Objective : To determine the concentration of an unknown diprotic acid using acid-base titration.

  In this experiment, a standard solution of a diprotic acid is prepared,   and then titrated with a 25.0cm³ sample of 0.10moldm⁻³ sodium hydroxide solution. The diprotic acid reacts with sodium hydroxide as follows:

H₂A (aq) + 2NaOH(aq)→ Na₂A(aq) + 2H₂O(l)

          From the equation, the number of moles of diprotic acid is half the number of moles of NaOH, then from titration, the volume of the acid needed to neutralize the NaOH is used to determine the molarity of the diluted diprotic acid.

        Concentration of the               diluted acid  

        

Concentration of the original acid =

 concentration of the diluted acid

Materials        : 50.0cm³ of diprotic acid, 25.0cm³ 0.10moldm⁻³ NaOH, distilled  water,   5mL  phenolphthalein indicator

Apparatus        : 100cm³ beaker, measuring cylinder, pipette, conical flask, burette, filter funnel, 250cm³ volumetric flask

Fair testing        : A constant  volume and concentration of NaOH is used throughout the experiment.

Procedure        

Phase 1

1. 50.0cm³ diprotic acid is measured by using measuring cylinder.
2. Then the acid is transferred into a 250cm³ volumetric flask.
3. The volume of the solution is made up to the calibration mark of the volumetric flask with distilled water. The contents of the volumetric flask is swirled repeatedly.

Phase 2

1. 25cm³ of 0.10moldm⁻³ of sodium hydroxide is pipetted into a conical flask.
2. A few drops of phenolphthalein indicator is added.
3. The burette is rinsed in the acid solution and then the burette is filled with fresh solution.
4. The alkali sample is titrated against the acid solution.
5. The experiment is repeated until 2 concordant volumes of acid is obtained.
6. The titration results are recorded in a table.

Results

Average volume of diluted acid added

                                         

Discussion

1. Number of moles of NaOH = 0.0250.1 =2.510⁻³ mol

From the equation, 1mole of diprotic acid reacts with 2 moles of sodium hydroxide.

 Number of moles of diprotic acid =   2.510⁻³ = 1.2510⁻³ mol

2. n = cv

1.2510⁻³ = c ()

  c = 0.0880 molL⁻¹

3. Concentration of the original diprotic acid = 0.0880 

   

   = 0.44 molL⁻¹

4. The experimental value is 0.44M and it is slightly less than the stated value 0.50M. This is because the volume of acid taken from the original acid is less than 50ml during the experiment. Hence the result is affected at the end of the experiment.  Another reason is that the stated value 0.50M might not be the true concentration of the diprotic acid due to unavoidable error in the process of earlier dilution of the initial concentration of the acid.
5. During the process of titration, the acid added is a little bit in excess because the colour of phenolphthalein takes some time to undergo change. Swirling of the solution should be longer to observe the colour change, however the rate of acid drop in this experiment is continuous without  pausing to observe the colour change.

Precaution method :

1. The burette is clamped vertically to ensure accurate reading.
2. The conical flask is frequently swirled to ensure that all the acid added  from the burette can react with the base and to ensure complete mixing of the reactants.
3. More than one accurate titration is carried out, to minimize error by getting accurate readings.
4. The unknown diprotic acid is handled carefully.

Conclusion :

The concentration of the diprotic acid can be determined through the acid-base titration. The concentration obtained in this experiment is 0.44M instead of the stated value 0.50M.