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Agar problems..... extremely desperate. - Low-phosphate agar not setting (Apr/12/2010 )

Hi all,

We're at our wits end with regards to this issue we're having at our lab. Trying to make low phosphate citrate (2.4 mg/L) media agar, which is not setting!!!! Have previously used Gibco microbiology agar, which is over a decade old, that has worked and has solidified the media. We've run of that and have tried used a wide variety of brands and none of the agar seem to be setting the media. On top of that the media is precipitating out. Please please please help! If anyone has any ideas why or have experienced similar things we'd like to hear from you. I hope we don't have to age our agar before we can use it for our stuff.


Check the pH of your medium. Low pH medium will inhibit gelling of agar.
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precipitation can occur because of the heat during autoclaving and interacting chemical compounds, maybe you should autoclave two or more solutions (i.e. phosphate citrate in water and agar in water) seperatly and mix it after autoclaving when cooled to 50C. But this depends on the ingredients of your agar.

check pH. If it is not too low like already suggested try to use more agar. not all agar brands have the same quality and purity so increasing the amount of agar often helps.

And are you sure your autoclave is working fine? If the agar is not dissolved properly it will not gel.


Thank you for the replies. Tried adjusting the pH and finally it worked!