I am trying to prepare a Minimal Media with lower and higher pH than usual. I had success with the pH 3 media but the higher pH one is giving me trouble.
Right now the media contains:
(NH4)2SO4
Citric Acid
MgSO4
FeCl3
Trace elements
Phosphate buffer at pH 13 (prepared with monosodium and disodium phosphate according to the HH equation for the given pH)
I usually weight The ammonium sulfate and Citric Acid, add the needed amount of water, add the MgSO4, FeCl3 and Trace element solution and then add the rest up to 1L of buffer. Then as soon as I check the pH again, it has been lowered back to 7-8 and as the first drop of base touches liquid, it starts to get cloudy and precipitate.
Any ideas on how to achieve my MM with pH 13 and overcome this situation? Thanks
Ps: more details about preparation can be provided if needed!
I don't think phosphate is a good buffer at that pH. Just because you can make a solution of the right pH doesn't mean it has any buffering capacity. A quick glance at phosphate buffer charts shows that phosphate is usually used in the pH 6-8 range, which is most likely the problem you are having.
That's probably your MgSO4, it won't like the base addition and with the Na2HPO4 will form an insoluble salt.
Leave it out, add everything else, pH and then add in the MgSO4 last.
Mykhaylo Semenov Petrov , You are precipating both Mg and Fe as phosphates at that pH. Likely also some of your trace elements.
Perhaps you can use borax as buffer.
Perhaps of some interest for your query:
https://www.researchgate.net/post/Im_looking_for_a_good_pH_13_buffer_recipe_for_hydrolysis_experiment
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