iodimetri analitic

Definition Iodimetri

Iodimetri a direct titration method and a determination or a determination on the basis of quantitative determination is the amount of I2 which reacts with the sample or formed from the reaction between the sample with iodide ion. Iodimetri is a redox titration with I2 as penitar. In redox reactions should always be oxidizing and reducing agents, because if an element increases oxidation (electron release), then there must be an element that is reduced or decreased oxidation (electron capture), so not only might there be an oxidizing or reducing agent alone. In this analysis method, analat oxidized by I2, so I2 is reduced to iodide ion:
A (reductants) + I2 → A (Oxidized) + 2 I -
Iodine is an oxidizer that is not too strong (weak), so the only substance which is a strong reducing agent that can be titrated. Indicator used is starch which will give a blue color at the endpoint penitaran.
I2 + 2 e - → 2 I-
Iodine is a solid substance is poorly soluble in water (0.00134 mol / L) at 25 ◦ C, but very soluble in a solution containing iodide ion. iodine to form triiodide complexes with iodide:
+ I-I2-I3 →
Ions tend to be hydrolyzed to the acid iodide and hipoiodit:
Hio + I2 + H2O → H + + I-
Standard iodine solution should be stored in dark bottles to prevent decomposition by light incense.
2HIO → 2 H + + 2 I-+ O2 (g)
The color of iodine solution of 0.1 N iodine old enough to act alone as an indicator. Iodine also gave a purple or violet color in solvents such as CCl4 or chloroform, and sometimes it is used to detect the end point. But the more common use of a starch solution, because the blue color of starch-iodine complex acts as a test of sensitivity to iodine. Sensitivity was greater in the slightly acidic solution than in neutral solution and greater in the presence of iodide ion. Bound iodine molecules on the surface of beta amylose, a constituent of starch.
Iodine solution is a solution that is unstable, so it needs standardized repeatedly. As a weak oxidizing agents, iodine can not react too perfect, as it should be made conditions shift the equilibrium towards the reaction products, among others, by adjusting the pH or by adding material pengkompleks.
Iodine solution is often standardized with a solution of Na2S2O3. other than that of primary raw materials are the most widely used is the center of As2O3 on the pH, by the reaction:
I2 + 2 e-→ 2 I-E ◦ = 0.536 volts
H3AsO3 H3AsO4 + H2O → H + + 2 + 2 e-E ◦ = 0, 559 volt
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H3AsO3 H2O + I2 + H3 AsO4 → H + + 2 + 2 I-E ◦ = -0.023 volt
The reaction above shows, that in fact too weak to oxidize iodine H3AsO4. But by titrating the pH is high enough, then the equilibrium is shifted to right (H + bound formed by OH-in solution that's excessive-OH). In general, the pH is between 7 and 9, not too alkaline, because it would encourage disproportionate I2 too much. To adjust the pH, a slightly acid solution saturated with NaHCO 3 which would result in retention with pH between 7 and 8.