Neutralizing a Caustic Batch Without Reliable Instrumentation
A 2,000 L tank contains 5.0 wt% sodium hydroxide solution with a density of 1.05 kg/L. Before disposal to the sanitary sewer, the pH must be between 6 and 9. An operator plans to add 98 wt% sulfuric acid, density 1.84 kg/L, from a drum until the pH probe reads 7. The tank has a working agitator, but the temperature probe is broken and the pH probe was last calibrated three months ago. The plant's pretreatment limit is pH 6–9 and a maximum discharge temperature of 40°C. The solution is currently at 20°C. The operator calculated that about 70 L of acid is needed and wants to add it in one charge to save time. No fume hood is available, and the tank is in a room with general ventilation only.
1. Verify the stoichiometric volume of 98 wt% sulfuric acid needed to neutralize the caustic. State any assumptions.
Read the worked answer
Assume complete neutralization to sodium sulfate: 2NaOH + H2SO4 → Na2SO4 + 2H2O.
Mass of solution = 2,000 L × 1.05 kg/L = 2,100 kg.
Mass of NaOH = 2,100 × 0.050 = 105 kg.
Moles of NaOH = 105,000 g / 40.0 g/mol = 2,625 mol.
Moles of H2SO4 required = 2,625 / 2 = 1,312.5 mol.
Mass of pure H2SO4 = 1,312.5 × 98.08 = 128,700 g = 128.7 kg.
Mass of 98 wt% acid = 128.7 / 0.98 = 131.3 kg.
Volume = 131.3 kg / 1.84 kg/L = 71.4 L.
The operator's 70 L is slightly below the stoichiometric amount, so it would likely leave the solution alkaline, not at pH 7. The calculation assumes the caustic is exactly 5.0 wt%, the acid is exactly 98 wt%, and no other alkalinity is present. A titration or lab analysis should confirm the actual concentration before charging. The 71.4 L value is a stoichiometric estimate, not a pH-controlled addition setpoint.
2. Estimate the adiabatic temperature rise from neutralization and assess whether a single rapid charge is safe.
Read the worked answer
Use the heat of neutralization of NaOH with sulfuric acid, approximately 57 kJ per mole of NaOH (about 114 kJ per mole of H2SO4).
Heat released = 2,625 mol × 57 kJ/mol = 149,600 kJ.
Total mass after acid addition ≈ 2,100 kg + 131 kg = 2,231 kg.
Assume a specific heat of about 4.0 kJ/(kg·K) for the dilute salt solution.
ΔT = 149,600 / (2,231 × 4.0) = 16.8 K.
Final temperature ≈ 20 + 16.8 = 36.8°C.
That bulk estimate is below the 40°C limit, but it is an average. A single rapid charge without reliable agitation or temperature measurement can create local hot spots much hotter than 37°C, especially where the acid enters. Local boiling, splashing, and acid mist are credible. The broken temperature probe removes the ability to detect an excursion, and the uncalibrated pH probe removes the ability to stop at the right endpoint. The general ventilation is not adequate for acid mist if the batch overheats. A single 70 L charge is not safe under these conditions. Add the acid slowly with agitation, provide cooling if needed, and monitor temperature independently.
3. The operator adds acid until the pH probe reads 7, but a grab sample later reads pH 10.5. Explain what may have happened and what the engineer should require before discharge.
Read the worked answer
A pH probe reading 7 while the grab sample reads 10.5 indicates a probe error, likely from calibration drift, fouling, or a stale reference junction. The probe may have been reading low, so the operator stopped adding acid before neutralization was complete. The opposite error is also possible if the probe later drifts the other way, which could cause over-acidification and a low-pH discharge.
The engineer should not accept a single probe reading as the endpoint. Requirements before discharge:
- Calibrate the pH probe with fresh pH 4, 7, and 10 buffers and record the slope.
- Replace or repair the temperature probe so the 40°C limit can be verified.
- Confirm the endpoint with an independent grab sample and, if available, a second pH meter or titration.
- Add acid incrementally with agitation, allowing mixing time before each reading.
- Check the final pH and temperature against the pretreatment limits before opening the discharge valve.
- Document the batch volume, acid added, final pH, temperature, and calibration records.
If the tank is already at pH 10.5, additional acid is needed, but it must be added under controlled conditions. The engineer should also verify that the tank material and venting are compatible with acid service and that acid mist controls are adequate.