Quick questions on Choosing analytical techniques for water quality: VCE Chemistry Unit 2

The progression in detection limit (highest concentration first) is: gravimetric > titration > colorimetry > UV-Vis > AAS.

Major analyte (above 100 mg L^-1): gravimetric analysis or titration. Both are direct mass-based methods and give excellent accuracy. Titration is faster; gravimetric is the gold standard for some species (sulfate as $BaSO_4$).

Metallic cations ($Pb^{2+}$, $Cu^{2+}$, $Fe^{3+}$, $Ca^{2+}$, $Zn^{2+}$): AAS is the standard. Colorimetry works after forming a coloured complex (e.g. iron with thiocyanate). Gravimetric and titration also work at higher concentrations.

Titration is the cheapest technique. A burette, an indicator and a standard solution are all that is needed; the technique is taught in every school laboratory. Gravimetric analysis is also cheap in equipment but slow.

1. Is the analyte at trace level (below about $1\ mg\ L^{-1}$)? If yes and it is a metal, use AAS. If yes and it is not a metal, use UV-Vis with a derivatising reagent.

Water samples carry dissolved salts, dissolved organics and suspended solids that interfere with each technique differently. AAS handles complex matrices well because the atomic line is highly specific. Colorimetry can be affected by background colour or turbidity (filter first). Gravimetric analysis suffers if other ions co-precipitate (e.g.

gravimetric analysis or titration. Both are direct mass-based methods and give excellent accuracy. Titration is faster; gravimetric is the gold standard for some species (sulfate as $BaSO_4$).

titration, colorimetry or UV-Vis. Titration if a clean endpoint exists. UV-Vis if the species absorbs in the UV or visible range or if a derivatising reagent can be added.

AAS for metals, UV-Vis after derivatisation for some non-metals. Gravimetric and titration are essentially useless at this level.

titration (EDTA for total hardness; acid-base for alkalinity).

Lead at $10$ micrograms per litre is well below any titration detection limit. AAS is the only reasonable choice.

AAS detects atoms in their elemental ground state and is only used for metals (and a few semi-metals). Chloride and other non-metallic anions are not in scope.

For a colourless analyte (such as $NO_3^-$) you must add a derivatising reagent that produces a coloured complex.

They are still routine in water laboratories where the concentration is in the right range, the matrix is clean and the technique is the most cost-effective option.

AAS for major-ion calcium at $80\ mg\ L^{-1}$ is overkill; an EDTA titration gives the same answer at a tenth of the cost.