This reaction is a typical example of a double displacement reaction whereby an aqueous solution of sodium sulfide reacts with an aqueous solution of copper (II) chloride to give copper (II) sulfide (black shiny crystals) and aqueous sodium chloride (a colourless solution). Sometimes, instead of a colourless filtrate, a yellow solution may be obtained, which occurs because of the decomposition of solid sodium sulfide to hydrogen sulfide.
Sodium sulfide is a yellow, yellow–pink or white solid flake with a sulfurous (rotten egg) smell which contains at least 30% water. Crystalline sodium sulfide is hydroscopic and absorbs moisture from air to form hydrogen sulfide. Discolouration of the solid occurs with time. When sodium sulfide is dissolved in water, hydrolysis occurs and the sulfide solution develops the characteristic rotten-egg odour. Dissolving an ageing sodium sulfide solid in water will result in more hydrolysis of the anion compared to a fresh sample. Instead of colourless, a yellow aqueous solution of sodium sulfide is observed. This solution has a strong sulfurous smell and the yellow colour fades with time.
When this sodium sulfide solution is made to react with copper (II) chloride solution, a black precipitate (copper (II) sulfide) in a yellow solution is formed. Filtration results in a yellow filtrate, which contains aqueous sodium chloride and sulfide/polysulfide ions in equilibrium.
Science ASSIST strongly recommends precipitating out the sulfide ions from the filtrate before performing evaporation. These ions can be removed through reaction with aqueous silver nitrate solution:
To the filtrate, add 0.1 M silver nitrate solution drop wise, until no further black precipitate is formed, filter the mixture and evaporate the filtrate to get the sodium chloride crystals.
Warning: If the filtrate containing sulfide ions is heated, the sulfide ions will be oxidized to sulfur dioxide, a very toxic and corrosive gas.
Science ASSIST also recommends:
- using a fume hood and appropriate PPE while preparing the copper (II) chloride and sodium sulfide solution,
- storing sodium sulfide in a tightly closed secondary containment container in the fridge,
- ensuring proper disposal of all chemicals involved in this experiment, and
- using the Risk Assessment Template for schools developed by Science ASSIST ( see Risk Assessment Template).
Disposal of waste chemicals
- The sulfides of copper and silver should be kept in the heavy metal wastes (solid) disposal bottle, which then should be disposed of by a licensed waste disposal contractor.
- Old degradable samples of solid sodium sulfide should also be disposed of by a licensed waste disposal contractor.
- Waste sodium sulfide solution should be treated with dilute ferric chloride solution to precipitate the sulfide ions as ferric sulfide, and then neutralized with sodium carbonate solution.
- The solid ferric sulfide is collected by either Buchner or gravity filtration, and when dry, placed in the heavy metal waste disposal bottle. The filtrate can be disposed of down the sink.
Due to the toxic and corrosive nature of sodium sulfide, other safer chemicals can be used to illustrate “Making Ionic compounds” such as:
- Reaction between aqueous solutions of copper (II) chloride and sodium phosphate.
- Reaction between aqueous solution of copper (II) chloride and sodium hydroxide.
- Reaction between aqueous copper (II) chloride and potassium carbonate.
Agarwal, Jyotsna 2012. ‘Effect of absorbability of iron contents by precipitated barium sulphate in the commercial crude sodium sulphide’ International Journal of Science and Technology, Volume 2 No.10, October 2012, https://web.archive.org/web/20170810141246/http://ejournalofsciences.org... (Original source no longer exist, replaced with Internet Archive version July 2018).
‘Inorganic chemistry/Qualitative analysis/test for anions’ May 2015, Wikibooks website https://en.wikibooks.org/wiki/Inorganic_Chemistry/Qualitative_Analysis/Tests_for_anions
‘Safety Data Sheet: sodium sulphide’, 28 May 2013 Chem-Supply website https://www.chemsupply.com.au/documents/ST0191CH6R.pdf
‘Standard Operating Procedures For Handling, Storage and Disposal of Sodium Sulfide’ October 2013, Drexel University Department of Safety and Health website https://www.materials.drexel.edu/media/129527/sop_0026_-_sodium_sulfide.pdf