Titration Curve of a Weak Acid with Strong Base
Objective: To obtain and analyze a pH versus volume curve for the titration of ethanoic acid with sodium hydroxide.
Materials:
pH sensor (Pasco) / phenolphthalein indicator solutionSPARK (Pasco) / ethanoic acid solution, ? M CH3COOH
graduated cylinders (100-ml and 10-ml) / buffer solutions (pH 4 and pH 10)
Beaker (250-ml and 400-ml) / Standardized sodium hydroxide solution (0.100 M)
Small glass funnel / magnetic stir plate
50 cm3 buret / Stir bar
ring stand and buret clamp / Wash bottle with distilled water
Test tube clamp
Plastic pipets
Volummetric pipet (20.00 ml)
Pre-Lab:
1. Watch the following video, which shows how to properly perform a titration:
http://www.dlt.ncssm.edu/core/Chapter16-Acid-Base_Equilibria/Chapter16-Labs/titration-lg.htm
2. Learn how to calibrate the pH sensor and SPARK by watching the video at
http://www.pasco.com/prodCatalog/PS/PS-2102_pasport-ph-sensor/index.cfm#resourcesTab
Procedure:
A. Buret set-up
1. Rinse the buret with ~ 5 ml of 0.100 M NaOH solution.
2. Rinse and fill the buret past the 0 ml mark with 0.100 M NaOH solution. Use a funnel for this step and make sure the stopcock is closed before filling.
3. Drain some of the NaOH through the tip into a waste beaker to fill the tip and bring the level of NaOH to exactly 0.00 ml. If the 0.00 ml mark is passed, use a pipet to add more NaOH until the meniscus is exactly at 0.00.
4. Obtain a clean, dry 250-ml beaker.
5. Using the volumetric pipet, place exactly 20.00 ml of ethanoic acid, CH3COOH into the 250-ml beaker.
6. Add ~ 80 ml of distilled water to the 250-ml beaker.
7. Add 3 drops of phenolphthalein indicator to the beaker.
8. Add a stir bar to the beaker.
B. Trial run
· No pH measurements need to be made in this run.
· The purpose of this run is to determine the approximate amount of titrant needed to reach the endpoint. This amount will be referred to as V ml.
1. Turn on the stirrer.
2. Open the stopcock valve and add sodium hydroxide until you reach you reach a permanent faint pink end-point.
3. Record the volume needed (V ml). This value (V) will be used later to help you judge the size of volume additions.
C. Calibrate the pH sensor
1. Obtain small samples of pH 4 and pH 10 buffer solutions (enough so that the pH sensor’s glass electrode will be fully submerged)
2. Following the instructions on p. 13-14 of the SPARK User’s Guide, perform a two-point calibration using the two buffer solutions.
3. Allow the sensor to rest in the pH 4 solution while you move on to the next section.
D. Buret and pH sensor set up
1. Repeat all steps in Part A and turn on the stirrer.
2. Clamp the pH electrode so that the bulb is in the solution but above the stir barn (see diagram).
3. Record the initial pH for 0.00 ml of 0.100 M NaOH added.
4. Add sodium hydroxide solution in 5 ml portions until about (V - 5) ml, recording the pH and exact buret volume after each addition. V is the volume determined in Part B Trial Run.
5. Now add sodium hydroxide in 1 ml increments and then 0.1 ml increments as you see the color change of the indicator start to persist. Do this through the end point.
6. After the equivalence point, continue adding sodium hydroxide in appropriate volumes while continuing the record the pH and buret volume until a pH of 12 is obtained or until the buret reading = 50.00 ml. Do not allow the NaOH to go below 50.00 on the buret.
Analysis:
1. Enter your data into a spreadsheet program such as Microsoft Excel or Open Office Calc. (Column A = Volume of NaOH; column B = pH)
2. Construct the following a plot of pH as a function of volume of base added.
The video at http://youtu.be/Cbo_h6r1Ric is a good guide. Although, for our analysis, you will need to:
a) change the graph type to “X-Y (Scatter) with Smoothed Line and Markers”
b) change the minimum unit and gridlines on both axes in order to get more lines on the graph (see “Titration Curve of a Strong Acid with Strong Base” handout as an example of what the axes and plot area should look like).
3. Use the Geometric Method to determine the pH and volume at the equivalence point and midpoint.
Data
Volume pH
0 / 3.45 / 4.36
10 / 4.84
15 / 5.26
16 / 5.37
17 / 5.51
18 / 5.71
19 / 5.97
19.1 / 6.01
19.2 / 6.05
19.3 / 6.07
19.4 / 6.11
19.5 / 6.26
19.6 / 6.28
19.7 / 6.33
19.8 / 6.39
19.9 / 6.47
20 / 6.52
20.1 / 6.59
20.2 / 6.8
20.3 / 6.82
20.4 / 7.03
20.5 / 7.38
20.6 / 7.43
20.7 / 8.32
21.7 / 10.58
22.7 / 11
23.7 / 11.2
24.7 / 11.33
25.7 / 11.42
26.7 / 11.48
27.7 / 11.54
28.8 / 11.58
29.9 / 11.61
30.9 / 11.65
31.9 / 11.67
32.9 / 11.72
33.9 / 11.74
34.9 / 11.75
39.9 / 11.82
44.9 / 11.86
49.9 / 11.9
Since you did complete this lab, you need to read over the procedure to determine what substance was in the flask(analyte) and buret(titrant). I have highlighted important information for you to look at.
Titration Analysis Questions
1. Was the equivalence point at pH =7? Explain why the pH of the equivalence point is not neutral even though the number of moles of base added is equal to the number of moles of acid.-do not use type of acid –base relationship. What are the ions doing in solution to make this observation?
2. Using your graph, what volume did it take to reach equivalence point? Make sure you used the graphical methods to determine this!
3. Calculate the exact concentration of acetic acid in the analyte using the exact concentration of NaOH and the volumes you measured in the experiment. Show work! Think basic titration calculations from Unit 4, MaVaNa=MbVbNb
4. If you titrated HCl instead of CH3COOH, would it take more, less, or the same amount of NaOH to neutralize? Explain.
5. Label the buffer region on your titration curve by circling the area on the curve. What buffer system is present? I’m looking for the chemical formulas of what is in the buffer
6. Identify the volume at midpoint by notating in the x-axis a dash. Make sure you are using graphical methods to determine this.
7. Write the algebraic equation for Henderson Hasselbach used to solve for midpoint & explain the significance of midpoint using terms of acid and conjugate base.
8. Determine the Ka of acetic acid from your data and calculate the % error based on the accepted value in your textbook. Look at page 680 for Ka of acetic acid(also known as ethanoic acid)
9. Using your textbook, page 743, table 16.1 state the range of pH in which the phenolphthalein goes from clear to pink.
10. Evaluate the use of phenolphthalein indicator for this titration. Can you suggest a better indicator?
11. Why would bromothymol blue have been a poor choice of an indicator for this titration? Use its range of pH in your explanation
Watch the Pre-lab video clip in preparation for the Green Crystal Titration by Wednesday.