PhreeqcUsers Discussion Forum
Processes => Oxidation and reduction equilibria => Topic started by: kishu5694 on March 01, 2021, 10:00:52 AM

Initially i only have concentration of Fe(2+) in a water. and using below equation, I have to find other element Concentration. Fe(2+) + e > Fe(3+)
So the question is
1) How do i calculate Fe(3+) and e concentration in wastewater.
2) How to find Fe(3+) concentration using redox potential.
Thank you in advance

I'm not sure what is appropriate to your system.
If you define Fe in SOLUTION and pe, then PHREEQC will distribute the iron between Fe(2) and Fe(3) based on the pe.
SOLUTION 1
pH 7
pe 8
Fe 1
If you start with Fe(2) and add an oxidant, like O2, then Fe(2) will oxidize to Fe(3) in the reaction calculation. Note that the oxygen will react to equilibrium among all redox states of all elements.
SOLUTION 1
pH 7
pe 5
Fe(2) 1
REACTION
O2 1
0.1 mmol
There are other possibilities, but perhaps this will get you started.

Thank you for you reply. I have some more Doubts.
Is there any specific equation for finding electron Concentration?
Can i use Below equation?
pe = log c(electron)
If yes then How can i find Fe(3+)? by theoretical calculation?
Also i use below eq
K_Fe+3 = 10^lgK_Fe+3*(g_Fe+2)/(g_Fe+3*g_em) (g stands for activity coefficient)
Fe+3 = 1000*K_Fe+3*((Fe+2/1000))/((e/1000))
But i dont have e concentration.

There are no good answers for you, so you should not spend too much time on this.
Here is my advice first. Fe(3) concentration is going to be low at neutral pH because of precipitation of ferric oxyhydroxide minerals. Therefore, you can assume iron is ferrous iron.
You can attempt to measure potential (Volts) with a platinum electrode. After correcting for the reference electrode, you can calculate the pe from the potential. However, the platinum electrode is not sensitive to all redox species, and measurement tends to be a long drift, perhaps to a stable value and perhaps not. I wasted a lot of time in my youth with these measurements.
You can also calculate pe from two redox states of the same element. Your Fe equation is an example. The most likely analyses are O2 (O(0)/H2O), NH3/NO3, and SO4/S(2). If you define concentrations of any redox pair in SOLUTION, PHREEQC will calculate a pe for that pair.
The problem is that redox elements may not be in equilibrium with each other, so you may get one pe from a platinum electrode, and a different pe from each redox pair.
There are colorometric techniques to measure Fe(2) and Fe(3). If you really, really want to know the concentrations of the two states of iron, then you should analyze them.

Thank you so much for your reply. It helps me a lot to decide the direction for my research. @dlparkhurst