Qualitative inorganic analysis

The qualitative analysis is concerned with the detection of chemical elements, functional groups or compounds, without regard to their proportions. This is done by detection reactions or instrumental ways.

• 2.1 oxidation ( redox )
• 2.2 precipitant ( precipitation reaction )
• 2.3 Displacement ( acid -base reaction )
• 2.4 Masking ( complex formation reaction)

Beginnings, development and methods of analytical chemistry

Again and again pushed people to unfamiliar liquids, objects, food and beverages, their unknown effect they wanted to investigate. While some conservative governors slaves inserting in Antiquity and the Middle Ages this kind of food analytical purposes as a taster - and probably spent - other monarchs held their courts often scholars. Context of experimental artists, court astrologers and theologians, doctors, herb women, quack, alchemists, master and magician gave further their often secret discoveries and experiences from generation to generation - and first systematic approaches developed with the advent of scientific study and research methods to study unknown samples to the materials contained in them.

Probably one of the oldest physical- analytical methods for the investigation of an unknown metal sample was perhaps the Archimedes' principle: the comparison of the density by immersion in water in order to distinguish genuine from adulterated gold can.

Also, chemical analysis already existed even before the chemistry established as a natural science. Pliny knew ferrous sulfate in verdigris prove by advocating Galläpfelsaft ( this is with ferrous ion, a black iron compound ).

After the discovery of nitric and sulfuric acid in the vitriol, alum and Salpetersiedereien ( Byzantium, 13th century ) could be silver and gold alloys and chemically different from each other: silver dissolves in nitric acid ( " aqua fortis " ) that gold is not. When you then dissolved in hydrochloric acid, nitric acid, aqua regia was discovered (Venice, 15th century ) - it solved itself the king of metals, gold. And with the help of copper salt solutions - for example, produced of aqua fortis and bronze - taught Andreas Libavius ​​(approx. 1550-1616 ), how to prove ammonia in water: copper salt solutions are stained deep blue by ammonia.

Robert Boyle developed in 1685 following the first course of analysis to study the quality of a water body without harmful taste samples:

Friedrich Hoffmann extended the analysis of transition in 1703 to the detection of saline (proof means: hell stone) and sulfur compounds (with the help of mercury and / or mercury salts ), and currently miner ( 1780 ) included the reagent system " of the analyst already litmus, Violet and Galläpfelsaft, sulfuric acid, oxalic acid, potash, lime, nitrate of silver, lead, sugar and alcohol.

In the 19th century, after all, a layman soon barely manageable repertoire of proof methods and detection reactions developed after the discovery of more and more elements. And to prevent that certain substances targeted detection reactions ( by discoloration, cloudiness, etc.) disturbed, developed chemists finally a separation system: They parted with the help of certain precipitant ( Gruppenreagentien ) the detected metal salts (cations) in groups of precipitates and solutions - the classical wet chemical cation separation transition originated. This was based on the basis of precipitation and acid -base reactions and methodically always targeted use always the same, efficient precipitation and detection agents in laboratories.

Inorganic Chemistry

The first examination of a small portion of the material mixture takes place after the outer properties such as

• Color,
• Nature,
• Crystal form
• May also smell and taste, but because of the potential toxicity of unknown substances strongly advise against odor and taste samples!

And then in preliminary tests

• Solubility
• The behavior during heating ( Lötrohrprobe )
• The coloring of borax or phosphorus pearl and
• The flame color tested.

The greater part is then brought into solution, because almost only from solutions can be characteristic of individual ions colorations and rainfall produce. Poorly soluble compounds are digested with strong acids or molten salts.

In the separation process, the dissolved ions with the help of reagents are divided into groups, within which more separations are carried out in order, finally, the isolated ions with special reagents to detect ( identify reaction, detection reaction ). These groups are referred to as precipitation groups. The cation separation transition is carried out in the study of chemistry in order from top to bottom:

• Reduction group

• Hydrochloric acid group

• Hydrogen sulfide group

• Urotropin Group

• Ammonium sulfide group

• Ammonium group

• Soluble group

Organic chemistry

Over the past centuries, many people have dealt with individual aspects of qualitative organic analysis. In this way, special reagents such as Fehling's solution, Lucas- sample have been developed for the detection of several important functional groups, and many others

The first comprehensive separation and analysis programs for qualitative organic analysis were written in the 20th century. The scheme of such analytical procedure is as follows:

• Separation of the unknown mixture analysis by column chromatography, distillation or precipitation or extraction ( ether separation process )
• Determination of the physical properties of the individual substances, ie Physical state, color, odor, Siede-/Schmelzpunkt, refractive index, pH of the pure substance and in aqueous solution. With the aid of certain literature or software, such as Beilstein, then the choice of possible materials can already be severely limited here.
• Elemental analysis by sodium information enables the detection of nitrogen ( Lassaigne sample), sulfur ( lead sulphide precipitation), and halogens ( precipitation with silver nitrate) in the original substance.
• Detection of functional groups, e.g., alcohol or amine, with the characteristic responses
• Clear identification by producing a characteristic derivative

Suppression of side reactions

Often in the detection reaction, an additional reagent is added in addition to the means of detection, which eliminates the sample substances which interfere with the detection reaction. The suppression of side reactions can happen in many ways. The main methods are:

Oxidation ( redox )

For example, if a test sample is carried out in acetate, to prove the presence of acetate ions in an unknown sample, then this material is triturated in a mortar and potassium hydrogen sulfate. This substance reacts with acetate to acetic acid, which is revealed by their odor:

This reaction is disturbed when a sample has additionally sulfide ions: in this case arises from sulfide and hydrogen sulfate which smells like rotten eggs poisonous gas hydrogen sulfide: an odor sample should be avoided in this case, of course. To eliminate one gives in addition some hydrogen peroxide added: It oxidizes the interfering sulfide to odorless sulfate.

Precipitation agent ( precipitation reaction )

Many heavy metal ions from interfering with evidence of anions by responding with the detecting agent color reactions. For suppression a salt sample is therefore boiled with soda (sodium carbonate ) and water and filtered. Soda solution is basic, since water reacts with carbonate:

Therefore, the filtrate ( Sodaauszug ) contains carbonate and hydroxide ions. Disturbing heavy metal ions form precipitates ( carbonates and hydroxides ) in this solution. The filtrate is known as Sodaauszug and contains anions of the sodium salts. These can now be detected without interference.

Also in cation separation transition working with precipitants to separate interfering cations from each other - primarily with hydrogen sulfide, which precipitates sulfides (see picture and see hydrochloric acid group, hydrogen sulfide group, ammonium sulfide group Ammoniumcarbonatgruppe ).

Displacement ( acid -base reaction )

Strong acids displace weak acids from their salts (see above). If evidence for anions such as sulfate is carried out, is one end, a one Sodaauszug from the sample. However, this contains from soda forth carbonate ions (see above). Carbonate reacts with the detecting agent, such as sulfate barium chloride solution to give a white precipitate (this makes for barium carbonate, white, which simulates a positive sulfate detection, because this creates barium sulfate, painters white):

For sulfate - proof hydrochloric acid is added: You displaces the carbonate ion by the formation of carbonic acid (carbon dioxide and water, acid -base reaction ):

If the hydrochloric acid Sodaauszug the sample solution still shows a white precipitate at Bariumchloridzugabe, it must come from ( barium ) sulfate ago ( precipitation reaction ).

Masking ( complex formation reaction)

Another method is to "mask" the interfering ion. If a sample contains, for example, cobalt ions, then these can be detected with the detecting agent ammonium thiocyanate and pentanol ( amyl alcohol ): When shaking the reaction mixture in a blue pentanol soluble cobalt thiocyanate complex is formed (see below evidence for cations):

However, the sample also contains iron salts, the formed in this test reaction, deep red iron - thiocyanate complex covers the blue color - iron interfere with detection ( complex formation reaction):

To mask you are in the detection of solid sodium fluoride added: It responds to a colorless, unreactive hexafluoroferrate complex - the iron has been "masked".

Modern

Nowadays qualitative rapid analyzes with specific sensitive reagents are common. The Instrumental Analysis has much more weight, even for very simple qualitative questions. More difficult questions about from the organic or biochemistry are usually solved by chromatography and spectroscopic methods.

Nevertheless, the first semester of chemistry studies devoted intensively to the cation separation transition and its detection reactions, because it conveys important physical skills.