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.

Experiment set up

See Notes for teachers and technicians

Safety tips

Avoid contact of all chemicals with eyes or skin.

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 1 l 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.

Questions and discussion of results

Figure 2 shows the titration curve of sodium carbonate with HCl. There are two abrupt pH changes in the curve. These correspond to the following successive reactions:

Na2CO3 + HCl -> NaHCO3 + NaCl    (conversion of carbonate into bicarbonate)

NaHCO3 + HCl -> CO2 + H2O + NaCl

The Calculated molarity of HCl in this experiment is 0.95 mol/l.

Figure 3 shows the titration curve of the reaction:

NaOH + HCl -> NaCl + H2O

This reaction involves strong acid (HCl) and strong base (NaOH). You can notice how the pH changes from a very high to very low pH value. In such reactions, the pH at the equivalence point is 7. Move the cursor on the screen and see that the steepest trace occurs at pH 7.

The calculated molarity of NaOH in this experiment is 1.01 mol/l.

Figure 4 shows the pH changes during the titration of baking powder with HCl. In contrast to the carbonate experiment, we can see here only one abrupt pH step, which corresponds to the conversion of bicarbonate into carbon dioxide. It is interesting to note that Figure 4 is similar to the second portion only of Figure 2.

Figure 5 shows the titration curve of vinegar against sodium hydroxide. Note that the pH of the solution increases during the titration due to the addition of NaOH.

The Reaction involved is:

CH3COOH + NaOH -> CH3COO-Na+ + H2O

The calculated concentration of acetic acid in vinegar in this experiment is 5.85 % (w/w)

Figure 6 is the opposite of Figure 3 where HCl is being titrated with NaOH. It is clear that the pH jump is larger in the case of titration of strong acids (e.g. HCl) than that in the titration of weak acids (e.g. acetic) with an alkali.

Questions

  1. Explain why the pH at equivalence point in Figures 3 and 6 is 7 whereas in Figure 5 is 8.86
  2. Calculate the pKa of acetic acid from Figure 5.
  3. Predict the titration curve if you titrate a mixture of 0.1 mol/l sodium carbonate and 0.1 mol/l sodium bicarbonate with HCl.
  4. What would happen if you did not calibrate the flow rate?

Further study

  1. Repeat part 3 with the concentrations of HCl and NaOH reduced 10 and 100 times. Study the effect of concentration on the pH change.
  2. Repeat part 6 with H3PO4 instead of HCl. (You should see a rising curve with two pH jumps corresponding to the first two hydrogen atoms of phosphoric acid)
  3. Plot the first derivative (dpH/dt) against time of all the previous experiments and locate the end point in each case. The end point in the first derivative curves are defined as the point at the maximum value. The first derivative curves can be obtained by graphing programs such as MicroCal Origin. (A sample is shown in Figure 7.)