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#02 Biochemistry Buffers Lecture for Kevin Ahern’s BB 450/550

#02 Biochemistry Buffers Lecture for Kevin Ahern’s BB 450/550

Kevin Kevin Ahern: Alright! Good deal. Let’s see. A couple things. Somebody asked if I would send, or several people asked
if I would send matching, problems for the 6th
edition of the textbook compared to the 7th and I did that and hopefully
that’s helpful to you. As I noted in my e-mail, sometimes the problems
change a little bit so I can’t tell
you they’re exactly the same problems but at least they’re the ones I
assigned the students last year so you can see
approximately where those are. I have put a couple of books
on reserve in the library. They’re not yet
available I think. The library takes a little while to get stuff out there, so, when I get the word from
the library they’re available, they’ll be available to you. And I’ve got a bunch
of 6th edition textbooks sitting around as well
so if there’s interest in those and you
want to take a look at one of those come see me, I can work with that also. Okay, not yesterday, Wednesday, Monday,
whatever it was… okay, Monday, I got through talking, about just sort
of an introduction about chemistry and
I started talking about solutions and pH and today I’ll spend
a fair amount of time talking about pH and buffers. It’s important that
we all get on the same, page with respect to this. Now somebody asked me a question just before class about, is it true I don’t let you
use calculators on an exam? And that’s true… you don’t use
calculators on exam. But you also won’t
have to come up with a number on an exam, okay? You’re not going to
have to do a logarithm in your head for example, okay, it’s not
something like that. But you will have to
know how to use equations and how those equations
tell you information. I’ll give you some
examples of that today. So even though there are problems you may be actually using
a calculator to do them, all the problems that
I will assign you, you will not need
a calculator to do. Yes, ma’am? Student: So we should commit
all the equations to memory? Kevin Ahern: Should you commit
all the equations to memory? That’s a very good question. In fact, I will give
you every equation that you need to
know on the exam. Ahh [class laughing] right? Okay, so you won’t need
to memorize any equations. You won’t have to crunch
a number as such, okay? You will have to know
how to use equations and get information
from those equations by their very nature. So that’s something you
will need to know how to do. The TAs will be helping you with that process and
of course I’m available to help you with
that process as well, so don’t sweat the
calculator component, okay? Alright, well
usually in my lectures what I have are a
series of figures I go through and show you
various principles and so forth, and this is one place today and today’s an unusual lecture in that I don’t really have much in the way of figures
I’ll be going through and showing you, simply because your book actually I think is rather short-sighted. It’s one of the few
places where the book is not very good in its
coverage of the subject. It’s not very good in
covering pH and buffers. And so it’s important
to understand these concepts because, as we shall see, understanding how the pH of a solution affects molecules, we understand
better how it affects charges of molecules
and ultimately how that affects
these molecules that, that are in proteins. Proteins, we are going to
come back to many times, are essential obviously
for cellular life, but their structure is
essential for their function. And one of the things
that we will see when we start talking
about proteins, and I’ll actually… [buzzing] you know, when they
redesigned this room, I was told that was
going to go away. Space aliens are
still here I guess. Okay, when they, proteins get different charges, they adopt different shapes. And so we change the
ability of a protein to function as we change
the pH of a solution. So, what I’m going to do today… [buzzing] oh, don’t do that… [rumbling continues] okay. What I’m going to
go through today is talking about pH and buffers, okay, so it’s very
important to understand that. Alright, well last
time I introduced the topic I said pH of course we know is a negative
log with a hydrogen ion concentration and
pOH is a negative log with a hydroxide
ion concentration. Freshman chemistry, okay? pH plus pOH is equal to 14, okay? And by the way, I think you should be able to add 14 and subtract 14 without
a calculator so there are, I can’t tell you, you won’t have to do simple math, but you won’t have
to do any crunching. Another concept I
want to introduce here, I talked last time a little about the fact there’s a difference between strong
acids and weak acids. And we will be mostly concerned in this course with weak acids. That is acids that do not
completely dissociate in water, okay, at a certain pH, alright? Now a prime example of that, acedic acid. As I said, acedic acid ionizes to
a very limited extent. If I put it into water, I might get one molecule in a thousand that
are ionizing, okay? I put HCl in water
and I get every one of them ionizing, meaning they come apart. So there is a
fundamental difference between weak acids
and strong acids. If I know how much
HCl I start with, I know how many
protons there are there. I don’t necessarily know
that if I have acetic acid. The number of protons
that I will have free in a solution where
there’s acetic acid will depend on the pH
of that solution, okay? The ionization of acetic
acid varies with pH, that’s number one. The ionization of a
weak acid varies with pH. Okay. Now we’ll see that mathematically and I hope you’ll think
in mathematical terms instead of memorization terms. I can tell you for example
if the pH is higher, we’re going to
have more ionization than if we have the
pH being low, okay? When I say ionized with
respect to a weak acid, I’m talking about
a proton coming off. That’s what ionizing
is all about. For HAc I can write HAc goes to H+ plus Ac-. Okay. That is an ionization
right there. I’m making two ions. Now, this equation
you’re going to see over and over and over because it’s important
that we understand what’s happening
in that ionization. Alright. Well how do we determine, are all weak acids the same? They’re not all the same. In fact, we see enormous variety
in the strength of weak acids. If I were to define
strength of weak acid to you, I would say that at a given pH, a relatively strong weak acid will ionize more than a
relatively weak, weak acid. Now we’ve got strong weak acids and weak, weak acids, alright. Nothing like
confusing the picture on the second day of class. Okay. That’s important, okay. Now, how do we compare those? Well we compare those
where there’s a measure of a strength of an acid
it’s a constant known as Ka. We won’t even concern
ourselves with that. We’re going to be
concerned with the negative log of Ka which is the pKa. So just like ph
is the negative log of a hydrogen ion concentration, pOH is the negative log of
the hydroxide ion concentration, pKa is the negative log of the Ka. We probably won’t talk
about Ka again after today. We will talk a lot about pKa. So what is pKa? pKa is the measure of
the strength of an acid. A strong acid like
HCl has no pKa, it completely comes apart, okay. A weak acid like, acetic acid has a
pKa of 4.76, okay. What does that mean? Okay, I’ll tell you what
that means in a second. But I’m going to compare
the pKa of acetic acid with that of formic acid, okay. Formic acid is also a weak
acid but its pKa is 3.75. And no, you don’t need
to memorize these numbers, you’ll be given them
if you need them. Okay. If I compare acetic
acid to formic acid, formic acid has a pka of 3.75, acetic acid has a pKa of 4.76, formic acid is stronger of a
weak acid than acetic acid is. Okay? Are we clear on that? So when comparing the two, I compare the two pKas, the one that has the lower
pKa is the stronger acid. Now we can go through, we can derive all the math, as necessary to do that, but we don’t really need to do that in this class, okay. Simple concepts about what Ka is. Well, what pKa is, I’m sorry. Okay. I said HCl, strong
acid, doesn’t have pKa. Okay. We don’t, we don’t
even consider it. Because it just
completely comes apart. We can’t mess with that. Alright. So, the lower the pKa,
the stronger the acid. Alright? [buzzing] I don’t know why it does that. Male student: Do you have
your phone in your pocket? Kevin Ahern: I do have
my phone in my pocket, but I don’t think that
should be doing that. Should I, should I mess with it? Male student #1: Yeah Male student #2: Yeah, that
will get feedback across. Kevin Kevin Ahern: I’ll
put it on airplane mode. I’ve tried this before but it
didn’t seem to make go away. [Buzzing continues] Female student: [inaudible] Kevin Kevin Ahern:
I’ll just jump. Okay. Now, I apologize for that noise. I think it’s something
that this room is haunted or something, it just doesn’t, it’s stuck there. Alright, let’s think about that, that weak acid, let’s think about
acetic acid for a moment and let’s think
about what happens with it in a solution. I said if I just
dump it into water, okay, a pretty small percentage of that HAc becomes H+
and Ac-. When I’m concerned about pH, the only thing I’m
concerned about is the H+, not the Ac-, right? If I have it sitting there and let’s say I put
a million molecules of acetic acid into
that aqueous solution of water and one
thousand of those, protons come off, I’m going to have
one thousand protons from there come off, I’m going to have one
thousand Ac minuses, right? And I’m going to have 999
thousand HAcs left behind, right? Everybody with me? Let’s say I add some sodium
hydroxide to that solution. Sodium hydroxide of
course is a strong base, and like a strong acid, a strong base completely
dissociates in water, so if I put a million molecules of NaOH into a solution of water, I get a million molecules of Na+ and a million molecules of OH-. Completely dissociates. So strong acid is equivalent to strong base in
terms of its strength. They completely come apart. Let’s imagine, if you will, that I put a
thousand of molecules, let’s make it fun, let’s put 449,000 molecules of OH- into that solution. Okay? What’s going to happen
to that solution? I’m going to have an
excess of OH-, right? Is the pH going to go way up? It’s gonna go up. But it’s not going to go up
as much as you might think. Why not? Go ahead. Female student: The
buffer’s to the limit? Kevin Ahern: Well,
you’re getting ahead of me in terms of definitions
of what a buffer is, but yes, buffers, there are buffers, this is acting as a buffer, but I don’t even want
to use that term yet. Why doesn’t the pH just
go through the roof? Female student: The
acedic acid dissociates? Kevin Ahern: Acetic
acid can dissociate. So I had 499, I’m sorry, I had 999 molecules of that Hac that was sitting there, right? Some of those could
give out protons, right? And what’s acedic, what’s OH going to do
when it hits a proton, well it’s gonna hit with
a proton and make water, thereby neutralizing it, right? So I had a thousand
molecules of H+ and I had a thousand
molecules of Ac-, and I add 449,000 molecules, why am I think 449, 499 thousand molecules of NaOH, 499 thousand
molecules of that Hac is going to give up protons. It’s going to give up protons. And at that point, I’m gonna have 500,000 molecules of Ac- and I’m going to have 500,000 of HAc. Right? 1,000 plus 499,000, right? With me? You can say, “yes, Kevin.” No you can’t obviously. Okay. At that point I have
500,000 molecules of HAc, I have 500,000 molecules of Ac-, and I’ve got a higher pH. That higher pH turns out
to have an important name. When I have equal numbers
of Ac- and HAc, I’ve reached the pKa. pH equals pKa when
salt equals acid. With me? pH equals pKa when
salt equals acid. So pKa is simply a pH. It’s a special pH. It’s the pH at which
salt equals acid. Now, I told you that acetic
acid had a pH of 4.76, right? That tells you something, it tells you that pKa, I said pH, has a pKa of 4.76. That tells you that
pKa is a constant. It’s a constant for a given acid. Whenever I have acedic acid, it will always be 4.76. It will never change. I can change the protons, I can change the hydroxide, but the pKa will be a constant. The pH will change,
but the pKa won’t. Alright? Well it turns out
there’s an equation that relates these
things I’m telling you just conceptually
at the moment, okay? The equation is an equation you’re going to hear a lot about, it’s called the Henderson
Hasselbalch equation. The Hendrson Hasselbach
equation states that, here we go, pH equals pKa
plus log of Ac- over HAc. More commonly we will say
that pH equals pKa plus log, the concentration of salt, divided by the
concentration of acid. I called the thing that has
lost the proton the salt. And I think you’ll
find it much easier to understand if you call it
the salt instead of the base. Okay? So, pH equals pKa plus log, the salt, over acid. Acid is the thing
that has the proton. Acid is not the proton. The acid is the thing
that has the proton. The difference between the salt and the acid is a single proton. There it is right there. Okay? Now, everybody understand
the terms I’m talking about? Alright? Salt, acid, Henderson
Hasselbalch equation. We’re going to use the
equation in just a second. How do I make
something into a salt? I take protons, I take
protons off of an acid, right? I can convert acid into
salt by taking protons off and in the example
I just gave you, how did I take
protons off the acid? I added a strong base. Okay? If I wanted to put
protons onto that salt, how do you suppose I would do it? I would add a strong acid. Okay? How would I protons on? Well, if I start dumping protons
into the reaction right here, what’s going to
happen to this equation on the basis of the
principle of Levoisier? Male student: It’s
going to push it left. Kevin Ahern: It’s going
to move to the left, which means I’m
going to make this and I’m going to lose this. Right? You already saw I
went to the right when I started
taking protons away, the solution starts
trying to make them up. Starts trying to replace them,
starts making more Ac-. There’s a one to
one relationship. For every molecule of
strong base I added, I lost one of these
and I made one of these. The same holds true
if I go the other way. If I go the other way and I add protons
to this solution, if I add 500 molecules of ACl, I’m going to lose 500 of these, I’m going to make 500 of these. Understanding that is
the most thing students screw up on buffer problems. The single most common
thing they screw up on. There’s a one to one relationship between adding and
subtracting acids and bases. That’s all there is to it. A very simple concept, okay? Now, let’s go back
to our equation. Our equation said pH
equals pKa plus the log, the concentration of salt, divided by the concentration
of the acid, alright? In the example I gave you, we had 500,000 molecules of salt and we had 500,000
molecules of acid. Let’s plug in to that equation. They’re in the same volume so the volume cancels out, we don’t have to worry
about concentration, we can actually
use numbers, okay? you have a hand up. Female student: Is that
a dash or a negative? Kevin Ahern: Uh, where? Female student: [inaudible] Kevin Ahern: This? That’s a dash. Yeah, that’s not a
negative, that’s a dash. That’s a good question, I didn’t notice there
was a dash there. Maybe I’ll remove that dash
so you don’t get confused. That’s a dash, not a negative pH, that’s just a dash. pH equals pKa plus
log of salt over acid. Now, Let’s think about that. Let’s plug in our terms. I want to find the pH of
the solution I’ve just, I’ve just defined for you. I just made a solution
of acetic acid. I have a pKa of
4.76 because that’s a constant for acetic acid. And I’ve got 500,000 molecules of Ac
– and 500,000 molecules of HAC. What’s the pH? 4.76! You can tell by either
the fact I just told you when the two are equal, if that makes sense, but you can also tell it more
importantly from the equation. The log of 500,000 over 500,000 is the same as the log of 1, and the log of 1
is equal to zero. Yes, you have to know that, but I’ll even put
that on the exam. Okay? That’s cool. Okay. That’s cool. So now I see mathematically
why the pKa is the pH at which the salt equals the acid because when the
salt equals the acid, this log term becomes
zero and pH equals pKa. Questions? I’m kinda going through
this kinda blig-da-bleh. Am I that clear? I know I’m not. Am I that fiersome? I probably am. Yeah? [laughing] Oh, is there a question? I’m sorry. Yeah? Male Student: You said
if you want to increase the [inaudible], you
add a strong base? Kevin Ahern: If I want to
increase the amount of salt, okay, I would have
to pull protons off of the acid using a strong base. Okay. Important concept and thank
you for asking the question. If I want to make acid from salt, I’ve got to put
protons in which means I’ve got to add a strong acid. Yes? Male Student: So to make acid, you have to use acid? Kevin Ahern: In order to make
acid in this system, I have to use protons from a strong
acid, that’s correct. Okay? So if I guess there’s no questions,
you guys are ready for a pop quiz. That means you’ve already
understood it, right? Or are there questions? Because I’ll find out really
quickly if you understood it or not. I guess if there
are no questions, you must understand it, therefore the pop quiz
is irrelevant, right? Nobody has a question? Male student: I have a question. Kevin Ahern: Okay, good. Male student: What’s the Na… Kevin Ahern: You just saved
the whole class right there, they should thank you. [class laughing] Male student: What’s the Na doing while it dissasociates from
the OH in a strong base? Kevin Ahern: So I add NaOH, what’s happening to the Na? Nothing, just sits there. Male student: Just hangs? Kevin Ahern: Just sits there. Female student:
And that goes to say [inaudible] strong
acid [inaudible]. Kevin Ahern: Mhm, the Cl’s
just going to sit there. And if you keep adding
strong acid and strong base, strong acid, strong base, you’re making NaCl, and NaCl, and you’ve got
a very salty solution, and that’s all that happens. Other good questions. So, you’re saved from a pop quiz. How about that? I won’t be so nice next time. Alright, now, what we’re
starting to understand, or what I hope to introduce next is the concept of what is
something called a buffer. Okay? You used the term up
here of what a buffer was, we need to talk about
what a buffer is. So I’m gonna find
a buffer for you and then we’re going to
go through some examples. Alright? A buffer. Buffers are absolutely essential. You’ll see why the
further we go along. Alright? Definition of a buffer, a buffer is a system that
that resists change in pH. It resists them. It doesn’t prevent them. It resists them. Okay? In the example I gave you, we dumped a whole
bunch of OH in there, but the pH didn’t go up very much and I’ll show you a graph
of that in a bit, okay? The system is
acting like a buffer. It’s preventing the pH
from going up as much as it would if the
buffer weren’t there. There’s a couple of problems that I’ve assigned in your book that will illustrate to you what a buffer is and
how a buffer works. And you’ll compare those, actually, it’s not in the book, it’s actually one
of the ones I made up for you on the system, okay? Where there are
the ones that Kevin made up on the site, alright? If you click on those, you’ll see a couple of
them that will illustrate to you how a system
that has a buffer differs from a system that
doesn’t have a buffer if you have the same
amount of protons. And you’ll see there’s a very
big difference between the two. Now, so I’ve defined
a buffer for you. A buffer is a system
that resists change in pH. Yes, ma’am. Female student:
Does it work the same whether the buffer
starts in the system or whether you add
the buffer later? Kevin Ahern: Does it
matter if you add the buffer first or if you add
the other stuff later. Turns out it
technically doesn’t, no. Good question. Yes, sir? Male Student: Approximately,
what’s the effective range pH-wise of
the average buffer? Kevin Ahern: Oh this
is a very good question also and thank you
for asking that. Very much. What’s the range of a buffer? is a buffer an infinite thing? Can a buffer resist
pH change forever? No, okay. Buffers have what
we call capacity. We’ll see some examples, in fact one of the
problems in your book actually illustrates
capacity to you. It’s going to confuse
you when you first do it ’cause it’s not
gonna make any sense. And that should be
a clue that something isn’t what you think it is. But to answer your question. What’s the range of the
effectiveness of a buffer? It’s mostly a definition thing. But effectively,
most buffers are good within one pH unit
above or below their pKa. So for acetic acid, the effective buffering range, that is where it’s best
at resisting change in pH, is from about 3.76
up to about 5.76. One pH unit of its pKa. Either way. Okay? And I haven’t given you an
example of how a buffer works yet, so I’m gonna do that in a second, but other questions just
in general about buffers? Okay. Alright. I keep needing to look up also, make sure there
are people up there. Yes? Male student: Will a buffer work
outside of its effective range? Kevin Ahern: Will a buffer work
outside its effective range? Yes, I mean, will
a buffer participate in accepting and donating
protons outside that range? Yes, but its effect
on controlling the pH will be minimized. So we won’t see as
strong a protective effect if we get
outside of that range. Good question. Alright. So how does a buffer work? Well, let’s think back
to this buffer that this system that I
just described to you. I’ve got a solution that
has 500,000 molecules of Ac- and it has 500,000 molecules of HAc. Equal numbers of salt and acid. Let’s imagine that I add
10,000 protons to this system. You can do the math
pretty quickly and say, “well, you’re gonna
have 501,000 molecules “of HAc because we’re making Hac “and I have 499,000 of Ac-
because I lost Ac-“, right? How much would the pH change? Well, it turns out it’s not
going to change very much. How would I know? I plug it into the Henderson
Hasselbalch equation and what I would see
is that I would have pH equals 4.76 plus
the log of 499,000 divided by 501,000, right? That’s very close to one. That means that log term
is very close to zero. Right? The buffer is protecting this. Where there’s excess protons, the buffer grabs them. Where’s there’s excess OH, the buffer makes protons. Very, very important concept. Female student: The buffer,
in that example you just gave, did you give us
what your buffer was, or did you just say you
added this much buffer? Kevin Ahern: So, acetic acid
is the buffer system here. Thank you for asking that also. Alright. Weak acid systems
make great buffers. Anything that has a pKa makes a great buffer in
a certain range. Okay? Anything that has a pKa makes a great buffer in
a certain range. Now, in this example
I just gave you, we don’t see the
pH change very much. It’s a very miniscule change that happens and that logirithm of it actually makes it
an even smaller change. If I did this, I would
ask you a reasonable question for me to ask you on an exam would be, “is the pH higher than the pKa? “Or is the pH
lower than the pKa?” Well I don’t have a calculator! You don’t have to have a
calculator to answer that question. How would you
answer that question? Male student: It’s lower. Kevin Ahern: It’s lower? Why would you say it’s lower? Male student: It’s been
acidified and the lower pH. Kevin Ahern: Okay. So he says it’s been acidified and it’s true that’s done that. But one of the
places where students confuse themselves is
which acidification. I want you thinking
mathematically. Mathematically, why is
the pH lower than the pKa. And it is lower than
the pKa, you’re right. Female student: [inaudible] Kevin Ahern: You’re
taking the log of a number that’s less than one. Exactly! 499,000 divided by 501,000 is a number that’s less than one. Just like 499 over
501 is less than one. The logirithm of a number less
than one is a negative number. So if I have 4.76
plus a negative number, I have to have the
pH lower than 4.76. Right? What if it were higher? What would I have to do to add to the solution to make it be, for example, 501,000
molecules of Ac- and 499,000 molecules of HAc? What would I have to
add to make that happen? I wouldn’t add salt. I would have to add HCl. I could add salt, I could add salt. But not based on
what I just told you. I didn’t change the total amount. I said 499 and 501. If I add salt, I’m gonna have
501 and 501, right? I’m going to have a
different total amount. When I add a strong acid, I turn salt into acid and
my total stays the same. One million molecules. So I add a strong, base to that to make that happen. Pull those protons off to switch the HAc into Ac-. Everybody clear on that? Okay. If I told you I had
a solution that had a pH of 4.3 and the system had a pKa of 4.9, more
salt or more acid? pH 4.3, pKa 4.9. What does that say
about the log term? What does the log
term have to be? Students: It has to be negative. Kevin Ahern: It
has to be negative. Negative log term,
what do I have to have? I have to have more acid, right? Has to be less than one, which means I have to have more, and by the way, HAc only holds for acetic acid, so to make it general, we call it A
– and HA, okay. So I have to have more A
– than I have HA. Now these are the kinds of things you can work completely
without a calculator. You can manipulate that
log term in your head. Bang. You got it right there. You don’t have to
have a calculator. It’s important for students to learn how to work problems, okay, to understand the
math of what’s there, not to see the
confusion of the numbers. That’s why I don’t want
you using a calculator. I want you to think
about these things. I want you to understand them at a real level and not a “a-duh-duh-duh-duh-duh-duh-duh-duh”
level. Because if those numbers that you “duh-duh-duh-duh-duh”
in don’t have meaning, then you get garbage out as well. You have to put
the right stuff in. So what I’m trying to get you to do is to understand how
to get that right stuff in. On my exam, all
you will have to do is get a solution to the point where it would go
into a calculator. If you get down to this
is the logirithm of 3, then the logirithm of 3 is
the answer and that’s it. You don’t have to
calculate that, okay? Make sense? Okay, um, let’s see. Other questions about that? I’m going to show
you some examples after if you don’t
have any questions. Nobody? Okay, alright. Let’s see a buffer
in action, right? Yes? Female student: [inaudible]. to assume that the concentration of Ac- plus HAc is .1 more? Kevin Ahern: Not
for a buffer, no. A buffer can have
any concentration. Female student: Okay. That was the concentration
of that specific problem. Kevin Ahern: Okay. Okay. Female Student: Okay. So just don’t always assume. Kevin Ahern: No, no. So buffers can have
any concentration. And that’s important
because buffers, I can make a buffer as
concentrated as I want to. If I say the word “buffer,” here’s something I want you
guys to pop in your heads. Alright? When I say the word buffer, I want you to think of two terms. Two terms I’ve been using
over and over, alright? Salt and acid. That should immediately pop
those two up in your head. When I say buffer, if I told you I have a
buffer that is .1 molar, the first thing that
should pop into your head is salt plus acid
equals .1 molar. Because when I have a buffer, I have to have both of those. I have to have salt
and I have to have acid. Now the actual
amounts of those you may have to calculate, alright, but when I say buffer, salt plus acid equals the
total amount of a buffer. Alright? In this case it was .1
but it could be a variety, it could be anything I make up. Yes? Connie. Connie: Just to verify, did you say if the buffer’s .1, then salt plus acid equals 1? Kevin Ahern: If I
say a buffer is .1, say .1 moles of buffer, if I said I had
.1 moles of buffer, then salt plus acid would
equal .1 moles, that’s correct. Okay? Makes sense? Okay, now. The concentration of
a buffer is important. Let’s think about
a .1 molar buffer. Let’s say I have a .1
buffer of acetic acid. My favorite acid, right? And it’s at its pKa value. Its pH is at its pKa value. What does that tell me
about the concentration of salt and acid in that? They’re equal and
they’re equal to what? .05 each, right? Half the total. What if I add, let’s
say .1 molar Acl to that in an equal volume. What’s going to
happen to that buffer? Based on what I just told you, what are you going to see happen? I add HCl, I’m going
to lose salt, right? I’m going to make acid. How much salt can I lose? .05, but I just added
twice that amount of HCl. Uh oh. When I start doing
my subtracting, I discover I have a negative amount of Ac-. Can I have a negative amount of Ac-? No. Something’s wrong here, right? Something’s wrong, Houston. Okay? Female student: How
much [inaudible]? Kevin Ahern: What I’ve done. I added .1 molar, okay. What I’ve done is
I’ve just given you an example where I’ve exceeded
the capacity of the buffer. Buffers have limited capacities. Alright? They’re not infinite. So one of the reasons we change the concentration of a buffer, okay, is so we can
change its capacity. Alright? Capacity’s important. Alright, so you’ll
see one of the problems in the book will actually exceed the capacity of a buffer. When you get to it, I think you’ll discover, you’ll figure out what it is. Yes? Male student: So regardless of
the concentration of the buffer, the capacity won’t
change [inaudible]. Kevin Ahern: I’m not sure
[inaudible] the question. Male student: So you
have 500,000 or you have .1 molar versus .05 molar, the capacity’s going
to stay the same? Kevin Ahern: The capacity
is defined by the salt and acid that is there. So if I more than that of
strong acid or strong base, I’m going to exceed its capacity. Connie? Connie: So it’s not dependant
on the concentration at all? Kevin Ahern: It is dependent
on the concentration. Absolutely. Connie: Oh, well,
because you said it’s usually within
one pH unit of the pKa. Kevin Ahern: Yep, yep. Connie: What if you have a really concentrated [inaudible]. Kevin Ahern: That’s a
mathematical question that you’re asking me to
define non-mathematically. So come see me, I’ll show you mathematically
what we’re talking about. With one pH unit, you’re
not exceeding capacity. The one pH unit
is going to define how much I can add to it to
to get to that one pH unit. Right? So I’m limited by that. Alright, okay. So buffers have capacity. I can’t exceed those
capacities because if I do, the pH is going to go boing! because I no longer
have a buffer. The buffer was providing me that, that protection against
massive change in pH. When I exceed the
buffering capacity, I don’t have a buffer anymore. It’s gone. The pH is going to go sproing. Yes, sir? Male student: Could it be
generalized that the capacity is going to be defined
both by concentration and volume combined
of the buffer? Kevin Ahern: So, the
capacity of a buffer is, so the question
that you’re asking is if I know the number
of moles total a buffer, that defines
ultimately the capacity because concentration
times volume gives me that and
the answer is yes. Okay. Now, I know you’re going
to have some struggles with this and I understand that. Please come see me, please come see the TAs, work through the problems, okay? I can guarantee you
there will be help in getting and understanding of this bigger picture, okay? It’s important to get
that bigger picture. Gotta keep an eye on the time… Okay, I promised to
show you a buffer plot. And this very high
quality graphic was drawn by yours truly. [class laughing] This was for you guys
in freshman chemistry, did a titration curve, right? Did you like titration
curves in freshman chemistry? You did! Okay, good. Most people don’t like them. Alright, so this
shows the relationship between the pH and the
amount of OH I added. In this case, I
started with a solution that had essentially
all acid to start with. How do I know that? Well, I had a low pH, and right down here the low pH. How would I know a low pH would have mostly acid to start with? How would I determine that? Nobody? If I said extra credit, then
everybody would jump right? Male student: What
was the question? [laughing] Kevin Ahern: I didn’t say
I was giving extra credit, I just said if I said I was
giving extra credit, okay? The question is how would I know down here I got most
things in the acid form? Female student: [inaudible] Kevin Ahern: But I’m not
talking about protons. Protons are not acid. I’m talking about HA. How would I know I have most
everything in the HA form? You have a friend. What is the friend? The friend is Henderson
Hasselbalch equation! Alright? If the pH is low, below the pKa, what happens to that ratio? More salt, more acid? Class: More acid. Kevin Ahern: More acid, right? Bingo. Henderson Hasselbalch tells me
and it tells me very quickly. The answers to
virtually every question I’m going to ask you will
be rooted in that equation. They’re gonna be
rooted there, okay? You wanna get familiar
with that equation. Now, let’s see what
happens to the solution. I start out with the solution, it’s got protons on, I start adding sodium hydroxide. What happens to the pH? The pH rises. It rises relatively
rapidly at first. Why does it rise
relatively rapidly at first? Male student: It’s
outside the buffer zone. Kevin Ahern: It’s outside
the buffering region. The buffering region
being plus of minus 1. In this case, the pKa I’ve
got for this is about 2.5. So I’m below 2.5, it rises relatively rapidly and then it starts to level off. And it’s leveling off because
it’s acting as a buffer. It’s resisting the change. I’m adding a lot more hydroxide, but the pH is not
going up very much. Once I started getting away from that region
by more than 1 unit, I all of a sudden see
the pH start to go boing. Every buffer plot is
going to look like this. This is a visual image
of what a buffer is doing. It’s resisting a change
in pH at a certain range. The maximum resistance is
right here where pH equals pKa, and that’s another
important concept. Not only is pKa the
pH at which the buffer has equal salt and acid, it’s also the place
where there’s maximum resistance to change in pH. We have maximum
buffering capacity. There’s a question here? Yeah. Male student: Molecularly,
why does it do that? Why doesn’t it just
continue on the more you add, the more it disassociates? Kevin Ahern: It is associating. Male student: I know,
but what does it plateau? What doesn’t it just continue the breaking of the bonds? I don’t know why does it pause. Kevin Ahern: Why does it
go up and then flatten? Male student: Yeah Kevin Ahern: Okay,
well two reasons. Alright? One, plug it in mathematically. Plug in a whole bunch of
different values of salt and acid in that ratio and you’ll see
that mathematically, that’s exactly what it does, so mathematically, that’s
the answer to your question. Male student: I’m
talking more molecularly. Like bond-wise. You know what I mean? Kevin Ahern: Well you’re
talking about ionization. Right? Okay. So ionization is happening because the absence
of presence of protons. It’s Lavoisier’s equation with the HAc going to H+ and Ac-. We put pressure on that
equation one way or another. That favors the ionization. Okay? Question? Male student: Wasn’t it
Le Chatelier’s Principle? Kevin Ahern: What did I say? Lavoisier’s… Le Chatelier’s Principle! God, I do that all the time. It is Le Chatelier’s principle. Sorry. Lavoisier was the practical
inventor of chemistry, not Le Chatelier. Yes, but thank you, it is Le Chatelier’s principle. Male student: Are there
ever systems where multiple buffers that have
overlapping ranges of effect are used
where you could have one say 1.5, 2.5, 3.5? Kevin Ahern: Yeah. So can you have multiple
buffers in there? The answer is you can. We’re going to do
buffers one at a time. I figure you’ve got
enough to think about. But yes, you can. And multiple buffers do
complicate the picture a lot. We’ll see starting on Friday, I’ll be talking
about amino acids. And amino acids have
multiple buffering regions. And so you’ll see a flattening, a flattening, a flattening. That can happen. And so that is a simply example
of what you’re talking about. Okay, now I promised you guys, any questions about this? We’ve gotten through a
good number of things. I want you to, the most important message
I want you to take across, get across from this, is that the ionization is going to be related to pH and
its relationship to the pKa. pH, the ionization of
a substance is related to the pH of the solution
it’s in and its pKa value. The more the pH is above the pKa, the more the
protons will be gone. The more it’s below there, the more the protons will be on. And you don’t need
to memorize that. Henderson Hasselbalch
tells you that. What I’m saying in words to you is what Henderson
Hasselbalch is telling you. That’s a lot of stuff, why don’t we finish
with some fun? I thought we might celebrate
Henderson Hasselbalch with a song to the tune of
“My Country Tis of Thee.” I’ve never sung
in front of a class before so let’s do this. [professor and class singing] Lyrics: Henderson Hasselbalch You put my brain in shock Oh woe is me The pKa’s can make Me lie in bed awake They give me really bad headaches Oh hear my plea Sale minus acid ratios Help keep the pH froze By buffering They show tenacity Compelte audacity If used within capacity To maintain things I know when H’s fly A buffer will defy Them actively Those protons cannot waltz When they get bound to salts With this the change in pH halts All praise to thee Thus now that I’ve addressed This topic for the test I’ve got know-how The pH I can say Equals the pKa In sum with log of S o’er A I know it now Kevin Ahern: Okay,
good place to stop. Thank you [END]

16 thoughts on “#02 Biochemistry Buffers Lecture for Kevin Ahern’s BB 450/550

  1. You can follow my course at the following URL –

    (you may have to manually type this URL in your browser as YouTube frequently inserts invisible characters into URLs that interfere with cutting and pasting)

  2. Thank you so very much. The equation makes a lot of sense…no need for memorization cause you efficiently explain the logic behind it.

  3. Kevin, I can't get your numbers to add up. If I put a million molecules of HAc in water, 1000 molecules disassociate to give me 1000 H+ and 1000 Ac- and 999,000 HAc remaining. Now I add 499,000 NaOH which completely disassociates, leaving me 499,000 OH-. I would think 1000 of my OH- would immediately associate with my 1000 H+, making water, leaving me 498,000 OH-, which would then react with my 999,000 HAc to give me 498,000 water and 498,000 Ac- and 501,000 HAc remaining. Add my original 1000 Ac- to give me a final tally of 501,000 HAc and 499,000 Ac- plus a bunch of water. I can't seem to arrive at the balance you describe in the video of 500,000 each for HAc and Ac-. Am I missing something? If I put them in balance, what happens to my 1000 H+ generated when I originally added the HAc to the water?

  4. in whole my live i considered all the teachers are fools but this professor i think he is genius i rise my hat for you

  5. 18:38 online it still says a – pH in the equation. Mislead me as well until I watched the video! Don't forget to change it! haha

  6. Hello Professor, I try to watch your videos and also read biochemistry free and easy but I am not sure if they go in line with your lectures because the second chapter in video lectures is amino acids while in the book it is energy, am I using the right resource? thank you

  7. I would just very much like to thank you, I was really getting very discouraged about the possibility of passing my biochem class as my professor is not doing well at getting concepts across. Your videos have already helped me understand and effectively apply the concepts to the homework and I'm feeling a lot more confident. Thank you for considering the students struggles and fears about the subject, putting us at ease, and making your amazing lectures accessible to all!

  8. thank you very much for your generous gesture of posting these videos nursing student and your videos are really useful for me.Greetings from Colombia

  9. Thank you so much for making these lectures available to anyone. I'm a teenager from Argentina and I find biochemistry extremely interesting. I like how passionate you talk about what you teach, it is making me love biochemistry even more!

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