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NSWChemistryQuick questions
Module 8: Applying Chemical Ideas
Quick questions on Colourimetry, UV-vis and AAS explained: HSC Chemistry Module 8
10short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is beer-Lambert law?Show answer
For a solution that absorbs light, the absorbance $A$ is related to the path length $l$ and concentration $c$ by:
What is building a calibration curve?Show answer
1. Prepare standards by serial dilution of a stock solution of the target species. Use at least five standards bracketing the expected concentration range. 2.
What is colourimetry?Show answer
The simplest version. A coloured filter (a piece of coloured glass or plastic) selects a band of visible light a few tens of nanometres wide. A photocell measures the light passing through the cuvette. Suitable for any solution with a visible colour.
What is uV-visible spectrophotometry?Show answer
A more capable instrument. A diffraction grating (monochromator) selects a narrow (about 1 nm) band anywhere from 200 to 800 nm. A photomultiplier or photodiode detector measures the transmitted intensity.
What is atomic absorption spectroscopy (AAS)?Show answer
The technique of choice for trace metal analysis. Three components are unique:
What is forgetting to use $\lambda_{\max}$?Show answer
Sensitivity drops away from the peak; using the wrong wavelength means a worse detection limit and a non-linear plot.
What is not zeroing on a matrix-matched blank?Show answer
If your sample is in acid, your blank should be the same acid. Water-only blanks under-correct.
What is working outside the linear range?Show answer
Beer-Lambert breaks down for $A > 2$. Dilute and re-measure.
What is reusing the wrong lamp on AAS?Show answer
A copper lamp cannot measure lead. The lamp must match the analyte.
What is treating AAS as good for non-metals?Show answer
AAS measures atomic absorption by metal atoms in a flame. It is not used for $Cl^-$, $SO_4^{2-}$ etc.