Strong Acid Strong Base Titration

Full text

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Adapted from works of Addest Technovation Pte Ltd

Strong Acid – Strong Base Titration using Datalogger Safety Reminders

• Wear protective gear while handling chemicals.

• Follow directions for using the equipment

• Dispose of all chemicals and solutions properly.

Objectives

1. To develop the skills to set up an MGA Workstation with Drop Counter Platform, pH Sensor and other relevant apparatus to automate the process of obtaining the titration graph.

2. To investigate the changes in pH value when strong base is gradually added into strong acid.

3. To determine the concentration of the base from the titration graph obtained using an MGA Workstation with pH Sensor.

Apparatus

• 1 MGA Workstation

• 1 pH Sensor

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Adapted from works of Addest Technovation Pte Ltd • 1 plastic burette (with two taps and a nozzle)

• 1 250 ml beaker labeled ‘R’ 3/4 filled with tap water

• 2 100 ml beaker

• 1 10 ml measuring cylinder

• 1 25 ml pipette

• 1 pipette filler

• 1 magnetic stirrer

• 1 magnetic stirring bar

About Drop Counter Platform:

There is a light source and a light sensor residing within the base of the Drop Counter Platform.

When a droplet falls through, it will block the light and the sensor will detect a decrease in the transmitted light intensity. This will cause the datalogger to register one count.

Chemicals

0.50 mol/dm3 sodium hydroxide, NaOH(aq)

Hydrochloric acid, HCl(aq), of unknown concentration

Procedure

Setup and Preparation

1. Switch on the MGA by pushing the on / off button at its right bottom as shown. The screen will lit up as shown.

2. Connect the pH Sensor to CH 1 of the MGA.

Connect the Drop Counter Platform to CH 2 of the MGA. Ensure that the flipper of the sensor connector

faces the white line as shown.

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Adapted from works of Addest Technovation Pte Ltd Calibration

3. Clamp the Drop Counter Platform to the vertical bar of the magnetic stirrer and tighten the screw to secure the platform in place as shown.

4. Place an empty beaker on the magnetic stirrer under the Drop Counter Platform.

5. Insert the tip of the burette into the designated slot on the Drop Counter Platform. The burette should remain upright.

6. Ensure that the upper tap of the plastic burette is closed, and fill the burette with 50 cm3 of NaOH(aq).

Note: The burette comes with 2 taps as shown on the right.

The upper tap will be used to control the drop rate of the solution while the lower tap will be used as an “On/Off” valve.

7. Open the lower tap on the burette completely and

carefully adjust the upper tap until you observe a drop rate of approximately 2 to 3 drops per second for your experiment.

Note: Leave the upper tap of the burette in that position and do not change it throughout the experiment.

8.

Close the

lower tap

completely. Replace the beaker

under the Drop Counter Platform with a 10 ml

measuring cylinder as shown.

9. Now go back to the Drop Counter Setting on the MGA

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Adapted from works of Addest Technovation Pte Ltd

10. Tap the "START" icon and completely open the lower tap on the burette so as to let the acid solution drip. (After tapping the "START" icon, the word "START" will be changed to "STOP".)

Note: Ensure that the Drop Counter can detect the drops by observing the number beside ‘No. of drops’ in the ‘Drop Counter Calibration’ window – the number should be increasing.

11. When the liquid level in the measuring cylinder is between 9 ml and 10 ml, completely close the lower tap. Then tap "STOP" as shown below Step 9 (right figure).

12. Read the total volume of the NaOH(aq) solution collected in the measuring cylinder as accurately as possible and enter this value beside "Total Vol. (ml)" as shown below Step 9 (right figure)

13. Tap "UPDATE". The average volume per NaOH(aq) droplet will be calculated and displayed, and also be saved in the MGA for use later.

14. Tap "Done" to complete the calibration.

Experimental Setup

15. Pipette 25.0 cm3 of HCl(aq) into another 100 ml beaker. Add a magnetic stirring bar into the beaker.

16. Top up the NaOH(aq) in the burette to 50 ml.

17. Replace the measuring cylinder with the beaker containing HCl(aq) of unknown concentration on the magnetic stirrer.

18. Turn on the magnetic stirrer, and ensure that the stirring bar stirs smoothly.

19. Remove the pH Sensor from the storage bottle.

Using the pH Sensor:

Unscrew its cap in the direction as indicated by the black arrow until it is detached from the storage bottle.

Gently pull out the pH Sensor.

Once the pH Sensor is removed from the bottle, use the spare cap provided (this cap comes with a black rubber stopper) to cover the bottle to prevent spillage of the storage solution if the bottle topples.

Caution:

Take extra care of the pH Sensor because the tip can be broken.

The solution in the storage bottle is to condition the sensor during storage. (Do not pour it away.)

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Adapted from works of Addest Technovation Pte Ltd

20. Place the storage bottle aside for ease of carrying out the experiment.

21. Dip the tip of the pH Sensor into the water in the beaker

labeled ‘R’ and rinse it for about 10 seconds. Then gently wipe the tip of the pH Sensor with a sheet of tissue paper.

22. Insert the pH Sensor through the designated slot on the Drop Counter Platform.

23. Adjust the Drop Counter Platform to ensure that the tip of the pH Sensor is fully immersed in the solution, while the magnetic stirring bar is still free to spin.

Data Collection

Note: At this point, you may notice that the MGA screen has gone blank. The MGA enters the “sleep” mode once it is not in use for 2 minutes. If this happened, press the left or the right arrow buttons (as circled on the picture) to wake it up. Do not switch off the MGA and then on again – you will lose the calibration data.

24. Press the green button on the MGA to start data collection (the word “Capturing” will appear on top of the screen).

25. Open the lower tap of the burette completely to start dripping. The graphs displaying the pH versus time (should be a curve) and volume of sodium hydroxide versus time (should be linear) will start to form on the MGA screen as shown.

26. When the graph of pH versus time is fully formed, press the red button to stop data collection (the word “Capturing” will disappear).

27. Close the lower tap on the burette.

28. To obtain the titration graph of pH versus volume from the 2 graphs that appear on the MGA Screen, tap the icon. Then tap the icon to enlarge the titration curve.

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Adapted from works of Addest Technovation Pte Ltd

30. Record the x-axis value observed on the MGA screen. This is the volume of NaOH(aq) added at end-point.

31. Turn off the magnetic stirrer, take out the magnetic stirring bar from the beaker and pour away the solution. Clean up the apparatus.

Saving Data

32. Ensure that an SD card has been inserted in, and tap the icon.

33. Save the file under a suitable filename with not more than 8 characters.

34. The file can be saved under 2 formats:

(a) *.MGA format – MGA datalogger native format

(b) *.CSV format – for use with Microsoft Excel

35. Tap "Additional Information" to input “Automated Titration” as the title of the experiment. Also key other information such as your name, class and comments, which will show up in the printout of the graph. Then tap "Done" after input.

36. Tap "Save" to save the file into the SD card.

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Adapted from works of Addest Technovation Pte Ltd Sketch of Titration Graph

pH

0

Volume of NaOH(aq) added at end-point = ______ cm3

Calculations

Figure

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References

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