TL;DR — Correct ≠ robust. Stress-test before you scale.
Your buffer hits pH 7.4 exactly. Ship it?
Not yet. A recipe can be right and fragile — perfect on paper, off the rails the moment something’s slightly off.
At scale, “slightly off” happens
- weighing tolerances
- concentrated-stock variation
- dilution mistakes
A 10% slip in one component is normal. The real question: how much does your pH move when it does?
The experiment: ±10%
Perturb each component ±10% and predict the pH at every point. Plot it:
- green — pH barely budges. Robust.
- red — pH falls off a cliff. Fragile.
Two recipes can both hit 7.4. One holds within 0.1 pH at ±10%; the other swings half a unit. Pick the green one — on purpose.
That’s QbD for buffers
You’re not just hitting a target. You’re choosing a formulation that forgives the real world — and you can show why.
Bottom line
Don’t ask “does it hit pH?” Ask “how hard is it to knock off pH?”