Question

In some communities the amount of arsenic (As³⁺) in the water supply has become an important health issue. One way to analyze this low concentration is via anodic stripping voltammetry (ASV). A 100.0 mL water sample is prepared and the arsenic in the water is reduced to As³⁺. Then a potential is used at the working electrode to reduce the As³⁺ to As⁰. Later, when the As⁰ was stripped from the electrode, 1.00 x 10^-4 coulombs were required. What is the ppb of arsenic in the water?

Answer 1

Solution

Coulomb mol relation is used to find moles of electron using Faradays constant. Reduction of As reaction is used to find mol ratio between As and electrons. Using mol ratio, moles of As are calculated. Volume is converted to L and moles of As is to microgram. Using ppb formula, the concentration in ppb is calculated.

Moles of electrons = Coulombs / Faradays constant

= 1.0 E-4 Coulomb x 1 mol / 96487 coulomb

=1.4391 E-9 ml electron.

As3+ + 3e- --- > As

1 mol As : 3 mol e-

Moles of As = 1.4391 E-9 mol e- x 1 mol As / 3 mol e-

= 4.797E-10 mol As

Mass of As in microgram

4.797E-10 mol As x (1 mol Al/ 74.922 g) x 1E6 micro gram / g  

= 0.03594 microgram

Ppb = 1 microgram / L

0.03594 micro gram / 0.1 L = 0.359 ppb

Answer is 0.359 ppb