A remarkable insoluble polyhalide salt

Polyhalogen ions are fairly well known and can be considered as complexes of halide ions with halogens. The best known example of such an ion is the triiodide ion, I3–.
Polyhalide ions are quite unstable and easily lose free halogen, or a mixed halogen compound and a simple halide ion remains behind. In aqueous solution, such ions also are quite unstable and tend to lose free halogen or hydrolyse, giving rise to formation of oxoacids and free halogen. These ions, however, can be stabilized in combination with large cations in a solid salt. An experiment with the rubidium salt of tetrachloroiodate(III) demonstrates the stabilizing effect of the large rubidium ions. The solid is fairly stable and only slowly releases iodine trichloride. When the solid is added to water, then, however, it quickly decomposes, giving hydrochloric acid, iodine and iodic acid.

In this experiment, an even more stable salt of the tetrachoroiodate(III) ion is made, using the large cation N(CH3)4+. With this cation, a nearly insoluble salt is formed, which even can be kept in plain water for multiple days without decomposition.

Required chemicals:

  • tetramethylammonium chloride, N(CH3)4 Cl
  • iodic acid
  • concentrated hydrochloric acid (at least 30% HCl)
  • sodium sulfite
  • sodium hydroxide
  • ammonia (5% NH3 is suitable)

Required equipment:

  • test tubes

  • little beaker or erlenmeyer

Safety:

  • Concentrated hydrochloric acid is corrosive. Avoid contact with skin. If by accident some of the acid comes in contact with skin, then quickly rinse the skin with plenty of water.
  • Iodic acid is a strong oxidizer.
  • Sodium hydroxide is very corrosive. Avoid contact with skin. If some of this comes in contact with skin, then rinse with water, until the slippery feeling has gone.
  • In this experiment, chlorine gas is produced. Do this experiment outside, or use very good ventilation. Avoid inhalation of chlorine gas.
  • Ammonia is irritating and fairly corrosive to skin.

Disposal:

  • All solid waste can simply be destroyed by dissolving in a solution of sodium sulfite or sodium hydroxide. The resulting solution can be flushed down the drain with a lot of water.

 

 
 

 

Demonstration of insolubility of tetramethylammonium tetrachloroiodate(III)

A very simple experiment demonstrates the low solubility of tetramethylammonium tetrachloroiodate(III) in aqueous solution.

Add some solid iodic acid to concentrated hydrochloric acid. When this is done, then chlorine gas is produced vigorously and the liquid turns golden yellow. A piece of iodic acid at the size of an average pea dissolves in 5 ml of concentrated hydrochloric acid within a few minutes.

In a separate test tube, dissolve a small amount of tetramethylammonium chloride in a few ml of concentrated hydrochloric acid. The solid quickly dissolves in the acid. To this solution, add a piece of iodic acid. The piece of iodic acid is covered with a bright yellow layer and then the reaction stops. Only a few small bubbles of chlorine gas are produced. When the liquid with the piece of iodic acid is shaken, then small pieces of yellow solid material split off and exposed iodic acid immediately is covered with the yellow solid. Even after two days of storage, still most of the iodic acid remains undissolved and a yellow precipitate is formed under a colorless liquid.


Production and isolation of solid tetramethylammonium tetrachloroiodate(III)

The above simple experiment demonstrates that making tetramethylammonium tetrachloroiodate(III) must be done in a different way. First, iodic acid must be dissolved in hydrochloric acid. This gives tetrachloroiodate(III) in solution as follows:

   HIO3 + 5H+ + 6Cl–→ICl4– + 3H2O + Cl2

The ICl4– remains in solution as golden yellow ions. These ions form a precipitate with tetramethylammonium ions, which are added after the iodic acid has dissolved.

The procedure for making solid tetramethylammonium tetrachloroiodate(III) is as follows:

Dissolve some iodic acid in hydrochloric acid. Take a pea-sized piece of iodic acid and add this to 5 ml or so of concentred hydrochloric acid. If at the end of the reaction, the iodic acid dissolves slowly, then add a ml or so of concentrated hydrochloric acid and if necessary, heat gently. Do not overheat, as that will decompose the tetrachloroiodate ion! At the end of the reaction this solution is golden yellow/orange. Be careful with this reaction. Quite some chlorine gas is produced. Avoid inhaling this gas.

Dissolve some tetramethylammonium chloride in water. Take an amount, comparable to 2 pea-sized pieces of solid material. The amount is not critical at all though. Add this solution to the golden yellow solution. If a concentrated solution of tetramethylammonium chloride is added, then almost all of the liquid forms a gel and you need a glass rod or plastic stick to mix the chemicals well. You get a bright yellow paste.

It is not easy to purify this material, it is a very sticky paste with a lot of absorbed water and acid. Best is to add all of this sticky paste to water and rinse this two or three times. With a plastic spoon or spatula, take the sticky paste out of the test tube and put this in an erlenmeyer with water. The result looks as follows:

   


The yellow material only very sparingly dissolves in water and does not give a strongly colored yellow solution. The initial yellow color is from excess tetrachloroiodate(III). If more tetramethylammonium chloride were added, then a nearly colorless solution would be obtained with the yellow precipitate in it. This was checked in another experiment at a smaller scale. The picture below shows the result of that experiment, in which a slight excess amount of tetramethylammonium chloride was used:

   



The yellow precipitate must be rinsed with water to increase its purity and then it must be filtered. The filtered solid must be kept on the filter and this filter must be put on a pile of paper tissues. If the filter is carefully folded and this folded filter is pressed between two piles of paper tissue, then a fairly dry solid is obtained, which is quite pure as well. Nearly all water (with impurities dissolved in it) are absorbed by the tissue paper.

After the above treatment, the solid must be carefully scraped from the filter and be put in a petri dish or on a watch glass. Then it must be set aside at a warm dry place for 24 hours or 48 hours (depending on ambient temperature and humidity of air). It is best to stir the solid material every few hours and to break apart larger crumbs of material to assure that the material becomes completely dry.

After two days, a yellow dry and non-sticky powdered solid is obtained. The material does not look crystalline, it looks like amorphous powder. The final result is shown in the picture below.

   

This is a small vial with a volume of 2.5 ml or so. The powder is quite fluffy and contains a lot of air. This amount was not weighed, but it certainly will not be more than 2 grams of solid.


Some experiment with the yellow solid

The yellow solid, obtained in this experiment, is quite interesting and allows a few interesting experiments to be done.


Addition of sodium sulfite

One interesting experiment is to add a solution of sodium sulfite to the yellow solid. Simply add some of the solid to a dry test tube and then add a dilute solution of sodium sulfite. When this is done, then the solid immediately turns black. The black solid in turn slowly dissolves and finally, a pale yellow solution is obtained, which has a faint smell of sulphur dioxide.

     

The pale yellow color is due to the presence of iodide ion in the presence of sulphur dioxide. This pale yellow color is not due to the presence of free iodine or of remains of the tetrachloroiodate(III), it is a complex of iodide ion and sulphur dioxide. For more information, see this page.


Addition of sodium hydroxide

Another interesting experiment is adding sodium hydroxide. When this is done, then the result is quite striking. For this experiment, some of the yellow solid must be taken and then a not too strong solution of sodium hydroxide must be added. The solid immediately turns bright orange and it quickly dissolves. It dissolves much faster than in a solution of sodium sulfite of similar concentration. The pictures below show a small amount of the yellow solid, to which a solution of sodium hydroxide is added.

     

The left picture shows the result, immediately after adding the solution of sodium hydroxide, the right picture shows the same little vial, half a minute later. The liquid remains yellow. This most likely is due to the presence of hypoiodite ions, but this only is a hypothesis, it was not tested by means of experiments. The nature of the orange compound is unknown (at least to me as the author of this web page).

Below follows a better picture of the orange solid. It was prepared by adding dilute sodium hydroxide to a fairly large chunk of wetted powder of tetramethylammonium tetrachloroiodate(III) and avoiding shaking or swirling of the test tube:

   

On shaking, the orange solid falls apart quickly and dissolves, giving a yellow solution.


Addition of ammonia

A final experiment was done by adding ammonia (5% by weight) to the yellow solid. This results in formation of a dark grey solid, which does not dissolve in water, nor in excess ammonia. The dark grey solid almost certainly must be so-called nitrogen triiodide, which in reality is not pure NI3, but NI3·NH3.

No attempt was made to isolate this grey solid or drying it,  because of the explosive and very unstable nature of NI3·NH3.

Discussion of results

Iodic acid reacts with concetrated hydrochloric acid and gives chlorine gas and tetrachloroiodate(III) ion:

     HIO3 + 5H+ + 6Cl– → ICl4– + 3H2O + Cl2

The ICl4– is not very stable and easily loses ICl3. In water it is hydrolyzed as follows:

     5ICl4– + 9H2O→3IO3– + 18H+ + I2 + 20Cl–

Adding a salt of tetrachloroiodate(III) to water results in formation of a brown solution of iodine and it even is possible that some solid iodine settles from the aqueous solution. Only at high concentrations of chloride and acid (i.e. in concentrated hydrochloric acid) this hydrolysis reaction does not occur.

The tetrachloroiodate(III) forms fairly stable salts with large cations. The potassium salt can be isolated, but in contact with air it quickly loses ICl3. The rubidium and cesium salts already are somewhat more stable, but still, these also easily lose ICl3 and quickly hydrolyse in aqueous solution. With the tetramethylammonium ion, however, a salt is produced, which binds the tetrachloroiodate(III) ion more strongly, because of its low solubility. In contact with air, the solid only has a very faint smell, while solid RbICl4 or KICl4 has a very strong choking smell of iodine trichloride.

The yellow solid, produced and isolated in this experiment is N(CH3)4 ICl4.

When the tetrachloroiodate(III) ion comes in contact with sulfite, then it immediately is reduced to iodine and chloride ions. When there is excess sulfite, then the iodine in turn is reduced to iodide ions.

     2ICl4– + 3SO32– + 3H2O→I2 + 3SO42– + 6H+ + 8Cl–

This explains the change of color from yellow to dark grey as soon as the sulfite is added to the yellow solid. The reaction is very fast, it is nearly instantaneous. If excess sulfite is present, then the iodine also is reduced, but this reaction takes more time:

     I2 + SO32– + H2O→2I– + SO42– + 2H+

Both the reduction of tetrachloroiodate(III) and the reduction of iodine produce acid and this in turn reacts with excess sulfite ions to water and sulphur dioxide. This explains the smell of sulphur dioxide in the experiment and it also explains the weak yellow color of the final solution, due to formation of the yellow complex I·nSO2–  (see this page).

The reaction with sodium hydroxide is not really clear to me. The final reaction can be explained, but the intermediate orange solid is unknown to me. Maybe it is due to formation of iodide, which forms a red-brown complex with iodine, which is trapped in the solid, but this is not tested by experiment.

The final reaction is hydrolysis as described above, where the acid is neutralized by the hydroxide ions and the iodine reacts to hypoiodite and iodide.

The reaction with ammonia is easy to explain. The reaction again is hydrolysis with formation of iodine and this iodine in turn reacts with ammonia in the well-known reaction in which so-called "nitrogen triiodide" is formed, which in reality is a combination of NI3 and NH3. 
   

 

   

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