SPECIAL TITRATIONS USING A pH ELECTRODE

Introduction

Certain hydrated metal ions in solution behave as Bronsted acids (proton donor) and the pH of their solutions will range from about 2 to 6. The final pH depends upon the nature of the metal ion, its concentration in the solution, and the presence of other species such as HCO3-, and NH3 in the solution. These metal ions can be monitored in various ways but one interesting way is to titrate them just like any acid or base while using a pH electrode to monitor the [H+] after each addition of the titrant. In this experiment you will determine the amount of zinc present in a starting solution that is approximately 0.05 M and the mass of zinc in a commercial mineral supplement.

You will plot the titration curves, calculate the concentration of the zinc in the solution from the end point of the titration, and determine the mass of zinc present in a vitamin tablet.

Materials

Standard 0.10 M solution of NaOH, 0.050 M solution of Zn2+ , actually Zn(N03)2, CBL, TI-8X calculator, pH electrode, 50 mL buret, 100 mL graduated cylinder, 10 mL graduated cylinder, 250 mL beaker, 250 mL flask, Saran wrap if available, Dl water, eyedropper pipettes, mineral or vitamin supplement tablet, and access to a balance.

Procedure

Since time is required to prepare and dissolve the vitamin tablet one partner should start on that while the other gets the solutions and prepares the burette and CBL system for the titration.

1. Gently crush the zinc containing tablet in a plastic cup and mass the cup and crushed tablet. Place the crushed tablet into a clean 250 mL flask and add 200 mL of Dl water. Set this flask aside to allow the contents to dissolve. Mass the empty plastic cup and determine the mass of the tablet being dissolved in the water in the flask. Note: Not all of the material will dissolve since part of the tablet is inert binder.

2. Fill a 50 mL burette with 0.10 M NaOH. Be sure to drain the liquid down to fill the tip. Read the burette to the nearest 0.01 mL.

3. You will use a CBL system as a recording pH meter. This CBL-pH meter will store your data and make an approximate graph of the results of your experiment as the titration is being performed. Use the Trigger/Prompt Titration procedure for this titration.

4. Place 5.0 mL of the starting Zn2+ solution and about 50 mL of Dl water in the 250 mL beaker. Remove the pH probe from its protective bottle by unscrewing the white cap. Place the pH probe into the solution in the beaker. Swirl the pH probe gently to ensure complete mixing. Use this solution as the first data point for the titration. Even though the CBL system is saving your data, record the data for each titration in your lab notebook.

5. Perform a titration using the 0.10 M NaOH.

6. To proceed with the titration, add one milliliter of base. Read the exact volume carefully, swirl or mix using the pH probe.

7. Repeat step 6 until you have added several milliliters past the endpoint.

** SOMETHING EXTRA**

8. For a more accurate determination of the concentration of the metal ion, perform a back titration. This titration is performed by titrating the excess base present in the final solution from step 8 using 0.10 M HCI. The amount of excess base will be determined and can be subtracted from the total base added in step 7. This indicates the amount of base that was actually combined with the metal ion.

Calculations

1. Graph the pH and volume of titrant data for each titration as part of your report using either the calculator or a piece of graph paper. Place the dependent variable, pH, on the y-axis and the independent variable, volume of titrant, on the x-axis.

2. Write the balanced chemical equation for the hydrolysis of Zn2+. Repeat this for any ion you may analyze.

3. Write the equation for the neutralization reaction between the metal solution and the base. Write the neutralization equation for the back titration.

4. Use the graphs to determine the endpoint volume of NaOH and endpoint volume of HCI for each back titration.

5. Calculate the [Zn2+] in the standard and supplement tablet solutions.

6. Determine the mass (mg) of zinc in the supplement tablet.

7. Get the theoretical mass of zinc from your teacher and calculate your % deviation (error).

Extra

1. After each titration, rinse the beaker with tap water and then two small amounts of Dl water. After the mineral supplement tablet has dissolved as much as it will, allow at least 15 minutes, stir the solution with a glass rod and place 100 mL of that sample solution into the beaker; do not add any additional water.

2. Titrate using approximately 1.0 mL amounts of 0.10 M NaOH until you are well past the endpoint. Record all data. Rinse the beaker and pH electrode and do a second titration with the other 100 mL sample of dissolved tablet. When you are completely finished, rinse the electrode and place it into the protective bottle.

More Findings

1. The phosphate ion will hydrolyze producing a solution with a high pH. You may titrate a Na3PO4 solution using 0.10 M HCI. Can you predict the shape of the titration curve before you start adding the acid to the sodium phosphate solution? Can you calculate the concentration of the phosphate ion in the starting solution?

2. What would the shape of the titration curve be different if you titrated a solution containing NaH2PO4 or Na2HPO4? Can you determine the concentration of these solutions using a titration with HCI?

3. Some fertilizers contain phosphate for plant utilization and some detergents contain phosphates as water softeners. Would this titration technique be suitable for determining the amount of phosphate in the fertilizer or the detergent? The only way to answer either of those questions is to evaluate a non poison containing fertilizer sample or a fertilizer or detergent sample. Ask your teacher for a sample to investigate.

4. The carbonate ion can be tested for using a solution of Na2CO3 and/or NaHCO3.

5. You could also use the pH electrode titration method to analyze water that has been in contact with iron nails lead shot, or pieces of zinc (try galvanized nails) and determine the concentration of each metal ion in those solutions. Those solutions are available from your teacher or you may prepare your own if you have samples of them. To do these titration's a dilute solution of NaOH is required; 0.0010 to 0.0020 M.

6. Determine the rate of loss of a metal into a solution. Should a blank solution of Dl water be analyzed?