[Polleke]

Are there any scientists here that use PEG or have worked with it and are able to explain me why a sample with PEG and a fungus becomes very low in pH.
(maybe the fungus has some importance in the proces, but maybe its just the PEG that can make a solution lower in pH at certain circumstances).


At the start the sample had a pH of about 7, after 5 months all of a sudden the sample reaches a pH not even higher then 3.

Also important to tell: I only noticed this drastical decrease in pH with samples stored at +20°C.

It didnt happen with samples stored at 5°C.

[Arkcon]

Polyethylene glycol is pretty resistant to microbial attack, but nothing made of carbon chains lasts forever in a bottle on the bench.  Five months may simply be too long a shelf life for your samples.

[Polleke]

So you are stating that the Micro-organism is changing the PEG and thus changing the pH.

So in other words : without the micro-organism the samples would still have a normal start pH.

And why do I notice this only for the samples stored at 20°C ? Maybe the temparure is also influencing the rate of changing of the PEG?

[Arkcon]

True enough, generally, reaction rates tend to increase as temperature increases.  It could be oxidation by the air, or some other contaminant, besides microbes.  I just figure your fungus is metabolizing something and it's products are low pH -- doesn't that seem likely, especially compared to frozen samples?

[Polleke]

True enough, generally, reaction rates tend to increase as temperature increases.  It could be oxidation by the air, or some other contaminant, besides microbes.  I just figure your fungus is metabolizing something and it's products are low pH -- doesn't that seem likely, especially compared to frozen samples?

Eum, so you think that if I kept the samples airtight I might not have seen this drastical reduction in pH? (ex.=> 1 sample kept closed from the air, the other one maybe opened once during 5 monhts in sterile conditions).
Meaning that the sample kept closed would not change so fast in pH?



What you say about : True enough, generally, reaction rates tend to increase as temperature increases.  I agree with that, the temperature does influence the speed of a reaction.

I just figure your fungus is metabolizing something and it's products are low pH

==> true too, but how come I only see this when PEG is used ? Why not when I use sucrose or another substance ?

If have more samples then the ones with PEG and I only noticed it in the samples filled with PEG thus meaning that the PEG does have a big influence in the pH change.
Or there has to be some sort of specific interaction between the fungus as PEG ?

I just figure your fungus is metabolizing something and it's products are low pH -- doesn't that seem likely, especially compared to frozen samples?

====>


eum, true too, the samples stored by lower temperature do not show a lower pH, so it could indeed be that the growth of the fungus is influencing the pH change, but then how do you explain this: the fungus grows better (and faster) when stored at 5°C ! And not when stored at 20°C! 
Dont you think this is strange then?




Anyway, I am not a chemist , so I might make big mistakes or make assumptions that are completely wrong. I am just trying to figure it out.