Reference no: EM133277064

Elementary Analytical Chemistry

Titration Analysis of a Polyprotic Acid or base with a pH probe

Adapted from the lab Using a pH Electrode for an Acid-Base Titration (Harris, 7. Using a pH Electrode for an Acid-Base Titration, 2016):

This lab will span two weeks. In this lab we will titrate polyprotic acids using the standard NaOH solution that you previously made. In this experiment you will use a pH electrode to follow the course of an acidbase titration. You will observe how pH changes slowly during most of the reaction and rapidly near the equivalence point. Section 11-5 of the textbook provides background for this experiment. Using the plot of pH vs Volume of NaOH, you will interpret the graph to find the equivalence points. Because we are working with polyprotic weak acids, we expect that the half-equivalence volumes will allow us to estimate the pKa for the first two ionization constants. This gives us the first pKa1 and pKa2 values.

In week two, we will use two mathematical treatments designed to clarify the locations of the equivalence points. We will compute the first and second derivatives of the titration curve to locate the end point. We will then use these values to find the half-equivalence points on our original plot to determine the pKa1 and pKa2 values. This gives us two more sets of data.

Next you will create Gran Plots to find the equivalence points, pKa1 and This will give you the last set of Ka values for comparison.

Make a note of the concentration that you calculated in your notebook. The analysis of the data will be using several techniques:
1. Qualitative Analysis of the curve to determine the equivalence points for each acid
2. Quantitative Analysis of the data using the first and second derivative plots. This is to determine the Equivalence points for the acid, that will be used to find the pKa1 and pKa2 for the assigned acid.
3. Quantitative Analysis of the data using Gran Plots for Analysis.

Week One: Data Collection Procedure:

1. Following instructions for your particular pH meter, calibrate a meter and glass electrode, using buffers with pH values near 10 and 4. Rinse the electrode well with distilled water and blot it dry with a tissue before immersing in any new solution.

2. The first titration is intended to be rough, so that you will know the approximate end point in the next titration. Pipet 25.0 mL of acid solution into a 250-mL beaker containing a magnetic stirring bar. Position the electrode in the liquid so that the stirring bar will not strike the electrode. If a combination electrode is used, the small hole near the bottom on the side must be immersed in the solution. This hole is the salt bridge to the reference electrode.

3. Allow the electrode to equilibrate for 1 min with stirring and record the pH. Using a 50 mL buret, titrate with standard 0.1 M NaOH until the pH stabilizes at or above a pH=11.5 , using a 50-mL buret. Add 0.5 mL of titrant at a time so that you can estimate the equivalence volume to within 0.5 mL. Offload your data to the computer to analyze its shape. Since you have a polyprotic acid, you should have more than one jump region. Note the volumes at which these jumps occurred.

4. Now comes the careful titration. Pipet 25.0 mL of unknown solution into a 250-mL beaker containing a magnetic stirring bar. Position the electrode in the liquid so that the stirring bar will not strike the electrode. Allow the electrode to equilibrate for 1 min with stirring and record the pH.

5. Add ~1.5-mL aliquots of titrant and record the exact volume, the pH, and the color 30 s after each addition. When you are within 2 mL of the equivalence point, add titrant in 2-drop increments. When you are within 1 mL, add titrant in 1-drop increments. Continue with 1-drop increments until you are 0.5 mL past the second equivalence point. The equivalence point has the most rapid change in pH. Add five more 1.5-mL aliquots of titrant and record the pH after each.

6. Save the data into an excel file for later analysis. At end of lab data collection day, upload your excel file for verification, and upload your lab notebook images. You may insert these images into a tab of the excel spreadsheet so that there will be only one file deliverable.

7. For next week, prepare a visual determination graph for each trial to determine rough equivalence points see video playlist in Canvas for analyzing graphs. For Week One, go to the Lab Module and click on the link for LV05A

Data Analysis A: pH vs Volume plot:

1.Construct a graph of pH versus titrant volume. Mark on your graph where the indicator color change(s) was observed. Note the anticipated pKa for each of the acid forms in the polyprotic acid. Watch the procedural Video for this process. Once the interpreted graph is completed, use your snipping tool to insert the image into your Word Document Lab Report.

LV05B. Graphical Analysis of pH Titration Data using Excel. (Online videos to assist in your analysis of your data)
Analysis: You will create three types of plot for each sample analyzed in L05.
1. pH vs Volume (completed in week1) is needed before lab
2. First Derivative plot
3. Second Derivative Plot
4. Gran Plot for each equivalence point.

Derivative and Gran Plot analyses of Titration Data.

There is a video playlist in the lab module labelled LV05B. This shows you how to create derivative and Gran Plots for your data. Apply these principles to your collected data to complete the rest of the lab.

Data Analysis B: Derivative Plot Analysis of your samples.
2. Following the example in Figures 11-5 and 11-6 of the textbook, compute the first derivative (the slope, ΔpH/ΔV) for each data point within ±1 mL of the equivalence volume. Watch the Video on Canvas for assistance with this using Excel. Once this plot is completed, use the snipping tool to insert it into your lab report document.

From your graph, estimate the equivalence volumes as accurately as you can, as shown in Figure 1.

Figure 1. Locating the maximum position of the first derivative of a titration curve.

3. Following the example in Figure 11-6, compute the second derivative (the slope of the slope, Δ(slope)/ΔV). Prepare a graph like Figure 11-7 in the textbook and locate the equivalence volume as accurately as you can.

4. Go back to your graph from step 1 and mark where the indicator color changes were observed.
Compare the indicator end point to the end point estimated from the first and second derivatives.

Data Analysis C: The Gran Plot Analyses: From Harris Lab The Gran Plot (Harris, 9. Analysis of an Acid-Base Titration Curve: The Gran Plot, 2016)

Using a Gran Plot to Find the End Point of a Titration

1. Construct a Gran plot (a graph of Vb10-pH versus Vb) by using the data collected between
0.8Ve and Ve. Do a Gran plot for each of the observed Ve for your acid. Draw a line through the linear portion of this curve and extrapolate it to the abscissa to find Ve. Use this value of Ve in the calculations below. Compare this value with those found with your indicator and estimated from the graph of pH versus Vb. Perform Gran Plots for the first and 2nd equivalence points for each trial.

2. Compute the slope of the Gran plot and use Equation 3 to find Ka for the acid species as follows: The slope of the Gran plot is -KaγHA/γA- In this equation, HA is (the weak acid) and A- is the conjugate base anion (the weak base). Because the ionic strength changes slightly as the titration proceeds, so also do the activity coefficients. Calculate the ionic strength at 0.95Ve, and use this "average" ionic strength to find the activity coefficients. There is a video in Canvas to assist you with this graph.

3. From the measured slope of the Gran plot and the values of γHA and γA- calculated using the DebyeHückel Equation, calculate pKa. (Hint: the ionic strength is related to the pH of the solution at this stage) Then choose an experimental point near ?Ve and one near ?Ve. Use Equation 1 to find pKa with each point. (You will have to calculate [A-], [HA], at each point. Hint: use your alpha equations and your pH.)

4. Compare the average value of pKa from your experiment with pKfor your acid listed in Estimated from your pH vs V plot.

5. Repeat the analysis of the second equivalence point, and find pKa2

Theory: Be sure to discuss the following concepts and terms:
• Endpoint, equivalence point, first derivative, second derivative, Gran Plot, polyprotic acid , activity coefficients, ionic strength.
Sample Calculations
• The visual graph, first and second derivative graphs are all methods of determining the equivalence points. Using the graphical analyses, find the pH from your titration curve at the ½ equivalence points. This pH is the pKa for the first two protonated forms.
• Use the slope of the Gran Plots for the first two equivalence points to determine the value of the Ka's. There is a helpful spreadsheet in your student files area to assist you with this.
• Find the Average of your Ka 1 results. Find the average of your Ka2 results.
• For each method, find the % difference from the average. Which technique appeared to give the best results?

Purpose: By collecting the data, what question were you looking to answer? (Note that learning a technique is not the purpose of this lab. You collected numbers so that you could use them to arrive at an answer. What was the question?)

Theory: Describe the rationale behind this procedure. In your discussion, you should consider the suggested readings and suggested terms and topics in each lab. Cite your sources.

Procedure: Give a brief description of your process. Remember that this is for someone who has not performed the experiment. Describe the apparatus and necessary steps.

Data: Provide Tables of Raw Data Collected for each device. No calculations have yet been performed on any raw data- this is exactly what the instrument provided, to the proper precision of the device.

If your data is better shown as a graph, display the graphed data here. Using the "Snipping Tool" in Windows or Apple screen shot that is cropped, cut and paste from your Excel plots to create figures in your report for plots and images. Caption each figure. (The snipping tool is used so that the curve-fitting and interpretation is preserved. If you cut and paste from excel, the graph markup is not preserved) Sample

Calculations:

Show an example of one of each of these. Show the formula that you used including all units and chemical species, show sample number substitution, then show your results: Use your equations template in Word to create these. Refer to the rubric and protocol for the expectation You may also include intermediate summary tables here if relevant.

Remember that Excel tables are not calculations- they are most similar to a calculator display of results.

Questions: Answer any directed questions not included in your results Error Analysis:

1. Discuss procedural or sample handling errors that may have led to incorrect measurement. Whether these occurred or not, what error sources should be considered? What difficulties or sources of error did you have when performing the experiment?

2. How do you think that your stated difficulties affected your resulting measurement?

3. Were there any data that were rejected? On what basis?

Attachment:- Titration Analysis of a Polyprotic Acid.rar