C12.2 Acid-base titration

What are titrations used for?

• A method to calculate the concentration of an acid or an alkali by determining how much acid is needed to neutralise an alkali (or vice versa)

• Titrations can also be used to prepare soluble salts from the reaction between dilute acid and an alkali solution

List the apparatus needed for a titration

• 25 cm3 volumetric pipette

• Pipette filler

• 50 cm3 burette

• 250 cm3 conical flask

• Small funnel

• 0.1 mol / dm3 sodium hydroxide solution

• Sulfuric acid of unknown concentration

• A suitable indicator

• Clamp stand, clamp & white tile

List the method for a titration

1. Use the pipette and pipette filler and place exactly 25 cm3 sodium hydroxide solution into the conical flask

2. Using the funnel, fill the burette with hydrochloric acid placing an empty beaker underneath the tap. Run a small portion of acid through the burette to remove any air bubbles

3. Record the starting point on the burette to the nearest 0.05 cm3

4. Place the conical flask on a white tile so the tip of the burette is inside the flask

5. Add a few drops of a suitable indicator to the solution in the conical flask

6. Perform a rough titration by taking the burette reading and running in the solution in 1 – 3 cm3 portions, while swirling the flask vigorously

7. Quickly close the tap when the end-point is reached

   • The endpoint is when one drop causes a sharp colour change

8. Record the volume of hydrochloric acid added, in a suitable results table as shown below

   • Make sure your eye is level with the meniscus

9. Repeat the titration with a fresh batch of sodium hydroxide

10. As the rough end-point volume is approached, add the solution from the burette one drop at a time until the indicator just changes colour

11. Record the volume to the nearest 0.05 cm3

12. Repeat until you achieve two concordant results (two results that are within 0.1 cm3

What are indicators used for?

• Indicators are used to show the endpoint in a titration

List common acid-base indicators (neutral colour, colour in acid, colour in base)

What indicator to use with a strong acid and a weak base?

• Methyl orange (pH range 3.1-4.4)

What indicator to use with a strong acid and a strong base?

• Bromythol blue (pH range 6-7.6) or phenolphthalein (pH range 8.2-10)

What indicator to use with a weak acid and a strong base?

• Phenolphthalein (pH range 8.2-10)

What is a titrant?

• A substance of known composition and concentration that is used to react with an analyte (in burette)

What is an analyte?

• A substance that is being investigated (in the flask)

What is the equivalence point?

• The point in a titration when there are equivalent amounts of titrant and analyte so the two substances can react completely with nothing left over

How to calculate pH of a strong acid given molarity?

-log[H+]

How to calculate pH of a strong base given molarity?

14 + -log[H+]

How to calculate moles given molarity and volume?

• mL to L → divide by 1000

• L to mL → multiply by 1000

How to calculate moles given mass and relative mass?

• Relative mass: find element on periodic table, add their atomic masses

  • If there is a subscript, multiply

  • If there is a coefficient, it is not affected