the karl fischer titrator uses what to determine an endpoint?
Karl Fischer titration
Terminal updated
Karl Fischer titration is a classic titration method in chemical assay that uses coulometric or volumetric titration to determine trace amounts of h2o in a sample. It was invented in 1935 by the German chemist Karl Fischer. [1] Today, the titration is washed with an automatic Karl Fischer titrator.
- Chemical principle
- Coulometric titration
- Volumetric titration
- Disadvantages and advantages
- See also
- Literature
- References
- External links
Chemical principle
The uncomplicated reaction responsible for water quantification in the Karl Fischer titration is oxidation of sulfur dioxide with iodine:
- H2O + SO2 + Iii → SOthree + 2HI
This elementary reaction consumes exactly i molar equivalent of water vs. iodine. Iodine is added to the solution until it is nowadays in excess, mark the end point of the titration, which can exist detected past potentiometry. The reaction is run in an alcohol solution containing a base, which eat the sulfur trioxide and hydroiodic acid produced.
Coulometric titration
The principal compartment of the titration cell contains the anode solution plus the analyte. The anode solution consists of an alcohol (ROH), a base (B), SOtwo and KI. Typical alcohols that may be used include ethanol, diethylene glycol monoethyl ether, or methanol, sometimes referred to every bit Karl Fischer grade. A mutual base of operations is imidazole.
The titration cell also consists of a smaller compartment with a cathode immersed in the anode solution of the main compartment. The two compartments are separated by an ion-permeable membrane.
The Pt anode generates I2 from the KI when current is provided through the electric circuit. The net reaction as shown beneath is oxidation of SO2 by I2. Ane mole of I2 is consumed for each mole of H2O. In other words, 2 moles of electrons are consumed per mole of h2o.
- 2I− → I2 + 2e−
- B·I2 + B·Thenii + B + H2O → 2BH+I− + BSOiii
- BSO3 + ROH → BHRSO4
The end point is detected most commonly by a bipotentiometric titration 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 I− merely fiddling Itwo. At the equivalence point, excess I2 appears and an precipitous voltage drib marks the end point. The amount of accuse needed to generate I2 and achieve the cease betoken tin then be used to calculate the amount of water in the original sample.
Volumetric titration
The volumetric titration is based on the same principles as the coulometric titration, except that the anode solution in a higher place now is used as the titrant solution. The titrant consists of an alcohol (ROH), base of operations (B), SO2 and a known concentration of I2. Pyridine has been used every bit the base in this case.
1 mole of Iii is consumed for each mole of H2O. The titration reaction gain as above, and the end point may be detected by a bipotentiometric method equally described in a higher place.
Disadvantages and advantages
The popularity of the Karl Fischer titration (henceforth referred to as KF) is due in big part to several practical advantages that information technology holds over other methods of moisture determination, such every bit accurateness, speed and selectivity.
KF is selective for water, considering the titration reaction itself consumes water. In contrast, measurement of mass loss on drying will detect the loss of any volatile substance. Yet, the strong redox chemistry (Then2/Iii) means that redox-agile sample constituents may react with the reagents. For this reason, KF is unsuitable for solutions containing due east.g. dimethyl sulfoxide.
KF has a high accurateness and precision, typically inside one% of bachelor water, e.m. 3.00% appears every bit ii.97–three.03%. Although KF is a destructive analysis, the sample quantity is minor and is typically limited by the accuracy of weighing. For case, in order to obtain an accuracy of 1% using a scale with the typical accuracy of 0.2mg, the sample must contain 20mg h2o, which is east.g. 200mg for a sample with 10% h2o. For coulometers, the measuring range is from 1–5ppm to about 5%. Volumetric KF readily measures samples upwardly to 100%, simply requires impractically large amounts of sample for analytes with less than 0.05% water. [two] The KF response is linear. Therefore, single-point calibration using a calibrated one% h2o standard is sufficient and no scale curves are necessary.
Little sample preparation is needed: a liquid sample can ordinarily be directly injected using a syringe. The assay is typically complete inside a infinitesimal. Notwithstanding, KF suffers from an fault called drift, which is an apparent water input that tin can confuse the measurement. The glass walls of the vessel adsorb h2o, and if whatever water leaks into the cell, the slow release of water into the titration solution tin continue for a long fourth dimension. Therefore, before measurement, it is necessary to carefully dry out 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 outcome.
KF is suitable for measuring liquids and, with special equipment, gases. The major disadvantage with solids is that the h2o has to be attainable and easily brought into methanol solution. Many mutual substances, especially foods such as chocolate, release water slowly and with difficulty, requiring additional efforts to reliably bring the total h2o content into contact with the Karl Fischer reagents. For case, a high-shear mixer may be installed to the prison cell in social club to break the sample. KF has problems with compounds with strong binding to water, as in h2o of hydration, for example with lithium chloride, so KF is unsuitable for the special solvent LiCl/DMAc.
KF is suitable for automation. Generally, KF is conducted using a separate KF titrator, or for volumetric titration, a KF titration prison cell installed into a general-purpose titrator. There are also oven attachments that tin can be used for materials that have problems beingness analyzed ordinarily in the cell. The of import aspect about the oven attachment is that the cloth doesn't decompose into h2o when heated to release the h2o. The oven zipper also supports automation of samples.
Using volumetric titration with visual detection of a titration endpoint is also possible with coloured samples by UV/VIS spectrophotometric detection. [three]
See also
- Titration
- Wet analysis
Literature
- H2o determination by Karl Fischer Titration by Peter A. Bruttel, Regina Schlink, Metrohm AG
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References
- ↑ Fischer, Karl (1935). "Neues Verfahren zur maßanalytischen Bestimmung des Wassergehaltes von Flüssigkeiten und festen Körpern". Angew. Chem. 48 (26): 394–396. doi:x.1002/ange.19350482605.
- ↑ "ASTM E203 – 16 Standard Exam Method for H2o Using Volumetric Karl Fischer Titration". www.astm.org.
- ↑ Tavčar, Eastward., Turk, Due east., Kreft, Due south. (2012). Simple Modification of Karl-Fischer Titration Method for Determination of Water Content in Colored Samples. Periodical of Analytical Methods in Chemistry, Vol. 2012, Commodity ID 379724.
External links
- EMD Chemicals AQUASTAR Tech Notes
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