In a moon cake package, there is often a packet of oxygen absorber provided with each moon cake. The oxygen absorber is usually used for food storage, because oxygen is the main substance which causes the degeneration of food.
Some food contains fat or oil. When the food is exposed in air, the fat and oil may be oxidized by oxygen. This process is called “rancidity” (The main theory of rancidity is not discussed in this report). After the rancidity, the product may be harmful to human. So oxygen absorber is needed to reduce the amount of oxygen in package.
Oxygen in air also can facilitate the growth of mold. As we all know, the mold on the food release toxic substances. It is dangerous for us to take in the food contains these toxic substance. When we use oxygen absorber, the concentration of oxygen is decreased. Low oxygen concentration can inhibit the growth of mold.
Besides, oxidation of food can cause the colour change of food. If the colour of foods changes, people will have a lower longing to have these food. Using oxygen absorber can prevent the change of colour.
When people discovered that oxygen can be the main cause of degeneration of food, scientists started to find out the solutions. This caused the fast development of oxygen absorber in the 20th century. Scientists combined two simple ideas: “Irons absorbs oxygen when it rust.” and “Prevent oxidation by eliminating oxygen.”, and then iron powder package was first acted as oxygen absorber. In 1977, the modified oxygen absorber, “AGELESS(r)(?????(r))”, was produced by a Japanese company, Mitsubishi Gas Chemical(tm). Nowadays, in the market, this oxygen absorber can be easily found in many packed food.
How can the oxygen absorber absorb oxygen in the food package? What is the composition of the oxygen absorber? In this activity, we will plan and carry out an investigation to find out the chemical nature of the oxygen absorber.
(In this project, we mainly discuss about the oxygen absorber “AGELESS(r)”, Type Z-PT, just like the picture above)
To analyse qualitatively an oxygen absorber sample taken from moon cake package by different methods.
Apparatus and Reagents Used:
Apparatus used: Watch glass; Magnet; Beaker; Test tube; Boiling tube; Delivery tube; Dropper; Crucible tongs; Test tube holder; Test tube rack; Funnel; Filter paper; Bunsen burner; Glass rod; etc.
Reagents used: Diluted Hydrochloric acid (dil. HCl);
Concentrated Hydrochloric acid (conc. HCl);
Acidified Potassium Permanganate Solution (KMnO4);
Very Diluted Nitric Acid (dil. HNO3);
Acidified Silver Nitrate Solution (AgNO3), and
Barium Chloride Solution (BaCl2)
Procedures and observations:
1. The oxygen absorber package was opened. The powder form solid was taken out. The features of oxygen absorber was observed directly first. It was found that the oxygen absorber is reddish brown and black powder.
2. A magnet was used to attract the oxygen absorber. It is found that the oxygen absorber can be attracted by magnet.
3. Dil. HCl was added to oxygen absorber in the test tube. After adding, the powder did not dissolve very well. The colour of solution just changed from colourless to very pale yellow/brown colour. After a while, the powder started to dissolve. A small amount of gas bubbles were evolved.
4. The oxygen absorber was under dry heat in a boiling tube, which was connected to limewater. After dry heating for a period of time, the limewater became milky.
5. The solution contained oxygen absorber was added into acidified KMnO4 solution. The colour of solution changed from purple to pale yellow.
6. The oxygen absorber was added into conc. HCl solution. The solution turned green quickly. White fume was emitted. After a while, some gas bubbles started to be released. Heat was also released.
7. The oxygen absorber was dissolved into dil. HNO3 first, and then the filtrate was added to acidified AgNO3 solution. White precipitate was given out.
8. The filtrate in Experiment 7 was added into BaCl2 solution. No observable change.
9. The oxygen absorber was added into acidified KMnO4 solution, and then was heated by Bunsen burner. The boiling tube was connected to limewater. Finally the limewater was turned milky.
10. Flame test was done for the oxygen absorber. It was found that golden yellow colour flame was seen.
Explanations of the observations above:
Explanation of Experiment 2:
From the observation, it can be concluded that the oxygen absorber contains some metal which can be attracted by magnet, such as iron (Fe), cobalt (Co) and nickel (Ni). By the introduction, the oxygen absorber can be deduced to contain iron powder.
Explanation of Experiment 3:
From the observations, it can be deducted that Fe in the oxygen absorber reacted with HCl to give out hydrogen gas (H2). Gas was released after a while because there was an oxide layer on the oxygen absorber, which blocked the Fe to react with HCl.
Fe + 2HCl FeCl2 + H2
Fe2O3 + 6HCl 2FeCl3 + 3H2O
Explanation of Experiment 4:
From the observation, it can be deducted that calcium carbonate (CaCO3) was formed, i.e. carbon dioxide (CO2) was produced during the reaction. The oxygen absorber may contain some carbon (C) or some carbonate (CO32-) solid, because under strong dry heat, both C and CO32- can release CO2.
C + O2 CO2
CO32- CO2 + O2-
Explanation of Experiment 5:
From the observation, it can be deducted that MnO4- disappeared, i.e. redox reaction occurred. The solution contain oxygen absorber may had reducing agent (R.A.), such as Fe2+.
5Fe2+ + 8H+ + MnO4- Mn2+ + 5Fe3+ + 4H2O
Explanation of Experiment 6:
From the observations, it can be deducted that the reaction between conc. HCl and oxygen absorber is exothermic. Conc. HCl was first reacted with oxide layer and then reacted with Fe to undergo displacement reaction.
(The chemical reaction is the same as the experiment 3)
Explanation of Experiment 7:
From the observation, it can be deducted that silver chloride (AgCl) solid formed, i.e. Cl- ion was present in the solution.
Ag+ + Cl- AgCl
Explanation of Experiment 8:
There was any observable change, it can be deducted that sulphate ion (SO42-) was absent in the solution.
Explanation of Experiment 9:
From the observations, it can be deducted that CO2 was produced in the reaction. As the temperature of solution cannot higher than 100?, thermo decomposition could not occur. Therefore, only redox reaction can take place. KMnO4 is a strong oxidizing agent (O.A.), the other reactant must be R.A., and C is the required chemical. Finally, it can be deducted that CO32- will be absent in oxygen absorber, and C was present in oxygen absorber.
Explanation of Experiment 10:
From the observation, it can be deducted that sodium or iron metal ion may be present in the oxygen absorber.
The possible composition of oxygen absorber:
From the observations of the experiments and the explanations above, the composition of the oxygen absorber can be deducted. First, it may contain Fe, which is the main active ingredient of oxygen absorber. This is the chemical reaction between Fe and O2 in air:
4Fe + 3O2 2Fe2O3
Besides, C may be one of the ingredients in the oxygen absorber. Carbon can react with oxygen to release CO2. The amount of oxygen can be reduced.
It was also found that Cl- was present in the oxygen absorber. By scientific deduction, it can be deducted that NaCl may be present in the oxygen absorber. Because the present of salt can increase the rate of rusting of iron. So the rate of oxygen absorption can also be increased.
The reason of the powder form for oxygen absorber instead of big solid form:
The powder form solid has a greater surface area than the big solid form. The rate of reaction, which is the rate of oxygen absorption in this case, can be greatly increased.
The suggestion for further work on oxygen absorber:
Undoubtedly, this kind of oxygen absorber is pretty good because it is harmless to the food, and it also has a satisfied efficiency to absorb oxygen (By the introduction about “AGELESS(r)”, this oxygen absorber can lower the concentration of oxygen to 0.1% or lower). However, when this oxygen absorber is used in food with large amount of water, the rate of rusting of Fe in the oxygen absorber is compulsively increased, the efficient time of oxygen absorber may be reduced. Therefore, I suggest that some compound which can absorb water should be added in it, such as calcium chloride (CaCl2). So the oxygen absorber can absorb more oxygen for longer time.
Errors and Improvements of experiments:
1. From the experiments above, it was found that an oxide layer was formed on the oxygen absorber powder. It was because the experimental oxygen absorber absorbs a large amount of oxygen before the experiment. This may cause the wrong deduction of the experiment. To improve this problem, in the next time, the experimental oxygen absorber should be kept in closed space, to reduce the amount of oxygen absorbed.
2. In this experiment, the present of sodium ion was supposed present in the oxygen absorber due to the observation of Experiment 10. However, the colour of flame of sodium is similar to that of iron, so it was difficult to test the present of sodium ion accurately. Therefore, the more accurate method is comparing the light emission spectrum between sodium and that of iron. In the spectroscope, we can get an accurate result by their different light spectrums.
3. Of course, the greatest method to determine the composition of oxygen absorber is to inject the oxygen absorber sample in to mass spectrometer. By the peaks in the graph plotted by mass spectrometer, we can know the composition immediately.
By the numerous experiments, the oxygen absorber may have iron powder, carbon powder and some sodium chloride salt.
1. Mitsubishi Gas Chemical Company, INC. (r)
2. doc88.com http://www.doc88.com/p-90129266292.html
3. Wikipedia Japan http://ja.wikipedia.org/wiki/%E8%84%B1%E9%85%B8%E7%B4%A0%E5%89%A4