Yes another Orp question

[dhoch]

Randy,

I've searched through your forum, and found some interesting info, but nothing that directly responds to this:

I have an ORP controller on my AQIII, as well as a handheld model (Ph/Temp/ORP combo unit by Hanna).

I have watched my ORP over time (you can see info on my parameters page at my web site: http://www.danj.com/reef), after calibrating my probe I noticed I was running in the 230-300 range (I've been running 100 mg of ozone via red sea).

I notice that after doing a water change ORP would drop from about 290ish to 235ish (you can see exactly when I'm performing water changes by looking at my graph referenced above 10 gal ~2-3 x / week).

Since I just got my Hanna handheld unit I thought I would test my water change water... guess what... it's ORP was 270... so now I'm confused (I was expecting it to be much lower).

When I add the 270 ORP water to a tank that has ORP ~290ish, why does the ORP drop to 235?

Dave

 

[Boomer]

As Randy well tell you that is not an easy answer. The new water has disrupted the oxidizers to reducers ratios and almost always there is a greater loss in oxidizers to reducers, due to the chemistry of the new water. You would think that new water, being "cleaner", would have a higher ORP but it does not. ORP does not follow some rule of avg. like one would think, i.e., 250 & 300 = 275. I might add there are other things in science that do not follow this "rule of avgs." Have you read through this yet ?

OPR and the Reef Aquarium
http://www.reefkeeping.com/issues/2003-12/rhf/feature/index.php

 

[dhoch]

Yeah I understand that it's not necisarrily a rule of averages (I didn't got through all that grad school for nothing), but I don't understand the drastic decrease. Why when adding 2 higher values do you end up with a lower value... THere has to be something really non-linear happening in those chemical equations.

I'm going to do a little xperiment and remove my tank from the equation (just take tank water, and new water and mix em and see what happens).

Dave

 

[Randy Holmes-Farley]

I agree that it is very complicated.

Even pH, however, does not always fall in between two solutions mixed together. Mixing pH 8.2 seawater and 8.3 baking soda in fresh water can result in a pH below 8.2. The reason is that bicarbonate is a stronger acid in seawater than in fresh water.

With respect to ORP, it is not very well established exactly what is being measured exactly. It may be certain redox active metals (like manganese, maybe iron), and that speciation may, for example, be very dependent on the binding of those metals by organics and even inorganics in solution. Such organics will be much higher in the tank water than in the raw salt water, and that may be a part of the explanation.

There may also be reactions between trace redox active ingredients in the two types of water that tend to make products that are not redox active. For example, iron ions reacting with hydroxide to precipitate as iron hydroxide. Fe+++ is less soluble than Fe++, and so precipitating Fe(OH)3 may reduce ORP.

In the end, I don't think you will be able to determine exactly what is happening on a molecular level.

The above article has a lot more on ORP.

 

[dhoch]

Thanks randy... Even thought it's not a lot help.

BTW it's too bad you are not traveling... We at WAMAS would have loved to have you give a talk at one of our meetings.

Dave

 

[Randy Holmes-Farley]

You're welcome and good luck with the ORP testing.

BTW it's too bad you are not traveling... We at WAMAS would have loved to have you give a talk at one of our meetings.

Sorry, there are just too many things needing my attention to travel much.

 

[ronc98]

Is it possible to get a 10-20 point drop in ORP without adding anything to the tank. Without Dosing ozone I however right around 280. From time to time I get an unexplained drop. Total tank volume is right about 400 gallons. PH and temp are all stable.

I do have three paris of fish in my tank that spawn about regularly I attributed the drop to this.

 

[Randy Holmes-Farley]

Sure, the activities of organisms in the tank might well cause ORP changes. Even a fish releasing a big poop might drop ORP.

 

[ronc98]

Thanks Randy.

They accure mostly at night when the lights are off so I was a bt concerned it was tank related or ORP meter related.

 

[DrBDC]

Now every morning I'll be thinking about the ORP of the toilet dropping.

 

[Randy Holmes-Farley]

They accure mostly at night when the lights are off so I was a bt concerned it was tank related or ORP meter related.

It might also be related to electrical/rf interference by lights, if it happens immediately when lights come on or turn off.

 

[ronc98]

It does not correspond to when lights or other devices go on or off. It is more random and mostly at night. I have pretty much eliminated all outside influences other then ORP itself.

 

[Randy Holmes-Farley]

Here's an old post about spawning and ORP:

http://reefcentral.com/forums/showthread.php?s=&postid=1600472#post1600472

 

[DrBDC]

Is the ph compensation turned OFF on the ORP probe? Also, is the temp compensation turned ON on the pH probe?

 

[ronc98]

There is temp compensation in the PH and no compensation for the ORP

 

[ronc98]

It is not a PH or temp issue. To big of swing. I have isolated it down to something happening in the tank itself.

Thanks Randy for the help.

 

[Randy Holmes-Farley]

:thumbsup:

Let us know if you find out exactly what is causing it.

 

[boxfishpooalot]

If your orp was say 150, would that definitely mean somthings wrong? What number would be usefull to start investigating?

 

[ronc98]

Randy,

Any idea how quick of a rise you should see in ORP once you start dosing ozone (50mg/hr) in a skimmer that has a flow through it of 300 gph with a total tank volume of 350 gallons?

 

[DrBDC]

From this thread:

http://www.reefcentral.com/forums/showthread.php?s=&threadid=517084&perpage=25&pagenumber=3

I found this:

Very little will be alive : below 100 millivolt
Very Bad : below 140 millivolt
Bad : 140 to 180 millivolt
Poor : 180 to 200 millivolt
Too Low : 200 to 220 millivolt
New salt water : 220 to 240 millivolt
Low : 220 to 270 millivolt
Medium : 270 to 310 millivolt
Good : 310 to 340 millivolt
Better : 340 to 360 millivolt
Best : 360 to 390 millivolt
High : 390 to 450 millivolt
Too high : over 450 millivolt
Dangerous : over 525 millivolt
Very dangerous : over 575 millivolt

What reference they are using for those numbers I have no idea.