Karl Fischer titration
Encyclopedia
Titration
Titration, also known as titrimetry, is a common laboratory method of quantitative chemical analysis that is used to determine the unknown concentration of an identified analyte. Because volume measurements play a key role in titration, it is also known as volumetric analysis. A reagent, called the...
method in analytical chemistry
Analytical chemistry
Analytical chemistry is the study of the separation, identification, and quantification of the chemical components of natural and artificial materials. Qualitative analysis gives an indication of the identity of the chemical species in the sample and quantitative analysis determines the amount of...
that uses coulometric
Coulometry
Coulometry is the name given to a group of techniques in analytical chemistry that determine the amount of matter transformed during an electrolysis reaction by measuring the amount of electricity consumed or produced....
or volume
Volume
Volume is the quantity of three-dimensional space enclosed by some closed boundary, for example, the space that a substance or shape occupies or contains....
tric titration to determine trace amounts of water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
in a sample. It was invented in 1935 by the German chemist Karl Fischer
Karl Fischer (chemist)
Karl Fischer was a German chemist. In 1935 he published a method to determine trace amounts of water in samples. This method is now called Karl Fischer titration and was originally performed manually but has been automated...
.
Coulometric titration
The main compartment of the titration cell contains the anodeAnode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
solution plus the analyte. The anode solution consists of an alcohol (ROH), a base (B), SO2
Sulfur dioxide
Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...
and I2. A typical alcohol that may be used is methanol
Methanol
Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...
or diethylene glycol monoethyl ether
2-(2-Ethoxyethoxy)ethanol
2-ethanol, also known under trade names Carbitol, Carbitol cellosolve, Transcutol, Dioxitol, Poly-solv DE, and Dowanal DE, is an industrial solvent. It is a clear, colorless, hygroscopic liquid. Structurally it is an alcohol and an ether, a triethylene glycol with missing one hydroxyl group with...
, and a common base is imidazole
Imidazole
Imidazole is an organic compound with the formula C3H4N2. This aromatic heterocyclic is a diazole and is classified as an alkaloid. Imidazole refers to the parent compound, whereas imidazoles are a class of heterocycles with similar ring structure, but varying substituents...
.
The titration cell also consists of a smaller compartment with a cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
immersed in the anode solution of the main compartment. The two compartments are separated by an ion-permeable membrane.
The Pt anode generates I2 when current is provided through the electric circuit. The net reaction as shown below is oxidation of SO2 by I2. One mole of I2 is consumed for each mole of H2O. In other words, 2 moles of electrons are consumed per mole of water.
- B·I2 + B·SO2 + B + H2O → 2BH+I− + BSO3
- BSO3 + ROH → BH+ROSO3−
The end point is detected most commonly by a bipotentiometric method. A second pair of Pt electrodes are 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- but little I2. At the equivalence point, excess I2 appears and an abrupt voltage drop marks the end point. The amount of current needed to generate I2 and reach the end point can 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 above now is used as the titrant solution. The titrant consists of an alcohol (ROH), base (B), SO2 and a known concentration of I2.One mole of I2 is consumed for each mole of H2O. The titration reaction proceeds as above, and the end point may be detected by a bipotentiometric method as described above.
Advantage of analysis
The popularity of the Karl Fischer titration is due in large part to several practical advantages that it holds over other methods of moisture determination, including:- High accuracy and precision
- Selectivity for water
- Small sample quantities required
- Easy sample preparation
- Short analysis duration
- Nearly unlimited measuring range (1ppm to 100%)
- Suitability for analyzing:
- Solids
- Liquids
- Gases
- Independence of presence of other volatiles
- Suitability for automation
In contrast, loss on drying will detect the loss of any volatile
Volatility (chemistry)
In chemistry and physics, volatility is the tendency of a substance to vaporize. Volatility is directly related to a substance's vapor pressure. At a given temperature, a substance with higher vapor pressure vaporizes more readily than a substance with a lower vapor pressure.The term is primarily...
substance.
The major disadvantage 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, and require additional efforts to reliably bring the total water content into contact with the Karl Fischer reagents.
Literature
- Water determination by Karl Fischer Titration by Peter A. Bruttel, Regina Schlink, Metrohm AG, Order Number 8.026.5013 (Free Monograph)