universe84a

Introduction of  Culture Media pH Assay

Culture Media pH Assay is very useful during culture media preparation for providing favorable pH of the medium. ”pH” indicates a number of ”H” hydrogen ions concentrations. pH 7 indicates neutral pH. pH above 7 indicates alkaline pH. pH below 7 indicates acidic pH. Normally tap water is pH 6.8.  Most of the pathogenic bacteria need a slightly alkaline pH of 6.2-7.4. Vibrio cholerae requires a more alkaline pH of 9.0. pH must be checked using pH paper when the media is prepared.

Types of pH paper used are-

1. Wide range pH paper
2. Narrow range pH paper
3. Litmus paper

All the different types of pH are commercially available. The color change is then compared with the color shade against the pH value that gives the exact pH value of the media.

During the preparation of a media after dissolving all the ingredients in water and making up to the final volume, a drop is soaked a piece (1-inch size of the pH paper ) and the color change is then matched.

pH Meter for Culture Media pH Assay

A special instrument called a pH meter is also available that can be used to check the pH of the media.

Procedure of Culture Media pH Assay

• Clean a beaker for holding the prepared media to be checked.
• Add the media in the clean beaker.
• Clean the electrodes with a piece of tissue paper. Checked the Ph of the reference standards first.
• Then check the media pH by immersing the electrode into the media. pH is ready by the galvanometer scale.

List of  pH indicators for Culture Media pH Assay

Indicator  – Range  -Colour change

Thymol blue –   1.2 – 2.8 -Red to Yellow

Bromophenol blue – 2.8 – 4.6 – Yellow to Violet

Bromocresol green -3.6 -5.2 -Yellow to blue

Methyl red: 4.4 -6.2: Red to Yellow

Litmus: 4.5 -8.3:  Red to Blue

Bromocresol purple: 5.2 – 6.8:Yellow to Violet

Bromothymol blue: 6.0 – 7.6: Yellow to blue

Neutral red:  6.8 – 8.0: Red to Yellow

Phenol  red: 6.8 – 8.4: Yellow to purple pink

Cresol red: 7.2  -8.8: Yellow to violet red

Thymol blue: 8.0 -9.6: Yellow to blue

Phenolphthalein: 8.3 – 10.0: Colourless to blue

Thymolphthalein: 9.3 -10.5: Colourless to blue

BDH ” universal”: 3.0 -11.0: Red- Orange- Yellow- green-blue

Andrade’s indicator: 4.0 -5.5: Yellow to dark reddish-pink

Some results of incorrect recondition for Culture Media pH Assay

Most of the laboratory media are available as dehydrated by hygroscopic power so the laboratory personnel should take all the precautions to add the correct volume of distilled water/de-ionized water to dissolve the powder forms. And they should also close the lid tightly after they take out the power. The above mentioned in the table are the points to consider for preparing good quality media.

Keynotes on Culture Media pH Assay

• All the pH indicators those range come under the alkaline range, the final color changes are the same as those names. e.g. Bromothymol blue-final color blue (alkaline range) except neutral red
• pH can be assayed by conventional technique (pH strip) and modern way using a digital pH meter as shown above image.