Properly Testing Phosphates in RODI

JustinS · Mar 30, 2015

Hi Randy,
I have several large systems that have problems with excessive algae and dinoflagellates. I have (4) 500 gallon systems ((15) 30 gallon tanks plus sump) with 2 small clownfish and one small bubble tip in each of the 30 gallon tanks that are target fed just a couple of pellets a day (like 4-5 tiny new life spectrum pellets) with anything missed immediately siphoned out. Every tank is covered in dinos and algae. The systems are setup to have a constant trickle of roughly 20 gallons each of freshly made seawater daily and are heavily skimmed with Orca Pro2 skimmers.

Ive been having a battle with my RODI company about the levels of phosphate in our DI water. They are under the impression that there is no way that there is phosphate getting past their system, but I am detecting almost 0.2 when testing for it. They claim that there must be an error in my testing. The massive amount of algae and dinos coupled with the tiny amount of food and large exchanges of water seems to contradict their claim, but I would like to verify my testing procedures as I know there can be errors when testing RODI.

I use a HACH 3900 spectrophotometer and the reactive phosphate reagent. I test both straight RODI and I also make up a fresh batch of seawater in a sterile container that has been acid washed and test that as a comparison. They do differ slightly.

My question is, am I using the correct test, and would this be an appropriate method to test for phosphates in the RODI? Do I need to do something different?

I have read your articles about phosphate in RODI and I do agree that food is probably the major source of phosphates in most cases. I know that topping off tanks with this water probably isnt a big deal, but wouldnt setting up a system with elevated water and constantly changing water with elevated phosphates be a problem? We can pull out enough algae that it exceeds the total biomass of the tanks by several times on a weekly basis and it still look like we hardly made a dent. I even have planted freshwater tanks with one fish using this RODI that are being overrun with algae
Thanks in advance.

Randy Holmes-Farley · Mar 30, 2015

0.2 ppm is starting to be significant.

The method sounds ok as long as it is being blanked properly.

You might test some water you think is low, like distilled from a store.

Do you detect phosphate in the tank itself?

JustinS · Apr 1, 2015

Thanks Randy,
I ran the test with some poland springs distilled yesterday and got a result of 0.06ppm, and then ran it again with some instant ocean mixed in and got a result of 0.16. So now Im starting to wonder if there is in fact some error in the test method, although I still have the unexplained algae and dino problems...

Randy Holmes-Farley · Apr 1, 2015

Can you describe in detail the process you are using?

Do you make a standard curve, or rely on an internal data set from Hach?

JustinS · Apr 10, 2015

I was using the internal program in the DR3900. The kit I used was this one Phosphorus (Reactive) TNT Reagent Set, Low Range | Hach USA - Overview | Hach Which I now see has a lower limit of 0.06mg/l. I follow the procedure outlined in the instructions here http://www.hach.com/asset-get.download.jsa?id=7639983835
Basically I was collecting the sample in acid washed and RODI rinsed containers and testing immediately. First blanking the sample, then adding the reagent, waiting 2 min and reading the sample. Then I would mix up a brackish solution with some instant ocean in an acid washed, RODI rinsed container and test that. The instant ocean sample was always a little higher.
I guess Im going to have to go through the validation procedure, but I wanted to make sure I was at least following the correct methodology for RODI. Im a little disappointed the reagents only go down to 0.06mg/l.

Randy Holmes-Farley · Apr 10, 2015

JustinS said:
I was using the internal program in the DR3900. The kit I used was this one Phosphorus (Reactive) TNT Reagent Set, Low Range | Hach USA - Overview | Hach Which I now see has a lower limit of 0.06mg/l. I follow the procedure outlined in the instructions here http://www.hach.com/asset-get.download.jsa?id=7639983835
Basically I was collecting the sample in acid washed and RODI rinsed containers and testing immediately. First blanking the sample, then adding the reagent, waiting 2 min and reading the sample. Then I would mix up a brackish solution with some instant ocean in an acid washed, RODI rinsed container and test that. The instant ocean sample was always a little higher.
I guess Im going to have to go through the validation procedure, but I wanted to make sure I was at least following the correct methodology for RODI. Im a little disappointed the reagents only go down to 0.06mg/l.

Yes, low phosphate readings by spectrometry are limited by the relatively low absorbance they create when reacted with the reagents.