Making sense of Phosphate readings

[asylumdown]

Normally I don't pay much head to nitrate and phosphate tests, and I normally only test the major ions once every couple of weeks. I'm paying extra special attention right now because things were so unbelievably out of whack, and then I took everything offline, so I'm doing all this testing to get things back on track. Once all my systems are back in place and my auto-dower is dialled in I'll probably go back to hardly ever testing.

And I'm going to stop trusting my hanna checker. With the rowaphos in there (and I used about 15-20% of the amount I would normally have put in if it was just regular bulk GFO), I was seeing a steady climb every day. I changed out the Rowaphos last night because I thought that maybe with such a small amount in the reactor it had honestly already been exhausted, but this morning it measured 0.21 after the conversion to PPM Phosphate, which shouldn't be possible unless the Roawphos was actually contributing phos rather than removing it. I've been using the same glass vial for each test though, and right after I got the 0.21 result, I swapped out the vial for one of the vials from my calcium kit and got a result of 0.008ppm phosphate.

I think my vial has been getting imperceptibly stained by each sequential test. I gave it a thorough wash with a bottle brush cleaner and laboratory soap two days ago, and I empty and rinse the vials with distilled water immediately after each test, but it's apparently not enough. I don't know how else to clean it, and it looks spotless to my eyes, but when one vial gives you 0.21ppm and another vial gives you 0.008 with the same water following the exact same protocol, it makes me not trust any of the results if the equipment alone can contribute such a wide margin of error.

As for the nitrate tests, today it was 2ppm. It's been a clear and steady increase since I took the pellet reactor offline and stopped doing massive daily water changes. More than anything I think it's just interesting to see, as my tank has never not had pellets on it so I really had no idea what it would do without them. I don't necessarily trust the numbers exactly, but I do trust the relative colour development which indicates that my nitrates are definitely rising. The Red Sea nitrate kit uses the same reagent protocol as my faculty's analytical hydrogeology lab (which really means it shouldn't be going down the sink as there's cadmium in it, but Red Sea doesn't tell you that), and while my lab would use a Hach spectrophotometer to measure the colour exactly, you can get a really good qualitative idea of the amount of nitrate in the sample just by looking at how red/pink it turns. And every day since wednesday my sample has been progressively pinker than the day before.

[Magickiwi]

Originally Posted by asylumdown

Normally I don't pay much head to nitrate and phosphate tests, and I normally only test the major ions once every couple of weeks. I'm paying extra special attention right now because things were so unbelievably out of whack, and then I took everything offline, so I'm doing all this testing to get things back on track. Once all my systems are back in place and my auto-dower is dialled in I'll probably go back to hardly ever testing.

And I'm going to stop trusting my hanna checker. With the rowaphos in there (and I used about 15-20% of the amount I would normally have put in if it was just regular bulk GFO), I was seeing a steady climb every day. I changed out the Rowaphos last night because I thought that maybe with such a small amount in the reactor it had honestly already been exhausted, but this morning it measured 0.21 after the conversion to PPM Phosphate, which shouldn't be possible unless the Roawphos was actually contributing phos rather than removing it. I've been using the same glass vial for each test though, and right after I got the 0.21 result, I swapped out the vial for one of the vials from my calcium kit and got a result of 0.008ppm phosphate.

I think my vial has been getting imperceptibly stained by each sequential test. I gave it a thorough wash with a bottle brush cleaner and laboratory soap two days ago, and I empty and rinse the vials with distilled water immediately after each test, but it's apparently not enough. I don't know how else to clean it, and it looks spotless to my eyes, but when one vial gives you 0.21ppm and another vial gives you 0.008 with the same water following the exact same protocol, it makes me not trust any of the results if the equipment alone can contribute such a wide margin of error.

As for the nitrate tests, today it was 2ppm. It's been a clear and steady increase since I took the pellet reactor offline and stopped doing massive daily water changes. More than anything I think it's just interesting to see, as my tank has never not had pellets on it so I really had no idea what it would do without them. I don't necessarily trust the numbers exactly, but I do trust the relative colour development which indicates that my nitrates are definitely rising. The Red Sea nitrate kit uses the same reagent protocol as my faculty's analytical hydrogeology lab (which really means it shouldn't be going down the sink as there's cadmium in it, but Red Sea doesn't tell you that), and while my lab would use a Hach spectrophotometer to measure the colour exactly, you can get a really good qualitative idea of the amount of nitrate in the sample just by looking at how red/pink it turns. And every day since wednesday my sample has been progressively pinker than the day before.

I use muriatic acid to wash my sample cuvettes. And I also think that they get stained after a few tests so that you can't trust the readings. I've dosed the Carib-Sea phosbuster twice in the last two weeks and according to the hanna checker my phosphate didn't go down at all the day after etc.

[asylumdown]

Well I did some digging to try and figure out what product from the Hanna reaction could be staining the glass vials and found this from the EPA methods manual on determining phosphate using the Ascorbic acid method that I'm pretty sure Hanna is using. (http://water.epa.gov/scitech/methods...od_365_3.pdf):

6.2 Acid Washed Glassware: All glassware should be rinsed with hot 1:1 HCl and rinsed with distilled water. The acid-washed glassware should be filled with distilled water and treated with all reagents to remove the last traces of phosphorous that might be adsorbed on the glassware. Preferably, this glassware should be used only for the determination of phosphorous and after use it should be rinsed with distilled water and kept covered until needed again. If this is done, the treatment with 1:1 HCl and reagents is only required occasionally. Commercial detergents should never be used.

What I take away from this is that the glass itself can adsorb some P and because of this I should not have rinsed my vials in tap water (I usually rinse in tap first, then finish with a rinse in distilled), I should not have used soap (even though it was lab soap), that there is some product of the reaction that will eventually need to be cleaned with an acid wash (possibly calcium sulphate based on other readings), and that the P from your sample can get adsorbed in to the glass over time.

It's probably not a big deal for the high range checker, but since this checker is measuring in the parts per billion, even a small amount of contamination could throw off the results. The fact that we only use a single powder pillow means that Hanna must have combined the molybdate and tartrate with the ascorbic acid in a single package. An analytical lab would store those separately, but perhaps that's why there's a 2 minute dissolution period before you start your 3 minute reaction timer.

[lastlight]

so loosely translated i should just go back to watching for increased growth in my chaeto and swap my gfo then?

[asylumdown]

Originally Posted by lastlight

so loosely translated i should just go back to watching for increased growth in my chaeto and swap my gfo then?

yah I think that's as good a method as any. I'm not sure how many hobbyists have the set up to safely do a wash with hot 1:1 HCl. We also are only measuring orthophosphate with our tests, so a teeny weeny amount of the P moving through our systems.

Don't get me wrong, I think there's value in knowing your orthophosphate concentrations, as having a high concentration of dissolved orthophosphate is as problematic as having no detectable orthophosphate but luxuriant and uncontrolled growth of problem algae, but I think we have to take the results from those tests with a gigantic grain of (sea) salt. Hanna has given us the spectrophotometer necessary to view P down to the ppb range, but for consistently accurate results you also need the analytical lab procedures called for at that resolution. If we were being really picky about it, we'd also want to centrifuge our sample first to make sure there was no microscopic particulates in the solution, but I doubt many hobbyists are going to run out and pick up a $10,000 centrifuge any time soon (even though they're freaking awesome).

If you're changing your GFO with any regularity, your orthophosphate concentration is going to be low, but that doesn't mean that your system's total P load will be low, and P will still be moving through the biological systems in your tank in ways you'll never be able to test for as a hobbyist, so it's really only one component of the whole tank maintenance thing.