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This experiment aims to generate the titration curves of some typical acid-base neutralization reactions. The presented simple setup produces titration curves, which are almost identical with those presented in textbooks of analytical chemistry.
A titration curve is a plot showing the changes of pH of the titrated solution versus the volume of the added standard solution (titrant). Acid-base titration curves can be constructed in several ways. One way is manual recording and plotting of pH values after each manual addition of an aliquot from the titrant solution. Another way is automatic recording and plotting of pH values continuously during automatic addition of the titrant. The last approach is the principle of operation of expensive automatic titration equipment. DrDAQ data logger connected to a PC with PicoLog data logging software allows the automatic recording and plotting of pH values. Continuous addition of the titrant solution can be realized by a peristaltic or syringe-type pump, which pumps the solution at a predetermined and fixed rate. A much cheaper alternative is to use an air pump (like that used in a home aquarium). The objectives of this experiment are manifold:
Glass combination pH electrode.
One beaker (125 ml).
Magnetic stirrer-magnet bar
Air pump (JUN ACO 9903) (can demonstrate the validity of the experiment) for higher accuracy and reliability a peristaltic or syringe pump is preferred.
1 L glass bottle with tight lid.
0.1 mol/L HCl.
0.1 mol/L NaOH.
0.1 mol/L Na2CO3.
Graduated cylinder, 25 ml.
5 ml graduated pipette.
25 ml pipette.
The system is connected as shown in Figure 1. The air pump propels the titrant solution with a fixed and known flow rate:
[Volume (V) of the titrant added after time (t) = flow rate (mL/sec) * time(t) (sec)]
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:
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 into and out from the glass bottle. It is recommended to use epoxy to seal the tubing in the lid of the bottle.
Use a high 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.
Note: Remember to wash the reaction beaker and pH electrode with distilled water before each titration.
Put a 25 ml graduated cylinder underneath the end of the tubing. Turn on the air pump, and collect a certain volume (e.g. 20 ml) of the titrant in the cylinder. Measure the required time (t). Calculate the flow rate (F) as follows:
F = V (ml) / t (s)
A flow rate of about 1–3 ml/min (0.0166–0.05 ml/sec) is appropriate. Do not change the settings once you have measured the flow rate.
M(HCl) = [(M × V)carbonate × 2] / [(t × F)]
This set up is almost the same as that provided with commercial automatic titrators, which have the integrated systems to:
Automatic titrators possess sophisticated mechanisms, which allow a variable flow rate for more precise end point location.
M(NaOH) = [M(HCl) × (t) × F] / V(NaOH)
Sodium Bicarbonate % (w/w)= [(M(HCl) × t × F × 84 × 2] × 100 / 5
% (w/w) = [(M(NaOH) × F × t × 60.05 × 10] / 1000