Quantitative Analysis Lab
10-08-266
Spring Quarter 20073
Experiment
BUFFERS
Purpose:
To acquaint the student with the theory, preparation and calculations associated with pH
buffer systems
Apparatus:
pH Meter and electrode
50 mL pipet
1.0 mL pipet
150 or 250 mL beakers
100 ml volumetric flasks
250 ml volumetric flasks
Chemicals:
Acetic acid (5 M)
Sodium hydroxide (0.100 M)
Hydrochloric acid (1.00 M)
Sodium carbonate (Solid)
Deionized water (Lab Sink supply)
Unknown Sodium Hydroxide Solutions (pH range 0.050 M to 0.20 M)
pH 4, 7 pH meter calibration buffers
Theory:
A buffer is defined as a solution that resists change in pH when a small amount of acid or base is added or
when the solution is diluted. This is very useful for maintaining the pH for a reaction at an optimum
value. (This can be very important in all areas of chemistry. Optimum pH might be required for
optimization of reaction conditions or for proper functioning of an assay). A buffer solution consists of a
mixture of a weak acid and its conjugate base or a weak base and its conjugate acid at predetermined
concentrations or ratios. In other words, we have a mixture of a weak acid and its salt or a weak base and
its salt. It can be prepared by mixing both compounds or can be prepared by forming the conjugate with
either the addition of strong acid or base as appropriate.
The Henderson-Hasselbalch equation is a form of the ionization constant equation that is useful for
calculating the pH of a weak acid solution containing its salt. A general form can be written for a weak
acid, HA, that ionizes to its salt, A- and H+:
HA(aq) + H20(l) = H3O+(aq) + A-(aq)
Ka 
[ H 3O  ][ A ]
[ HA]
Taking the log of both sides of the equation and with a little math manipulation we get
pH = pKa + log [A-]/[HA]
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If the solution is diluted, the ratio of base to acid remains constant so the pH of the solution does not
change. The buffer will be able to neutralize the same number of moles of acid or base. However, the
buffer capacity of the system, the amount of acid or base that can be added without causing a large
change in pH, would be decreased.
Buffer Capacity =  = (number of moles of OH- or H3O+ added)
(pH change)(volume of buffer in L)
The buffering capacity is maximum where pH = pKa. In general, the buffering capacity is
satisfactory over the pH range of pKa ± 1.
The mixture of a weak acid and its salt may also be obtained by mixing an excess of weak acid with some
strong base to produce the salt by neutralization or by mixing an excess of salt with a strong acid to
produce the weak acid component of the buffer.
Procedure:
Part A
Step 1:
Dilute the appropriate amount of the stock Acetic acid ( CH3COOH) to make 250
ml of 0.20 M solution and 250 ml of 0.020 M solution. Measure the original pH
of each solution. Calculate the theoretical values of these solutions.
Step 2:
Prepare 250 ml of 0.10 M NaOH and 250 mL 0.010 M NaOH. Calculate the the
theoretical values of these solutions.
Step 3:
Prepare a buffer solution by combining 50 mL of 0.20M CH3COOH with 50 mL
of 0.10M NaOH. Measure the pH of this buffer. Calculate the theoretical value of
this buffer solution. Compare the experimental and theoretical values.
Step 4:
Add 1.0 ml of 1.00 M HC1 to the buffer prepared in step 3. Also add 1.00 mL of
1.00 M HCl into a beaker of 100 mL distilled water. Measure the pH of the
resultant solutions. Compare the experimental and theoretical values.
Step 5:
Add 1.0 mL of a 1.00 M HCl to a buffer solution prepared from the combination
of 50 ml 0.020 M CH3COOH and 50.0 mL of 0.010 M NaOH. Compare the
experimental and theoretical values.
Part B
Using sodium carbonate make a buffer solution (0.10 M) at the appropriate pH range, you will
need to look up the pKa’s. The calculations and methodology are to be designed by you and
approved by the instructor. You will need to prove that it is indeed a buffer solution by the
addition of a base solution of an unknown molarity. You will use your resulting pH to determine
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the molarity of the unknown base.
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Name _________________
Quantitative Analysis
Section _________
Report Sheet
Buffer Experiment
Part A
Table I. Results for Acetate Buffer Experiment
Exp # Reading # >
Trial 1
1
pH 0.20 M Acetic Acid
2
pH 0.020 M Acetic Acid
3
pH 0.10 M NaOH
4
pH 0.0010 M NaOH
5
Step 3: Buffer Solution pH
6
Step 4: Water pH after HCl
addition
7
Step 4: Buffer pH after HCl
addition
8
Step 5: Buffer Solution pH
9
Step 5: Buffer pH after HCl
addition
Trial 2
Trial 3
Theoretical pH
Part B
Table II. Results for Carbonate Buffer Experiment
11
Reading # >
1
2
12
Buffer Solution pH
13
Buffer after base addition
pH
14
Calculated base Molarity
3
Theoretical pH
Attach notebook pages with calculations, showing all work. Include a brief conclusion in your
notebook..
BE PREPARED BEFORE LAB!!!!!!
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