The dilemma when measuring phosphate

[JonasRoman]

Hi
This picture from an anonymous aquarist is a perfect example of what I often say: ICP tests can't measure phosphate directly. They measure phosphorus and calculate the phosphate from that, which means the result will always be falsely high to some degree. Photometry gives you the true value, but there's a catch: if you mail in a sample, the phosphate can be consumed in the vial before it reaches the lab. Summa summarum: mailed-in ICP results give you falsely high calculated phosphate to some extent, while mailed-in photometry often gives you falsely low readings to some extent. You really need to test phosphate yourself, right away, using a photometer (or a great test kit and very good eyes).

Jonas

 

[Dan_P]

Hi
This picture from an anonymous aquarist is a perfect example of what I often say: ICP tests can't measure phosphate directly. They measure phosphorus and calculate the phosphate from that, which means the result will always be falsely high to some degree. Photometry gives you the true value, but there's a catch: if you mail in a sample, the phosphate can be consumed in the vial before it reaches the lab. Summa summarum: mailed-in ICP results give you falsely high calculated phosphate to some extent, while mailed-in photometry often gives you falsely low readings to some extent. You really need to test phosphate yourself, right away, using a photometer (or a great test kit and very good eyes).

Jonas

Agree that determining the phosphate concentration is vexing.

Just a note, when a sample is sent to a vendor for analysis and if it is sterile filtered or stabilized, the affect of biological activity is minimized.

 

[JonasRoman]

Agree that determining the phosphate concentration is vexing.

Just a note, when a sample is sent to a vendor for analysis and if it is sterile filtered or stabilized, the affect of biological activity is minimized.

Its never sterilised, the bacterial activity is there even if it passes a microfilter, but I agree that the drawbacks described above is lower when doing this. But the problem is that you never know how much it impacts or not.

 

[Dan_P]

Its never sterilised, the bacterial activity is there even if it passes a microfilter, but I agree that the drawbacks described above is lower when doing this. But the problem is that you never know how much it impacts or not.

Unless you mess up the sterile filtration, it effectively stops biological activity (but not viruses or enzymes). The technique is a standard one.

 

[Dan_P]

Its never sterilised, the bacterial activity is there even if it passes a microfilter, but I agree that the drawbacks described above is lower when doing this. But the problem is that you never know how much it impacts or not.

A second thought. Since none of these measurements come with a standard deviation or confidence interval, you don’t have evidence that different phosphate measurements are statistically different. This might just be a case of noisy data. Much frustration in this area may just be caused by a large standard deviation. This may not be the case for ICP where total phosphorous is measured.

 

[EnterName]

Its never sterilised, the bacterial activity is there even if it passes a microfilter, but I agree that the drawbacks described above is lower when doing this. But the problem is that you never know how much it impacts or not.

A second thought. Since none of these measurements come with a standard deviation or confidence interval, you don’t have evidence that different phosphate measurements are statistically different. This might just be a case of noisy data. Much frustration in this area may just be caused by a large standard deviation. This may not be the case for ICP where total phosphorous is measured.

The statistics nerd in me agrees, but to be fair: The photometric result of 0.001ppm is 1.7% of the 0.058ppm MS result.

If a difference of 98.3% is within the standard error, then I don't think testing would make much sense at all. Of course the sample size is 1 right now and it could just have been coincidence, but there is a reason why labs started offering photometric PO4 tests.

 

[JonasRoman]

The statistics nerd in me agrees, but to be fair: The photometric result of 0.001ppm is 1.7% of the 0.058ppm MS result.

If a difference of 98.3% is within the standard error, then I don't think testing would make much sense at all. Of course the sample size is 1 right now and it could just have been coincidence, but there is a reason why labs started offering photometric PO4 tests.

Yes, this case is maybe extreme but just one randomly choosen so its for sure more. This case huge difference may be because of organics and bacteria in sample water. As you never know nor can control that you can not 100% trust the mailed po4 results. I mean, the sample contains N aswell. And combo N and P in a living water =consumption of both. There is for sure at least some carbon to manage this.

In my personal cases I always see difference in those directions that this example show us, but not so much. More like 25% deviation from true results. I do not dose carbon.

When I test at home and just let water stand, po4 is reduced to 0 quite fast. I have made a small study of this. :-) (filtered water )

Jonas

 

[JonasRoman]

A second thought. Since none of these measurements come with a standard deviation or confidence interval, you don’t have evidence that different phosphate measurements are statistically different. This might just be a case of noisy data. Much frustration in this area may just be caused by a large standard deviation. This may not be the case for ICP where total phosphorous is measured.

Agree on that but as this is a pro lab I assume that the photometric method at least have a accuracy and precision of 0.02 mg/l. But yes if that "machine " is worse than that its more or less useless for such a professional test.
The P I guess acc and prec is as others for icp oes, like 2-3% , that would be +- 0.005 mg/l PO4. Good enough ofcourse.

Jonas

 

[JonasRoman]

This good discussion encourages me to do a study again. At the moment I have 0.1 ppm in my tank.That's quite high.So I would take an opportunity now to see how fast it will be lowering when I take the water side. I will do one test with filtered water and one without. I dose no carbon.

 

[CHSUB]

Yes, my most recent ICP had po4 (photometer) @ .008; Hanna had .06….

 

[EnterName]

Yes, my most recent ICP had po4 (photometer) @ .008; Hanna had .06….

Did you simply fill the sample vial by dipping it into the water or did the kit come with a syringe and filter?

 

[CHSUB]

Did you simply fill the sample vial by dipping it into the water or did the kit come with a syringe and filter?

Syringe and filter included. No3 with Hanna LR and ICP were both 0.1 ppm, so results consistent. Only po4 was different from my hobby testing vs ICP.

 

[EnterName]

Did you simply fill the sample vial by dipping it into the water or did the kit come with a syringe and filter?

Syringe and filter included. No3 with Hanna LR and ICP were both 0.1 ppm, so results consistent. Only po4 was different from my hobby testing vs ICP.

Do you remember what your hobby PO4 test said?

 

[CHSUB]

Do you remember what your hobby PO4 test said?

Yes, measured all parameters at same time I sent ICP, Hanna PO4 was .06 ppm. Also Salifert calcium read 390 ppm and ICP had calcium at 468 ppm…way off also.