Acid-base titrations

Introduction

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:

1. To construct acid-base titration curves in a very similar way to that offered by automatic titrators.
2. To learn some of the principles associated with acid-base titration curves by using DrDAQ as an educational tool.
3. To use the generated titration curves to determine the concentration of some analytes in common samples such: as acetic acid in vinegar, and sodium bicarbonate in baking powder.

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.

Figure 1: Experiment setup

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:

[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:

• 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 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.

Carrying out the experiment

Note: Remember to wash the reaction beaker and pH electrode with distilled water before each titration.

Part 1: setting and determination of the flow rate

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.

Part 2: determination of unknown HCl concentration (standardization of HCl)

• Fill the 1l glass bottle with the unknown HCl solution (˜1 mol/l).
• Pipette 5 ml of 1.0 mol/l Na₂CO₃ solution into a 125 ml glass beaker.
• Add about 50 ml of distilled water.
• Immerse the glass pH electrode in the solution.
• Turn on the magnetic stirrer.
• Set the PicoLog to monitor pH at a frequency of one sample every 2 seconds.
• Simultaneously, start recording with DrDAQ and start the flow of titrant (just turn on the air pump).
• Note that the initial pH is alkaline (sodium carbonate is a basic salt).
• Observe how the pH falls slowly through the entire interval before the end point and how the pH changes abruptly over a very limited time around the end point.
• Observe the advantage of the PicoLog autoscaling feature in this application.
• Note that the curve shows two pH drops at equal time intervals (for equal volume added).
• Measure the time (t) required for complete neutralization of the sodium carbonate (second end point).
• Calculate the molarity (M) of HCl solution from the following expression:

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:

• Deliver the titrant,
• Monitor the pH,
• Plot the curves and
• Detect the end point.

Automatic titrators possess sophisticated mechanisms, which allow a variable flow rate for more precise end point location.

Part 3: determination of the concentration of sodium hydroxide solution

• Pipette 5 ml of the unknown sodium hydroxide solution into a 125 ml glass beaker.
• Add about 50 ml of distilled water.
• Use the same HCl used in the previous part.
• Repeat as above.
• Note that the starting pH is very high (strong alkali).
• Only one large pH jump is observed.
• Locate the time (t) of the end point (the steepest point in the curve that corresponds to pH 7 in this case). (The PicoLog cursor will help you to define the end point.)
• Calculate the molarity (M) of NaOH solution from the following expression

M_(NaOH) = [M_(HCl) × (t) × F] / V_(NaOH)

Part 4: determination of the content of sodium bicarbonate in commercial baking powder

• Suspend a 5 g portion of baking powder in 100 ml of distilled water.
• Shake well and pipette 50 ml aliquot into a 125 ml glass beaker.
• Titrate as above using the same HCl solution.
• Note that the initial pH of the bicarbonate solution is substantially lower (~7.2) than that of the carbonate solution described in part 2.
• The % (w/w) of sodium bicarbonate is calculated from the following expression:

Sodium Bicarbonate % (w/w)= [(M_(HCl) × t × F × 84 × 2] × 100 / 5

Part 5: determination of acetic acid content in vinegar

• Fill the glass bottle with NaOH solution determined in part 3 to be used as titrant.
• Calibrate the flow rate (F).
• Pipette 10 ml of commercial vinegar into the 125 ml glass beaker.
• Dilute with about 50 ml of distilled water.
• Repeat as above.
• Observe that the initial pH is in the acidic region. This is due to the presence of acetic acid in the vinegar.
• Calculate the % concentration of acetic acid from the following experssion:

% (w/w) = [(M_(NaOH) × F × t × 60.05 × 10] / 1000

Part 6: comparison between the titration of acetic acid and HCl with NaOH:

• Pipette 10 ml aliquot of HCl solution used in parts 2–4 in a 125 ml beaker.
• Dilute with about 50 ml of distilled water.
• Repeat as in part 5.
• Observe that the initial pH is in the acidic region and that only one pH jump occurs.