Preparing Solutions - (Oct/29/2012 )

Hello,

I have the following recipe for a solution that I need to prepare:

KH₂PO₄ ----------0.6%
(NH₄)₂SO₄ --------- 2.0%
NaCl ---------------1.2%
MgSO₄ x 7 H₂O---0.25%
CaCl₂ x 7 H₂O ----0.16%

If I am to understand this correctly. I could take 100 mls and add 0.6g, 2.0g, 1.2g, 0.25g, and 0.16g of appropriate item correct? It's just weird to see percentages.

Thanks!

Correct - these are called weight per volume (often abbreviated to w/v) and are typically expressed as a percent. You can also have volume per volume percentages.

Thanks for your help!

I prepared the solution, but it precipitated after I autoclaved it. I'm thinking it's a pH problem, but how do I know what it is supposed to be? Or am I supposed to assume the pH is netural?

Could be a pH problem, but it is probably more likely that the precipitate is a combination of the different salts (my guess probably potassium sulphate). You could filter in place of autoclaving.

Hi,

Wouldn't filtering take out the benefit of the Potassium Phosphate from the media though?

I tried to bring the solution to neutral pH and it made it worse with even more ppt. So, I went back the other way to acidic which helped dissolve the remaining ppt. However, did I just mess up the salt solution, thus, my media that I'll be putting my salt solution into?

How can I figured out what the pH is supposed to be?

Add magnesium sulfate and calcium chloride after autoclaving. You can filter sterilize the stocks or autoclave them separately. It's a quite common method in minimal media.

Add them after cooling. You may see something cloudy forming at the time you add magnesium sulfate and calcium chloride, just mix the solution well and it will disappear.