Friday, 22 March 2013

22:47 -

Experiment 1: Acid Base Experiment


GUIDE 1: ACID BASE EXPERIMENT

1) Acid base titration

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

Introduction
Weak acid is different from strong acid because it cannot dissociate completely in water. Weak acid such as acetic acid and organic acid can be partially dissociated. Therefore there is only small amount of weak acid molecules dissociates, while in strong acid, all acid molecules exists as H+ concentration in weak acid depends on the coefficient of equilibrium.


The Henderson-Hasselbalch equation describes the relationship of pH as a measure of acidity with the acid dissociation constant (pKa), in biological and chemical systems. The equation is especially useful for estimating the pH of a buffer solution and finding the equilibrium pH in acid-base reactions.
The equation is given by:

Here, [HA] is the molar concentration of the undissociated weak acid, [A⁻] is the molar concentration of this acid's conjugate base and  is pKa is - log (Ka) where Ka is the acid dissociation constant.


Materials:
0.1M acetic acid = 4.76
0.1M phosphoric acid 2.15, 7.20, 12.35
0.1M amino glycine acid=2.53 (carboxylic) 9.78 (amino)
0.1M NaOH
Calibrated pH meter

Methods:
1.      0.1M NaOH was filled in a burette.
2.      25mL of the three acids was titrate separately with NaOH
3.      The pH was measured every time 1mL of NaOH was added.
4.      The findings was recorded.




Result

Titration of Acetic Acid

Volume of NaOH(ml)
pH
1
3.53
2
3.78
3
3.94
4
4.28
5
4.23
6
4.30
7
4.38
8
4.46
9
4.54
10
4.59
11
4.67
12
4.74
13
4.80
14
4.88
15
4.95
16
5.02
17
5.11
18
5.18
19
5.26
20
5.38
21
5.52
22
5.67
23
5.87
24
6.67

Titration curve of Acetic acid


                                                           From  the graph the  pKa is 4.9

Titration of Amino glycine

Volume of NaOH
pH
1
8.29
2
8.67
3
8.90
4
9.03
5
9.19
6
9.29
7
9.43
8
9.48
9
9.56
10
9.64
11
9.70
12
9.79
13
9.87
14
9.93
15
10.00
16
10.06
17
10.16
18
10.22
19
10.30
20
10.34
21
10.48
22
10.58
23
10.71
24
10.80
25
11.15
26
11.28
27
11.49
28
11.71
29
11.87
30
12.01
31
12.04
32
12.10
33
12.13
34
12.20
35
12.21
36
12.26
37
12.30
38
12.33
39
12.35
40
12.38
41
12.41
42
12.42
43
12.46
44
12.47
45
12.49
46
12.50
47
12.51
48
12.53
49
12.54
50
12.57
51
12.58
52
12.60
53
12.62
54
12.63
55
12.64
56
12.65
57
12.66
58
12.67
59
12.68
60
12.69
61
12.70
62
12.71
63
12.72
64
12.73
65
12.73
66
12.74
67
12.74
68
12.75
69
12.75
70
12.76
71
12.77
72
12.78
73
12.79
74
12.80
75
12.80
76
12.81
77
12.81
78
12.82
79
12.84
80
12.85
81
12.85
82
12.85
83
12.85
84
12.86
85
12.87
86
12.87
87
12.87
88
12.88
89
12.88
90
12.88
91
12.89
92
12.90
93
12.90
94
12.90
95
12.90
96
12.90
97
12.90
98
12.91
99
12.91
100
12.91
101
12.91
102
12.91
103
12.91
104
12.92
105
12.92
106
12.92
107
12.92
108
12.93
109
12.93
110
12.93
111
12.93
112
12.93
113
12.94
114
12.95
115
12.94
116
12.94
117
12.94



































































































































Titration curve of Amino glycine




From the graph the pKa is 10



Discussion


A weak acid only partially dissociates from its salt. The pH will rise, but as it reaches a zone where the solution seems to be buffered, the slope levels out. After this zone, the pH rises sharply through its equivalence point and levels out again like the strong acid or strong base reaction. Once the equivalence point is reached the strong base affects the increase of pH rapidly.

The first is the half-equivalence point. This point occurs halfway through a buffered region where the pH barely changes for a lot of base added. The half-equivalence point is when just enough base is added for half of the acid to be converted to the conjugate base. When this happens, the concentration of H+ ions equals the Ka value of the acid. Take this one step further, pH = pKa.

The second point is the higher equivalence point. Once the acid has been neutralized, notice the point is above pH=7. When a weak acid is neutralized, the solution that remains is basic because of the acid's conjugate base remains in solution.
pKa values can be obtained from the titration data by the following methods.First ,the pH at the point of inflection is the pKa value and this may be read directly. The other way is by definition the pKa value is equal to the pH at which the acid is half titrated. The pKa can therefore be obtained from the knowledge of the end point of the titration.


2) Making pH indicator

 PROCEDURE:

2. Application: Making pH indicator
This experiment explores the extraction of natural indicators from common flowers, fruits, and vegetables and the pH at which these natural indicators change color. Some indicator solutions and papers will indicate both an acid and a base, while others are specific to just one. From the pH indicator that we made, we can test the solution prepared.
Materials
i. 0.1 M HCl solution
ii. 0.1 M NaOH solution
iii. 2-propanol
iv. red cabbage and turmeric
Extracting the indicator
1.      Finely chopped the red cabbage and turmeric in different mortar.
2.      Add about 10 mL of solvent and macerate. (Suggested solvents: water, 2-propanol, 50-50 water/2-propanol mixture,or acetone)
3.      Filter and collect the filtrate from the macerated plant samples.
Testing the pH range of the indicator
1. Label 13 test tubes from 1 to 13.
2. Place 9.0 mL of distilled or deionized water in all test tubes except #1 and #13.
3. Prepare solutions in the acid range in the following manner:
a. Place 10.0 mL of 0.1 M HCl in test tube #1. (pH = 1)
b. Transfer 1.0 mL of 0.1 M acid from test tube #1 to test tube #2 and mix thoroughly. (pH = 2)
c. Transfer 1.0 mL of acid solution from test tube # 2 to test tube #3 and mix thoroughly. (pH = 3)
d. Continue making the serial dilutions by transferring 1.0 mL of the most recently diluted acid solution to the next test tube until six acid solutions of pH 1 to 6 have been prepared. Be sure to mix each thoroughly before the transfer.
4. Add 10.0 mL distilled or deionized water to test tube #7. (pH = 7)
5. Prepare solutions of base in the following manner:
a. Place 10.0 mL of 0.1 M NaOH in test tube #13. (pH = 13)
b. Transfer 1.0 mL of 0.1 M NaOH from test tube #13 to test tube #12 and mix thoroughly. (pH = 12)
c. Continue making serial dilutions of the base going from pH 12 down to pH 8 by transferring 1.0 mL of the most recently diluted basic solution to the next test tube and mixing thoroughly each time.
6. Label the wells of a spot plate from 1 to 13. Transfer a few drops of each of the solutions prepared in steps 3, 4, and 5 to the corresponding well in the spot plate.
7. Add a drop or two of the flower/fruit/vegetable extract indicator to each well. Observe the pH at which the indicator changes color.

Testing the pH of other liquids
Once the pH ranges of the indicators have been determined, they can be used in acid-base titrations or to test the pH of household chemicals.

RESULTS:
Type of pH Indicator:
pH Indicator
Cabbage pH Indicator

Tumeric pH Indicator
Testing pH of other liquids:
Soy Sauce (Turmeric pH 7: Cabbage pH 9)

Vinegar(Cabbage 1: Turmeric 1)

Soap (Cabbage  12: Turmeric 13)

Talcum Powder (Cabbage 9: Turmeric 9)
DISCUSSION:
Acids, in general, are compounds that release hydrogen ions when they are put in water. Hydrogen ions (H+) are hydrogen atoms that have picked up a positive electric charge. Strong acids such as hydrochloric acid release lots of H+. Weak acids like the citric acid release only a few H+. Bases, in general, are compounds that release hydroxide ions when they are put in water. Hydroxide ions are made of a hydrogen atom attached to an oxygen atom, with a negative charge. They can be symbolized as OH-. When equal amounts of H+ and OH- in water, they combine to make water (H2O). If there is not enough OH- to combine with all the H+, the water will have extra H+ and be acidic. On the other hand, if there is not enough H+ to combine with all the OH-, the water will have extra OH- and be basic.
Everything has pH value and we could estimate the pH range of some substances by using the natural pH indicators where we will be using the tumeric and red cabbage. It is known that tumeric could be use as pH indicators and the coulur changes will determine the pH range of that particular substances being test.Curcume Longa is the scientific names for tumeric and it is weel knowm being used in Asia where the tumeric being used as antiseptic towards wounds and cuts so turmeric does has many advantages to wards human.Suprisingly only recently the scientist are giving attention towards the properties contain within tumeric.

Red cabbage contains a pigment molecule called flavin which is 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). What makes anthocyanin an indicator is that each form of the compound reacts with light differently, so that each form is a different color.
Following the procedures we test four several substances which are vinegar, soap, talcum and sot sauce respectively. All of the substance has different colour changes that enable us to compatre the colour changes with the pH indicator that has been set up earlier on so that we could estimate the pH range of the substance. Concepts of this pH indicator is quiet simple as it regarding the rate of dissociation either more towards the release of hydrogen ions or the hydroxide ions.If the hydrogen being release in greater amounts so the pH would be lower showing that the substances ia acidic with the pH range of 1.0- 6.9. Then the range for hydroxide ion would be 7.1-14 which means that substance is alkaline and the rate of dissociation of hydroxide ions is very high.
Next we focus on the colour changes and in tumeric the colour would be from yellow to red or dark pink which is the yellow would be the most acidic with pH value of one as for pH value 14 the colour would be pink or red.This colour is the main indicators that helps us to determine the pH range of the substances so it has to be well prepared according to the procedures.But unfortunately our indicators are not good as we can not set apart the colour from pH value 1 until 11 so the observation of colour changes and the indicators cannot being done accurately.

In red cabbage, the color changed is observable from  pinkish to greenish. When in red or pinkish show that the substance is acidic while greenish is basic. When test with talcum powder it’s show purple color which indicate slightly neutral. If we compared the result obtained by turmeric and red cabbage, red cabbage is better pH indicator as it can detect or differentiate between the base and acid. Turmeric only can indicate base.
Based on the results the pH for vinegar is acidic  with pH 1,then soap  with pH 14 followed by talcum with slightly alkaline and the pH value is 8 and pH value 7 is for the soy sauce.It can be seen that the colour changes of some of the substances is not really good and it is hard to determine the pH range but still we manage ti estimate the proper range and being able to use the tumeric indicators.
CONCLUSION:
Red cabbage is a better pH indicator than turmeric as red cabbage is able to indicate acid and base clearly. 
REFERENCES:
http://sciencesquad.questacon.edu.au/activities/colourful_chemistry.html
http://www.ehow.com/how_8497638_make-turmeric-ph-indicator.html
http://www.carolina.com/teacher-resources/Interactive/red-cabbage-guice-homemade-ph-indicator/tr10851.tr
http://chemistry.about.com/od/acidsbase1/a/red-cabbage-ph-indicator.htm