671:, which is an apparent water input that can confuse the measurement. The glass walls of the vessel adsorb water, and if any water leaks into the cell, the slow release of water into the titration solution can continue for a long time. Therefore, before measurement, it is necessary to carefully dry the vessel and run a 10–30-minute "dry run" in order to calculate the rate of drift. The drift is then subtracted from the result.
546:
20:
664:, the measuring range is from 1–5 ppm to about 5%. Volumetric KF readily measures samples up to 100%, but requires impractically large amounts of sample for analytes with less than 0.05% water. The KF response is linear. Therefore, single-point calibration using a calibrated 1% water standard is sufficient and no calibration curves are necessary.
295:
674:
KF is suitable for measuring liquids and, with special equipment, gases. The major disadvantage with solids is that the water has to be accessible and easily brought into methanol solution. Many common substances, especially foods such as chocolate, release water slowly and with difficulty, requiring
690:
KF is suitable for automation. Generally, KF is conducted using a separate KF titrator, or for volumetric titration, a KF titration cell installed into a general-purpose titrator. There are also oven attachments that can be used for materials that have problems being analyzed normally in the cell.
659:
KF has a high accuracy and precision, typically within 1% of available water, e.g. 3.00% appears as 2.97–3.03%. Although KF is a destructive analysis, the sample quantity is small and is typically limited by the accuracy of weighing. For example, in order to obtain an accuracy of 1% using a scale
192:
This elementary reaction consumes exactly two molar equivalent of water vs. iodine. Iodine is added to the solution until it is present in excess, marking the end point of the titration, which can be detected by potentiometry. The reaction is run in an alcohol solution containing a base, which
541:{\displaystyle {\begin{aligned}{\ce {2 I-}}&\longrightarrow {\ce {I2 + 2 e-}}\\{\ce {B}}\!\cdot \!{\ce {I2 + B}}\!\cdot \!{\ce {SO2 + B + H2O}}&\longrightarrow {\ce {2 BH+I^- + BSO3}}\\{\ce {BSO3 + ROH}}&\longrightarrow {\ce {BHRSO4}}\end{aligned}}}
554:
method. A second pair of Pt electrodes is immersed in the anode solution. The detector circuit maintains a constant current between the two detector electrodes during titration. Prior to the equivalence point, the solution contains
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The popularity of the Karl
Fischer titration (henceforth referred to as KF) is due in large part to several practical advantages that it holds over other methods of moisture determination, such as accuracy, speed and selectivity.
585:
The volumetric titration is based on the same principles as the coulometric titration, except that the anode solution above now is used as the titrant solution. The titrant consists of an alcohol (ROH), base (B),
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Little sample preparation is needed: a liquid sample can usually be directly injected using a syringe. The analysis is typically complete within a minute. However, KF suffers from an error called
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The important aspect about the oven attachment is that the material doesn't decompose into water when heated to release the water. The oven attachment also supports automation of samples.
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may be installed to the cell in order to break the sample. KF has problems with compounds with strong binding to water, as in water of hydration, for example with
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KF is selective for water, because the titration reaction itself consumes water. In contrast, measurement of mass loss on drying will detect the loss of
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Using volumetric titration with visual detection of a titration endpoint is also possible with coloured samples by UV/VIS spectrophotometric detection.
83:
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with the typical accuracy of 0.2 mg, the sample must contain 20 mg water, which is e.g. 200 mg for a sample with 10% water. For
652:) means that redox-active sample constituents may react with the reagents. For this reason, KF is unsuitable for solutions containing e.g.
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Fischer, Karl (1935). "Neues
Verfahren zur maßanalytischen Bestimmung des Wassergehaltes von Flüssigkeiten und festen Körpern".
622:. The titration reaction proceeds as above, and the end point may be detected by a bipotentiometric method as described above.
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additional efforts to reliably bring the total water content into contact with the Karl
Fischer reagents. For example, a
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immersed in the anode solution of the main compartment. The two compartments are separated by an ion-permeable membrane.
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from the KI when current is provided through the electric circuit. The net reaction as shown below is oxidation of
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The elementary reaction responsible for water quantification in the Karl
Fischer titration is oxidation of
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Simple
Modification of Karl-Fischer Titration Method for Determination of Water Content in Colored Samples
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and reach the end point can then be used to calculate the amount of water in the original sample.
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appears and an abrupt voltage drop marks the end point. The amount of charge needed to generate I
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solution plus the analyte. The anode solution consists of an alcohol (ROH), a base (B),
796:"ASTM E203 – 16 Standard Test Method for Water Using Volumetric Karl Fischer Titration"
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816:. Journal of Analytical Methods in Chemistry, Vol. 2012, Article ID 379724.
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55:. Today, the titration is done with an automated Karl Fischer titrator.
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290:. In other words, 2 moles of electrons are consumed per mole of water.
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The titration cell also consists of a smaller compartment with a
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Chemical method to determine trace amounts of water in a sample
183:{\displaystyle {\ce {2 H2O + SO2 + I2 -> H2SO4 + 2 HI}}}
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in a sample. It was invented in 1935 by the German chemist
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The main compartment of the titration cell contains the
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830:EMD Chemicals AQUASTAR Tech Notes
230:diethylene glycol monoethyl ether
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1:
614:is consumed for each mole of
593:and a known concentration of
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776:. Royal Society of Chemistry
626:Disadvantages and advantages
236:, sometimes referred to as
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552:bipotentiometric titration
770:"Karl Fischer's Titrator"
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254:The Pt anode generates
23:A Karl Fischer titrator
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33:Karl Fischer titration
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205:Coulometric titration
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743:Angew. Chem
662:coulometers
559:but little
41:coulometric
839:Categories
728:References
715:Literature
201:produced.
45:volumetric
845:Titration
780:3 January
704:Titration
517:⟶
466:−
443:⟶
395:⋅
369:⋅
353:−
324:⟶
314:−
242:imidazole
141:⟶
37:titration
698:See also
639:volatile
602:Pyridine
234:methanol
249:cathode
226:ethanol
74:) with
76:iodine
669:drift
522:BHRSO
232:, or
211:anode
49:water
782:2023
685:DMAc
197:and
751:doi
636:any
509:ROH
494:BSO
475:BSO
268:by
43:or
27:In
841::
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643:SO
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588:SO
452:BH
401:SO
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219:SO
177:HI
158:SO
113:SO
78::
69:SO
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