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Offlinerungi
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Registered: 07/11/01
Posts: 212
Last seen: 15 years, 4 months
henderson hasselbach ph of media
    #6131061 - 10/04/06 12:44 AM (17 years, 7 months ago)

Objective: to make a 250 ml bath of media buffered at PH 4.
Question: What volume of 0.1M H2PO4, 0.1M HPO4, and distilled H2O in mililiters to obtain a final volume of 250 ml?
The agar and sugars(powder) will be disolved in this 250 ml volume of liquid and then sterilized.

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Offlinewietstocker
product ofevolution

Registered: 07/14/03
Posts: 319
Loc: Cranium, PNW
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Re: henderson hasselbach ph of media [Re: rungi]
    #6131511 - 10/04/06 07:30 AM (17 years, 7 months ago)

That is actually a really good question. How do you make up a buffer at a pH that lies between the pK's of two forms of a triprotic acid?


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InvisiblePinback
Stranger
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Registered: 07/20/02
Posts: 836
Loc: Europe
Re: henderson hasselbach ph of media [Re: rungi]
    #6132523 - 10/04/06 01:52 PM (17 years, 7 months ago)

As a rule of thumb, a buffer shows good buffering capabilities only around the acids pKa ± 1 pH unit, so a phosphate buffer (of the mono-/dibase) would not be suitable. The amount of acid/base you add would also not only depend on the pH, but also on the buffer strength. The ratio of acid/base is however determined by pH alone.

Check this link for other buffers: http://www.sigmaaldrich.com/Brands/Fluka...al_Buffers.html
and here if you are too lazy to use the Henderson-Hasselbach formula: http://dbr.csoft.net/chem/bufcalc.php

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Invisiblefastfred
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Registered: 05/17/04
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Re: henderson hasselbach ph of media [Re: rungi]
    #6135053 - 10/05/06 01:00 AM (17 years, 7 months ago)

I don't think you'll be able to come up with that pH buffer using H2PO4/HPO4. You can use the H-H equation to figure out how much to use for what pH, but a buffer will not work well unless the pH is close to the pKa.

For a pH 4 buffer you could use formate, benzoate, acetate, or propionate with good results. You also need to make sure your buffer is biocompatable.

pH 4 media seems like kind of an odd idea in the first place. I don't think that much of anything would grow in it. The media is also only going to get more acidic during growth.


-FF

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