LAB 3 -ACID BASE AND TITRATION

LAB 3 -ACID BASE AND TITRATION

Objective:

u To determine the pH value of acid and base solution through titration with standard solution.

u To apply the correct technique in titration.

u To carry out acid base titration using phenolphthalein as indicator.

Introduction :

A titration is a procedure used in analytical chemistry to determine the amount or concentration of a substance. In a titration one reagent, the titrant, is added to another slowly. As it is added a chemical stoichiometric reaction occurs until one of the reagents is exhausted, and some process or device signals that this has occurred. The purpose of a titration is generally to determine the quantity or concentration of one of the reagents, that of the other being known beforehand. In any titration there must be a rapid quantitative reaction taking place as the titrant is added, and in acid-base titrations this is a stoichiometric neutralization. The type of titration is simply the type of chemical reaction taking place, and so in this section we consider acid-base titrations.

Acid-Base Titration Reactions

All acid-base titration reactions, as all acid-base reactions, are simply exchanges of protons. The reaction could be strong acid + strong base  (neutral) salt, as in the case of

HCl + NaOH  NaCl + H₂O,

although the reaction would be correctly written as

H₃O⁺ + OH^-  H₂O

since strong acids and strong bases are totally dissociated to protons and hydroxide ions in water.

For reactions which are

strong acid + weak base  (acidic) salt,

such as the example

HCl + CH₃NH₂  CH₃NH₃⁺Cl^-,

or

strong base + weak acid  (basic) salt,

such as the example

NaOH + CH₃COOH  Na⁺CH₃COO^- + H₂O,

the cations and anions could be omitted as they do not actually participate in the reaction. (Some chemists call these bystander ions.)

Virtually all acid-base titrations are carried out using a strong acid or strong base. In most cases the strong acid or strong base is used as the titrant. It is less common, but equally feasible, to place the strong acid or strong base in the titration vessel and use the weak acid or weak base as the titrant. A weak acid-weak base titration would have only a small pH change at the equivalence point. This small change is difficult to detect, and for this reason weak acid-weak base titrations are uncommon.

Standards in Acid-Base Titrations

One of the substances involved in a titration must be used as a standard for which the amount of substance present is accurately known. The standard can be present either in the form of a pure substance or as a standard solution, which is a solution whose composition is accurately known. A standard can be prepared in only two ways: use a primary standard or standardize by titration against some previously standardized solution. A primary standard is some substance such as oxalic acid which can be precisely weighed out in pure form, so that the number of moles present can be accurately determined from the measured weight and the known molar mass. For example, we might prepare a 0.1000 molar solution of primary standard oxalic acid by weighing out exactly 0.1 moles of oxalic acid and diluting to one liter in a volumetric flask.

The standard solutions used in an acid-base titration need not always be primary standards. A standard solution which has been prepared by quantitative dilution of a primary standard is an excellent secondary standard solution. Secondary standards can also be prepared by titration against a primary standard solution.

Apparatus and Material :

Apparatus	Materials
Phenolphthalein indicator Buret Pipette 250ml beakers pH meter White paper towel	0.1M NaOH 0.1M CH3COOH Acetic acid

Methodology :

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

1. The buret was filled with 0.1 M NaOH. 25.00 mL of 0.1 M CH₃COOH was pipetted into a 250 mL beaker and 3-4 drops of phenolphthalein indicator was added. To best observe colour changes, the beaker was placed on a white paper towel.
2. The solution was titrated by adding the NaOH titrant in 1-2 mL increments. The beaker was carefully swirled with each addition.
3. The colored form of phenolphthalein began to stay for a while and then disappeared. At this point, the NaOH dropwise was added until the acetic acid is a very light colour. This was the endpoint for phenlophthalein.
4. The pH of solution in the beaker at this end point was measured and recorded. Then the pH probe was rinsed with distilled water and the probe tip was replaced into its vial.
5. Colour change observed during the titration was recorded. The pH and added NaOH volume at that indicator’s endpoint was used to estimate the target point when conducting the following procedure.
6. The pH / volume data was transferred to an Excel file for later analysis.
7. The saved data in Excel file was used to plot a graph of pH vs. Volume.

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

1. All the steps above were repeated with phosphoric acid.

Result :

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 is a technique where a solution of known concentration is used to determine the concentration of an unknown solution. Usually known solution is poured into the burette and titrant to the unknown solution until the reaction is complete. Knowing the volume of titrant added allows the determination of the concentration of the unknown. Often, an indicator is used to usually signal the end of the reaction, the endpoint.

By adding the sodium hydroxide, which is a basic solution, to the acetic acid, which is an acidic solution, a neutralization reaction occurs. An indicator known as phenolphthalein, is also added to the vinegar. This indicator turns the solution to a dark pink when excess NaOH is added to make the solution more basic. Thus, when the solution turns light pink, this indicates that it has successfully been neutralized. The amount of NaOH used to standardize the vinegar can then be used to determine the amount of acetic acid in the vinegar as they are both at a 1:1 ratio as seen in the above equation. Thus, the moles of NaOH used to neutralize the acid must equal the number of the moles of acetic acid present in the vinegar.

The purpose of this investigation was to determine to molar concentration of acetic acid in vinegar. This experiment showed that the concentration of acetic acid is 0.44mol/L (±3.87%). However, the exact value of the amount of acetic acid present in a 100mL sample of vinegar is 0.883mol/L. The 50% error in this lab can be accounted for through flaws within the procedure of the experiment.

The reactions that occurred in during the experiment were neutralization reactions, meaning that the moles of acid equaled the moles base at the end of the experiment. This factor was used to calculate the molar concentration of the acetic acid by applying it to the formula ‘moles = concentrations x volume’. This series of calculations also contributed towards the uncertainty associated with the final answer as each step approximately doubles the % uncertainty.

Conclusion:

In conclusion, from the result we found out the indicator that was used, change the colour of the solution shows that it was the endpoint. The monoprotic acid is weak acid which lead the curve of the pH have only one equivalent point after pH 5.57 the rapidly to 11.21. For the second acid which is phosphoric acid which is strong acid, we manage to found three equivalent point as it change rapidly at 3.49, 10.94 and lastly 12.51.  The strong acid acid is the solution which hydrogen ion completely ionise meanwhile the weak acid acid is the solution which hydrogen ion partially ionise. 

Reference:

1. NA, (2016), Titration. Retrieved on 27 November 2018 from https://chem.libretexts.org/Ancillary_Materials/Demos%2C_Techniques%2C_and_Experiments/General_Lab_Techniques/Titration 

2. NA, (2018), Acid-Base Titration. Retrieved on 27 November 2018 from https://www.sparknotes.com/chemistry/acidsbases/titrations/section1/ 

3. The Editors of Encyclopaedia Britannica, (ND), Titration Chemical Process. Retrieved on 27 November 2018 from https://www.britannica.com/science/titration