Practical no. : 1
Experiment title : Acid Base Experiment
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
Abstract:
The purpose of acid base laboratory experiment was to determine equivalance points. pKa points for a strong acid. HCl, titrated with a strong base, NaOH using a drop approach in order to determine completely accurate data. The pH is measured every time 1ml of NaOH added. The pKa of acetic acid theoretically is at 4.76. Using a concentration of 0.1M NaOH, we had the largest NaOH volume before the largest pH increase at 19.00ml. For the largest NaOH volume after the largest pH increases we used 20.00ml of base. The pKa of amino glicine acid theoretically is at 2.53(carboxylic) and 9.78(amino). This means that the graph of titration of amino glicine acid having two inflection point in which this acid can be attached to two different group of carboxylic and amino. However, from this experiment, the first inflection point are skip and only one inflection point or midpoint can be seen. Using a concentration of 0.1M NaOH, there is no point that we can observed the pH increase drastically. The pH started to be constant at pH 9.83. Unlike other labs, this lab allowed for the most precise titration results possible. The reason for this is that separate drops were entered one at a time and their volume was computed in order to add to the total. However, due to the fact of adding drops one at a time, time became an issue while completing the experiment. One titration took nearly 30 minutes in order to complete flawlessly.
Introduction:
An acid-base titration is a neutralization reaction
that is performed in the lab in the purpose of to determine an unknown
concentration of acid or base. The general purpose of a titration is to
determine the amount of particular substance in a sample. Weak acid is
different from strong acid as it cannot dissociate completely in the water. Due
to this, H+ concentration in weak acid depends on the coefficient of
equilibrium. When a weak acid is titrated with a strong base, or weak base is
titrated with a strong acid, the titration curve is unique for the weak acid or
the weak base. Hence, a titration curve can be used to determine the ionization
constants for weak acids and weak bases.
Materials:
0.1M
acetic acid =4.76
0.1M
phosphoric acid 2.15, 7.20, 12.35
0.1M
amino glicine acid=2.53 (carboxylic) 9.78 (amino)
0.1M
NaOH
Calibrated
pH meter
Methods:
Result:
pH of Acids
NaOH (ml)
|
Acetic
acid
|
Amino
glycine acid
|
1
|
3.53
|
8.16
|
2
|
3.75
|
8.47
|
3
|
3.91
|
8.72
|
4
|
4.08
|
8.90
|
5
|
4.21
|
9.04
|
6
|
4.27
|
9.16
|
7
|
4.34
|
9.31
|
8
|
4.42
|
9.40
|
9
|
4.49
|
9.46
|
10
|
4.54
|
9.54
|
11
|
4.60
|
9.63
|
12
|
4.71
|
9.71
|
13
|
4.75
|
9.80
|
14
|
4.84
|
9.83
|
15
|
4.91
|
9.89
|
16
|
4.96
|
9.96
|
17
|
5.06
|
10.02
|
18
|
5.08
|
10.12
|
19
|
5.15
|
10.18
|
20
|
5.27
|
10.27
|
21
|
5.40
|
10.34
|
22
|
5.52
|
10.45
|
23
|
5.61
|
10.54
|
24
|
5.84
|
10.66
|
25
|
6.22
|
10.78
|
26
|
9.25
|
10.94
|
27
|
11.13
|
|
28
|
11.34
|
|
29
|
11.53
|
|
30
|
11.70
|
|
31
|
11.81
|
|
32
|
11.87
|
|
33
|
||
34
|
12.01
|
|
35
|
12.06
|
|
36
|
12.10
|
|
37
|
12.15
|
|
38
|
12.18
|
|
39
|
12.21
|
|
40
|
12.24
|
|
41
|
12.27
|
|
42
|
12.29
|
|
43
|
12.32
|
|
44
|
12.33
|
|
45
|
12.36
|
|
46
|
12.40
|
|
47
|
12.41
|
|
48
|
12.42
|
|
51
|
12.44
|
|
54
|
12.48
|
|
57
|
12.48
|
|
60
|
12.50
|
|
63
|
12.51
|
|
66
|
12.52
|
|
69
|
12.53
|
|
72
|
12.61
|
Graph of Acetic Acid
Discussion:
In this experiment, a solution of acetic acid which
is a weak acid has been titrated with a 0.1 M
solution of NaOH solution. During the titration, the concentration of acid
will be decreasing because of the reaction with the increment of NaOH. Also,
since the conjugate base of the weak acetic acid was the product of this
reaction, the concentration of base will be increasing at the end of the
experiment. The quantities of both acid and base concentrations became equal at
some point. The equality occurred halfway to the equivalence point which means
half of the weak acid has been converted into its conjugate base, so the molar
quantities will be identical at this point.
The
Henderson- Hasselbalch equation:
pH
= pKa + log [base] / [acid]
plays an important role in order to find the
ionization constant. However, the equality of both acid and base concentrations
[acid]
= [base]
Log
[base] / [acid] = log 1= 0
The Henderson-Hasselbalch equation was reduced to
pH=
pKa
Acetic acid
is a monoprotic since it has only has 1 reflection point or 1 pKa point. By
analyzing the graph of acetic acid, at the half way point (or also known as
inflection point or pKa point),
the pH turns out to be 4.76 as 13.50 ml NaOH has been added. Hence, the
pKa for the acetic acid must also be 4.76 (by applying the
Henderson-Hasselbalch equation, pH= pKa). The acid ionization
constant, Ka, can be calculated;
pKa
= - log Ka
Ka = antilog (-4.76)
= 1.74 x 10-5
The equivalent point is a midpoint of the
vertical line. However, in this graph, the equivalent point was not reach yet
since the experiment was stop after 26 ml of NaOH was added. By looking at the
actual graph of acetic acid, the equivalent point was marked as 27.02 ml of
NaOH has been added into the weak acid solution. 
Titration curves are obtained when the pH of given volume of a sample solution varies after successive addition of acid or alkali. The curves are usually plots of pH against the volume of titrant added or more correctly against the number of equivalents added per mole of the sample. This curve empirically defines several characteristics (the precise number of each characteristic depends on the nature of the acid being titrated:
1) The number of ionizing groups,
2) The pKa of the ionizing group(s),
3) The buffer region(s).
Amino Acids are Weak Polyprotic Acids. They are present as zwitter ions at neutral pH and are amphoteric molecules that can be titrated with both acid and alkali. All of the amino acids have an acidic group (COOH) and a basic group (NH2) attached to the α carbon, and also they contain ionizable groups that act as weak acids or bases, giving off or taking on protons when the pH is altered. The strong positive charge on the amino group induces a tendency for the carboxylic acid group to lose a proton, so amino acids are considered to be strong acids. Some amino acids have other ionizable groups in their side chains and these can also be titrated.
When an amino acid is dissolved in water it exists predominantly in the isoelectric form. The isoelectric point, pI, is the pH of an aqueous solution of an amino acid at which the molecules have no net charge. In other words, the positively charged groups are exactly balanced by the negatively charged groups. When this dissolved amino acid is titrated with acid, it acts as a base, and with base, it acts as an acid which makes them an amphoteric molecule.
The Simple amino acids, like glycine, have two dissociation steps: first, the loss of H+ from the acidic carboxyl group at low pKa value for each dissociable group of an amino acid can be determined from such a titration curve by extrapolating the midpoint of each buffering region (the plateau) within the curve. Also revealed from the diagram is a point on the curve where the amino acid behaves as a neutral salt. Specifically, this point is known as the isoelectric point (pI), and is loosely defined as the pH where the amino acid is predominantly a zwitterion. Furthermore, the pI can be approximated as halfway between the two points of strongest buffering capacity and can be estimated by:
where K1 and K2 are the dissociation constants for the deprotonation of glycine’s carboxylic acid and amino groups.
Since pK1 and pK2 of amino glycine acid are given, we can calculate the isoelectric point (pI),
pI = (1/2)(2.35 + 9.78)
= 6.0
Tables of pKa and pI values of each amino acid are readily available and can be used as standards to identify an unknown amino acid. Furthermore, identification of the regions of the titration curve require a thorough knowledge of the protonation and deprotonation process of an amino acid and an understanding of the definition of an isoelectric point. In summary, titration curves are helpful in the identification of amino acids as follows:
- The number of pKa values differentiates polar and nonpolar amino acids from charged amino acids.
- The position of the pKa values for charged amino acids allows one to identify positively charged from negatively charged amino acids.
- Comparisons between experimental and literature pKa values can allow the identification of a specific amino acid.
These ionizations follow the Henderson-Hasselbalch equation:
Since amino glycine acid having two pKa value,
pKa1 = 2.35
pH = 2.35 + log (0.1M NaOH / 0.1M amino glycine acid)
pH = pKa
= 2.35 (carboxylic)
pKa2 = 9.78
pH = 9.78 + log (0.1M NaOH / 0.1M amino glycine acid)
pH = pKa
= 9.78 (amino)
When the concentration of the unprotonated form equals that of the unprotonated form, the ratio of their concentrations equals 1, and log 1=0. Hence, pKa can be defined as the pH at which the concentrations of the protonated and unprotonated forms of a particular ionizable species are equal. The pKa also equals the pH at which the ionizable group is at its best buffering capacity; that is the pH at which the solution resists changes in pH most effectively.
The pK is the pH at the midpoint of the buffering region (where the pH changes only slightly upon addition of either acid or base). The pK is the pH corresponding to the inflection point in the titration curve. The end point of a titration curve represents the observed end of the titration. The isoelectric point (isoelectric pH; pI) is the pH at which the amino acid has a net zero charge. For a simple diprotic amino acid, the pI falls halfway between the two pK values. For acidic amino acids, the pI is given by ½(pK1 + pK2) and for basic amino acids it’s given by ½(pK2 + pK3).
In this experiment we are finding out the titration curve of the amino acid Glycine.
Glycine is a diprotic amino acid which means that it has two dissociable Protons, one on the α amino group and the other on the carboxyl group. In the case of Glycine, the R group does not contribute a dissociable Proton.
The dissociation of proton proceeds in a certain order which depends on the acidity of the proton: the one which is most acidic and having a lower pKa will dissociate first. So, the H+ on the α-COOH group (pKa1) will dissociate before that on the α-NH3 group (pKa2).
Conclusion:
- The strength of an acid refers to its ability or tendency to lose a proton. A weak acid is an acid that dissociates incompletely. It does not release all of its hydrogen in a solution, donating only a partial amount of its protons to the solution. These acids have higher pKa than strong acid, which release all of their hydrogen atoms when dissolved in water.
- Acetic acid is a monoprotic acid and its pKa value is 4.76 whereas amino glycine acid is a polyprotic acid and its pKa values are 2.35 and 9.78.
References:
Helmenstine, A., M. (2013). Acid-Base Titrations. Retrieved Mac 18, 2013, from http://
chemistry.about.com/od/chemistryquickreview/a/titrationcalc.htm
Experiment title : Making pH indicator
Abstract
pH indicator experiment is conducted to observed the pH range of household chemicals based on the natural indicator. In this experiment, we used two types of materials as indicator color. The two materials are red cabbage and turmeric.The natural indicator also indicate the presence of acid and base. pH range from 1 to 6 is acidic and pH range from 8 to 13 is basic.
Introduction
pH is a measure of hydrogen ion concentration, acidity or alkalinity of a solution. Aqueous solution at 25°C with a pH less than seven are acidic, while those with a pH greater than seven are basic or alkaline. A pH level of is 7.0 at 25°C is defined as neutral because the concentration of H3O+ equals the concentration of OH− in pure water. The most common method to get an idea about the pH of solution is to use an acid base indicator. An indicator is a large organic molecule that works somewhat like a “color dye". Whereas most dyes do not change color with the amount of acid or base present, there are many molecules, known as acid - base indicators, which do respond to a change in the hydrogen ion concentration. Most of the indicators are themselves weak acids.
Materials and apparatus:
·
0.1M HCl
·
0.1M NaOH
·
Solvent
·
Distilled water
·
Mortar
·
Red cabbage
·
Turmeric
·
Test tube & rack
·
Spot plate
Methods:
Extracting the indicator
Testing the pH range of the indicator
Testing the pH of other liquids
Result
1. Acid Base Titration
2. Application : Making pH indicator
Preparation of indicator with turmeric
Preparation of indicator with red cabbage
Testing the acidity of household materials with turmeric and red cabbage.
R - Red cabbage
T - Turmeric
Comparison of household materials with both indicator
Discussion
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).An acidic solution is any aqueous solution which has a pH < 7.0 ([H+] > 1.0 x 10-7 M). In this experiment, hole on white tiles that labelled from 1 to 6 for both turmeric and red cabbage indicate the color of an acid for different types of household materials. Pati oren, asam jawa and sos cili is categorized as acidic that range from 1 to 6. The soap was the only one household materials that shown the basic result. The other household materials such as mineral water, flour, shampoo, toothpaste just show the pH range between pH 1 to pH 11 (turmeric indicator) and between pH 3 to pH 11 (red cabbage indicator).
Red cabbage juice contains a natural pH indicator that changes colors according to the acidity of the solution. Red cabbage juice indicator is easy to make, exhibits a wide range of colors. 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. Therefore, it is possible to determine the pH of a solution based on the color it turns the anthocyanin pigments in red cabbage juice.
Indicators, such as turmeric, work by changing their color with changes in pH. There are many different pH indicators and each indicator changes color at a particular pH level. Often an individual indicator will only undergo one or two color changes. This usually means that using a single pH indicator will tell us the general pH range of an unknown solution. Usually several different indicators must be used and the results compared in order to accurately determine the pH. The turmeric indicator changes color between roughly a pH of 7.4 and 8.6. If turmeric is exposed to neutral or acidic substances (those with a pH of less than 7.4) it will retain its yellow coloration. However, if turmeric is exposed to more alkaline substances (those with a pH greater than 8.6) it becomes a dark pink/red. In this experiment turmeric shows brown color for basic solution.
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).An acidic solution is any aqueous solution which has a pH < 7.0 ([H+] > 1.0 x 10-7 M). In this experiment, hole on white tiles that labelled from 1 to 6 for both turmeric and red cabbage indicate the color of an acid for different types of household materials. Pati oren, asam jawa and sos cili is categorized as acidic that range from 1 to 6. The soap was the only one household materials that shown the basic result. The other household materials such as mineral water, flour, shampoo, toothpaste just show the pH range between pH 1 to pH 11 (turmeric indicator) and between pH 3 to pH 11 (red cabbage indicator).
Besides, we also found that the red cabbage was the best indicator compared to the turmeric because it shown more range of color that can indicate the pH range.
Conclusion
As a conclusion, pH change with the change of H+ ion . pH for acid range from 1 to 6 and pH for base range from 8 to 14. pH 7 is neutral.
References
1 comments:
you guys made me feel like a real biochemist
Post a Comment