The ammonium ion is a positively charged polyatomic cation whose chemical formula is NH 4 +. The molecule is not flat, but is shaped like a tetrahedron. The four hydrogen atoms make up the four corners.
Ammonia nitrogen has a pair of unshared electrons capable of accepting a proton (Lewis base), hence the ammonium ion is formed by the protonation of ammonia according to the reaction: NH 3 + H + → NH 4 +
Figure 1: Structure of the ammonium ion.
The name ammonium is also given to substituted amines or substituted ammonium cations. For example, methylammonium chloride is an ionic salt of the formula CH 3 NH 4 Cl where the chloride ion is attached to a methylamine.
The ammonium ion has properties very similar to the heavier alkali metals and is often considered a close relative. Ammonium is expected to behave like a metal at very high pressures, such as inside gas giant planets like Uranus and Neptune.
The ammonium ion plays an important role in protein synthesis in the human body. In short, all living things need proteins, which are made up of about 20 different amino acids. While plants and microorganisms can synthesize most amino acids from nitrogen in the atmosphere, animals cannot.
For humans, some amino acids cannot be synthesized at all and must be consumed as essential amino acids.
Other amino acids, however, can be synthesized by microorganisms in the gastrointestinal tract with the help of ammonia ions. Thus, this molecule is a key figure in the nitrogen cycle and in protein synthesis.
Properties
Solubility and molecular weight
The ammonium ion has a molecular weight of 18.039 g / mol and a solubility of 10.2 mg / ml of water (National Center for Biotechnology Information, 2017). Dissolving ammonia in water forms the ammonium ion according to the reaction:
NH 3 + H 2 O → NH 4 + + OH -
This increases the hydroxyl concentration in the medium by increasing the pH of the solution (Royal Society of Chemistry, 2015).
Acid base properties
The ammonium ion has a pKb of 9.25. This means that at pH higher than this value it will have an acid behavior and at lower pH it will have a basic behavior.
For example, when dissolving ammonia in acetic acid (pKa = 4.76), the free electron pair of nitrogen takes a proton from the medium, increasing the concentration of hydroxide ions according to the equation:
NH 3 + CH 3 COOH ⇌ NH 4 + + CH 3 COO -
However, in the presence of a strong base, such as sodium hydroxide (pKa = 14.93), the ammonium ion yields a proton to the medium according to the reaction:
NH 4 + + NaOH ⇌ NH 3 + Na + + H 2 O
In conclusion, at pH less than 9.25, nitrogen will be protonated, while at pH greater than that value it will be deprotonated. This is of utmost importance in understanding titration curves and understanding the behavior of substances such as amino acids.
Ammonium salts
One of the most characteristic properties of ammonia is its power to combine directly with acids to form salts depending on the reaction:
NH 3 + HX → NH 4 X
Thus, with hydrochloric acid it forms ammonium chloride (NH 4 Cl); With nitric acid, ammonium nitrate (NH 4 NO 3), with carbonic acid it will form ammonium carbonate ((NH 4) 2 CO 3) etc.
It has been shown that perfectly dry ammonia will not combine with perfectly dry hydrochloric acid, the moisture being necessary to cause the reaction (VIAS Encyclopedia, 2004).
Most of the simple ammonium salts are very soluble in water. An exception is ammonium hexachloroplatinate, the formation of which is used as a test for ammonium. The salts of ammonium nitrate and especially perchlorate are highly explosive, in these cases ammonium is the reducing agent.
In an unusual process, ammonium ions form an amalgam. Such species are prepared by electrolysis of an ammonium solution using a mercury cathode. This amalgam eventually breaks down to release ammonia and hydrogen (Johnston, 2014).
One of the most common ammonium salts is ammonium hydroxide, which is simply ammonia dissolved in water. This compound is very common and is found naturally in the environment (in air, water, and soil) and in all plants and animals, including humans.
Applications
Ammonium is an important source of nitrogen for many plant species, especially those that grow in hypoxic soils. However, it is also toxic to most crop species and is rarely applied as the sole nitrogen source (Database, Human Metabolome, 2017).
Nitrogen (N), bound to proteins in dead biomass, is consumed by microorganisms and converted into ammonium ions (NH4 +) that can be absorbed directly by the roots of plants (eg rice).
Ammonium ions are usually converted to nitrite ions (NO2-) by nitrosomonas bacteria, followed by a second conversion to nitrate (NO3-) by Nitrobacter bacteria.
The three major sources of nitrogen used in agriculture are urea, ammonium, and nitrate. The biological oxidation of ammonium to nitrate is known as nitrification. This process involves several steps and is mediated by obligate aerobic, autotrophic bacteria.
In flooded soils, oxidation of NH4 + is restricted. Urea is broken down by the enzyme urease or chemically hydrolyzed to ammonia and CO2.
In the ammonification step, the ammonia is converted by ammonifying bacteria into the ammonium ion (NH4 +). In the next step the ammonium is converted by nitrifying bacteria into nitrate (nitrification).
This highly mobile form of nitrogen is most commonly absorbed by plant roots, as well as microorganisms in the soil.
To close the nitrogen cycle, nitrogen gas in the atmosphere is converted into biomass nitrogen by Rhizobium bacteria that live in the root tissues of legumes (e.g. alfalfa, peas and beans) and legumes (like alder). and by cyanobacteria and Azotobacter (Sposito, 2011).
Through ammonium (NH4 +), aquatic plants can absorb and incorporate nitrogen into proteins, amino acids and other molecules. High concentrations of ammonia can increase the growth of algae and aquatic plants.
Ammonium hydroxide and other ammonium salts are widely used in food processing. The Food and Drug Administration (FDA) regulations state that ammonium hydroxide is safe ("generally recognized as safe" or GRAS) as a yeast agent, pH control agent, and finishing agent. superficial in food.
The list of foods in which ammonium hydroxide is used as a direct food additive is extensive and includes baked goods, cheeses, chocolates, other confectionery products (eg candy), and puddings. Ammonium hydroxide is also used as an antimicrobial agent in meat products.
Ammonia in other forms (eg ammonium sulfate, ammonium alginate) is used in condiments, soy protein isolates, snacks, jams and jellies, and non-alcoholic beverages (PNA potassium nitrate association, 2016).
The measurement of ammonium is used in the RAMBO test, particularly useful in diagnosing the cause of acidosis (Test ID: RAMBO Ammonium, Random, Urine, SF). The kidney regulates acid excretion and systemic acid-base balance.
Changing the amount of ammonia in the urine is an important way for the kidneys to do this. Measuring the level of ammonia in the urine can provide insight into the cause of an acid-base balance disturbance in patients.
The level of ammonia in the urine can also provide a lot of information about the daily acid production in a given patient. Since most of an individual's acid load comes from ingested protein, the amount of ammonia in the urine is a good indicator of dietary protein intake.
Urine ammonia measurements can be particularly useful for the diagnosis and treatment of patients with kidney stones:
- High levels of ammonia in the urine and a low urinary pH suggest ongoing gastrointestinal losses. These patients are at risk for uric acid and calcium oxalate stones.
- A little ammonia in the urine and high urine pH suggests renal tubular acidosis. These patients are at risk for calcium phosphate stones.
- Patients with calcium oxalate and calcium phosphate stones are often treated with citrate to elevate urine citrate (a natural inhibitor of calcium oxalate and calcium phosphate crystal growth).
However, since citrate is metabolized to bicarbonate (a base), this drug can also increase the pH of the urine. If the urine pH is too high with citrate treatment, the risk of calcium phosphate stones may be inadvertently increased.
Monitoring urine for ammonia is one way to titrate the citrate dose and avoid this problem. A good starting dose of citrate is about half the ammonium excretion in the urine (in mEq of each).
The effect of this dose on urine ammonium, citrate and pH values can be monitored and the citrate dose adjusted based on response. A drop in urine ammonia should indicate whether the current citrate is sufficient to partially (but not completely) offset the daily acid load of that given patient.
References
- Database, Human Metabolome. (2017, March 2). Showing metabocard for Ammonium. Recovered from: hmdb.ca.
- Johnston, FJ (2014). Ammonium salt. Retrieved from accessscience: accessscience.com.
- National Center for Biotechnology Information. (2017, February 25). PubChem Compound Database; CID = 16741146. Retrieved from PubChem.
- PNA potassium nitrate association. (2016). Nitrate (NO3-) versus ammonium (NH4 +). retrieved from kno3.org.
- Royal Society of Chemistry. (2015). Ammonium ion. Recovered from chemspider: chemspider.com.
- Sposito, G. (2011, September 2). Soil. Recovered from encyclopedia britannica: britannica.com.
- Test ID: RAMBO Ammonium, Random, Urine. (SF). Recovered from encyclopediamayomedicallaboratorie.com.
- VIAS Encyclopedia. (2004, December 22). Ammonium Salts. Recovered from encyclopedia vias.org.