A cationic chlorine species, ClO₂⁺, known as chloryl ion

• chloride, Cl^–
• hypochlorite, ClO^–
• chlorite, ClO₂^–
• chlorate, ClO₃^–
• perchlorate, ClO₄^–

All of these are well-known. Chlorine, however, also can form a cation, the ion ClO₂⁺ which is known as chloryl. This ion contains chlorine in oxidation state +5 and can be considered derived from chlorate, from which one oxygen ion is taken away, leaving a cation. This indeed is possible, but only in the presence of extreme reagents, which have a very strong affinity for the oxygen atom. This ion is extremely moisture-sensitive and even in the presence of the tiniest amount of moisture, it decomposes. In this experiment some of this cationic species is made and some properties of this ion are demonstrated. No attempt is made to isolate salts of this cation. This is possible, but only using extreme reagents in special equipment with meticulous exclusion of water. This is not possible in the average home lab.

Be very careful with this experiment. The compounds, used in this experiment are extremely corrosive and there is a risk of violent decomposition with the possible splattering of the extremely corrosive solution. Never scale up this experiment!

Required chemicals:

• potassium chlorate
  
• oleum with 20% SO₃
  
• chlorosulfonic acid
• concentrated sulfuric acid
• red phosphorus (optional)
  

The acids must have high purity and may not contain any organic material, which can act as reductor. Drain cleaner acid should not be used, it may lead to dangerous and violent reactions.

Required equipment:

• test tubes
• glass pipette with balloon for transferring the acids to the test tube
  

Safety:

• Potassium chlorate is a strong oxidizer and forms dangerous mixtures with many reducing compounds, including paper, wood, and many other things which can burn. In this experiment, however, a very small amount is used and the risk of fire is limited.
• Oleum is extremely corrosive! Avoid contact of this with skin or any other body part at any cost! Exposure will lead to instant severe burns and extreme pain.
• Chlorosulfonic acid is extremely corrosive, just like oleum. Again, avoid contact of this with skin or any other body part at any cost! Exposure will lead to instant severe burns and extreme pain.
• Avoid direct mixing of oleum and chlorosulfonic acid with water. They react explosively with high risk of splattering of extremely corrosive acid droplets. Best way to dispose of any remains is dilution with 5 times its volume of concentrated sulfuric acid (96% or so) and then carefully pouring this acid in a lot of cold water.
• Oleum and chlorosulfonic acid produce dense white fumes in contact with air. They react with water vapor and in this reaction a mist of sulfuric acid (and in the case of chlorosulfonic acid, also hydrochloric acid) is produced. Avoid inhaling any of this mist!
  

Disposal:

• After the experiment, dilute all liquid waste with approximately 5 times its volume of concentrated sulfuric acid.
  
• Fill a bucket with at least a few liters of cold tap water.
  
• Very slowly, pour the concentrated acid from the test tube(s) in the cold water.
• Pour the waste down the drain. It is not particularly toxic.

Preparation of a solution of potassium chloryl sulfate

Take a perfectly dry test tube and put 1 ml of 20% oleum in the test tube. Use a pipette for transferring this acid to the test tube. Use high quality oleum, which is (nearly) colorless. It may not contain any organic residues or charred material.

Below is a picture of the oleum, used in this experiment. It is a colorless liquid, p.a. reagent grade. At the bottom of the bottle one can see that it is colorless, the yellow color is due to the backside of the label, which is somewhat yellow.

Crush a little amount of potassium chlorate, such that a fairly fine powder is obtained. If the solid already is a fairly fine powder, or consists of little (less than around 0.3 mm size) crystals, then it can be used as is.

Add the potassium chlorate to the test tube with oleum. Only add a tiny amount! A little pile of 2 mm diameter and 2 mm height is more than enough. This will be 20 mg or so.

After adding the potassium chlorate, the solid slowly dissolves. Around the crystals, there is a scarlet red color. In thin layers, the newly formed product looks bright orange/red. On dilution, the solution also looks bright orange/red. No gas is produced.
This is in strong contrast with adding potassium chlorate to concentrated (appr. 96%) sulfuric acid. If that is done, then some orange color can be observed, but there also is production of quite some bright yellow gas, which is ClO₂. With sulfuric acid, the chlorate ions are protonated and HClO₃ is formed, which is not stable under these conditions. This disproportionates (more details are given below in the explanation of the observed results) and one of the products is ClO₂.

Some of the small crystals of potassium chlorate were sticking to the walls of the test tube, which were wetted with oleum. These little crystals slowly dissolve and produce an intensely colored solution. The main body of the solution is bright orange/red.

Production of a little more of the red compound

After this experiment, some confidence was obtained, that this compound does not decompose or explode at once. It is sufficiently stable to be observed and it was decided to make a little more of this. For that purpose, the following steps were performed.

Take a perfectly dry test tube and put 1 ml of 20% oleum in the test tube and add 5 ml or so of reagent grade concentrated sulfuric acid to that. Swirl the test tube to mix both liquids. This results in formation of a colorless liquid, which hardly fumes in contact with air. This liquid is sulfuric acid, with just a few percent of free SO₃ in it. Immediately after mixing, this liquid is quite hot. This heat is due to the reaction between the SO₃ in the oleum and the few percent of water in the concentrated sulfuric acid.

Let the liquid cool down and then pour the liquid into the test tube with the orange solution. Swirl the test tube with the orange solution. The result is a paler orange liquid, due to the dilution with the newly added acid. No noticeable heat is produced. Apparently, all water in the concentrated sulfuric acid was used up already in the reaction with the oleum in the other test tube.

Add a small spatula of potassium chlorate to the pale orange liquid and swirl the test tube. Do not use a large amount! A pile of 3 to 4 mm with a height of a few mm is more than enough. The total amount of potassium chlorate should not exceed 70 mg or so. Add the solid in small portions and dissolve one portion, before adding the next.

After adding all potassium chlorate and dissolving this, a beautiful deep orange/red solution is obtained, which sticks to the glass easily and has a very intense color, even when viewed through a thin layer, sticking to the glass. No gas is observed, not even a faint yellow color of ClO₂ can be observed above the liquid. There also is no smell of ClO₂.

This solution is estimated to have an SO₃ content of approximately 5% by weight. The exact concentration is not important. As long as there is any free SO₃, there will be no water. SO₃ and water simply cannot coexist.

This solution has ClO₂⁺ ions. These ions are called chloryl ions. Besides these, there are K⁺ ions from the KClO₃. According to literature, chloryl ions have a red color. Chloryl can also be present in covalent form, such as in ClO₂F. In that case, it is colorless.

A few experiments with the red/orange solution of chloryl ions

A few experiments were performed with this red solution:

• Dripping the solution on some red phosphorus
  
• Adding some of the solution to a large excess amount of water
• Keeping the solution in contact with air for a while
• Adding some chlorosulfonic acid to the orange/red solution

In the experiments, it was surprising to see that the reactivity of the solution is not that extreme. Its reactivity is mainly because of the SO₃-content, and much less so due to the presence of the chloryl ions.

The result of the first experiment is that no visible reaction occurs. This is in strong contrast with the situation in which red phosphorus, potassium chlorate and sulfuric acid are mixed. In the latter case, there is an explosion.

The result of the second experiment is as expected. On dilution, a lot of heat is produced (reaction of the SO₃ and sulfuric acid with water). The resulting solution is (nearly) colorless. The chloryl ions are hydrolysed to colorless chloric acid, which in turn dissociates to chlorate ions and hydrated hydrogen ions.

When the solution is kept in contact with air, then it fairly quickly (in several minutes) loses its orange color. It absorbs water vapor from the air and in this process the chloryl hydrolyses.

The final experiment was adding chlorosulfonic acid to the red solution. When the chlorosulfonic acid is added to the solution carefully, then two layers are formed, with a diffuse border between the layers. The density of the chlorosulfonic acid is lower than the density of the red solution. It floats on top, as a pale yellow liquid.

   

The gas, which is produced is colorless (or very pale green, hard to observe). There is a strong smell of chlorine though, so most likely it is chlorine gas. At least it contains quite some chlorine gas. No ClO₂ is produced. The latter has an intense yellow color and a distinct odor, which is not observed. After shaking, the production of gas quickly ceases. At each shake, again a little gas is produced. The liquid has a yellow color, as shown in the picture above. At the end of this experiment, the liquid was poured in a bucket full of water. It dissolves in the water, making a hissing noise (it contains a lot of chlorosulfonic acid), producing a colorless solution.

Discussion of results

When potassium chlorate is added to concentrated sulfuric acid (96% or so), then a lot of yellow gas is produced and at the same time, white fumes are produced. The following reaction occurs:

     KClO₃  +  H₂SO₄  $→$  HClO₃  +  K⁺  +  HSO₄^–

HClO₃ is quite a strong acid, but sulfuric acid is stronger and this protonates the chlorate ion in potassium chlorate. HClO₃ is not stable as a covalent molecule. Only when it can act as acid and form H⁺ and ClO₃^– it is stable. This is the case in aqueous solutions up to appr. 30% by weight. The covalent, not dissociated compound, disproportionates as follows:

     3 HClO₃  $→$  HClO₄  +  2 ClO₂  +  H₂O

The ClO₂ escapes as bright yellow gas. Both ClO₂ and anhydrous HClO₄ are very unstable and frequently, this reaction leads to explosion. The H₂O immediately is absorbed by excess H₂SO₄.

When potassium chlorate is added to sulfuric acid, containing free SO₃, then a different reaction occurs:

     KClO₃  +  SO₃  $→$  ClO₂⁺  +  K⁺  +  SO₄^2–

The SO₄^2– ion, then in turn, will react with H₂SO₄, present in the liquid, to HSO₄^– ions.

The ion ClO₂⁺ has a red color. It is called chloryl ion.

Solid salts of the chloryl ion have been prepared, but doing that can only be done in a very well equipped lab and is not something for the home chemist. Some examples of chloryl salts, which have been isolated are ClO₂SbF₆ and ClO₂ClO₄. These are red ionic compounds.

The chloryl ion is very reactive and as soon as it comes in contact with water, it is hydrolysed:

     ClO₂⁺  +  2 H₂O  $→$  HClO₃  + H₃O⁺

If there is a large excess amount of water, then the HClO₃ dissociates and acts as acid. This is quite a strong acid, so most of it will be ionized as follows:

     HClO₃  +  H₂O  $→$  ClO₃^–  + H₃O⁺

If only a little water is added, then the HClO₃ decomposes, giving ClO₂ and HClO₄.

The reaction with chlorosulfonic acid is a simple redox reaction. Chlorosulfonic acid contains chlorine in oxidation state -1 and the chloryl ion contains chlorine in oxidation state +5, so there is an easy target for oxidation. The complete redox reaction is as follows:

    ClO₂⁺  +  5 HSO₃Cl  + HSO₄^–  $→$  3 Cl₂  + 3 H₂S₂O₇

In the equation is written H₂S₂O₇, which most likely is a simplification of what really happens. In oleum, there is a complicated equilibrium mix of pyrosulfate, free SO₃, HSO₄^– and H₂SO₄. The essence of this equation is that the chloryl ion oxidizes the chlorosulfonic acid and forms chlorine gas.

Unfortunately not much information about chloryl ion and its compounds can be found online. Wikipedia has a brief page on it, giving its color and some basic information.

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