Redox titration

Redox titration (also called oxidation-reduction titration) is a type of titration based on a redox reaction between the analyte and titrant.

Redox titration may involve the use of a redox indicator and/or a potentiometer.

Example

An example of a redox titration is treating a solution of iodine with a reducing agent and using starch as indicator. Iodine forms an intensely blue complex with starch. Iodine (I₂) can be reduced to iodide (I⁻) by e.g. thiosulphate (S₂O₃²⁻), and when all iodine is spent the blue colour disappears. This is called aniodometric titration.

Most often the reduction of iodine to iodide is the last step in a series of reactions where the initial reactions are used to convert an unknown amount of the analyte (the substance you want to analyse) to an equivalent amount of iodine, which may then be titrated. Sometimes other halogens than iodine are used in the intermediate reactions because they are available in better measurable standard solutionsand/or react more readily with the analyte. The extra steps in iodometric titration may be worth while because the equivalence point, where the blue turns colourless, is more distinct than some other analytical methods. and bromate R and B, and the amount of thiosulfate T, where B or T are known andR is unknown – but we have to make sure that R is less than B. (All amounts are in mole.) Then R = B -T/6

An oxidizing agent (also called an oxidant, oxidizer or oxidiser) can be defined as either:

1. a chemical compound that readily transfers oxygen atoms, or
2. a substance that gains electrons in a redox chemical reaction

In both cases, the oxidizing agent becomes reduced in the process.

In simple terms:

• The oxidizing agent is reduced.
• The reducing agent is oxidized.
• All atoms in a molecule can be assigned an oxidation number. This number changes when an oxidant acts on a substrate.
• Redox reactions occur when oxidation states of the reactants change.

Oxidizing is when Oxygen molecules are added to a compound. It then becomes Oxidized. Removing Oxygen molecules, makes it un-oxidized.

A reducing agent (also called a reductant or reducer) is the element or compound in a redox(reduction-oxidation) reaction (see electrochemistry) that reduces another species. In doing so, it becomes oxidized, and is therefore the electron donor in the redox. For example consider the following reaction:

2 [Fe(CN)₆]⁴⁻ + Cl₂ → 2 [Fe(CN)₆]³⁻ + 2 Cl⁻

The reducing agent in this reaction is ferrocyanide ([Fe(CN)₆]⁴⁻): it donates an electron, converting toferricyanide ([Fe(CN)₆]³⁻), simultaneous with the reduction of chlorine to chloride.

In organic chemistry, reduction more specifically refers to the addition of hydrogen to a molecule, though the aforementioned definition still applies. For example, benzene is reduced to cyclohexane in the presence of a platinum catalyst:

C₆H₆ + 3 H₂ → C₆H₁₂

In organic chemistry, good reducing agents are reagents that deliver H_2.

A redox indicator (also called an oxidation-reduction indicator) is an indicator that undergoes a definite color change at a specific electrode potential.

The requirement for fast and reversible color change means that the oxidation-reduction equilibrium for an indicator redox system needs to be established very fast. Therefore only a few classes of organic redox systems can be used for indicator purposes.

There are two common type of redox indicators:

• metal-organic complexes (Ex. phenanthroline)
• true organic redox systems (Ex. Methylene blue)

Sometimes colored inorganic oxidants or reductants (Ex. Potassium manganate, Potassium dichromate) are also incorrectly called redox indicators. They can’t be classified as true redox indicators because of their irreversibility.

Almost all redox indicators with true organic redox systems involve a proton as a participant in their electrochemical reaction. Therefore sometimes redox indicators are also divided into two general groups: independent or dependent on pH.

pH independent redox indicators

Indicator	E0, V	Color of Ox form	Color of Red form
2,2'-Bipyridine (Ru complex)	+1.33 V	colorless	yellow
Nitrophenanthroline (Fe complex)	+1.25 V	cyan	red
N-Phenylanthranilic acid	+1.08 V	violet-red	colorless
1,10-Phenanthroline (Fe complex)	+1.06 V	cyan	red
N-Ethoxychrysoidine	+1.00 V	red	yellow
2,2`-Bipyridine (Fe complex)	+0.97 V	cyan	red
5,6-Dimethylphenanthroline (Fe complex)	+0.97 V	yellow-green	red
o-Dianisidine	+0.85 V	red	colorless
Sodium diphenylamine sulfonate	+0.84 V	red-violet	colorless
Diphenylbenzidine	+0.76 V	violet	colorless
Diphenylamine	+0.76 V	violet	colorless
Viologen	-0.43 V	colorless	blue

pH dependent redox indicators

Indicator	E0, V at pH=0	E0, V at pH=7	Color of Ox form	Color of Red form
Sodium 2,6-Dibromophenol-indophenol or Sodium 2,6-Dichlorophenol-indophenol	+0.64 V	+0.22 V	blue	colorless
Sodium o-Cresol indophenol	+0.62 V	+0.19 V	blue	colorless
Thionine (syn. Lauth's violet)	+0.56 V	+0.06 V	violet	colorless
Methylene blue	+0.53 V	+0.01 V	blue	colorless
Indigotetrasulfonic acid	+0.37 V	-0.05 V	blue	colorless
Indigotrisulfonic acid	+0.33 V	-0.08 V	blue	colorless
Indigo carmine (syn. Indigodisulfonic acid	+0.29 V	-0.13 V	blue	colorless
Indigomono sulfonic acid	+0.26 V	-0.16 V	blue	colorless
Phenosafranin	+0.28 V	-0.25 V	red	colorless
Safranin T	+0.24 V	-0.29 V	red-violet	colorless
Neutral red	+0.24 V	-0.33 V	red	colorless