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what is the principle involved in determination of moisture
content?
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Answer Posted / shoukatalli
fundamental principle behind it is based on the Bunsen
reaction between iodine and sulphur dioxide in an aqeous
medium.
karl fischer reagent which reacts quantitatively and
selectively with water to measure moisture contents.
karl fischer reagent consist iodine ,sulphur dioxide,a base
and a solvent such as alcohol classic karl fischer reagents
contained pyridine ,a noxious carcinogen as the bas. the
reagents most frequently used today are pyridine free and
contain Imidazole or primary amine instead.
The alcohol reacts with sulfur dioxide and base to form an
alkyl sulphate salt, which is then oxidized by iodine to an
alkyl sulphate salt. This oxidation reaction consumes
water. The reactive alcohol is typically methanol.
How does it work?
Water and iodine are consumed in a 1:1 ratio on the above
reaction. Once all of the water present is consumed, the
presence of excess of iodine is detected Voltametrically by
the titrators indicator electrode. That signals the end-
point of the titration.
The amount of water present in the sample is calculated
based on the concentration of iodine in the Karl Fischer
titrating agent (i.e. titer) and the amount of Karl Fischer
reagent consumed in the titration.
The rate of reaction depends on the pH value of the solvent
or working medium.when pH is between 5 and 8 the titration
proceeds normally. However when the pH is lower than 5 the
reaction speed is very slow. On the other hand, when pH
higher is than 8, titration rate is fast,but only due to an
interfering esterification side reaction which produces
water, resulting in an vanishing eandpoint,Thus the optimal
pH range f or the Karl Fischer reaction is from 5 to 8 and
highly acidic or basic samples need to be buffered to bring
the overall pH into that range.
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