The arsenate is an ion with the

chemical formula A chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, such as pare ...

. Bonding in arsenate consists of a central

arsenic Arsenic is a chemical element; it has Symbol (chemistry), symbol As and atomic number 33. It is a metalloid and one of the pnictogens, and therefore shares many properties with its group 15 neighbors phosphorus and antimony. Arsenic is not ...

atom, with

oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical Electrical charge, charge of an atom if all of its Chemical bond, bonds to other atoms are fully Ionic bond, ionic. It describes the degree of oxidation (loss of electrons ...

+5, double bonded to one oxygen atom and single bonded to a further three

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

atoms. The four oxygen atoms orient around the arsenic atom in a tetrahedral geometry.

Resonance Resonance is a phenomenon that occurs when an object or system is subjected to an external force or vibration whose frequency matches a resonant frequency (or resonance frequency) of the system, defined as a frequency that generates a maximu ...

disperses the ion's −3 charge across all four oxygen atoms. Arsenate readily reacts with metals to form arsenate metal compounds. Arsenate is a moderate

oxidizer An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or " accepts"/"receives" an electron from a (called the , , or ''electron donor''). In ot ...

and an

electron acceptor An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. Electron acceptors are oxidizing agents. The electron accepting power of an electron acceptor is measured by its redox potential. In the ...

, with an

electrode potential An electrode is an electrical conductor used to make contact with a nonmetallic part of a Electronic circuit, circuit (e.g. a semiconductor, an electrolyte, a vacuum or a gas). In electrochemical cells, electrodes are essential parts that can c ...

of +0.56 V for its reduction to

arsenite In chemistry, an arsenite is a chemical compound containing an arsenic oxyanion where arsenic has oxidation state +3. Note that in fields that commonly deal with groundwater chemistry, arsenite is used generically to identify soluble AsIII anions ...

. Due to arsenic having the same valency and similar

atomic radius The atomic radius of a chemical element is a measure of the size of its atom, usually the mean or typical distance from the center of the nucleus to the outermost isolated electron. Since the boundary is not a well-defined physical entity, there ...

phosphorus Phosphorus is a chemical element; it has Chemical symbol, symbol P and atomic number 15. All elemental forms of phosphorus are highly Reactivity (chemistry), reactive and are therefore never found in nature. They can nevertheless be prepared ar ...

, arsenate shares similar

geometry Geometry (; ) is a branch of mathematics concerned with properties of space such as the distance, shape, size, and relative position of figures. Geometry is, along with arithmetic, one of the oldest branches of mathematics. A mathematician w ...

and reactivity with

phosphate Phosphates are the naturally occurring form of the element phosphorus. In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthop ...

. Arsenate can replace phosphate in biochemical reactions and is

toxic Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a subst ...

to most

organisms An organism is any living thing that functions as an individual. Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have been pr ...

Natural occurrence

Arsenates occur naturally, in hydrated and

anhydrous A substance is anhydrous if it contains no water. Many processes in chemistry can be impeded by the presence of water; therefore, it is important that water-free reagents and techniques are used. In practice, however, it is very difficult to achie ...

form, in a variety of

minerals In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2011): M ...

. Examples of arsenate-containing minerals include

adamite Adamite is a zinc arsenate hydroxide mineral, Zn2 As O4O H. It is a mineral that typically occurs in the oxidized or weathered zone above zinc ore occurrences. Pure adamite is colorless, but usually it possess yellow color due to Fe compoun ...

alarsite Alarsite (AlAsO4) is an aluminium arsenate mineral with its name derived from its composition: aluminium and arsenate. It occurs as brittle subhedral grains which exhibit trigonal symmetry. It has a Mohs hardness of 5-5.5 and a specific gravit ...

annabergite Annabergite is an arsenate mineral consisting of a hydrous nickel arsenate. It is considered a member of the vivianite group and known for its ability to form crystals in a characteristic apple-green color. History Annabergite has been kn ...

, erythrite and legrandite. When two arsenate ions balance the charge in a formula, it is called diarsenate for example zinc diarsenate, .

Uses

Arsenate-based

pesticides Pesticides are substances that are used to pest control, control pest (organism), pests. They include herbicides, insecticides, nematicides, fungicides, and many others (see table). The most common of these are herbicides, which account for a ...

such as lead hydrogen arsenate were commonly used until their replacement by newer pesticides such as DDT and subsequent ban by multiple regulatory bodies due to health concerns.

Transition metal In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. The lanthanide and actinid ...

arsenate compounds are often brightly coloured and have been used to make

pigments A pigment is a powder used to add or alter color or change visual appearance. Pigments are completely or nearly solubility, insoluble and reactivity (chemistry), chemically unreactive in water or another medium; in contrast, dyes are colored sub ...

Copper arsenate Copper arsenate (Cu3(AsO4)2·4H2O, or Cu5H2(AsO4)4·2H2O), also called copper orthoarsenate, tricopper arsenate, cupric arsenate, or tricopper orthoarsenate, is a blue or bluish-green powder insoluble in water and alcohol and soluble in aqueous a ...

was a minor compound used in the Egyptian blue pigment used by the

ancient Egyptians Ancient Egypt () was a cradle of civilization concentrated along the lower reaches of the Nile River in Northeast Africa. It emerged from prehistoric Egypt around 3150BC (according to conventional Egyptian chronology), when Upper and Lower ...

and Romans. Cobalt violet pigment was made from cobalt arsenate before its toxicity led to its replacement by

cobalt phosphate Cobalt phosphate is the inorganic compound with the formula Co3(PO4)2. It is a commercial inorganic pigment known as cobalt violet.Hugo Müller, Wolfgang Müller, Manfred Wehner, Heike Liewald "Artists' Colors" in Ullmann's Encyclopedia of Industr ...

Chromated copper arsenate Chromated copper arsenate (CCA) is a wood preservative containing compounds of chromium, copper, and arsenic, in various proportions. It is used to impregnate timber and other wood products, especially those intended for outdoor use, in order to pr ...

(CCA) has been a widely used

wood preservative Wood preservation refers to any method or process, or even technique, used to protect the wood and extend its service life. Most wood species are susceptible to both biological (''biotic'') and non-biological (''abiotic'') factors that cause d ...

since the 1930s. Safety concerns have led to the phasing out of CCA-treated wood for

residential A residential area is a land used in which houses, housing predominates, as opposed to industrial district, industrial and Commercial Area, commercial areas. Housing may vary significantly between, and through, residential areas. These include ...

projects in many countries. CCA remains a common and economical treatment choice for non-residential uses such as

agriculture Agriculture encompasses crop and livestock production, aquaculture, and forestry for food and non-food products. Agriculture was a key factor in the rise of sedentary human civilization, whereby farming of domesticated species created ...

Speciation

Depending on the pH, arsenate can be found as trihydrogen arsenate (that is

arsenic acid Arsenic acid or arsoric acid is the chemical compound with the chemical formula, formula . More descriptively written as , this colorless acid is the arsenic analogue of phosphoric acid. Arsenate and phosphate salts behave very similarly. Arsenic ...

), dihydrogen arsenate (), hydrogen arsenate (), or arsenate (). Trihydrogen arsenate is also known as

. At a given pH, the distribution of these arsenate

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

can be determined from their respective acid dissociation constants. :(p''K''_a1 = 2.19) :(p''K''_a2 = 6.94) :(p''K''_a3 = 11.5) These values are similar to those of

phosphoric acid Phosphoric acid (orthophosphoric acid, monophosphoric acid or phosphoric(V) acid) is a colorless, odorless phosphorus-containing solid, and inorganic compound with the chemical formula . It is commonly encountered as an 85% aqueous solution, ...

. Hydrogen arsenate and dihydrogen arsenate predominate in aqueous solution near neutral pH. The reduction potential (pe) of a solution also affects arsenate speciation. In natural waters, the dissolved oxygen content is the main factor influencing reduction potential. Arsenates occur in oxygenated waters, which have a high pe, while arsenites are the main arsenic species in

anoxic waters Anoxic waters are areas of sea water, fresh water, or groundwater that are depleted of dissolved oxygen. The US Geological Survey defines anoxic groundwater as those with dissolved oxygen concentration of less than 0.5 milligrams per litre. Anoxic ...

with a low pe. A Pourbaix diagram shows the combined influence of pH and pe on arsenate speciation.

Contamination

Arsenates, along with arsenites, are a significant source of

contamination Contamination is the presence of a constituent, impurity, or some other undesirable element that renders something unsuitable, unfit or harmful for the physical body, natural environment, workplace, etc. Types of contamination Within the scien ...

in some natural water sources and can lead to

arsenic poisoning Arsenic poisoning (or arsenicosis) is a medical condition that occurs due to elevated levels of arsenic in the body. If arsenic poisoning occurs over a brief period of time, symptoms may include vomiting, abdominal pain, encephalopathy, and water ...

with repeated exposure. Countries with high levels of arsenic minerals in

sediment Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...

and rock, such as

Bangladesh Bangladesh, officially the People's Republic of Bangladesh, is a country in South Asia. It is the List of countries and dependencies by population, eighth-most populous country in the world and among the List of countries and dependencies by ...

, are especially at risk of arsenate contamination.

Arsenate poisoning

Arsenate is harmful to

humans Humans (''Homo sapiens'') or modern humans are the most common and widespread species of primate, and the last surviving species of the genus ''Homo''. They are Hominidae, great apes characterized by their Prehistory of nakedness and clothing ...

and

animals Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia (). With few exceptions, animals consume organic material, breathe oxygen, have myocytes and are able to move, can reproduce sexually, and grow from a ...

as it interferes with the normal functioning of

glycolysis Glycolysis is the metabolic pathway that converts glucose () into pyruvic acid, pyruvate and, in most organisms, occurs in the liquid part of cells (the cytosol). The Thermodynamic free energy, free energy released in this process is used to form ...

and the

Krebs cycle The citric acid cycle—also known as the Krebs cycle, Szent–Györgyi–Krebs cycle, or TCA cycle (tricarboxylic acid cycle)—is a series of biochemical reactions that release the energy stored in nutrients through acetyl-CoA oxidation. The e ...

. Arsenate replaces inorganic

in the step of

that produces 1,3-bisphosphoglycerate from

glyceraldehyde 3-phosphate Glyceraldehyde 3-phosphate, also known as triose phosphate or 3-phosphoglyceraldehyde and abbreviated as G3P, GA3P, GADP, GAP, TP, GALP or PGAL, is a metabolite that occurs as an intermediate in several central pathways of all organisms.Nelson, D ...

. This yields 1-arseno-3-phosphoglycerate instead, which is unstable and quickly hydrolyzes, forming the next intermediate in the pathway, 3-phosphoglycerate. Therefore, glycolysis proceeds, but the ATP molecule that would be generated from 1,3-bisphosphoglycerate is lost – arsenate is an uncoupler of glycolysis, explaining its toxicity. As with other arsenic compounds, arsenate binds to

lipoic acid Lipoic acid (LA), also known as α-lipoic acid, alpha-lipoic acid (ALA) and thioctic acid, is an organosulfur compound derived from caprylic acid (octanoic acid). ALA, which is made in animals normally, is essential for aerobic metabolism. It i ...

, inhibiting the conversion of

pyruvate Pyruvic acid (CH3COCOOH) is the simplest of the alpha-keto acids, with a carboxylic acid and a ketone functional group. Pyruvate, the conjugate base, CH3COCOO−, is an intermediate in several metabolic pathways throughout the cell. Pyruvic ...

into

acetyl-CoA Acetyl-CoA (acetyl coenzyme A) is a molecule that participates in many biochemical reactions in protein, carbohydrate and lipid metabolism. Its main function is to deliver the acetyl group to the citric acid cycle (Krebs cycle) to be oxidation, o ...

, blocking the

and therefore resulting in further loss of ATP.

References