Ring test

A ring test is a chemical reaction for the qualitative detection of nitrate ions in aqueous solutions . To do this, the sample solution is acidified in a test tube with dilute sulfuric acid, a few drops of iron (II) sulfate solution are added and then concentrated sulfuric acid is carefully placed under a layer . A place at the boundary layer of the sample solution and sulfuric acid, redox reaction take place: nitrate ions to nitrogen monoxide reduces and the iron (II) ions (Fe ²⁺ ) to iron (III) ions (Fe ³⁺ ) oxidation :

{\ displaystyle \ mathrm {3 \, Fe ^ {2 +} + NO_ {3} ^ {-} + 4 \, H ^ {+} \ rightarrow NO + 3 \, Fe ^ {3 +} + 2 \, H_ {2} O}}

Ring test positive

In the further course of the reaction, nitrogen monoxide is deposited on excess iron (II) ions complexed with water . The pentaaquanitrosyl iron (II) complex formed in the process leads to a reddish-purple to brown, ring-shaped color along the test tube wall, which gives the reaction its name.

{\ displaystyle \ mathrm {[Fe (H_ {2} O) _ {6}] ^ {2 +} + NO \ rightarrow [Fe (H_ {2} O) _ {5} NO] ^ {2 +} + H_ {2} O} \!}

In the complex, the nitrogen monoxide is not present as a radical , as is usually the case , but in the form of the NO ⁺ ion. Since this is formed by transferring an electron from nitrogen monoxide to iron, the iron is in the + I oxidation state, which is otherwise very rare for iron .

( Note: Depending on whether one regards the NO as a 1, 2 or 3-electron ligand in terms of complex chemistry , the iron has the oxidation state + III, + II or + I. There are more recent studies that say that iron - against the textbook opinion - rather has the oxidation level + II or + III).

Depending on the concentration, the color can be reduced to a pale purple, so a negative sample is very helpful for comparison. Nitrite ions interfere with this detection, as the solution turns brown when the iron (II) solution is added. Any nitrite ions present are removed by boiling with urea . Nitrite ions can be detected qualitatively with Lunge's reagent . Concentration precipitation can also occur along the layer boundary. This is usually white and no longer reveals subtle positive results. The presence of iodide or bromide can simulate a positive result, as oxidation to iodine or bromine occurs at the layer boundary.

Individual evidence

↑ Jander, Blasius, Strähle: Introduction to the inorganic-chemical practical course . 14th edition. Hirzel, Stuttgart 1995, ISBN 978-3777606729 .
^ AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 102nd edition. Walter de Gruyter, Berlin 2007, ISBN 978-3-11-017770-1 , p. 1655.
↑ Hsiu-Yao Cheng, Shyang Chang, and Po-Yu Tsai: On the “Brown-Ring” Reaction Product via Density-Functional Theory . In: J. Phys. Chem. A . tape 102 , no. 2 , 2004, p. 358-361 , doi : 10.1021 / jp031136x .
↑ Alicja Wanat, Thorsten Schneppensieper, Grażyna Stochel, Rudi van Eldik, Eckhard Bill, and Karl Wieghardt: Kinetics, Mechanism, and Spectroscopy of the Reversible Binding of Nitric Oxide to Aquated Iron (II). An Undergraduate Text Book Reaction Revisited . In: Inorg. Chem. Band 41 , 2002, p. 4-10 , doi : 10.1021 / ic010628q .

Web links

Wikibooks: Practical course in inorganic chemistry / Nitrate detection using ring test - learning and teaching materials

[1] Jander, Blasius, Strähle: Introduction to the inorganic-chemical practical course . 14th edition. Hirzel, Stuttgart 1995, ISBN 978-3777606729 .

[2] AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 102nd edition. Walter de Gruyter, Berlin 2007, ISBN 978-3-11-017770-1 , p. 1655.

[3] Hsiu-Yao Cheng, Shyang Chang, and Po-Yu Tsai: On the “Brown-Ring” Reaction Product via Density-Functional Theory . In: J. Phys. Chem. A . tape 102 , no. 2 , 2004, p. 358-361 , doi : 10.1021 / jp031136x .

[4] Alicja Wanat, Thorsten Schneppensieper, Grażyna Stochel, Rudi van Eldik, Eckhard Bill, and Karl Wieghardt: Kinetics, Mechanism, and Spectroscopy of the Reversible Binding of Nitric Oxide to Aquated Iron (II). An Undergraduate Text Book Reaction Revisited . In: Inorg. Chem. Band 41 , 2002, p. 4-10 , doi : 10.1021 / ic010628q .