Acetate Buffer Preparation: Lab Procedure and pH Measurement

Part A: Acetate Buffers

Obtain 35mL acetic acid, 35mL sodium acetate, and 15mL sodium hydroxide

Connect a pH probe to CH1 of the LabQuest interface. Open the LoggerPro software from the ‘LABS’ folder

Open the Data Collection menu, , and change the Mode to Events with Entry, and the Column Name (as well as the Short Name) to Ratio

Use a graduated pipet and a set of clean test tubes to prepare a series of buffers from your ‘stock’ solutions as shown in Table 6.1

Note: Before using the pipet to measure the next reagent, make sure to rinse the pipet very well with DH2O, and then with about 1mL of the reagent to be measured, in order to avoid cross contamination.

Rinse the pH electrode with D H2O collecting all rinsing in a large beaker. Wipe the outer body of the electrode with a paper towel. DO NOT touch or wipe the glass bulb; gently shake off excess rinse water from the bulb.

Autoscale the graph and select the linear fit button, . Sketch the shape of this graph (Graph 2) and record the equation of the line and correlation coefficient.

Click inside the expression box, and select log from the drop-down menu of the [Functions>] button. From the [Variables (Column)>] button select Ratio. Click 'DONE' to apply the changes

Left click the mouse on the x-axis title of the graph and select log Ratio to view the plot.

Click 'STOP' when finished. Autoscale the graph, , and sketch the shape of the curve in your report sheets (Graph 1).

Select Data/New Calculated Column and change the Column and Short Name to log Ratio.

Place the pH probe in the buffer and click 'collect'. When the reading stabilizes, click 'keep' . Type the corresponding ratio into the edit box, followed by 'OK' to store the first data pair.

Measure and record the pH of the remaining buffers the same way, rinsing the probe in-between samples.

Part B: Buffer Effectiveness

Use a graduated pipet to add 1.0 mL of 1.0 M NaOH to each buffer prepared in Part A

Calculate and report the new mole ratio of each component in Table 6.2

Click 'COLLECT' followed by [Store Latest Run].

Obtain the pH of each solution as described in Part A. Click 'STOP' when finished. Record all pH values in Table 6.2 before moving on.

Part C: Buffer Capacity

Use graduated pipets to prepare 10 mL of 1:1 acetic acid/sodium acetate buffer in a clean test tube

Open a new file, File/New and click on the Data Collection button . Change the Mode to Events with Entry, the Column and Short Name to NaOH and the Units to mL. Click 'DONE' to apply the changes.

Click 'COLLECT' and measure the pH of the buffer. When the reading stabilizes, click 'KEEP'.

Type-in the total volume of NaOH added (0 in this case), followed by to store the first data pair.

. Use a pipet to add 0.5 mL of 1.0 M NaOH to the buffer. Swirl gently to mix and measure the pH.

Click 'KEEP' and enter the total volume of NaOH added.

Continue adding base in 0.5 mL increments up to a total volume of 4.0 mL as shown in Table 6.3. Record the pH values.

Click 'STOP' when finished. Autoscale the graph. Examine the plot to find the volume of NaOH that corresponds to a pH change of 1 unit higher than the initial pH and record this on your data sheet. Sketch the graph on your data sheet (Graph 3).

Clean-Up:

Discard all solutions to the waste container provided. The pH electrode rinsing may be disposed down the sink.

Wash the test tubes and remove the masking tape. Replace test tubes upside-down in the rack to dry