Acid-base chemistry, titration curves, buffer capacity, reagent demand calculations, and the control strategies that keep pH within setpoint in industrial and wastewater applications.
Natural waters resist pH change through the carbonate buffer system. Understanding buffer intensity is essential for sizing reagent doses and predicting control behaviour.
The carbonate buffer system dominates pH behaviour in most natural waters:
H₂CO₃ (carbonic acid) and HCO₃⁻ (bicarbonate) buffer around pH 6.3HCO₃⁻ and CO₃²⁻ (carbonate) buffer around pH 10.3The buffer intensity β quantifies how much acid or base is needed to change pH by one unit:
β = buffer intensity (eq/L per pH unit)CT = total carbonate species concentration (mol/L)K₁ = first dissociation constant of carbonic acidBuffer intensity is at a maximum at pH = pK₁ (6.35) and pH = pK₂ (10.33), and at a minimum near pH 8.3 — the equivalence point of the bicarbonate system. This is why pH 7–8.5 is the hardest region to control: the water has low buffer capacity and a small reagent addition causes a large pH swing.
Water at pH 7.5, alkalinity = 100 mg/L as CaCO₃ (2.0 × 10−3 mol/L), 20°C. Estimate buffer intensity.
Converting a pH target into a reagent mass requires knowing the water's acid/base demand — typically expressed through acidity, alkalinity or titration to endpoint.
The alkalinity of water is its capacity to neutralise acid. For most waters, alkalinity ≈ [HCO₃⁻] + 2[CO₃²⁻] + [OH⁻] − [H⁺]. In mg/L as CaCO₃:
Q = water flow rate (m³/h)ΔAlk = alkalinity to be neutralised (mg/L as CaCO₃)Macid = molar mass of acid reagent (g/mol)n = equivalents of H⁺ per mole of acid (1 for HCl, 2 for H₂SO₄)P = reagent purity (fraction, e.g. 0.93 for 93% H₂SO₄)Q = 200 m³/h, initial pH = 9.2, target pH = 7.0. Water alkalinity = 180 mg/L as CaCO₃. At pH 9.2, most alkalinity is HCO₃⁻; to reach pH 7.0 requires neutralising ≈ 70% of bicarbonate = 126 mg/L as CaCO₃. Use 93% H₂SO₄ (M = 98 g/mol, n = 2).
Q = 50 m³/h, pH = 3.5, target pH = 7.0. Acidity titration to pH 7 = 450 mg/L as CaCO₃. Use 50% NaOH (M = 40 g/mol, n = 1, P = 0.50).
Each reagent has a different reaction speed, buffering effect, cost profile and handling risk. The choice depends on target pH, flow, existing alkalinity and regulatory constraints.
| Reagent | Reaction | Speed | Buffer effect | Best for |
|---|---|---|---|---|
| H₂SO₄ (93–98%) | 2H⁺ + SO₄²⁻ | Instant | Adds sulphate — no buffer | Large flows, pH > 8, industrial |
| HCl (32%) | H⁺ + Cl⁻ | Instant | Adds chloride — no buffer | Food-grade, smaller flows |
| CO₂ (liquid) | CO₂ + H₂O → H₂CO₃ | 2–5 min | Forms HCO₃⁻ — buffers near pH 6.3 | Potable water, gentle pH reduction |
| NaOH (25–50%) | OH⁻ + H⁺ → H₂O | Instant | No buffer; raises Na⁺ | Industrial effluent, fast correction |
| Ca(OH)₂ (slurry) | OH⁻ + H⁺ → H₂O | 10–30 min | Adds Ca²⁺ + HCO₃⁻ — buffers | Large flows, mining, lowest cost |
| Na₂CO₃ (soda ash) | CO₃²⁻ + H⁺ → HCO₃⁻ | 5–10 min | Strong buffer near pH 8.3 | Moderate pH raise, boiler water |
CO₂ dosing for potable water is increasingly popular because it is gentle, food-safe and self-buffering. The stoichiometry is:
The practical dose is 5–20 mg/L depending on initial pH and alkalinity. CO₂ is delivered as liquid in pressurised cylinders or bulk tanks, vaporised and injected through a diffuser. No hazardous chemical handling — but the injection system must be gas-tight and the contact tank vented.
pH is the most nonlinear control variable in water treatment. Fixed-gain PID fails without compensation.
The process gain of a pH loop is the slope of the titration curve at the operating point. Near pH 7 (the equivalence point of most waters), the gain is enormous — a tiny reagent addition causes a large pH swing. At pH 4 or pH 10, the gain is modest.
Design rules for pH control:
Acid mine drainage: Q = 100 m³/h, pH = 3.0, target = 7.5. Titratable acidity = 800 mg/L as CaCO₃. Hydrated lime slurry at 5% w/w (50 g/L Ca(OH)₂).
PID tuning, gain scheduling and split-range control for pH loops.
Read MoreWhere the target reagent dose comes from for neutralisation reactions.
Read MoreConvert reagent mass into pump flow and stroke settings.
Read MorePump type and material compatibility for acid, base and lime duty.
Read MoreSend us your water analysis, flow rate, target pH range and available reagents. We will calculate reagent demand, size the dosing pumps, specify the control loop and deliver a P&ID.
Our expertise spans multiple industries with sector-specific water treatment solutions.