0.5 Acid and bases to buffers  (Page 2/2)

To determine the amount of conjugate acid and base needed to make a buffer of a certain pH, the Henderson-Hasselbach must be employed.

$\text{pH}={\text{pK}}_a+\text{log}(\frac{[\text{base}]}{[\text{acid}]})$ (4)

With a given pH and known pK _a , the solution of the Henderson-Hasselbach equation gives the logarithm of a ratio which can be solved by performing the antilogarithm of pH/pK­ _a .

${\text{10}}^{\text{pH}-{\text{pK}}_a}=\frac{[\text{base}]}{[\text{acid}]}$ (5)

Experimental procedure

Materials required

• pH electrode and pH 7 buffer for calibration
• burette
• 250 mL beaker
• magnetic stirrer
• 0.4 M and 0.1 M NaOH
• 0.2 M phosphoric acid
• buffer solutions (pH 4 and pH 7)

Part i. demo during prelab lecture: drink anyone?

1. Six wine glasses are filled with the same “mystery” liquid.
2. Each glass takes on a different color of the rainbow, despite the fact that the same liquid was added to each.

Part ii. titration of phosphoric acid

1. Obtain a pH probe and connect it to pH/mV 1 on the Microlab interface. Open the MicroLab program and select “Microlab Experiment.” Choose “Add Sensor.” This will bring up a window where you need to select “pH /D.O.”, click on the appropriate port of the interface, and choose “pH” out of the two options below. To calibrate the pH probe, click “next.”
2. Take a sample of two buffer pH standards at pH 4.00 and 7.00. Calibrate the pH probe with these two solutions. This is done by selecting “Add Calibration Point” and entering the correct pH value noted on the bottle. Note that the pH probe should always be rinsed with deionized water and carefully patted dry before being inserted into a solution so as to avoid cross contamination. A large waste beaker is useful to have for rinsing. After the two points are entered, select linear calibration, and save the calibration data.
3. In a dry beaker, obtain 30 mL of a 0.2 M phosphoric acid solution. Use a graduated cylinder to add 50 mL of deionized water to a 250 mL beaker. Rinse your 10 mL volumetric pipette with the phosphoric acid solution and pipette 10 mL of the acid into the water. Rinse and fill your 25 mL burette with 0.4 M NaOH. The initial burette reading should be 0 mL. Remember to clear the air out of the tip of the burette.
4. Place the beaker on the magnetic stirrer and add a stir bar. Position the burette ready for titration. Insert the pH probe. Turn on the magnetic stirrer and adjust the stirring rate to moderate speed (without allowing the stir bar to splash or hit the probe).
5. On the Microlab main screen choose “Add Sensor” and select “Keyboard” under the sensor drop box. Click “Next.” This will bring up a prompt in which you should enter “KBD” in the top box and “mL” in the bottom “units” box, and then hit “Finish.”
6. Drag the keyboard sensor from the top left of the screen to “Data Source 1” on the x-axis of the graph. Drag the pH sensor to “Data Source 2” on the y-axis of the graph. Drag the pH sensor to the box in the bottom right corner.
7. Click “Start.” Enter your starting volume, 0 mL, in the window that appears, and hit enter. The window will not disappear. Slowly add a small volume of NaOH to the beaker, approximately 0.5 mL, enter the reading on your burette into the box, and hit enter. Repeat this process until both peaks have been observed and the pH has stabilized.
8. To save your data, choose “export data” under File, and select “comma separated value.” For help with plotting the data and derivative of the data see the “Data Analysis” section below.

Part iii. buffers (use the same microlab program)

1. Using equations (4 and 5), calculate the ratio of concentrations of ${\text{Na}}_2{\text{HPO}}_4$ and ${\text{NaH}}_2{\text{PO}}_4$ to produce 100 mL of buffer solution with pH = 6.91. Show your calculations to your TA before proceeding.
2. Prepare your buffer solution from 0.1 M ${\text{Na}}_2{\text{HPO}}_4$ and 0.1 M ${\text{NaH}}_2{\text{PO}}_4$ solutions.
3. Insert the pH probe in your buffer solution and wait until the reading becomes stable and write down the value in your report form. Don’t worry if the pH reading isn’t exactly 6.91. The important thing is that there isn’t a drastic change in pH upon addition of acid or base.
4. Pour 50 mL of the buffer solution into another beaker so that you have two beakers each with 50 mL of your buffer solution.
5. Add 1 mL of 0.1 M NaOH to the first beaker and mix the solution with a glass rod. Wait until the pH reading becomes stable and write down the value in your report form. If the pH of your buffer solution changes by more than 0.3 pH units, you will need to redo the calculations and re-prepare the buffer solution in order to get an acceptable result.
6. Add 1 mL of 0.1 M HCl to the second beaker and mix the solution with the glass rod. Insert the pH probe into the second beaker. Wait until the pH reading becomes stable and write down the value in your lab report form. If the pH of your buffer solution changes by more than 0.3 pH units, you will need to redo the calculations and re-prepare the buffer solution in order to get an acceptable result.

Data Analysis

Two plots need to be made from the data taken in Part II. One plot, pH vs. volume of NaOH, can be made directly from the data that is initially present. The data needs to be further analyzed to make the plot of the first derivative. This plot should be ( $\Delta$ pH / $\Delta$ vol NaOH) vs. volume of NaOH. Remember to include a title and axis labels.