Making sense of Phosphate readings

[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.