Pico Technology

Target groups:

Primarily, College students (General Chemistry, Introduction to Analytical Chemistry and Instrumental Analysis Courses) Partially, High school students.

Required knowledge

1. Expression of solution concentration (e.g., Molarity and percentage concentration).
2. Acid-Base equilibria.
3. Basics of volumetric analysis.

Safety tips

Avoid contact of all chemicals with eyes or skin.

Equipment required

1. DrDAQ.
2. Glass combination pH electrode.
3. One beaker (125 ml).
4. Magnetic stirrer-magnet bar
5. Air pump (JUN ACO 9903) (can demonstrate the validity of the experiment) for higher accuracy and reliability a peristaltic or syringe pump is preferred.
6. Tygon Tubing.
7. 1 l glass bottle with tight lid.
8. 0.1 mol/l HCl.
9. 0.1 mol/l NaOH.
10. 0.1 mol/l Na₂CO₃.
11. Vinegar.
12. Graduated cylinder, 25 ml.
13. 5 ml graduated pipette.
14. 25 ml pipette.

Teachers notes

Experiment setup

The system is connected as shown in Figure (1). The air pump propels the titrant solution with a fixed and known flow rate:

In this way, the amount added of the titrant becomes a linear function of time, the variable which can be recorded with DrDAQ and PicoLog.

The flow rate is kept constant by fixing the following variables:

• The speed of the pump,
• The setting of the control tap,
• The height of the tube above the level of the air pump (this is not important with other types of pumps).

Make sure that the inlet air stream lies above the solution level in the glass bottle, do not let air bubble in to the titrant solution.

Make sure that there are no air leaks around the Tygon tubing coming in to and out from the glass bottle. It is recommended to use epoxy to seal the tubing in the lid of the bottle.

Use high a stirring rate and position the glass pH electrode as far as possible from the falling drops of the titrant to minimize local concentration of the titrant in the vicinity of the glass pH electrode.

Once all the parts are collected, the setup requires about half an hour.

Each part of the experiment requires about 10 minutes including washing the beaker with distilled water between runs.

Answers

1. Figure 3 and 6 show titrations involving a strong acid and a strong base. The solution at the equivalence point contains their salt (NaCl) which is neutral, i.e., pH 7. Whereas, Figure 5 shows the titration of a weak acid (acetic) against strong base (NaOH). At this equivalence point, the solution contains their salt (sodium acetate) which is a basic salt the pH of which is > 7 , 8.86 in this case.
2. pKa of acetic acid can be calculated by determining the pH at half neutralization. At this point pH =pKa (theoretical value = 4.74, the experimental value 4.6).
3. The second step will be as twice as the first step.
4. You can still get the titration curve but you cannot tell the volume of the titrant required to reach the equivalence point, and of course cannot calculate the unknown concentration.

Results

Figure 2: Titration curve of sodium carbonate against HCl

Figure 3: Titration curve of NaOH against HCl

Figure 4: Titration curve of baking powder against HCl

Figure 5: Titration curve of Vinegar against NaOH

Figure 6: Titration curve of HCl against NaOH

Figure 7: The first derivative of figure 2, for more accurate end point location.