Iodometry

Iodometry is a chemical analysis method which is used for the quantitative determination of various substances. It belongs to the titrimetric method of analysis and is based on the conversion of iodide ion (I- ) in iodine ( I2) or vice versa, that the following equilibrium reaction:

With the Iodometry can be determined quantitatively both I- oxidizing and a reducing effect on I2 analytes. The iodometric must be clearly differentiated from the Iodatometrie, in the latter case the redox properties of iodate ions are used.

Determination of reducing action analytes and Preparation of the iodine standard solution

Reducing action analytes can reduce I2 to iodide ions. The higher the amount of substance of analyte in the sample, the more iodine may be reduced. The determination of such analytes can be done either by direct titration or by back-titration.

Direct titration

In the direct titration of the sample is titrated directly with Iodmaßlösung. From the volume consumption of iodine solution until the equivalence point is on the analyte using the stoichiometric laws are recalculated. A direct titrations offers when the analyte reacts completely and rapidly with iodine. Thus, an accurate detection of the equivalence point is possible.

Determination by back-titration

The determination of reducing action analytes can also be done using a back titration. For this, the sample is mixed with a defined excess ( ie known concentration and known volume ) of iodine standard solution. After brief standing can then be determined by titration with sodium thiosulfate solution, the unreacted residual amount of iodine. Such back-titration offers when the analyte reacts only slowly with iodine and therefore can not be directly titrated with it. An example is the determination of mercury ( I) chloride, which reacts with an iodine / iodide solution, a complex compound:

Similarly, mercury can be determined (II ) salts.

Preparation of the standard solution and titer determination

Iodine itself is only poorly soluble in water. Therefore, in the preparation of the standard solution, the iodine is added to a solution of potassium iodide. In this salt solution, iodine dissolves much better with the formation of triiodide ion:

Since the precise weighing of iodine is difficult because of the high vapor pressure of the volatile character or on the weighing pan, which is often made ​​by means Iodmaßlösung potassium iodate and potassium iodide. For this to exact mass of ponderable well Kaliumiodats is added to a solution with an excess of potassium iodide. After acidification forms in a comproportionation the desired content of iodine. The iodine standard solution is unstable, where the reduction of iodine is particularly caused by light irradiation. To better protect from light, it preserves the solution expediently in amber glass containers. At regular intervals, the actual content of iodine in the standard solution should be checked. For this, an exactly weighed mass are used to arsenic (III ) oxide solution as a primary standard, which is transferred to the iodine quantitatively arsenates and arsenic acid. Due to the toxicity of arsenic compounds, the actual content may alternatively be determined with sodium thiosulfate pentahydrate as a solid or with a sodium thiosulfate solution of known strength.

Determination of oxidising analytes

Oxidizing analytes oxidise iodide ions to iodine, the resulting amount of iodine is a measure of the analyte. An excess of iodide ions, it is in this case added to the sample in order to avoid a corresponding defect, which would limit the formation of iodine. Under these conditions, the analyte can be completely react with iodide ions, the latter remains a residual amount in the sample mixture included. The higher the amount of analyte material, the higher is the resulting amount of substance of iodine. The exact ratio of analyte to arising of iodine varies depending on the analyte and can be read out from the coefficient ratio of the reaction equation. Example of the iodometric determination of Cu2 :

The molar amount of I2 is exactly half as large as the number of moles of analyte ( Cu 2 ). The resulting chemical reaction at the iodine quantity of fuel is determined quantitatively by titration. These are now iodine-containing sample in the weakly acidic pH range ( in alkaline thiosulfate is oxidized to sulfate) titrated with sodium thiosulfate solution ( Na2S2O3 ). In this case, the iodine is converted back to iodide:

At the equivalence point of this reaction is just complete, and from the volume of consumption of standard solution up to this point can be concluded that the amount of iodine present -been. The higher the consumption of standard solution, the higher the amount of substance of iodine and was ultimately the higher the amount of substance of the analyte. Since it is not directly the analyte is titrated, it is in the iodometric determination of oxidising analytes to be an indirect titration.

Indication of the equivalence point

Depending on what type is determined by analyte, is formed at the equivalence point iodine or being the last remaining iodine chemically reacted. Although dissolved iodine is a pale yellow color, but for an accurate endpoint detection ( emergence / disappearance of the yellow color ) is the intrinsic color too weak. Instead, a few drops of starch solution is added to the sample just before reaching the equivalence point. The intense blue color of the starch-iodine complex now allows accurate detection of the equivalence point.

Value of the iodometric

The iodometric is a universal process, as many analytes possess reducing or oxidizing properties. Here, however, is also the biggest problem of the method. The determination is often affected by other substances present in the sample, also reducing or oxidizing properties may have iodine or iodide ion. An example of an important field of application is the determination of the iodine value. Hereby, the number of double bonds in long chain alkene or an unsaturated fatty acid can be measured.