Determination of the concentration of chloride ions in seawater.

The concentration of chloride ions – for example in seawater – can be found by titration with standard silver nitrate solution. The silver ions form a precipitate of silver chloride:

Ag⁺ (aq) + Cl^- (aq) à AgCl (s)

It is not possible to see when the precipitation finishes, so a precipitation indicator is necessary. A solution of potassium chromate(VI) is used which, if the concentrations are quite carefully controlled, begins to precipitate red silver chromate(VI) when all the chloride ion has been precipitated:

2Ag⁺ (aq) + CrO₄^2- (aq) à Ag₂CrO₄ (s)

Seawater is a convenient material to use; if the real material is unavailable a solution of sodium chloride containing about 35 g dm^-3 of sodium chloride will suffice, or perhaps the synthetic seawater obtainable from aquarium shops.

1 Pipette 25.0 cm³ of seawater (or equivalent) into a 250 cm³ graduated flask, make to the mark with pure water and mix well.

2 Pipette 10.0 cm³ of the diluted seawater into a 250 cm³ conical flask, and add 10 drops of 2 mol dm^-3 potassium chromate(VI) solution.

3 Titrate the mixture with standard 0.05 mol dm^-3 silver nitrate solution until the precipitate just develops a reddish tint.

4 Repeat to obtain three consistent titres.

Results:

Mean titre/cm³:

Calculation:

Find the concentration of chloride ions in the original seawater in mol dm^-3.

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