EXPERIMENT 1: PREPARATION OF BUFFER SOLUTIONS
TABLE OF CONTENTS Bil. 1 2 3 4 5 6 7 8
Title Summary Pictorial Methodology Results Discussions Tutorial Calculations Conclusions References
Page 2 3 5 6 8 11 15 16
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EXPERIMENT 1: PREPARATION OF BUFFER SOLUTIONS
1.0 SUMMARY
In this experiment, the objective is to prepare buffer solutions and test the effects of added acid or base upon the pH. It is also to review acid-base chemistry and calculation. The experiment is started by preparing the buffer solutions citric acid and sodium citrate. Weight out each substances (3.560g of citric acid and 9.255g of sodium citrate) and add distilled water to make a buffer solution. Then, determine the pH of the solution using the pH electrode. The pH value calculated is 5.00 compared with the experiment, the pH value obtained by the pH electrode is 4.96. This slight different occur may come from several error that happened while handling the solution or while conducted the pH electrode. As conclusions, buffer solutions are solutions that resist change in hydronium ion and the hydroxide ion concentration upon addition of small amounts of either acid or base, or upon dilution.
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EXPERIMENT 1: PREPARATION OF BUFFER SOLUTIONS
2.0 PICTORIAL METHODOLOGY
2.1 Preparation of buffer solution
2.1.1
Solid citric acid and sodium acetate have been mixed together to form a solution. Then the solution has been stirred using mechanical stirrer.
2.1.2
500 mL of a 0.1 buffer solution with a pH of 5.0 using citric acid, C 6H8O7 and sodium acetate, NaC 2H3O2 were prepared. The mixed solution of citric acid and sodium acetate then dilute with distilled water in the volumetric flask.
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EXPERIMENT 1: PREPARATION OF BUFFER SOLUTIONS
2.2 Testing of Buffer Solution
2.2.1
The buffer solution was added into 100ml beaker and the pH was measured using the pH electrode.
2.2.2
50ml deionized water was added into two different beakers and the pH was measured. The first beaker was added with 1.0 ml of 1M HCL and the second beaker was added with 1.0 ml of 1M NaOH. The pH of each solution was measured.
2.2.3
The above steps was repeated but by using buffer solution instead of deionized water. The pH of each solution was measured.
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EXPERIMENT 1: PREPARATION OF BUFFER SOLUTIONS
3.0 RESULT
Solution
pH Initial
After adding HCl
After adding NaOH
Deionised water
5.76
2.32
12.09
Buffer
4.96
4.84
5.06
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EXPERIMENT 1: PREPARATION OF BUFFER SOLUTIONS
4.0 DISCUSSION
The pH value of the deionised water was easily influenced by the present of atmosphere CO2. Theoretically, the pH value of deionised water is 5.5. The pH value might change if the deionised water was left exposed to the atmosphere and this will cause the change in pH value which will rise up to 7.00. Based on the experiment, the initial pH value for the deionised water obtained was 5.76. This shows that the deionised water has been well preserved. When the strong acid, HCl was added into the deionised water, the pH value obtained is 2.32 which mean the deionised water has turn into acid. The same phenomenon occurs while strong base, NaOH was added into the deionised water. The pH value obtained is 12.09. The result can be obviously seen, as the deionised water was turned into acid and base as the strong acid and strong base was added respectively. The Buffer solution was prepared by mixing the buffer agent’s form (HA) which is citrate -
acid with its conjugate base (A ) which is sodium citrate. The pH value calculated is 5.00 compared with the experiment, the pH value obtained by the pH electrode is 4.96. This slight different occur may come from several error that happened while handling the solution or while conducted the pH electrode. As HCl was added into the Buffer solution, the pH value changed from 4.96 to 4.84. It shows that the difference occur is quite small which is only 0.12. At the same time while NaOH was added, the pH value changed from 4.96 to 5.06. The buffer solution is still resist the pH value from change, as the difference is only 0.1. Although the pH value of Buffer solution obtained is different from the calculated, the activity of the Buffer solution is still can obviously be seen. The change of pH value is only in the range of 0.1-0.12. The error occurred gives a slight different as the calculated. The prepared Buffer solution may have a little contamination causing the pH value to become 4.96 (which is supposed to be 5.00). Second possible error may occur while handling the Buffer solution with the pH electrode. The data was taken before it is stable or the pH electrode itself was not well cleaned. Another error might happened was during the chemical was taken. The chemical taken might not be precise as desired (parallax error).
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EXPERIMENT 1: PREPARATION OF BUFFER SOLUTIONS
As the recommendation, before using any apparatus, it is necessary to clean it up first. This is to ensure that the apparatus is free contamination. The cleaning processes have to take charge while handling the pH electrode as well. This is a must in order to obtain a desirable pH value. As for the deionised water, try not to leave it exposed to the air in a long period of time.
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EXPERIMENT 1: PREPARATION OF BUFFER SOLUTIONS
5.0 TUTORIAL 2-
1. Describe how you would prepare a (CH 3CO2H / CH3CO ) buffer with a pH of 5.2 given that 2the pKa (CH3CO2H / CH3CO ) is 4.76. 2-
-Find the mole ratio of CH 3CO2H / CH3CO calculation as follows:
using Henderson-Hasselbalch equation. The
Henderson-Hasselbalch equation:
pHp a log
CHC
CHCH
Rearrange the equation:
log
CHC -
pHpH- p p a
CHCH
(1)
Given the pKa and pH for citric acid: pKa= 4.76
(2)
pH = 5.2
(3)
Substitute (2) and (3) into (1)
log
CHC
CHCH
-
log
CHC
CHCH
(4)
Antilog equation (4)
CHC
CHCH
CHC
CHCH
So the mole ratio of citric acid to citrate ion is CHC- CHCH
(5)
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EXPERIMENT 1: PREPARATION OF BUFFER SOLUTIONS
Given: n n
V= 500 ml = 0.5 L
(6)
(7), then Expand equation (7) into
nCH C- CHCH
(8)
Rearrange equation (5)
(9)
nCHC- CHCH
Substititute (9) into (8) nCH C- CHCH
mol
(10)
Solve equation (10) CHCH
CHCH
CHCH mol Using equation (9) to solve no of mole sodium citrate nCHC CHCH nCHC ( ) nCHC mol Molar mass for CHCH = 60.05 g/mol
Molar mass for CHC - = 82.03 g/mol Mass of citric acid = mol 60.05 g/mol = 7.999g Mass of sodium citrate = mol gmol = 30.089 g - Weight both of the citric acid and sodium acetate and put it inside the 500 ml volumetric flask. -Then add distilled water into the volumetric flask till neck of flask. Dissolve the acid and salt. - Add the distilled water till calibration mark.
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EXPERIMENT 1: PREPARATION OF BUFFER SOLUTIONS
2. What is buffer capacity? What determines the capacity of a buffer? Buffer capacity is the ability of buffer solution to resist pH changes. Buffer capacity increases as the molar concentration of acid or salt increases. The buffer capacity also increases as the pH is closer to pKa. The buffer capacity is expressed as the molar concentration of sodium hydroxide required to increase the pH by 1.
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EXPERIMENT 1: PREPARATION OF BUFFER SOLUTIONS
6.0 CALCULATIONS
1. Calculation for 500 ml of 0.1M of buffer solution with pH 5 Calculation of buffer solution can be done by using Henderson-Hasselbalch equation to deter mine the mole ratio of the citric ion and citrate ion. The calculation as follows: Henderson-Hasselbalch equation:
pHp a log
A HA
rearrange the equation: -
A
log
HA
pHpH- p p a
(1)
Given the pKa and pH for citric acid: pKa= 4.77
(2)
pH = 5
(3)
Substitute (2) and (3) into (1) A
log
HA A-
log
HA
(4)
Antilog equation (4)
A HA A HA
So the mole ratio of citric acid to citrate ion is A- HA
(5)
Given: n V= 500 ml = 0.5 L
(6)
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EXPERIMENT 1: PREPARATION OF BUFFER SOLUTIONS
n
(7)
Expand equation (7) into n
(8)
Rearrange equation (5)
(9)
Substititute (9) into (8)
mol
(10)
Solve equation (10)
mol Using equation (9) to solve no of mole sodium citrate
() mol Molar mass for citric acid = 192.13 g/mol Molar mass for sodium citrate = 294.1 g/mol Mass of citric acid = mol gmol *Mass of citric acid = 3.560 g Mass of sodium citrate = mol x gmol *Mass of sodium citrate = 9.255 g
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EXPERIMENT 1: PREPARATION OF BUFFER SOLUTIONS
2. For pH of buffer after adding adding acid The equation of ionization of citric acid: C H CH
C H
C H
“ICE” chart: Assume all acid consume Mol Initial
C6H8O7 0.01853 mol
Change Equilibrium
+0.003mol 0.02153 mol
+
H 0.001 L(1 M) = 0.001 mol 3 x 0.001 mol=0.003 -0.003 mol x
-
C6H5O7 0.03147 mol
-0.003 mol 0.02847 mol
Find Ka a x
( x )
Convert to pH pH log pH log pH (Refer to http://www.chem.purdue.edu/gchelp/howtosolveit/Equilibrium/Buffers.htm#Bufferaddacid )
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EXPERIMENT 1: PREPARATION OF BUFFER SOLUTIONS
3. For pH of buffer after adding base Ionization equation of sodium citrate: C H H C H Assume all base is consumed. Mol Initial
C6H8O7 0.01853 mol
Change Equilibrium
+0.001mol 0.01953 mol
-
OH 0.001 L(1 M) = 0.001 mol -0.001 mol 0
-
C6H5O7 0.03147 mol +0.001 mol 0.03247 mol
C H CH C6H8O7 0.01853 -x 0.01953-x
Initial Change Equilibrium x( x )
x x
xx pH log x pH
+
H 0 +x x
-
C6H5O7 0.03147 -x 0.03247-x
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EXPERIMENT 1: PREPARATION OF BUFFER SOLUTIONS
7.0 CONCLUSIONS
As for the conclusions, buffer solutions are solutions that resist change in hydronium ion and the hydroxide ion concentration (and consequently pH) upon addition of small amounts of either acid or base, or upon dilution. A buffer is a mixture of a weak acid and its conjugate base. Because the solute can either absorb protons or release protons, the buffer solution can maintain the pH at a constant value. From the results obtained, it can be concluded that the initial pH value for the deionised water obtained was 5.76. This shows that the deionised water has been well preserved.When the strong acid, HCl was added into the deionised water, the pH value obtained is 2.32 which mean the deionised water has turn into acid. The same phenomenon occurs while strong base, NaOH was added into the deionised water. The pH value obtained is 12.09. The result can be obviously seen, as the deionised water was turned into acid and base as the strong acid and strong base was added respectively. As HCl was added into the Buffer solution, the pH value changed from 4.96 to 4.84. It shows that the difference occur is quite small which is only 0.12. At the same time while NaOH was added, the pH value changed from 4.96 to 5.06. The buffer solution is still resist the pH value from change, as the difference is only 0.1. Although the pH value of Buffer solution obtained is different from the calculated, the activity of the Buffer solution is still can obviously be seen. The change of pH value is only in the range of 0.1-0.12. Due to some errors that occur, the experimental data values obtained are opposed to the theoretical value. Therefore, some recommendations have been made so that the errors can be overcome. Lastly, the two main objectives of this experiment are fully achieved.
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EXPERIMENT 1: PREPARATION OF BUFFER SOLUTIONS
8.0 REFERENCES
1. (Deionized Water With Acid and Base [online] Available from http://www.finishing.com/2400-2599/2576.shtml [31st August 2012])
2. (Buffer Solutions [online] Available from http://chemed.chem.wisc.edu/chempaths/GenChem-Textbook/Buffer-solutios-620.html [31st August 2012]) 3. (Buffer Solutions [online] Available fromhttp://en.wikipedia.org/wiki/Bu fromhttp://en. wikipedia.org/wiki/Buffer_solution ffer_solution[31st [31st August 2012]) 4. (Buffer Solutions [online] Available from www.chemguide.co.uk/physical/acidbaseeqi www.chemguide.co.uk/p hysical/acidbaseeqia/buffers.html a/buffers.html[31st [31st August 2012]) 5. (Preparation of pH Buffer Solutions [online] Available from http://delloyd.50megs.com/moreinfo/bu http://dello yd.50megs.com/moreinfo/buffers2.html ffers2.html [31st August 2012])
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