Titration

General experimental setup :
above: burette with standard solution ,
below: Erlenmeyer flask (better titration flask ) with sample solution

The titration ( titrimetry , volumetry or also dimensional analysis ) is a method of quantitative analysis in chemistry . A known substance, the concentration of which is unknown (sample solution), is converted in a targeted chemical reaction with a standard solution , the concentration of which is precisely known. The volume of the standard solution used is measured and the unknown concentration of the sample solution is calculated using the stoichiometry .

The process is also possible with little equipment and is therefore used early in basic training. Since the measurement results are very precise with optimized titration methods and the titration can be easily automated, it is widely used in chemical analysis.

In medicine and pharmacology is under the process titration dose adjustments , the dose understood gradually a drug to regulate upward or Abdosierung, until optimal results reached ( dose titration , engl. Dose titration ).

The founders of titration include Joseph Louis Gay-Lussac , François Antoine Henri Descroizilles , Claude-Louis Berthollet and Louis-Nicolas Vauquelin in France at the end of the 18th and beginning of the 19th centuries.

General procedure

With a burette , a reagent of known concentration (the standard solution , also called titrator, titrant or titrans) is added dropwise to a sample solution (Titrand) until the equivalent amount is reached (also called equivalence point or end point). The endpoint detection can be done in a variety of ways using chemical and physical methods and also differentiates between the different types of titration. The volume used can be read off the burette. Before starting the actual titration, the content ( molar concentration c in [mol / l]) of the standard solution is precisely determined and a correction factor, the so-called titer , is determined in order to increase the accuracy of the measurement.

Reaction types

Titrations can be differentiated according to the type of chemical reaction .

Acid-base titration

→ Main article : Acid-base titration

An acid-base reaction occurs during titration . The endpoint can be identified by adding pH indicators and changing the color. It is also possible to measure the pH value with electrodes and to determine the end point by plotting the pH value and used standard solution.

Precipitation titration

There are precipitation reactions used to determine. The reaction of silver ions Ag ⁺ with chloride ions Cl ^- ( argentometry ) sometimes indicates the end point by agglomerating the milky precipitate (methods according to Gay-Lussac and Liebig ), sometimes supported by the addition of a dye such as eosin or fluorescein ( titration according to Fajans ) or through the formation of a colored product such as iron rhodanide (after titration according to Volhard ) or silver chromate ( titration according to Mohr ).

A special case is hydrolytic precipitation titration, in which titration is carried out with an alkali salt of a weak acid . An example of this is the determination of the total hardness with potassium palmitate solution , in which the palmitation reacts hydrolytically with water to form hydroxide ions after the equivalence point is exceeded .

Complexometric determination

→ Main article : Chelatometry

The determination is based on complex formation reactions . Dyes can be added or the color change can be monitored photometrically through the formation of a complex and thus also determined using instruments. Titration with EDTA is widespread .

Redox titration

→ Main article : Redox titration

In some cases, redox reactions can be used for the determination. Well-known methods are manganometry , iodine atomometry , bromatometry or cerimetry , each of which is named after the standard solution used.

Phase transfer

The 2-phase titration according to Epton is used to determine ionic surfactants in aqueous solution. The end point is the color change of a dye mixture in the organic chlorinated phase.

Polyelectrolyte titration

It is used to determine the cationic requirement of polyelectrolytes . As titrant is in anionic suspensions polyDADMAC and cationic Kaliumpolyvinylsulfat used.

evaluation

In order to evaluate the titration, the amount of substance n _M of the substance present in the standard solution is usually first calculated from the volume V _T of standard solution used up to the equivalence point using the formula

${\displaystyle n_{\text{M}}=c\cdot V_{\text{T}}}$

certainly. For the calculation, the so-called titer f _T (if specified) must be taken into account for the concentration :

${\displaystyle c_{\text{Ist}}=c_{\text{Soll}}\cdot f_{\text{T}}}$

In order to determine the amount of substance n _{P of} the substance to be determined in the sample solution, the equivalent numbers of the standard solution (z _M ) and the sample solution (z _P ) must be included:

${\displaystyle n_{\text{P}}={\dfrac {z_{\text{M}}}{z_{\text{P}}}}\cdot n_{\text{M}}}$

The following formula can be used to convert mass to amount of substance or vice versa:

${\displaystyle n={\dfrac {m}{M}}\ \Leftrightarrow \ m=M\cdot n}$

Endpoint detection

• chemical indicators (visual)
  • Color change of indicators
  • Precipitation (precipitation reactions)
• physical indicators (instrumental)
  • Potentiometric titration is based on the sudden change in the electrochemical potential
  • pH value measurement using a glass electrode
  • Bi amperometry
  • Conductometric endpoint determinations are based on measurements of electrical conductivity
  • Thermometric or enthalpic endpoint determinations

Types of titration

Direct titration

With direct titration , the sample solution and reagent solution are immediately reacted with one another. The sample solution is presented and titrated directly with the reagent solution. In the case of inverse titration , on the other hand, a measured amount of reagent solution is titrated with the sample solution.

Indirect titration

In indirect titration , the substance to be examined is converted in a chemical reaction before the titration. The substance to be determined is converted into a precisely defined other substance in a chemical reaction, which is then determined titrimetrically. A further distinction is made between back titration , in which the sample solution is completely converted with a certain volume of reagent solution and then the unused part of the reagent solution is determined by a titration, and substitution titration , in which a substance to be determined is initially replaced by another substance (e.g. Replacement cation) is released ("substituted"), which can then be back-titrated.

Special titrations

• Amine number
• Boehm titration
• Bromine number
• Chemical oxygen demand (COD)
• Epton titration
• Ester number ( EZ ; amount of KOH in mg, required for the saponification of esters in 1 g fat)
• Halbtitration for determining the acidity constant (pK _S ) an acid
• Iodine number
• Karl Fischer method for determining the water content
• Conductivity titration
• Neutralization number ( NZ ; amount of KOH in mg, required for the neutralization of all free acids in 1 g of fat)
• Oil demand
• Peroxide number ( POZ , amount of oxygen bound as peroxide in mval O ₂ per kg, which can easily be split off as active oxygen)
• Acid number ( AN ; amount of KOH in mg, required for the neutralization of the free organic acids in 1 g of fat)
• Saponification number
• anhydrous titration

Automated titration

Automatic titrator

The different reaction types and types of titration can be implemented in the automated laboratory reactor system. A laboratory automation system records the condition with the help of a suitable probe (pH glass electrode, conductivity probe, turbidity probe, color probe, ...) and controls the addition of the reagent solution via a dosing pump . The amount added is usually determined by automatically measuring the weight decrease in the reagent storage container.

Volumetry without titration

A particularly simple type of volumetry is used to determine the volume of a gas by displacing a corresponding volume of liquid. It should be noted that the liquid must be selected to match the gas to be determined. The nitrogen determination with an azotometer, for example, uses this by using a potassium hydroxide solution as a liquid : this absorbs the carbon dioxide and water that are also produced during the determination and allows the nitrogen volume to be read off directly on a scale on the azotometer burette.

See also

• Hägg diagrams

Individual evidence

1. ↑ Entry on titration . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.T06387 .
2. ↑ See dose titration of a drug and patient friendliness. Hanseatic Higher Regional Court Hamburg, judgment of February 11, 2010 - 3 U 122/09. Rechtslupe.de, accessed on January 8, 2013.
3. ↑ Gerhard Schulze, Jürgen Simon, Jander year measurement analysis , 17th edition, de Gruyter, Berlin, 2009, p. 2.
4. ↑ ^{a b c d} Otto-Albrecht Neumüller (Ed.): Römpps Chemie-Lexikon. Volume 2: Cm-G. 8th revised and expanded edition. Franckh'sche Verlagshandlung, Stuttgart 1981, ISBN 3-440-04512-9 , pp. 1265-1270.

literature

• Gerhart Jander , Karl Friedrich year : measurement analysis . 17th edition, de Gruyter, Berlin 2009, ISBN 978-3-11-019447-0 .
• Leo Gros, Peter A. Bruttel, Marcus von Kloeden: Practical training in titration. Metrohm AG.
• Christian Haider: Electrodes in potentiometry. Metrohm AG.
• Peter A. Bruttel: Non-aqueous titrations of acids and bases with potentiometric endpoint determination. Metrohm AG.
• Wolfgang Richter, Ursula Tinner: Practical aspects of modern titration. Metrohm AG.

Web links

Wiktionary: Titration  - explanations of meanings, word origins, synonyms, translations

Commons : Titration  - album with pictures, videos and audio files

Wikibooks: Practical course in inorganic chemistry / Basics of titrimetry  - learning and teaching materials

• Virtual titration
• "Potentiometric titrations in theory and practice" by Alfons Reichert (PDF, 860 KiB)
• Video: titration . Institute for Scientific Film (IWF) 1985, made available by the Technical Information Library (TIB), doi : 10.3203 / IWF / C-1587 .