Arsenic-based pigments

Beautifully colored precipitates are formed from solutions

Scientific name: Three arsenic based pigments are formed in the reactions of arsenic compounds with different salts

YouTube

Arsenic-based pigments

by Hegelrast

Safety

Arsenic compounds are extremely toxic. Be very careful to not get it on the skin! Wear eye protection goggles and gloves.

Always follow general safety recommendations. Please note that conducting chemistry experiments you must comply with the relevant legal procedures in your country.

Reaction formula

2 Na3AsO4 + 3 Co(NO3)2 → Co3(AsO4)2 + 6 NaNO3

6 AsO2- + 4 Cu2+ + CH3COO- → 3 Cu(AsO2)2*Cu(CH3COO)2

As2O3 + Na2CO3 → 2 NaAsO2 + CO2

AsO2- + S2O32- + H+ → As2S3 (simplified)

Step-by-step instruction

WARNING! This experiment is dangerous! You should NOT perform this at home. ONLY carry out this experiment, if you are a trained chemist, and you understand local safety and legal requirements, which are required to perform such experiments

  1. The first synthesized substance in this experiment is cobalt (II) arsenate. Prepare about 50 ml of cobalt nitrite (pink) solution and sodium arsenate (colorless) solution.
  2. Mix them in a beaker. The formation of purple precipitate is observed.
  3. The second pigment is paris green – green colored cooper (II) acetarsenite. For this experiment we need arsenious oxide, sodium carbonate solution, copper sulfate solution and acetic acid solution.
  4. To obtain cooper (II) acetarsenite first we need to get sodium arsenite solution. For this purpose add some arsenious oxide to sodium carbonate solution (about 20 ml). A little heating may be required for better dissolving.
  5. Then take copper sulfate solution and mix it with acetic acid solution. Put it on a magnetic stirrer. 6.Slowly pour the sodium arsenite solution to the mixture of copper sulfate and acetic acid solution. On this stage you should avoid the formation of precipitate and mix the components very carefully. A bright blue clear solution will be obtained.
  6. Heat this solution to boiling for a few minutes.
  7. The mixture turns green and solidifies because of the cooper (II) acetarsenite formation.
  8. The last synthesized substance in this experiment is arsenise sulfide - orpiment. Prepare about 50 ml of mixed sodium arsenate and acetic acid solution and about 25 ml of thiosulfate solution.
  9. Put an empty beaker on a magnetic stirrer.
  10. Pour both solutions in the beaker. Wait for a little.
  11. A yellow precipitate is formed.
  12. To isolate synthesized pigments the suspensions should be filtered and precipitates should be left to dry.

Scientific background

Most arsenic compounds are very poisonous. Nevertheless some years ago arsenic-based pigments were very popular as paints. This experiment shows how to obtain some of them: cobalt (II) arsenate (or erythrite), cooper (II) acetarsenite (or paris green) and arsenise sulfide (or orpiment).

First of them - cobalt (II) arsenate is a pink-purple pigment. This substance is formed in the double displacement reaction between arsenate and cobalt(II) ions:

2 Na3AsO4 + 3 Co(NO3)2 → Co3(AsO4)2 + 6 NaNO3

The second has a rich green color and is produced in a cocrystallization process between copper(II) acetate and copper(II) arsenate:

6 AsO2- + 4 Cu2+ + CH3COO- → 3 Cu(AsO2)2*Cu(CH3COO)2

The source of AsO2- ions is sodium arsenite which is formed when arsenious oxide is added to sodium carbonate solution:

As2O3 + Na2CO3 → 2 NaAsO2 + CO2

The last synthesized substance in this experiment is arsenise sulfide – orpiment. Orpiment is a golden-yellow pigment. It is formed in a clock reaction between acidified arsenate solution and a thiosulfate solution. The mechanism of this reaction is very complex and involves a number of intermediate products. The overall process could be simplified as follows:

AsO2- + S2O32- + H+ → As2S3 (simplified)

Danger:
Coolness:
Difficulty:

Published on 09 March 2015

  • Fire
  • Heating with fire
  • Explosion
  • Poisoned gas
  • Organic
  • Electricity
  • Solution
  • Oxidation reduction
  • Color change
  • Precipitate
  • Gassing
  • Catalyst