The Remarkable Chemistry of Potassium Superoxide
research conducted by Andrew Allen
research director: Dr. Mike Anderson (primary) and Dr. Bruce Mattson

Potassium superoxide is a yellow paramagnetic solid that is air and moisture sensitive.  Its formula is KO2 and because potassium is +1, the superoxide has a charge of -1.  Potassium superoxide has a number of remarkable physical and chemical properties that we present at this web page.

Part 1. Physical properties of potassium superoxide.

Potassium superoxide is a yellow solid.  If we were to leave the solid out in the air, it would soon decompose to a white solid due to either carbon dioxide or water vapor in the air. 

Potassium superoxide is paramagnetic.  The simple explanation is that with a formula of O2-2, there are simply an odd number of electrons in the anion (6 e- + 6 e- + 1 e- = 13 e- ), hence it is paramagnetic.  From a molecular orbital  explanation, the peroxide ion has a molecular orbital configuration of (sigma)s2 (sigma*)s2 (sigma)p4 (pi)43 (pi*)3.  The unpaired electron in the pi* orbital accounts for the paramagnetic behavior or potassium superoxide.  This also accounts for the yellow color of the substance. In order to demonstrate the paramagnetism of postasium superoxide, we suspended a test tube of the substance from a thread as shown below.  When we bring a neodymium magnet up close to the material, there is a noticeable attraction towards the magnet.  The small picture that below these two is a composite picture of the tube with and without the magent present.

Composite picture of the KO2-filled test tube with (foreground) and without (background) the neodymium magent present.

Part 2. Chemical properties of potassium superoxide.  

For the reactions that we are about to describe, we used the following apparatus.  We constructed a glass tube from a disposible glass pipet.  The tube was plugged on one end with some glass wool and then filled with about 0.6 g KO2 followed by another glass wool plug. The photo shows how two short lengths of latex tubing are inserted inside the pipet.  The glass tubes are connected to syringes as shown in a number of the following photos. 

Experiment Series 1. Conversion of carbon dioxide to oxygen:

2 CO2(g) + 4 KO2(s) --> 3 O2(g) + 2 K2CO3(s)

The above reaction was conducted in the following sequence of pictures.  The syringe pictured on the left contains CO2, the KO2 tube is in the middle and the receiver syringe is on the right.  Note the yellow color of KO2 changing to white K2CO3 as the CO2 is passed through the solid.  In terms of moles, we used approximately 2 mmol of CO2 and 8 mmol KO2.  Thus we expect that CO2 is the limiting reagent. The receiver syringe is expected to contain O2.

The gas produced and collected in the right syringe, expected to contain O2 is tested, along with CO2 with universal indicator.  This is a picture of the universal indicator solution that is used for the tests.

The solution above was divided evenly into two beakers as pictured below.  On the left we have the receiver syringe from the above reaction, expected to contain O2On the right we have another syringe filled with CO2

The product syringe does not change the pH of the solution, while the same amount of CO2 lowers the pH appreciably. 

Next, the product gas is tested using the traditional glowing splint test for O2, as shown in the next sequence of three pictures.

Buring splint blown out and...

... plunged into the receiver syringe, proving the presence of O2.

The limewater test.  Both CO2 and product gas (O2) are bubbled through test tubes containing Ca(OH)2(sat'd), commonly called limewater.  The solution turns cloudy if CO2 is present as the gas reacts with Ca(OH)2(sat'd) to form CaCO3(s).  The test is positive for the CO2 in the reactant syringe as shown in the left test tube, but negative for the gas in the product syringe, shown at right.

In this final test, the two gases (unreacted CO2 and product gas O2) are mixed with 6 M NaOH(aq), Oxygen does not react, however CO2 does, forming Na2CO3(aq) + H2O(l).  The result is that all of the CO2(g) "disappears" as it becomes part of the solution phase.  In this photo, the syringe on the left contained 60 mL CO2(g) + 10 mL 6 M NaOH.  Note that all of the CO2(g) has reacted and is gone.  The receiver syringe on the right
contains 60 mL O2(g) produced from the reaction + 10 mL 6 M NaOH. No reaction occurs.

In this next sequence of three pictures, NO(g), the colorless gas in the left syringe, is being passed through the KO2(s) forming red-brown nitrogen dioxide, NO2(g). The reaction is highly exothermic.

In this final sequence of five pictures, SO2(g), the colorless gas in the left syringe, is being passed through the KO2(s) forming, we presume, sulfur trioxide, SO3(g).  The reaction is highly exothermic.  More work is planned here.

In this closeup, the reaction area is shown as the dark area to the left.  The areas moves rightward as the reaction proceeds.  Red "mini-flames" are noted in the KO2 tube!  (We have not successfully captured them on film yet, however.)