Using ORP to measure interesting things in Tank Water

taricha

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In normal Tank water, ORP is slow to equalize and is affected by so many things that the number itself tells you very little of interest.
But if you add a strong oxidizing agent to a sample of tank water, then the ORP can be really informative - it can tell you how that oxidizer is reacting with substances in the sample. Chlorine (bleach) and its reactions are well-studied in water treatment and it reacts quickly with several things that show up in normal tank water and could be worth knowing.

(I'll type up a procedure in a follow-up post for anyone else who wants to try it. I'm curious what other people's tank water looks like.
edit: procedure detailed in post 11)

Let's look at how ORP behaves with chlorinating a few different samples. Each data point is an addition of bleach solution done every 30 seconds. The available chlorine or free chlorine added is noted as Cl2 (ppm)

ORP_Cl2_SW.png


It takes barely any Cl2 to raise the ORP of distilled water (Blue)
My tank water (Green)
1L of that tank water placed in a beaker with 10mL GAC and bubbled for 24 hours greatly reduced the amount of Cl2 reactive compounds in my tank water (Red line)
Fresh Instant Ocean at matching 1.026sg to my tank water, bubbled for 3 hours contains an interestingly large amount of Cl2 reacting compounds. (Yellow)

So what sorts of things is the Cl2 reacting with? Since I.O. is complicated, I'll use the GAC-treated tank water as a baseline.
ORP_Cl2_NH3_Gln.png


In Red are the GAC-bubbled tank water samples at 24 and 48hrs. Very little additional removal of organics occurred from 24-48 hr.
In Teal, the GAC baseline water spiked with 0.6ppm total ammonia (calculated as NH3ppm) has this really eye-catching shift at 600-650mV.
In Purple is the baseline water spiked with 1.67ppm of the amino acid Glutamine.

As you can see, the chemistry of chlorination is really sensitive to these N-containing compounds, and the details of the reaction show up in the ORP. The rapid jump in the ORP of the chlorinated ammonia sample apparently correlates to the destruction of chloramine (google image from a paper).

In fact the amount of additional Cl2 required to raise the ORP of ammonia and amino acids seems to agree with chemistry theory fairly well.
If you convert concentrations in the above graph to moles, you get in the ballpark of 1.28 Cl2 per ammonia (theory is 1.5) and about 2.04 Cl2 per glutamine (theory is 2 - in short time).

What else does and doesn't react with Chlorine that might matter for tank water.
The major nutrients PO4 and NO3 seem totally unreactive with Cl2 over a short period.

Cl2_NO3_PO4.png

Adding PO4 of 1ppm and NO3 of 10ppm didn't have any effect on the amount of Cl2 required to raise ORP of tank water (within uncertainty).

What does affect it?
Ammonia and N-containing organics like amino acids and proteins. I don't have details on proteins but I can say that it's sensitive to what accumulates in skimmate, what is removed by GAC and what's added to the water 1hr after feeding fish flake (but not NO3 & PO4).

Cl2_ORP_feed_skim_GAC.png

(note: after seeing more data, I would now probably pick a lower target than 700mV for this measure - too flat at 700mV, but the above data illustrates the point regardless.)

I'll wrap up for now. Anyone else tried similar stuff?
(credit to @Dan_P for prodding me into looking at Cl2 consumptions of aquarium water)
 
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Dan_P

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In normal Tank water, ORP is slow to equalize and is affected by so many things that the number itself tells you very little of interest.
But if you add a strong oxidizing agent to a sample of tank water, then the ORP can be really informative - it can tell you how that oxidizer is reacting with substances in the sample. Chlorine (bleach) and its reactions are well-studied in water treatment and it reacts quickly with several things that show up in normal tank water and could be worth knowing.

(I'll type up a procedure in a follow-up post for anyone else who wants to try it. I'm curious what other people's tank water looks like.)

Let's look at how ORP behaves with chlorinating a few different samples. Each data point is an addition of bleach solution done every 30 seconds. The available chlorine or free chlorine added is noted as Cl2 (ppm)

ORP_Cl2_SW.png


It takes barely any Cl2 to raise the ORP of distilled water (Blue)
My tank water (Green)
1L of that tank water placed in a beaker with 10mL GAC and bubbled for 24 hours greatly reduced the amount of Cl2 reactive compounds in my tank water (Red line)
Fresh Instant Ocean at matching 1.026sg to my tank water, bubbled for 3 hours contains an interestingly large amount of Cl2 reacting compounds. (Yellow)

So what sorts of things is the Cl2 reacting with? Since I.O. is complicated, I'll use the GAC-treated tank water as a baseline.
ORP_Cl2_NH3_Gln.png


In Red are the GAC-bubbled tank water samples at 24 and 48hrs. Very little additional removal of organics occurred from 24-48 hr.
In Teal, the GAC baseline water spiked with 0.6ppm total ammonia (calculated as NH3ppm) has this really eye-catching shift at 600-650mV.
In Purple is the baseline water spiked with 1.67ppm of the amino acid Glutamine.

As you can see, the chemistry of chlorination is really sensitive to these N-containing compounds, and the details of the reaction show up in the ORP. The rapid jump in the ORP of the chlorinated ammonia sample apparently correlates to the destruction of chloramine (google image from a paper).

In fact the amount of additional Cl2 required to raise the ORP of ammonia and amino acids seems to agree with chemistry theory fairly well.
If you convert concentrations in the above graph to moles, you get in the ballpark of 1.28 Cl2 per ammonia (theory is 1.5) and about 2.04 Cl2 per glutamine (theory is 2 - in short time).

What else does and doesn't react with Chlorine that might matter for tank water.
The major nutrients PO4 and NO3 seem totally unreactive with Cl2 over a short period.

Cl2_NO3_PO4.png

Adding PO4 of 1ppm and NO3 of 10ppm didn't have any effect on the amount of Cl2 required to raise ORP of tank water (within uncertainty).

What does affect it?
Ammonia and N-containing organics like amino acids and proteins. I don't have details on proteins but I can say that it's sensitive to what accumulates in skimmate, what is removed by GAC and what's added to the water 1hr after feeding fish flake (but not NO3 & PO4).

Cl2_ORP_feed_skim_GAC.png

(note: after seeing more data, I would now probably pick a lower target than 700mV for this measure - too flat at 700mV, but the above data illustrates the point regardless.)

I'll wrap up for now. Anyone else tried similar stuff?
(credit to @Dan_P for prodding me into looking at Cl2 consumptions of aquarium water)
A really nice demonstration!

Any plans to see whether the Cl2/ORP method could be used to adjust a skimmer or GAC water flow to optimize organic matter removal?

Recently, I was reading about the supposed benefits of adding H2O2 to an aquarium. I wonder if you relooked at H2O2 addition, you might see something in the ORP data we have all missed. For example, I grew heathy GHA in the presence of H2O2 but there is a popular notion that H2O2 harms GHA. What does the H2O2-ORP profile in tank water look like when it kills vs when it is harmless?
 
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taricha

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well, It's already convinced me that I need to either get my skimmer in good working shape, or start running regular GAC.

h2o2 is interesting enough to look at. Apparently everyone else's probe says h2o2 makes ORP decrease. Mine disagrees and says it goes the boring correct direction - higher ORP with the oxidizer.
 

Randy Holmes-Farley

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well, It's already convinced me that I need to either get my skimmer in good working shape, or start running regular GAC.

h2o2 is interesting enough to look at. Apparently everyone else's probe says h2o2 makes ORP decrease. Mine disagrees and says it goes the boring correct direction - higher ORP with the oxidizer.

Interesting. What source of H2O2?
 
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taricha

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Interesting. What source of H2O2?
Multiple. Drugstore brand, Dollar Store brand and Flinn (educational science supplier). Dug into documentation and they seem to all use same stabilizer.

So I can't explain discrepancies with the rest of the hobby.
 

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Multiple. Drugstore brand, Dollar Store brand and Flinn (educational science supplier). Dug into documentation and they seem to all use same stabilizer.

So I can't explain discrepancies with the rest of the hobby.

Maybe you are the only one with their ORP robe plugged in correctly. lol
 

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Multiple. Drugstore brand, Dollar Store brand and Flinn (educational science supplier). Dug into documentation and they seem to all use same stabilizer.

So I can't explain discrepancies with the rest of the hobby.

FWIW, you are the only person I've seen dose tank water, not a reef tank.

Wonder if that is the difference?
 
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taricha

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FWIW, you are the only person I've seen dose tank water, not a reef tank.

Wonder if that is the difference?
I did it in the tank too. I actually left my probe measuring in the tank for weeks to try to recreate the effect others get.

The only other thought that I have is that my probe is not a saltwater hobby probe. It's a single junction, many in the hobby are double junction to deal with being in saltwater continuously.

Edit: ...and now I can't replicate it. I guess h2o2 lowers ORP and Chemistry is just wrong :p
Or maybe now I forgot how to plug my probe in right...
 
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taricha

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So here's a step by step on how to do this for anyone else who wants to check their water for this "poor man's Ammonia/organic nitrogen" test.

1. Take a bottle of bleach with printed concentration on it (like Clorox), make a 1/300 dilution of it (59.8mL Distilled + 0.200mL bleach) in a clean rinsed glass container.
2. Use the "available Chlorine" on the label to calculate the concentration of your bleach solution:
My bottle 7.5% bleach "7.1% available chlorine"
0.071 x 1/300 = 0.000237 = 237ppm available chlorine, or free chlorine, or Cl2 for short.
3. Take a clean and well-rinsed glass container and add 100mL tank water sample.
4. Brush and rinse your ORP meter, and place it in the 100mL tank water.
Use a 1.00mL syringe, add measured amounts of your bleach solution to the 100mL tank water, stirring with the ORP probe. ORP will quickly respond to bleach in seconds.

5. Go until you get the ORP to 625mV in about 5 minutes.

6. Calculate Cl2 ppm added.
(bleach solution Cl2 ppm) x (mL of bleach solution added) / (mL of Tank Water sample + mL of bleach solution) = Cl2 ppm added to reach 625mV
so for one of my tank water samples shown below...
(237ppm Cl2) x (1.45mL solution) / (100mL Sample + 1.45mL solution) = 3.4ppm Cl2 to reach 625mV

Fine print: the first time you do it, the amount of Chlorine needed will be too high, because residues in the beaker and on the probe had to be chlorinated as well. Let it sit at the high Cl2 level for a couple of minutes, and then repeat for an accurate measure.


To repeat, or do another sample
Discard the previous sample, rinse the beaker with a splash of distilled water.
"quick reset" the ORP meter if you don't want to wait for it to slooooowly equalize with tank water again, swirl it for a few seconds in a half cup of tap water with a drop of Prime/dechlorinator/water conditioner (thiosulfate). It'll get the ORP back to under 200 in a few seconds, then rinse the ORP meter (tap water is fine), place in tank water until you're ready for next measurement.




Tracking a Feeding experiment:
And here's an example of using it to track the organic nitrogen (protein, aminos etc) in the tank from a feeding of fish food, and how the tank water returned to almost original levels after 4 hours.

Frozen food was one ~3 gram cube of mysis + one cube of brine (omega one) in 70gal system. No additives, no preservatives. Tank water samples filtered before testing, so this is dissolved nutrients only.
ORP_Cl2_feed4hr.png

(this is actually an ORP vs time graph, I converted to ORP vs Cl2 added)

Feeding Data.png


You can see that even in my fairly dirty tank water, a small amount of added N (calculating from food labels, I added 380mg protein in 265L = ~1.4ppm Protein.)
It seems reasonable that the feeding of this amount of organic N - some portion of which would be soluble - increased the Cl2 consumed in my water by the 0.8ppm that I measured in the above data.
 
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Dan_P

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So here's a step by step on how to do this for anyone else who wants to check their water for this "poor man's Ammonia/organic nitrogen" test.

1. Take a bottle of bleach with printed concentration on it (like Clorox), make a 1/300 dilution of it (59.8mL Distilled + 0.200mL bleach) in a clean rinsed glass container.
2. Use the "available Chlorine" on the label to calculate the concentration of your bleach solution:
My bottle 7.5% bleach "7.1% available chlorine"
0.071 x 1/300 = 0.000237 = 237ppm available chlorine, or free chlorine, or Cl2 for short.
3. Take a clean and well-rinsed glass container and add 100mL tank water sample.
4. Brush and rinse your ORP meter, and place it in the 100mL tank water.
Use a 1.00mL syringe, add measured amounts of your bleach solution to the 100mL tank water, stirring with the ORP probe. ORP will quickly respond to bleach in seconds.

5. Go until you get the ORP to 625mV in about 5 minutes.

6. Calculate Cl2 ppm added.
(bleach solution Cl2 ppm) x (mL of bleach solution added) / (mL of Tank Water sample + mL of bleach solution) = Cl2 ppm added to reach 625mV
so for one of my tank water samples shown below...
(237ppm Cl2) x (1.45mL solution) / (100mL Sample + 1.45mL solution) = 3.4ppm Cl2 to reach 625mV

Fine print: the first time you do it, the amount of Chlorine needed will be too high, because residues in the beaker and on the probe had to be chlorinated as well. Let it sit at the high Cl2 level for a couple of minutes, and then repeat for an accurate measure.


To repeat, or do another sample
Discard the previous sample, rinse the beaker with a splash of distilled water.
"quick reset" the ORP meter if you don't want to wait for it to slooooowly equalize with tank water again, swirl it for a few seconds in a half cup of tap water with a drop of Prime/dechlorinator/water conditioner (thiosulfate). It'll get the ORP back to under 200 in a few seconds, then rinse the ORP meter (tap water is fine), place in tank water until you're ready for next measurement.




Tracking a Feeding experiment:
And here's an example of using it to track the organic nitrogen (protein, aminos etc) in the tank from a feeding of fish food, and how the tank water returned to almost original levels after 4 hours.

Frozen food was one ~3 gram cube of mysis + one cube of brine (omega one) in 70gal system. No additives, no preservatives. Tank water samples filtered before testing, so this is dissolved nutrients only.
ORP_Cl2_feed4hr.png

(this is actually an ORP vs time graph, I converted to ORP vs Cl2 added)

Feeding Data.png


You can see that even in my fairly dirty tank water, a small amount of added N (calculating from food labels, I added 380mg protein in 265L = ~1.4ppm Protein.)
It seems reasonable that the feeding of this amount of organic N - some portion of which would be soluble - increased the Cl2 consumed in my water by the 0.8ppm that I measured in the above data.
Great write up. I hope you get a few takers so that we can compare numbers across systems. I am getting closer to buying an ORP probe
 

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Nice experiment, but i dont think you can trust the amount of acid in bleech bottle in such way. The values will be all over the place, not because of test sample differencess, but difference in koncentration in bleech bottles.
 
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taricha

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The values will be all over the place, not because of test sample differencess, but difference in koncentration in bleech bottles.
I bought some off-brand bleach for sterilizing phyto once. No label concentration, and digging through a bunch of online manufacturer documentation it said "between 1 and 5% sodium hypochlorite" so I never buy off-brand again.
Clorox says right on the bottle what the concentration is, and a chemical test for free chlorine of a diluted solution of the bleach gives an answer in good agreement to the label - at least in a fresh bottle, haven't tested an old one.

(no acid comes into play here, the pH of these saltwater samples is essentially stable during chlorination).
 
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My point is: If people buy of the shelf bleach and dilute it by a factor 300, their measurement values can not be easily compared to each other. (Measurements will be all over the place!)
 
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taricha

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My point is: If people buy of the shelf bleach and dilute it by a factor 300, their measurement values can not be easily compared to each other. (Measurements will be all over the place!)

oh, I see.
Yeah that's why the calculations regarding the bottle label are necessary...
2. Use the "available Chlorine" on the label to calculate the concentration of your bleach solution:
My bottle 7.5% bleach "7.1% available chlorine"
0.071 x 1/300 = 0.000237 = 237ppm available chlorine, or free chlorine, or Cl2 for short.

6. Calculate Cl2 ppm added.
(bleach solution Cl2 ppm) x (mL of bleach solution added) / (mL of Tank Water sample + mL of bleach solution) = Cl2 ppm added to reach 625mV
so for one of my tank water samples shown below...
(237ppm Cl2) x (1.45mL solution) / (100mL Sample + 1.45mL solution) = 3.4ppm Cl2 to reach 625mV


So in the end, we can compare concentration of Free Chlorine apples to apples.
In theory, anyway!
 
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taricha

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My skimmer is badly in need of maintenance, so I scrubbed down the cup and neck, then measured to see if I can detect my skimmer actually cleaning and removing this Chlorine consuming organic material from the water.

An hour after feeding I collected water that was entering the skimmer chamber and water leaving the skimmer standpipe. Measured two samples of each.

ORP_Cl2_skimmer.png


Small, but yep. Now that I cleaned the skimmer there's a measurable difference in the water before and after the skimmer.
After skimming, the water consumes less Cl2, i.e. needs less added Cl2 to reach the same high ORP.
 
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Got a little more data on how much Ammonia, Amino Acids, and Proteins affect this Chlorine - ORP measure...
Amm_amino_prot.png


Ammonia needs about 5.6ppm Cl2 per ppm NH3 to raise ORP to the high target level.
Amino acids, Glutamine and Glutamic acids both needed 1.1-1.2ppm Cl2 per ppm of the amino acid.
Two protein standards ovalbumin and bovine serum albumin both needed in the ballpark of 0.20ppm Cl2 per ppm of the protein.

Given that I can get the Cl2 measures consistent to 0.1 or 0.2ppm Cl2 (see the immediate previous post), then that implies a limit of detection of 0.5 to 1ppm Protein, 0.1-0.2ppm Amino acid, and I'll just say less than 0.05 ppm Ammonia.

One more thing I want to do ... I've wondered what the actual amount of amino acids is in a coral food supplement like this?
AcroPower (Two Little Fishies)
CoralAmino (brightwell)

(Brightwell at least says it's "Crude Protein 1.00%", Two Little Fishies says nothing about amounts.)
 

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Got a little more data on how much Ammonia, Amino Acids, and Proteins affect this Chlorine - ORP measure...
Amm_amino_prot.png


Ammonia needs about 5.6ppm Cl2 per ppm NH3 to raise ORP to the high target level.
Amino acids, Glutamine and Glutamic acids both needed 1.1-1.2ppm Cl2 per ppm of the amino acid.
Two protein standards ovalbumin and bovine serum albumin both needed in the ballpark of 0.20ppm Cl2 per ppm of the protein.

Given that I can get the Cl2 measures consistent to 0.1 or 0.2ppm Cl2 (see the immediate previous post), then that implies a limit of detection of 0.5 to 1ppm Protein, 0.1-0.2ppm Amino acid, and I'll just say less than 0.05 ppm Ammonia.

One more thing I want to do ... I've wondered what the actual amount of amino acids is in a coral food supplement like this?
AcroPower (Two Little Fishies)
CoralAmino (brightwell)

(Brightwell at least says it's "Crude Protein 1.00%", Two Little Fishies says nothing about amounts.)

I think this experiment will be difficult to interpret because a protein will have lower reactivity to chlorine ((as hypochlorite/hypochlorous acid) than the component amino acids if present as individual amino acids.


 
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