Nitrate

In inorganic chemistry, a nitrate is a salt of nitric acid. In organic chemistry the esters of nitric acid and various alcohols are called nitrates.

Chemical properties

The nitrate ion is a polyatomic anion with the empirical formula N O₃⁻ and a molecular mass of 62.01 daltons; it consists of one central nitrogen atom surrounded by three identical oxygen atoms in a trigonal planar arrangement. The nitrate ion carries a negative one formal charge and can be represented as a hybrid of the following resonance structures:

The nitrate ion is the conjugate base of nitric acid. A nitrate salt forms when a positively charged ion attaches to the negatively charged oxygen atoms of the ion, forming an ionic compound. Almost all nitrates are soluble in water at standard temperature and pressure.

Occurrence and history

Solid nitrates are not very abundant in Nature as they are very soluble. They can appear where nitrogen containing ground water is evaporating (e.g. in soils of arid regions, on animal shed walls). Nitrification bacteria in the soil are also needed for the process. The first commercially exploited source was India. While the British Empire had a reliable supply, the continental powers had to collect scrapings from walls and barns, install saltpetre farms (based on aging and leaching manure and urine). The famed tax-collecting duties of Lavoisier actually consisted of being the commissioner of this Saltpeter Administration. Later the large deposits of sodium nitrate in the Atacama desert of northern Chile acquired economic significance.

Until early in the 20th century there was no means of chemical synthesis of nitrates. Chile was a major exporter, and European countries with burgeoning populations due to the industrial revolution were dependent on its nitrates for use as fertilizer to feed their people. They were needed for modern military explosives as well. These two critical uses proved to be crucial in world history -- almost. Had the Germans not devised the Haber process for producing nitrate, they would not have been able to feed their civilian population and armies, nor continued to make explosives. The First World War might have ended as a direct result of embargo of essential raw materials. With the aid of inorganic chemistry, however, the war continued. Nowadays most nitrates are produced from ammonia synthetised from atmospheric nitrogen.

Uses

Agriculture

Nitrates such as potassium nitrate (saltpeter) and ammonium nitrate are important nitrogen carriers in fertilizers; lesser amounts of calcium nitrate, magnesium nitrate and sodium nitrate are applied. These nitrates must be used quickly by plants because they are easily lost through leaching or denitrification by bacteria. Nitrate pollution has become an environmental issue in rivers and oceans. According to the Black Hawk County Green Party, the Cedar River (Iowa) has the highest nitrate levels of any river in the world.

Explosives

Nitrates are also oxidizing agents. When mixed with hydrocarbons or carbohydrates, nitrates can form a flammable or even explosive mixture. For example, potassium nitrate is the oxidizing ingredient in black gunpowder.

Food

Nitrates are also used in curing meats.

Medicine

History

The first organic nitrate was discovered by Italian chemist Ascanio Sobrero in 1847. He was a chemistry professor at the University of Torino and worked under Théophile-Jules Pelouze who himself had worked on discovery of explosives. Sobrero called the discovered compound pyroglycerin and surprisingly in private correspondence as well scientific papers advocated against its use considering it too dangerous to handle and use. He described strong headache after putting nitroglycerin on the tongue.

In 1849 German Constantin Hering, tested nitroglycerin in healthy volunteers and he also observed headaches with its use. As a homeopath he thought it a good remedy for headaches (reasoning that "like cures like").

Later, in 1859 Frederick Guthrie worked with amyl nitrate and noticed that accidental inhalation of it led to face and neck flushing and heart palpitations. It was linked with vasodilatation. This inspired T. Lauder Brunton to used amyl nitrate in coronary heart disease. In 1867 he published in the Lancet a scientific paper On the Use of Nitrite of Amyl in Angina Pectoris (1867;ii:97-98) . Another therapeutic observation was tolerance with repeated use.

Use

In medicine, organic nitrates such as glyceryl trinitrate, isosorbide mononitrate (ISMN) and isosorbide dinitrate (ISDN) are particularly useful for prevention and treatment of angina pectoris. However they can cause a dangerous reaction if taken within 24 hours of taking sildenafil citrate (Viagra) or similar drugs. A list is available at ATC code C01#C01DA Organic nitrates.

Nitrate poisoning (acquired methemoglobinemia) can be treated with Methylene blue.

Related materials

Nitrates should not be confused with nitrites, the salts of nitrous acid. Organic compounds containing the nitro functional group (which has the same formula and structure as the nitrate ion save that one of the O⁻ atoms is replaced by the R group) are known as nitro compounds.

Effects on aquatic life

High levels of nitrate in water can potentially cause the death of fish. Over 30PPM of nitrate can inhibit growth, impair the immune system, cause stress, and reduce energy levels. The principal pathway of entering aquatic systems is through surface runoff from agriculatural or landscaped areas which have received excess nitrate fertilizer. After entering the receiving waters nitrates also can induce algal blooms and eutrophication, causing other ecosystem modification in addition to the mortality effects stated above. Nitrates are a component of total dissolved solids, an indicator of water quality in widespread use.

Nitrate is often labelled as NO3 by Aquarists.

External links

Oxoanions | Nitrates | Nitrogen metabolism | Curing agents

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