And how are the chemicals properly disposed? All three reagents consist of strongly alkaline solution of heavy metal compounds. According to the Globally Harmonized System GHS , they are classified as toxic if swallowed and irritating to skin and eye.
Since they are very toxic to aquatic life, the must not be drained in the sink but have to be collected and neutralized. They identified glucosone i. The spectrum of products seems to be rather sensitive towards temperature, pH and complexing agents. Based on this assumption, the wrong conclusion for glucose is drawn, leading to the above mentioned misinterpretation.
The chemistry of these reactions is but rather complex and can hardly be dealt with in detail even at the High School level. First investigations of the oxidation of glucose by iodine in alkaline solutions date from the early 20th century. The latter is hydrolysed to gluconate under alkaline conditions. The colour disappears, if glucose is added to the solution. The active oxidizing agent in the reaction is not iodine itself but hypoiodous acid, HOI.
The latter is formed from iodine in reaction 1 : J. An investigation of the reaction rate at different pH values by Ingles und Israel supported this hypothesis.
At higher pH, c HOI is lowered by reaction 2 and 3. The iodate ion formed in 3 does not oxidise glucopyranose. At lower pH-values equilibrium 1 is shifted to the left side and thus c HOI is lowered as well. In order to keep the pH-value in a narrow range close to the pH for a maximum reaction rate, an appropriate buffer solution should be used. For the purpose of qualitative analyses, three different solutions are needed in order to perform an iodine test.
The concentrations of iodine, sodium bicarbonate and sodium carbonate can be varied, the values given here are those used for the present work. The iodine test is positive not only for reducing sugars. Several other compounds react with the iodine or the hypoiodous acid. The most prominent are given here:. The reaction of glucose and also of other reducing sugars with a weakly alkaline solution of iodine is a versatile tool, offering a variety of useful applications to chemistry in School classes and even undergraduate University courses.
If direct contact with the eyes is avoided and it is not swallowed, the iodine test solution is harmless.
Hence, no glassware is needed. Simple small beakers made of plastics are suitable reaction vessels. It is these two properties, which render the iodine test suitable even for a use in Primary School, where different foods can be tested for reducing sugars see 4.
At the High School level, reducing and non-reducing sugars can be distinguished. The reaction, following a simple and straightforward stoichiometry see Figure 1 , can be understood at that level. Also, hydrolysis of the glycosidic bond in sucrose can be investigated. As well, the increase of the rate of a chemical reaction with increasing concentration of temperature can be easily deduced.
Finally, the positive iodine test of a solution of fructose, a ketose, and its reduced rate can be rationalized in terms of the de Bruyn-van Ekenstein-rearrangement. It might be a challenge to avoid that pupils conclude A from B. In that case, the iodine test with ascorbic acid or malt sugar can demonstrate, that other compounds can be the cause for decolourisation as well. Two experiments are proposed for the above mentioned purpose. The healthy ones are free of sugar, but all chewing gums taste sweet.
How can we distinguish between them without tasting? Experimental details are given in Chapter 5. The kids are instructed to observe the different behaviour of a chewing gum containing sugar compared to the sugar-free chewing gum. It should become clear to them, that only the sugar containing chewing-gum will lead to a decolourization of the iodine test solution.
In a second stage, they should use the test to experimentally recognize sugar-containing chewing gum. Experiment 2: Is it table sugar that gives ripe grapes a sweet taste? The iodine test allows a distinction between table sugar and grape sugar. The former is a non-reducing sugar, i.
In contrast, if grape juice from ripe grapes is added, the blue colour will rapidly disappear. The same happens, if grape sugar is added. The kids should deduce from their observations that the sweet taste of the grapes cannot be caused by table sugar only.
Instead, that is what they have to be told, grapes contain grape sugar. At this stage, kids can come up with the conclusion, that chewing gums cannot contain only table-sugar. Students at High School should be able to master a more abstract view on the chemistry of simple carbohydrates. Given the name of the compound, they are supposed to deduce the correct Fischer or Haworth formula Figure 4 of the mono- and disaccharides. Subsequently, they have to decide by the criterion of a present hemiacetal group, whether it is a reducing or a non-reducing carbohydrate.
In a simplified way, the reaction of a reducing sugar with the iodine test solution see Figure 1 could be presented with the iodine molecule as the oxidizing agent, the hydroxide ion acting just as proton acceptor. Experiment 3: Reducing and non-reducing sugars.
Starch is a carbohydrate found in plants. It consists of two different types of polysaccharides that are made up of glucose units which are connected in two different ways. One is the linear amylose and the other is the branched amylopectin pictured below. Amylose is the compound that is responsible for the blue color.
Its chain forms a helix shape, and iodine can be bound inside this helix pictured below. The colors are caused by so-called charge transfer CT complexes. Molecular iodine I 2 is not easily soluble in water, which is why potassium iodide is added. Together, they form polyiodide ions of the type I n — , for example, I 3 — , I 5 — , or I 7 —. Plants make compounds called carbohydrates which have a wide variety of uses including foods and fuels. All carbohydrates contain the elements carbon, hydrogen and oxygen.
Two of the most common carbohydrates are glucose and starch. Glucose C 6 H 12 O 6 is a simple sugar unit. From the formula, you can see that it contains twice as many hydrogen atoms as carbon atoms. See the graphic below, which show four views in turning from a the side to an end view. Amylose in starch is responsible for the formation of a deep blue color in the presence of iodine. The iodine molecule slips inside of the amylose coil. Iodine - KI Reagent: Iodine is not very soluble in water, therefore the iodine reagent is made by dissolving iodine in water in the presence of potassium iodide.
This makes a linear triiodide ion complex with is soluble that slips into the coil of the starch causing an intense blue-black color.
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