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

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).

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]

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 $\text{mg/L}$.

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.