Determining The PKa Value Of Acetic Acid - LD Didactic

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Determining the pKa value of acetic acid

can also be carried out with Mobile-CASSY 2, Pocket-CASSY, Mobile-CASSY and Micro-CASSY

Load settings

Load example

Danger

Acids and bases are caustic. Wear protective goggles and acid-resistant gloves.

Experiment description

A measure for the strength of moderately strong or weak acids called the pKa value is determined from its titration curve with a strong base. Starting from the dissociation equation for the acid,

HA + H2O    H3O+ + A−

we obtain for the point of half equivalence (where half of the acid has reacted with the base):

[A−] = [HA].

From the equilibrium equation

it therefore follows that

pKa = pH.

Thus the pKa value can easily be read after the equivalence point and the point of half equivalence of the titration have been determined.

Equipment list

1	Sensor-CASSY	524 010 or 524 013
1	CASSY Lab 2	524 220
1	Chemistry box or pH adapter S	524 067(2)
1	pH electrode with BNC plug	667 4172
1	Magnetic stirrer	666 8451
1	Beaker, 250 ml, squat shape	664 103
1	Stand rod, 450 mm, M10 thread	666 523
1	Graduated pipette, 10 ml	665 997
1	Pipetting ball	666 003
1	Burette, 25 ml	665 845
1	Burette filling funnel	665 816
1	Burette clamp simple	666 559
1	Crossed bosshead	666 543
1	Universal clamp, 0...25 mm	666 551
1	PC with Windows 10/11

Chemicals

Acetic acid, c = 0.1 mol/l	e.g. 500 ml: 671 9560
Caustic soda solution, c = 0.1 mol/l	e.g. 500 ml: 673 8410
Buffer solution pH 4.00	e.g. 250 ml: 674 4640
Buffer solution pH 7.00	e.g. 250 ml: 674 4670
Distilled water

Experiment setup (see drawing)

Plug the pH box with the single rod pH electrode into input A of the Sensor-CASSY, which is connected to the PC.

Set up the titration assembly using the stand material, the magnetic stirrer, the beaker and the burette. Put about 100 ml of distilled water into the beaker and add exactly 10 ml of 0.1 M acetic acid by means of the pipette. Fill 0.1 M caustic soda solution into the burette through the funnel up to the zero mark.

Attach the pH electrode so that on the one hand the measuring diaphragm is completely immersed and on the other hand the glass membrane cannot be damaged by the rotating stirrer bar. In order to mark the equivalence point, you may add some drops of a phenolphthalein solution as an indicator.

Calibration

In order to obtain accurate measuring results, you have to calibrate the pH electrode when you use it for the first time. Afterwards the calibration should be repeated from time to time:

Load settings

• Select Correct in Settings pHA1.
• Rinse the pH electrode with distilled water, immerse it in the buffer solution pH 7.00, and agitate it briefly.
• Enter 7.00 as the first target value. When the measured value is stable, press the Correct Offset button.
• Rinse the pH electrode with distilled water, immerse it in the buffer solution pH 4.00, and agitate it briefly.
• Enter 4.00 as the second target value. When the measured value is stable, press the Correct Factor button.
• Mark the Sensor-CASSY, the pH electrode, and the pH box in order that they can be used at the same input later on (otherwise the stored calibration will not be applicable).

Carrying out the experiment

Load settings

• If you want to apply other volume steps than 0.2 ml, the entry can be changed in Settings V in the field Formula, e.g. by entering "0.5*n-0.5" for steps of 0.5 ml.
• Determine the first measured value with .
• Continuously add drops of caustic soda solution at a slow rate, and record the pH value in steps of 0.2 ml with .

Evaluation

At the equivalence point (V = Veq), the acetic acid has entirely reacted with the caustic soda solution:

HAc + NaOH → Na+ + Ac− + H2O

In order to exactly determine the equivalence point with CASSY Lab, press the right mouse button in the diagram, select Other Evaluations → Find Equivalence Point. After marking the curve range, you will obtain a graphical display of the equivalence point and the pKa value as determined via the point of half equivalence (V = Vheq). The associated values are written below in the status line and can be inserted as text at an arbitrary place in the diagram.

Further experiment options

By neutralizing a hydrochloric acid solution of the same pH value as the present acetic acid solution (approx. 10 ml of 0.01 M HCl) and comparing the two titration curves, the concept of a buffer solution can be introduced. The strongly diluted hydrochloric acid is already neutralized after 1 ml of NaOH has been added, whereas for neutralizing the acetic acid of similar acidity approx. 10 ml of NaOH are required. Determining the pKa value of the hydrochloric acid would not be meaningful.

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