Help Index
pH Titrations- A Laboratory Experiment Simulation.

Help Index

Menu Commands

Keyboard Equivalents

Calculations


Menu Commands

Titration

Options

Results


Titration

The Titration Menu Items are:

New : This allows you to begin a new titration.

Reactants : This brings up a dialogue box with a range of acid-base types and some common reagents and buffers. You can select to enter known concentrations or ask to use an unknown concentration and then test your skills by calculating it from the titration curve.

For example, if you select HCl with KOH, then you are prompted for the molarities of both acid and base. If you had asked to titrate an unknown concentration, in this case of the KOH, then you would be prompted only for the concentration of the HCl.

In these cases, you are allocated a sample number which should be recorded so that the tutor can determine whether you correctly calculated the unknown concentration.

Quit: This will end the laboratory exercise.


Options

The Options Menu Items are:

Show Grid. This option displays a grid over the titration curve.

Deliver More.

Deliver Less.

Together, these allow for a change in titre size. There are five different settings available and once you have reached the minimum volume no further decreases are possible. This is indicated by the greying out of the option. Note, if you continuously hold down the tap, the burette continues to drain and the final titre is calculated only after you release it, so for the most precise curve you need to work with small titres, that is small turns of the tap and with the deliver less option greyed out.


Results

The Results Menu Items are:

Copy Data as Text

Copy Graph as Picture

These options allow you to paste either the titration values or the graph to the WINDOWS clipboard for use in other applications.

For example you may wish to copy the data into EXCEL so that you can calculate a first derivative of the curve to more accurately determine the endpoint.


Keyboard
Keyboard Equivalents

Alt T and Alt S are alternative ways for turning on the tap and stirrer respectively.


Calculations

Here is an example of a titration curve for a difunctional acid (H2A) titrated against KOH.

From the lines from positions A,B,C and D, the concentration of the acid and its pK values can be determined.

Determination of Concentration

The reactions which occur during the titration are:

H2A	->	HA-	+	H+

HA-	->	A2-	+	H+

Point B, represents the position of the end-point of the reaction between H2A and HA-, that is, when one mole of protons has been removed from H2A by reaction with one mole of base and the point where HA- now fully exists.

If you used the default concentration of KOH (0.100 mol dm-3) then the number of moles of base is:

0.100 x B / 1000

= B x 10-4

If we let the concentration of the acid be represented by X and given that the volume of acid added to the beaker is 16 cm3, then the number of moles of acid is given by:

X x 16 / 1000

Since at point B the number of moles of base is equal to the number of moles of acid, these two expressions are equivalent and by reading off the volume at B from the curve it is possible to calculate X.

For example, if B=16 cm3, then X=0.100 mol dm-3.

Point D represents the position of the end-point of the reaction between HA- and A2-. This must be at twice the volume at B, since both protons have now been removed and each would require the same amount of base to neutralise them.

Determination of pK values

The calculations needed to determine the pK values can best be seen by writing the equilibrium expressions for the reactions above in terms of pK and pH.

For the first step:

Ka = [H+].[HA-]/ [H2A]

or	[H2A]/[HA-] = [H+]/Ka

hence	log10 [H2A]/[HA-] = pKa - pH

When the concentrations of H2A and HA- are equal, then the left hand side of this expression becomes zero (log 1=0.0) and at that point the pKa1 is equal to the pH.

From the titration curve, this is represented by point A where the volume is half that of B. At this point, half of the original H2A is converted to HA- and there is a 50:50 mixture of both species present in the solution.

Similarly the point C, which is 1.5 times the volume at point B and 3 times the volume at A, can be used to determine pKa2 since at this point there is a 50:50 mixture of the species HA- and A2- and so again the pK can be read off the pH axis.

In the example given, pK1 is =1.6 and pK2 =3.9.