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#568437 07/04/24 09:49 AM
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I’m trying to decide on a phosphate measurement system. I understand the desired level is less than 0.1 ppm although some references indicate 0.04 ppm to avoid eutrofication on. All but one system I have seen are barely able to distinguish that small amount, if at all. There is one system I have seen that looks like it would be able to detect that amount, but it is advertised for use in salt water, with no reference to fresh. I would like to make these measurements myself. What are people using?

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Phosphorus is IMO a tricky complex chemical to accurately determine for the average non-laboratory analyst. Accurately measuring phosphorus is not an easy chemical to accurately test compared to alkalinity, hardness, pH and nitrogen.


For testing phosphorus there are two basic types: “soluble” and is preferred over “dissolved” or “filterable” and the term “particulate” as chemically bound forms is preferred over “non-filterable.”

If samples are to be tested later, the samples should technically be preserved to reduce the phosphorus concentration from changing due to various biological and chemical reactions. Be aware that several substances can interfere with the phosphorus analysis in samples.

First, the type of phosphorus for testing is converted to dissolved orthophosphate. Second, the dissolved orthophosphate is then determined colorimetrically. If one visually evaluates the color change and does not use a colorimeter or spectrophotometer then the results are “ball park” range at best. Thus the accuracy of the results needed is determined by the type of test used.

Done correctly total phosphorus and total soluble phosphorus require a digestion step prior to the measurement of the orthophosphate form. The three digestion methods most commonly used during total phosphorus analysis include perchloric acid, nitric acid-sulfuric acid, and persulfate oxidation methods. Phosphorus occurs in association with organic matter, which must be effectively oxidized to release phosphorus as orthophosphate.

The amount of particulate (non-filterable) phosphorus or soluble (filterable) types depends on the type and size of filter (0.45um) used in the analytical procedure. While this filter pore size is small enough to exclude all bacteria, colloidal particles will still pass through the filter—meaning that very small chemical precipitant particles will pass through the filter paper and be measured as filterable or “soluble.”


https://www.waterrf.org/sites/default/files/file/2022-09/Measuring-Low-P-Concentrations.pdf

Last edited by Bill Cody; 07/04/24 04:51 PM.

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While I majored in Chemistry and did a ton of lab analysis then, I stayed away from glassware (I broke it too often) in career, so I appreciate the complexity if this issue, especially in nature where you have almost no knowledge of all contributing factors. That looks like a good reference so I’ll give it a good read. Thanks

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Bill Cody. Your reference makes me happy that I avoided that field! The vey last paragraph sums it up well:

"Measurements from various laboratories for phosphorus concentrations less than 50 µg/L
have shown a large variability and have proven to be a challenge. While laboratories follow
standard procedures, some variability in results still exist. Variations above 100 percent are
not uncommon. A new standard procedure for phosphorus analysis is needed to eliminate
this variability.'

And these are tests under certified laboratory conditions, not hobby tests!

50 ug/L is 0.05 ppm, which is commonly quoted as the lowest value that is safe from the process of eutrophication. So the answer to my question seems to be: if you see any phosphorous in your water with any hobby test, it's probably too much.

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For slow or slowing eutrophication of your water feature regarding phosphorus, I really like the insightful summary by 'rivershed':
Quote
50 ug/L is 0.05 ppm, which is commonly quoted as the lowest value that is safe from the process of eutrophication. So the answer to my question seems to be: if you see any phosphorous in your water with any hobby test, it's probably too much.

Last edited by Bill Cody; 07/06/24 01:00 PM.

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