Iodometric titration is synonymous with redox titration method. Iodine is a universal laboratory reagent because it reacts directly with an array of organic and inorganic substances. Since iodometric titration is a form of redox or oxidation-reduction reaction, it can accurately measure the amount of oxidising or reducing agents in a chemical reaction. Also, it can be reversed to either direction in an iodine/iodide reaction.
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Standard Iodine Solution
Iodine is a volatile, water-repellent element, but it can undergo dissolution in the presence of excess iodides, creating iodide ions. It minimises the strength of a given solution by removing diluting material (free iodine), which makes it stable enough to be used in the laboratory. So, iodine solution is prepared by soaking elemental iodine into iodide solution. Standard iodine solution cannot be easily prepared because of its high volatility and water repellency, but it can be easily normalised against sodium thiosulfate or arsenic oxide.
Sodium Thiosulfate Solution
Pure sodium thiosulfate can be prepared more easily. The exact composition of a given sample, though, is difficult to sort through because the water of hydration present in a sodium thiosulfate crystal is highly temperature and humidity dependent. Sodium thiosulfate solution is normalised against potassium dichromate or potassium iodide. Normally, sodium thiosulfate solution offers relatively low redox independent pH, so a handful of carbonate is added to maintain solution pH above 7 for an optimised result.
Iodometric Titration of Copper
In iodometric determination of copper, iodides are oxidised into iodine in the presence of copper (II) ions that are reduced to cuprous ions. This volumetric analysis is normally accomplished with sodium thiosulfate solution. The reaction should be held in an environment of ten to fifteen per cent ammonia and acetic acid, so that it can act like an acetic buffer solution. For best results, an additive (such as copper thiocyanate) is mixed in, which lowers the density of cuprous ions, increasing the oxidation power of the system. Also, the solution should not contain other substances that could be potentially oxidising or reducing agents.
End Point Detection with Starch
Though iodine is sparingly soluble in water, its presence can easily be detected by the eyes because it produces a yellowish colour in aqueous solutions. But during the final point of titration, the concentration of iodine gets diminished; as a result, the yellow coloured solution of iodine turns very pale. This could be misleading, so a starch solution is used for the end point detection.
Sources of Errors
During iodometric titration, diligent observation is required. Iodine easily evaporates at room temperatures, so the flask containing iodine should not be opened during the reaction. People performing titration should also not be hurried because they may overlook important steps.
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