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MAKING A BUFFER
INTRODUCTION
REFERENCE: Harris Text, Section 9-5, pages 196 205
For this lab youwill be assigned to make a
It willbe up to you to determine just how to do this buffer witha particular pH and buffer capacity.
and to carry out the procedure you devise. Pay
particular attention to lecture notes on similar problems.
Consider how to approach this before coming to lab – make an effort to understand
Section 9-5 of the Harris text. Here is an example assignment
to help
you:
Prepare a pH7 t0.1 buffer such that 100 mL of this buffer solutioncan absorb between 35
t3 mmol of strong acid or strong base to effect a pH change of one unit.
In other words, the starting pH of your buffer must be between 6.9 and 7.1. At that point
you would test the buffer capacity by addition of 1MHCI or 1 MNaOH. If the bufer pH is 7.0to
begin with, andyouadd 1M HCI,then the pH must change to 6.0 somewhere after adding 32 mL
of HCl and before adding 38 ml of HCI(35 +3). If youtest your buffer capacity by adding NaOH
the same thing applies except the pH of the buffer would have to change from 7.0 to 8.0. So there
are two things which need to be considered: 1) how to attain the desired starting pH; and 2) how
to attain the desired buffer capacity. After that is accomplished, how to prepare the buffer that you
determined would work must be considered. When you are ready to use apH meter, consult your
instructor.
Once you have the buffer solution prepared to achieve the desired goals, infom your
instructor and he or she will watch you test your buffer for initial pH and buffer capacity. Use a
graduated cylinder to add either 1 M NaOH or 1 MHCIfor testing of buffer capacity.
You will use pH electrodes to monitor the pH. Please treat the pH electrodes as the
is very
breakable and sensitive instruments they are. Always be aware that the tip of the electrode
deionized
of
stream
a
with
electrode
fragile andact accordingly. Be sure to thoroughly rinse the pH
careful to
water whenever you move the electrode from one solution to another, and please be
the electrode
always leave the electrode soaking in the buffer solution you found it in. This keeps
hydrated and prolongs the life of the electrode.
pH electrode that
The stockroom should have already performed a 2-point calibration of the
IS necessary for accurate measurementS.
7-2
Buffer Preparation
base,
of a weak acid and its conjugate(A) and
mixture
a
either
of
comprised
is
weak acids
Kecallthata buffer
coniugate acid Below is a list of available
its
and
base
weak
a
of
mixture
or a
bases (B)for buffer preparation:
Substance
Ammonium chloride (A)
Ethanolamine hydrochloride (A)
Imidazole (B)
Potassium hydrogen phthalate (A)
Sodium acetate trihydrate (B)
Sodium chloroacetate (B)
Sodium formate (B)
Sodium dihydrogen phosphate (A)
Formula
pka (conjugate acid)
NH,CI
HCzH,NOCI
CsH,N2
KHC&H4O4
NaCzH302(H0)3
NaCzH2CIO2
NaCHO2
NaH,PO4
9.2
9.5
7.0
5.4
4.8
2.9
3.8
7.2
Notes:
bases, but the listed pKa Values are
i. More than half the listed substances are
for the conjugate acids.
component either the acid or base.
ii. Each substance is a source of one buffer
See the
strong acid (base).
You create the conjugate by reacting with
“Preparing a Buffer in
discussion on the top of page 202 in the Harris under
Real Life!”.
buffer that satisfies the indicated criteria:
a
prepare
to
questions
basic
3
answer
to
need
You
prepare the buffer?
What would be the appropriate Conjugate acid base system to
1.
base are required.
2. How many mmol of conjugate acid/conjugate
correct number of mmol of base and acid?
3. How will |make the buffer with
then proceed to make and
Once you’ve figured out the answers to these three questions,
a total solution 1 unit from pKa.
We will designate the base as A and the acid as HA. Although the hydrogen phthalate ion is
amphiprotic, it is the conjugate acid component of the buffer, and the listed pKa corresponds to its
ionization as a weak acid. The phthalate ion, CaHO4, is the conjugate base component. We will
also designate [A]and [HA]to be amounts (mmol) rather than concentrations (mmo/mL). Recall
that volumes do not have a direct effect on buffer solution
behavior.
I’lshoW youtwo ways to go about this.
Approach number 1: The Math Approach
This involves solving simultaneous equations. Two conditions
(solved).
1. The
initial
pH
of
the
buffer
(equations) must be fulfilled
should
be
5.6
5.6 = 5.4+ log
10 ml
2. The pH of the buffer should be 4.6 after adding
4.6 = 5.4 + log
of 1M HCI (10 mmol HCI).
([A]- 10
[HA] + 10)
SO
7-4
completely with the conjugate
reacts
mmol)
(10
acid added
This is because all the strong
solution.
buffer
base (phthalate ion) in the
A +H’’ HA
youare testing buffer capacity
if
example,
For
same.
the
Important: Not all situationsare different
occur and the 2nd equation must
would
reactlon
(B/BH*) the
a
a base,
by raising the pH with Involved.
using a baselconjugate acid
you’re
if
And
reflect the chemistry
approach is the same so continuing on…
general
the
But
different.
reactions would also look
the
to
for [A]and [HAJ, it is important
The rest is math. Solve
potassium hydrogen phthalate (in this example).
know that both originate from
Rearranging equation 1:
5.6 – 5.4 = logHA = 0.2
1002 =1.58
Rearranging equation 2:
l-10 =-0.8
loo(4
=
5.4
4.6
HAI+ 10/
10 0:8
0158
HAJ+10)
be solved by rearranging followed by
can
unknowns
2
with
equations
Now the 2 highlighted
is one suggested approach without
here
this,
do
to
way
one
than
more
substitution. There is
details.
equation 1
1 solve for [A] (in terms of [HA) in
solve for [HA]
2nd substitute that into equation 2 and
3d solve for [A]
Approach number 2: The Chemistry Approach
Starting out with the buffer equation as before, given the choice of an acid withbepke = 5.4.
5.6
buffer
should
the
of
initial
1. The
pH
5.6 =5.4+log
The first question to ask is what ratio of A/HA gives us a pH of 5.6? Solve the above equation.
0.2 =log (
1002= )=1.58
Spring 2024
So for every mole of HA, 1.58 moles of A is required to achieve a pH of 5.6. But how mnuch of
these conjugates do we need for the desired buffer capacity? Recal 10 mmol of acid or base to
qive a ApH unit of 1.As before acid should be added so the pH will pass through the pka. Choose
reasonable number and see what happens.
Iteration 1- Say 10 mmol of HA and 15.8 mmol of A:.
Remember the AHAratio must be 1.58 to achieve the specified buffer p. 10mmol of HA was
chosen based on a random assessment that it was the buffer capacity number.
When adding strong acid (HCI) the following reaction occurs: A+ H*>HA
Initial
Change
Equil
A
H+
15.8
-10
5.8
HA
10
10(added)
+10
20
Now find the pH of the solution after the strong acid addition.
4.86
pH = 5.4 + log
capacity is too great and must be
The pH didn’t change quite enough which means the buffer but
remember to keep the ratio the
reduced. How to do that? Decrease the mmol of HA and A,
same, 1.58.
Iteration 2- Say 8 mmol of HAand 12.6 (8 x 1.58) mmol of A.
Initial
Change
Equil
A
H*
HA
12.6
-10
2.6
10(added)
+10
18
4.56
pH = 54 + log(o=
18.
be. (About 8 mmol of HA, 12,6
should
composition
buffer
the
what
you
Pretty close. This tells
iterations than this.
mmol of A). It may take youmore
answer this. But there is one nore thing
to
required
information
the
have
Question 3: Now you
have to create its partner from
you
and
conjugates
the
of
one
to think about. Youare only given
nothing! Once you’ve got that figured out…
something from
what you have. And youcan’t create
weigh it out ona 3
Do a mol’ gconversion and
To perform the experiment:
point (mg)balance.
in 50 – 75 mL of H20.
the calculated amount above
Record the masS prior to
1. Dissolve
dissolving.
me before using the pH meter.
Consult
solution.
this
of
2.. Measure the pH
3. Add 1M HCI (or1 M NaOH) to bring pH to 5.6 (this example). Use your 10mL graduated
cylinder, keep track of how much strong acid or base is required to achieve the specified
pH.
4. Call me to watch youtest the buffer capacity of the buffer that you prepared in step 3 by
adding 1 MHCI or 1 MNaOH using your 10 mL graduated cylinder.
5. If meets buffer specs as given in assignment you are done. If not check calculations,
hydrogen phthalate (in this
concepts, etc. Redo lab by modifying the amount of potassiumexpected.
from
deviated
capacity
buffer
the
example)according to how
CHM 2730, SP24
Exp 7 report instructions
Prepare a narrative report following the guidelines below. (Understanding section 9-5 should really
help
you with this.)
1. List assigned buffer number (from table) and specifications: buffer pH, starting
solution,
2.
capacity.
Discuss the preparation of your buffer solution. Include the following:
a
b.
Calculations showing how the mmol of each buffer component (conjugate) was determined.
Describe how the buffer was prepared. ldentify each substance and the amount (mass or volume)
used to prepare the buffer.
3. Experimental results: prepare a table that includes the following data:
a
the original buffer pH;
b
the final buffer pH after testing capacity. If you had to add more acid/base to reach the target pH,
indicate the pH after you added allof your acid/base, and indicate how much more you had to
add to reach the target pH.
Include the cumulative amount (mmol) of strong acid or base added with the corresponding pH.
4. Compare the amount of strong acid or base used to achieve the specified original buffer pH and the
amount
calculated. Likewise, compare the amount of strong acid or base needed to change the buffer
pH by 1 unit with the amount calculated. If you added strong acid or base beyond that needed to
change the buffer pH by 1unit, briefly describe how the pH changed as extra strong acid or base was
added. You do not need to rationalize or explain any significant discrepancies between experimental
results and calculated quantities. However, if you made any adjustments to Compensate for any
anticipated (e.g., hygroscopic buffer source) or observed (e.g., preparing/testing a new buffer solution
based on an initial outcome) discrepancies, briefly describe what you did.
dropbox by
Prepare your report in Word and upload the completed report to the corresponding D2L
or hand written
the due date/time. Calculations can be illustrated using the equation editor in Word
sized and
properly
and inserted into the document as images. (Make certain inserted images are
question
legible.) Report should be written in complete sentences, and complete paragraphs. Each
above can be labeled in the report.
Prepare a
Starting
Assignment
buffer
number
having a
pH of
from
6.7
Imidazole
4
that has a
capacity
of
25±2
mmol
to change
the pH by
Conjugate
1unit
acid
pka of
from
6.7 to 7.7
7.0
PH -G-2-72
PH
Pha t
CA
6-7-2o
log.
LA3
-03
pH s Pka tlog CA25
LHA
CHaY 25
CA325
CHA]725
25
7-1
S”ol9 =
Calt 25
CHA3-2s
S.0019/ A-25
[A+25
S019HA – 25 -296 = 05o1g SA
25
+125 -28o
-oSolq HA
O.SalA
[HA3 33-32nmolaeid
o-SoI19 (33·32)
LA
I6-626 + 33-32
=50Mmdl
o-65x 68 o0? (Tm ida Zole)
Totladed
Acid
mlAcid
Co)Base
M Mol
3ml
3ml
pH
q-8
3m
q-4
8-3
3m
Im H
8-3
3ml
3ml
2m]
Im
Het
64
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