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Eh (pe) in an acid Solution (7 replies)

9 months ago
Unterstarm 9 months ago

Is it possible to infer the Eh (pe) of a solution from pH, metals and TDS?

Dizzy Flores
9 months ago
Dizzy Flores 9 months ago

I think it is possible to make some general inferences based on pH and metals speciation provided that the samples are carefully handled and not contaminated by exposure to the atmosphere. That said, Eh is difficult if not impossible to reliably quantify IMO. Eh in natural waters is a mixture of potentials from multiple redox couples that may or may not be in equilibrium. Significant dissolved iron at circum neutral pH is a pretty reliable indicator of redox potentials below 200 mV, but I would caution trying to put too fine a point on it. Often you can establish Eh thresholds that the samples are above or below, but there is a lot of uncertainty associated with this type of analysis. I tend to rely on sensitivity analyses to help me understand where the critical thresholds are and if I can make any reliable inferences from the data.

Helena Russell
9 months ago
Helena Russell 9 months ago

No you measure Eh.

Bill Rico
9 months ago
Bill Rico 9 months ago

I come down somewhere in the middle of the comments; sometimes yes, sometimes no.

9 months ago
Oberstorm 9 months ago

This is all theory crafting that hardly applies to real life. Show me how you can reliably measure arsenate/arsenite or ferric/ferrous couples, and then correctly infer Eh from the original sample. Whatever the results of these analyses, I would never have confidence in the inferred Eh without an actual measurement of ORP and pH.

John Koenig
9 months ago
John Koenig 9 months ago

I have to agree with you, sometimes yes, sometimes no. Considering that the platinum electrode is generally most sensitive to the ferrous ferric couple, and completely insensitive to a whole bunch of couples that may poise your system; say CH4/bicarbonate in ANY anaerobic biogeochemical system, measuring Fe II in the field with total iron in the lab gives you a bounding number, as does measuring dissolved oxygen in the field. I have been doing ion exchange of arsenite/arsenate in the field for a while now. Sometimes it makes sense; sometimes there is redox disequilibria related to local Dhamkohler numbers. Ultimately, it’s a multi-methods weight of evidence approach in my opinion. Runnels did some work on dissolved gasses that truly trashed field ORP using a platinum electrode in some systems.

I start with this. I don't "trust" any field ORP until I know it was cleaned and calibrated before the measurement. Second, if the chemical analysis doesn't have any dissolved Fe or Mn, you’re probably not going to measure an accurate redox, so you need a trace metals analysis too. Next, there has to be alternate redox couples to confirm, DO at a minimum, other pairs may or may not have success, so plan on having a complete water quality analysis too. Fourth, look to the solids to see if any of it makes sense.

It's theorization to assume that a platinum electrode measurement of ORP is meaningful at all, ever, anywhere.

Marshal Dienes
9 months ago
Marshal Dienes 9 months ago

Plot of measured vs. calculated Eh provides a good representation of the very questionable relationship between the two methods.

9 months ago
Unterstarm 9 months ago

Many thanks for your comments were very useful for me.

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