API Phostphate Test Question

[W_Oz]

I was just curious, I know what my PO4 levels are like in my tank thanks to my work. However I did out of curiosity buy a few test kits from a couple of different companies and just to have them laying around if I need to check something. One of these kits was an API PO4 kit and I'm a little stumped by the thing.

While I know that full development time is important, I kinda can't help myself and sort of monitor the progress the color reagents are making while I wait. I've noticed that this kit will hit a high number then drop back to a much lower number. (e.g. it will read 2ppm then drop back to ~0.5ppm). I realize these kits are only so reliable, however I'm just curious if this is "normal" behavior for this product as these readings can be misleading without another system in place to verify results.

[doug z]

I got a zero reading on mine..

I guess I need a more sensitive test.. LOL

[fins_n_fur]

Having read various threads in planted tank forums, the common opinion seems to be that widely sold, less expensive test kits for some elements aren't really accurate, especially for phosphates. If you need/want more accurate readings, you need to spend more money. I will see if I can find a brand name or two for you to check out.

[Alfcea]

Just one question out of curiosity...

Do you know what the kit contains? I mean, the reagent?

[W_Oz]

45% Sulfuric Acid and Ammonium heptamolybdate are the listed ingredients for bottle 1 and bottle 2 is <97% glycerol and >3% stanous chloride-0 H2O (Anhydrous).

I realized this was an inexpensive kit when it was purchased, I decided on a whim to go buy one and the stores in the area only had this one at the time. The "good" LFS normally carries a PO4 kit which is around 3x more expensive however they were out at the time. I can't for the life of me recall the brand but the gent I get all my answers from at this shop said they're one of the better test kits and tend to be quite accurate.

Edit: The brand that the shop normally carries is "Salifert"

[Alfcea]

Hmmm....

It seems that the heptamolybdate [Mo6O24]6- directly reactws with both organic and inorganic phosphate to form phosphomolybdate [Mo5P2O23]6-, which is coloured... I am clueless as of what the SnCl2 would be for, though... as a reducing agent, maybe?

Technically, what do you do? Do you add the reagent in bottle 1 to a sample of the water to be tested, wait for a colour change and then add a few drops of bottle 2 and wait for another colour change?.... hmmm....

Sorry about this rambling.......

[fins_n_fur]

I finally remembered the more precise test kits...made by LaMotte--signigicantly more expensive the API, but perhaps worth if you need. I think there is another high-end accurate brand as well.

Edit: Hach, I believe the brand is.

[kaybee]

How low of a level of phosphate are you trying to measure may determine which test kit or device to get. Depending on that some test kits may not be relevent regardless of how 'accurate' they are if they can't accurately detected low-range phosphate. (I'll try to explain).

For testing phosphate in a reef aquarium (as one example) the Lamotte kit wouldn't be a good choice (in my opinion) because (excluding 0ppm), the lowest level of phosphate it can detect is 0.2ppm). How are lower levels (> 0ppm but < 2.0)represented in the results, and would "0ppm" actually be 0ppm?

When trying to discern lower levels and when levels lower than 0.1ppm and preferably no greater than 0.03ppm are desired as detection grades 0, 0.2, 0.4ppm, etc do not provide the required resolution. I change out my phosphate removing media when PO4 reaches 0.07ppm so the test kit (or device) would need to be able to distinguish 0.07ppm from a 0ppm or 2.0ppm

So accuracy as well as scale of resolution (if it's called that) may need to be considered as well. Of course there may be requirements were the goal isn't necessarily to detect minute levels of phosphate, but just something to think about (I'm not sure what types of PO4 levels are desired in a FW planted tank).

[W_Oz]

I know Hach well, they make the Free/Residual Cl2 tests we use at the lab as well as our pocket Spectrophotometer used in that test.

For the API kit, you add X drops of reagent one, swirl to homogenize, then add X drops again of reagent 2, swirl to homogenize. Wait Y minutes and read against a color card.

I believe the SnCl is there to cause a precipitate to form with the phosphomolybdate. After the development period the solution becomes turbid, which I believe is also why the glycerol is there to keep the precipitate suspended by increasing the viscosity of the solution in the vial. I could be wrong though of course. lol

Anyway, I'm not interested in keeping my phosphates at zero, or close to it, so I don't need a test with an accurate bottom end, just a decent middle. I do intentionally put phosphate in the tank to keep the plants happy though I've let my fert dosing slide since I've been sick and haven't felt able to keep up with the dosing schedule. I'll have to get back on that now that I'm back on my feet.