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Ph. vs. ORP

RichT

RichT

Officer Emeritus
Officer Emeritus
Curious, why dose it appear that Ph is practically inverse to ORP? It's pretty obvious on the Aqua Notes graphs.

Also, my Ozone Generator doesn't come on for days. Is it possible for Purigen to keep the ORP between 380 and 410 on it's own?

Thanks

PS...Please be gentle all knowing Chem-heads, I used High School Chemistry class as nap time. Still can't figure out how I passed that class. :p
 
B

Boomer

Rich it may be best for you to try to read through this. I know some of it will be deep but you should get the idea.

ORP and the Reef Aquarium
http://reefkeeping.com/issues/2003-...ature/index.htm

I doubt that it is the Purigen. When is the last time you cleaned that probe ? ORP probes need to be cleaned often. When algae grows on them they release O2 from their photosynthesise which raise the ORP.
 
C

Carlo

Already a couple of posts since I started this but since it's already typed.

Trying to keep it simple or at least understandable but pH and ORP are both complex areas. I'll try and cover it from a few bases but keep it understandable. :)

But first a NOTE: If you are using a Neptune controller and I think you are. In the setup there is parameter for PH compensation on the ORP calibration section of the controller. I run my turned off which I've personally found to make the ORP more consistant with other ORP monitoring devices I've had. Not a big difference either way and ORP value is relative at best.

Your observation is correct, Redox Potential (ORP) is inversely influenced by pH.

From Curt (of Neptune) http://www.reefcentral.com/forums/showthread.php?s=&threadid=1200443
"pH and ORP are inversely correlated. For every 1 unit of pH change, the ORP will change by 58 mV."

Can't get simpler then that! But Randy mentions it's more like 59 mV.

Couple or Terms reworded from http://www.wetwebmedia.com/redox.htm:

Redox is short for REDuction OXidation. Redox is a measure of a systems capacity to oxidize material. It is measured indirectly as the ability of an aquatic system to conduct electricity, in millivolts (mV, 1/1000 of a volt).

Redox POTENTIAL is a value defining how much gain or loss a system MIGHT do.

ORP is you guessed it, Oxidation-Reduction Potential.

From the wetwebmedia.com link above: "I like John Tullock's definition of Redox offered in "The Reef Tank Owner's Manual" ...a measure of the ability of a system to eliminate wastes". Not totally accurate scientifically, but very valuable as a subjective description."

Captive systems, being overcrowded and overfed lose redox potential with accumulation of wastes, loss of dissolved oxygen, respiration, etc

Removing organics through skimming, chemical filtration, cleaning filter media or substrates/rocks reduces reductive INFLUENCES, effectively raising ORP. Cutting back on feeding or bio-load will also raise the ORP.

Allowing the organics to build up will have the opposite affect and the ORP will fall.

So think of things like "waste" and dissolved organics in the tank as ORP REDUCERS. On the other side are OXIDIZERS. Without getting into chemicals lets just say that UV & Ozone injectors can be considered OXIDIZERS. It's far more complex then that however.

Randy has said as a conceptual type thing: "ORP is a measure of the relative fighting ability of the oxidizers and the reducers."

NOW THE GOOD STUFF

pH can impact the ORP readings in aquaria. Often, ORP goes down as pH rises. A typical aquarium ORP reading will change on the order of 59 mv/pH unit. The easiest way to understand this is to simply think of pH as a measure of hydrogen ions (H+) in solution, and to think of H+ as being on the side of the oxidizers.
above paragraph taken from taken and modified from http://reefkeeping.com/issues/2003-12/rhf/feature/index.php

As the OXIDIZERS are added the ORP goes up and so does the H+, which cause the pH to go down.

Carlo

PS Here's some additional information on uses of dissolved oxygen, pH and ORP if interested.
Check out the rH calculator at http://www.reefcentral.com/calc/rh.php
rH = ((ORP + 205) / 29.58) + (2 * pH)
Here's more information on rH: http://www.h-minus-ion.org/rH-score-1.html
 
C

Carlo

The Purigen (carbon too) can help raise ORP by removing waste/organics but I'd highly doubt to any large degree unless you just added fresh media. Without Ozone running I doubt you would be close to 410 mV.

I'd agree with Boomer it's time to clean the probe and suggest a recalibration (pain in the but) if you can.

Carlo
 
RichT

RichT

Officer Emeritus
Officer Emeritus
Thanks guys, I briefly scanned both Boomers link and Carlo's info and plan on digging in tonight when I get home. Both appeared to be pretty well Chem-inept friendly. I will also check out/clean the prob when I put it in the ingenious styro probe float. My ORP was at 440 this morning. Although the generator did come on briefly when I dosed some Randy's part 3A, the ORP, per the graph, continued to rise even after the generator shut off at 385. Even though my ACIII is set to control the generator between 385 and 380, it usually runs between 400 and 410 as the PH tops off at around 8.2 or so.

Thanks again for your help.

One more ? Is there a common household product I can check the ORP against?
 
B

Boomer

One more ? Is there a common household product I can check the ORP against?

No but we all wish there was one. You will have to buy one :) There are some links in the article.
 
C

Carlo

Actually I use a VERY COMMON household product and it seems to be pretty accurate if you mix correctly. Keep in mind your use of ORP is relative to begin with. What I mean by that is that it's not like comparing pH on my system to pH on your system and saying we need to be in 8.2-8.4 range or Nitrates should be < 1ppm type thing.

With ORP it's more a "range" you go for so some give and take is already built in. With that said.

Measure out 5 parts of RO/DI water and add 1 part household CLOROX BLEACH (get the kind with out additives) for a total of 6 parts. This will measure out to just about a perfect 700 mV. Now it's not an ideal range but works. Ideally we would have a reference solution closer to the target level of the tank like 400 mV. Try and make enough solution so you have 3/4 cup.

Swirl you probe in this for a while to make sure there are no air bubbles and also to stir the solution pretty well and then walk away and let the ORP settle.

CALIBRATE YOUR MONITOR/CONTROLLER
You should be able to use this reading to calibrate your monitor/controller to the 700mV solution above.

Neptune 3 "backdoor" calibration:

1) With the controller in the run display place the probe in the 700mV reference solution. Wait 30-60 minutes for the probe to stabilize. Note the value.
Let y = 700 - ORP reading on neptune panel
2) Go to the second screen of the selftest display. Press the up button twice, the down button twice, and then the select button. You should now be in the undocumented manual calibration menu.
3) Go to the ORP manual cal. Adjust the offset up or down by 'y'.
4) Exit the manual cal menu.
5) Go to the run display. With the ORP probe still in the 700 mV calibration solution, the ORP should read 700.

Just for reference I've done this numerous times and then put the probe in true 400 mV calibration solution and it was always with 5 to 10 mV. That's much better then the typical "drift" you can get in a week without cleaning the probe.

Hope that helps,
Carlo
 
C

Carlo

Hope everything was helpfull.

This is actually a "wopper" of a question to try and explain. Both pH and especially ORP are both very complex subjects by themselves let alone trying to explain the interaction of the two without getting "deep".

It's probably the hardest to explain question I've seen here on our forums. So with that said you'll might have some questions unanswered after reading though the different links and if so just ask and maybe someone can explain it better then the online stuff so it makes better sense.

Carlo
 
RichT

RichT

Officer Emeritus
Officer Emeritus
Carlo said:
This is actually a "wopper" of a question to try and explain
Click to expand...
Just trying to do my part to keep the Chem-heads busy. ;D

Carlo said:
just ask and maybe someone can explain it better then the online stuff so it makes better sense.
Click to expand...
Thanks Carlo, will do.



Ahhhhhh.....I've just been redirect and had my priorities adjusted for me. So no reading tonight. At least my hair won't hurt when I go to bed. LOL
 
C

Carlo

Just have a cold bottle of "medicine" on standby to ease the pain. Preferably one of the green bottle meds! ;D

Carlo
 
B

Boomer

Carlo and Rich

Bleach is something that can be used for ORP and is often used in the swimming pool industry. Although some what crude you can get close enough if that is OK with someone. There are even some calculators out to help you. I'll see if I can find some. For me there is to much to go wrong. Even if your probe value is giving what seems a fairly accurate reading in OPR it does not tell you if you probe is really working properly. This is where the Quinhydrone ORP method comes in, for testing ORP probes. You really can't calibrate a ORP probe. You are just setting it to a solution of say 400mV. It is like you bathroom scale. It does not read zero but 5 lbs, so you just turn the dial till it reads zero. Or you put a 100 lb weigh on it and it reads 110 lbs so you turn the dial till it says 100lbs. All meaning, different people making up their bleach solution differently, since there is no real way, without a special probe, to test the pH of the RO/DI water and not all measuring device are created equal, such as dry cup vs. wet cups. Thus, the offset error of say Carlo's 5-10 mV, may be more or even less. Of course, one may argue that even "if it was off 25 mV using Ozone it is not that big of deal really right" and I would agree. As it is the trend of OTP not really its value, as long as it is not to high.

Issues with bleach

Another factor to consider when making ORP measurements is that they can be pH- dependent (remember that pH is a measure of hydrogen ions). For example, chlorine exists in solution as hypochlorous acid (OCl- ). Depending on the pH, this hypochlorous acid will shift its equilibrium to provide more or less free chlorine (this accounts for chlorine reacting more strongly at low pH values-as pH is lowered, more free chlorine is generated).Even though the concentration of chlorine remains constant, its oxidizing power is pH-dependent. .


Chlorine vs pH plots
http://www.sensorex.com/support/education/ORP_education.html

Measuring ORP
http://www.sensorex.com/support/technical_articles/treating_water_at_orp_speed.html

Quinhydrone Method
http://www.sensorex.com/products/orp_electrodes/accessories/ORP_solutions.html
 
C

Carlo

I would totally agree with everything you just said Boomer. In all honesty the method you just mention is what Neptune recommends or many other people use the Pinpoint calibration solution at 400mV (about ideal range) which should be dead accurate and no room for error like the Quinhydrone mixture.

I only mentioned the bleach thing because of the homemade solution question.

I say go for the cleaning and homemade solution but follow it up with the Pinpoint 400mv solution to fine tune it. If for no other reason, it's a lot closer to tank ORP values then the 700mV is.

Carlo
 
RichT

RichT

Officer Emeritus
Officer Emeritus
Lucked out over the weekend and was able to pick up some RedoxSol-450 at That Fish Place. Appreciate the info though everyone.



Brushed off the electrode, stuck it in the bottle, waited 5 minutes, read 445 on the meter. Added 5 to the ORP offset (ACIII) and she read 451. So I'm pretty confident the probe and meter are close enough.

Thanks again.
 
C

Carlo

Very nice Rich. I just got in more of the 400mV pinpoint solution today myself. Any solution in the 400-450 range is going to be much better then the 700 range DIY solution I mentioned earlier.

Curious, did you adjust your ORP values earlier with the 700 DIY solution before getting the 445mV reading or was this without any tinkering previously? That's an excellent reading either way.

Carlo
 
B

Boomer

Rich

I have never seen that stuff before. It would be interesting to see if you "calibrated" your OPR probe with that and then tried the PinPoint, same for Carlo, to see how close the meter reads. We have found that some solution are waaaaaaay off, not yet so much in ORP, hardly no tests but pH cal solutions and the same may hold true for that.

On another note. We have recently discovered that the PinPoint 53 mS used for refracts, *seems* to be nothing more than a solution of seawater @ 35 ppt. As it works on refracts, EC meters and even hydrometers :)
 
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