Acid-Base Titrations

Learning Objectives (OCR Spec 2.1.4)

By the end of this lesson you should be able to:

• Prepare a standard solution of known concentration
• Carry out an acid-base titration using a pipette, burette and suitable indicator
• Calculate concentrations, volumes and reacting masses from titration data
• Calculate percentage uncertainties in apparatus
• Evaluate experimental procedures and identify sources of error

What Is a Titration?

A titration is a quantitative technique used to determine the concentration (or purity) of one solution by reacting it with a solution of accurately known concentration (a standard solution, or titrant). For acid-base titrations we monitor the point at which the acid and base have exactly reacted - the end-point, which should match the theoretical equivalence point.

Preparing a Standard Solution

A standard solution is made from a primary standard - a pure, stable, non-hygroscopic solid of known formula. Examples include anhydrous sodium carbonate, Na2CO3, and potassium hydrogen phthalate (KHP).

Procedure

1. Calculate the required mass: mass = concentration x volume x Mr
2. Weigh the solid in a clean, dry weighing bottle on an analytical balance (+/- 0.001 g)
3. Transfer to a 250 cm3 beaker and dissolve in ~100 cm3 of distilled water with stirring
4. Rinse the weighing bottle with distilled water and add washings to the beaker ("quantitative transfer")
5. Transfer the solution to a 250 cm3 volumetric flask via a funnel, rinsing the beaker and funnel
6. Make up to the mark with distilled water (meniscus level with the graduation line)
7. Stopper and invert several times to mix thoroughly

Performing a Titration

Apparatus

Apparatus	Purpose	Uncertainty (typical)
25.00 cm3 volumetric pipette	Deliver fixed volume of one reagent	+/- 0.06 cm3
250.0 cm3 volumetric flask	Make the standard solution	+/- 0.30 cm3
50.00 cm3 burette	Deliver variable volume of titrant	+/- 0.05 cm3 per reading (so +/- 0.10 cm3 per titre)
Analytical balance	Weigh solid	+/- 0.001 g
Conical flask	Hold the analyte (colour change observed against white tile)	-

Procedure

1. Rinse pipette with the solution it will deliver; pipette 25.0 cm3 into a clean conical flask
2. Add 3-4 drops of suitable indicator (methyl orange for strong acid/weak base; phenolphthalein for weak acid/strong base)
3. Rinse the burette with the titrant; fill the burette and take an initial reading to 2 decimal places (0.00 or 0.05 cm3)
4. Run a quick "rough" titration to find the approximate end-point
5. Repeat, adding titrant dropwise near the end-point and swirling continuously until you obtain concordant titres (within 0.10 cm3 of each other)
6. Discard the rough titre; average the concordant titres to obtain the mean titre

Why Rinse?

• Rinse pipette and burette with their own solutions (not water) so any residual water does not dilute them
• Rinse conical flask with distilled water only - adding more of one reagent would change the moles present

Calculations

Step 1: Moles of Known Solution

n = c x V (V in dm3) or n = (c x V)/1000 (V in cm3)

Step 2: Use Stoichiometry

Use the balanced equation to find moles of the other reactant.

Step 3: Calculate the Required Quantity

For unknown concentration: c = n/V For percentage purity: %purity = (theoretical mass / actual mass) x 100

Worked Example 1: Finding an Unknown Concentration

Q: 25.0 cm3 of a sodium hydroxide solution of unknown concentration is titrated against 0.100 mol dm-3 sulfuric acid. The mean titre is 22.50 cm3. Calculate the concentration of the NaOH solution.

Equation: 2NaOH + H2SO4 -> Na2SO4 + 2H2O

Step 1: Moles of H2SO4 = (0.100 x 22.50) / 1000 = 2.25 x 10^-3 mol Step 2: Moles of NaOH = 2 x 2.25 x 10^-3 = 4.50 x 10^-3 mol Step 3: [NaOH] = (4.50 x 10^-3) / (25.0/1000) = 0.180 mol dm-3

Worked Example 2: Percentage Purity

Q: 1.25 g of impure sodium carbonate is dissolved in water and made up to 250.0 cm3 in a volumetric flask. 25.0 cm3 of this solution is titrated against 0.100 mol dm-3 HCl, requiring a mean titre of 22.80 cm3. Calculate the percentage purity of the sample.

Equation: Na2CO3 + 2HCl -> 2NaCl + H2O + CO2