Analytical Chemistry

The analytical chemistry is the part of the Chimie which relates to the analysis of the products, i.e. the recognition and the characterization of known or unknown products. That goes from the follow-up of production (to check that a chain manufactures a product in conformity with the specifications) to the police investigation (to determine the nature of a trace, source of a ground, a painting…).

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Description

The word “analysis” comprises the suffix “- lysis” which means “to break up” (cf Pyrolyse, Hydrolyze, electrolysis). Indeed, one of the first concerns of chemistry since Antoine Lavoisier was to determine the elements, i.e. the products of which all the bodies are made up. It was thus necessary to find methods to divide the complex bodies, then to characterize the elementary bodies resulting from this decomposition.

Until the beginning of the 20th century, analytical chemistry consisted in making react the unknown product with products known to determine its nature. The introduction of quantitative methods , by using the concepts of the physical Chemistry, marked a renewal of the analytical chemistry (for example, in 1943, Gaston Charlot developed the method which bears its name to replace the traditional tests with the Sulfure of hydrogen). Nowadays, one uses readily methods calling upon the Physique, which make it possible to determine and quantify a whole range of elements in only one operation.

Separation, purification: analyzes immediate

Before analyzing a compound, one takes a sample of it, then one separates the various components from the mixture. If the mixture consists of several phases, one starts by separating these phases. For example, one can separate the solid phase from the liquid phase by filtration or sifting. The separation of a homogeneous mixture uses the differences in physical properties between the components. For example, one easily extracts salt from a salt-sand mixture by means of water, because salt is water soluble and sand is not it. On the other hand, the iron filings and sand are both insoluble in water: one will not be able to thus separate them by difference in solubility in this liquid. However, only the iron filings is magnetic, one will be able to thus recover it by magnetic separation. One can separate from the liquid components by successive or split distillations. In certain cases, successive crystallizations make it possible to separate the solid components.

The Chromatographie is the method of separation most often applicable. It has a great number of alternatives according to the nature of the coating of the column used for the analyzes and the interaction component-sample. The two principal types of chromatography are the chromatography by permeation of freezing and the chromatography by exchanges of ions. The first method consists in separating the molecules according to their size; in the second method, the particles are separate according to their load. Gas chromatography separates the volatile components from a sample and the liquid chromatography/liquid separates the neutral molecules from small size in solution.

The chromatography makes it possible to purify a body or a component before its proportioning or to eliminate the compounds which would obstruct its proportioning. It is useless to purify a compound before its analysis if the method of analysis acts only on the studied compound. For example, to determine the pH (concentration in ions oxonium) of blood with an electrode of glass does not require preliminary stage of separation.

The calibration constitutes another preparatory stage for the qualitative analyzes and quantitative. The answer and the sensitivity of mechanical or electronic equipment to the required component must be calibrated by using a pure component or a sample containing a known quantity of the component.

Classification of the chemical analyzes

Analytical chemistry can be classified in various ways. Before approaching the various methods of the conventional chemical analysis, one must in general proceed to a certain number of generally known operations under the name of " Immédiate" analyzes;. They are essentially physical methods (especially) even chemical (when they are sufficiently specific) of which the goal is to separate the various chemical species present in a sample. Crushing, sifting, gas classification, distillation, crystallization, filtration, centrifugation etc are among many others of the operations of the Proximate Analysis. The chromatographic methods and the similar methods (the such electrophoresis S) are extremely powerful techniques of separation and belong to the whole of the techniques suitable for the Proximate Analysis. Modern approach of the methods known as " not destructives" where the sample is treated as a whole whose consumption remains negligible with respect to the total mass of this one, offers obviously the economy of the proximate analysis, preserves this sample for purposes of check analysis so necessary, but encounters frightening difficulties the such effects of matrix and the problems of the calibration. It is convenient to distinguish in any sample whatever it is the two following terms:

what one seeks to determine: Analyte,
all the remainder: the Matrix.

Essentially any sample is thus single, because it is enough that one varies with respect to the other so that the analytical problem yesterday known is transformed into a " new a priori inconnu".

The analyzes can thus be classified:

according to the type: analyzes qualitative or quantitative the qualitative analysis can be by traditional methods or instrumental

the sample is subjected to the analysis either to know the identity of its components or to determine the contents of its components. If one does not know with which type of material one deals, it can be necessary to make a qualitative analysis before making a quantitative analysis.

according to the manner of carrying out it: analyzes traditional, or Titrage, or analyzes instrumental

the techniques of traditional analysis (or not-instrumental) use, in general, of the quantitative reactions in aqueous phase or the cubic measures in gas phase. The instruments used are simple like the graduated Verrerie (test-tube S graduated, Pipette S graduated…), balances S analytical, PH-mètre S… They are, in general destructive. When they call upon aqueous solutions, one speaks about “wet process”.

techniques of instrumental analysis, from which various spectrometries, use an equipment which allows the determinations basing itself on physical properties of the Analyte S. These analyzes are carried out either on the sample such as it is (it is then nondestructive), or on solids prepared, or on solutions of the samples.

according to the target product: mineral Analyze or organic

the mineral analysis applies to the product not-organics but also to minerals contained in organic materials such as for example the Plomb in the gasoline.

according to the quantity of samples used: macro or Microanalysis.

According to the technique used, this quantity perhaps about some Gram S or of the fractions of milligram. Techniques of microanalysis were especially developed in qualitative analysis (reactions on drops of solution).

according to the posterior conservation of the sample: destructive Analysis or not destructive.

In general, the traditional analysis is destructive, and the instrumental analysis is not it always.

according to the automacity: analyzes manual or automatic

the machine analysis is used much in industry to follow and direct the parameters of a process, for example the content of Carbon monoxide of a Combustion gas, the quality of the product or the quality of the environmental rejections.

It is also applied in the laboratories which receive many samples in the same way standard: follow-up of the parameters of water quality for example.

See too

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