Practical - Carrying out an accurate acid-alkali titration

There are a number of ways that you could carry out an accurate acid-alkali titration. Different acids and alkalis could be used in this practical. This outlines one way to carry out the practical using sodium hydroxide solution and hydrochloric acid.

It is important in this practical to use appropriate apparatus to make and record volumes accurately. This includes the safe use and careful handling of substances, such as when mixing.

Aims

To investigate the relative concentrations of a solution of an alkali (eg sodium hydroxide) and a solution of an acid (eg hydrochloric acid).

Method

  1. Use a small funnel to fill a burette with dilute hydrochloric acid. Make sure the tap at the bottom of the burette is closed before filling.
  2. Open the tap to let some acid into the part of the burette below the tap.
  3. Record the starting volume of acid in the burette. You should read the bottom of the meniscus.
  4. Use a pipette and pipette filler to add some sodium hydroxide to a conical flask.
  5. Add two or three drops of indicator to the conical flask.
  6. Open the burette tap and add the acid to the conical flask. Swirl the conical flask. Continue adding the acid until the indicator just changes colour. Record the final volume of acid in the burette.
  7. Repeat steps 4 to 6 with a clean conical flask. In step 6, when the indicator is close to changing colour, add the acid a drop at a time to obtain a more accurate volume.
A burette containing dilute hydrochloric acid in a a beaker containing sodium hydroxide and hydrochloric acid, placed on top of white paper.

Results

Record the burette volumes in a suitable table. For example:

RunEnd volStart volAcid used vol
Rough26.00 cm31.00 cm3
125.60 cm30.00 cm3
225.80 cm30.00 cm3

Analysis

  • calculate the volume of acid used in each titration
  • calculate the mean (average) volume of acid needed to neutralise 25 cm3 of sodium hydroxide solution
Question

What is the mean volume of hydrochloric acid which reacts with 25 cm3 of sodium hydroxide in this practical?

The volume of hydrochloric acid used is (25.60 + 25.80) ÷ 2 = 25.70 cm3.

Evaluation

Question

What do your results tell you about the concentration of the acid?

The volume of acid used was slightly larger than the volume of the alkali. Therefore the acid must be slightly less concentrated than the alkali.

Question

How could this experiment be adapted to make a pure sample of sodium chloride?

25 cm3 of sodium hydroxide solution would be reacted with 25.70 cm3 of hydrochloric acid with no indicator. This would make a solution of pure sodium chloride, which would be evaporated to leave the solid salt.

Hazards, risks and precautions

It is important in this practical activity to use appropriate apparatus and methods. This includes the safe use and careful handling of substances.

Evaluate the hazards and the precautions needed to reduce the risk of harm. For example:

HazardPossible harmPrecaution
Hydrochloric acid and sodium hydroxide solutionBoth solutions are corrosive and damage skin and clothesUse dilute solutions, wear eye protection, wash hands after contact with solutions
Burette and pipetteMay break when used, producing broken glass that could cutTake care, especially when clamping the burette
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