Choosing the correct wavelength in spectrophotometry is very important because it affects how accurately you can measure a substance’s concentration.
Here’s how it is done — simply explained:
1. Use the wavelength where the substance absorbs light the most
- Every substance absorbs light best at a certain wavelength — this is called the λmax (lambda max).
- At λmax, even small concentration changes cause large changes in absorbance, giving more accurate results.
2. Find λmax using a scan
- First, run a wavelength scan of the sample.
- The instrument records absorbance at many wavelengths.
- The peak (highest point) on the graph shows λmax — use that wavelength for your measurement.
3. Avoid wavelengths where other substances absorb
- If your sample has more than one compound, choose a wavelength where only your target substance absorbs and others don’t — this avoids interference.
4. Consider the color of the solution
- The color of the solution gives a clue:
- A blue solution absorbs orange/red light (around 600–700 nm).
- A yellow solution absorbs blue/violet light (around 400–450 nm).
5. Follow standard methods when available
- In labs, standard methods or literature often list the best wavelength for common substances — these can be used directly.
In short:
Choose the wavelength where the substance absorbs the most light (λmax), avoid interference from other substances, and use standard values when available. This ensures accurate and reliable spectrophotometric results.