First rule out a measurement error
The pH on the specification sheet is the value after sterilisation at 25 °C, usually given as a range such as 7.3 ± 0.2. Common false alarms:
- Measuring the medium while it is still at 50 °C. Each 10 °C can shift the reading by 0.1 or more and can damage the electrode.
- An uncalibrated electrode or expired buffers.
- Measuring before sterilisation and forgetting that autoclaving lowers pH by about 0.1 to 0.2.
When adjustment is genuinely needed
- Adjust before sterilisation, with 1 N NaOH to raise and 1 N HCl to lower, adding small amounts and stirring well before each reading.
- For agar media, adjust after the agar has dissolved while the medium is still liquid.
- If more than 0.3 units of correction is needed, suspect the weighing or the water quality before adding more base.
Why most formulations need no adjustment
The buffering components (phosphates, carbonates and others) are built into the dehydrated medium. Prepared at the stated concentration in purified water, the pH falls within specification. A need for correction usually points to water quality (tap water, acidic deionised water) or a weighing error.
Related products
Phosphate-buffered formulations such as Buffered Peptone Water and Tryptic Soy Broth tolerate preparation errors better; unbuffered formulations such as Peptone Water and Nutrient Broth are more sensitive to water quality.