Module 8 colour stuff

What is complexation?

A metal ion bonds with one or more ligands to form a complex ion; this can change colour or solubility.

Give an example of complexation.

AgCl(s) dissolves in NH₃ because Ag⁺ forms the soluble complex ion [Ag(NH₃)₂]⁺.

What is the core idea of colorimetry?

Use how much visible light a coloured species absorbs to determine its concentration.

A solution appears a certain colour. What does that tell you about the light it absorbs?

It transmits the colour you see and absorbs other wavelengths, especially the complementary colour.

Why is the complementary colour usually chosen in colorimetry?

The solution absorbs it strongly, giving a more sensitive absorbance measurement.

What is the basic pathway through a colorimeter?

Light source → coloured filter → sample/cuvette → detector → diecorder.

What does the filter do in a colorimeter?

Selects a suitable range of visible wavelengths for the sample to absorb.

What does absorbance actually represent?

How much of the incident light is absorbed rather than transmitted through the sample.

What is the Beer-Lambert law?

A = εlc, where ε = molar absorptivity, l = path length and c = concentration.

According to Beer-Lambert law, how do concentration and path length affect absorbance?

Increasing concentration or path length increases absorbance, if other conditions remain constant.

Why does increasing concentration increase absorbance?

More absorbing particles are present in the light path, so more light is absorbed.

How is absorbance related to incident and transmitted light?

A = log₁₀(I₀/I), where I₀ is incident light intensity and I is transmitted light intensity.

What is a calibration curve really showing?

The relationship between known concentration and measured absorbance under fixed conditions.

How do you construct a calibration curve?

Prepare standards of known concentration → measure absorbances → plot concentration on x-axis and absorbance on y-axis → draw a line of best fit.

How do you use a calibration curve to find an unknown concentration?

Measure the unknown absorbance and interpolate that value on the calibration line to read its concentration.

Why must standards and an unknown be measured under the same conditions?

So differences in absorbance are due to concentration rather than wavelength, path length or instrument conditions.

What should you do if an unknown absorbance lies outside the calibration range?

Dilute the unknown into the calibration range, remeasure it, then account for the dilution factor.

What is the purpose of a blank/reference?

To account for absorption from the solvent, cuvette and background so measured absorbance is due to the analyte.

What should a blank contain?

Everything in the sample except the analyte being measured.

What is UV-Visible spectrophotometry?

A technique measuring UV-Vis absorption. Pathway: Light source → monochromator → sample → detector → absorbance/spectrum.

What is the main difference between a colorimeter and UV-Vis?

A colorimeter uses coloured filters and visible light, while UV-Vis uses a monochromator to select precise wavelengths across UV and visible regions.

What is a monochromator and why is it useful?

A component that separates light into wavelengths and selects a narrow wavelength, allowing more precise measurements than a coloured filter.

What is λmax and why is it useful?

The wavelength of maximum absorbance; measuring there usually gives the greatest sensitivity to concentration changes.

What is a chromophore, and why does it matter in UV-Vis?

The part of a molecule that absorbs UV/visible light; different chromophores can absorb different wavelengths and produce different spectra.

What are the main practical errors in colorimetry/UV-Vis?

Dirty cuvettes, fingerprints, bubbles, wrong wavelength, inaccurate standards/dilutions or inconsistent cuvettes/path lengths.

What is AAS used for in Module 8?

To determine the concentration of a metal ion in an aqueous sample by measuring its absorption of light.

What is the core principle of AAS?

Atoms of each element absorb characteristic wavelengths of light; greater concentration of target element produces greater absorbance.

Why must the sample be atomised in AAS?

To convert the metal species into free gaseous atoms that can absorb their characteristic wavelengths.

Why is AAS element-specific?

Each element has unique electron energy levels and therefore absorbs characteristic wavelengths of light.

What is the basic pathway through AAS?

Element-specific light source → atomised sample → monochromator selects characteristic wavelength → detector measures transmitted light.

What does the monochromator do in AAS?

Selects the characteristic wavelength being measured so absorption by the target element can be isolated.

What relationship is used to determine metal-ion concentration in AAS?

Higher concentration → higher absorbance; known standards are used to construct a calibration curve.

How do you determine an unknown metal-ion concentration using AAS?

Measure standards → construct absorbance vs concentration calibration curve → measure unknown absorbance → interpolate its concentration.

Why are standards and the unknown measured under the same AAS conditions?

So differences in absorbance are due to concentration rather than changes in experimental conditions.

Why is AAS useful for environmental analysis?

It can quantitatively measure very low concentrations of metal ions in samples such as water.