water results, some react with water (low results), and some react with iodine (high result). Aldehydes and ketones react with methanol to produce acetals and ketals. Water is a by-product of this reaction, and its production leads to erroneously high water contents:
Acetal formation: CH3CHO + 2CH3OH → CH3CH(OCH3)2 + H2O (14.4) Ketal formation : (CH3)2CO + 2CH3OH → (CH3)2C(OCH3)2 + H2O (14.5) This can be remedied by using commercially available one-component reagents that contain 2-methoxyethanol and 2-chloroethanol rather than methanol. In the presence of SO2 and base, aldehydes, undergo what is called the bisulfite addition.
REFERENCES 227
This reaction consumes water.
Bisulfite addition: CH3CHO + H2O + SO2 + NR → HC(OH)SO3(HNR) (14.6) The kinetics of this type of reaction are slow, so this can be avoided by starting the reaction immediately after sample addition.
Many samples have redox potentials such that they can be oxidized by iodine. Therefore, the iodine in the titrant may be consumed by readily oxidizable samples that will give a false high value for the water content. Some common substances that can be oxidized by iodine are ascorbic acid, arsenite (AsO2
?
),
arsenate (AsO4
3?
), boric acid, tetraborate (B4O7
2?
), carbonate (CO3
2?
), disulfite
(S2O5
2?
), iron(II) salts, hydrazine derivatives, hydroxides (OH?), bicarbonates
(HCO3
?), copper(I) salts, mercaptans (RSH), nitrite (NO2 ?), some metal
oxides, peroxides, selenite (SeO3
2?), silanols (R3SiOH), sulfite (SO3 2?), tellurite
(TeO3
2?), thiosulfate (S2O3
2?), and tin(II) salts. For situations such as these
where the material under analysis reacts with iodine, an oven can be used to liberate the moisture from the sample, which is then carried into the reaction vessel and titrated without interference. REFERENCES
1. United States Pharmacopoeia, USP 25, Chapter _921_, Water Determination. 2. Fundamentals of the Volumetric Karl Fischer Titration, Mettler Toledo Application Brochure 26.
3. Riedel-deHaen Hydranal Manual.