Titration Procedure

A Titration Curve


Click to see video  Click to view video  Click to view RealViewer (low resolution) video   Click to read script

To measure the change in pH during a titration., a pH electrode can be connected to measure the concentration of hydrogen ions.

Procedure:

1.  Put 25.00 mL of 0.1000 M HCl into a beaker that contains a magnetic stir bar.   Add 2 or 3 drops of phenolphthalein indicator.
2.  Set up a buret that contains 0.1000 M NaOH.  Set up a pH electrode and pH meter to read the pH of the solution in the beaker.
3.  Allow the base to run slowly into the acid.  Record the pH for each mL of added base, until the endpoint is approached.  Near the endpoint record the pH for each drop of added base.  Past the endpoint, again record the pH for each mL, until a total of 50.00 mL of base has been added.

Click to see time lapse video  Click to view video  Click to view RealViewer (low resolution) video

Results:

As the endpoint of the reaction is approached, the pink color of the indicator begins to persist for longer time periods.  The change in pH as the volume of base changes is called a titration curve.  In this time lapse video of a titration, notice how slowly the pH changes, except as the reaction passes through the endpoint.  Also notice how quickly the phenolphthalein changes color at a pH of just over 8.

You can also calculate the theoretical titration curve for an acid-base titration (the concentration of the acid decreases with each addition of base).  This spreadsheet Click to load spreadsheet template can be used to calculate the theoretical cuves for different kinds of titrations.  To use it you will need to know the Ka values for the acid.  The Ka is the equilibrium constant for the dissociation of the acid.  For a theoretical acid HB, the dissociation reaction is:

HB equilsymbol.gif (859 bytes) H+ (aq) + B- (aq) Ka = [H+][B-]
           [HB]

Here are some Ka values for different acids.  The acids are listed in order of strength from the strongest at the top, to the weakest near the bottom.   Very strong acids (above the dividing line) have an unmeasureable Ka in water since they completeley dissociate into ions.  To calculate the titration curve for these acids, enter a Ka value of 10 (or more) in the spreadsheet.  Polyprotic acids (such as phosphoric acid) have more than one endpoint.   To calculate the titration curve for these acids, enter each Ka for the step-wise dissociation of the acid into the spreadsheet.

Acid (All ions are aqueous) Ka
 
Perchloric HClO4 H+ + ClO4- Very large acidstrength1.gif (1093 bytes)
Hydrochloric HCl H+ + Cl- Very large
Sulfuric H2SO4 H+ + HSO4- Very large
Nitric HNO3 H+ + NO3- Very large
Hydronium ion H3O+ H+ + H2O 1.0
Sulfurous acid  1 H2SO3 H+ + HSO3- 1.3 x 10-2
Hydrogen sulfate ion HSO4- H+ + SO42- 1.3 x 10-2
Phosphoric H3PO4 H+ + H2PO4- 7.5 x 10-3
Hydrofluoric HF H+ + F- 7.1 x 10-4
Nitrous HNO2 H+ + NO2- 4.5 x 10-4
Acetylsalicylic (aspirin) C9H7O4H H+ + C9H7O4- 3.0 x 10-4
Methanoic (formic) HCOOH H+ + HCOO- 1.7 x 10-4
Ascorbic (vitamin C) C6H7O6H H+ + C6H7O6- 3.0 x 10-4
Benzoic C6H5COOH H+ + C6H5COO- 6.5 x 10-5 acidstrength2.gif (1080 bytes)
Ethanoic (acetic) CH3COOH H+ + CH3COO- 1.8 x 10-5
Carbonic H2CO3 H+ + HCO3- 4.3 x 10-7
Hydrogen sulfide (hydrosulfuric) H2S H+ + HS- 9.1 x 10-8
Hydrogen sulfite ion HSO3- H+ + SO32- 6.3 x 10-8
Dihydrogen phosphate ion H2PO4- H+ + HPO42- 6.2 x 10-8
Boric H3BO3 H+ + H2BO3- 5.8 x 10-10
Ammonium ion NH4+ H+ + NH3- 5.6 x 10-10
Hydrogen carbonate ion HCO3- H+ + CO32- 4.8 x 10-11
Hydrogen phosphate ion HPO42- H+ + PO43- 4.8 x 10-13
Dihydrogen borate ion H2BO3- H+ + HBO32- 1.8 x 10-13
Hydrogen borate ion HBO32- H+ + BO33- 1.6 x 10-14
Hydrogen sulfide ion 2 HS- H+ + S2- 1    x 10-19

1 Sulfurous acid, H2SO3 has never been isolated as a chemical species.  The reaction here is really for SO2 + H2O H3O+ + HSO3-

2 Hydrogen sulfide ion's ionization is very low and hard to measure.  This value is an estimate only.