Neutralising a Waste Acid Drum Before Drain Disposal
The first-year chemistry laboratory of a polytechnic has accumulated 5.0 L of waste hydrochloric acid left over from titration practicals; its measured pH is 2.0. College rules permit disposal through the laboratory drain only after the waste has been brought into the pH 6 to 8 band. The store holds 500 g of sodium bicarbonate (NaHCO3, 84 g/mol) and about 200 g of sodium hydroxide pellets (NaOH, 40 g/mol). A demonstrator proposes dropping in NaOH pellets until pH paper turns green, arguing that it is quicker and uses less material. Unknown to him, the same drum also received rinse water from a practical in which 0.01 mol of copper(II) sulphate had been dissolved. The laboratory in-charge must decide the reagent, the quantity, and whether this drum may be drained at all, and must record the reasoning for the safety file.
1. Calculate the mass of sodium bicarbonate required and state whether the stock is sufficient. Write the reaction.
Read the worked answer
pH 2.0 means [H+] = 10^-2 mol/L = 0.010 mol/L. HCl is a strong acid and fully dissociated, so acid present = 0.010 × 5.0 L = 0.050 mol.
NaHCO3 + HCl → NaCl + H2O + CO2 is a 1:1 reaction, so 0.050 mol of bicarbonate is needed.
Mass = 0.050 × 84 = 4.2 g.
The 500 g stock is more than a hundred times the requirement, so availability is not a constraint - the argument for NaOH 'because we have less of it' solves a problem that does not exist.
Practical notes: add the solid in small portions to the stirred acid (never acid to solid), keep the drum vented because CO2 is evolved, and re-check pH after 15 minutes of stirring, since dissolved CO2 makes the first reading misleadingly low.
2. The NaOH plan would also neutralise the acid. Give the reasons it is the poorer choice here.
Read the worked answer
Chemically it works: 0.050 mol × 40 g/mol = 2.0 g NaOH would neutralise the batch.
But the risks outweigh the saving:
- Dissolving NaOH is strongly exothermic; pellets dropped into acid cause local boiling and spattering of acid mist.
- Near equivalence the strong acid–strong base curve is almost vertical, so one pellet too many can jump the drum from pH 3 to pH 11, breaking the upper limit and needing re-acidification.
- Pellets sink and cake on the bottom, so a pH reading at the top of the drum is not the bulk pH.
- Caustic pellets are a contact hazard for a student helper; bicarbonate is a benign food-grade solid.
Bicarbonate is effectively self-limiting: excess bicarbonate plus the CO2 produced buffers the liquor near pH 8, so overshoot is far less likely. In a teaching laboratory, the safer reagent is the correct reagent.
3. State what the copper(II) sulphate changes about the decision, and what you would record.
Read the worked answer
0.01 mol of Cu2+ in 5 L is 0.002 mol/L, i.e. 0.002 × 63.5 ≈ 0.127 g/L, or about 127 mg/L copper.
Neutralising to pH 7 does nothing to remove it - the ion stays in solution and passes down the drain. Copper is toxic to aquatic life and typical drain limits are of the order of 1–3 mg/L, so this batch is roughly 40–100 times over the limit even when perfectly neutral. The acid is a small problem; the copper is the real one.
Decision to record: do not drain. Segregate the drum as heavy-metal waste. If no contractor is available, raise the pH to about 9–10 with bicarbonate/carbonate so copper precipitates as Cu(OH)2 or basic copper carbonate, filter the sludge, dry it and hand it to the authorised recycler; only the clear filtrate may then be considered for drain disposal. Keep the mass balance on file.