Iodometry is a method of volumetric chemical
analysis, a titration where the appearance or disappearance of elementary
iodine indicates the end point.
Usual reagents are sodium thiosulfate as titrant, starch as an indicator (it forms blue complex with iodine molecules -
though polyvinyl alcohol has started to be used recently as well), and an iodine
compound (iodide or iodate, depending on the desired reaction
with the sample).
The principial reaction is the reduction of iodine to iodide by thiosulfate:
- I2 + 2 S2O32− → S4O62− + 2 I−
A common and illustrative use of iodometry is the measurement of concentration of chlorine
in water. Chlorine in pH under 8 oxidizes iodide to iodine. An overabundance of potassium iodide is
added to the known amount of sample in acidic environment (pH < 4, the reaction is not complete in more alkaline pH). Starch
is added, forming blue clathrate complex
with the liberated iodine. The blue solution is then titrated with thiosulfate until the blue color vanishes.
Two possible sources of error can influence the outcome of the iodometric titration. One is the air oxidation of acid-iodide solution and the other is the volatility of I2. The first one can be
eliminated by adding an excess of sodium carbonate in the reaction vessel. This removes
oxygen in the vessel by forming carbon dioxide(which is
heavier than air). The other error can be reduced by using an excess of iodide solution which captures liberated iodine to form triiodide ions, I3−.
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