What is Arsenic?

Arsenic (As) is an element, which means that it is a chemical that can’t be broken down into simpler chemicals (so it is not a compound or molecule that is made up of other elements).

Elements in the same column usually have similar properties, so arsenic has similarities to phosphorus (P), which is a part of the DNA molecules (see below) that make up your genes – that similarity might explain how it is taken up by cells. Arsenic can hitch a ride in the cell’s transporter system for P.
Arsenic has an atomic number of 33. That means it has 33 protons in its nucleus and 33 electrons buzzing around the nucleus, when it is uncharged.

Photo of DNA — The picture shows a model of the DNA molecule. The orange blobs are the phosphorus atoms that link the nucleotides (individual DNA molecules) together in a long chain. In this model, two chains stick together to form a double helix.

The molecular weight of arsenic is 75, so one mole of As atoms has a mass of 75 grams.

Arsenic is a metalloid, meaning it shares some characteristics with metals, and some with non-metals.

Metals are good conductors of electricity, but the conductivity goes down as the temperature goes up. Non-metals do not conduct electricity. Elements that are not very good at conducting electricity but can still do it, and that conduct electricity better at higher temperature fit in between these groups and are called metalloids. Since these compounds conduct electricity poorly, they are also called semiconductors. Semiconductors are used a lot in the electronics industry.
Biological molecules are made up mostly of non-metals such as carbon (C), hydrogen (H), oxygen (O), nitrogen (N), sulfur (S), and phosphorus (P), although metals are needed in much lower amounts.

We are mainly interested in arsenic because of its toxicity. Toxic chemicals prevent or affect processes that are required for life. They somehow stop or change a function that is required for the animal, person or plant to live. Arsenic is a very potent toxin.

Potent means that it has a strong effect even in very low amounts – so a tiny bit of arsenic can do a lot of damage.
About 100 mg of As is enough to kill a grown person – that’s about 1/20th of a teaspoon!

Arsenic is a naturally occurring compound. It is found in the Earth’s crust (the solid outer layer that we live on) at a concentration of around 2-5 parts per million. It is the 20th most abundant element on Earth.

So for every million grams of earth crust, there would be between 2 and 5 grams of As present.
Put another way, it’s like 2-5 grains of salt in a half cup of sugar, or 2-5 drops of lemonade in a 13 gallon barrel of water.

Arsenic is not evenly distributed though, so some places have much higher arsenic concentrations, and other places are lower. Arsenic is often associated with mineral ores that are mined, like copper, gold and zinc. Arsenic concentrations are also high at hotsprings and other geothermal sources.

The picture on the left is a mine and the one on the right is near the outlet of a hotspring. The orange colour at the edge of the water is where a high arsenic precipitate has formed

Even though it is toxic, arsenic has been used for many purposes, including in medicine (believe it or not!), agriculture, glass production, as a wood preservative, and in the electronics industry. This is discussed further in the “Where arsenic comes from” section.

Arsenic can exist in different forms. Organic forms of arsenic are associated with organic carbon.

Biological molecules, like the protein, DNA and lipids that make up your body, are based on carbon.
These are called organic molecules, and they include all carbon-containing molecules except for carbon dioxide (CO₂) or carbonic acid (H₂CO₃). The organic carbon is shown in green below.

Arsenic can be incorporated into organic compounds like monomethylarsonic acid (CH₃AsO(OH)₂), arsenobetaine ((CH₃)₃As⁺CH₂COOH), arsenocholine ((CH₃)₃As⁺CH₂CH₂OH), and Paris Green (Cu(CH₄COO)₂.3Cu(AsO₂)₂).

Inorganic forms of arsenic include many solid minerals, such as orpiment (As₂S₃) and arsenopyrite (FeAsS). There are also soluble inorganic forms like arsenious acid (H₃AsO₃), and arsenic acid (H₃AsO₄), which are the compounds of concern in drinking water. Arsenious acid has a valence state of +3, which may be written as As(III), and arsenic acid is As(V), with a valence state of +5.

Valence state:

The valence state describes how many bonds can be made with the atom. Bonds are formed between elements when they share or exchange one or more electrons. You can think of the shared valence electrons in a covalent bond as the glue that holds the elements in a molecule together. They’re shown as lines in the structure diagram of arsenious acid. The bonds between the O and H atoms are assumed.

Now, back to valence: oxygen has a valence state of –2 (except in molecular oxygen, O₂, where its valence state is 0). That means it can donate two electrons to form bonds with other elements. As(III) has a valence state of +3, meaning it has three places available to form bonds by sharing electrons with atoms that have a negative valence state. H has a valence state of +1.
If you add the valences of each atom in the molecular formula for arsenious acid, H₃AsO₃, you can see that the molecule, which is uncharged, has a total valence state of 0 (3x(+1) for the hydrogen atoms; 1x(+3) for the arsenic(III) atom; 3x(-2) for the oxygen atoms = 3+3-6 = 0). If you look at the structure of the molecule, you can see that As forms a bond with each oxygen atom, and each oxygen atom also bonds with a hydrogen atom with its other valence electron.
In arsenic acid, arsenic has a valence state of +5, so in addition to the three bonds it forms with the OH groups, it also makes a double bond with the remaining oxygen atom in the formula. So, again, all of the oxygen atoms are sharing two valence electrons, each of the hydrogens accept one valence electron from oxygen to form a bond, and the arsenic makes a total of 5 bonds by sharing valence electrons from the oxygen atoms. If you add up the valence states of all of the atoms in the molecule, you should again get 0.

Arsenic acid and arsenious acid are the forms that are normally found in water – though they may lose some of their H⁺ atoms depending on the pH.

pH is the scale that measures how acidic a solution is. Hydrogen ions (H⁺) are acid groups, and the pH is a scale that tells you how much H⁺ there is in solution.
Solutions with a low pH (less than 7, which is neutral) are acidic. The lower you go, the more acidic, the higher the concentration of H⁺ ions in solution.
Solutions with a high pH are basic. They have a very low concentration of H⁺ ions in solution. Basic solutions are also high in OH^– ions.
In water, if you multiply the H⁺ and OH^– molar concentrations, they always equal 1×10^-14. That means when the H⁺ concentration is high, the OH^– concentration must be low, and vice versa.

If arsenic acid, or some other acid for that matter, loses a proton (H⁺) the remaining part of the molecule has a negative charge. At near-neutral pH, which is common for natural waters, arsenic acid loses one or two H⁺ ions, giving the rest of the molecule a charge of –1 or –2 (H₂AsO₄^– or HAsO₄^2-).

Note: this time if you add up the valence states of all the atoms you should get –1 for H₂AsO₄^– and –2 for HAsO₄^2-.

Arsenious acid remains mostly uncharged until the pH is raised to about 9. Above that pH, it will start losing H⁺ ions.

The difference in charge at normal environmental pH means that the two forms behave differently in the environment.

Negatively charged molecules are attracted to positively charged sites on the surface of soil particles or rocks. Many rocks have positively charged iron, aluminum and manganese binding sites on their surfaces. Negatively charged molecules can associate with the positively charged surface sites, because opposite charges attract each other. That means the negatively charged molecules get “stuck” on the surface and don’t move with the water. They will continue to be trapped in the soil as long as there are free binding sites on the soil surface. Once the sites are all filled or “saturated”, As(V) will also be mobile. The uncharged molecules of arsenious acid are free to travel in the water and are more mobile. That means the uncharged form is more likely to end up in your tap water.

Note that there are also plenty of negatively charged binding sites on soil particles as well – but they won’t affect the arsenic species we are discussing here. The arsenic also reacts with surface sites in ways that are not related to charge, but this is the simplest way to imagine the reaction.

The surface chemistry of soils is really interesting and complicated. It affects water quality and the movement of all sorts of pollutants in the environment, but that discussion is beyond the scope of this web page.

So you can see the importance of understanding the different forms of arsenic. The different forms will travel differently in the environment. But what we’ve seen so far is only part of the story. Another important factor is that some forms of arsenic are more toxic than others. We will discuss toxicity further in the section on Health effects.