Hanna Instruments HI 903 - Manual (Page 144)

9-16
METHODS OPTIMIZATION
on the type of alcohol included in the solvent. As the chain length of an alcohol’s alkyl or
substituted alkyl group increases, the alcohol’s reactivity toward ketones and aldehydes
decreases (i.e. methanol is the most reactive).
Acetal or ketal formation can be prevented by the use of methanol-free reagents specially
produced for this purpose. Reagents for aldehyde and ketone analysis replace methanol with
higher alcohols, ethers, halogenated alkanes or similar combinations.
9.3.2 Choosing and Modifying a Solvent
The solvent plays an important role in the KF titration. It must react with sulfur dioxide to
form the reactive methyl sulfite species, dissolve the sample and/or extract water, and it
should help prevent side reactions from occurring. The most common solvent is methanol.
Co-solvents can be added to increase sample solubility in one-component solvents, as long
as the mixture contains at least 20 - 30% methanol. In a two component reagent system,
50% solvent for two component system and 50% co-solvent can be used. This ensures that
there is enough sulfur dioxide and base for the Karl Fischer reaction to take place.
In general a solvent should be chosen in accordance with the sample composition:
Fats, oils and long-chain hydrocarbons have limited solubility in methanol. Co-solvents of
long-chain alcohols (n-decanol) or chloroform should used;
Carbohydrates and proteins have poor solubility in methanol, formamide can be used as a
co-solvent;
Analyzing acids or bases may take the pH outside the optimal range and additional buffering
may be required; a commercial Karl Fischer ‘Buffer’ reagent can be added or extra imidazole
can be added for acid samples and salicylic acid can be added to the solvent for basic samples;
For analysis of ketones or aldehydes, the methanol can be replaced with special “K” reagents
that contain mixtures including 2-chloroethanol, chloroform, ethanol or 1-methoxy-2-propanol.
9.3.3 Water Standards
Water standards are used to standardize the titrant and to verify the titrator’s performance
and analyst technique. Water standards are an integral part of ISO 9000, GMP, GLP and FDA
guidelines for water determination.
The most commonly utilized water standard for volumetric Karl Fischer titration is sodium
tartrate dihydrate. Available as a highly-purified, non-hygroscopic powder, sodium tartrate
dihydrate has a stable water content of 15.66 ± 0.05%. The compound is, however, sparingly
soluble in methanol requiring at least 3 minutes of stirring for complete dissolution.
If high precision or NIST traceability is required, water standards sealed in glass ampules
are also commercially available. Although they are more expensive, sealed standards come
pre-analyzed and certified by the manufacturer and are available in a wide range of
concentrations.
The experienced analyst can also use very small volumes of deionized water as a standard.
Due to the very water-sensitive nature of a Karl Fischer titration, only a few milligrams of
water are required for a typical standardization or system verification. A great deal of skill is
therefore required in determining the mass of the water introduced into the titration vessel in
order to achieve highly accurate results.