
This technique is particularly valuable in biochemistry for tracking the progress of reactions where a substrate is converted into a colored product, or where a colored substrate is consumed over time.

Schematic diagram of a colorimeter showing the light path through a filter, cuvette, and detector.

2. Underlying Principles

The Beer-Lambert Law provides the mathematical foundation for colorimetry, stating that absorbance is directly proportional to the concentration of the absorbing species and the path length of the light. The formula is expressed as $A = \epsilon c l$ , where $A$ is absorbance, $\epsilon$ is the molar absorptivity, $c$ is concentration, and $l$ is the path length.
Absorbance and Transmission have an inverse logarithmic relationship. While transmission decreases exponentially as concentration increases, absorbance increases linearly, making absorbance the preferred unit for creating calibration graphs.
The relationship between the two is defined by the equation $A = \log_{10}(\frac{100}{\%T})$ . This means that a solution with $100\%$ transmission has an absorbance of $0$ , while a solution with $10\%$ transmission has an absorbance of $1$ .

3. Instrumentation & Components

The Light Source provides a broad spectrum of light, which must be narrowed down to a specific wavelength to ensure maximum sensitivity for the substance being measured.
A Filter (or monochromator) is used to select the specific color of light that the sample absorbs most strongly. For example, if a solution appears blue, it is reflecting blue light and absorbing orange light; therefore, an orange filter should be used to maximize the absorbance reading.
The Cuvette is a small, transparent rectangular vessel that holds the liquid sample. It is essential that the cuvette is made of material that does not absorb light at the chosen wavelength and that its surfaces are clean and free of scratches.

4. Methodology: The Calibration Curve

To determine an unknown concentration, a Calibration Curve must first be constructed using a series of standard solutions with known concentrations. These standards are typically prepared through serial dilution, where each subsequent solution is a specific fraction of the previous one's concentration.
The colorimeter is first "zeroed" using a blank solution (usually distilled water or the solvent used), which sets the baseline absorbance to $0.00$ or transmission to $100\%$ . This ensures that any light absorbed by the solvent or the cuvette itself is subtracted from the final measurement.
Once the absorbance values for the standards are recorded, they are plotted on a graph of Absorbance (y-axis) versus Concentration (x-axis). The resulting straight line allows the user to find the concentration of an unknown sample by locating its absorbance on the y-axis and reading the corresponding value on the x-axis.

5. Key Distinctions

Feature	Absorbance ( $A$ )	Transmittance ( $T$ )
Definition	Light 'trapped' by the sample	Light 'passing through' the sample
Relationship to Concentration	Directly Proportional (Linear)	Inversely Exponential
Mathematical Range	$0$ to $\infty$ (typically $0-2$ in practice)	$0\%$ to $100\%$
Ideal for Graphing	Yes, produces a straight line	No, produces a curve

Blank vs. Sample: A blank contains everything except the colored solute being measured, whereas the sample contains the solute. Using a blank accounts for background interference.
Filter Color vs. Solution Color: The filter used is always the complementary color of the solution. This ensures that the light passing through is the wavelength that the solute is most efficient at absorbing.

6. Exam Strategy & Tips

Always mention the 'Blank': In exam questions regarding experimental design, you must explicitly state that the colorimeter was calibrated using a blank solution to set the absorbance to zero. This is a frequent marking point.
Filter Selection: If asked which filter to use, identify the color of the solution and choose the color on the opposite side of the color wheel. Using the same color filter as the solution would result in very low absorbance and poor sensitivity.
Linearity Check: Ensure that your calibration curve passes through the origin $(0,0)$ . If a solution has zero concentration of the colored solute, its absorbance should be zero after blanking.

7. Common Pitfalls & Misconceptions

Dirty Cuvettes: Fingerprints or smudges on the clear sides of a cuvette will scatter light, leading to an artificially high absorbance reading. Always handle cuvettes by their frosted sides or wipe them with a lint-free tissue.
Air Bubbles: Small bubbles clinging to the inside of the cuvette can interfere with the light path, causing erratic or incorrect readings. Tapping the cuvette gently can help dislodge them.
Concentration Limits: The linear relationship of the Beer-Lambert law often fails at very high concentrations because the molecules are too crowded, leading to non-linear absorbance. In such cases, the sample must be diluted before measurement.

Colorimetry

Summary

1. Definition & Core Concepts

2. Underlying Principles

3. Instrumentation & Components

4. Methodology: The Calibration Curve

5. Key Distinctions

6. Exam Strategy & Tips

7. Common Pitfalls & Misconceptions

Colorimetry

Summary

1. Definition & Core Concepts

2. Underlying Principles

3. Instrumentation & Components

4. Methodology: The Calibration Curve

5. Key Distinctions

6. Exam Strategy & Tips

7. Common Pitfalls & Misconceptions