Most coolant failures are not sudden. A coolant does not stop protecting on a calendar date — its inhibitor package quietly wears out, and by the time the radiator corrodes or the water pump weeps, the damage is already done. The good news: two numbers on a routine test report tell you how much inhibitor life is left before that happens. They are pH and reserve alkalinity. For distributors and importers, reading them turns a blind 'replace every two years' into a measured decision — and turns a spec sheet into a tool for telling reliable suppliers apart.
Why inhibitor life is the real expiry date
Coolant is mostly water and ethylene glycol. Those two move heat and set the freeze and boil points. But the part that keeps the engine alive is the inhibitor package — the organic acids, silicates, phosphates or borates that stop aluminium, cast iron, copper, brass and solder from corroding. Every kilometre and every heat cycle consumes a little of that package. When it runs out, the metal starts eating itself. The freeze point can still read fine and the boiling point can still read fine, yet the coolant has become a near-neutral liquid that accelerates corrosion. That is why pH and reserve alkalinity matter more than the numbers printed on the front label.
pH: the first, cheapest number
pH measures how acidic or alkaline the coolant is, on a 0–14 scale. Fresh coolant is deliberately alkaline (basic), because that state suppresses corrosion. As inhibitors deplete, pH drifts — usually downward, toward neutral. A coolant sitting at pH 7.0 is effectively 'dead' on the corrosion front, even if it still freezes and boils correctly.
| Technology | Typical fresh pH | What the drift means |
|---|---|---|
| IAT (silicates / phosphates) | 10.0–11.0 | Falls toward neutral as silicates drop out of solution |
| HOAT (hybrid organic) | 8.5–10.5 | Wide range; track against the product's own baseline |
| OAT (carboxylate) | 8.5–9.5 | Drifts down slowly; long life but still finite |
| Phosphate / OAT tropical blends | 8.5–9.5 | Watch the low end in hot, high-load markets |
Reserve alkalinity: the number that actually predicts failure
pH tells you the current state; reserve alkalinity (RA) — sometimes called total reserve alkalinity — tells you how much buffer is left. It measures how much acid the coolant can still neutralise before corrosion takes over. In practice it is reported as the millilitres of 0.1N acid needed to titrate a sample to the endpoint: a larger number means more buffer remaining. It is the single best early-warning signal that the inhibitors are running out.
| Sample | Typical fresh RA (mL) | Replace when |
|---|---|---|
| IAT coolant | 10–20 | falls below roughly 5–6 mL |
| OAT / HOAT coolant | 15–28 | falls below roughly 6–8 mL, or below half of fresh |
| Heavy-duty / fleet formulas | 20–35 | below the OEM limit (often 4–6 mL) |
| Any coolant near end of life | < 5 | treat as spent — replace |
How to read the two numbers together
- 1Both in range → the coolant is healthy; keep to its service interval and re-test next period.
- 2pH normal but RA falling → the buffer is being consumed; schedule a change before the inhibitors run out.
- 3pH dropping toward 7 → the inhibitor is gone; replace now, regardless of the RA reading.
- 4Either number far outside the product's own baseline → investigate contamination (fuel, oil, hard water) before a routine change.
A field check distributors and fleets can run
- Keep a refractometer for concentration and a pH strip or dip pen for pH — both cost little and need no lab.
- Ask the supplier for reserve alkalinity on the spec sheet, and re-test a sample from each batch you receive.
- Keep a one-line log per fleet vehicle or per batch: concentration, pH, RA, date. Trends beat single readings.
- Set a hard floor — for example RA below 5 mL or pH below 7.5 — below which product is pulled; use your own number, matched to the formula you stock.
What to ask a supplier before you import
- pH and reserve alkalinity on the TDS, with units and a fresh baseline — not just the freeze point.
- The test method used, so you can reproduce the numbers or have them third-party verified.
- Whether RA is stated as a minimum, and what service limit the formulation is designed to meet.
- Batch-to-batch consistency data, because a spec is only useful if the drum in your warehouse matches it.
Request coolant specifications
BOLITCH supplies tropical-spec coolant with measured specifications — including pH and reserve alkalinity on the TDS — so you can verify inhibitor life instead of guessing it. Request the full spec sheet, MSDS (available on request) and samples at bolitch.com or email sale@bolitch.com.