As the name implies, iodine is involved. The technique of iodometry is commonly used in experiments where the amount of oxidizing agents in a water body needs to be quantified. This makes it the most suitable method since hypochlorite or peroxide, found in bleach, is a strong oxidizing agent. The end point is determined using starch as indicator which forms a blue-black colour with iodine and fades to colourless when the reaction is complete.
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Iodometry is used to determine the concentration of oxidising agents through an indirect process involving iodine as the intermediary. In the presence of iodine, the thiosulphate ions oxidise quantitatively to the tetrathionate ions. To determine the concentration of the oxidising agents, an unknown excess of potassium iodide solution is added to the weakly acid solution. The iodine, which is stoichiometrically released after reduction of the analyte, is then titrated with a standard sodium thiosulphate solution Na2S2O3.
This suspension is a watery solution of starch with a few drops of bactericide added to prevent decomposition, as this would stop the starch behaving as an indicator. Once the bond between the iodine I2 and the helical chain of beta-amylose is formed it turns an intense blue.
Colour of the starch solution in the presence of I2. In the presence of I- ions the starch solution is colourless. Source: Istituto comprensivo di Tubirgo Important considerations Iodometric titration needs to be done in a weak acid environment which is why we need to remember that: 1.
Sodium thiosulphate needs a neutral or weak acid environment to oxidise with tetrathionate in an alkaline solution we would get sulphate oxidation ; 3. In a strong acid environment thiosulphate decomposes to S2; 4. Oxidation is a chemical process which is catalysed by various factors presence of oxygen, levels of unsaturation in the oil, presence of metals, temperature and leads to the formation of hydroperoxides.
Determining the concentration of these chemicals is important because hydroperoxides have a negative effect on the acceptability of the fat matrix used, and on the food fried in it, and they also decompose easily, forming molecules which are dangerous for human health.
Reaction of radicals responsible for formation of hydroperoxides in edible fats and oils. The reaction is illustrated as the sum of the two half-reactions in fig. The iodine released is titrated using sodium thiosulphate at a known concentration with a starch indicator blue colour.
The number of equivalents of titrated iodine is the same as the number of hydroperoxides present in the sample as shown in the reaction in fig. Thiosulphate is added until the blue colour disappears and the solution turns colourless. The turning point indicates that all the iodine released has been titrated.
Iodimetry The term iodimetry, on the other hand, refers to titration using an iodine solution and is useful for determining substances that have reducing properties.
Standard iodine solutions are of fairly limited use compared to oxidants because of their small electrode potential. Application of iodometry One interesting application of iodometry in the food industry is for determining sulphur dioxide SO2 in wine. Sulphur dioxide has several important functions: regulates the fermentation of the alcohol; acts as an antioxidant; is useful for stopping the fermentation of the must; aids in the extraction of polyphenolic substances; lowers the fermentation temperature.
Forms of SO2 in wine Once sulphur dioxide is added to wine it does not remain free but oxidates in part and in part combines with other molecues: Free SO2: found as such, or in the form of sulphurous acid H2SO3 or potassium bisulphite, which is less efficient than gaseous sulphur dioxide and has no smell. The free form either as a gas or an acid is the most important because it inhibits the action of microoganisms and acts as an antioxidant.
Oxidated sulphur dioxide appears in the form of sulphur trioxide SO3 , sulphuric acid or potassium bisulphate. The SO2, therefore, can combine with sugars, proteins and polyphenols. Combined sulphur dioxide is in equilibrium with the free form.
This means that any reduction in the free form will result in a significant quantity of the combined form moving towards the free form. Thi is another of the advantages of using sulphur dioxide because it guarantees the stability of the product over time. Principles of the method The total sulphur dioxide in the wine sample is determined through direct iodimetric titration using starch as the indicator.
With red wines it is difficult to see the equivalence point because their intense red colour makes it difficult to perceive the colour change in the indicator. Formula for the determination of sulphur dioxide Le lezioni del Corso.
Difference Between Iodometry and Iodimetry
Summary What is Iodometry? In iodometry, iodides react with another oxidizing agent in an acidic medium or neutral medium. When this reaction takes place, iodide we add iodide in the form of KI oxidizes to iodine and the other species will undergo reduction by iodide. Then we can titrate the released iodine with another species. This titrating species is a standard solution of a reducing agent, which is capable of reducing iodine back to iodide form.