Arsenic sulfides: As₄S₄ and As₂S₃

On this web page some chemical properties of the two most common arsenic sulfides are demonstrated. In the past, both sulfides were used as pigments. In many Renaissance paintings, these pigments can be found for yellow and orange colors. Nowadays, these compounds hardly are used anymore, due to their great toxicity, and also due to their limited stability towards other chemicals and light. There are better alternatives now for yellow and orange colors.

In the bulk state, As₄S₄, called realgar, is a bright red crystalline compound, often covered in a thin layer of orange powder. Realgar crystals slowly fall apart into fine powder on exposure to bright light of wavelengths in the range of 500 nm to 670 nm. This powder is ochre/orange, and it is an allotrope of realgar, called pararealgar.

Realgar is known already for a very long time. Even in biblical times it was known already, named σανδαράκη (sandarákē, “realgar”). It was mixed with wax to make seals for envelopes of important documents and it was a highly valued mineral. It was also used as a poison.

The other sulfide, As₂S₃, is called orpiment. It also is known for a very long time already. In the Roman empire it was called auripigmentum. From this, the word orpiment is formed over the centuries. This compound was also used for making seals for important documents, and it also was used to make the tips of arrows toxic. In ancient times it was already known that orpiment is toxic.

The following picture shows crystals of both realgar and orpiment. The grid on the paper has a side of 5 mm.The crystals both have a natural origin. It is remarkable that a natural compound can be so pure.

In the experiments, described on this web page, crunched crystals of these minerals are used. Both crystalline solids are very brittle and quite soft. With a metal spoon and some pressure, it is easy to crack crystals like these. On AliExpress, I could obtain 100 grams of crunched orpiment crystals for less than $10, including shipping. Small crystals of chinese orpiment are quite cheap. Realgar is much more expensive and large crystals of orpiment, like the one shown in the picture also are more expensive. The cheap orpiment from China, however, has very good purity, especially if one considers the fact that it is a natural compound. Below, a sample of 100 grams of small crystals of orpiment is shown. This material is used in the experiments, described further below.

I also crushed a small piece of realgar with a metal spoon and after that, I used a plastic tool to make it finer. The color changes from red to orange, when the particles become finer. Lateron in the experiments, it becomes clear, however, that the crunching of the realgar was not perfect, still some larger particles remained, which were hard to dissolve.

The experiments on this webpage are interesting and show some colorful chemistry of the element arsenic. Experiments with arsenic are not common at all on the internet and most amateur chemists do not have any arsenic compounds. The experiments, however, must be carried out with great care. Avoid exposure to powdered realgar or orpiment and be sure not to get any of the solutions in this experiment on your skin. If you get an arsenical solution on your skin, quickly rinse it with water for several minutes.

With the discovery that orpiment is quite easy to obtain for decent prices, the element arsenic has become much more accessible than it was otherwise. The element and its compounds afford some interesting experiments, but should only be handled by the somewhat more experienced home chemist.
 

Required chemicals:

• arsenic sulfide, As₄S₄, realgar
• arsenic sulfide, As₂S₃, orpiment
• sodium hydroxide
• bleach
• hydrochloric acid

Required equipment:

• test tubes
• beakers
• magnetic stirrer
  

Safety

• Both arsenic sulfides are toxic
• The solutions are particularly toxic, especially the alkaline ones, because they attack skin and arsenic can be absorbed easily through them. In case of contact with these solutions, rinse with water for a long time, until the slippery feeling is gone.
• Avoid inhalation of powdered arsenic sulfides, when crushing the crystals.
• Sodium hydroxide is very corrosive to skin and especially to the eyes. Avoid contact with the eyes at any cost. If sodium hydroxide comes in contact with the skin, rinse with water, until the slippery feeling disappears.
• Hydrochloric acid is corrosive.
• Do not add strong reductors like zinc or sodium borohydride to the solutions, containing arsenic! The use of such reductors usually leads to formation of hydrogen, and when arsenic is present in solution, then small amounts of arsine, AsH₃, may be formed, which is extremely poisonous!
  

Disposal:

• The waste should be collected and not be flushed down the drain. Bring the collected waste to a proper waste processing facility.
  

Dissolving realgar

For this experiment, powdered realgar (crunched crystals) are used. A closeup is shown here:

Realgar is soluble in solutions of sodium hydroxide. The solutions should be warm. The reaction is not very fast, but finally, all realgar can be dissolved. Initially, the solid dissolves with a very deep red color, but very quickly (in boiling hot solutions of NaOH this only takes a few seconds), the solution decomposes and a dark brown precipitate is formed.

Prepare a solution of approximately 10% sodium hydroxide by weight.

Heat this liquid, so that it becomes quite hot, but not so hot that it is close to boiling.

To this liquid add a little crushed realgar.

The realgar slowly dissolves, giving first a chololate brown color, but when more dissolves, it becomes nearly black. Below follow a few videos, of the dissolving of the realgar.

• Adding realgar to warm 10% solution of sodium hydroxide
• Approximately 1.5 minute later
• Approximately 3 minutes later

This experiment also shows that realgar is quite hydrophobic. It is not easily wetted and remains floating on the surface of the solution. This also hinders quite dissolving of the material.

Another experiment was done by adding a boiling hot and more concentrated solution (appr. 20% by weight) of sodium hydroxide to some solid realgar in a test tube. This is shown in a video, where one sees a transient deep red color, but very quickly the solution becomes very dark brown and turbid. 

The contents of the test tube and the contents of the beaker were added to each other and put in a test tube, which was swirled to make an homogenous liquid with the dark brown precipitate. This test tube was allowed to stand for two days. After two days, a dark brown precipitate is in the bottom of the test tube, together with a smaller quantity, sticking to the glass (the test tube was not positioned perfectly vertically). The precipitate is very fine.

Dissolving elemental arsenic

The brown solid, formed in these solutions, is elemental arsenic, very finely divided. On standing, it forms a flocculent precipitate. This precipitate can easily be dissolved in bleach. This can easily be demonstrated.

Swirl the test tube with the very dark brown precipitate from the previous experiment. Take approximately 25% of the dark liquid from the test tube and pour this in a small beaker.

To this turbid brown liquid add some bleach (with approximately 10% active chlorine), while stirring.

The stirring solution quickly turns much paler. the arsenic dissolves, but some milky turbidity is formed. The solution also contains sulfide, and this is oxidized. Some of this sulfide is oxidized to sulfur, while most is oxidized further to (most likely) sulfate. The sulfur becomes visible as pale turbidity. The below images show the color of the solution with intervals of one to two seconds.

In a video of the dissolving arsenic you can see this change of color over a period of 25 seconds. The video also shows that the crushed realgar was not crushed perfectly. Some larger red crystalline pieces can be seen on the bottom of the small beaker. The video also shows that the coarser pieces of realgar do not readily dissolve in the bleach.

Dissolving orpiment

Orpiment is more easily dissolved in hot dilute solutions of sodium hydroxide than realgar. Even somewhat bigger crystals of orpiment can be dissolved in boiling hot solutions of sodium hydroxide in a minute or so.

Put some orpiment in a test tube.

Add some water.

The picture shows that the larger pieces of orpiment collect at the bottom of the test tube, but the finer particles remain floating on the water. Just like realgar, the orpiment also is not that easily wetted.

Add approximately 20% by weight of sodium hydroxide. This is not critical at all, it certainly will also work with 10% or 30%.

Carefully heat the test tube with the sodium hydroxide, water, and orpiment. Swirl well, while heating. Stopper the test tube loosely with a piece of wadding to avoid formation of small droplets with arsenic before heating. If the test liquid boils, then the small droplets do not escape into the air, while any water vapor and hot air can escape from the test tube.

After approximately 1 minute of heating and swirling all of the orpiment has dissolved, including the larger pieces at the bottom. The resulting liquid is nearly clear, it contains a very small amount of brown solid, which settles at the bottom after a while. The liquid itself is very pale yellow.

The amount of brown material is really small. This most likely is due to tiny amounts of realgar, as an impurity of the orpiment. Realgar impurity is quite common with natural orpiment as shown below.

Formation of arsenic sulfide in acidic solution

Arsenic sulfide actually is quite a remarkable compound. Many sulfides have the property that they are stable at high pH, but are decomposed in acids. With orpiment, arsenic(III) sulfide, this is the other way around. As the above experiment shows, orpiment can easily be dissolved in alkaline solutions. When such a solution is added to an acid, then arsenic sulfide (of higher purity and purer color) precipitates again. This experiment is really remarkable, because the binding of the sulfide is so strong, that there only is a very faint smell of hydrogen sulfide on addition of the sulfide solution to dilute acid. The experiment is as follows:

Prepare a solution of 5% hydrochloric acid (actually, any strong acid will do). Prepare 50 ml or so of the acidic solution and put this in a beaker.

Put a stir bar in the beaker with the acid, and put it on a stirrer.

Pour the liquid from the previous experiment in the acid, while the stirrer is on.

When the above steps are performed, then a bright yellow and pure precipitate of arsenic sulfide is formed. This precipitate can be filtered and dried and in this way a very finely powdered purely yellow pigment can be obtained. In the past, this process was used to make artificial very pure yellow pigment, free of brown or reddish impurities and with very small particle size. The formation of the yellow pigment is shown in a video. The pigment is formed at once.

Dissolving arsenic sulfide by adding bleach

The arsenic sulfide in the acidic solution is remarkably stable. If some of the yellow precipitate is transferred to concentrated hydrochloric acid (30% by weight, slightly fuming), then it still does not dissolve and no smell of hydrogen sulfide can be observed. This is in strong contrast to the similar antimony sulfide, which does dissolve in concentrated hydrochloric acid and gives a strong smell of hydrogen sulfide.
When bleach is added to the dilute acid with the arsenic sulfide, then the solid does dissolve. The resulting solution is totally clear and pale green (due to excess bleach and the presence of excess chlorine, formed in the reaction between the dilute acid and the bleach). The dissolving of the arsenic sulfide in acid+bleach is shown in a video. After two minutes of stirring all arsenic sulfide is dissolved.

Discussion of results

Realgar is a red compound, which on crunching can be converted to a bright orange powder. Realgar is not stable to light. Wavelengths in the range from approximately 500 nm to 670 nm convert this compound to an allotropic form. In this process, the crystals crumble to powder with an ochre/orange color. The two allotropes of As₄S₄ have the following structures (left is the red realgar, right is the ochre/orange pararealgar):

Orpiment is a yellow compound, with a 2D polymeric structure. It forms sheets of arsenic atoms and sulfur atoms in a 2 to 3 ratio (see picture below, purple stands for arsenic, yellow for sulfur).

Solution of realgar in sodium hydroxide

Realgar dissolves in a solution of sodium hydroxide. In this compound, arsenic has an average oxidation state, equal to +2. In the solution, however, the soluble arsenic species has arsenic in oxidation state +3. This only is possible if for every three atoms of arsenic, one is converted to the element (oxidation state 0), so that the others can go to +3. The dissolving process goes through complex intermediate structures with a deep red color of varying composition. These complex structures are unstable and fall apart in simple anionic species, which at the very high pH of 10% solutions of NaOH are derived from orthoarsenite, AsO₃^3–, with one or two oxygen atoms replaced by sulfur atoms. So, in solution one gets partial thioarsenites. On average, the following reaction equation can be given

     3 As₄S₄ + 24 OH^– → 4 AsOS₂^3– + 4 AsO₂S^3– + 12 H₂O + 4 As

In reality, the reaction will be more complicated, and almost certainly, there will also be AsO₃^3– ions and AsS₃^3– ions in solution, but on average the ratio of the number of oxygen atoms and sulfur atoms in the (thio)arsenite ions will be 1. Maybe a better representation of the reaction is

     3 As₄S₄ + 24 OH^– → 8 "AsO_1½ S_1½^3– " + 12 H₂O + 4 As

Here, "AsO_1½ S_1½^3– " stands for all (partial) thio-orthoarsenite ions, which on average have the given formula. These (thio)arsenite ions are colorless. The precipitated arsenic is very dark brown, nearly black.

Solution of orpiment in sodium hydroxide

Orpiment also dissolves in a solution of sodium hydroxide. Here, there is no need for the arsenic to change its oxidation state, and hence no disproportionation is observed in this reaction. All arsenic remains in oxidation state +3 and all of it goes in solution in the form of partial thioarsenites.

     As₂S₃ + 6 OH^– → 2 "AsO_1½ S_1½^3– " + 3 H₂O

The solution of orpiment in dilute sodium hydroxide is colorless. If there is some brown coloration and turbidity, then this can be attributed to realgar impurities of the orpiment sample. Realgar impurities are common in samples of orpiment.

Reaction of thioarsenites with acids

When thioarsenates are acidified, then all sulfide is very tightly bound to arsenic. Arsenic in oxidation state +3 very tightly binds sulfide and even concentrated hydrochloric acid is not capable of releasing the sulfide as H₂S from the arsenic. Only if there is excess sulfide, it is released as H₂S in acidic solution, but as long as there still is arsenic(III), which is not fully bound to sulfide, then any sulfide present is bound. So, on acidification, some of the following reactions occur:

     2 AsS₃^3– + 6 H⁺ → As₂S₃ + 3 H₂S

     2 AsOS₂^3– + 6 H⁺ → As₂S₃ + 2 H₂O + H₂S

     AsOS₂^3– + AsO₂S^3– + 6 H⁺ → As₂S₃ + 3 H₂O

H₂S in turn reacts with arsenites and thioarsenites, with the help of some acid, to form As₂S₃ and water. An example of such a reaction is the following:

     2 AsO₂S^3– + H₂S + 6 H⁺ → As₂S₃ + 4 H₂O

All these reactions finally lead to formation of As₂S₃ and binding of all sulfide. These reactions best can be summarized as follows:

     2 "AsO_1½ S_1½^3– " + 6 H⁺ → As₂S₃ + 3 H₂O

These reactions all are driven by the fact that the solubility of As₂S₃ is extremely low in water and acidic solutions. So, if any As₂S₃ is formed, it precipitates and does not take part in any reaction anymore. This drives the reaction to an end, until either all arsenic is consumed, or all sulfide (in the form of H₂S in the acidic solutions) is consumed.

Oxidation of arsenic by bleach and oxidation of As₂S₃ by bleach

Arsenic is fairly easily oxidized. When bleach is present, then the arsenic is oxidized, all the way to its +5 oxidation state, and at the high pH of the experimental conditions, this is in the form of orthoarsenate ion:

    2 As + 5 ClO^– + 6 OH^– → 2 AsO₄^3– + 3 H₂O + 5 Cl^–

As₂S₃ is also easily oxidized by bleach. If that reaction occurs in acidic solution (as was the case in this experiment), then the arsenate will appear as arsenic acid. The sulfur is mostly oxidized to sulfate, but some of it is only oxidized to sulfur. In the acidic solution, the main reaction can best be represented as follows:

    As₂S₃ + 14 HClO + 6 H₂O → H₃AsO₄ + 3 HSO₄^– + 14 Cl^– + 17 H⁺

In a side reaction, there also is formation of some elemental sulfur.

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