LABORATORY REPORT 1

SBF 3013 BIOCHEMISTRY

LABORATORY 1

TITLE: ACID BASE (1. Acid Base Titration)

Name	Matric number
Sharifah Awanis bt Syed Mohd Aswad	D20141067053
Maureen Santih ak Ambang	D20141067070
Heidi Amelda ak Lagat	D20141067086

LECTURER: ASSOCIATE PROFESSOR DR ROSMILAH BINTI MISNAN

INSTRUCTOR: NUR ATIEKAH BINTI AZAHARI

Laboratory 1 : Acid base

1.      Acid Base Titration

Introduction:

Titration is the slow addition of one solution of a known concentration (called a titrant) to a known volume of another solution of unknown concentration until the reaction reaches neutralization. In this experiment we used weak acids to react with NaOH.

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, while in strong acid, all acid molecules exist as H⁺  and A^-  in a solution. Due to this property, H⁺  concentration in weak acid depends on the coefficient of equilibrium.

The pH can be measured by emerging the tip of pH meter into the solution and read the value in the recorder. The higher the concentration of H⁺ , the lower the pH value.

Objectives:

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.

Materials

0.1M acetic acid (pKa=4.76)

0.1M phosphoric acid (pKa= 2.15, 7.20, 12.35)

0.1M NaOH

Calibrated pH meter.

Methods:

Results:

0.1 M acetic acid

Volume of NaOH	pH
1	3.59
2	3.92
3	4.09
4	4.13
5	4.31
6	4.35
7	4.44
8	4.51
9	4.60
10	4.65
11	4.66
12	4.83
13	4.84
14	4.95
15	5.12
16	5.21
17	5.21
18	5.26
19	5.26
20	5.46
21	5.68
22	5.89
23	6.30
24	6.62
25	8.90

0.1 M phosphoric acid

Volume of NaOH	pH
1	2.09
2	2.09
3	2.09
4	2.22
5	2.28
6	2.36
7	2.45
8	2.55
9	2.56
10	2.62
11	2.68
12	3.02
13	3.45
14	4.12
15	6.10
16	6.43
17	6.51
18	6.55
19	6.68
20	6.70
21	6.90
22	7.10
23	7.31
24	7.65
25	7.85
26	8.10
27	8.45
28	9.41
29	11.05
30	11.63
31	11.87
32	12.10
33	12.14
34	12.24
35	12.27
36	12.30
37	12.35
38	12.40
39	12.45
40	12.50

Graph pH vs Volume of NaOH (acetic acid)

Graph pH vs Volume of NaOH (phosphoric acid)

Discussion:

Titration curve for acetic acid and phosphoric acid are shown on the graph papers. When NaOH is added to acetic acid, acetate ion is produced. During titration, the concentration of the acetic will be decreasing. The curve shows a slight linear rising. Now, the solution contains acid and its conjugate base which is acetate ion. This conjugate base acts as a buffer that resists the change in pH upon dilution and addition of both acid and base.

    
From the graph of acetic acid, 1ml of NaOH is the initial point, while at 24.9ml (pH=7.95) is the equivalent point. From 1ml of NaOH to 25ml of NaOH is called as buffer zone. At the equivalent point, all acetic acid is neutralized and only the acetate ion is present in the solution. The equivalent point is the midpoint of the vertical line.

Then, the first point from the titration curve of phosphoric acid is the pure pH of the phosphoric acid. When NaOH is added into the phosphoric acid, H3PO4- is produced. The content in the solution contains acid and its conjugate base which is dihydrogen phosphate. This conjugate base also acts as buffer which resists the change in pH upon dilution and addition of both acid and base.

1ml of NaOH indicates the initial point, at 14.8ml (pH= 5.5) of NaOH indicates the first equivalent point. From point 1ml of NaOH to 15ml of NaOH, it is called as buffer zone.  7.5 ml is the midpoint between 1ml to 15ml which is equal to pKa1 (2.5). At the first equivalent point, all H3PO4-    is being neutralized and only some H2PO4- is present in the solution.

When more NaOH is added, HPO4(2-)  is produced.  is the conjugate base that acts as buffer. At 29.5ml (pH=11.35)  of NaOH is the second equivalent point. From 15ml to 29 ml is called as second buffer zone. The pH of the midpoint between 15ml to 29ml equals to pka2, which is 7.1 at 22 ml. At the equivalent point, only HPO4(2-)     is present in the solution.

When more NaOH is added further, PO4(3-)  will be produced. The solution contains acid and its conjugate base which is phosphate ion. It is acts as buffer. From the graph plotted, we can only see until second equivalent point. But, the graph plotted has reached the third pKa3 (12.35) at 37ml of NaOH.

The graph is different for each acid is because of the difference types of acid that is used. Both of the acid are weak acid. Theoretically, the pKa value for acetic acid is 4.76 which is only one pKa, while phosphoric acid has three pka values that are 2.15, 7.20 and 12.35. The pKa value also indicates the inflection point for the acid. From the graphs plotted, the change in slope for acetic acid graph is small, while for the phosphoric acid graph, the change in slope is larger or steeper. With this comparison, it shows that phosphoric acid is stronger than acetic acid but both acids are weak acids.

Conclusion:

In conclusion, a strength of the acid refers to the ability of the acid to lose a proton. Weak acid is an acid that dissociates incompletely. It also has higher pKa value compared to strong acid. In this experiment, acetic acid is a monoprotic acid because it has only one pKa value which is 4.76. Whereas, phosphoric acid is a polyprotic acid because it has three pKa values which are 2.5, 7.1 and 12.35.

References:

1. Pharmacopoeia, B. (2016). British pharmacopoeia.

2. Berend, K., De Vries, A. P., & Gans, R. O. (2014). Physiological approach to assessment of acid–base disturbances. New England Journal of Medicine, 371(15), 1434-1445.

3. Povar, I., & Spinu, O. (2015). Acid-Base Buffer Properties of Heterogeneous Multicomponent Extraction Systems. Solvent Extraction and Ion Exchange, 33(2), 196-209.

Reflection:

Name: Heidi Amelda

I have learnt how to be patience during conducting this experiment because it requires a lot of time to do the experiment. We faced a little problem during conducting this experiment but we able to handle it. Great teamwork from my group members really help me a lot.

Name: Maureen Santih

In this experiment, we faced some problems such as we do not have enough pH meter that we have to share it with the other group. This make our work become slower as we need to measure the pH of the phosphoric acid and acetic acid every time after adding with 1ml of NaOH Until they reached pH 7(acetic acid) and pH 14(phosphoric acid). We also make some mistakes that we overly titrate more than 1ml of NaOH into the posphoric acid and acetic acid. This may effect the results of the experiment. What I get from this experiment is I get to know that a strength of the acid refers to the ability of the acid to lose a proton. Phosphoric acid is stronger than acetic acid, but both are weak acids. Overall, the experiment is important for me to understand the acid and base.

Name: Sharifah Awanis

This experiment need a lot of patient because i need to measure the pH of the acid repeatedly. The pH value also increased slowly so that it takes a long time to achieve pH 7 for acetic acid and pH 14 for phosphoric acid. with the help of my groupmate, we successfully done this experiment and got the data that we need.