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reefer1970
09/27/2017, 08:01 PM
My tank has been at .66 phosphates for weeks while using gfo. Switched to BRS high capacity gfo last night and was excited to see results today. Was .87 in tank and .75 out of gfo reactor!?. Tested my RO water and it was .07 and I "believe" my syringe and plastic cup were clean. I'll test the RO again but can I just use distilled water for a zero ppm dilution? Also, I did feed a good dose of Phyto Feast yesterday, does anyone know if that is notorious for having phosphates?


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bertoni
09/27/2017, 08:26 PM
Distilled water should be fine for that purpose.

dkeller_nc
09/28/2017, 12:41 PM
I addressed some of your question in your previous post. Presuming that you're using the Hanna phosphorus checker, then yeah, you're RO water is plenty clean enough to use as a blank.

However, one thing to note is that it's generally not necessary to add the phosphate reagent to the blank. And if you're not adding the phosphate reagent to the blank, then it doesn't matter how much phosphate the blank has in it.

Finally, realize that a lot of tank water has color compounds in it that interfere with the blue color that the Hanna checker is looking for to quantitate your phosphate.

So based on the above note, the proper way to use the Hanna system is to fill the vial with tank water (but no powdered reagent), blank the meter, remove the vial (the same vial), add the powdered reagent, agitate the vial for 2 minutes, replace it in the meter in the same orientation as you blanked the meter, and press and hold the button until the countdown clock starts.

If this is not what you're doing, you may well be chasing a ghost as far as your phosphate readings are concerned.

Dan_P
09/28/2017, 04:38 PM
Finally, realize that a lot of tank water has color compounds in it that interfere with the blue color that the Hanna checker is looking for to quantitate your phosphate.

The Hanna Phosphorous Checker is not going to detect the color of aquarium water, but that being said, the color will have no effect on the analysis because the Checker blank is aquarium water and that nulls out any absorption from the water.

dkeller_nc
09/28/2017, 05:54 PM
The Hanna Phosphorous Checker is not going to detect the color of aquarium water, but that being said, the color will have no effect on the analysis because the Checker blank is aquarium water and that nulls out any absorption from the water.

Yes, if correctly performed. Perhaps I've misunderstood, but it seemed the OP wanted to use RODI as the blank. There is actually some degree of adsorption of typical aquarium water at the adsorption maximum of the molybdenum-blue complex of 700nm, though I agree with you that it's not much.

Dan_P
09/28/2017, 06:34 PM
Yes, if correctly performed. Perhaps I've misunderstood, but it seemed the OP wanted to use RODI as the blank. There is actually some degree of adsorption of typical aquarium water at the adsorption maximum of the molybdenum-blue complex of 700nm, though I agree with you that it's not much.

Got it. I have never been able to measure a color absorption of aquarium water though I haven't given up thinking about approaches. Before using GAC, aquarium water in a five gallon white bucket definitely had a tinge of yellow compared to new salt water, but that is about a two foot optical path. This could be quantified with digital image colorimetry but a five gallon sample and five gallon sample is not convenient for routine measurements.

reefer1970
09/28/2017, 06:45 PM
I'm not using rodi as a blank, but as a dilution. The phosphates are too high for my Hanna checker so I'm using half rodi (or distilled) and half tank water. The Hanna is zeroed out by the color of the sample prior to adding the reagent. Does this sound correct?


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bertoni
09/28/2017, 06:48 PM
The dilution idea is fine, although the results will lose precision.

Dan_P
09/28/2017, 06:58 PM
Yes, if correctly performed. Perhaps I've misunderstood, but it seemed the OP wanted to use RODI as the blank. There is actually some degree of adsorption of typical aquarium water at the adsorption maximum of the molybdenum-blue complex of 700nm, though I agree with you that it's not much.

By the way, the light source for the Checker has a wavelength of 525 nm, the maximum for the phosphomolybdate complex is around 850 nm (700 nm works but 850 nm absorbs ~50% more). This never made sense to me, shining a green light through a solution that absorbs red. The ULR Phosphorous Checker does work well for measuring the absorption of nitrate test solutions (red-pink solutions absorb green).

Dan_P
09/28/2017, 06:59 PM
The dilution idea is fine, although the results will lose precision.

Why?

bertoni
09/28/2017, 07:16 PM
There's going to be some lack of precision in the dilution itself. Also, the ULR is not rated for freshwater, so I'm going to guess the software is going to have accuracy issues.

dkeller_nc
09/28/2017, 07:50 PM
By the way, the light source for the Checker has a wavelength of 525 nm, the maximum for the phosphomolybdate complex is around 850 nm (700 nm works but 850 nm absorbs ~50% more). This never made sense to me, shining a green light through a solution that absorbs red. The ULR Phosphorous Checker does work well for measuring the absorption of nitrate test solutions (red-pink solutions absorb green).

Yeah, I remember your investigations into using it for quantitative nitrate assays. Clever bit of work, btw.

And yeah, I'm aware of the actual absorbance maximum. The kind of weird thing is the original paper set the assay up for 700nm. I guess it was just the limitations of the old halogen lamp sources for Vis/IR or the photomultiplier tube's sensitivity in the old spectrophotometers.

reefer1970
09/28/2017, 08:57 PM
Operator error maybe? Maybe you put in 4.9 ml of rodi and 5.1 of tank water?
Btw does anyone know if a weak battery in the checker would give false readings?


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bertoni
09/28/2017, 10:31 PM
A weak enough battery could cause a wide range of problems.

dkeller_nc
09/29/2017, 07:08 AM
I'm not using rodi as a blank, but as a dilution. The phosphates are too high for my Hanna checker so I'm using half rodi (or distilled) and half tank water. The Hanna is zeroed out by the color of the sample prior to adding the reagent. Does this sound correct?


Ah, I understand now. Yes, so long as you're blanking the meter on the same diluted sample that you're running the test on, then you're good.

This is a guess, because even with an advanced chemistry degree I can't say for sure without running experiments, but you should be OK with a 1:1 dilution of your tank water with RODI. I'd be a lot less certain if you were running a 1:10 dilution of tank water to RODI, or you were running it on freshwater alone.

But, I really wouldn't bother. Presuming that you are trying to drop the phosphate below the meter's upper range (200 ppb, IIRC), then I'd suggest simply continuing with water changes and GFO until the phosphate eventually drops into the meter's range.