Some less well known chemistry of selenium

The chemistry in some sense resembles that of sulphur, but there also are remarkable differences. Some aspects of the chemistry of selenium are not known very well and certainly cannot be regarded as common chemistry.

In this experiment, a sulphur-selenium compound is prepared in solution, which most likely can best be compared to polysulphide, but in literature, there is no mention of such compound.

Another compound is prepared, which most likely is selenium oxybromide.

Selenium can be purchased in the form of small corpuscles or in the form of a very fine black powder. Both forms of selenium are the same allotrope, the dark grey allotrope, which does not dissolve in any solvent without reaction. In the experiments, described in the first part of this web page, a high quality sample of selenium is used, as shown in the picture above.

Required chemicals for part 1 of this webpage:

• selenium

• sodium sulfide

• dilute sulphuric acid

• sodium hydroxide

• sodium peroxodisulfate

Required chemicals for part 2 of this webpage:

• sodium selenite

• selenium dioxide

• moderately concentrated hydrobromic acid (40% by weight)

• sodium hydroxide

• sodium sulfite

Required equipment:

• test tubes
• small beaker

Safety:

• Moderately concentrated hydrobromic acid and dilute sulphuric acid are corrosive.
• The soluble compounds of selenium are exceedingly toxic and there is a risk of cumulative effects. Be 100% sure that there will be no exposure at all to these compounds!
• Sodium sulfide and sodium hydroxide both are strongly alkanine and as such are very corrosive to skin and especially the eyes.
• Sodium sulfide has a horrible smell of rotton eggs and at low pH the sulfide is released as hydrogen sulfide. In this experiment, the amount of used sulfide is too low to allow dangerous concentrations of hydrogen sulfide, but the horrible smell definitely will be noticed, unless one works in a good fume hood.

Disposal:

• Do not flush the solutions in the sink. Keep them until they are brought to a proper chemical waste processing facility.

Part 1: A selenium-sulfide anionic species

When selenium is added to a solution of a sulfide at sufficiently high pH, then the selenium slowly dissolves and a deep red solution is obtained. Probably the selenium only dissolves when free sulfide ion is available, hence the need for a sufficiently high pH. Plain sodium sulfide is capable of dissolving the selenium, its solution contains sufficient sulfide ion.

The experiment can easily be carried out:

Take a small amount of selenium (a few tens of mg is sufficient) and take a spatula full of sodium sulfide. Dissolve the sodium sulfide in water, and add this to the selenium.

Swirl the solution carefully for 5 minutes or so. The selenium slowly dissolves and finally one obtains a clear deep red solution.

The picture above shows some of the solution in a wide (28 mm) test tube. It shows how dark the solution is, even with the very small amount of starting material.

A few tenths of ml of the above solution were further diluted and put in a standard (18 mm) test tube with three times its volume of water. This results in a clear red/brown solution. This sulfide/selenium anionic species has a beautiful color.

When this solution is diluted much more (by pouring it in a beaker, filled with water), then it partially decomposes, giving a yellow/brown solution, which is not totally clear.

Selenium-sulfide anionic species unstable at low pH

The selenium-sulfide anionic species is unstable at low pH. This is nicely shown when the clear brown/red liquid (as shown above in the last picture) is acidified with some dilute acid (e.g. 10% to 20% sulphuric acid).

An orange precipitate is formed, which clumps together at the bottom in larger orange/red/brown lumps.

When this precipitate is heated, then it darkens somewhat to a more reddish color and the particles are clumping together even more.

Selenium can be oxidized easily

The precipitate in the hot solution easily dissolves when a peroxodisulfate is added as oxidizer. On addition of excess sodium peroxodisulfate, the precipitate quickly becomes much lighter in appearance (probably because sulfide is oxidized to sulphur, which also precipitates) and after this, it dissolves, until the liquid is almost clear and very pale yellow. The final color, however, and also the final remains of turbidity, do not appear easily.

The liquid remains light yellow, and also slightly turbid. The picture above shows a last remain of a piece of selenium, which due to the heat has turned black, and which has bubbles of oxygen on it, due to decomposing peroxodisulfate. The selenium quickly dissolves, what remains most likely is finely dispersed sulphur.

It can nicely be demonstrated that this solution is not totally clear. When the light of a green laser diode is allowed to pass through the liquid, then a clearly visible beam of light can be observed, indicating the presence of larger particles. A clear solution does not show such a beam.

Selenium-sulfide species is stable at very high pH

To the remaining part of the dark red solution (first test tube picture on this page) a fairly large amount of solid sodium hydroxide was added and the solid was allowed to dissolve. The liquid heats up considerably due to the heat of hydration of the sodium hydroxide.

When the strongly alkaline solution is added to a large volume of water, then the liquid remains clear. This is in contrast with the situation of adding the liquid without sodium hydroxide to a large volume of water. The picture below shows a beaker to which the alkaline liquid is added. Due to the higher density of the liquid, it does not completely mix with the water immediately.

A particularly nice effect is obtained when a small amount of dilute sulphuric acid is carefully dripped into the middle of beaker. The dilute acid sinks to the bottom, but at the soft interface between the water and the brown solution with the selenium-sulfide species in it, a yellow precipitate is formed, which remains floating in the water, in the interface area. The precipitate remains there for many minutes, without any visible change.

The yellow color most likely is due to the very fine dispersion of the precipitated selenium.

When more acid is added and the contents of the beaker are stirred with a rod, then the liquid becomes totally opaque and the color of the liquid becomes like the color of orange juice. This yellow/orange precipitate either is very finely dispersed selenium, or it is a mix of selenium and sulphur.

Part 2: Selenium bromide in aqueous solution

In this part of the experiment, another interesting species of selenium is shown. This is not based on elemental selenium, but on selenious acid, H₂SeO₃. This acid can be made from selenium dioxide, or from sodium selenite.

Selenious acid forms an intensely colored red species with bromide ion, but only when the concentration of the bromide ion is very high. On dilution with water, the color disappears and a colorless (or pale yellow) solution remains.

Two experiments were carried out. In one experiment, solid Na₂SeO₃ is added to 40% HBr. In another experiment, solid SeO₂ is added to 48% HBr.

Sodium selenite in hydrobromic acid

The first experiment is simply adding solid sodium selenite to 40% hydrobromic acid. Only a tiny amount of sodium selenite is used, and even with this tiny amount a fairly intensely colored solution is obtained. The upper picture shows the solid in a standard test tube, the lower picture shows the result of adding the 40% hydrobromic acid and briefly swirling the test tube, until all of the solid has dissolved. A nice red/brown solution is obtained, which has an intense color. The liquid, sticking to the glass still exhibits a clearly visible yellow color.

When water is added, even just a small amount, then the color disappears and the solution becomes pale yellow. The pictures below show the result of adding a small amount of water, and adding somewhat more water. The solution does not become totally colorless, a pale yellow color remains and the liquid also is very slightly turbid. The only explanation for this is that there is a tiny amount of colloidal selenium in the pale yellow liquid.

This is the result of adding a small amount of water and not shaking:

After shaking, the liquid is as follows:

When more water is added, then the yellow color does not disappear, it just is diluted as expected:

Selenium dioxide in hydrobromic acid

An even deeper color can be obtained from selenium dioxide in 48% hydrobromic acid. This combination of chemicals allows even higher concentration of bromide and stronger acidity. The two pictures below show how little selenium dioxide is used and what is the result of adding the hydrobromic acid.

The liquid has a really strong color and the liquid, sticking to the glass shows an intense yellow color, indicating that its color density is high.

When approximately the same volume of water is added to the deep red liquid, then the liquid almost becomes colorless. The liquid, sticking to the glass, which is not touched by the water still shows the bright yellow color.

The selenium (which is at oxidation state +4), present in the hydrobromic acid solution can be reduced very easily. Simply adding some solid sodium sulfite results in immediate reduction of the selenium to its elemental form. The coarser pieces of selenium are brick-red, the more finely dispersed selenium has an orange color:

Discussion of results

The results of these experiments are interpreted by myself, the author of this webpage, and is not backed up by literature, so take this explanation with some grains of salt .

Part 1: Selenium in solution of sodium sulfide

In the first experiment, I expect that the reaction between selenium and the solution of sodium sulfide is very similar to the reaction of sulphur with a solution of sodium sulfide. A well-known reaction is the following:

   S^2- + nS → S_n+1^2-

In this reaction, so-called polysulfides are formed. Solutions of polysulfides have a deep golden yellow color. The more sulphur is dissolved in a solution of a sulfide, the deeper the color. The value of n in this equation can be as high as 4 under normal conditions and under more special conditions, according to literature, higher values are reported.

Analogous to the polysulfide reaction, I expect that with selenium the following reaction occurs:

   S^2- + nSe → SSe_n^2-

The value of n is not known, it most likely equals 1, but more could be possible. Finding out possible values of n requires much more detailed research, in which one determines how much selenium can be dissolved in a solution with a known content of sodium sulfide. The possible range for n almost certainly depends on the concentration of the sulfide.

When a polysulfide solution is acidified, then the polysulfide ions decompose according to the following equation:

   S_n+1^2- + 2H⁺ → H₂S_n+1 → H₂S + nS

Most likely, with the selenium-sulfide anionic species the following occurs:

   SSe_n^2- + 2H⁺ → H₂S + nSe

As a side reaction, some H₂Se and S might be formed as well, but whether this occurs or not must be investigated further. It also must be investigated whether the reaction goes through an intermediate H₂SSe_n or not.

Surprisingly, I could not find any information about such reactions in literature.

Remark: A similar experiment was carried out with tellurium in a solution of sodium sulfide. Tellurium, however, does not react at all. Even after minutes of boiling, no tellurium dissolves and the solution remains colorless.

Part 2: Selenium oxybromide in aqueous solution

When SeO₂ or Na₂SeO₃ is added to an aqueous acidic solution, then the acid H₂SeO₃ is formed, which is a fairly weak acid. This acid reacts with hydrobromic acid, giving a deep red/brown compound. I think that the following equilibrium reaction occurs:

   SeO(OH)₂ + 2H⁺ + 2Br^– ↔ SeOBr₂ + 2H₂O

The equilibrium only is to the right when the concentration of bromide and acid is sufficiently high, even at moderate concentration, the equilibrium is at the left for almost 100%.

Literature mentions SeOBr₂ as a yellow/red compound, so this matches the observations.

This behavior definitely has no analogue in sulphur chemistry. Both thionylchloride and thionylbromide react vigorously with aqueous solutions and are driven completely towards H₂SO₃/SO₂, even when traces of water are present. Any water is destroyed irreversibly by these compounds and no reverse reaction occurs. Adding SO₂ to strong hydrobromic acid certainly does not lead to formation of thionylbromide.

An acidic solution, containing selenium in the +4 oxidation state is exceptionally easily reduced in the presence of chloride or bromide. A mild reductor like sulfite (which leads to sulphur dioxide in the acidic medium) immediately reduces the selenium to its elemental state:

   H₂SeO₃ + 2SO₂ + H₂O → 2H₂SO₄ + Se

This reaction only occurs at high speed in the presence of chloride or bromide. So, these ions have some catalytic effect and this most likely has to do with the equilibrium in which SeOCl₂ or SeOBr₂ is formed.

Reduction of H₂SeO₃ by SO₂ is instantaneous at room temperature, when the reaction is carried out in aqueous HCl or HBr. The reaction only occurs slowly and requires heating when the reaction is carried out in dilute sulphuric acid of similar acidity.

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