pH drift
Updated
The most useful diagnostic in the property: pH moves before anything else is visible.
The earliest warning you get
A formula that has changed chemically has usually changed pH, and pH is measurable in seconds.
So a stability program that checks nothing else should check pH, and a batch that is out of specification on pH is telling you something before the customer does.
What causes it
An unbuffered formula, which will drift with any small change in a raw material.
Raw material lot variation, particularly in materials sold as a solution at a nominal concentration.
Absorption of carbon dioxide from air, which drags an alkaline product down over time and is worse in a partly empty container.
Reaction with the container, which is rare in plastic and real in some metals.
Degradation of an ingredient, which is the one that matters because it means something else has happened too.
Why it matters
Cleaning performance changes, because the mechanism is pH-dependent at both ends.
The preservative may fall outside its effective range, so a drift produces a microbial problem downstream.
Container compatibility changes.
And the hazard classification and the labeling can change, which is a compliance problem rather than a quality one.
Catching it
A target and a tolerance on the formula sheet, and a reading on every batch.
Retained samples measured at intervals: a month, three months, six, twelve. That series is what tells you whether a formula is stable, and it takes minutes.
Buffer the formula if it drifts, rather than adjusting each batch back, which treats the symptom.
And keep the meter calibrated, because a drifting meter looks exactly like a drifting product.