Is there a buffer at acidic pH? (pH 1 and 2) - (Sep/04/2013 )
mdfenko on Tue Sep 24 13:43:02 2013 said:
this discussion is, however, more of an academic exercise than useful.
Not at all.
OP wants to run an experiment in a constant, low pH. You suggest adding buffer. That will work, but it means adding substances that are potentially interfering with the researched system. I suggest checking first if the solution pH is not already stable enough on its own, in which case there is no need to add anything.
Difference between experiment run with or without addition of potentially interfering ions is not academic, it is highly practical.
mdfenko on Tue Sep 24 11:43:48 2013 said:
El Crazy Xabi on Thu Sep 5 23:33:43 2013 said:
Don't know what will you use it for but many polyprotic acids may be used for that. I work with acidophiles and the media itself does not include anything that is added because its buffering properties. Sulfuric-based solutions around pH2 are quite stable regarding the pH. Ferric iron can buffer too, at pH around 2.3
Borek on Mon Sep 23 21:50:39 2013 said:
i think there may be some confusion about what buffers are and how they work.Do you really need a typical buffer made of conjugate base and acid pair? What you need is a solution that resists pH changes - or has a high enough buffering capacity. At low and high pH buffering capacity of a solution of a strong acid (base) is quite often high enough so that you don't need anything else. Google for buffer capacity plot and you'll see what I mean.
here are a couple of publications that will explain buffer theory and practice:
I don't know why you cite me but...
Some polyprotic acids have one of the pKa extremely low, that's why they can be the base for low pH buffers...
To make a buffer you don't really need to add both conjugated base and acid but to bring the pH of the solution close to the buffering range for the given speciation. You can make a buffer with H3PO4 and NaOH if you adjust the pH to 7.0, it is not different to add NaH2PO4/Na2HPO4. It has to be with the speciation and not with the compounds you add
The sulfuric acid itself has a pKa2 of 1.9 so itself is a buffering agent at pH close to 2.
And Fe3+ constitutes one of the main buffering systems in AMD, ARD, the Tinto River and biomining operations keeping the pH around 2.1-2.3: Fe3+ + 3H2O = Fe(OH)3 + 3H+
El Crazy Xabi on Wed Sep 25 04:15:59 2013 said:
I don't know why you cite me but...
Some polyprotic acids have one of the pKa extremely low, that's why they can be the base for low pH buffers...
To make a buffer you don't really need to add both conjugated base and acid but to bring the pH of the solution close to the buffering range for the given speciation. You can make a buffer with H3PO4 and NaOH if you adjust the pH to 7.0, it is not different to add NaH2PO4/Na2HPO4. It has to be with the speciation and not with the compounds you add
The sulfuric acid itself has a pKa2 of 1.9 so itself is a buffering agent at pH close to 2.
And Fe3+ constitutes one of the main buffering systems in AMD, ARD, the Tinto River and biomining operations keeping the pH around 2.1-2.3: Fe3+ + 3H2O = Fe(OH)3 + 3H+
apologies, i'm not sure why i included your post, must have been a slip of the mouse. you are, of course, correct.
Borek on Tue Sep 24 15:56:50 2013 said:
mdfenko on Tue Sep 24 13:43:02 2013 said:
this discussion is, however, more of an academic exercise than useful.
Not at all.
OP wants to run an experiment in a constant, low pH. You suggest adding buffer. That will work, but it means adding substances that are potentially interfering with the researched system. I suggest checking first if the solution pH is not already stable enough on its own, in which case there is no need to add anything.
Difference between experiment run with or without addition of potentially interfering ions is not academic, it is highly practical.
of course it's academic. the op got the answer to the question asked (which was for a buffer to use at the pH's required for the experiment).
the op was not interested in a hcl-kcl system because the ionic strength would be too high (i'm paraphrasing) as would your suggestion of using hcl to maintain the pH at 1.
hcl alone is not considered a viable option to maintain low pH in biological systems.
so, while you are correct that you can maintain pH by overwhelming the system with acid (or base), it is still academic for the purpose presented here.
mdfenko on Wed Sep 25 15:25:27 2013 said:
apologies, i'm not sure why i included your post, must have been a slip of the mouse. you are, of course, correct.
OK, no worries
mdfenko on Wed Sep 25 15:25:27 2013 said:
the op was not interested in a hcl-kcl system because the ionic strength would be too high (i'm paraphrasing) as would your suggestion of using hcl to maintain the pH at 1.
My understanding was that the OP was afraid of using KCl, not of using HCl. Please note I never suggested HCl, it was only used as an example in calculations that showed pure acid solution keeps pH stable. Every other strong acid will work the same way, although monoprotic acids have some advantages - see next paragraph.
When it comes to ionic strength - you may want to rethink this argument. Of all possible solutions with pH 1.0 0.1M solution of a strong monoprotic acid has the lowest possible ionic strength (you can't get pH of 1.0 without 0.1M of H+ and a counterion, so ionic strength of 0.1 is an absolute minimum that will be always present). All systems you have suggested will have a higher ionic strength, so high ionic strength argument works in favor of using the simplest system, not one of those you suggested.
hcl alone is not considered a viable option to maintain low pH in biological systems.
I admit I am coming from the analytical chemistry background, so it is possible there are other reasons to not use a monoprotic acid alone in biological systems, but so far you have not named any. Can you point me to a credible source supporting this notion?
KCl/HCl is one of the few buffers for pH 2 or below, if not the only one, that are commonly listed. Indeed HCl itself wouldn't make any sense as buffer unless you go to pH -7... in theory... in a depreciable ionic strength system. Good point Borek, I didn't realise initially, I was starting to think about the ionic strength too.
Adding a chloride (KCl), though you may add NaCl, LiCl,... being equally suitable for this, increases even more the ionic strength of the system. Common ion effect applied to buffers.
KCl/HCl buffer is probably not used to keep anything alive but as far as I know it is used in enzymology and biochemistry in general.
ionic strength is a combination of the ion and counter-ion. 0.1M hcl would have an ionic strength of 0.2M.
a polyprotic acid, such as phosphoric acid (3 ionizable hydrogens), would be able to present 0.1M hydrogen ions with a counter ion concentration of ~0.033M. the ionic strength of this buffering system (ignoring any other components of the medium) would be 0.133M. sulfuric acid (2 ionizable hydrogens) would be able to present 0.1M hydrogen with a total ionic strength of 0.15M.
as for literature regarding commonly used biological buffers, i already posted some. on the other hand, it is unusual to work at such a low pH in biological systems (unless you are working with acidophiles).
El Crazy Xabi on Fri Sep 27 01:50:09 2013 said:
KCl/HCl is one of the few buffers for pH 2 or below, if not the only one, that are commonly listed
I always wonder when I see such recipes if they were not designed blindly by someone not understanding what it is all about. Like - buffer is made by mixing acid and salt, let's mix acid and salt to get a pH 1.0 buffer. KCl presence doesn't change anything - with, or without it, solution has exactly the same buffering capacity. KCl only increases the ionic strength. Could be it has some other properties, but I have no idea what purpose it can serve.
mdfenko on Fri Sep 27 12:30:51 2013 said:
ionic strength is a combination of the ion and counter-ion. 0.1M hcl would have an ionic strength of 0.2M.
By definition ionic strength is half of the sum of concentrations of all ions multiplied by their charges squared - see http://en.wikipedia.org/wiki/Ionic_strength or another ionic strength discussion (or any other source). I remember reading that it was defined this way to make ionic strength equal concentration for 1:1 electrolytes.That means for 0.1M HCl ionic strength is 0.5*(0.1*12 + 0.1*12) = 0.1.
a polyprotic acid, such as phosphoric acid (3 ionizable hydrogens), would be able to present 0.1M hydrogen ions with a counter ion concentration of ~0.033M. the ionic strength of this buffering system (ignoring any other components of the medium) would be 0.133M. sulfuric acid (2 ionizable hydrogens) would be able to present 0.1M hydrogen with a total ionic strength of 0.15M.
You are ignoring squaring of the charges. For triprotic acid ionic strength would be 0.5*(0.1*12 + 0.033*32) = 0.1985, for diprotic acid ionic strength would be 0.5*(0.1*12 + 0.05*22) = 0.15 (which is accidentally the same value you got). Both solutions have ionic strength higher than 0.1.
as for literature regarding commonly used biological buffers, i already posted some
None of the two documents you posted addresses the problem. One of them lists HCl/KCl buffer for pH 1.0 (see my post above), but doesn't say anything about whether using just HCl is wrong (and if - why).
(i learned a little bit more about calculating ionic strength, thanks)
i would guess, then, that the primary reason to use a polyprotic buffer salt is that you can maintain pH with a lower molarity than with a monoprotic "buffer" (buffering capacity).
on the other hand, using straight hcl with proteins (if that's the purpose of the medium) may endanger the peptide bonds and certain amino acids (hcl is used to hydrolyze peptide bonds for amino acid analysis, albeit at higher concentrations).