Qualitative analysis

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The qualitative analysis deals with the detection of chemical elements , functional groups or compounds without considering their quantitative proportions. This happens through detection reactions or by instrumental means. The quantitative analysis examines how much of certain substances is present in a sample

Beginnings, development and methods of analytical chemistry

Again and again people came across unknown liquids, objects, foods and beverages whose unknown effects they wanted to investigate. While some cautious rulers in antiquity and the Middle Ages used slaves as food taster for such food analysis purposes - and probably also consumed them - other monarchs often kept scholars at their courts. Tasting and experimental artists, court astrologers and theologians, doctors, herbalists, quacks, alchemists, masters and magicians passed on their often secret discoveries and experiences from generation to generation - and with the advent of scientific investigation and research methods, the first systematic approaches to investigation developed unknown samples for the substances they contain.

Archimedes , Domenico Fetti , 1620, Old Masters Staatliche Kunstsammlungen Dresden

Probably one of the oldest physical-analytical methods for examining an unknown metal sample was perhaps the Archimedean principle : the comparison of the density by immersion in water in order to be able to distinguish real from falsified gold.

Chemical analysis methods also existed before chemistry was established as a natural science. Pliny already knew how to detect iron sulfate in verdigris by using gall apple juice (this forms a black iron compound with iron- II ions ).

After the discovery of nitric and sulfuric acid in the vitriol, alum and nitric boilers (Byzantium, 13th century), silver and gold alloys could also be chemically distinguished from one another: silver dissolves in nitric acid (“separating water”), gold does not. When hydrochloric acid was then dissolved in nitric acid, aqua regia was discovered (Venice, 15th century) - it even dissolved the king of metals, gold . And with the help of copper salt solutions - made z. B. from separating water and bronze - taught Andreas Libavius (approx. 1550–1616) how to detect ammonia in water: The ammonia solution turns copper salt solutions deep blue.

In 1685 Robert Boyle developed the following first analysis to examine the quality of a body of water without tasting harmful tastes:

  1. Measurement of temperature and determination of density (measuring volume and weighing)
  2. Careful determination of color, odor and effect on the skin
  3. Determination of moving particles in the water with the help of magnifying glasses and the effect of air on the water sample,
  4. Test with gall apple juice (if the water contains iron salts, it turns black, with copper salts it turns red and / or cloudy),
  5. Test with violet or red cabbage juice ( anthocyanins as an indicator )
Friedrich Hoffmann

Friedrich Hoffmann expanded the analysis in 1703 to include the detection of table salt (detection means: Höllenstein ) and sulfur compounds (with the help of mercury and / or mercury salts), and at Bergmann's time (around 1780) the analyst's set of reagents already included litmus, violet and gall apple juice, Sulfuric acid, oxalic acid , potash , lime water, hellstone, lead sugar and alcohol .

In the 19th century, after the discovery of more and more elements, a repertoire of detection methods and detection reactions that was soon barely manageable for laypeople developed . And in order to prevent certain substances from interfering with specific detection reactions (through coloration, cloudiness, etc.), chemists finally developed a separation system: With the help of certain precipitants (group reagents ), they separated the metal salts ( cations ) to be detected into groups of precipitates and solutions classical wet chemical cation separation was created. This was based on precipitations and acid-base reactions and the methodologically more targeted use of the same, more efficient precipitation and detection agents in laboratories.

Inorganic chemistry

The first test of a small part of the substance mixture is based on external properties such as

  • Colour,
  • Nature,
  • Crystal shape,
  • possibly also smell and taste, but because of the possible toxicity of the unknown substances, smell and taste tests are strongly discouraged!

and then will be in preliminary samples

The larger part is then completely in solution brought, because almost entirely of solutions, the characteristic of individual ions staining and allow rainfall produce. Hardly soluble compounds are broken down with strong acids or molten salts .

In the separation process, the dissolved ions are divided into groups with the help of reagents , within which further separations are carried out in order to finally detect the isolated ions with special reagents (identification reaction, detection reaction ). These groups are called precipitation groups. In chemistry studies, the cation separation is carried out in the order from top to bottom:

Group name Reagent separated ions Action principle
  • Reduction group
 N 2 H 4 Pt 2 + / 4 + , Pd 2 + / 4 + some precious metals are reduced to metal by hydrazine and can then be dissolved and identified again
  • Hydrochloric acid group
 HCl Ag + , (Pb 2+ ), (Hg 1+ ) see hydrochloric acid group Precipitation of poorly soluble chlorides in acid
  • Hydrogen sulfide group
 H 2 S in hydrochloric or acetic acid solution Cu 2+ , Sn 2 + / 4 + , Cd 2+ , Hg 1 + / 2 + , Pb 2+ , As 3 + / 5 + , Sb 3 + / 5 + see hydrogen sulfide group Precipitation of poorly soluble sulphides in acidic
  • Urotropin group
 C 6 H 12 N 4 Fe 3+ , Al 3+ , Cr 3+ see ammonium sulfide group , urotropine group Precipitation of sparingly soluble hydroxides of trivalent cations in alkaline
  • Ammonium sulfide group
 (NH 4 ) 2 S. Co 2+ , Ni 2+ , Zn 2+ , Mn 2+ see ammonium sulfide group Precipitation of other sparingly soluble sulfides, now in the alkaline
  • Ammonium carbonate group
 (NH 4 ) 2 CO 3 Ba 2+ , Sr 2+ , Ca 2+ see ammonium carbonate group Precipitation of poorly soluble carbonates in alkaline
  • soluble group
 directly, because many ions have been separated off by the previous precipitations Na + , NH 4 + , K + , Mg 2+ , Li + see detection reactions , soluble group

Organic chemistry

In the past centuries, numerous people have dealt with individual aspects of qualitative organic analysis. In this way, special reagents have been developed for the detection of various important functional groups , e.g. B. Fehling's solution , Lucas sample u. v. a. The first comprehensive separation and analysis courses for qualitative organic analysis were written in the 20th century. The scheme of such an analysis is as follows:

Suppression of side reactions

In the case of a detection reaction, an additional reagent is often added in addition to the detection agent, which removes substances in the sample that interfere with the detection reaction . The suppression of side reactions ( interference suppression ) can be done in a variety of ways. The main methods are:

Oxidation (redox reaction)

If z. B. a preliminary test for acetate is carried out to prove the presence of acetate ions in an unknown sample, then this substance is rubbed in a mortar with potassium hydrogen sulfate. This substance reacts with acetate ions to form acetic acid , which is revealed by its smell:

Acetate is protonated by hydrogen sulfate. Acetic acid and sulfate are formed .

This reaction is disrupted if a sample also shows sulfide ions: In this case, the poisonous gas hydrogen sulfide, which smells like rotten eggs, is produced from sulfide and hydrogen sulfate: In this case, of course, an odor test should not be performed. To eliminate the disturbance, you also add a little hydrogen peroxide : It oxidizes the disruptive sulfide to the odorless sulfate.

Precipitant (precipitation reaction)

CoNO 3 (pale pink), CoS (black), Co (OH) 2 (red-brown), cobalt carbonate (blue)

Many heavy metal ions interfere with the detection of anions by entering into color reactions with the detection medium. To eliminate interference, a salt sample is therefore boiled with soda ( sodium carbonate ) and water and filtered. Soda solution is basic because water reacts with carbonate:

Dissociation equilibrium of the carbonate ion in water ( acid-base reaction )

The filtrate ( soda extract ) therefore contains carbonate and hydroxide ions. Interfering heavy metal ions form precipitates ( carbonates and hydroxides ) in this solution . The filtrate is called soda extract and contains the anions as sodium salts. These can now be detected without interference.

Precipitants are also used in the cation separation process to separate interfering cations from one another - primarily with hydrogen sulfide, which precipitates sulfides (see figure and see under hydrochloric acid group , hydrogen sulfide group , ammonium sulfide group , ammonium carbonate group ).

Displacement (acid-base reaction)

1. barium hydroxide, 2. barium carbonate, 3. barium sulfate

Strong acids displace weak acids from their salts (see above). If you have evidence for anions such as B. sulfate is carried out, one uses a soda extract from the sample. However, this contains carbonate ions from the soda (see above). Like sulphate, carbonate reacts with the detection agent barium chloride solution to form a white precipitate (barium carbonate, white, which simulates a positive sulphate detection, is formed, because barium sulphate , painter's white) is formed:

Hydrochloric acid is added to detect sulphate : it displaces the carbonate ion by forming carbonic acid (carbon dioxide and water; acid-base reaction ):

If a white precipitate still occurs in the hydrochloric acid soda extract of the sample solution when barium chloride is added, this must come from the (barium) sulfate ( precipitation reaction ).

Masking (complex formation reaction)

Iron (III) solution and iron (III) thiocyanate

Another method is to “mask” the interfering ion. If a sample z. B. contains cobalt ions, then these can be detected with the detection agent ammonium thiocyanate and pentanol (amyl alcohol): When the reaction mixture is shaken, a cobalt thiocyanate complex is formed, which is blue-soluble in pentanol (cf. under Evidence for cations ):

Cobalt cations react in an aqueous medium when thiocyanate ions are added to form the pink pentaaquathiocyanatocobalt (II) complex, which dissolves in pentanol with a blue color.

However, if the sample also contains iron salts, the deep red iron-thiocyanate complex that forms during this detection reaction covers the blue color - iron interferes with the detection ( complex formation reaction ):

Iron (III) ions and thiocyanate ions react in an aqueous medium to form the blood-red pentaquathiocyanato ferrate (III) complex.

Solid sodium fluoride is added to mask the detection : It reacts to form a colorless, non-reactive hexafluoroferrate complex - the iron has been "masked".

Modern

Nowadays, rapid qualitative analyzes with specifically sensitive reagents are common. Also has instrumental analysis more weight significantly, even for very simple qualitative questions. More difficult questions, for example from organic or biochemistry, are usually solved using chromatography and spectroscopic methods .

Nevertheless, the first semesters of the chemistry course are intensively devoted to the cation separation process and its detection reactions, because it imparts important knowledge of the subject.

See also

literature

Web links

Wikibooks: Internship Inorganic Chemistry # Qualitative Analysis  - Learning and teaching materials
Wikibooks: Inorganic Chemistry for Students  - Learning and Teaching Materials

(Basics of detection reactions in inorganic chemistry and chemistry in general)

Individual evidence

  1. ↑ O. Neunhoeffer: Analytical separation and identification of organic substances . 2nd Edition. De Gruyter, Berlin 1965 DNB 453569323
  2. ↑ H. Laatsch: The technique of organic separation analysis - an introduction . Georg Thieme Verlag, Stuttgart / New York 1988, ISBN 3-13-722801-8 .