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The pH of my prepared medium is off. How do I adjust it?

Written and reviewed by the STBIO technical teamPublished

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

  1. Adjust before sterilisation, with 1 N NaOH to raise and 1 N HCl to lower, adding small amounts and stirring well before each reading.
  2. For agar media, adjust after the agar has dissolved while the medium is still liquid.
  3. 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.

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.

Written and reviewed by the STBIO technical teamSTBIO MEDIA has produced peptones and microbial culture media since 2002, with ISO 22000 and Halal certification and an in-house QC laboratory.

Further reading

FDA BAM Culture Media Cross-Reference: Which Media Each BAM Chapter Uses and the STBIO EquivalentAerobic plate count, coliforms and E. coli, Salmonella, Listeria monocytogenes, Staphylococcus aureus, Bacillus cereus, Vibrio, Clostridium perfringens, yeasts and moulds and Cronobacter: the media named in each FDA Bacteriological Analytical Manual chapter, matched to Taiwan-made dehydrated media for US laboratories qualifying a second source.Read moreWhich Culture Media Do Taiwan FDA Food Microbiology Methods Specify? Cross-Reference for 21 Announced MethodsAerobic plate count, coliforms, E. coli, Enterobacteriaceae, Salmonella, Staphylococcus aureus, Listeria monocytogenes, lactic acid bacteria, yeasts and moulds, Bacillus cereus, Vibrio parahaemolyticus, Clostridium perfringens and Cronobacter: the media named in each Taiwan FDA (TFDA) announced method, matched to Taiwan-made products.Read moreWhat Are Casamino Acids? Casamino Acids vs Tryptone vs Casein PeptoneCasamino acids are casein hydrolysed completely with hydrochloric acid, consisting almost entirely of free amino acids. How they differ from tryptone and enzymatic casein peptone in hydrolysis, tryptophan, salt and use.Read more