Free solvated electrons

Approximately 150 years ago, it was discovered that sodium can be dissolved in liquid anhydrous ammonia, giving a deep blue solution, while no hydrogen gas is produced. Further studies revealed that this deep blue color is caused by the presence of free electrons e−, solvated by ammonia molecules. The sodium, added to the ammonia dissolved and produces ions Na+ and e−.

For a long time it was believed that the free electron only could exist at low temperatures in liquid ammonia, but later it was discovered that certain organic amines also allow formation of free electrons, also at room temperature. Such solutions are moderately stable and can be kept around for hours. In this experiment, such a solution is produced from lithium andethylenediamine, NH 2CH2CH2NH2. Some properties are demonstrated. The deep blue color of the free solvated electrons also is prominently displayed at room temperature and in other solvents than liquid ammonia.

           

The picture shows a very dilute solution of dissolved lithium in ethylenediamine. This dilute solution contains ions Li+ and e−, both solvated by ethylenediamine, so it would be better to write Li(en)n+ and e(en)n−, where n is some unspecified number, and (en) stands for the ethylenediamine ligand.


Required chemicals:

  • lithium
  • ethylenediamine (the chemical must be pure, not a solution in water)

Required equipment:

  • test tubes
  • small tank or dish for holding water

Safety:

  • Ethylenediamine is quite a strong base, hence it is corrosive. This compound is intensely strongly fuming and has a smell reminiscent to the smell of ammonia (although it is less pungent). Avoid contact with skin and avoid inhaling the fumes.
  • Lithium is very reactive. Its reaction with water is highly exothermic. Avoid touching the metal with bare hands, especially if your hands are wet or somewhat sweaty. Also avoid as much as possible, contact of lithium with air, it is oxidized very quickly.

Disposal:

  • No particularly toxic waste is produced in this experiment. The waste can be flushed down the drain with a lot of water.

 

 

 


Putting a piece of lithium in ethylene diamine

This experiment is a very simple one. Simply take a piece of lithium, pea-sized or slightly larger. The picture below shows the lithium, used in this experiment. It is shiny lithium , stored under pressurized butane gas in a very well sealed bottle.

       


One such a piece of lithium (appr. 3.5 mm diameter, 8...10 mm length) was taken and put in a test tube. When this is done, then the piece quickly loses its metallic lustre and becomes covered by an oxide layer. It is oxidized by oxygen and water vapor from the air:

          


To this, appr. 2 ml of pure ethylenediamine was added. When this is done, then first there is very slow production of hydrogen. This almost certainly is due to the presence of a small quantity of water in the ethylenediamine. The production of hydrogen is very slow. The liquid also obtains a grey color with a somewhat bluish hue.

         


After a while, the color of the piece of lithium changes and becomes more pronounced blue/cyan. Production of hydrogen gas at this point slows down even further.

         

Also some solid particles appear, which give the lithium a somewhat rough appearance. This solid material most likely is formed while the lithium was oxidized in the air and may also be due to impurities in the ethylenediamine. In a later stage of the experiment, these solid particles peel off when more of the lithium dissolves.





Formation of free solvated electrons

Once all water has been used up, free solvated electrons make it into solution, further away from the lithium metal. Initially, the blue color of the solvated electrons only can be seen very near the lithium, but in later stages, more and more of the solvated electrons are formed and remain in solution.

Initially, the electrons can only be observed transiently. Around the lithium, a fairly high concentration of free electrons is formed. The lithium is covered by a very dark blue, nearly black, layer. When the test tube is shaken vigorously, then the blue color of free solvated electrons can be observed briefly, but the color very quickly fades again, when the shaking stops. This is demonstrated by a video: transient appearance of solvated electrons. Download size is just over 3 MByte. Apparently, in the liquid there is something dissolved, which very quickly oxidizes the free electrons. This dissolved species most likely is still remaining water, which quickly reacts with the free electrons, giving hydroxide ions and hydrogen gas.

After several times of shaking the test tube in this way, the fading of the blue color of free electrons takes more time. At a certain point in time, the fading takes several seconds and the process can be followed nicely. The effect is quite striking. Each time when the test tube is swirled, the liquid becomes very dark blue, nearly black, while at the glass wall above the liquid no color can be observed at all, and the nearly black color fades in a few seconds to colorless. Below follows a sequence of a few pictures, which shows the changes over a period of 4 seconds. The time between two images is 500 milliseconds, the first image (top left) is just after shaking vigorously.

       

       

       

A video demonstrates this effect even better: slow oxidation of free solvated electrons. Download size is nearly 6 MByte. The video also shows that oxidation of the free electrons takes more time after each turn of shaking of the test tube. The compound which oxidizes the free electrons is used up and its concentration becomes lower and lower.

At a certain point, the color does not fade anymore after shaking. The liquid does not contain any oxidizing agent anymore which consumes the electrons. At this stage, there also is a deep blue color of the liquid, sticking to the glass walls of the test tube. This liquid, however, quickly loses color and this most likely is due to oxidation by oxygen and/or watervapor from the air. The picture sequence below shows the liquid, immediately after shaking the test tube and each further picture is make exactly 1 second after the previous one. The total sequence covers 5 seconds.

       

       

The time it takes before the color has faded from the glass of the test tube does not change noticeably after many turns of shaking. This is because the test tube is open and fresh air can get in. The blue color quickly fades in contact with air, due to aerial oxidation. A video is made of this process as well: electrons oxidized by air. Download size is nearly 6 MByte.





Free electrons in water: immediate oxidation

In this final experiment, the very dark blue liquid with the free electrons is added to water. As soon as the liquid comes in contact with water, the color disappears and a colorless solution is produced.

Finally, the piece of lithium itself is thrown in the water. It quickly dissolves, while fizzling violently. The reaction between lithium and water is quite vigorous and no traces of free solvated electrons can be observed at all in this reaction.

A video is made of this process as well: electrons oxidized by water. Download size is appr. 10 MByte.



 

Discussion of results

In all of their reactions, alkali metals are strong reductors and frequently their reaction is written as follows:

     M → M+ + e−

Here M is the alkali metal ion. This usually is considered a so-called half-reaction and this reaction equation is combined with another equation, in which electrons are consumed, so that the net combined reaction equations  have the same charge at the left and right and have no production, nor consumption of free electrons. This kind of equations do not describe what happens really, but are a tool to help find the correct and complete reaction equation in real redox reactions.

In this experiment, however, the electrons really are produced and exist as such in solution. They are solvated by the solvent, in this case ethylenediamine. So, here we have the following reaction equation, which is not merely a bookkeeping device or mathematical model, but describes what happens in reality:

     Li(s) → Li+(solv) + e−(solv)

Here, (s) stands for solid, and (solv) stands for the fact that the lithium ions and electrons are solvated and can be considered as complexes with the metal ion and the free electron as central ion and ethylenediamine molecules as ligands. The solvated lithium ions are colorless and the solvated electrons have a deep blue color.

The most remarkable property of the free electrons is that their color is independent of the solvent in which they appear and also independent of temperature. At low concentration, they have a deep blue color. At higher concentrations, from appr. 0.05 mol/l and up, the color shifts from deep blue to bronze-like. In this experiment, however, such high concentrations could not be achieved. The bronze-like solutions have an electrical conductance, nearly as good as metals, but the blue solutions, according to literature, also have a very good electrical conductance, several times better than any ordinary salt of the same alkali metal in the same solvent.

 

 

   

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