Making salts from acids and alkalis

A soluble salt can be prepared by reacting an acid with a soluble reactant. This is usually a dilute solution of an alkali such as sodium hydroxide or ammonia. The main steps are:

  1. Carry out a titration. This is to determine the volumes of acid and alkali that must be mixed to obtain a solution containing only salt and water.
  2. Mix the acid and alkali in the correct proportions, as determined in step 1, but this time without including an indicator.

Pure dry crystals can be produced by crystallisation, then filtration to remove excess solution, followed by drying on a watch glass or in a warm oven.

Carrying out a titration

Apparatus

  • a pipette to accurately measure the volume of a reactant before transferring it to a conical flask
  • a burette to add small, measured volumes of one reactant to the other reactant
  • a suitable indicator
Apparatus needed for titration: burette, pipette, conical flask and pipette filler.A pipette filler is needed to use a pipette safely

Method

This is an outline method for carrying out a titration in which hydrochloric acid is added to sodium hydroxide solution.

  1. Use the pipette and pipette filler to add a measured volume of sodium hydroxide solution to a clean conical flask.
  2. Add a few drops of indicator and put the conical flask on a white tile.
  3. Fill the burette with hydrochloric acid and note the starting volume.
  4. Slowly add the acid from the burette to the alkali in the conical flask, swirling to mix.
  5. Stop adding the acid when the end-point is reached (when the indicator first permanently changes colour). Note the final volume reading.
  6. Repeat steps 1 to 5 until you get concordant titres. More accurate results are obtained if acid is added drop by drop near to the end-point.

Results

Record the results in a suitable table. The one here also shows some sample readings.

RunEnd volStart volTitre
Rough26.85 cm31.00 cm325.85 cm3
124.60 cm30.00 cm324.60 cm3
224.90 cm30.60 cm324.30 cm3
325.50 cm30.80 cm324.70 cm3

Readings should be recorded to two decimal places, ending in 0 or 5 (where the liquid level is between two graduations on the burette). The titre is the volume added (the difference between the end and start readings).

Note that two titre values in the table are ticked. These titre values are concordant - they are within 0.20 cm3 of each other. Only concordant titre values should be used to find the mean titre.

The mean titre is found by taking an average, ignoring the rough run, and run 2 (because it is not concordant):

Mean titre = \frac{\textup{(24.60~+~24.70)}}{\textup{2}}

= 24.65 cm3

Titration must be used to obtain a solution of a salt and water only, when using an acid and an alkali. This is because there is no insoluble excess reactant that could be removed by filtration.