Error analysis of PO4 kits

[mat167]

I'm curious as to the range of error involved in our hobby grade tests. From what I've gathered, the accuracy involved doesn't justify paying top dollar for a photometer or colorimeter when a 20 dollar test has a similar potential for error. If I remember correctly, the hanna photometer was rated with an accuracy at +/- 0.04 ppm, which seems impressive unless we take into account the fact that we like our PO4 under 0.03 ppm.

I have never used one of these and I am by no means attempting to discredit these products. I really am just curious as to how a test with a greater error value than the number we wish to measure can be useful, or trusted.

Maybe it's just the way I'm looking at the situation. I liken this situation to having a ruler, that can measure 1 mm to 10 cm. If we wish to measure 4 mm, but the marks on the ruler are only guaranteed to be accurate within +/- 6 mm, then it becomes a significant source of error and we ought to question the reliability of our measurement.

Sorry if this post came out of the blue, or maybe this has been covered by someone else. It's been a long day and I probably won't be able to sleep until I at least put this out there. All I'm hoping for is for this to be at least comprehensible, and if I should be so lucky to have an individual with a keen interest in error analysis explain the flaw in my reasoning, then I'll sleep well.

 

[tmz]

Don't know. the hach test kit which is quite expensive may have a better accuracy rating than a hobby grade colorimeter. I think the other hobby test kits are less precise in low ranges than the colorimeter and it is hard to distinguish color shades with any precision They dont publish that information. I am sure you can get a lab grade colorimeter with less of a margin if you wish to spend more money .

 

[Billybeau1]

All hobby grade test kits have some margin for error. Nature of the beast.

The best of all of the hobby grade titration kits I have used is the Salifert. If the water is clear you likely have very low phosphate. Any tint of blue means you have work to do. That's good enough for me.

I have not tried a photometer so I do not know how it compares.

 

[KevChem]

Even with the best reagents and spectrophotometer the phosphate test is only good to about 0.010ppm P (about 0.030ppm PO4). Then there is the uncertainty as you pointed out. The test kits are all based on the same basic chemical reaction (phosphomolybdenum blue). The basis of the 0.030ppm recommendation is the bottom basement of the measurable level for these type of test kits.

Additionally, these test kits only measure free phosphate - not any P bound to phospholipids, DNA, ATP, etc. So there is likely more P in the tank than what you measure anyway.

Personally, I think if you take the steps necessary to minimize P, you shouldn't have to worry too much about the test result.

Kevin

 

[mat167]

Thanks guys, that clears things up a bit. I've been using salifert since I started, and I've always found it to be more than adequate. It was just one of those nagging questions that would have persisted until I asked it eventually.

 

[tmz]

FWIW I get a consistent clear water reading with the slalifert(0ppm) contrasted to ..04 to .07ppm per the Hanah colorometer for the same sample water. Crosschecked several times over several months.

This suggests at least anecdotaly weaker low range sensitivity by that test kit than by that colorimeter. OR simply my inability to see any blue in it as opposed to the ease of a digital readout.

 

[MrPike]

Additionally, these test kits only measure free phosphate - not any P bound to phospholipids, DNA, ATP, etc. So there is likely more P in the tank than what you measure anyway.

Exactly.

Its for this reason I don't even bother testing for phosphates. If I see algae, I know they are present and work to reduce them.

 

[tmz]

While hobby grade tests don't measure organic phosphate, it is the inorganic phosphate they measure that inhibits calcification when it takes the place of a calcium ion on a forming calcium carbonate crystal. This has an adverse effect on calcareous organisms including corals.

So,I think measuring inorganic / orthophosphate, commonly referred to as PO4, to guide efforts (feeding, gfo, carbon dosing, etc) is a good idea. Insuring it is below at most 0.10 ppm ,if that's the best the test will tell me, is important.
Organics containing phosphate are generally exportable via skimming granulated activated carbon and other organic removers and carbon bond breakers to some extent.

 

[MrPike]

Inorganic Phosphate bound to liverock is not testable with our hobby kits, but is still a problem. Especially if you glue a stony coral directly to it.