CHN analyzer

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

In a pure compound can determine its empirical formula from the percentage content of the elements with known molecular mass. Furthermore, the elemental analysis in the research and production of chemical products is also used for purity control of organic and inorganic substances.

History

Organic Elemental Analysis The first apparatus for an organic elemental analysis were developed by Antoine Laurent de Lavoisier. Joseph Louis Gay -Lussac and Louis Jacques Thénard improved the apparatus significantly, the apparatus was reduced, and as an oxidant was potassium chlorate, and later copper -oxide ( with Döberreiner 1815 ) used (II ), to prevent measurement errors caused by nitrogen oxides they used copper shavings.

Jöns Jakob Berzelius (1813-1817) first used anhydrous calcium chloride to bind the generated in the incineration process water.

A significant improvement of the elemental analysis was achieved by Justus von Liebig, his description was a large reading group accessible. Liebig used glass beads, which contained an aqueous potassium hydroxide solution and were used for binding of the carbon dioxide, so that with this apparatus, the carbon content of an organic compound could be well determined. Furthermore Liebig used a multi-divided coal stove and a bayonet- shaped combustion tube. Nitrogen was determined volumetrically separated. The analysis results were very accurate with this apparatus and analysis required less time than the previous equipment.

Subsequent improvements of the apparatus were: By Varrentrap and Will (1841 ), the determination of the nitrogen as ammonia, by Glaser, the entry of oxygen gas into the combustion tube and the storage of the substance sample close to the mouth, by Dennstedt (1900), the electric heater, by Fritz Pregl (1912-1917), (Nobel prize in Chemistry in 1923, for which he developed micro- analysis of organic substances ).

The importance of organic elemental analysis

An organic substance must first be uniformly isolated before elemental analysis was performed by physical separation methods (distillation, sublimation, chromatography, crystallization). By knowing the elemental compositions of many organic compounds later guesses to the sum formula, sometimes to the structural formula of an organic molecule could be made.

Without the organic elemental analysis, it would not have been possible to determine the structures of organic materials. For the development of organic chemistry, the elemental analysis was of critical importance.

Description of the earlier elemental analysis

An earlier apparatus for elemental analysis consisted of a refractory glass tube (50 cm length) in the front part was a porcelain dish with just the weighed material sample (about 0.5 g) - enclosed by two Asbestpropfen asked, behind - until the end of the pipe - there was copper (II ) oxide. Liquid substances were in a small, extended to a tip glass ball filled (the fabric is then evaporated by heating ). The tube was at the ends with perforated stopper containing a thin glass tube closed. At the front part was an apparatus for generating oxygen, attached a balanced U-tube with dried calcium chloride with the back. Behind the calcium chloride tube, the resulting gas was still guided by a balanced vessel with concentrated potassium hydroxide solution. First, the copper oxide was heated, and then the sample was heated at a sufficient flow of oxygen.

After the fuel combustion, the weight gain was determined on calcium chloride tube. The liberated during the reaction, water was bound by calcium chloride. The hydrogen of the water derived from the organic compound. The increase in weight of the potassium hydroxide solution is based on the uptake of carbon dioxide. The carbon of the carbon dioxide derived from the organic compound.

The hydrogen content of the water in the combustion of organic sample can be easily calculated:

Hydrogen content: 2,02:18,0 = 0.1119

With this factor, the weight increase had to be multiplied in the calcium chloride tube ( the resultant water), after the combustion, in order to obtain the weight of a part of hydrogen of the organic compound.

For carbon resulting analog:

Carbon content: 12.01: 44.0 = 0.2729

The multiplied weight amounts can then be set in relation to the initial weight, the difference is the oxygen, nitrogen, phosphorus, halogen content of the compound.

To determine the ratio of the formula weight of the respective mass of the individual elements was divided by the atomic weight of the element. They sought now by integer multiples of each item in the ratio formula, we obtained the empirical formula or molecular formula.

Digestion methods

Depending on the element to be determined various laboratory methods have been developed, so nitrogen is usually determined as ammonia by titration, plus there are special reaction vessels ( Kjeldahl flask, Fresenius - piston ) with which losses are avoided, which would distort the analysis result otherwise.

Combustion methods

Current state of the art for CHNS analysis in this area is the so-called combustion analysis. With this the sample to be determined is first accurately weighed with a balance, and then catalytically combusted at high temperatures ( up to 1800 ° C by making use of Reaktionsexothermen ) with pure oxygen.

Directly thereafter, the combustion gases formed ( oxidation products ) by means of a carrier gas (usually pure helium ) over a about 600 - 900 ° C and hot copper or tungsten contact (as shavings or granules) made ​​and nitrogen oxides contained in the gas stream (NOx) to completely molecular nitrogen (N2 ) is reduced. Subsequently, the defined combustion gases (CO2, H2O, SO2, N2) in specific columns (called Adsorptions-/Desorptions-Säulen, Eng. Purge and trap ) or gas chromatography separated and fed one quantified and a thermal conductivity detector (TCD or TCD).

Since in this measurement method, the order of the elements is ( respectively detected as so-called peaks) in a sample measurement is technically defined precisely, it allows both unambiguous identification (qualitative determination) and on the peak areas ( integral over time ) of the measuring signals at the same time the amount detection (quantitative determination) of the individual elements as C, H, N, S. with the help of known sample weight can then be the respective mass fraction calculated exactly (in percent or ppm ) of the elements in the sample analyzed.

Another measuring method works instead of the full gas separation with gas specific detectors ( usually IR detectors ) for each of CO2, H2O and SO2. For the determination of the nitrogen (N2), here a thermal conductivity detector (TCD or TCD) is used. A rare find flame ionization detector (FID) using.

A distinction is made in the laboratory analysis or between micro - elemental, which for small Substanzeinwaagen of about 0.01 - optimized 10 mg and macro - elemental, which are designed for higher Substanzeinwaagen of up to about 5 g.

Nitrogen determination

An offshoot of classical elemental analysis, there are also pure Stickstoffanalysatoren by the Dumas method for determining the content of N / protein. 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 N2 is quantified ( after separation and / or absorption of interfering gases such as water, SO2, and CO2 if necessary ) also by a thermal conductivity detector (TCD or TCD), a further gas separation or additional detectors are not required. The carrier gas can here as an alternative to pure helium, for example, CO2 can be used.

Oxygen determination by high-temperature pyrolysis

In contrast to CHNS determination ( only pure helium or forming gas as a carrier gas ) at high temperatures (typically about 1200 ° to 1400 ° C) in a finely divided carbon contact ( carbon black ) is used for determining the oxygen content in a sample under inert or reducing conditions quantitatively carbon monoxide formed (CO). This CO is then separated as in the CHNS analysis, a specific column or GC column from also formed during the pyrolysis of nitrogen N2 and measured by a thermal conductivity detector (TCD or TCD) and quantified. Alternatively, can be effected, for example, a CO specific IR detector CO quantification.

184864
de