Adjusting pH
Updated
A short step where the commonest mistake is doing it too enthusiastically.
Adjust at the end, on the bulk
Once everything is dissolved and the batch is at its working temperature, because pH reads differently warm and because every addition moves it.
Not on a sample, on the bulk. A corrected sample tells you the direction and not the amount.
How to do it
Use a dilute solution of the adjuster rather than the concentrate. A 10 percent solution gives you control; neat caustic gives you a pocket at pH 14 that destroys fragrance locally before it disperses.
Add slowly, with mixing, and wait. pH takes time to equilibrate through a batch and the reading drifts for a minute or two after each addition.
Measure with a calibrated meter, at a recorded temperature.
And approach the target from one side, slowing as you near it.
Overshooting
The characteristic failure, and it is expensive because correcting back adds salt.
Every overshoot and correction adds counter-ions, and salt changes viscosity, can reduce solubility and can push a surfactant system off its peak.
So a batch adjusted back and forth three times is not the same product as one adjusted once, even at the same final pH.
If you overshoot badly, it is frequently better to add more of the unadjusted base formula than to titrate back.
What to record
The reading before adjustment, the amount of adjuster added, the final reading, and the temperature.
A batch that needed noticeably more adjuster than usual is telling you a raw material lot has changed, which is information you want before the next batch rather than after.