# Detection reaction

A detection is a method of analytical chemistry , which serves a fabric sample (the analyte ) to investigate qualitatively. This is to be distinguished from quantitative methods of determination and methods of structural analysis .

AgCl as a white precipitate ; right: AgCl dissolved in ammonia water
CuSO 4 (left), evidence as Cu (NH 3 ) 4 (deep blue) as Cu 2 [Fe (CN) 6 ] (brown-red)

The detection reaction is a chemical reaction of the detection reagent used , which indicates the presence of an analyte . The pictures on the right show, for example, the formation of a precipitate in the test tube as a detection reaction for chloride anions with the help of silver salt solution and ammonia water , including two detection of copper salts as a copper tetrammine complex (deep blue solution, complex formation reaction ) and as copper hexacyanidoferrate-II (brown precipitate, precipitation reaction ). Other types of material conversions that can be used as typical detection reactions are redox reactions and acid-base reactions .

With the detection (the detection reaction ), a sample can be examined with no or relatively simple apparatus for:

• individual components contained in it (qualitative),
• their approximate amount or concentration (semi-quantitative) as well as
• structural features, e.g. B. functional groups
• Species in which an element is present (e.g. chlorine as chloride or hypochlorite or elemental)

Thus, chemical elements , possibly existing ions and functional groups with the help of many "rapid test" (test strip or wet chemical detection reactions) identified in the sample. In addition to various measurement methods , sensory perception is of central importance , while methods of instrumental analysis are used for concentration analysis and structural analysis (in research, production ( analysis ) and chemistry lessons ) . These include B. instrumental determination methods of chromatography , spectrometry , photometry , osmometry , refractometry , volumetry , gravimetry and electroanalytical methods .

## methodology

The methods include precipitation reactions , redox reactions , displacement reactions , complex formation reactions and flame tests . If necessary, the sample has to be prepared before carrying out the detection reaction or to be cleaned of interfering accompanying substances.

In inorganic-analytical chemistry, the qualitative detection of substances in material samples takes place, for example. B. in the form of carrying out the cation separation process (cf. under qualitative analysis and in the following article section).

## Application areas and history

Quantitative determinations of substances are often carried out with similar detection reactions, but aim to determine the content of the previously qualitatively detected substances (see under: Quantitative Evidence ). These often only occur in traces (<1%), just above the limit concentration (GK) or the detection limit (EG) of the detection reaction, so that physical analysis methods must be used ( gas chromatography , atomic absorption spectroscopy , etc.). Nowadays, trace substances in the ppb range can also be recorded (1 ppb = 1:10 9 ; see under: Quantitative analysis , instrumental analysis , analytical chemistry ).

To be able to carry out qualitative as well as quantitative verifications of substances that were only present in traces was previously of great importance in chemistry. The example of arsenic shows this importance with regard to criminology: the Marsh test is a detection reaction in chemistry and forensic medicine for arsenic, which was developed in 1832 by the English chemist James Marsh . Before Marsh's sample was discovered, arsenic was a popular murder poison because it was difficult to detect. Another detection of arsenic is the Bettendorf sample (via tin (II) chloride ).

Through the instrumental analysis and its methods such as B. the spectroscopic method has decreased the importance of detection reactions in analytics. However, they are still of didactic importance for conveying subject-specific content and methods (see under: chemistry lessons ).

## Detection reactions

The detection reaction is therefore primarily a preliminary investigation for quantitative determination or for structural elucidation. As a rule, it has the function of a rapid test that provides certain information about the sample's properties.

### Evidence of Ions

Iron (II) sulphate (slightly yellow-greenish) and iron (III) chloride (yellow-brownish) and their detection with blood liquor salts

Detection reactions for ions can take the form of redox reactions , acid-base reactions , complex formation reactions or precipitation reactions .

Some salts are e.g. B. very poorly soluble in water. This is used to detect ions through precipitation. To do this, both an aqueous solution of the substance to be examined (sample, analyte ) and a reference solution (sample solution) are mixed with the detection agent. The ions contained in the reference solution react with the detection agent, as do any ions contained in the sample solution. If the sample solution reacts like the reference solution, the proof is positive. So fall z. B. Iron (II) cations when an aqueous solution of the red blood liquor salt is added as a poorly water-soluble, intensely blue salt.

A distinction is made in terms of the analyte in inorganic matter:

### Evidence of gases

Detection reactions exist not only for anions and cations, but also for gases:

#### hydrogen

The oxyhydrogen sample is suitable for the detection of hydrogen . The unknown gas is ignited. If you hear a bang or a loud whistle, it is hydrogen:

${\ displaystyle \ mathrm {2 \ H_ {2} + O_ {2} \ longrightarrow 2 \ H_ {2} O}}$

#### oxygen

Oxygen can be detected with the glow chip sample . A smoldering wood chip clearly glows in a gas mixture with a high proportion of oxygen.

#### carbon dioxide

Proof of carbonate with barite water

A calcium hydroxide solution is used to detect carbon dioxide . To do this, the gas is passed into saturated lime water or barite water and a colorless solid ( calcium carbonate ) precipitates if it contains carbon dioxide.

${\ displaystyle \ mathrm {CO_ {2} + H_ {2} O \ longrightarrow H_ {2} CO_ {3}}}$
${\ displaystyle \ mathrm {H_ {2} CO_ {3} + Ca (OH) _ {2} \ longrightarrow CaCO_ {3} +2 \ H_ {2} O}}$

or: ${\ displaystyle \ mathrm {CO_ {2} + Ba (OH) _ {2} \ longrightarrow BaCO_ {3} + H_ {2} O}}$

#### ammonia

Gaseous hydrogen chloride or concentrated hydrochloric acid is used to detect ammonia . This creates ammonium chloride , which precipitates as a white mist.

${\ displaystyle \ mathrm {NH_ {3} + HCl \ longrightarrow NH_ {4} Cl}}$

### Evidence for acids and bases

Indicator strips for measuring the pH value (acid-base detection)

An acid-base indicator is a substance that changes color to indicate changes in the pH value caused by acids or bases . Acid-base indicators are therefore most frequently used for titrations (see under Acid-base titration ).

Acids and bases in aqueous solution contain oxonium or hydroxide ions . This can be proven with a universal indicator , for example . The indicator changes its color depending on the pH value of the sample substance. Other acid-base indicators include phenolphthalein and bromothymol blue .

### Evidence of water

Water is created, for example, as a condensate of gaseous water vapor or as a reaction product from the neutralization of acids and alkalis. It can be detected with anhydrous copper (II) sulfate : anhydrous, white copper sulfate turns light blue when water is added. A copper pentahydrate complex is created in which four water molecules act as ligands of the central ion:

${\ displaystyle \ mathrm {CuSO_ {4} +5 \ H_ {2} O \ longrightarrow [Cu (H_ {2} O) _ {4}] SO_ {4} \ cdot H_ {2} O}}$

This is a complex formation reaction .

Another very sensitive detection of water based on complex formation is the pink coloration of blue (i.e. anhydrous) cobalt chloride, which is used to produce water test strips from blue cobalt chloride paper.

## literature

• Michael Wächter: chemistry laboratory . Verlag Wiley-VCH, Weinheim 2011, pp. 215-241, ISBN 978-3-527-32996-0
• Gerhart Jander , E. Blasius: Introduction to the inorganic-chemical practical course . S. Hirzel Verlag, Stuttgart 2005 (15th edition), ISBN 3-7776-1364-9
• Gerhart Jander, E. Blasius: Textbook of analytical and preparative inorganic chemistry . S. Hirzel Verlag, Stuttgart 2002 (in 15th edition), ISBN 3-7776-1146-8
• Michael Wächter: Substances, particles, reactions . Verlag Handwerk und Technik, Hamburg 2000, pp. 154–169, ISBN 3-582-01235-2
• Bertram Schmidkonz: Practical course in inorganic analysis . Verlag Harri Deutsch, Frankfurt 2002, ISBN 3-8171-1671-3
• Eberhard Gerdes: Qualitative Inorganic Analysis. A companion for theory and practice . Springer, Berlin 2001 (2nd edition), ISBN 3-540-67875-1
• Thomas Bitter (editor): Elements of Chemistry I - teaching material for high school . Ernst Klett Schulbuchverlag GmbH, Stuttgart 1986 (1st edition), ISBN 3-12-759400-3