Elemental analysis

from Wikipedia, the free encyclopedia

The elemental analysis is a branch of analytical chemistry . It is the method for determining the elements of the non-metals carbon , hydrogen , oxygen , nitrogen and sulfur contained in organic and inorganic compounds , as well as phosphorus and halogens . A distinction is made between the mere determination of the constituents ( qualitative elemental analysis ) and the determination of the percentage content or mass fraction of the elements found ( quantitative elemental analysis ).

In the case of a pure compound, you can determine its molecular formula from the percentage content of the elements with a known molecular weight . Elemental analysis is also used in research and production of chemical products to control the purity of organic and inorganic substances.

history

Organic elemental analysis The first apparatus for organic elemental analysis were developed by Antoine Laurent de Lavoisier . Joseph Louis Gay-Lussac and Louis Jaques Thénard significantly improved the apparatus, the apparatus was made smaller and potassium chlorate , later also copper (II) oxide (with Döberreiner around 1815), was used as an oxidizing agent, and copper shavings were used to prevent measurement errors due to nitrogen oxides.

Jöns Jakob Berzelius (1813–1817) was the first to use anhydrous calcium chloride to bind the water produced during the combustion process.

Five-ball apparatus

Justus von Liebig achieved a considerable improvement in elementary analysis , and his description was made available to a large readership. Liebig used glass spheres in the form of a five-ball apparatus , which contained an aqueous potassium hydroxide solution and served to bind the carbon dioxide formed, so that the carbon content of an organic compound could be determined with this apparatus. Liebig also used a multi-split coal stove and a bayonet-shaped combustion tube. Nitrogen was determined volumetrically separately. The analysis results were very precise with this apparatus and analyzes required less time than the previous apparatus.

Later improvements to the apparatus were: By Varrentrap and Will (1841), the determination of nitrogen as ammonia, by Glaser, the entry of oxygen gas into the combustion tube and the storage of the substance sample near the mouth, by Dennstedt (1900), the electrical heating, by Fritz Pregl (1912–1917), ( Nobel Prize in Chemistry 1923, for the microanalysis of organic substances he developed ).

The importance of organic elemental analysis

An organic substance must first be uniformly isolated using physical separation methods ( distillation , sublimation , chromatography , crystallization ) before an elemental analysis is carried out. Knowledge of the elementary compositions of many organic compounds made it possible to later make assumptions about the molecular formula , sometimes also the structural formula , of an organic molecule.

Without the organic elemental analysis it would not have been possible to determine the structures of organic substances. Elemental analysis was of crucial importance in the development of organic chemistry.

Description of the earlier elemental analysis

An earlier apparatus for elemental analysis consisted of a hard-to-melt glass tube (50 cm long), in the front part a small porcelain bowl with the precisely weighed material sample (approx. 0.5 g) - enclosed by two asbestos plugs - was placed, behind it - up to the end of the tube - there was copper (II) oxide. Liquid substances were filled into a small glass ball with a pointed tip (the substance then evaporates when heated). The tube was closed at the ends with pierced stoppers containing a thin glass tube. An apparatus for generating oxygen was attached to the front part and a balanced U-tube with dried calcium chloride was attached to the rear part. Behind the calcium chloride tube, the resulting gas was passed through a balanced vessel with concentrated potassium hydroxide solution. First the copper oxide was heated, then the sample was heated with a sufficient flow of oxygen.

After the material had been burned, the increase in weight was determined on the calcium chloride tube. The water released during the reaction was bound by calcium chloride. The hydrogen in the water comes from the organic compound. The weight gain of the potassium hydroxide solution is based on the uptake of carbon dioxide. The carbon in the carbon dioxide comes from the organic compound.

The hydrogen content of the water when the organic sample is burned can easily be calculated:

Hydrogen content: 2.02: 18.0 = 0.1119

This factor had to be used to multiply the weight gain in the calcium chloride tube (the resulting water) after combustion in order to obtain the part by weight of hydrogen of the organic compound.

For carbon it results analogously:

Carbon content: 12.01: 44.0 = 0.2729

The multiplied amounts of weight can then be related to the initial weight, the difference gives the oxygen, nitrogen, phosphorus and halogen content of the compound.

To determine the ratio formula , the respective weight of the individual elements was divided by the atomic weight of the element. If you now search for integer multiples for each element in the ratio formula , you get the sum formula or also the molecular formula.

Digestion Methods

Depending on the element to be determined, various laboratory methods have been developed; nitrogen is usually determined as ammonia by titration ; there are special reaction vessels ( Kjeldahl flasks , Fresenius flasks ) with which losses are avoided which would otherwise falsify the analytical result.

Combustion methods

The current state of the art for CHNS analysis in this area is so-called combustion analysis. In this case, the sample to be determined is first precisely weighed in with a balance and then catalytically burned with pure oxygen at high temperatures (up to 1800 ° C using reaction exotherms).

Immediately afterwards, the combustion gases (oxidation products) formed are fed with the help of a carrier gas (mostly pure helium) over a copper or tungsten contact (as chips or granules) at a temperature of approx. 600 - 900 ° C and the nitrogen oxides (NO x ) contained in the gas flow are completely added molecular nitrogen (N 2 ) reduced. Then the defined combustion gases (CO 2 , H 2 O, SO 2 , N 2 ) are separated in specific separation columns (so-called adsorption / desorption columns, English purge and trap ) or by gas chromatography and one after the other a thermal conductivity detector (TCD or TCD) supplied and quantified.

Since the sequence of the elements (each detected as so-called peaks) in a sample measurement is technically precisely defined with this measurement method, this allows both clear identification (qualitative determination) and the measurement signals to be measured using the peak areas (integral over time), as well as at the same time (quantitative determination) of the individual elements as C, H, N, S. With the help of the known weight, the respective mass fraction (in percent or ppm) of the elements in the analyzed sample can then be precisely calculated.

Another measurement method works instead of complete gas separation with gas-specific detectors (mostly IR detectors) for CO 2 , H 2 O and SO 2 . A thermal conductivity detector (TCD or TCD) is used here to determine the nitrogen (N 2 ). Flame ionization detectors (FID) are also used less frequently.

In laboratory analysis, a distinction is made between micro-elemental analyzers, which are optimized for small substance weights of around 0.01-10 mg, and macro-elemental analyzers, which are designed for higher substance weights of up to approx. 5 g.

Nitrogen determination

As an offshoot of the classic elemental analysis, there are also pure nitrogen analyzers based on the Dumas method for determining the nitrogen or protein content. These devices are preferably used in the analysis of agricultural products, for soil and plant analysis as well as in food analysis. In these devices, the molecular nitrogen N 2 (after separation and / or absorption of interfering gases such as water, SO 2 and possibly CO 2 ) is also quantified on a thermal conductivity detector (TCD or TCD); no further gas separation or additional detectors are required required. As a carrier gas may alternatively z to pure helium here. B. CO 2 can also be used.

Oxygen determination by high temperature pyrolysis

In contrast to the CHNS determination, the oxygen content in a sample is determined quantitatively under inert or reductive conditions (only pure helium or forming gas as carrier gas) at high temperatures (mostly approx. 1200 to 1400 ° C) on a finely divided carbon contact (gas soot) Carbon monoxide (CO) is formed. As in the CHNS analysis, this CO is then separated from the nitrogen N 2, which is also produced during the pyrolysis, via a specific separation column or GC column and measured and quantified on a thermal conductivity detector (TCD or TCD). Alternatively, the CO quantification can e.g. B. can also be done using a CO-specific IR detector.

Web links

Individual evidence

  1. ↑ Note: for better linguistic delimitation, a distinction is made in the chem. Analysis in general the term elemental analysis of elemental analysis : elemental analysis means the organic elemental analysis of the non-metals CHNS and O described here , less often P and halogens, while elemental analysis means the analysis of other elements of the periodic table (e.g. . Metals) as they are z. B. can be recorded with the ICP or AAS in atomic spectroscopy .
  2. ↑ Pogg. Ann. 21 (1831), 1-43.
  3. ↑ Since tungsten can interfere with the measurement of sulfur, it is only suitable as a reducing agent for CHN analysis. Therefore, copper is used to measure (CHN) S.
  4. ↑ This form of the reduction of nitrogen oxides at a hot copper contact to N 2 goes back to the work of Jean Baptiste Dumas and is also known in analytics as the Dumas method .

literature

  • Justus Liebig: Instructions for the analysis of organic bodies. Vieweg, Braunschweig 1837. Facsimile
  • Justus Liebig: About a new apparatus for analyzing organic bodies, and the composition of some organic substances. In: Annals of Physics. Volume 21, 1831, pp. 1-47.