Higher ORP and more Stable pH..Wow

[sditch]

I just wanted to share some of the interesting changes that has started going on with my 120 sps tank recently after a year of clockwork stable parms such as ph,temp and ORP monitor levels.

I wanted to make some changes to my one year old tank in order to try and get a more nutrient low enviroment without using zoevit, Ozone(Used it for 4 months) or UV so I started using Bio Digest and BioTrace from Prodibio. After two weeks my ORP has jump from about 300 to 340 and that was with a 10% water change after the first week (Knowing how the water change dumps your ORP for days).

I then noticed my pH was seamed more stable, no more .3 - .4 24 hour swings (unless my probe is false) its more like 7.89 - 8.04 in a 24 hour period.

I also can see a great improvment in the water clarity and reminds me of when I used ozone. My 250DE 14k looked less yellow

Whats interesting is I also remove almost all (alot) of cyto from my refugium when I started because I heard it would die once using Prodibio. So just a nice 30" Long Refugiem with mirical mude and some light with little cyto.

Oh, I also started using Sponge Power and Coral Vitilizer but really don't she how that could help being there nutrients.

Just thoght it was interesting, This Prodibio must be improving the water quility without question unless removing the cyto did. And why is my pH more stable durning the day and night? Could removing a 5gal bucket of Micro Alge like cyto from my refuigiem cause these.

I was always told that the microalge helps keep a stable pH during the night.

Interesting to me, below you can see my parms each hour for the past two weeks. Can you spot the water change ?

01:00 03/15/2008 79.6 8.02 227
02:00 03/15/2008 79.3 8.01 228
03:00 03/15/2008 79.1 8.01 229
04:00 03/15/2008 79.3 7.96 231
05:00 03/15/2008 79.6 7.92 233
06:00 03/15/2008 79.7 7.92 231
07:00 03/15/2008 79.7 7.88 234
08:00 03/15/2008 79.3 7.90 234
09:00 03/15/2008 79.1 7.90 235
10:00 03/15/2008 79.3 7.90 234
11:00 03/15/2008 80.0 7.92 234
12:00 03/15/2008 79.9 7.90 235
13:00 03/15/2008 79.9 7.93 234
14:00 03/15/2008 80.0 7.93 235
15:00 03/15/2008 80.2 7.97 233
16:00 03/15/2008 80.4 8.00 232
17:00 03/15/2008 80.6 8.01 232
18:00 03/15/2008 80.7 8.04 232
19:00 03/15/2008 80.7 8.06 234
20:00 03/15/2008 80.7 8.08 232
21:00 03/15/2008 80.6 8.11 230
22:00 03/15/2008 80.1 8.04 230
23:00 03/15/2008 79.8 8.00 232
00:00 03/16/2008 79.4 8.01 232
01:00 03/16/2008 79.1 8.00 233
02:00 03/16/2008 79.2 7.98 234
03:00 03/16/2008 79.4 7.94 235
04:00 03/16/2008 79.5 7.93 235
05:00 03/16/2008 79.7 7.90 237
06:00 03/16/2008 79.7 7.89 238
07:00 03/16/2008 79.7 7.87 237
08:00 03/16/2008 79.7 7.87 238
09:00 03/16/2008 79.7 7.87 238
10:00 03/16/2008 80.0 7.90 238
11:00 03/16/2008 79.8 7.95 238
12:00 03/16/2008 79.7 7.99 236
13:00 03/16/2008 79.7 8.00 238
14:00 03/16/2008 79.5 8.05 237
15:00 03/16/2008 79.4 8.06 235
16:00 03/16/2008 79.4 8.11 236
17:00 03/16/2008 79.2 8.13 221
18:00 03/16/2008 79.5 8.13 235
19:00 03/16/2008 80.0 8.11 235
20:00 03/16/2008 79.7 8.09 236
21:00 03/16/2008 79.3 8.06 238
22:00 03/16/2008 79.1 8.08 237
23:00 03/16/2008 79.3 8.04 241
00:00 03/17/2008 79.6 8.01 240
01:00 03/17/2008 79.8 7.98 239
02:00 03/17/2008 79.6 7.95 240
03:00 03/17/2008 79.1 7.92 243
04:00 03/17/2008 79.2 7.90 244
05:00 03/17/2008 79.4 7.88 246
06:00 03/17/2008 79.6 7.87 247
07:00 03/17/2008 79.8 7.86 248
08:00 03/17/2008 79.5 7.88 248
09:00 03/17/2008 79.1 7.92 247
10:00 03/17/2008 79.3 7.94 247
11:00 03/17/2008 79.9 7.96 246
12:00 03/17/2008 79.9 7.97 247
13:00 03/17/2008 79.9 8.02 245
14:00 03/17/2008 80.0 8.04 245
15:00 03/17/2008 80.2 8.06 244
16:00 03/17/2008 80.3 8.07 245
17:00 03/17/2008 80.3 8.11 245
18:00 03/17/2008 80.4 8.12 247
19:00 03/17/2008 80.5 8.11 247
20:00 03/17/2008 80.2 8.08 246
21:00 03/17/2008 79.9 8.06 247
22:00 03/17/2008 79.7 8.05 249
23:00 03/17/2008 79.5 8.03 251
00:00 03/18/2008 79.3 8.01 251
01:00 03/18/2008 79.1 7.99 253
02:00 03/18/2008 79.2 8.00 252
03:00 03/18/2008 79.4 7.97 254
04:00 03/18/2008 79.6 7.95 254
05:00 03/18/2008 79.9 7.93 256
06:00 03/18/2008 79.5 7.94 256
07:00 03/18/2008 79.3 7.97 256
08:00 03/18/2008 79.1 7.97 256
09:00 03/18/2008 79.4 7.95 255
10:00 03/18/2008 79.8 7.93 257
11:00 03/18/2008 80.1 7.97 257
12:00 03/18/2008 80.3 8.00 256
13:00 03/18/2008 80.6 8.03 257
14:00 03/18/2008 80.7 8.04 257
15:00 03/18/2008 80.7 8.04 257
16:00 03/18/2008 80.8 8.07 258
17:00 03/18/2008 80.9 8.08 258
18:00 03/18/2008 81.0 8.07 259
19:00 03/18/2008 81.1 8.10 258
20:00 03/18/2008 80.8 8.05 260
21:00 03/18/2008 80.5 7.98 262
22:00 03/18/2008 80.1 7.94 265
23:00 03/18/2008 79.8 7.96 264
00:00 03/19/2008 79.4 7.93 267
01:00 03/19/2008 79.2 7.92 264
02:00 03/19/2008 79.3 7.88 264
03:00 03/19/2008 79.6 7.85 268
04:00 03/19/2008 79.8 7.83 268
05:00 03/19/2008 79.9 7.82 269
06:00 03/19/2008 79.6 7.83 271
07:00 03/19/2008 79.0 7.84 270
08:00 03/19/2008 79.1 7.87 269
09:00 03/19/2008 79.2 7.85 270
10:00 03/19/2008 79.3 7.88 268
11:00 03/19/2008 79.9 7.92 268
12:00 03/19/2008 79.7 7.97 269
13:00 03/19/2008 79.3 7.99 268
14:00 03/19/2008 79.1 8.04 266
15:00 03/19/2008 79.8 8.03 267
16:00 03/19/2008 79.8 8.05 268
17:00 03/19/2008 79.5 8.09 267
18:00 03/19/2008 79.3 8.09 267
19:00 03/19/2008 79.2 8.10 263
20:00 03/19/2008 79.3 8.07 262
21:00 03/19/2008 79.4 8.01 264
22:00 03/19/2008 79.4 7.97 266
23:00 03/19/2008 79.5 7.94 265
00:00 03/20/2008 79.5 7.91 268
01:00 03/20/2008 79.6 7.87 269
02:00 03/20/2008 79.6 7.94 266
03:00 03/20/2008 79.5 7.90 267
04:00 03/20/2008 79.4 7.89 268
05:00 03/20/2008 79.3 7.91 267
06:00 03/20/2008 79.1 7.90 268
07:00 03/20/2008 78.8 7.90 269
08:00 03/20/2008 78.6 7.92 269
09:00 03/20/2008 78.3 7.90 269
10:00 03/20/2008 78.2 7.93 266
11:00 03/20/2008 78.6 7.96 269
12:00 03/20/2008 79.0 7.97 266
13:00 03/20/2008 79.6 7.95 267
14:00 03/20/2008 79.9 7.92 269
15:00 03/20/2008 79.5 7.91 272
16:00 03/20/2008 79.2 7.88 273
17:00 03/20/2008 79.6 7.87 274
18:00 03/20/2008 79.9 7.87 273
19:00 03/20/2008 79.7 8.10 261
20:00 03/20/2008 79.3 8.03 264
21:00 03/20/2008 79.2 7.97 266
22:00 03/20/2008 79.4 7.98 268
23:00 03/20/2008 79.5 7.92 271
00:00 03/21/2008 78.4 8.02 217
01:00 03/21/2008 78.4 8.15 216
02:00 03/21/2008 78.5 8.11 218
03:00 03/21/2008 78.6 8.06 219
04:00 03/21/2008 78.6 8.04 221
05:00 03/21/2008 78.6 8.03 221
06:00 03/21/2008 78.5 8.00 223
07:00 03/21/2008 78.4 7.98 224
08:00 03/21/2008 78.4 7.98 224
09:00 03/21/2008 78.2 7.95 226
10:00 03/21/2008 78.1 7.99 226
11:00 03/21/2008 78.5 8.00 225
12:00 03/21/2008 79.0 8.00 223
13:00 03/21/2008 79.5 8.01 221
14:00 03/21/2008 80.0 8.02 220
15:00 03/21/2008 79.5 8.00 220
16:00 03/21/2008 79.2 7.99 221
17:00 03/21/2008 79.1 8.03 220
18:00 03/21/2008 79.2 8.05 219
19:00 03/21/2008 79.2 8.07 219
20:00 03/21/2008 79.1 8.03 220
21:00 03/21/2008 79.2 7.99 220
22:00 03/21/2008 79.3 7.98 222
23:00 03/21/2008 79.4 7.93 224
00:00 03/22/2008 79.5 7.90 226
01:00 03/22/2008 79.6 7.89 227
02:00 03/22/2008 79.7 7.85 230
03:00 03/22/2008 79.8 7.81 232
04:00 03/22/2008 79.4 7.79 233
05:00 03/22/2008 79.0 7.81 232
06:00 03/22/2008 79.1 7.79 235
07:00 03/22/2008 79.2 7.78 234
08:00 03/22/2008 79.2 7.79 235
09:00 03/22/2008 79.2 7.80 235
10:00 03/22/2008 79.4 7.81 236
11:00 03/22/2008 79.7 7.88 236
12:00 03/22/2008 79.8 7.91 233
13:00 03/22/2008 79.8 7.93 233
14:00 03/22/2008 79.8 7.96 231
15:00 03/22/2008 79.9 7.97 232
16:00 03/22/2008 80.0 7.98 232
17:00 03/22/2008 79.8 7.91 233
18:00 03/22/2008 79.7 7.92 234
19:00 03/22/2008 79.8 7.92 232
20:00 03/22/2008 79.5 7.90 233
21:00 03/22/2008 79.3 7.80 236
22:00 03/22/2008 79.1 7.82 236
23:00 03/22/2008 79.2 7.79 240
00:00 03/23/2008 79.3 7.75 240
01:00 03/23/2008 79.5 7.77 240
02:00 03/23/2008 79.5 7.79 242
03:00 03/23/2008 79.6 7.79 243
04:00 03/23/2008 79.7 7.78 243
05:00 03/23/2008 79.8 7.79 245
06:00 03/23/2008 79.8 7.80 244
07:00 03/23/2008 79.5 7.81 245
08:00 03/23/2008 79.1 7.84 245
09:00 03/23/2008 79.1 7.84 244
10:00 03/23/2008 79.1 7.85 244
11:00 03/23/2008 79.2 7.86 244
12:00 03/23/2008 79.8 7.89 242
13:00 03/23/2008 79.8 7.88 242
14:00 03/23/2008 79.8 7.95 241
15:00 03/23/2008 79.8 7.98 241
16:00 03/23/2008 79.7 8.04 243
17:00 03/23/2008 79.7 8.04 251
18:00 03/23/2008 79.5 8.05 252
19:00 03/23/2008 79.3 8.06 251
20:00 03/23/2008 79.0 8.08 251
21:00 03/23/2008 79.2 8.03 254
22:00 03/23/2008 79.3 8.01 257
23:00 03/23/2008 79.6 8.00 258
00:00 03/24/2008 79.7 7.97 261
01:00 03/24/2008 79.8 7.97 262
02:00 03/24/2008 79.9 7.95 264
03:00 03/24/2008 79.7 7.95 266
04:00 03/24/2008 79.6 7.93 267
05:00 03/24/2008 79.5 7.93 270
06:00 03/24/2008 79.4 7.93 270
07:00 03/24/2008 79.2 7.93 272
08:00 03/24/2008 79.0 7.94 271
09:00 03/24/2008 78.8 7.93 272
10:00 03/24/2008 78.6 7.95 272
11:00 03/24/2008 78.6 7.96 269
12:00 03/24/2008 79.1 8.02 269
13:00 03/24/2008 79.6 8.02 271
14:00 03/24/2008 79.9 8.04 273
15:00 03/24/2008 79.6 8.07 274
16:00 03/24/2008 79.3 8.11 279
17:00 03/24/2008 79.3 8.09 286
18:00 03/24/2008 79.7 8.11 287
19:00 03/24/2008 79.9 8.11 288
20:00 03/24/2008 79.7 8.07 292
21:00 03/24/2008 79.2 8.03 297
22:00 03/24/2008 79.3 8.01 300
23:00 03/24/2008 79.4 7.98 300
00:00 03/25/2008 79.6 7.95 301
01:00 03/25/2008 79.6 7.93 303
02:00 03/25/2008 79.6 7.91 306
03:00 03/25/2008 79.5 7.89 310
04:00 03/25/2008 79.4 7.94 308
05:00 03/25/2008 79.3 7.90 311
06:00 03/25/2008 79.1 7.88 314
07:00 03/25/2008 78.9 7.86 315
08:00 03/25/2008 78.8 7.87 315
09:00 03/25/2008 78.6 7.89 313
10:00 03/25/2008 78.4 7.91 311
11:00 03/25/2008 78.5 7.91 306
12:00 03/25/2008 79.0 7.97 310
13:00 03/25/2008 79.6 7.97 306
14:00 03/25/2008 79.8 8.00 308
15:00 03/25/2008 79.6 8.06 308
16:00 03/25/2008 79.4 8.10 308
17:00 03/25/2008 79.2 8.11 314
18:00 03/25/2008 79.0 8.10 316
19:00 03/25/2008 79.5 8.11 313
20:00 03/25/2008 79.7 8.05 319
21:00 03/25/2008 79.8 7.98 324
22:00 03/25/2008 79.8 7.95 328
23:00 03/25/2008 79.8 7.92 331
00:00 03/26/2008 79.8 7.89 331
01:00 03/26/2008 79.8 7.86 334
02:00 03/26/2008 79.8 7.84 336
03:00 03/26/2008 79.8 7.82 338
04:00 03/26/2008 79.7 7.80 342
05:00 03/26/2008 79.5 7.79 339
06:00 03/26/2008 79.4 7.78 344
07:00 03/26/2008 79.3 7.77 344
08:00 03/26/2008 79.2 7.75 347
09:00 03/26/2008 79.1 7.75 345
10:00 03/26/2008 79.0 7.75 344
11:00 03/26/2008 79.0 7.79 244
12:00 03/26/2008 79.5 7.87 316
13:00 03/26/2008 80.0 7.90 322
14:00 03/26/2008 79.8 7.93 323
15:00 03/26/2008 79.7 7.94 322
16:00 03/26/2008 79.6 7.96 323
17:00 03/26/2008 79.5 7.97 328
18:00 03/26/2008 79.3 7.96 330
19:00 03/26/2008 79.2 7.96 332
20:00 03/26/2008 79.4 7.93 333
21:00 03/26/2008 79.6 7.88 337
22:00 03/26/2008 79.7 7.87 339
23:00 03/26/2008 79.7 7.85 343
00:00 03/27/2008 79.7 7.84 344
01:00 03/27/2008 79.8 7.82 347
02:00 03/27/2008 79.7 7.82 347

 

[Randy Holmes-Farley]

Do you know what is in the supplements that you added?

 

[sditch]

The Sponge Power and Coral Vitilizer are Zeovit suppliments and they do not go in great detail to tell you whats in them. I do know the Coral Vitilizer has some ajents that can lighten up corals.

The Prodibio has good nuitrent hungry eating bacteria from what I understand as well. Not reall sure Randy, I just know that Im starting to see clearer water and higher ORP like I use to see when I used ozone.

The Corals are lighting up alittle and my guess would be from the Coral Vitilizer.

 

[Randy Holmes-Farley]

I just wonder if there are any oxidizing (ORP raising) species in any of those additives.

 

[sditch]

It could be, would that be a bad thing?

 

[hypernesia]

www.prodibio.fr/anglais/accueil.htm
This link tells you about the prodibio and what bacteria strains are used. I am currently on the edge about trying it. I haven't looked up the sp. of bacteria yet to verify all of their claims although some are Nitrosomas, and most of us know about those

 

[sabbath]

BioTrace from Prodibio is listed for fresh water on their site. Did you use it in saltwater?

 

[hypernesia]

Bioptim is for saltwater but the page on their site is currently down, there is a thread on here from a couple of years ago about a guy (Iwan) I think from Switzerland who was using this and parts of the Zeovit system and his tank was gorgeous. A guy from the UK on that thread was using the Biotrace The Biodigest is bacteria and the Bioptim / Biotrace is bacterial food.

 

[sabbath]

<a href=showthread.php?s=&postid=12200500#Post target=_blank>Originally posted</a> by hypermedia
Optima is for saltwater but the page on their site is currently down, there is a thread on here from a couple of years ago about a guy (Iowan) I think from Switzerland who was using this and parts of the Soviet system and his tank was gorgeous. A guy from the UK on that thread was using the Bio trace The Bio digest is bacteria and the Optima / Bio trace is bacterial food.

OK, I clicked on this link http://www.prodibio.fr/anglais/accueil.htm
when you hover over it. It shows for fresh?

 

[hypernesia]

I was just at the website if you click to use, then hover over, the Bioptim link it says Marine Water. Maybe just a glitch but for sure the Trace is for fresh and the Optim is for Marine. Marine Depot sells the system, check it out just put Prodibio into the search on their website, not bad pricing and way cheaper than Zeovit

 

[sditch]

I have only started using it for 2 weeks so Im sure it will take some time but so far everything looks happy. The water looks alittle clearer so far so good.

I will try it to see if I can get lower nuitrient levels without going zeovit.

 

[Randy Holmes-Farley]

It could be, would that be a bad thing?

Not necessarily, but it is a very different conclusion than if the ORP was raised for other reasons. IMO, there is no benefit from raising ORP by simply adding high redox chemicals, but there might be from other changes to husbandry practice that raised it.

 

[m2434]

BTW, In the data provided, there is not actually any significant change in pH variance, ORP or temp....

it may be more obvious if you look at the data like this:


min max p
day PH PH Hdiff

1 7.88 8.11 0.23
2 7.87 8.13 0.26
3 7.86 8.12 0.26
4 7.93 8.10 0.17
5 7.82 8.10 0.28
6 7.87 8.10 0.23
7 7.93 8.15 0.22
8 7.78 7.98 0.20
9 7.75 8.08 0.33
10 7.93 8.11 0.18
11 7.86 8.11 0.25
12 7.75 7.97 0.22
13 7.82 7.84 0.02


min max
day ORP ORP ORPdiff

1 227 235 8
2 221 241 20
3 239 251 12
4 251 265 14
5 262 271 9
6 261 274 13
7 216 226 10
8 226 240 14
9 240 258 18
10 261 300 39
11 301 331 30
12 244 347 103
13 344 347 3



min max
day Temp Temp tempdiff

1 79.1 80.7 1.6
2 79.1 80.0 0.9
3 79.1 80.5 1.4
4 79.1 81.1 2.0
5 79.0 79.9 0.9
6 78.2 79.9 1.7
7 78.1 80.0 1.9
8 79.0 80.0 1.0
9 79.0 79.8 0.8
10 78.6 79.9 1.3
11 78.4 79.8 1.4
12 79.0 80.0 1.0
13 79.7 79.8 0.1

 

[sditch]

m2434,

Please keep in mind I only uploaded the data from when I started the adjetives(2 Weeks). I may agree the pH may not be that big of a swing put the ORP has inproved for sure, You can see the rise upo to Day 10, then you have to consider a water change that was done.

After a water change it takes serveal days to get back to normal.

 

[m2434]

Yes, there appears to be a relationship between time and ORP.

As stated above, looking at daily pH swings, there isn't any
significant association between daily pH swings, time, temp swings or ORP swings.

However, on an hourly scale, a relationship with pH and ORP, as well as ph and temp can be seen. Also, a relationship with ORP and temp, independent of pH, can be seen in the data (maybe due to temp sensitivity of the probe??).

So, with more data, after accounting for temp and ORP swings, there may be a daily pH trend. There just isn't anything definitive yet...

 

[sabbath]

Interesting, I always thought orp was affected by Ph. Maybe it is temp? Here is my controllers graph from just now. Note; I added some corals around 4 oclock.

 

[Randy Holmes-Farley]

ORP is impacted by pH for well known reasons. I discuss it here:

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

from it:

The Theoretical Relationship Between ORP and pH

One of the complications of ORP is that the measured value can sometimes depend on pH. Whether ORP does depend on pH or not, and to what extent, is determined by the exact redox reactions that are involved in controlling the ORP in that solution. There have been equations proposed that purport to "correct" ORP for changes in pH, giving a new parameter, sometimes called rH. This parameter was proposed in the 1920's by W. M. Clark.7 One form of this correction is shown below:

rH = mV / 29 + (2 x pH)

and sometimes a correction for changes in oxygen concentration is thrown in:

rH = mV / 29 + (2 x pH) + [O2]

where [O2] is the concentration of O2 in ppm. The use of rH, however, presupposes a detailed understanding of the reactions involved, and is simply wrong for general use (as shown below). In a book8 that he published 40 years after his initial publication, Clark stated:

"At this point the author must confess to the introduction of rH. He conceived that there might be occasions when it would be convenient to speak of relative oxidation-reduction intensity without having to specify both potential AND pH...
...Unfortunately both the original intent and the obvious limitations have been overlooked by many who have converted their potentials for SPECIFIC SYSTEMS to rH numbers...
...In brief, rH has become an unmitigated nuisance."

Nevertheless, many people still use rH. Since it is imbedded in many articles relating to aquarists, it is worth understanding where the pH dependence comes from, and why it is not always the same.

As an example of a solution where the redox is not pH dependent, take a solution of Fe++ and Fe+++ in water, with no other redox active species. In that case, the ORP is exactly determined by the relative concentration of the two iron species, and is unchanged with pH.

Fe+++ + e- <---> Fe++

Specifically, the defining equation here is:

(does not copy well)

The main thing that is clear from this equation is that the ORP is independent of pH, and only depends on the relative concentrations of Fe++ and Fe+++.

The easiest way to think of the lack of pH dependence here is to recognize that neither H+ nor OH- participate in the reaction at all. So changing the pH has no direct impact on the reaction.

For many reactions where oxygen is an important participant, however, that is not the case:

O2 + 4H+ + 4e- <---> 2H2O

In this reaction, H+ does participate. Consequently, the oxidizing power is related to pH. As H+ is raised (by lowering pH), the reaction is driven to the right. One way to think of this is by LeChatlier's Principle where increasing the concentration of one species drives the reaction to the other side. In this case, lowering the pH increases the oxidizing power of the oxygen, and consequently raises the ORP. This result is the basis for the development of rH for many systems.

It is beyond the scope of this article to go into the detailed mathematics behind the pH dependence of ORP measurements, but Pankow does cover such issues in great detail in Aquatic Chemistry Concepts.9 For our purposes, an important result is that the magnitude of the change in ORP with pH depends entirely on the number of H+ involved in the reaction per electron. In the case of the Fe+++/Fe++ situation, this value is zero. For the oxygen/water reaction, the value is 1.0. The standard definition of rH assumes that this ratio is exactly 1.0. Consequently, it may not apply to many redox reactions that take place in aquaria.

Shown below are some typical reactions that also take place in aquaria. First, the oxidation of acetic acid to carbon dioxide, again with one H+ per electron (this reaction typifies many reactions involving neutral organic materials):

2CO2 + 8H+ + 8e- <---> CH3COOH + 2H2O

but if the same reaction proceeds with acetate, the reaction is:

2CO2 + 7H+ + 8e- <---> CH3COO- + 2H2O

and the ratio of H+ to e- is no longer 1.0, but is now 0.875.

For the various reactions of the nitrogen cycle, we have ratios that vary from 1.0 to 1.33:

NO2- + 7H+ + 6e- <---> NH3 + 2H2O

NO2- + 8H+ + 6e- <---> NH4+ + 2H2O

NO3- + 2H+ 2e- <---> NO2- + H2O

N2 + 6H+ + 6e- <---> 2NH3

N2 + 8H+ + 6e- <---> 2NH4+

The iodide/iodate reaction fits the 1.0 ratio:

IO3- + 6H+ + 6e- <---> I- + 3 H2O

Some other redox reactions that have other ratios:

MnO2 + 4H+ + 2e- <---> Mn++ + 2H2O

SO4-- + 10H+ + 8e- ---> H2S + 4H2O

SO4-- + 9H+ + 8e- ---> HS- + 4H2O

So if one really wants to understand how ORP would change with pH, one would have to know what the species are in aquaria that control redox. If it is a mixture of species, then the end result will come back as a complex averaging of the different reactions involved. Unfortunately, the species involved have not been clearly defined for seawater. In aquaria, which vary considerably in the concentrations of many redox active species, the situation is even more complicated.

The Empirical Relationship Between ORP and pH in Aquaria

While understanding the details of the theoretical relationship between pH and ORP is complicated, measuring it for a single aquarium is fairly easy. Figure 1 shows simultaneous plots of pH and ORP values over the course of several days in the aquarium of Simon Huntington. Clearly, the measured ORP and the pH are on exactly opposite cycles, as one would expect from a system where reactions involving oxygen are important (and as is shown by rH).

Does the reaction exactly follow the one H+/e- rule? Maybe not exactly. Figure 2 shows a plot of rH as a function of time using Simon's data. If the effects of pH on ORP were exactly removed by calculating rH using:

rH = mV / 29 + (2xpH) + 6.67

then one might expect rH to not have a diurnal cycle. In this figure, the data suggest that there is still a diurnal dependence to rH, possibly due to pH effects. I have seen data from other aquaria as well, and in those cases the same holds: that rH largely compensates for ORP changes with pH, but not perfectly. Since things other than pH (such as O2) may change during the night and day in aquaria, this experiment may be confounded by these other variables.

Simon also ran an additional experiment on his aquarium. He took a water sample, and added either sulfuric acid or sodium hydroxide to it to adjust pH. In this experiment, the other factors that might cycle diurnally in an aquarium are constant. The results are shown in Figures 3 and 4. The fact that the ORP goes almost exactly back to where it was at the start, despite the pH excursions, suggests that the acids and bases are not altering the "base" ORP, but are only impacting ORP through pH.

The ORP moves inversely to pH, as expected (Figure 3). But, the fact that the rH is generally not flat as the pH is changed (Figure 4), but rather tracks with pH changes, suggests that the mathematical conversion used (rH = mV / 29 + (2xpH) + 6.67) is overcorrecting for pH changes. That result in turn implies that the pH dependence of ORP may be less than predicted by the H+/e- ratio of 1.0. Perhaps this result indicates that in Simon's aquarium, some reactions with an H+/e- ratio below one are important in controlling ORP.

Overall, my suggestion for aquarists using ORP measurement devices is to be aware of how pH can influence ORP measurements, but to not overly emphasize specific pH corrections.

 

[sditch]

Thats good stuff Randy,

When I used ozone I always noticed it was hard to raise the ORP durning the day, but at night the ORP would reach its hightest levels while the pH was also at its lowest levels.

I also believe that the my Aqua controller uses the temp for the ORP and pH or one of the two. I noticed this when my temp sensor went bad the AC3 was showing some really weird numbers on ORP or pH, maybe both. Once I got the temp sensor swapped things when back to normal.

So I would guess the AquaControllers rely on temp for calaburation for pH or ORP.

 

[Randy Holmes-Farley]

I do not think that ORP has a direct temperature component, but your meter may have automatically corrected the pH for temperature.

 

[m2434]

Well, I'd say it is not compensating based on the data, or there is a strong bias in the compensation.

Also, I have two questions. What is your lighting schedule sditch? I ask because there is a very strong phase-relationship between the light cycle and pH and I'm guessing you have MH's or some other hot lights. I believe that this is causing a slight phase-relationship between temp and the light cycle.

Second, there appears to have been a water change around hour 143, but there is also a big dip in ORP after hour 244, do you know why this is?

It would be good to have alk data, but I realize this would be tough without an electronic alk monitor...