Iodo-Iodimetric Analysis

        Chemical reactions involving the oxidation reduction is widely used by titrimetric analysis. Ions of various elements may be present in conditions of varying oxidation, resulting in the possibility of many redox reactions. Many of these reactions are eligible to be used in titrimetric analysis and its applications quite a lot. 
The titrimetric iodometric analysis which indirectly to substances that are like iron oxidizing III, copper II, where this substance is added to oxidize iodide to form iodine. Iodine is formed will be determined through the use of thiosulfate standard solution.

       Definition of Reduction - Oxidation process of reduction - oxidation (redox) is a process involving transfer of electrons from one reactant to another reactant.
reduction
    Reduction is the capture of one or more electrons by an atom, ion or molecule.
oxidation
     Oxidation is the release of one or more electrons from an atom, ion or molecule. There are no free electrons in a chemical system, and the release of electrons by a chemical substance is always accompanied by electron capture by the other, in other words always followed by oxidation reduction reaction. In the oxidation reduction (redox) changes the valence of the substances that hold the reaction. Here occurred the transfer of electrons from a reducing pair to pair oxidizing. 
The second half reaction of a redox reaction can generally be written as follows: red → Ox + n é where red indicates the reduced form (also called a reductant or reducing agent), Ox is the oxidized form (oxidant or oxidizing agent), n is the number of electrons transferred and é are electrons. Redox reactions are widely used in titrimetric analysis of inorganic substances or organic. To set the end point on the redox titration can be done with the help of potentiometric or indicator.
Examples of redox reactions:

5Fe²⁺ + MnO₄ + 8H⁺ → 5Fe³⁺ + Mn²⁺ +4H₂O

Where: 5Fe²⁺ → 5Fe³⁺ + 5e → an oxidation reaction
           MnO₄ + 8H⁺ + 5e → Mn²⁺ + 4H₂O is a reduction reaction
Is the titrimetric iodometric analysis which indirectly to substances that are like iron oxidizing III, copper II, where this substance is added to oxidize iodide to form iodine. Iodine is formed will be determined through the use of thiosulfate standard solution.

Oxidator + MnO₄ + 8H⁺ + 5e → Mn²⁺ + 4H₂O 

KI → I₂ + 2e

I₂ + Na₂S₂O₃ → NaI + Na₂S₄O₆

  Iodimetri is a titrimetric analysis that directly used for the reducing agents or sodium thiosulfate with iodine solution or by addition of excess standard solution. Excess iodine is titrated back with thiosulfate solution.
Reductant + I2 → 2I-
Na2S2O3 + I2 → NaI + Na2S4O6

Theory of Indicators Amylum
  The indicators used in this method is an indicator of starch or amylum. Amylum is a strong indicator of iodine, which is blue when a substance containing iodine positive. Reason wearing amylum as indicators, including:
- The price is cheap
- Easily obtained
- Changes color when TAT is clear
- Reaction is spontaneous (without heating)
- Can be used at once in the iodo-iodimetri
 
While the weakness of this indicator are:
- Unstable (easily hydrolyzed)
- Easily damaged (infected bacteria)
- It is difficult soluble in water

How to make amylum indicators:
Prepare a 250ml glass beaker, fill with 100ml distilled water. Then enter the 3 grams of starch in 250 ml glass beaker. Heat while stirring until the temperature of 400C, 400C stop after stirring but still heated to a temperature of 600C. Enter the glass beaker into a black plastic bag and then put in a drawer. Let stand about 5 minutes to settle and form three layers, after taking the center of the cold solution as indicator. The reaction mechanism of the reaction mechanism is the reaction stages that describe a whole set of chemical reactions.
Iodo-iodimetri reaction mechanism: 

 2 Cu²⁺ + 4 I^- → 2 CuI + I₂

I₂ + 2 S₂O₃^2- → 2 I- + S₄O ^2-

I₂ + I^- → I₃ ^-

Amylum + I₃^- → AmylumI₃ ^- (blue)

Things Must Be Considered:

1. Titration should be carried out in the cold, in erlenmeyer without catalyst in order to reduce the oxidation of I-by O2 from the air into I2.
2. Na2S2O3 is a secondary standard solution should be standardized first.
3. The addition of the indicator at the end of titration (just before TAT).
4. Titration can not be done in a medium strong acid hydrolysis amylum because it will happen.
5. Titration can not be done in strong alkaline medium because of I2 will oxidize thiosulfate to sulfate.
6. Na2S2O3 solution must be protected from light because light helps bacterial activity that interferes thioparus. 

 
 credits: Book of Basic Praticum of Chemical Engineering 2011