![]() Sodium thiosulphate is used to reduce Iodine back to Iodide before the Iodine can complex with the Starch to form the characteristic blue-black colour. ![]() This clock reaction uses Sodium, Potassium or Ammonium persulfate to oxidise iodide ions to iodine. When the bisulphite is fully consumed, the iodine will survive (i.e., no reduction by the bisulfite) to form the dark blue complex with starch. However, the iodine is reduced immediately back to iodide by the bisulfite: The iodate in excess will oxidize the iodide generated above to form iodine: In this protocol, iodide ion is generated by the following slow reaction between the iodate and bisulfite: Adding more thiosulfate will have the opposite effect it will take longer for the blue color to appear.Īn alternative protocol uses a solution of iodate ion (for instance potassium iodate) to which an acidified solution (again with sulfuric acid) of sodium bisulphite is added. Increasing the pH, or the concentration of iodide or hydrogen peroxide will shorten the time. Once the thiosulfate ion has been exhausted, this reaction stops and the blue color caused by the triiodide - starch complex appears.Īnything that accelerates the first reaction will shorten the time until the solution changes color. When the solutions are mixed, the second reaction causes the triiodide ion to be consumed much faster than it is generated, and only a small amount of triiodide is present in the dynamic equilibrium. I 3 -(aq) + 2 S 2O 3 2-(aq) → 3 I -(aq) + S 4O 6 2-(aq)Īfter some time the solution will rapidly change color to a very dark blue, almost black. In the second, fast reaction, triiodide is reconverted to iodide by the thiosulfate. In the first, slow reaction, the triiodide ion is produced. There are two reactions occurring in the solution. To this is added a solution containing potassium iodide, sodium thiosulfate, and starch. This reaction starts from a solution of hydrogen peroxide with sulfuric acid. 3 Persulfate/Peroxydisulphate variation.This instant darkening effect, reminiscent of the sudden onset of night in Egypt, gave this reaction its poetic name of “Egyptian Night. When the forming iodine has already reacted with all of the available sodium thiosulfate, it begins to react with starch, forming a dark-blue starch-iodine complex. The solution remains clear because the forming iodine reacts with sodium thiosulfate: Hydrogen peroxide oxidizes iodide ions to molecular iodine: Several oxidation-reduction reactions take place in the solution, which can be described as follows Colorless solutions are mixed and, following a short pause, the liquid rapidly turns dark blue, almost black. The “Iodine Clock” reaction (sometimes called “Egyptian Night”) is an oxidation-reduction reaction. The volume of solution C is variable – adding more or less of solution C will affect the speed of the reaction. The experiment: combine 100 mL of solution A and 50 mL of solution B in a pitcher. ![]() Preparing a hydrogen peroxide solution (solution C): combine 400 mL distilled water, 100 mL 15% hydrogen peroxide solution, and 5 mL concentrated sulfuric acid in a 500 mL beaker. Add 500 mL distilled water and stir thoroughly. Preparing a solution of sodium thiosulfate and potassium iodide (solution B): combine 1 g sodium thiosulfate and 5 g potassium iodide in a 500 mL beaker. Add 400 mL freshly-boiled water and stir. Preparing a starch solution (solution A): Combine 5 g starch and 100 mL cold distilled water in the 500 mL beaker.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |