TITLE
Experiment 1: Acid Base Titration
INTRODUCTION
Titration is the quantitative measurement of an analyte in solution by reacting it completely with a standardized reagent. Acid and base react until one of the reactants is consumed completely. A solution of base of known concentration can therefore be used to titrate an acid solution of unknown concentration. Likewise, an acid solution of known concentration can be used to titrate a base solution of unknown concentration.Weak acid is an acid that cannot dissociate completely in water. Example of weak acid are acetic acid and organic acid. While, strong acid can completely dissociate in water. Example of strong acid is hydrochloric acid.
pH is a measure of the concentration of hydrogen ions in a solution. Strong acids like hydrochloric acid at sort of concentrations that normally use in the lab have a pH around 0 to 1. The lower the pH, the higher the concentration of hydrogen ions in the solution.
OBJECTIVE
1. To observe the property of weak acid with pH changes.
2. To learn how to use pH meter correctly.
3. To learn how to prepare buffer system.
4. To experience how to titrate acid-base
PROCEDURE:
1. ACID-BASE TITRATION
2. APPLICATION:MAKING pH INDICATOR
2. 9.0 ml of distilled water was placed in test tubes #2- #12.
3. Solutions were prepared in the acid range in the following manner:
4. For test tube #7 it remain with distilled water as neutral pH (pH=7).
5. Solutions were prepared in the base range in the following manners:
6. After finished the dilution, the well of a spots plate was labeled from 1-13 and then, the solution that have been prepared in step 3,4 and 5 were dropped a few in the well according the number.
7. A few drops of the red cabbage extract indicator were transferred to each well. then, the colour change were observed. This steps was repeated with the turmeric extract indicator to see which one is the best indicator.
8. Then, this indicator was used to test the pH of household chemical. The result was recorded.
RESULT
1. ACID-BASE TITRATION
2. APPLICATION: MAKING pH INDICATOR
DISCUSSION
1. ACID-BASE TITRATION
Point A (2 mL of NaOH) indicates the initial point, while point C (25 mL of NaOH) is the 1st equivalence point. From A to C, it is called as 1st buffer zone. The pH of the point midway between A and C (Point B- 12.5 mL of NaOH) equals to pKa1, which is 2.5. At the 1st equivalence point, all H3PO4 is being neutralized and only some H2PO4- is present in the solution.
When more NaOH is added, HPO4 2- is produced. The solution now contains acid and its conjugate base (hydrogen phosphate- as buffer).
When more NaOH is added further, PO4 3- is produced. The solution now contains acid and its conjugate base (phosphate ion- as buffer).
The graph is different for each acid is because of the difference in types of acid. Both of the acid is weak acid, but for acetic acid, it has one pKa value that is 4.76, while phosphoric acid, it have three pka value that are 2.15, 7.20 and 12.35. The pKa value also indicates the inflection point for the acid.
OBJECTIVE
1. To observe the property of weak acid with pH changes.
2. To learn how to use pH meter correctly.
3. To learn how to prepare buffer system.
4. To experience how to titrate acid-base
PROCEDURE:
1. ACID-BASE TITRATION
1.
0.1M of NaOH is filled in the burette.
2.
25 ml of acetic acid is titrated with NaOH.
3.
The pH is measured for every 1 ml of NaOH added
to the acid.
4.
The result is recorded in the table below.
5.
Experiment is then repeated with 0.1M of
phosphoric acid.
2. APPLICATION:MAKING pH INDICATOR
Extracting the red cabbage and turmeric.
Chopped and crushed the red cabbage and turmeric with water, 2-propanol, 50-50 water/2-propanol mixture, or acetone until their extracts come out.
Testing the pH range of the indicator.
1. Test tubes were labeled from 1-13.
2. 9.0 ml of distilled water was placed in test tubes #2- #12.
3. Solutions were prepared in the acid range in the following manner:
a) 10.0ml of 0.1M HCl was placed in test tube #1 (pH=1).
b) Then, 1.0ml of o.1M acid from test tube #1 was transferred to test tube #2 and mixed them up thoroughly (pH=2).
c) After that, 1.0ml of acid from test tube #2 was transferred to test tube #3 and mix them up thoroughly (pH=3).
d) The making serial dilution was continued by transferring 1.0ml of the most recently diluted solutions to the next test tube until 6 acid solutions of pH 1-6 have been prepared.
4. For test tube #7 it remain with distilled water as neutral pH (pH=7).
5. Solutions were prepared in the base range in the following manners:
a) 10.0ml of 0.1M NaOH in test tube #13 (pH=13)
b) 1.0ml of 0.1M NaOH in the test tube #13 was transferred to the test tube #12 and mixed them up thoroughly.
c) Then, continued making the serial dilutions of the base going from pH 12 down to pH 8 by transferring 1.0ml of the most recently diluted basic solution to the next test tube and mixed thoroughly each time.
6. After finished the dilution, the well of a spots plate was labeled from 1-13 and then, the solution that have been prepared in step 3,4 and 5 were dropped a few in the well according the number.
7. A few drops of the red cabbage extract indicator were transferred to each well. then, the colour change were observed. This steps was repeated with the turmeric extract indicator to see which one is the best indicator.
8. Then, this indicator was used to test the pH of household chemical. The result was recorded.
RESULT
1. ACID-BASE TITRATION
Titration of Acetic Acid
The table below shows the volume of NaOH used and the corresponding pH.
Volume of NaOH used (mL)
|
pH
|
1.0
|
3.50
|
2.0
|
3.77
|
3.0
|
3.90
|
4.0
|
4.04
|
5.0
|
4.10
|
6.0
|
4.27
|
7.0
|
4.33
|
8.0
|
4.44
|
9.0
|
4.52
|
10.0
|
4.59
|
11.0
|
4.63
|
12.0
|
4.72
|
13.0
|
4.80
|
14.0
|
4.87
|
15.0
|
4.93
|
16.0
|
5.00
|
17.0
|
5.07
|
18.0
|
5.15
|
19.0
|
5.23
|
20.0
|
5.30
|
21.0
|
5.39
|
22.0
|
5.53
|
23.0
|
5.70
|
24.0
|
6.20
|
25.0
|
6.27
|
26.0
|
6.91
|
27.0
|
9.70
|
Titration of Phosphoric Acid
The table and graph below show the relationship of the volume of NaOH used and the corresponding pH.
Volume of NaOH used (mL)
|
pH
|
2.0
|
2.29
|
5.0
|
2.28
|
10.0
|
2.42
|
15.0
|
2.67
|
20.0
|
3.16
|
25.0
|
5.38
|
30.0
|
6.43
|
35.0
|
6.92
|
40.0
|
7.31
|
45.0
|
7.91
|
50.0
|
10.68
|
55.0
|
11.56
|
60.0
|
11.86
|
65.0
|
12.02
|
70.0
|
12.20
|
75.0
|
12.31
|
80.0
|
12.40
|
85.0
|
12.47
|
90.0
|
12.50
|
95.0
|
12.56
|
102.0
|
12.61
|
107.0
|
12.65
|
112.0
|
12.80
|
2. APPLICATION: MAKING pH INDICATOR
Red Cabbage pH Indicator Colors
pH
|
1
|
2
|
3
|
11
|
12
|
13
|
Color
|
Red
|
Pink
|
Purple
|
Blue
|
Blue-Green
|
Greenish
Yellow
|
Turmeric pH
Indicator Colors
pH
|
2
|
4
|
6
|
8
|
11
|
12
|
Color
|
Yellow
|
Yellow
|
Yellow
|
Yellow
|
Light
dark red
|
Dark
red
|
Household using Red
Cabbage pH Indicator Colors
pH
|
2
|
12
|
Color
|
Pink
|
Blue-Green
|
Products
|
Vinegar
|
Cleaner
solution
|
Household using Turmeric pH Indicator Colors
pH
|
2
|
12
|
Color
|
Yellow
|
Light
Dark Red
|
Products
|
Vinegar
|
Cleaner
solution
|
DISCUSSION
1. ACID-BASE TITRATION
Titration Curve of Acetic Acid
The titration curve for the titration of acetic acid is shown above. The 1st point on the curve corresponds to a solution of acetic acid only. The pH at this point is the pH value for pure acetic acid. When some NaOH is added, acetate ion is produced. The curve shows a slight linear rising. The solution now contains acid and its conjugate base (acetate ion). This conjugate base acts as a buffer which resists the change in pH upon dilution and addition of both acid and base.
HC2H3O2 + OH-
Point A (1 mL of NaOH) indicates the initial point, while point C (27 mL of NaOH) is the equivalence point. From A to C, it is called as buffer zone. The pH of the point midway between A and C (Point B- 13 mL of NaOH) equals to pKa, which is 4.8. At the equivalence point, all acetic acid is being neutralized and only some acetic ion is present in the solution.
Titration Curve of Phosphoric Acid
The titration curve for the titration of phosphoric acid is shown in Figure 1.2. The 1st point on the curve corresponds to a solution of phosphoric acid only. The pH at this point is the pH value for pure phosphoric acid. When some NaOH is added, H2PO4- is produced. The solution now contains acid and its conjugate base (dihydrogen phosphate). This conjugate base acts as a buffer which resists the change in pH upon dilution and addition of both acid and base.
Point A (2 mL of NaOH) indicates the initial point, while point C (25 mL of NaOH) is the 1st equivalence point. From A to C, it is called as 1st buffer zone. The pH of the point midway between A and C (Point B- 12.5 mL of NaOH) equals to pKa1, which is 2.5. At the 1st equivalence point, all H3PO4 is being neutralized and only some H2PO4- is present in the solution.
When more NaOH is added, HPO4 2- is produced. The solution now contains acid and its conjugate base (hydrogen phosphate- as buffer).
Point E (48 mL of NaOH) is the 2nd equivalence point. Therefore, from C to E, it is called as 2nd buffer zone. The pH of the point midway between C and E (Point D- 36.5 mL of NaOH) equals to pKa2, which is 7.0. At the 2nd equivalence point, only HPO4 2- is present in the solution.
When more NaOH is added further, PO4 3- is produced. The solution now contains acid and its conjugate base (phosphate ion- as buffer).
Point G (90 mL of NaOH) is the 3rd equivalence point. From E to G, it is called as 3rd buffer zone. The pH of the point midway between E and G (Point F- 69 mL of NaOH) equals to pKa3, which is 12.2. At the 3rd equivalence point, only PO4 3- and excess unreacted OH- are present in the solution.
The point of the inflection at the
graph shows that at that point, there are equal quantities of acid and base
that have been added to the solution. This means that the pH is equals to the
pKa of the acid. In addition to that, near to this point, the pH changes
detected is little with the addition of the base.
The graph is different for each acid is because of the difference in types of acid. Both of the acid is weak acid, but for acetic acid, it has one pKa value that is 4.76, while phosphoric acid, it have three pka value that are 2.15, 7.20 and 12.35. The pKa value also indicates the inflection point for the acid.
Titration curve depends on pKa or acid strength. Inflection point decrease with weaker acid.
As referred to the graph, the change in slope for acetic acid graph is small, while in phosphoric acid, the change in slope is larger. With this comparison, it shows that phosphoric acid is more stronger than acetic acid, yet, both are weak acids.
2. APPLICATION:MAKING pH INDICATOR
REFERENCES
Indicators are substances, which indicate the completion
(equivalence point or end point) of a chemical reaction by change in color. For
example, in volumetric analysis, during the titration of sodium hydroxide with
hydrochloric acid (titrant in the burette), phenolphthalein turns pink to
colorless when the entire of sodium hydroxide gets neutralized by
hydrochloric acid and the pH becomes neutral. For this experiment, we have to
make our own pH indicator using red cabbage and turmeric. Before that, we must
know what is the meaning of pH indicator.
A pH indicator is an indicator used
in the titration of an acid and a base which is also called
a neutralization titration. A pH indicator indicates the
neutralization point or end point or equivalence point by a change in color
which is distinct. Usually the pH indicator that we used in experiment is
litmus paper. There are two types of litmus
papers also, blue and red. Blue litmus is used for acids because by dipping it
in acids it turns red. When the solution is basic in nature red litmus is used
which will turn blue.
We were using red cabbage
and turmeric to make our own pH indicator. Then we make a comparison between
these two natural pH indicator which one is the best pH indicator. Firstly, we
were using red cabbage because red cabbage juice
contains a natural pH indicator that changes colors according to the acidity of
the solution. Besides that these natural pH indicator is easy to make and also cheap. Red cabbage contains a pigment
molecule called flavin (an anthocyanin). This water-soluble pigment is also
found in apple skin, plums, poppies, cornflowers, and grapes. Very acidic
solutions will turn anthocyanin a red color.
Neutral solutions result in a
purplish color. Basic solutions appear in greenish-yellow. Therefore, it is
possible to determine the pH of a solution based on the color it turns the
anthocyanin pigments in red cabbage juice. The color of the juice changes in
response to changes in its hydrogen ion concentration pH is the -log[H+]. Acids
will donate hydrogen ions in an aqueous solution and have a low pH (pH < 7).
Bases accept hydrogen ions and have a high pH (pH > 7).
From the results that we obtained, we can see that red cabbage is a
better pH indicator compared to turmeric. This is because red cabbage indicator
solution show more colour change compared to turmeric indicator solution. We
should get more colour change but we only can see colour red,pink light purple,
light blue and light green for red cabbage indicator. While for turmeric
indicator, we can only see the colour changing for alkali solution only.
Actually turmeric indicator only turns
dark red in the presence of an alkali, but remains yellow in the presence of an
acidic or neutral solution.
The differences between using litmus paper and
using neutral pH indicator is that litmus paper can be stored over many years
and remains usable. Although red cabbage is cheap, very colour intensive and
can actually indicate the pH of a solution in more fine grained steps, the
problems is it works best with fresh cabbage and the extract is only usable
for a couple of weeks. However it is very useful home made pH indicator and can
be an interesting experiment for science class in school.
The ability to indicated the pH over a
couple of weeks or month for home made pH indicator depending on how we stored
the solution. Other than using red cabbage and turmeric to make our own pH
indicator, we can also use radish peels. Radish peels are stable for many month
and turmeric continues to work for many years.Radish work the same or similar
as red cabbage in the sense that it can indicate the pH in many fine grained
steps while for turmeric it just has two colours similar to litmus. As
mentioned earlier, turmeric pH indicator can work for many years but the best
way to store turmeric over longer periods is to store it as powder.
Then, to
use it we just have to mix the powder with water. After that, we are using the
red cabbage pH indicator and turmeric pH indicator to test on our household
products. Our group only using two household product which is vinegar and
toilet cleaner solution. The result that we obtained when using red cabbage pH
indicator is pink colour for vinegar and blue colour for toilet cleaner
solution. While results for turmeric pH indicator is yellow colour for vinegar
and dark red for toilet cleaner solution. It proved that, vinegar is an acid
while toilet cleaner solution is alkali.
CONCLUSION
1. ACID BASE TITRATION
A solution that contains a weak acid and its conjugate base has the special property of being able to resist changing its pH when either a base or an acid is added to it. Such a solution is called a buffer because the solution is protected, or buffered, from pH changes even when H3O+ or OH– ions are added to the solution. Buffers help biochemists study biomolecular reactions in the laboratory by stabilizing the pH of solutions used for experiments.
2. APPLICATION : MAKING pH INDICATOR
In conclusion, the red
cabbage shows that it is the best indicator of pH to refer. This is because, Red cabbage contains a pigment molecule called flavin
(an anthocyanin). This water-soluble pigment is also found in apple skin,
plums, poppies, cornflowers, and grapes. Very acidic solutions will turn
anthocyanin a red color. Neutral solutions result in a purplish color. Basic
solutions appear in greenish-yellow.
Therefore,
it is possible to determine the pH of a solution based on the color it turns
the anthocyanin pigments in red cabbage juice. The color of the juice changes
in response to changes in its hydrogen ion concentration pH is the -log[H+].
Acids will donate hydrogen ions in an aqueous solution and have a low pH (pH
< 7). Bases accept hydrogen ions and have a high pH (pH > 7).
REFERENCES
Anne Marie Helmenstine. Red
Cabbage pH Indicator ( 2013). Access from
http://chemistry.about.com/od/acidsbase1/a/red-cabbage-ph-indicator.htm
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Anonymous. Red Cabbage
(2012). Access from
http://en.wikipedia.org/wiki/Red_cabbage#pH_indicator on March 18, 2013.
Anonymous. Red Cabbage
Juice: A Homemade pH Indicator (2012). Access from
http://www.carolina.com/teacher-resources/Interactive/red-cabbage-guice-homemade
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18, 2013.
(2010, ). Ph indicators. Retrieved from TutorVista.Com. Retrived on 18 March 2013,
(2004-2013) Self made acid/base indicators, ph indicators. Retrived on
18 March 2013,
(2010, ). Ph indicators. Retrieved from TutorVista.Com. Retrived on 18 March 2013,
http://chemistry.tutorvista.com/analytical-chemistry/ph indicators. html#close_ iframe#close_ iframe
Helmenstine
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website:
http://chemistry.about.com/od/acidsbase1/a/red-cabbage-ph-indicator.htm
John Wiley & Sons : Essential Biochemistry, acid, base & pH.(2004). Retrieved from,
http://www.wiley.com/college/pratt/0471393878/student/review/acid_base/7_buffers.html on 16 March 2013
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on 19 March 2013.
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