ACID- BASE TITRATION

DEPARTMENT OF BIOLOGY

FACULTY OF SCIENCE & MATHEMATICS

UNIVERSITI PENDIDIKAN SULTAN IDRIS

SBK3013

PRINCIPLE IN BIOCHEMISTRY

EXPERIMENT 1: ACID BASE TITRATION

NAME	MATRIC NO.
MUHAMMAD FARIS BIN ISMAIL SAZEMI	D20141067089
MAYURIE PHUTHARANT A/P SURIN	D20141067078
NUR AFIQAH SYAHMINA BT MOHD KAMAL	D20141067091

GROUP: A

LECTURER’S NAME: DR. ROSMILAH MISNAN

INSTRUCTOR: NUR ATIEKAH BT AZAHARI

DATE AND TIME: 17 MARCH 2017

 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.

                The pH value can be measure by using the solution into the pH meter and after every drop of NaOH into the solution the value of pH is taken. The higher the concentration of H⁺, the lower the pH value.

MATERIAL

0.1 M Acetic acid

0.1 M Phosporic acid

PROCEDURE

(1) 0.1M NaOH is filled into a 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 as in the table below

(5) Experiment is then repeated with 0.1M of phosphoric acid

RESULT

1) Titration of Acetic Acid

Volume of NaOH used (mL)	pH
Initial	3.10
1.0	4.37
2.0	5.01
3.0	5.58
4.0	12.33

Graph of pH versus volume of base for acetic acid titration

2) Titration of Phosphoric Acid

Volume of NaOH used (mL)	pH
Initial	1.64
1.0	1.98
2.0	2.72
3.0	6.66
4.0	7.58
5.0	11.11
6.0	12.40
7.0	12.79
8.0	13.18
9.0	13.40
10.0	13.52
11.0	13.60
12.0	13.68
13.0	13.76
14.0	13.78
15.0	13.80
16.0	13.86
17.0	13.87
18.0	13.89
19.0	13.90
20.0	13.93
21.0	13.97
22.0	13.98
23.0	13.99
24.0	14.00

Graph of pH versus volume of base for phosphoric acid titration

DISCUSSION

Titration of Acetic acid

The graph of titration of acetic acid is shown above. Acetic acid is monoprotic acid which can donate only one hydrogen atom per molecule. At the first point of the curve, it only shows the solution of acetic acid only. The pH value at the point is the pH for pure acetic acid. When some volume solution of NaOH is added, hence acetate ion is produced. The curve will rise and show a slight linear. The solution will now contain both acid and its conjugate base which is the acetate ion. Acetate ion will act as a buffer that resist the change in pH value upon dilution or addition of both acid and base.

HC₂H₃O₂ + OH^- à  C₂H₃O₂^- + H2O

At point of addition 1 mL of NaOH, it indicates the initial point .The pH of the point midway is equal to the pKa for acetic acid which is 4.76 approximately. The buffer zone is the middle part of the curve which is flat and the addition of any base or acid will not affect the pH of the solution drastically. The value of buffer zone is between 3.76 to the 5.76. At equivalence point (addition 4 mL of NaOH), all acetic acid is being neutralized and only some acetic ion will be present in the solution.

Titration of Phosphoric acid

The graph of titration of phosphoric acid is shown above. Phosphoric acid is polyprotic acid that can donate more than one hydrogen atoms per molecule. The first point on the curve showed the solution of phosphoric acid only. The pH value at this point is the pH 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 or addition of both acid and base.

At point of addition 1mL NaOH it indicates the initial point. At the first flat curve of the graph which is indicate as the first buffer zone. The first pKa for phosphoric acid is 2.15 at the point midway on the first flat curve. The buffer zone is between pH 1.15 to 3.15. 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). At the second flat curve part of the graph indicate as 2nd buffer zone. The value of second buffer zone is between pH 6.2 to 8.2.  The pH of the point midway between the flat curves equals to pKa2, which is 7.20. 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). The third flat curve part of the graph is called as 3rd buffer zone. The pH of the point midway between the flat curves equals to pKa3, which is 12.35. The buffer zone is range between pH 11.35 to 13.35. At the 3rd equivalence point, only PO₄ ^3- and excess unreacted OH- are present in the solution.

The determining of pKa is by looking at the pH at the half-titration point which equal to the pKa of the weak acid. The pKa value also indicates the inflection point for the acid. Near to this point, the pH changes detected is little with the addition of the base.

The graph is different for each acid because acetic acid are monoprotic acid which have only one pKa value which is 4.76. On the other hand, phosphoric acid are polyprotic acid which indicate three pKa values which are 2.15, 7.20 and 12.35 respectively.

CONCLUSION

The pKa value that we get from the experiment is not much different to the value of the pKa that state for the acetic acid and phosphoric acid.

REFERENCES

Perrin, D. D., Dempsey, B., & Serjeant, E. P. (1981). pKa prediction for organic acids and 

           bases (Vol. 1). London; New York: Chapman and Hall.

Goldman, J. A., & Meites, L. (1964). Theory of titration curves: Locations of points at which 

           ph= pka on potentiometric acid-base titration curves; end-point errors in titrations to 

           predetermined ph values. Analytica Chimica Acta, 30, 28-33.