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Is there a buffer at acidic pH? (pH 1 and 2)

buffers acidic pH pH 1 pH 2

Best Answer mdfenko, 05 September 2013 - 03:49 AM

depending on which way you need to resist change in pH (toward more acidic or more basic, buffering is best at pK+ or -1.0 pH unit):

 

pyrophosphate has pK1=0.9; pK2=2.0 you can use it for both pHs

 

phosphoric acid has pK1=1.0

 

oxalic acid pK1=1.2

 

histidine and maleic (not malic) acid both have pK1=1.8

 

aspartic and glutamic acid both have pK1=2.1

 

lysine has pK1=2.2

 

glycine has pK1=2.4

 

let us know if you require any other suggestions

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#1 dustbunny000

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Posted 04 September 2013 - 10:59 AM

I want to make a buffers at pH 1 and 2. Does any one know of a buffer I can use? I found a protocol which uses HCl and KCl, but I really do not want to use KCl because of my system.

 

Thanks!



#2 hobglobin

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Posted 04 September 2013 - 11:05 AM

Citric acid with HCl will be around 2, lower I don't know.


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#3 dustbunny000

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Posted 04 September 2013 - 11:30 AM

It's really funny that you said that because that is actually what I'm doind right now! I use citrate buffer for a pH 3 solution and I already have the stocks. I do need that pH 1 data point though!



#4 hobglobin

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Posted 04 September 2013 - 11:42 AM

can't you use the ready-made buffers of the ph-meter also for this? there you have pH 1 and 2 and you're sure to have it right.
For pH 1 Sigma-Aldrich says hydrochloric acid / potassium chloride as ingredients.


One must presume that long and short arguments contribute to the same end. - Epicurus
...except casandra's that belong to the funniest, most interesting and imaginative (or over-imaginative?) ones, I suppose.

That is....if she posts at all.


#5 mdfenko

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Posted 05 September 2013 - 03:49 AM   Best Answer

depending on which way you need to resist change in pH (toward more acidic or more basic, buffering is best at pK+ or -1.0 pH unit):

 

pyrophosphate has pK1=0.9; pK2=2.0 you can use it for both pHs

 

phosphoric acid has pK1=1.0

 

oxalic acid pK1=1.2

 

histidine and maleic (not malic) acid both have pK1=1.8

 

aspartic and glutamic acid both have pK1=2.1

 

lysine has pK1=2.2

 

glycine has pK1=2.4

 

let us know if you require any other suggestions


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#6 El Crazy Xabi

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Posted 05 September 2013 - 03:33 PM

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



#7 Borek

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Posted 23 September 2013 - 01:50 PM

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.



#8 mdfenko

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Posted 24 September 2013 - 03:43 AM

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

 

 

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.

i think there may be some confusion about what buffers are and how they work.

 

here are a couple of publications that will explain buffer theory and practice:

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Edited by mdfenko, 24 September 2013 - 03:45 AM.

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#9 Borek

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Posted 24 September 2013 - 04:49 AM

I believe I have quite good grasp about what the buffers are and what they do. As far as I can tell nothing that is written in the documents you have attached contradicts what I wrote (quite the opposite*).

 

Buffer solution is a solution which keeps the pH constant when you add small amounts of acids and/or bases. Typically we achieve this effect by using a solution that contains a conjugate acid/base pair - that guarantees that adding small amounts of acid/base (and neutralizing acid or its conjugate base) doesn't change pH of the solution by much. 

 

Question is - what is important, fact that the solution keeps constant pH, or fact that it contains a conjugate pair? For practical purposes it is the first thing that matters - pH stability. How it is achieved is secondary. Unfortunately, typically buffers are taught by telling students "buffer is a solution containing a conjugate pair", which is just a way of making a secondary characteristic the most important part of the problem. But when you look at numbers it is obvious that the conjugate pairs is not always necessary.

 

Let's say I have a 100 mL of 0.1M acetic acid buffer at pH 4.75. I add 1 mL of 0.1 M HCl. pH drops down from 4.75 to 4.73 - pretty nice buffering effect, pH stays almost constant, that's what we wanted, that's what we use buffers for.

 

Now let's take another solution, 100 mL of 0.1M HCl - with pH of 1.00. I add 1 mL of 0.1M HCl, just like I did previously - and pH doesn't change at all.

 

Nothing strange, after all, I am mixing pH 1.0 solution with pH 1.0 solution, so perhaps I am just trying to cheat and it was accidental, and I will get another result if I will try to add strong base? Let's see.

 

Acetic buffer (identical like the one used before) - but I am adding 1 mL of 0.1 M NaOH. pH changes from 4.75 to 4.77.

 

0.1M HCl, 100 mL, adding 1 mL of 0.1 M NaOH - pH changes from 1.00 to 1.01. See, the change is even lower than it was in the case of solution that contained a buffer! So perhaps 0.1M HCl solution already BEHAVES like a buffer?

 

And that's where the buffering capacity gives an answer. Please check the plot I mentioned in the previous post. It clearly shows that solutions of low or high pH (which means they already contain some strong acid or base) do behave as if they were containing a buffer, so there is no need to add another one, consisting of a conjugate pair. It works only for highly acidic and highly basic solutions, say pH below 2.0 and above 12.0, but when it works - it works.

 

You will rarely see this discussed in the textbooks, as they mostly deal with buffers working in the middle of the pH scale, where the best approach is to use typical buffer consisting of an acid and its conjugate base. Unfortunately, the only book I am 100% sure it contains similar discussion is rater obscure - see it here.

 

*Please note that Stoll and Blanchard paper clearly states on the second page, that as they discuss pH 3-11, they ignore presence of H+ and OH-. You can't do that when you are dealing with pH 2 solution.

 

Note: I ignored ionic strengths of the solutions to not muddy water, but even including it won't change the conclusion.



#10 mdfenko

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Posted 24 September 2013 - 05:43 AM

then it appears that the confusion is based on terminology, specifically "conjugate acid" and "conjugate base".

 

when dealing with acid-base interactions the proton acceptor is referred to as the "conjugate base" and the proton donor is referred to as the "conjugate acid" (see this wikipedia page).

 

so, the examples you give are still dealing with conjugate acids and bases (except your example of adding 0.1M hcl to 0.1M hcl).

 

this discussion is, however, more of an academic exercise than useful.


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#11 Borek

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Posted 24 September 2013 - 07:56 AM

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.



#12 El Crazy Xabi

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Posted 24 September 2013 - 08:15 PM

 

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

 
 

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.

i think there may be some confusion about what buffers are and how they work.
 
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+


Edited by El Crazy Xabi, 24 September 2013 - 08:19 PM.


#13 mdfenko

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Posted 25 September 2013 - 07:25 AM


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.

 

 

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.


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#14 El Crazy Xabi

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Posted 25 September 2013 - 05:52 PM

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



#15 Borek

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Posted 26 September 2013 - 01:22 PM

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?







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