Arsenic is known as a pollutant in environment and a toxic element. However, in December 2010 resear — Analytical Chemistry Chemistry Question
Arsenic in water
Arsenic is known as a pollutant in environment and a toxic element. However, in December 2010 researchers of the National Aeronautics and Space Administration (NASA) of USA reported a species of bacterium in Mono Lake, California, that can use arsenic instead of phosphorus in biological molecule structures. It seems that monitoring concentration and identities of arsenic species in water will become even more important in near future.
In natural waters, arsenic is present in the form of oxoacids: Arsenous or arsenic acid with oxidation states of III and V, respectively. The source of arsenic in natural waters is often of geological origin. Arsenous acid and arsenic acid have the following dissociation constants.
H3AsO3: Ka1 = 5.1 ⋅ 10–10
H3AsO4: Ka1 = 5.8 ⋅ 10–3 Ka2 = 1.1 ⋅ 10–7 Ka3 = 3.2 ⋅ 10–12
In aqueous systems, oxidation state of arsenic is dependent on the presence of oxidants and reductants, dissolved oxygen plays an important role. World Health Organization (WHO) has established a maximum total arsenic concentration of 10 µg⋅ dm-3 in drinking water; this value has been adapted by many countries.
In a water sample obtained from a river as a potential source of drinking water, pH value is found to be 6.50. Using atomic absorption spectrometry, speciation analysis is also performed and arsenic(III) and arsenic(V) mass concentrations are found to be 10.8 µg⋅dm-3 and 4.3 µg⋅dm-3, respectively.
Calculate the total molar concentration for arsenic(III) and arsenic(V) inorganic species in the system, assuming that these are the only arsenic species present.
Model Answer
Total concentration of As(III): (10.8 µg dm-3) × (1 g / 10^6 µg) × (1 mol / 74.92 g) = 1.44 ⋅ 10-7 mol dm-3
Total concentration of As(V): (4.3 µg dm-3) × (1 g / 10^6 µg) × (1 mol / 74.92 g) = 5.74 ⋅ 10-8 mol dm-3
What will be the predominant molecular or ionic species for arsenic(III) at pH = 6.50? Write the formula(s).
Model Answer
For H3AsO3, pKa1 = 9.29
pH = 6.50 is significantly lower than 9.29.
Predominant species is H3AsO3.
What will be the predominant molecular or ionic species for arsenic(V) at pH = 6.50? Write the formula(s).
Model Answer
As(III) will be oxidized to As(V) if oxidants are added. The content will become less toxic, thus it is advantageous to have oxidants in the medium.
Calculate the molar concentration(s) of arsenic(III) species suggested in 9.2.
Model Answer
Since H3AsO3 is the single predominant species, the concentration will be 1.44 ⋅ 10−7 mol dm-3.
Calculate the molar concentration(s) of predominant arsenic(V) species suggested in 9.3.
Model Answer
pH = pKa2 + log([HAsO4 2–]/[H2AsO4 –])
[HAsO4 2–] + [H2AsO4 –] = 5.74 ⋅ 10–8
6.50 = 6.96 + log(x / (5.74 ⋅ 10–8 – x))
x = [HAsO4 2–] = 1.48 ⋅ 10–8
[H2AsO4 –] = 5.74 ⋅ 10–8 – 1.48 ⋅ 10–8 = 4.26 ⋅ 10–8
Arsenic(III) is known to be significantly more toxic to human as compared to arsenic(V). Is it advantageous or disadvantageous to have oxidizing agents such as dissolved oxygen in water?
Model Answer
As(III) will be oxidized to As(V) if oxidants are added. The content will become less toxic, thus it is advantageous to have oxidants in the medium.