Extractions – like getting potassium metal from bananas, are a popular activity for both home and professional chemists. They are particularly popular on social media, where a scientist will start with an innocuous object like a glove, banana, coffee, or over-the-counter medicine and end up with an exotic metal or compound. In this article we learn how to how to extract potassium from bananas.
A recent experiment, producing elemental potassium (and ultimately potassium hydroxide) from bananas has a plethora of interesting chemistry and is the subject of this article.
The extraction procedure was performed by Cody Don Reeder of Utah in the United States, utilizing a previous design he developed for the reduction and distillation of low-melting point alkali metals. A video of the procedure can be watched below, and it on his popular youtube channel, “Cody’s Lab”. Cody is known for performing innovative chemical experiments, often with alkali metals, mercury, gallium, or precious metals like gold or platinum.
Extract Potassium Metal from Bananas – procedure
Let’s look at the steps he took to isolate potassium metal, starting only with bananas. Both the banana flesh and the peel were used, with equal success.
- Remove moisture
The first step was to remove as much moisture as possible. This was facilitated by increasing the surface area as much as possible before heating them in a drying oven. Drying in air also works if the surface area exposed to air is large enough, and the humidity is low enough. 110° celsius can be considered an optimal temperature for removing moisture from organic material such as bananas.
- Pyrolysis of the organic material
In the second step, the dried bananas were carbonized in a furnace, probably at around 400° – 600° celsius. The long chains of carbons, hydrogen and oxygen in the cellulose, sugars, and starches break down into charcoal and gases such as carbon dioxide, methane, acetic acid and various carbonyl compounds. All volatile compounds were driven off, leaving us with “banana briquettes”. Similar briquettes are being used as biofuel in places like Thailand and Uganda.
- Combustion of remaining organic material
Third, any remain non-volatile organic material was burned in air, facilitating combustion with oxygen and reducing any remaining organic material to carbon.
Steps 2 and 3 could be combined, and you could try to burn the bananas directly after drying them. However, in other attempts we’ve seen this can take quite a while, and may result in a less reduced product.
- Separation of soluble and insoluble compounds
At this point, any potassium in the charcoaled remains of the bananas can easily be dissolved in water. It is most likely in the form of potassium carbonate, but some chloride could be present along with various sodium salts.
Exploiting solubility differences is actually key for many extractions, and often organic solvents will be needed to facilitate this. However, in this case plain distilled water works fine. Cody uses Kroger’s distilled water, which happens to be the distilled water of choice for the ChemTalk team.
After soaking the banana ashes in distilled water, the slurry is filtered. The filtrate will contain only soluble inorganic salts, mainly potassium and to lesser degree, sodium salts.
- Separation of sodium and potassium salts
The solubility of potassium perchlorate, KClO4, is about 10.6 grams per liter at 10° celsius. However, the solubility of sodium perchlorate, NaClO4, is a whopping 1,830 grams per liter at that temperature. We can use this physical property to separate out the potassium from the solution generated in step 4. In fact, practically all perchlorates are quite soluble in water except for potassium, rubidium, and cesium perchlorate which can be helpful when needed to separate them out.
Most of the potassium will be in the form of potassium carbonate, K2CO3. By adding ammonium perchlorate, we have the following four compounds in equilibrium:
2NH4ClO4 + K2CO3 <-> 2KClO4 + (NH4)2CO3
By taking advantage of Le Chatelier’s principle and removing the potassium perchlorate via precipitation in cold water, we can drive that equilibrium to the right. This works better if the water level is reduced to a minimum first, as Cody does in the video.
- Decomposition of potassium perchlorate
Potassium perchlorate will mostly decompose to KCl at 400°-500° Celsius, although some papers report some KClO3 being formed, and also that the decomposition is sometimes incomplete. Since KClO4 readily reacts with organic material, this decomposition is best performed with no contaminants.
Reaction: KClO4 + heat -> KCl + 2O2(g)
- Reduction of potassium chloride and distillation of potassium metal with lithium
Being able to isolate alkali metals in their elemental form outside of an industrial setting is the holy grail for many scientists. A select few have been able to have success in this area, by taking advantage of three facts. First, that the standard oxidation potential of lithium metal is greater than that of the other alkali metals. Two, that lithium metal is more readily available, and three – that the other metals have a lower boiling point than lithium.
From a practical point of view, this means that a molten mass of lithium metal and NaCl, KCl, RbCl, or CsCl can react in an oxidation-reduction reaction. The reaction can be driven towards the reduction of the lithium ion, if the sodium, potassium, rubidium, or cesium metal can be vaporized and therefore removed from the equilibrium. This process can be extremely difficult and dangerous when dealing with such reactive metals at high temperatures. However, Cody engineered a device specifically for this purpose, using molten tin as a safety buffer, and it was used to convert the KCl from step 6 into pure potassium metal (which was then shaped into a banana). For a safer reaction you can try at home, read about copper replacing zinc.
Reaction: KCl + Li -> K(g) + LiCl
- Oxidation of potassium metal with hydrogen oxide
Potassium will be oxidized to potassium hydroxide immediately upon contact with water. Or in this case, a nearby pond or creek. Although KOH is dangerous to life in high concentrations, adding such an insignificant amount to a large body of water will have no effect on the pH of the water and may even improve conditions for the local denizens of the ecosystem.
Reaction: 2K + 2H2O -> 2KOH + H2(g)
The reason that potassium can explode upon contact is water is very interesting. People usually attribute it to ignition of the hydrogen gas produced, but it is actually from the rapid formation of positively charged alkali ions forming along visually stunning spikes, which energetically repel each other, causing a “Coulomb explosion”.
And that’s the chemistry behind getting potassium metal from bananas. If you want to see the process of how to extract potassium from bananas for yourself, you can watch the video here. Let us know in the comments if this type of chemistry excites you.
Photo credit of the header images goes to Dennis SK.