About your ORP article

[cspark]

Thanks for your great article.
This is the easiest explanantion I ever read for the theory of ORP.

By the way, I found some confusion about your explanation for ORP- pH relationship.

I think the sentences quoted below are wrong, It should go opposite direction.

In page 3,

The oxidizers are typically strongest near the end of the light cycle when the pH and the oxygen concentration peak.

Near the end of the light cylce, when the pH ..is peak,
oxidizers are weakest, because of reduced [H+] concentration.

In page 10,

In this case, lowering the pH increases the oxidizing power of the oxygen, and consequently lowers the ORP.

lowering the pH, increases the ORP..

Let me know if I was wrong..

 

[Boomer]

Near the end of the light cylce, when the pH ..is peak,oxidizers are weakest, because of reduced [H+] concentration.

But there is an increase in hydroxides (OH-), which are oxidizers. For every net lose in pH (H +) there is and exact oppostie gain in pOH (OH-)

lowering the pH, increases the ORP.


Yup, that is an error for sure, the graphs depicks that very well, lower pH = higher ORP

 

[cspark]

But there is an increase in hydroxides (OH-), which are oxidizers. For every net lose in pH (H +) there is and exact oppostie gain in pOH (OH-)

OH- is weaker (very weak) oxidizer compare with O2

But Oxidation power of O2 is strongly depend on the pH level.

So, loosing oxidation power by decreasing [H+] will be predominant to the gaining oxidation power by increasing [OH-]


Also Compare the concentration of the Oxygen and OH- in the pH ~8 soultion

Oxygen is order of mM, OH- is 10exp-6 M

 

[Randy Holmes-Farley]

I think the sentences quoted below are wrong, It should go opposite direction.

The oxidizers are typically strongest near the end of the light cycle when the pH and the oxygen concentration peak.

Quite right. The oxygen is highest at the end of the light cycle, that part is right. But the H+ is highest at the end of the dark cycle, and that part is thereby wrong since it is the H+ that are helping the other oxidizers. Thanks for pointing out that misstatement. I'll see if I can have that sentence removed.

In this case, lowering the pH increases the oxidizing power of the oxygen, and consequently lowers the ORP.

Yes, that is a typo that should read "raises the ORP". I'll have that changed as well.

Thanks.

 

[reefkeeper1]

Great Article

Great Article

Randy,

After a few years in the hobby, I had pretty much conceptualized most of the water parameters that are tested by reef aquarists (pH, alkalinity, calcium, magnesium, etc.), but ORP had always seemed like something I really didn't have a handle on. Your straightforward description of ORP and particularly the oxidation and reduction examples finally made things clear to me.

 

[cspark]

...

...

Thanks Randy,

I just want add one more thing for your article which you didn't mention.

It is for the measurement of the ORP.

Most frequent error involving the ORP measurment will be the neglecting the maintenance of the electrode.

Especially, the internal solution of the Ag/AgCl reference electrode should be refilled after certain period.

That is the saturated KCl solution. But chloride ion gradually diffuse out into the tank, Its concentration becomes low, resulting decrease of reference electrode voltage.

If this situation get worse, polarizing ability of the electrode lost.
So, resulting ORP value becomes stuck to single value or little move.

 

[Randy Holmes-Farley]

but ORP had always seemed like something I really didn't have a handle on.

I understand. That's how I felt before putting the article together.

Your straightforward description of ORP and particularly the oxidation and reduction examples finally made things clear to me.

Thanks, I'm glad it was helpful.

 

[Randy Holmes-Farley]

Especially, the internal solution of the Ag/AgCl reference electrode should be refilled after certain period.

I agree that is important if it is refillable. The last ORP electrode that I had (one sold for aquarists by Coralife) wasn't refillable. Do you have a refillable one? I'm not sure what brands might be refillable. The one that I see in the Cole Parmer catalog is also sealed.

In general, I prefer to see folks use sealed electrodes (for pH or ORP) if they are dipping them into their aquaria, to avoid the possibility of dipping it below the fill hole (and thereby getting something other than saturated KCl into it).

 

[cspark]

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....

In general, I prefer to see folks use sealed electrodes (for pH or ORP) if they are dipping them into their aquaria, to avoid the possibility of dipping it below the fill hole (and thereby getting something other than saturated KCl into it).

Randy, Chloride ion is diffusing out through the junction portion of the electrode, which located in the bottom tip.

No matter how deep put the electrode, there is alway exchanging of the ion through this junction.

Most of the reference electrode or combination electrode on the market are refillable although it looks like sealed one.

There is a plastic sleeve on the top portion of the electrode, It look like attached part onto body, But just push it down, it will move, and you will see the hole in the side.

Even the cheapest one on the market (Hanna ORP monitor ?) comes with refillable one.

Also, the platinum part of the electrode should be polished periodically. It should be shiny when you see it against the light.

The professional ORP electrode has a platinum disk instead of platinum wire, because polshing is much easier with disk electrode than wire electrode.

 

[Randy Holmes-Farley]

Most of the reference electrode or combination electrode on the market are refillable although it looks like sealed one.

Well, mine was fully sealed, and so is the one that I am referring to from Cole Parmer. I don't know what ones you have, but I don't doubt that both types are available, just as they are for pH.

http://www.coleparmer.com/catalog/p...=&par=&cat=1&sch=875&sel=5900175&lstBool=true

From that description:

"These economical combination electrodes are ideal for your general-purpose applications in the lab or field. They have the impact-resistant qualities of an epoxy body. Electrodes provide a fast, stable response and are ideal for prolonged ORP readings. Sealed, gel-filled design requires virtually no maintenance. Electrodes measure 127 mm L x 12 mm dia. Electrodes withstand temperatures to 176Ã"šÃ‚°F (80Ã"šÃ‚°C)."

 

[Randy Holmes-Farley]

The two issues mentioned in the first post have been corrected in the posted article.

 

[cspark]

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....

Sealed, gel-filled design requires virtually no maintenance.

That kind of the electrodes in the broucher are designed for the laboratory use,
i.e. for the one time or prolonged use, not for the continuous 365/24 use. Should be stored in saturated KCl solution when not in use.

Any ions in the internal filling solution will eventually and gradually diffuse out from the reference electrode, it will create major error in ORP measurement.

In the salt water tank, a protein waste from the living creature can diffuse into the reference electrode chamber, react with silver ion to form the protein-silver complex, which in turn causing the shift of the electrode potential. That is the another reason to replace a internal solution periodically.

Some professional use double junction model in order to avoid this error.

Again, it will be good habit to check the internal solution. For me,
I always put extra KCl crystal to make sure that the internal solution is in the saturated state

 

[Randy Holmes-Farley]

For me, I always put extra KCl crystal to make sure that the internal solution is in the saturated state

Sounds like a good plan.

 

[Boomer]

cspark

The professional ORP electrode has a platinum disk instead of platinum wire, because polishing is much easier with disk electrode than wire electrode

Many sold in this hobby are of this type disk or band an not wire. Yes, they need to be polished, something I learned from Mike Ross

Most frequent error involving the ORP measurement will be the neglecting the maintenance of the electrode.

I sure as heck will agree with that

So, resulting ORP value becomes stuck to single value or little move.

Yes, I have seen this happen, it is like it gets "locked-in" to a value and won't change much no matter what you put it in. My first probe did that in a FOT/UGF , stuck to 275 mV.

Awhile back I invited Mike Ross here, he is Director of Research and Development at Sensorex (manuf.of many of the pH and ORP probes used in this hobby)


He had this to say

Mike says.....
Single vs double junction references

With respect to ORP reference electrodes. Most references are silver silver/chloride. The generate a constant output. The coral life units were Ag/AgCl. The noble metal responds to the reducing or oxidizing reactions, and are based on change from the reference electrode. Platinum is normally used as it does not enter into the reactions. While other reference types could be used, I would not recommend trying them
.

Most ORP electrodes are combination. That means they are a measuring cell and reference in 1 body. Noble metals are used. Platinum is the most common. Purity of the platinum is also critical. Cheap electrodes will tend to use plating or mixes, not high grade pure platinum as we do at Sensorex. Gold and silver are also used, but only in special applications where platinum can be affected by the solutions. These are mainly in waste treatment....not in Aquaculture. Stick with Platinum!

The rule is: use single junction unless you have solutions which interact with silver or chloride. The common materials that cause problems are: heavy metals, proteins and sulfides. Aquariums can be considered as a protein problem. My recommendation is go with double junction

Mike Ross

 

[simonh]

I have a double-junction ORP electrode (made by Sensorex I beleive). I documented the readings in ORP buffers when the probe was new and hope to do the same at every few months to see how much it has drifted. At present I find a 2 week cleaning with electrode cleaner necessary of the ORP probe for it to maintain sensible readings.

I am also collecting data each time I calibrate my pH probes (one from Cole-Parmer one Sensorex) on the offset/slope so I can see how much it shifts in time. From my understanding a large offset tends to indicate blocking of the junction or contamination of the KCl ?

 

[Randy Holmes-Farley]

From my understanding a large offset tends to indicate blocking of the junction or contamination of the KCl ?

I think that is correct, yes.

Do you know what the electode cleaner is?

 

[simonh]

Do you know what the electode cleaner is?

It's the Oakton electrode cleaner "for the remove of Membrane/Junction Protien Deposits on pH and ORP electrodes". It contains Pepsin and Hydrochloric Acid.

 

[Randy Holmes-Farley]

Thanks. That sounds like a good mix for reef aquarium exposures of CaCO3 and proteins.

 

[simonh]

Cole Parmer Catalogue# A-00653-06 (link)

 

[Randy Holmes-Farley]

:thumbsup: