What is Colorimetry?

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Grade Level:

Class 12

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Definition

What is it?

Colorimetry is a scientific technique used to measure the concentration of a coloured compound in a solution. It works by shining light through the solution and measuring how much light is absorbed, as darker solutions absorb more light.

Simple Example

Quick Example

Imagine you have two glasses of Rooh Afza. One has a little syrup, and the other has a lot, making it much darker. Colorimetry is like having a special 'eye' that can tell exactly how much syrup is in each glass by how dark it looks.

Worked Example

Step-by-Step

Let's say we want to find the concentration of a coloured dye in a water sample.

1. First, prepare 'standard' solutions of the dye with known concentrations (e.g., 10 mg/L, 20 mg/L, 30 mg/L).
---2. Measure the absorbance of each standard solution using a colorimeter. For example:
- 10 mg/L solution: Absorbance = 0.2
- 20 mg/L solution: Absorbance = 0.4
- 30 mg/L solution: Absorbance = 0.6
---3. Create a calibration curve by plotting absorbance (Y-axis) against concentration (X-axis). You'll see a straight line.
---4. Now, take your unknown water sample and measure its absorbance. Let's say it reads 0.5.
---5. Locate 0.5 on the Y-axis (absorbance) of your calibration curve. Draw a horizontal line to intersect the plotted line.
---6. From the intersection point, draw a vertical line down to the X-axis (concentration).
---7. Read the concentration value from the X-axis. In this case, it would be around 25 mg/L.
---Answer: The concentration of the dye in the unknown water sample is approximately 25 mg/L.

Why It Matters

Colorimetry helps scientists and engineers in many fields. In medicine, it's used to test blood sugar levels or detect diseases. In environmental science, it helps check water quality, ensuring our drinking water is safe. Chemical engineers use it for quality control in industries, making sure products are consistent.

Common Mistakes

MISTAKE: Assuming all coloured solutions absorb light equally, regardless of concentration. | CORRECTION: Remember that a more concentrated solution (darker colour) will absorb more light, while a less concentrated solution (lighter colour) will absorb less light.

MISTAKE: Not using a 'blank' or 'control' solution when calibrating the colorimeter. | CORRECTION: Always use a 'blank' (usually the solvent, like water) to zero the colorimeter. This removes any absorbance from the solvent itself, ensuring only the coloured substance is measured.

MISTAKE: Using any random light colour to measure absorbance. | CORRECTION: For best results, use a light colour that is strongly absorbed by the solution, which is usually the complementary colour to the solution's actual colour (e.g., if the solution is blue, use red light).

Practice Questions

Try It Yourself

QUESTION: A red solution has an absorbance of 0.3. If the concentration is doubled, what would you expect the absorbance to be? | ANSWER: 0.6

QUESTION: Why is it important to use a 'blank' solution (e.g., pure water) when performing colorimetry on a water sample containing a coloured impurity? | ANSWER: To zero the colorimeter and account for any absorbance by the water itself, ensuring only the impurity's absorbance is measured.

QUESTION: If a blue solution shows maximum absorbance when green light is passed through it, what might be wrong with the setup? | ANSWER: Blue solutions absorb red/orange light most strongly. If it's absorbing green light most, the setup might be incorrect, or the solution isn't truly blue, or the wrong wavelength filter is being used. It should absorb the complementary colour.

MCQ

Quick Quiz

What does a colorimeter primarily measure?

The temperature of a solution

The amount of light absorbed by a solution

The volume of a solution

The pH of a solution

The Correct Answer Is:

A colorimeter measures how much light a coloured solution absorbs. This absorption is directly related to the concentration of the coloured substance in the solution. It does not measure temperature, volume, or pH.

Real World Connection

In the Real World

In India, colorimetry is used in many labs. For example, in a pathology lab, a technician might use a colorimeter to check glucose levels in a patient's blood sample to diagnose diabetes. The blood sample reacts with a chemical to produce a colour, and the intensity of this colour, measured by the colorimeter, tells the doctor the sugar concentration. It's also used in food quality control to check for artificial colours in sweets or spices.

Key Vocabulary

Key Terms

ABSORBANCE: The amount of light a substance takes in | CONCENTRATION: How much of a substance is dissolved in a given amount of solution | CALIBRATION CURVE: A graph showing the relationship between absorbance and known concentrations | BLANK: A control sample, usually the solvent, used to zero the instrument

What's Next

What to Learn Next

Great job understanding colorimetry! Next, you should explore 'Spectrophotometry'. It's a more advanced technique that builds on colorimetry, allowing scientists to measure light absorption across a wider range of colours (wavelengths), giving even more detailed information about a substance.