LAB 3 : TITRATION

LAB 3 TITRATION

INTRODUCTION

In an acid-base titration, the important information to obtain is the equivalence point. If there are a given number of moles of acid in the titration flask, the equivalence point is reached when that same number of moles of base have been added from the burette.The molarity of the base can then be calculated since the number of moles of base added is the same as the number of moles of acid in the flask, and the volume of the base added is also known.  Similarly, if the number of moles of acid in the titration flask is unknown, it can be calculated for the equivalence point if the molarity of the base and the volume of base added are known.

Often the pH of the solution will change dramatically at the equivalence point. An acid-base indicator works by changing color over a given pH range.  If an indicator which changes color near the equivalence point is chosen, there is also a dramatic change in the color of the indicator at the equivalence point because the pH changes so rapidly.  

In a potentiometric acid-base titration, an indicator is not necessary. A pH meter is used to measure the pH as base is added in small increments (called aliquots) to an acid solution.A graph is then made with pH along the vertical axis and volume of base added along the horizontal axis.  From this graph the equivalence point can be determined and the molarity of the base calculated.

OBJECTIVES

1. To perform a potentiometric titration of an acidic solution of known molarity.

2. To graph the volume of base added vs the pH and to determine the equivalence point.

EQUIPMENTS

Phenolphthalein indicator

Buret

Pipette

250ml beakers

pH meter

White paper towel

*Please calibrate the pH meter prior to use as described in Experiment 1

METHODOLOGY

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

1. The burette was filled with 0.1M NaOH. 25.00 mL of 0.1 M CH3COOH was pipette into a 250 mL beaker and 3–4 drops of phenolphthalein indicator were added. The beaker WAS placed on a white paper towel to best observe color changes.

2. The solution was titrated by adding the NaOH titrant in 1–2 mL increments.The beaker was swirl carefully with each addition.

3. The colored form of the phenolphthalein will begin to stay for a while and then disappear. At this point NaOH was added dropwise until the acetic acid is a very light color. This is the endpoint for phenolphthalein.

4. The pH of the solution in the beaker were measured and recorded at the end point. Then the pH probe was rinsed with distilled water and the probe tip was replaced into its vial.

5. Any color change observed were recorded during the titration. The pH and added NaOH volume at that indicator’s endpoint was being used to estimate the target point when conducting the following procedure.

6. The pH/volume data were transfered to an Excel file for later analysis. The volumes store in the A column and the pH values in the B column of the spreadsheet, beginning in cells A1 and B1.

7. Using the saved data in Excel file , a plot of pH vs. Volume of NaOH added to observe the equivalence point and half equivalence pointwere prepared.

8. To verify the equivalence point, the inflection point was determined by calculating the change in pH per change in volume, ΔpH/ΔV, for each recorded volume, as shown in Figure 2 for the NaOH/HCl titration. ΔpH/ΔV vs. the volume of titrant added was plotted. To perform this, the volume  and  pH  values  are  in  columns  A  and  B were checked on the Excel spreadsheet. In cell C1, typed: =(A1+A2)/2. Click on cell C1 and drag down to the C cell