LAB 3 : TITRATION

Title : Titration

Introduction

     Titration is the slow addition of one solution of a known concentration that called as titrant to a known volume of another solution of unknown solution until the reactions reaches neutralization. Which is indicated by a color change. While acid - base titration are usually used to find the amount of a known acidic or basic substance through acid base reactions. The analyte that is titrand is the solution with an unknown molarity. The reagent is the solution with known molarity that will react with the analyte.

      Strong acid like hydrochloric acid has a concentration that normally around 0 to 1. And it can completely dissociate in water easily. While the weak acid such as acetic acid cannot dissociate completely in water. pH is the measure of the concentration of hydrogen ions in a solution. So the lower the pH the higher the concentration of hydrogen ions in the solution. 

Objectives

A) To observe the property of weak acid with the pH changes
B) To find the pH of acid with the calibrated the pH by using the pH meter.


Materials and apparatus

• Phenolphthalein indicator
• burette
• pipette, 
• 250 mL beakers
• pH meter
• White paper towel 

Methodology

A) Titration of monoprotic acid (acetic acid) with NaOH

1. Filled the burette with 0.1 M NaOH. 25.0 mL of 0.1 M CH3COOH was pipette into a 250mL beaker and added 3-4 drops of phenolphthalein indicator. Observed the color changes with placed the beaker on a white paper towel.    

      2. By adding the NaOH titrant in 1-2 mL increments, the solution was titrated. Carefully swirl the beaker with each addition.

      3. The coloured form of the phenolphthalein will begin to stay for a while and then  dissapear. At this point add the NaOH dropwise until the acetic acid is a very light color. This is the end point of for phenolphthelein.

      4. Measured and recorded the pH of the solution in the beaker at this end point, Then rinsed the pH probe with distilled water and replace probe tip into its vial.

       5. Observed and recorded any color changed during the titration. The pH and added NaOH volume at that indicator's endpoint should be used to estimate the target point when conducting the following procedure.

     

       6. Using the data, plot the pH vs. volume of NaOH added to observe the equivalencepoint and half equivalence point.

B) Titration of polyprotic acid (phosphoric acid) with NaOH.

Repeated all the steps above with phosphoric acid.

RESULT AND DATA

A) Titration of acetic acid with NaOH

Volume of Naoh	pH
3 mL	3.67
6 mL	4.07
9 mL	4.29
12mL	4.55
15 mL	4.74
18 mL	5.07
21 mL	5.53
24 mL	10.96
27 mL	11.58
30 mL	11.81
33 mL	12.11
36 mL	12.24
39 mL	12.29
42 mL	12.40
45 mL	12.39

   

B) Titration of phosphoric acid with NaOH

Volume of NaOH	pH
3 mL	1.68
6 mL	1.53
9 mL	1.64
12 mL	1.68
15 mL	1.82
18 mL	1.96
21 mL	2.07
24 mL	2.29
27 mL	2.46
30 mL	2.69
33 mL	3.55
36 mL	5.54
39 mL	6.20
42 mL	6.34
45 mL	6.69
48 mL	6.86
51 mL	7.03
54 mL	7.14
57 mL	7.41
60 mL	7.74
63 mL	7.87
66 mL	8.59
69 mL	10.56
72 mL	11.44
75 mL	11.48
78 ml	11.60
81 ml	11.70
84 ml	11.80
87 ml	11.87
90 ml	11.93
93 ml	12.05
96 ml	12.08
99 ml	12.11
102 ml	12.16
105 ml	12.20
108 ml	12.24
111 ml	12.30
114 ml	12.32
117 ml	12.36
120 ml	12.38
123 ml	12.41
126 ml	12.43
129 ml	12.45
132 ml	12.51

 


DISCUSSION

A) Titration curve of acetic acid

 The titration curve for this titration shows that at the 1^st point on the curve corresponds to a solution of acetic acid only. And the pH at this point is the pH value for pure acetic acid. The acetic ion produced by adding the NaOH and the curve shows a slight linear rising. At that moment, the solution contains acid and its conjugate base that we called as acetate ion. The conjugate base act as buffer which is resist the change in pH upon dilution and addition of both acid and base.

 HC2H3O2+ OH-          C2H3O-2 +H2O.

At point where is the volume of NaOH used is 3 mL indicates the initial point. While the volume of NaOH used at point 21 mL to point 24 mL are the equivalance point .At point 15 ml which is the point that equals to pKa, that is 4.74. At the equivalence point all acetic acid is being neutralized and only some acetic ion is present in the solution.

B) Titration curve of phosphoric acid

The first point on the curve correspond to a solution of phosphoric acid only. The pH at this point is the pH value for pure phosphoric acid . When some NaOH is added the H2PO-4 is produced. The solution now contains acid and its conjugate base. As the conjugate acts as buffer, it resist the change in pH upon dilution.

At point 3 ml  of volume of NaOH used  which indicates as initial point while at point 36 mL of volume of NaOH used is the 1st equivalence point. And at these points it were called as 1st buffer zone. The pH of the point midway between them (point 27 mL) equals to pKa1, which is 2.46. At the equivalence point, all H3PO4 is being neutralized and only some H2P0-4 is present in the solution.

When more Naoh is added, HP02-4 is produced. The solution now contains acid and its conjugate base. Point 66 mL of volume of NaOH used is the 2nd equivalence point. From 36 mL to 66 mL of volume of NaOH used , they called as 2nd buffer zone. The pH of the point midway between them which is 51 mL of volume of NaOH used equals to pKa2, which is 7.03. At the 2nd equivalence point only HP02-4 is present in the solution.

More NaOH is added, P03-4 is produced. The solution is now contains acid and its conjugate base .

At point 69mL volume of NaOH used is the 3rd equivalence point. From 72 mL to 105 mL volume of NaOH called as 3rd buffer zone. The pH of the midway is point 105 ml volume of NaOH used equals to pKa3 , which is 12.2. At the 3rd equivalence point , only P04 3-. and excess unreacted OH-  are present in the solution.

The graph is different for each acid this is due to the differences in types of acid. Both are the weak acid yet the acetic acid , it has one pKa value that is 4.74. While phosphoric acid have 3 pKa value that are 2.46, 7.03 and 12.2. The pKa value also shows the the inflection point for the acid.

Based on the graph, the change in slope for acetic acidgraph is small while in phosphoric acid is bigger.  With this comparison, it shows that phosphoric acid is more stronger than acetic acid eventhough both are weak acid.

CONCLUSION

In conclusion, a solution that contains a weak acid and conjugate is being able to resist changing its pH when either a base or an acid is added to it. This kind of solution called as buffer because the solution is protected from pH changes even when the H₃0⁺ or OH^- ions added to the solution. This help to stabilizing the pH of the solution. 

 REFERENCES

Chemistry LibreTexts, (Oct, 2016), Acid- Base Titration. Retrieved from 
https://chem.libretexts.org/Ancillary_Materials/Demos%2C_Techniques%2C_and_Experiments/General_Lab_Techniques/Titration/Acid-Base_Titrations

Khan Academy, (2016), Titration curve and equivalence point. Retrieved from
https://www.khanacademy.org/test-prep/mcat/chemical-processes/titrations-and-solubility-equilibria/a/acid-base-titration-curves