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Literatur-Referate Karl-Fischer-Titration
• Interference-free coulometric titration of water in lithium
bis(oxalato)borate using Karl fischer reagents based on N-methylformamide
William Larsson, Jan-Christoph Panitz, Anders Cedergren
A non-alcoholic coulometric reagent based on N-methylformamide
(NMF) was shown to eliminate the severe interference effect caused
by the alcohol component of the conventional Karl Fischer (KF) reagent
on the battery electrolyte lithium bis(oxalato)borate (LiBOB). For sample
amounts up to 240 µg of water, the stoichiometry of the KF reaction deviated
only slightly from the ideal 1:1 ratio for the best reagent composition.
Both solid and dissolved (in acetonitrile, tetrahydrofuran (THF) and
ethylene carbonate/ethyl methyl carbonate) LiBOB were titrated successfully
using a Metrohm 756 KF Coulometer with a diaphragm cell.
The detection limit was estimated to be 0.5-1 µg of water using 100 mL
of reagent in this system.
Talanta 69 (2006) 276–280
• Coulometric Karl Fischer titration of trace water in diaphragm-free
cells
William Larsson, Anders Cedergren, Naoki Matsuda
Factors influencing the accuracy and precision of diaphragm-free Karl
Fischer coulometric determinations of low µg-amounts of water have
been studied using the Metrohm 756 (pulsed current) coulometer and
eight different types of commercial coulometric reagents and some
modifications of these. As in the case of diaphragm-free coulometric
titration of large amounts of water, the positive errors, due to the formation
of oxidizable reduction products (of sulfur dioxide) in the cathode
reaction (besides hydrogen), were found to be minimized by the use of
highest possible pulse current (in the range 100-400 mA) in combination
with the fastest possible titration rate. Most accurate (102-103%)
and precise results (typical relative standard deviation 1.8%) were obtained
for reagents containing very large concentrations of imidazole
in combination with the presence of modifiers like hexanol, chloroform
and propylene glycol (i.e. the HYDRA-POINT reagents). Similar results
were obtained when this type of reagent was mixed 60/40 with xylene
according to the ASTM recommendation for water determinations in petroleum
products like crude and lubricating oils. Addition of decanol to
this type of reagent mixture was found to reduce the influence from the
oxidative reduction products significantly. A reduction of the error from
3.6% relative to 1.6% was achieved by addition of 9% (v/v) of decanol to
a 60/40 reagent mixture of HYDRA-POINT Coulometric Gen (containing
hexanol as modifier) and xylene. For larger concentrations of decanol
the pulse current had to be lowered to 100 mA and this might explain
why no further improvement was observed. An additional attempt to
minimize the interference by lowering the concentration of sulfur dioxide
in the reagents gave no significant effect. However, by means of a
home-built computer-controlled coulometric instrumentation based on
continuous instead of pulsed current (including a large cathodic current
density) it was possible to achieve recovery rates close to 100% for
the best reagents investigated. The reason for this improvement is discussed.
Talanta 65 (2005) 1349–1354
• Assessment and assurance of quality in water measurement by
coulometric Karl Fischer titration of petroleum products
Pavlina G. Ivanova, Zara V. Aneva
The precise and accurate determination of water in petroleum products
– gasoline, diesel oil and aromatic hydrocarbons – is of significant
importance for their normal functional and operational characteristics
for economics as well as in meeting international requirements.
The standard method ISO 12937 for the measurement of water by the
coulometric Karl Fischer titration method was subject to within-laboratory
assessment. Thereby it was established that the analytical procedure
could not always be relied upon, especially when low levels of
water were determined. The aim of this work is to assess each stage
of the test procedure depending on the particular problems defined
by the samples, using high-reactive Karl Fischer reagents and coulometric
cells with and without a diaphragm, and to indicate the major
factors causing the uncertainty. A reduced combined uncertainty was
achieved by modifying the sample preparation step, the administration
of the moisture in the coulometric cell, and sample injection manipulation.
Experiments showed that the cell without the diaphragm ensures
lower and uniform uncertainty of the measurements in the range of
0.01 to 10 mg H 2 O in comparison to the cell with a diaphragm and
therefore is more appropriate for the determination of low water levels
in the petroleum products. A procedure that utilized a mixture of
the reagent and toluene (10:6) was able to resolve the problem of two
phases of formation and reduced conductivity of the reagent in direct
titration of diesel oils. The results suggest that the modified procedure
ensures relative expanded uncertainty equal to or less than 2.0%
(n=5, confidence interval close to 95%) and 99.4% recovery for petroleum
products investigated in repeatability conditions.
Accreditation and Quality Assurance 10 (2006) 543–549
Literatur-Referate potentiometrische Titration
• Development and validation of an alternative titration method for
the determination of sulfate ion in indinavir sulfate
Breno de Carvalho e Silva, Lígia Maria Moreira de Campos, Gerson
Antônio Pianetti
A simple and rapid precipitation titration method was developed and
validated to determine sulfate ion content in indinavir sulfate raw material.
0.1 mol/L lead nitrate volumetric solution was used as titrant employing
potentiometric endpoint determination using a lead-specific
electrode. The United States Pharmacopoeia Forum indicates a potentiometric
method for sulfate ion quantitation using 0.1 mol/L lead perchlorate
as titrant. Both methods were validated concerning linearity,
precision and accuracy, yielding good results. The sulfate ion content
found by the two validated methods was compared by the statistical
t-student test, indicating that there was no statistically significant difference
between the methods.
Química Nova 28 (2005) 54–56
30 Metrohm Information Heft 2/2006