ACIDS AND BASES:

TITRATIONS

Learning Objectives

Upon completion of this experiment, students will have experienced:

1. Generate the titration curve for a strong acid and a weak acid with a strong base

2. Use the titration curve to determine the equivalent mass and pK of the weak acid.

Discussion

Titrations:

One of the most commonly used procedures is titration. An acid base reaction results in the formation of a salt in water. When this reaction is measured volumetrically, the concentrations of the unknown solution can be determined as discussed in Experiment 30. However, when the pH is measured as a result of the titration, a curve showing the relationship between pH and added titrant can be used to determine the equilibrium constant for an acid or base. The titration curve has 4 regions each with unique properties.

Region 1. Before the titrant is added to the solution.

In this region of the titration curve, only the initial acid or base is present and therefore, the calculation of the pH can be determined from the initial concentration, as discussed above. Strong acids [HA] = [H⁺], strong bases [BOH] = [OH^1-]. For weak acids or bases, the equilibrium constant, K, and the ICE table allows for the determination of the [H⁺] or [OH^1-].

Region 2: Buffer Region

After the first drop of the titrant is added but before the equivalence point, this is also called the "Buffer Region". In this region, the reaction of Acid + Base = salt + water determines the amount of acid remaining after base is added and the amount of salt produced. For example if 10 mL of 0.1 M acetic acid solution is titrated with 0.2 M NaOH, then the number of moles of each substance must be determined and the resulting concentration.

Region 3: At the equivalence point

At the equivalence point, the moles of acid being titrated is equal to the moles of base titrant added. Both the reactants of the solution are used up and only the salt has been produced. For a strong acid being titrated with a strong base, this means that the salt produced has limited affect on the pH and the pH at the equivalence point is the pH of water, pure water has a pH of 7.

Region 4: after the equivalence point or over titration region.

In this region, the titrant is being added in excess of the reactant and since the titrant is a strong base, its concentration takes precedence over the concentration of hydroxide ion being produced above and the pH is dependent on the amount of hydroxide added in excess of the equivalence point. The pH is then determined directly from the [OH^1-] of excess NaOH added.

These regions repeat for each hydrogen ion present in the polyprotic acids, but the basic structure remains. For a diprotic acid (two hydrogen ions), the regions would be Region 1; Region 2 – buffer region for the first hydrogen ion, equivalence point of the first hydrogen ion; buffer region for the second hydrogen ion, equivalence and finally Region 4, over titration.

We can identify the equivalence point by finding the point of the inflection point of the curve. If we use an indicator that changes color that occurs near the pH of the inflection point, we would see the color change very near the equivalence point, where the number of moles of base at the equivalence point is equal to the number of moles of hydrogen in the acid. But this is a less accurate method of determining the actual equivalence point. Using a pH titration curve we can determine the actual equivalence point, the volume of titrant added and therefore the moles of each species at the equivalence point.

Pre laboratory assignments

Complete the pre laboratory writing assignment in your laboratory notebook. The prewriting must be completed before the laboratory session begins. The writing should include a purpose/hypothesis of the experiment, a discussion or theory, a detailed step by step summary of the procedure explaining what data will be collected and how you will collect it. Data Tables or space to collect the data should be included within the procedure discussion. Look up any SDS on chemicals that you are not familiar.

Attempt the pre laboratory problems.

Procedure

Part V. Titration curve of an unknown acid.

Record and plot the pH versus volume data for the addition of a 0.5 M NaOH solution added to an unknown acid solution.

Calculate and Report the concentration of the acid solution.