Phosphates heat test

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I have to say I am very surprised at what an odd test this is and the inaccurate way it is described.

There are so many issues I hardly know where to begin.

First, I’ve not really noticed that folks have ICP values for P in great excess of what they see by kits, but icp will detect both organic firms and inorganic forms. Normal test kits detect only inorganic forms.

Some test kits (e.g., Hach PO-24) can detect organic forms by digesting organics completely with an oxidizing compound, an acid persulfate, at high temperatures. Then detecting as usual for inorganic phosphate. Very few reefers do this and I wouldn’t even know what organic phosphate levels are optimal.

The linked test heats the water but doesn’t digest it. It may breakdown some of the most fragile organic phosphates into detectable forms, but much of it won’t degrade to inorganic P so readily, hence the tedious Hach procedure. So it is measuring inorganic plus some fraction of organic P, including the organic P in whole living bacteria. I really have no idea what values that would be. If you want total P, use icp or a validated method like Hach.

Beyond that, the link talks about depot phosphate in the tank, including minerals. Those depots are potentially huge and important, but obviously not at all detectable by this heating of water method. So why mention them as important, and then not be able to detect them (which the link doesn’t even clarify)

So I see various possible ways to test phosphate, but this heated water one doesn’t seem useful to me.

Most distressing, the directions seem to state that the “depot factor” as they measure it is an indication of phosphate bound to rock. Nonsense. 1 ppm inorganic phosphate and no organic phosphate will have a depot factor if 1 , but will have massive amounts of phosphate bound to rock and sand surfaces in potentially great excess to that in the water itself. One of our members recently tested that for themselves. The amount of pure inorganic phosphate that can bind is huge.
 
Avast
Thanks Randy!
It seems a very strange thing also for me but the microwave effect over organic particles has raised some doubts on my mind.
I’ve made the test using my Hanna ULR and the result was a 1 ppb diference between heated and normal water which for me means that both tests were done correctly and there was no difference but since the ULR test for Phosphorus other doubt came so I’ve decide to ask.
In conclusion, it’s not today that I’ll find which amount of P I have deposited in my tank. :-)
 
Can there be a reason why the sample is heated in a microwave oven and not boiled on a stove? Can its be that way that this method was posted before the ICP test was in common use? Would love to hear more of the theoretical things around this. @Hans-Werner - do you have more information about this?

I can agree in a lot of what Randy says but on the other hand - Fauna Marin and H. Schuhmacher are in Europe known to be very serious and will not - IMO - state anything without having something on their feet - right or wrong - I do not know but it could be interesting to hear their argues.

Sincerely Lasse
 
Yes Lasse, I also have that and more Companies in high consideration and I can send them, privately, some questions about it.

I just ask for comments since as described it’s a quite simple approach for a question that almost every reefer has, I never heard about it and don’t understood how it works.

I understood everything Randy said and if the Company can give more information and demonstrate how it works it will be a remarkable contribution for the hobby.
 
AS
FWIW, if you want to read about microwave and other methods of breaking down organic phosphates, you can read some on it here:

Determination of phosphorus in natural waters: A historical review
http://www.jlakes.org/config/hpkx/news_category/2017-05-27/1-s2.0-S0003267016302860-main.pdf

2.3. Sample digestion For the determination of total phosphorus (TP; unfiltered samples) and total dissolved phosphorus (TDP; filtered samples) it is necessary to convert all of the phosphorus-containing species into a detectable form. For natural waters the most common method of detection is the “molybdenum blue” chemistry with spectrophotometric detection which determines molybdate-reactive orthophosphate (PO4 3) using either a batch or flow-based approach (see x 3.1) [50]. Inductively coupled plasma based methods (ICP-AES and ICP-MS) can also be used if the concentration of P is sufficiently high. The process of conversion, which involves the breaking of PeOeP, CeOeP and CeP bonds in condensed and organic P compounds is called digestion and is typically achieved by thermal oxidation with hydrolysis using either an autoclave, digestion block or microwave [51] or by UV photo-oxidation with or without heating. Autoclaving is generally simple and quick, gives reproducible results and is carried out in sealed vessels to minimise contamination [44,47]. Peroxydisulfate is the preferred oxidant and was first used for the determination of TP in seawater in the 1960s [52]. Since then a variety of peroxydisulfate methods, in either acidic [53] or basic [54] media, have been reported [47]. Autoclaving with alkaline peroxydisulfate, rather than acidic peroxydisulfate, is recommended for the simultaneous determination of TP and total nitrogen (TN) and for the digestion of marine waters due to the oxidation of chloride to free chlorine by peroxydisulfate in acidic media, which reduces its oxidising power [55]. When alkaline peroxydisulfate digestion is used, autoclaving or thermal heating should be continued until S2O8 2 is converted to hydrogen sulfate (HSO4 ; pKa ¼ 1.99) so that a low pH is reached in the latter stages of the digestion and acid hydrolysis of condensed phosphate species is achieved [51]. An acidic peroxydisulfate method was reported by Gales et al. [56] and simplified by Eisenreich et al. [57]. The method gives good recoveries and is simple and easy to use and is therefore recommended for the determination of TP and TDP in fresh waters [58]. UV photo-oxidation can be used for the digestion of marine and freshwaters [41,59] but if the sample contains condensed polyphosphates, heating with HCl or peroxydisulfate after UV irradiation is recommended [60]. UV photo-oxidation also gives good recoveries when incorporated into a flow injection (FI) manifold P. Worsfold et al. / Analytica Chimica Acta 918 (2016) 8e20 11 [61,62]. Microwave digestion has also been used in flow systems in conjunction with spectrophotometric detection [51,63] and ICP-MS detection [64]. The above methods provide a quantitative measurement of TP or TDP but if information on specific classes of P compounds is required, a more selective method of sample treatment is necessary, e.g. the use of phosphate cleaving enzymes such as acid and alkaline phosphatases, and this is discussed in x 3.3.
 
Chewy.com
FWIW, article 51 in the above link (mentions microwaving) is not microwaving alone, but actually the sort of oxidation I was referring to:

Determination of total phosphorus and nitrogen in turbid waters by oxidation with alkaline potassium peroxodisulfate and low pressure microwave digestion, autoclave heating or the use of closed vessels in a hot water bath: comparison with Kjeldahl digestion
https://www.sciencedirect.com/science/article/pii/S000326700200346X

"The evaluation of the use of alkaline peroxodisulfate digestion with low pressure microwave, autoclave or hot water bath heating for the determination of total phosphorus and nitrogen in turbid lake and river waters is described. "
 
So it’s a lazy Saturday. Thought I’d try it just for something to do.

Followed instructions exactly as listed and tested with Red Sea PO4. Samples taken at same time.

Unheated: shows estimated .05 ppm
5E19E2BB-7D5F-43DA-BB8B-A3BF4E84A99D.jpeg


Heated: shows estimated.06 ppm
9BB42FBC-DB29-4F80-B255-0B4B5911AD4F.jpeg


I know it says to use a PO4 kit but I used the Hanna Phosphorus ULS to validate the Red Sea.

Unheated: 12 ppb which = .037 ppm PO4
055735EC-B052-4463-895E-260B15D77A0D.jpeg


Heated: 16 ppb which = .049 ppm
AA804AB3-19BF-476F-A1FC-92B6B0C5BF47.jpeg


So the results consistently show a bit higher reading on the heated sample. The change (what they call Depot factor) was 1.2 on the Red Sea and 1.3 on the Hanna. The Red Sea test reads slightly higher than the Hanna as always, but given testing error the results are hard to know conclusively.

Interesting, but I don’t see any real value. If I accept the results, then I have little stored phosphate which sounds good to me and seems consistent with how my tank acts over time, but conclusive? I don’t think so. Randy’s points seem to make more sense to me.
 
Randy Holmes-Farley said:
FWIW, article 51 in the above link (mentions microwaving) is not microwaving alone, but actually the sort of oxidation I was referring to:

Determination of total phosphorus and nitrogen in turbid waters by oxidation with alkaline potassium peroxodisulfate and low pressure microwave digestion, autoclave heating or the use of closed vessels in a hot water bath: comparison with Kjeldahl digestion
https://www.sciencedirect.com/science/article/pii/S000326700200346X

"The evaluation of the use of alkaline peroxodisulfate digestion with low pressure microwave, autoclave or hot water bath heating for the determination of total phosphorus and nitrogen in turbid lake and river waters is described. "
Click to expand...

Thanks Randy but you should know that Chemistry it’s not everyone strongest area of expertise... [emoji4]
But simplifying I believe I understand that with microwave we oxidate some of part of the organics and the Phosphorus to be detected increase. That should explain why I’ve got a higher reading on the heated sample or not because the difference was just 1ppb!!

Tomorrow I’ll test with a friend’s Phosphate tester and let’s see what’s the results.
 
Alfrareef said:
Thanks Randy but you should know that Chemistry it’s not everyone strongest area of expertise... [emoji4]
But simplifying I believe I understand that with microwave we oxidate some of part of the organics and the Phosphorus to be detected increase. That should explain why I’ve got a higher reading on the heated sample or not because the difference was just 1ppb!!

Tomorrow I’ll test with a friend’s Phosphate tester and let’s see what’s the results.
Click to expand...

It might be that, or random test error. But I do not doubt that there is often a rise in phosphate on heating.

I just don't see how one interprets this value, and certainly don't agree that it has anything to do with their claim that

"The purpose of the PO4 Heat Test is to help aquarists gain a better understanding of the amount of deposits trapped inside calciferous surfaces. By using this method, aquarists can see the depot factor of their aquarium and take action sooner to prevent an over-accumulation of surface-bound P and PO4."
 
Tidal Gardens
As an example of the inherent fallacy here in using the "depot factor" to assess " surface-bound P and PO4" (their words in quotes), let's look at this thread where one of our members looks at how much inorganic phosphate is absorbed onto rock. There is no substantial organic phosphate in this experiment.

https://www.reef2reef.com/threads/phosphate-absorption-rates-in-aragonite.352405/

He found that 5 pounds of Florida mined aragonite would take up inorganic phosphate from the water, and over time, he added the equivalent of 57.4 ppm of phosphate, but only 0.16 ppm remained in the water. The rest is bound to the rock surface. Presumably, the "depot factor" defined in that heating method would be about 1, but there is 359 times as much inorganic phosphate bound to the rock surface as remaining in the water. :(

https://www.reef2reef.com/threads/phosphate-absorption-rates-in-aragonite.352405/page-4#post-4501106
 

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