Microsoft Word - WaterChemistry

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Half-reaction E^0 /V

NO 3 - + 3H+ + 2e- HNO 2 + H 2 O +0.94

2HNO 2 + 4H+ + 4e- H 2 N 2 O 2 + 2H 2 O +0.86

N 2 O 4 + 2H+ + 2e- 2HNO 2 +1.065

2HNO 2 + 4H+ + 4e- N 2 O + 3H 2 O +1.29

The data can be extended to include products in lower oxidation states. For example:

H 2 N 2 O 2 + 2H+ + 2e- N 2 + 2H 2 O; E^0 = 2.65V

Oxidation reactions usually result in the formation of the nitrate ion, with nitrogen in oxidation state
+5. For example, oxidation with permanganate ion can be used for quantitative analysis of nitrite
(by titration):

5NO 2 - + 2MnO 4 - + 6H+ → 5NO 3 - + 2Mn2+ + 3H 2 O

The product of reduction reactions with nitrite ion are varied, depending on the reducing agent used
and its strength. With sulfur dioxide, the products are NO and N 2 O; with tin (II), Sn2+, the product is
hyponitrous acid, H 2 N 2 O 2 ; reduction all the way to ammonia (NH3) occurs with hydrogen sulfide.
With the hydrazinium cation, N 2 H 5 +, hydrogen azide, HN 3 , an explosive compound, is produced:

HNO 2 + N 2 H 5 + → HN 3 + H 2 O + H 3 O+

which can also further react with nitrite:

HNO 2 + HN 3 → N 2 O + N 2 + H 2 O

This reaction is unusual in that it involves compounds with nitrogen in four different oxidation states.

Coordination Complexes
The nitrite ion is known to form coordination complexes in at least five different ways.

1. When donation is from nitrogen to a metal center, the complex is known as a nitro- complex.


2. When donation is from one oxygen to a metal center, the complex is known as a nitrito-
   complex.


3. Both oxygen atoms may donate to a metal center, forming a chelate complex.


4. A nitrite ion can form an unsymmetrical bridge between two metal centers, donating through
   nitrogen to one metal, and through oxygen to the other.


5. A single oxygen atom can bridge to two metal centers.

Alfred Werner studied the nitro-nitrito isomerism (1 and 2) extensively. The red isomer of cobalt
pentamine with nitrite is now known to be a nitrito complex, [Co(NH 3 ) 5 (ONO)]2+; it is metastable and
isomerizes to the yellow nitro complex [Co(NH 3 ) 5 (NO 2 )]2+. An example of chelating nitrite (3) was
found in [Cu(bipy) 2 (O 2 N)]NO 3 - "bipy" is the bidentate ligand 2,2'bypyridyl, with the two bipy ligands
occupying four coordination sites on the copper ion, so that the nitrite is forced to occupy two sites
in order to achieve an octahedral environment around the copper ion.