Hanna Instruments HI 903, Быстрый старт

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9-15
METHODS OPTIMIZATION
9.3 Karl Fischer Reagent System
A wide variety of Karl Fischer reagents exist on the market today, each designed and formulated
for specific sample matrices and titration conditions. Karl Fischer reagent systems consist of a
solvent and a titrant. The solvent is the liquid to which the sample is added in the reaction
vessel. The titrant is the iodine-containing liquid pumped into the cell during the titration.
9.3.1 Reagent System Classification
Reagent systems are classified as either one-component or two-component depending on whether
the sulfur dioxide and base are included in the titrant or with the solvent. In one-component
systems, also known as composites, the titrant contains all of the reactants needed to conduct
the titration (iodine, sulfur dioxide and a base) dissolved in an alcohol or ether. In a two-
component reagent system, the solvent already contains the sulfur dioxide and the base
while the titrant is typically a solution consisting of iodine and methanol.
9.3.1.1 One-Component Reagent Systems
One-component reagents are less stable than two-component systems, typically having only a
two-year shelf life, but they provide several significant advantages. The major advantage is that
the titrant is providing the sulfur dioxide and the base. The constant supply of reaction components
from the titrant allows a high level of flexibility with respect to the chemical composition of the
solvent and provides a nearly limitless solvent capacity for water. One-component solvent systems
can be easily customized, creating mixtures specially adapted to specific sample characteristics
without having to worry about providing appropriate levels of sulfur dioxide and buffer
components. Common solvent mixtures include ethanol, chloroform, xylene, toluene, and
long chain alcohols such as hexanol and decanol.
9.3.1.2 Two-Component Reagent Systems
Two-component reagents have advantages of their own. They are more stable and have a
longer shelf life than one-component systems. The sulfur dioxide is pre-mixed in excess with
an alcohol-based solvent, therefore the necessary reactive sulfite esters are present in vast
excess prior to the start of a titration. This results in higher titration speeds and greater
accuracy for low levels of water. In addition, having the base present in excess in the solvent
prior to sample addition results in a higher solvent buffer capacity.
9.3.1.3 Reagents for Aldehydes and Ketones
The addition of a sample containing aldehydes or ketones to a methanol-based Karl Fischer
solvent results in side reactions that adversely affect titration results. When alcohols react
with the carbonyl groups of aldehydes and ketones they form acetals and ketals via a reaction
that releases water. The generation of water during a titration will falsely inflate water content
results and could lead to vanishing endpoints.
While ketones are less reactive than aldehydes, the reactivity of both species is inversely