Bicarbonate

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In inorganic chemistry, bicarbonate (IUPAC-recommended nomenclature: hydrogencarbonate<ref>Template:Citation</ref>) is an intermediate form in the deprotonation of carbonic acid. It is a polyatomic anion with the chemical formula Template:Chem2.

Bicarbonate serves a crucial biochemical role in the physiological pH buffering system.<ref name="veq">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

The term "bicarbonate" was coined in 1814 by the English chemist William Hyde Wollaston.<ref>William Hyde Wollaston (1814) "A synoptic scale of chemical equivalents", Philosophical Transactions of the Royal Society, 104: 1-22. On page 11, Wollaston coins the term "bicarbonate": "The next question that occurs relates to the composition of this crystallized carbonate of potash, which I am induced to call bi-carbonate of potash, for the purpose of marking more decidedly the distinction between this salt and that which is commonly called a subcarbonate, and in order to refer at once to the double dose of carbonic acid contained in it."</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The name lives on as a trivial name.

Chemical propertiesEdit

The bicarbonate ion (hydrogencarbonate ion) is an anion with the empirical formula Template:Chem2 and a molecular mass of 61.01 daltons; it consists of one central carbon atom surrounded by three oxygen atoms in a trigonal planar arrangement, with a hydrogen atom attached to one of the oxygens. It is isoelectronic with nitric acid (Template:Chem2). The bicarbonate ion carries a negative one formal charge and is an amphiprotic species which has both acidic and basic properties. It is both the conjugate base of carbonic acid (Template:Chem2); and the conjugate acid of Template:Chem2, the carbonate ion, as shown by these equilibrium reactions:

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A bicarbonate salt forms when a positively charged ion attaches to the negatively charged oxygen atoms of the ion, forming an ionic compound. Many bicarbonates are soluble in water at standard temperature and pressure; in particular, sodium bicarbonate contributes to total dissolved solids, a common parameter for assessing water quality.<ref>Template:Cite book</ref>

Physiological roleEdit

File:Riassorbimento bicarbonati e respirazione cellulare.svg

Template:CO2 produced as a waste product of the oxidation of sugars in the mitochondria reacts with water in a reaction catalyzed by carbonic anhydrase to form Template:Chem2, which is in equilibrium with the cation Template:H+ and anion Template:Chem2. It is then carried to the lung, where the reverse reaction occurs and Template:CO2 gas is released. In the kidney (left), cells (green) lining the proximal tubule conserve bicarbonate by transporting it from the glomerular filtrate in the lumen (yellow) of the nephron back into the blood (red). The exact stoichiometry in the kidney is omitted for simplicity.

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Bicarbonate is a vital component of the pH buffering system<ref name="veq"/> of the human body (maintaining acid–base homeostasis). 70%–75% of Template:CO2 in the body is converted into carbonic acid (Template:Chem2), which is the conjugate acid of Template:Chem2 and can quickly turn into it.Template:Cn

With carbonic acid as the central intermediate species, bicarbonate – in conjunction with water, hydrogen ions, and carbon dioxide – forms this buffering system, which is maintained at the volatile equilibrium<ref name="veq"/> required to provide prompt resistance to pH changes in both the acidic and basic directions. This is especially important for protecting tissues of the central nervous system, where pH changes too far outside of the normal range in either direction could prove disastrous (see acidosis or alkalosis). Recently it has been also demonstrated that cellular bicarbonate metabolism can be regulated by mTORC1 signaling.<ref>Template:Cite journal</ref>

Additionally, bicarbonate plays a key role in the digestive system. It raises the internal pH of the stomach, after highly acidic digestive juices have finished in their digestion of food. Bicarbonate also acts to regulate pH in the small intestine. It is released from the pancreas in response to the hormone secretin to neutralize the acidic chyme entering the duodenum from the stomach.<ref>Berne & Levy, Principles of Physiology</ref>

Bicarbonate in the environmentEdit

Bicarbonate is the dominant form of dissolved inorganic carbon in sea water,<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> and in most fresh waters. As such it is an important sink in the carbon cycle.

Some plants like Chara utilize carbonate and produce calcium carbonate (Template:Chem2) as a result of biological metabolism.<ref>Template:Cite journal</ref>

In freshwater ecology, strong photosynthetic activity by freshwater plants in daylight releases gaseous oxygen into the water and at the same time produces bicarbonate ions. These shift the pH upward until in certain circumstances the degree of alkalinity can become toxic to some organisms or can make other chemical constituents such as ammonia toxic. In darkness, when no photosynthesis occurs, respiration processes release carbon dioxide, and no new bicarbonate ions are produced, resulting in a rapid fall in pH.Template:Cn

The flow of bicarbonate ions from rocks weathered by the carbonic acid in rainwater is an important part of the carbon cycle.

Other usesEdit

The most common salt of the bicarbonate ion is sodium bicarbonate, Template:Chem2, which is commonly known as baking soda. When heated or exposed to an acid such as acetic acid (vinegar), sodium bicarbonate releases carbon dioxide. This is used as a leavening agent in baking.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Ammonium bicarbonate is used in the manufacturing of some cookies, crackers, and biscuits.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

DiagnosticsEdit

In diagnostic medicine, the blood value of bicarbonate is one of several indicators of the state of acid–base physiology in the body. It is measured, along with chloride, potassium, and sodium, to assess electrolyte levels in an electrolyte panel test (which has Current Procedural Terminology, CPT, code 80051).<ref>Template:Cite book</ref>

The parameter standard bicarbonate concentration (SBC_e) is the bicarbonate concentration in the blood at a P_aCO₂ of Template:Convert, full oxygen saturation and 36 °C.<ref>Acid Base Balance (page 3) Template:Webarchive</ref>

File:Reference ranges for blood tests - by molarity.png

Reference ranges for blood tests, comparing blood content of bicarbonate (shown in blue at right) with other constituents.