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ph? when i should corect the ph of media?


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6 replies to this topic

#1 blondy11

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Posted 22 October 2011 - 10:48 AM

My question is when should correct ph of my media? Before or after reaching the final volume in the media?

#2 pito

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Posted 22 October 2011 - 01:24 PM

After autoclaving ;)


What kind of media are you preparing?


A lot of people add it after then reached the final volume. Normally you only add 1 or 2 (maybe up to 10) drupplets.. This isnt going to make such a big difference....

If you add the drupplets before you reach the final volume, the pH will change anyway if you then add some more water....

If you don't know it, then ask it! Better to ask and look foolish to some than not ask and stay stupid.


#3 bohr86

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Posted 22 October 2011 - 08:25 PM

After autoclaving Posted Image


What kind of media are you preparing?


A lot of people add it after then reached the final volume. Normally you only add 1 or 2 (maybe up to 10) drupplets.. This isnt going to make such a big difference....

If you add the drupplets before you reach the final volume, the pH will change anyway if you then add some more water....

I'll agree with you ~

#4 blondy11

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Posted 22 October 2011 - 10:53 PM

the media is MRS broth, MRS agar, Nutrition broth and ather
I always adjusted ph before autoclaving, and then only spill the media
if i do it after autoclaving how the media will be sterile?

#5 pito

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Posted 23 October 2011 - 03:09 AM

the media is MRS broth, MRS agar, Nutrition broth and ather
I always adjusted ph before autoclaving, and then only spill the media
if i do it after autoclaving how the media will be sterile?


Adjusting after autoclaving: its a tricky thing because you risk contamination.. and its just something people dont like.. its too much of a fuss... (however if you need to work at a certain pH level.. sometimes you do need to do this).
You can do this by working sterile, under the hood.. But its a fuss (a lot more timeconsuming) and you risk contaminating the media.

However: in most cases (with some experience) you can adjust the pH so that it would be correct after autoclaving...
In most cases the pH will drop a bit due to the autoclaving. So you adjust the pH a little bit before autoclaving so it would be exact what you want.
For example: I often prepare medium with pH level 7,4 because I know that after autoclaving it would be +- 7,2 (and I need 7,0-7,2).
If I would make it (before autoclaving) at pH 7,2 before autoclaving, I risk ending up at 6,9-7,0.
This is just an example, in most cases the pH doesnt matter that much and you can work in a larger range.

You see what I mean?


How you mean, you spill the media?
You mean you adjust the pH before autoclaving and then you use the media?
Or you mean that you adjust the pH before autoclaving and then "remove" some of the media from the bottle to end up with (eg) 500ml ?

If you don't know it, then ask it! Better to ask and look foolish to some than not ask and stay stupid.


#6 blondy11

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Posted 23 October 2011 - 03:35 AM

I mean ""adjust the pH before autoclaving and then you use the media"" and distribute in Petri dishes

From everything said PH is adjusted after reaching the final volume of the media?

#7 pito

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Posted 23 October 2011 - 09:34 AM

I mean ""adjust the pH before autoclaving and then you use the media"" and distribute in Petri dishes

From everything said PH is adjusted after reaching the final volume of the media?


Yes, its indeed after you reach the final volume that you adjust the pH.
There are two reasons why:
- adjusting the pH before you reach the final volume has no real sense because you would change the pH anyway if you add some more water
- when adjusting the pH you add at max 10 druplets.. this is nothing compared with 500ml

(ok, you can adjust the pH right before you reach the final volume by (for example) using 495ml and then adjusting it and then adding the 5ml - x drupllets still needed to get 500ml... but face it: you dont measure the volume that accurate (or are you really going to pipette out Xdrupplets?:P.. Its not that you can be 100% sure that its 500ml.. there is an error anyway on the flasks etc you use. So it would not be more accurate then when adjusting after you reached the final volume)

If you don't know it, then ask it! Better to ask and look foolish to some than not ask and stay stupid.





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