Automatic Acid-Base Titration

Equipment

Computer with Logger-Pro from Vernier Software, Serial Box Interface (SBI), combination pH electrode, pH amplifier, magnetic stir bar, stir plate, (2) 250 mL beakers, 10 mL graduated cylinder, constant drip rate titrant dispenser.

Reagents

Deionized water, 0.1 M acetic acid, 0.25 M sodium hydroxide, calibration buffers (optional, but recommended, phenolphthalein indicator solution (this is optional, but it emphasizes the endpoint in conjunction with the titration curve). Any combination of acid and base may be used, depending upon what type of titration curve you wish to produce.

Presentation

1. Connect the SBI to the serial port of the computer, plug in the power supply , connect the pH amplifier to the SBI (port 1 is the default location, but port 2 may be used if you change the setting in Logger-Pro), connect the pH electrode to the amplifier.
2. Load an experimental setup that already has the experimental parameters set or load the default version and set the parameters as needed (pH axis limits 0-14, time axis limits 0-8 minutes, sample rate=70 samples per minute, 5 point averaging).
3. Calibrate the pH electrode according to the Logger-Pro instructions if it's deemed necessary.
4. Add 10 mL of the acetic acid solution to the beaker, add 50 mL of deionized water and the stir bar to the beaker.
5. Place the second beaker under the titrator tip. Adjust the drop rate until it is approximately 1 drop every 2.5 seconds.
6. Place the beaker containing the acetic acid on the stir plate.
7. Adjust the stirring rate so that a very small vortex is created.
8. Place the rinsed pH electrode into the beaker. Allow it to rest on the bottom of the beaker. Make certain that the stir bar does not touch the electrode.
9. Click on the collect button and start the titrator to dripping.

Hazards

Solid sodium hydroxide and concentrated solutions can cause severe burns to eyes, skin, and mucous membranes. Concentrated acetic acid is a corrosive poisonous liquid. Both the liquid and the vapor are flammable. Inhalation of the fumes will cause irritation of the respiratory tract.

Discussion

This demonstration illustrates the shapes of titration curves without the tediousness of making titrant additions, collecting the resulting pH data and plotting it. After the initial setup, all of these steps are automatic. There are a few details of this demonstration that should be clarified. A typical titration plots the pH versus the amount of titrant added. This titration plots the pH versus the time. The assumption being made is that the time is proportional to the volume of added titrant. This assumption is true if the rate of titrant addition is constant throughout the titration. If a simple buret had been used, this would be a poor assumption. The drip rate is proportional to the head pressure which is proportional to the difference in heights between the top of the titrant and where it emerges from the dispensing tip. This height can vary in a buret by as much as 50 cm throughout the titration. This difficulty is greatly minimized by the use of a container with a much larger diameter. This makes the change in height for a given dispensed volume much smaller than for a buret. A 1 liter solvent bottle was used. It has an approximate diameter of 9 cm. This demonstation is designed to use approximately 10 ml of titrant. This corresponds to a height change of less than 0.2 cm. If the bottle is raised above the dispensing tip, so that there is a 20 cm difference in heights, this corresponds to less than a 1% change in relative height throughout the titration. If you were interested in doing so, you could determine the drip rate and use the time to calculate the volume of titrant added.
The details of the constant drip rate titrant dispenser construction are as follows: 1)Obtain a container with a diameter of approximately 9 cm. The height of the container is not critical. 2)Obtain a 1 hole rubber stopper that will fit the mouth of the bottle snugly. 3)Cut and bend a piece of glass tubing so that it fits in the one hole stopper, reaches nearly to the bottom of the bottle, and the top of the tube protrudes from the top of the stopper and bends at least 90 degrees. 4)Connect a 30-50 cm length a Tygon tubing to the end of the glass tube. 5)Connect the other end of the Tygon tube to the removable teflon stopcock assembly of a buret. 6)Push a syringe needle through the stopper and fit a Luer stopcock onto the needle.
The stopcock and needle assembly allow air to flow into the bottle to replace the titrant as the titration proceeds, allows you to close the bottle for storage and allows you to fit a syringe onto the needle to increase the pressure inside the bottle in order to force the titrant up through the glass tube and down to the tip, thus starting the siphon. The drip rate can be adjusted by rotating the stockcock slightly. The bottle may be raised or lowered to help with the adjustment. Once the proper drip rate is achieved, use a tubing clamp to stop the flow rather than turning off the stopcock.
Once the data collection is completed, you can discuss the shape of the various regions of the titration curve and explain the chemistry behind them. The relatively flat region at the beginning of the titration changes pH very little since you are creating a buffer of acetic acid and its conjugate base, acetate ion.

C₂H₄O_{2 (aq)} + OH^-_(aq) H₂O_(l) + C₂H₃O₂^-_(aq)

As you approach the equivalence point, the pH begins to change more rapidly. The acid component of the buffer is being exhausted and finally at the equivalence point, the acid component is completely reacted.

References