Development of Karl Fischer Reagents

8. Other Salts
Key Points
Chapter II: Applications - 1
Some salts react with Karl Fischer reagent and others do not. Direct
measurement is possible with salts that do not react with Karl Fischer
reagent and that can be dissolved or dispersed in a titration solvent. As
a general rule, moisture content can be determined easily by using the
moisture vaporization method. First, however, it is necessary to take the
heating temperature into consideration because of the need to check
for such factors as intramolecular dehydration. Procedures for the
various types of salts are described below.
Nitrates can be measured directly using volumetric titration. A generalpurpose
dehydrated solvent is used. Depending on the solubility of the
substance, a solvent formulated for sugars may be used. Examples of
substances for which this method is suitable include ammonium nitrate,
sodium nitrate, potassium nitrate, chromium nitrate, cobalt nitrate and
mercury (I) nitrate.
Sodium nitrite cannot be measured directly as it reacts quantitatively
with Karl Fischer reagent as shown in the following formula.
NaNO2+0.5I2+SO2+CH3OH � NaI+NO+HSO4CH3
Chromates and bichromates also react with Karl Fischer reagent, but
the reactions are not quantitative. Potassium permanganate is insoluble
and will not react. Sodium tetraborate produces a similar reaction with
boric acid, and direct measurement is not possible.
Na2B4O7+7I2+7SO2+19CH3OH�2NaI+12HI+7HSO4CH3+4B(OCH3)3
Sodium arsenite reacts in the same way with oxides (anhydrous
arsenites.)
NaAsO2+2I2+SO2+2CH3OH � NaI+AsI3+2HSO4CH3
Primary phosphates can be measured directly using volumetric titration.
The behavior of secondary phosphates reflects their weak basicity, but
they do not cause any significant interference. Tertiary phosphates
cause interference because of their strong basicity and they must be
neutralized with acid before Karl Fischer titration.
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