Welcome to Colorimetry: Quantitative Chemical Analysis
Have you ever noticed how adding more water to a glass of fruit squash makes its colour lighter, while adding more squash concentrate makes it deep and dark? That simple observation is the heart of colorimetry!
In your CCEA A2 Unit 9: Analytical Chemistry Techniques portfolio, colorimetry is one of the most powerful tools you will use. It allows you to measure exactly how much of a coloured substance is dissolved in a liquid. Whether checking iron levels in blood, testing contaminants in water, or analysing pharmaceutical drugs, colorimetry turns colour intensity into precise, quantitative data.
Don't worry if analytical chemistry feels daunting at first. We will break down every principle, instrument component, and step of your practical portfolio workflow so you can build complete confidence.
---1. Core Principles of Colorimetry
What is Colorimetry?
Colorimetry is an absorption-based quantitative analytical technique used to determine the concentration of a coloured solute in a solution. It works by shining light at a specific visible wavelength (within the visible spectrum of \(380\text{ nm}\) to \(780\text{ nm}\)) through a liquid sample and measuring how much of that light is absorbed or transmitted.
The Governing Principle: The Beer-Lambert Relationship
Colorimetry relies on a fundamental scientific law known as the Beer-Lambert Law. In simple terms:
• Absorbance (\(A\)) is directly proportional to the concentration (\(c\)) of the absorbing solute.
• Absorbance (\(A\)) is also directly proportional to the path length (\(l\)) of the sample holder (cuvette).
Mathematically, we express this proportionality under monochromatic light as:
\(A \propto c\)
When the path length of the cuvette is kept constant (standard cuvettes have a path length of \(1.0\text{ cm}\)), doubling the concentration of the coloured solute will double the absorbance value, provided the solution remains dilute.
The Complementary Colour Rule (Filter Selection)
Why do coloured solutions have colour? A solution appears a certain colour because it transmits (lets through) light of that colour while absorbing light of the opposite colour on the colour wheel (its complementary colour).
To get the most accurate and sensitive measurement, the colorimeter must shine light of the wavelength that the solution absorbs most strongly.
Rule: Always select a filter that is the complementary colour of the solution being tested.
• Example: A blue solution transmits blue light and absorbs orange/red light. Therefore, you must use a red or orange filter to achieve maximum absorbance and sensitivity.
• Common Mistake to Avoid: Never use a blue filter for a blue solution! The blue light would pass straight through without being absorbed, giving an absorbance reading close to zero.
Memory Tip: Think of "Opposites Attract". To measure a colour, choose its opposite colour on the optical wheel!
Key Takeaway for Section 1: Colorimetry measures light absorption at visible wavelengths (\(380\text{--}780\text{ nm}\)). Absorbance (\(A\)) is directly proportional to concentration (\(c\)), and we always select a filter with the complementary colour to maximize sensitivity.
---2. Inside the Colorimeter: Key Instrumentation
A standard laboratory colorimeter consists of five main parts arranged in a straight optical path:
1. Light Source:
A stable lamp (typically a tungsten filament lamp) that produces a continuous beam of visible white light containing all visible wavelengths.
2. Collimator / Lens and Filter (Wavelength Selector):
The lens focuses the light into a parallel beam. The optical filter selects a narrow band of visible light corresponding to the complementary colour of the analyte, blocking unwanted wavelengths.
3. Sample Holder (Cuvette Well):
Holds the cuvette containing the liquid sample. Standard laboratory cuvettes have a fixed optical path length of \(1.0\text{ cm}\).
4. Photodetector:
A photosensitive cell or photodiode that detects the light transmitted through the cuvette and converts that light energy into an electrical signal.
5. Readout / Digital Display:
Converts the electrical signal into a readable quantitative value, displayed either as:
• Absorbance (\(A\)): A unitless (arbitrary units) measurement of the light absorbed.
• Percentage Transmittance (\(\%T\)): The percentage of light that successfully passes through the solution.
Key Takeaway for Section 2: Light leaves the tungsten source → passes through the complementary filter → travels through the \(1.0\text{ cm}\) cuvette → hits the photodetector → outputs an Absorbance (\(A\)) or \(\%T\) value on the display.
---3. Step-by-Step Practical & Portfolio Workflow
For your Unit A2 9 portfolio, you will carry out colorimetric investigations in the laboratory. Follow this structured step-by-step method:
Step 1: Preparation of Standard Solutions
To determine an unknown concentration, you first need a benchmark. You prepare a primary standard or concentrated stock solution and perform accurate dilutions (serial or direct) using volumetric glassware (e.g., volumetric flasks and graduated pipettes). This creates a series of solutions of known, increasing concentrations across the desired analytical range.
Step 2: Reagent Addition / Chromogen Formation (If Applicable)
What if the substance you want to measure is colourless or only weakly coloured? You react the analyte with a specific complexing or chromogenic reagent. This chemical reaction forms an intensely coloured complex (a chromogen) suitable for colorimetric measurement (such as forming coloured transition metal complexes).
Step 3: Zeroing / Blanking the Instrument
Before testing any standards, you must calibrate the colorimeter using a solvent blank (a cuvette containing distilled/deionised water and all reagents except the analyte itself).
• Place the blank into the cuvette holder.
• Adjust the meter to read exactly \(0.00\text{ Absorbance}\) (or \(100\%\text{ Transmittance}\)).
Why is this crucial? Blanking cancels out any background absorbance caused by the solvent, reagents, or cuvette walls, ensuring that subsequent readings reflect only the analyte.
Step 4: Measuring the Standards
Measure the absorbance (\(A\)) of each standard solution in order, from lowest concentration to highest concentration. Ensure you handle the cuvette identically each time.
Step 5: Constructing the Calibration Curve
Plot your experimental data on a graph:
• x-axis: Concentration of the standard solutions (with appropriate units, e.g., \(\text{mol dm}^{-3}\) or \(\text{g dm}^{-3}\)).
• y-axis: Absorbance (\(A\)) (unitless / arbitrary units).
• Draw a straight line of best fit that passes directly through the origin \((0,0)\). A straight line through the origin confirms that the system obeys the Beer-Lambert relationship (\(A \propto c\)).
Step 6: Determining the Unknown Sample Concentration
Once your calibration graph is drawn:
1. Measure the absorbance of your unknown sample using the identical wavelength filter and cuvette orientation.
2. Interpolation Method: Locate the measured absorbance value on the y-axis, move horizontally across to intersect the line of best fit, and read straight down to the x-axis to find the concentration.
3. Gradient Method: Alternatively, calculate the gradient of the line of best fit:
\(m = \frac{\Delta A}{\Delta c}\)
Then calculate the unknown concentration using:
\(c_{\text{unknown}} = \frac{A_{\text{unknown}}}{m}\)
Key Takeaway for Section 3: Always zero the instrument with a solvent blank, plot Absorbance (\(y\)) against Concentration (\(x\)), draw a straight line through \((0,0)\), and interpolate to find the unknown concentration.
---4. Common Practical Pitfalls & How to Avoid Them
In Unit A2 9 portfolio reports, high marks are awarded for demonstrating awareness of experimental errors and analytical limitations. Keep these key issues in mind:
1. Incorrect Filter Selection
• Pitfall: Choosing a filter of the same colour as the solution.
• Solution: Always select the complementary colour to ensure high absorbance and high sensitivity.
2. Incomplete Blanking
• Pitfall: Using only pure distilled water to zero the instrument when colour-forming matrix reagents are present in the assay.
• Solution: The blank must contain the solvent plus all added reagents minus the analyte. This eliminates systematic overestimation of absorbance.
3. Cuvette Handling Artifacts
• Pitfall: Touching the clear optical faces of the cuvette leaves greasy fingerprints; water droplets or scratches scatter light, causing artificially high absorbance readings.
• Solution: Always hold cuvettes by their frosted/ribbed faces. Wipe the optical sides clean with a dry, lint-free tissue before placing them in the well. Ensure the cuvette is placed in the exact same orientation for every reading.
4. Air Bubbles
• Pitfall: Small air bubbles clinging to the inside surfaces of the cuvette scatter the light beam, causing unstable or elevated readings.
• Solution: Tap the cuvette gently on a bench surface or invert carefully to dislodge bubbles before taking a reading.
5. High Concentration and Linearity Limits (Deviations from Beer's Law)
• Pitfall: At high solute concentrations, solute molecules interact with each other, causing the calibration curve to bend and plateau (deviating from linearity).
• Solution: Beer's Law only holds true for dilute solutions. If your unknown sample's absorbance is too high and falls in the plateau region, perform an accurate dilution (e.g., a \(1:10\) dilution), re-measure its absorbance, and multiply the calculated concentration by the dilution factor.
Quick Review Summary
• Colorimetry measures visible light absorption (\(380\text{--}780\text{ nm}\)) to find solute concentration.
• Beer-Lambert Law: Absorbance is directly proportional to concentration (\(A \propto c\)) along a fixed \(1.0\text{ cm}\) path length.
• Filter Choice: Must be the complementary colour to the solution.
• Blanking: Sets the baseline (\(0.00\text{ Absorbance}\) / \(100\%\text{ Transmittance}\)) to cancel out background absorption.
• Calibration Graph: Plot Absorbance on the y-axis against Concentration on the x-axis; draw a line of best fit through \((0,0)\) and interpolate unknown values.