For titrations involving iodine, starch is commonly used as an indicator. Iodine with starch generates a dark blue complex. In iodimetry, the endpoint corresponds to a rapid color change from red to blue. In iodometry, the appearance or disappearance of elementary iodine indicates the equivalence point.
The titration is based on the reaction between a known-concentration titrant and an unknown-concentration sample analyte, in which an indicator is applied to detect the endpoint. Acid-base, redox, precipitation, and complexometric titrations are the four major forms of titration.
When a solute (oxidizing agent) is added to excess iodide to acquire iodine, the amount of iodine produced is determined by titration with a sodium thiosulfate solution, which is known as iodometry.
It is a method of volumetric chemical analysis based on indirect titration. The iodometric titration experiment or practical is performed to determine the concentration of solute is used to produce iodine. This testing is more common when the amount of oxidizing agents in a water body needs to be determined.
Frequently Asked Questions (FAQ):
Why starch is used as an indicator in iodometric titration?
In chemistry a starch solution is used as an indicator in an iodometric titration because it can absorb the I2 that is released. When titrated with standardized thiosulfate solution, this absorption causes the solution's color to change from deep blue to pale yellow, which represents the titration's endpoint point.
What is the function of the starch indicator?
To detect the presence of iodine, a starch solution is usually used as an indicator. A dark blue starch-iodine complex is formed when starch and iodine are present together.
Why starch is used as the best indicator in iodometric titration?
Because it forms a compound with iodine rapidly, is commonly available, and is non-toxic, hence starch is suitable for iodometric titration.
Why is a starch added at the end of the reaction of iodometric titration?
Because the starch-iodide complex is not very soluble in water, therefore when the iodine concentration is low the starch is added towards the end of an iodine titration. This prevents errors caused by I2 from remaining adsorbed on the complex and going undetected.
Why do we not use starch at the beginning of iodometric titration?
If the starch was added earlier in the titration, an iodine-starch complex would form, resulting in a reddish-brown endpoint. By alkali and heat the blue color produced is also eliminated, starch indicator titrations should be performed at a pH below 08.00 and ambient temperature.
Why starch indicators should be prepared freshly?
Since starch is readily biodegradable, it must be prepared freshly. A reducing agent, glucose, is the hydrolysis product of starch. Consequently, using partially hydrolyzed starch solutions in redox titrations can be problematic.
How do you make a starch indicator solution?
To prepare starch indicator solution, take 01.00 gm of starch in 40 ml of distilled water, mix well, and make up the volume up to 100 ml distilled water. Boil it for one minute, stirring thoroughly. Allow time for the temperature to drop. Decant the supernatant and use it as an indicator solution if a precipitate forms.
What is the role of KI in iodometric titration?
Because iodide will be oxidized into iodine in the presence of an oxidation agent, hence potassium iodide is employed in iodometric titration.
What are the advantages of using starch as an indicator in iodometric titration?
As an indicator, starch can help produce a sharper color change at the endpoint. In the presence of iodide ions, starch reacts with I2 to form an intensely colored blue complex that is visible at very low iodine concentrations, making it an excellent indicator in both direct and indirect iodometric titrations.
What observations indicate a positive test for starch?
The presence of starch is indicated by a blue-black color shift (Positive result). A negative result, or yellow color, indicates a lack of starch in the solution and, thus, the reaction's completion.
References:
- Wikipedia contributors. (2022, January 2). Iodometry. In Wikipedia, The Free Encyclopedia. Available Here
- Starch indicator - definition - Encyclo. (2022). Available Here
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