Why is NH4 + an acid

The ammonium-Ion NH4+ is a cation that reacts chemically in a similar way to alkali metal ions and forms salts with the corresponding formula, for example ammonium nitrate (ammonium nitrate) NH4NO3 or the salmiac (ammonium chloride). It is the conjugate acid to the base ammonia NH3. Ammonia / ammonium equilibrium in acidic solution:

Ammonium in nature

In nature, ammonium is primarily formed when proteins break down. It is used by fish and most other aquatic organisms as an end product, e.g. B. via the gills, excreted. It is also released as an end product when dead biomass is decayed by bacteria.

In the soil and in water, ammonium is bacterially (Nitrosomonas) oxidized first to nitrite and by another bacterial species (Nitrobacter) to nitrate and thus “detoxified”. This process is called nitrification and is very desirable in the soil. Nitrification is also an important part of self-cleaning in bodies of water.

Ammonia is toxic to fish even in low concentrations. Ammonium levels in the water of 0.5 to 1 mg / l are therefore classified as dangerous for fish, depending on the pH value of the water. If the ammonium content is above 1 mg / l, a body of water is not suitable for fishing purposes.

Ammonium in the body

The ammonium ion is similar to the potassium ion (K+) both in size and charge, which means that it can take its place in the organism. But since it reacts in a different way, e.g. B. synapses that are potassium-controlled cannot be split off again, it blocks them permanently, which leads to the effect of ammonium as a neurotoxin in all organisms that have potassium-controlled synapses.

Ammonium in the urine indicates calculus.

Ammonium in chemistry

Ammonium forms a dissociation equilibrium with ammonia. Because of the involvement of an oxonium ion, this equilibrium is dependent on the pH value. The proportion of ammonia increases with increasing pH value and increasing temperature.

During electrolysis with a mercury cathode, ammonium forms an ammonium amalgam with the mercury. When heated, this amalgam decomposes with the formation of ammonia. It is due to the fact that ammonium is a charged molecule (cation) can not, Ammonium metal in the same way as sodium or potassium.

To test a substance for ammonium, it is mixed with a little caustic soda or sodium hydroxide (cross match as a preliminary test for ammonium cations, see under Evidence for cations). The ammonia released can either be smelled or it can be detected by the basic discoloration of a moist pH test strip above the reaction mixture.

Sensitive evidence is the reaction with Nessler's reagent, which, however, also responds to amines. A sensitive and selective quantitative determination takes place according to DIN mostly with the help of the Berthelot reaction with the formation of a blue indophenol, the concentration of which can be measured colorimetrically.

The pKa value of ammonium is 9.2.

See also

Categories: Ammonium Compound | Functional group