ORP Control for Metal Removal with Sulfides 2

[harbor]

I'm looking for verification of an explanation that was given to me regarding the use of an ORP Controller to feed a sulfide (NaHS, DTC, etc.) to remove metals.

I'm aware that ORP measures the Oxidation/Reduction Potential, so using an ORP Controller to feed bleach or metabisulfite to a treatment system makes sense.

Here is the reasoning I was given for using ORP to feed sulfide based metal removal chemicals.

..." your not so much measuring Oxidation/Reduction as you are measuring the water's ability to conduct electricity due to the amount of dissolved metal in the water. If the pH remains constant, as the dissolved metal concentration increases, the ORP reading increases (in mV). As the metal is precipitated and removed, the conductivity (in mV) is reduced. For example, if you have a mix of dissolved metals in the water the ORP reading may be +200 mV; as you add the sulfide the metals are precipitated and the corresponding ORP reading is decreased until all the metals have been precipitated and removed - and now the ORP is at a lower value - maybe +10 mV..."

This description seems to make sense - until I think that the meter is designed to measure Oxidation or Reduction reactions.

It’s been suggested that a proportional chemical metering pump might work, except I worry that the dissolved metal concentration in the untreated waste water changes at various times (e.g. 9AM - Cu = 10 ppm, 11AM - Cu = 0.5 ppm, 3PM - Cu = 35 ppm). The OPR meter is hyped as a cure to metal fluctuations - when the metal concentration is high the chemical pump keeps pumping until the metals are gone; if the metals are low the chemical pump shuts off sooner. It seems to make sense and offers something the proportional pump maybe can't address.

My issue is with the explanation of how the ORP meter works - and how it relates to Oxidation / Reduction Potential.

Any enlightenment on how ORP "really" works in these applications would be greatly appreciated?

 

[m777182]

If a piece of metal is immersed into a solution of its ions a reaction will take place where atoms of metal will release electrones and as cations dissolve in a solution.The surpluse of electrones on the metal piece is called electrode half cell potential and can be measured;
(look to a physical chemistry book).
If there are ions of same species but in different oxidation states (like Fe2+ and Fe3+ )in a solution then
two reactions will simultaneously take place:in our example Fe2+ ions will each release an electron and become Fe3+ and in the second reaction Fe3+ ions will each accept an electron and become Fe2+. Both reactions are characterized by its own electrode potential. In an equilibrium an electrode made of an inert metal (like Pt or similar) can measure the net potential of such a redox reaction system.It is proportional to ln(activity of oxydant)/ln (activity of reducent). For approximate calculations we may replace the term activity by concentration. The potential of an inert electrode is therefore proportional to ln(concentration(oxydant)/concentration(reducent)).The proportionality factor includes R*T/(z*F),where R=8,316J/mol K,[T]=deg K,F=Faraday's constant and z =number of electrons that participate in a redox reaction.
The equilibrium among oxydant and reducent specia very often depends on pH and always on temperature of the solution. With an addition of either an oxydant or a reducent the ratio of ln(concentration(oxydant)/concentration (reducent)) will be changed and the potential of the inert electrode will measure it. It will be changed as well by an addition of an alkali or an acid, when pH dependant. And it will be changed by precipitation of either oxydizing or reducing specia because both concentrations and their logaritmic ratio will change.
The instrument for ORP measurement is basicaly similar to a pH meter so that most instruments can be simply switched to one or the other function.
Conductivity presents a summation of all present individual ionic conductances so it will be influenced by an addition of an electrolyte or by precipitation of a compound and almost always on T.
I hope the very rouhg explanation will help you a bit.
m777182

 

[harbor]

m777182,
Thanks for your input - it will help.