Lab Report 1: Titration

INTRODUCTION

Acid is a solution containing hydrogen ions. The higher the concentration of hydrogen ions, the lower the ph of acid. Hydrochloric acid is an example of  strong acid and ethanoic acid is an example of  weak acid. Strong acids are fully ionised  but weak acids are only partially ionised in solution. At the same concentration, strong acids have a higher concentration of hydrogen ions compare to weak acids.

Beside that, atypical ph meter has two basic components:
A) The meter itself, which can be a moving-coil meter (one with a pointer that moves against a scale) or a digital meter (one with a numeric display).
B)  One or two probes that inserted  into the solution you're testing.

For make sure ph meters to be accurate, they have to be properly calibrated (the meter is accurately translating voltage measurements into ph measurements), so they usually need testing and adjusting before we can start to use them. Calibration of a ph meter is by dipping it into buffers (test solutions of known ph) and adjust the meter accordingly. A buffer solution  is an aqueous solution containing a weak acid and its conjugate base or a weak base and its conjugate acid. A buffer's  ph changes very small when a small amount of strong acid or base is added to it. It is used to avoid any change in the ph of a solution, regardless of solute. Buffer solutions are used as in keeping ph at a nearly constant value in a wide variety of chemical applications.

This experiment is mainly based on titration. We titrate base into an acid and in this experiment, we titrate sodium hydroxide with three types of acid separately. These acid are acetic acid, phosphoric acid and  amino glycine acid. The ph of each acid when be titrated with sodium hydroxide will be recorded by using ph meter. 

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

1. 0.1M NaOH was fill in a burette.
2. 25ml of the two acids was titrate separately with NaOH.
3. The pH were measured every time when the 1ml of  NaOH was added.
4. The findings was recorded.

RESULTS

Volume of NaOH used (ml)
pH after every 1ml of NaOH added
Acetic acid
Phosphoric acid
0
0.75
2.21
1
0.80
2.44
3
0.87
2.63
4
0.93
2.72
5
0.95
2.82
6
0.98
2.84
7
1.00
2.96
8
1.04
3.04
9
1.10
3.13
10
1.18
3.20
11
1.28
3.28
12
1.40
3.35
13
1.55
3.40
14
1.71
3.42
15
1.87
3.55
16
2.53
3.64
17
4.26
3.74
18
4.70
3.87
19
4.95
3.99
20
5.13
4.15
21
5.28
4.38
22
5.41
4.80
23
5.52
7.34
24
5.57
8.88
25
5.74
9.36
26
5.87
9.57
27
6.00
9.74
28
6.16
9.85
29
6.36
9.94
30
6.65
10.02
31
10.08
32
10.14
33
10.18
34
10.22
35
10.28
36
10.31
37
10.33
38
10.36
39
10.38
40
10.40
41
10.42
43
10.46
45
10.52
47
10.56
49
10.58
51
10.62
53
10.66
55
10.68
57
10.68


Acetic Acid Graph


DISCUSSION
In this experiment, we were conducting a titration experiment using weak acid and strong base. The weak acid that was used in this experiment is acetic acid and phosphoric acid. Acetic acid is a monoprotic acid, while phosphoric acid is a polyprotic acid. The difference between this two type of acid is the amount of hydrogen that need to be neutralized. 

Since, acetic acid is a monoprotic acid, which mean only one hydrogen need to be neutralized it only have one midpoint. While for phosphoric acid, hydrogen that need to be neutralized  is 3 hydrogen, therefore phosphoric acid have 3 midpoint. Midpoint is a point of pH that resist changes. 

First, we start the titration for acetic acid and sodium hydroxide. The pKa that was obtained from this tiration is 4.76. The graph is a sigmoid curve graph. We used 30mL of sodium hydroxide to obtain the pH of 6.65 of the acetic acid.

After that, we titrate phosphoric acid with sodium hdroxide. The pKa that was obtained from this titration is 7.2. The phosphoric acid is supposed to have 3 midpoint since it is polyprotic acid. But in this experiment we only obtain 1 midpoint due to some error during the experiment. We used 57mL of sodium hydroxide to reach the pH of 10.68 for the phosphoric acid. 

There are few precaution steps that should be highlighted during this experiment which is, we have to make sure that the pH meter was washed with distilled water and was wiped until dry using tissue paper every time we need to measure the pH. This is because, we want to maintain the accuracy of the pH meter for every solution after the titration. 

Based on the result above, we can say that acetic acid need less sodium hydroxide compare to phosphoric acid as it have less hydrogen to be neutralized. 


CONCLUSION

 From the experiment, we can conclude that, acetic acid is a monoprotic acid that have pKa = 4.76 and phosphoric acid is a polyprotic acid that have pKa = 7.2.

REFERENCES

 1.  Theory and Problems of Biochemistry, 2nd ed. New Delhi : Tata McGraw-Hill Publishing Company Ltd., 2003.
2. Van Nostrand's scientific encyclopedia, Tenth edition / Glenn D. Considine, editor-in-chief ; Peter H. Kulik, Hoboken, N.J. : Wiley-Interscience, 2008.
3. http://www.science.uwaterloo.ca/~cchieh/cact/c123/stacids.