the selection and use of appropriate analytical techniques (gravimetric analysis, volumetric analysis, colorimetry, UV-visible spectroscopy and atomic absorption spectroscopy) to determine the concentration of analytes in a water sample, including comparing the suitability of techniques for major and trace analytes

What this dot point is asking

VCAA wants you to choose the most appropriate analytical technique for a given analyte in a water sample, and to compare techniques on the criteria that matter: detection limit, accuracy, type of analyte (ion, metal, coloured complex), cost and time. The five techniques in scope are gravimetric analysis, volumetric analysis (titration), colorimetry, UV-visible spectroscopy and atomic absorption spectroscopy (AAS).

The answer

The five techniques at a glance

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

Choosing by concentration range

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 $BaSO_4$).

Moderate analyte (1 to 100 mg L^-1): 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.

Trace analyte (below 1 mg L^-1, especially below 0.1 mg L^-1): AAS for metals, UV-Vis after derivatisation for some non-metals. Gravimetric and titration are essentially useless at this level.

Choosing by type of analyte

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.

Anions ($Cl^-$, $SO_4^{2-}$, $NO_3^-$, $PO_4^{3-}$): gravimetric (chloride as $AgCl$, sulfate as $BaSO_4$, phosphate as $Mg_2P_2O_7$) and titration (chloride by Mohr or Volhard). AAS does not directly measure anions. UV-Vis works for $NO_3^-$ and $NO_2^-$ at low concentration and after suitable colour development.

Coloured organic species (food dyes, natural pigments, some pollutants): colorimetry or UV-Vis directly.

Total hardness, alkalinity, acidity: titration (EDTA for total hardness; acid-base for alkalinity).

Choosing by cost and practicality

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.

Colorimetry uses a simple visible-light photometer; modest cost; widely used in field water-quality kits.

UV-Vis spectrophotometers are common laboratory instruments and reasonably priced; can replace colorimeters for most coloured analytes.

AAS is the most expensive, requires a dedicated technician, separate hollow-cathode lamps for each element, and a fuel-oxidant flame or graphite furnace. The cost is justified for trace-metal work that simpler techniques cannot do.

A practical decision flow

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.
2. Does the analyte absorb in the UV-Vis range, or can it be reacted to form a coloured product? If yes and the level is moderate, use colorimetry or UV-Vis.
3. Is there a clean acid-base, precipitation, redox or complexometric titration available, and is the level moderate to high? Use titration.
4. Does the analyte form a clean, insoluble precipitate, and is the level high? Use gravimetric analysis.

Compatibility with the water matrix

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. $BaSO_4$ can occlude small amounts of nitrate). Titration suffers if the matrix has buffering capacity that blurs the endpoint.

Common traps

Recommending titration for parts-per-billion contaminants

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

Recommending AAS for chloride

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.

Forgetting that colorimetry needs a coloured species

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

Treating gravimetric and titrimetric methods as outdated

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.

Stating a "best" technique without considering cost

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.

In one sentence

Choose a water-analysis technique by matching the analyte's concentration (gravimetric and titration for major analytes, colorimetry and UV-Vis for moderate, AAS for trace metals) to the analyte's type (metallic cations to AAS, anions to gravimetric or titration, coloured molecules to colorimetry or UV-Vis) and the practical constraints of cost, time and sample matrix.

Examples in context

Example 1. Nitrate monitoring in the Werribee Irrigation District. Southern Rural Water monitors nitrate in irrigation runoff entering Port Phillip Bay, where excess nitrogen contributes to algal blooms. Nitrate is colourless, so direct UV-vis at $220 \, \text{nm}$ or, more commonly, colorimetric reduction to nitrite by cadmium followed by reaction with Griess reagent to produce a pink azo dye (max absorbance $543 \, \text{nm}$) is used. With $\epsilon = 4.6 \times 10^4 \, \text{L} \, \text{mol}^{-1} \, \text{cm}^{-1}$, a sample of absorbance $0.230$ in a $1.00 \, \text{cm}$ cuvette gives $c = 5.00 \times 10^{-6} \, \text{mol/L}$ or $0.31 \, \text{mg/L}$ NO$_3$-N, below the ANZECC trigger of $0.7 \, \text{mg/L}$ for lowland rivers.

Example 2. Multi-method survey of Yarra River trace metals. Melbourne Water uses three complementary methods at each Yarra River monitoring station. AAS measures dissolved lead and copper at ppb levels using element-specific hollow-cathode lamps and graphite-furnace atomisers. Anion chromatography detects chloride, sulfate and nitrate simultaneously, typical at $1-100 \, \text{mg/L}$. Gravimetric analysis (residue on evaporation at $180^{\circ}\text{C}$) gives total dissolved solids, typically $200 \, \text{mg/L}$ in the lower river. Each method matches the concentration range: AAS for parts-per-billion metals, chromatography for moderate anions, gravimetric for major totals. Cost per sample varies from $80$ dollars (gravimetric) to $250$ dollars (AAS suite).

Try this

Q1. State the most appropriate analytical technique to determine each: (a) $5 \, \text{ppb}$ lead in drinking water; (b) sodium chloride concentration in seawater at $35 \, \text{g/L}$; (c) phosphate in fertilised wastewater at $5 \, \text{mg/L}$. [3 marks]

• Cue. (a) AAS (or ICP-MS). (b) Volumetric (Mohr titration) or gravimetric. (c) Colorimetry with molybdate reagent.

Q2. A sample of $50.0 \, \text{mL}$ of treated wastewater is analysed for sulfate by gravimetric precipitation as $\text{BaSO}_4$. The dried mass is $0.1167 \, \text{g}$. Calculate concentration of sulfate in $\text{mg/L}$. [3 marks]

• Cue. $n(\text{BaSO}_4) = 5.00 \times 10^{-4}$ mol; mass $\text{SO}_4^{2-} = 5.00 \times 10^{-4} \times 96.1 = 48.05 \, \text{mg}$; concentration $= 48.05 / 0.0500 = 961 \, \text{mg/L}$.

Q3. A water sample contains both $\text{Ca}^{2+}$ at $50 \, \text{mg/L}$ and $\text{Cu}^{2+}$ at $50 \, \mu\text{g/L}$. (a) Suggest a suitable method for each. (b) Justify each choice. (c) Calculate moles per litre of each. [2+2+2 marks]

• Cue. (a) Ca: EDTA titration or AAS. Cu: AAS or ICP-MS. (b) Ca higher concentration suits titration (cheaper, adequate); Cu trace level suits AAS (low detection). (c) $[\text{Ca}^{2+}] = 1.25 \times 10^{-3}$; $[\text{Cu}^{2+}] = 7.87 \times 10^{-7} \, \text{mol/L}$.