This is my first time making ampicillin stock solution for LB agar plate. What I have in the lab is ampicillin anhydrous from Alfa Aesar.
The desired stock concentration is 100 mg/ml (10 ml in total), I calculated how much NaOH I need to dissolve the ampicillin(1 mol ampicillin : 1 mole of NaOH). After dissolving all the ampicillin / the solution becomes clear, the pH was ~8.5 so I added HCl to adjust the pH to 7.0. However, as I was adding HCl drop by drop, the solution started to precipitate (although it went away eventually). Does that mean the stock solution is no longer good?
I will probably buy ampicillin sodium salt when this batch is over, looks like a lot of people have a hard time dissolving / adjusting pH with the anhydrous....
Ampicillin is quite robust so most likely it will work fine. You can test fairly easily by seeing whether or not Amp sensitive bacteria will grow on an LB agar plate containing 50-100 ug/mL from your stock solution.
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