Chemistry - How a buffer manages to keep things within a narrow span?

Discussion in 'General Science' started by atferrari, Oct 12, 2016.

  1. atferrari

    Thread Starter AAC Fanatic!

    Jan 6, 2004
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    In the past I got training to repair certain instruments, involving the usage of buffer solutions (chemistry realm). Having took them for granted I did not care what they actually were. Having signed an NDA I cannot say much more. Sorry.

    Last night I tried to understand, not precisely what a buffer is but the basics of the mechanism that guarantee to keep things within a narrow span of something even if you add (who knows what) to them.

    I hate when this happens (too often for my liking) but I could not understand what I've read. Chemistry ranks high in my stumbling blocks list.

    Usually I do not resort to analogies but maybe here I could use help in that way. I will appreciate any explanation as long you do not want me flying too high.

    Additional questions:

    Inside any living animal, do buffers exist as such or are they a creation of someone in the lab? Maybe Nature already got it patented before and we are just imitating it?

    I've seen several buffers listed with specific names. Does it mean that you cannot design a buffer for any substance you want? In the past I thought of them as being kind of "building blocks" as in electronics. Are they?

    JohnP, Wayneh, Gopher?
     
    Last edited: Oct 12, 2016
  2. jpanhalt

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    Jan 18, 2008
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    How theoretical do you want to get? Have you read about the Henderson-Hasselbalch equation (https://en.wikipedia.org/wiki/Henderson–Hasselbalch_equation ). Wikipedia is not the best place to go for a start or easiest to understand in my view. I am sure there are far more bench level explanations on the internet.

    Query 1: Absolutely, buffer's play a role in all living animals and probably all living things.. For one thing, most enzymatic reactions require a limited pH range to work effectively. In humans. the primary buffer is carbonic acid/bicrabonate; although, other anions also play a role. Your body is kept at approximately pH 7.4 (https://en.wikipedia.org/wiki/Bicarbonate_buffer_system). Even small deviations from that can be quite serious.

    Query 2: There are thousands of defined buffers. The buffered pH range goes from very low to quite high. Some have special properties, like being non-nucleophilic and non-reactive with certain things in the solution. Some may be patented, but there is nothing to keep you from designing your own buffer. Some may carry a person's name, others are named as a disambiguation a principal component (e.g., Tris, TAPS, MES).

    John
     
  3. jpanhalt

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    Jan 18, 2008
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    It occurred to me that you probably had searched Wikipedia and found it confusing. Let me give a thought experiment, that while not entirely accurate and ignoring equilibra may illustrate how a buffer works.

    Let's assume a strong acid like HCl completely ionizes in water to H+ and Cl-. A 0.01 molar (M) solution would produce a H+ ion concentration of 0.01 M. That is, a pH of 2. Now, let's assume you have another weaker acid called "acetic acid" that only ionizes 1% in water. Thus, a 0.01M solution of that acid would have 1% of 0.01M ionized to H+ or 0.0001M of H+, which equals pH4.

    Now, let's assume that the salt of an acid, regardless of whether the acid was strong or weak is fully ionized in water. That is a 0.01M solution of NaCl or NaOAc (OAc = acetate) would give a 0.01M solution of Na+ ion. Now for a big (and wrong) assumption. Let's assume that dissolving either salt in water does not affect the pH.

    For an experiment, let's say you have a solution of sodium acetate and with a little acetic acid at pH 4. What happens if you add HCl to that solution? The sodium ion in the solution can add to the Cl- of HCl to give NaCl. That remains ionized and does not affect the pH. But what happens to the H+? That free H+ can combine with an acetate to give acetic acid, which is only 1% ionized compared to HCl that is 100% ionized. Thus, the acetate anion "soaks" up H+ so the pH is not lowered as much as it would have been had acetate not been there ((i.e., the concentration of free H+ is not as high as it would be without the acetate anion) .

    Does that help? The more accurate method (Henderson-Hasselbalch) considers ion activity rather than simply concentration and the fact that salts do affect the pH.

    John
     
  4. wayneh

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    Sep 9, 2010
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    My "aha moment" when coming to understand buffers is the concept of equilibrium. Push on one side of the reaction arrow, and the composition shifts to the other side. Push on the other side, the composition shifts back.

    A buffer is a special case, because it is able to keep a solution in pH equilibrium within a narrow range of pH. It can absorb disruptions in the form of acid or base additions, and self-adjust without much change to the pH. As long as the capacity is not titrated, the new pH is not far from the previous one.

    Once the capacity is titrated, the solution is of course still in equilibrium but now the pH is not as immune to additional acid or base.
     
  5. BR-549

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    Sep 22, 2013
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    Sorta like a pH inertia of the solution. A pH resistance to change.
     
  6. wayneh

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    Maybe a little more like a glass full of water and ice. The temperature will hold pretty steady until all the water freezes or all the ice melts.
     
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  7. killivolt

    Active Member

    Jan 10, 2010
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    Ice and Water 32˚F, great way to test a Thermometer. It's (Plus or Minus - 500 btu's or within a 1/2 degree)

    Good explanation. While still above my head :oops:

    kv
     
  8. wayneh

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    Sep 9, 2010
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    Well, the ice and water in this analogy are the counterparts of the weak acid and conjugate base of a buffer. You can't move the pH until you get rid of all of one or the other.

    The "conjugate base" is just a big word for the anion A- that's left when the H+ proton dissociates from the weak acid into the solution.
    HA ⇌ H+ + A−
     
  9. #12

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    Nov 30, 2010
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    You chemistry guys always make me feel ignorant.:(
     
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  10. wayneh

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    That's the first lesson we learn in chemistry. We will always be ignorant.
     
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