API Phosphate kit

In communicating with API technical support about the KH kits, I mentioned the other kits we are using.  One of their scientists graciously provided information about the phosphate and nitrate/ammonia/nitrite kits. I've provided that information below.

I did a full.. generalized.. write up on the phosphate kit because the ingredients were not proprietary. So to answer your question on phosphate, it is going to read the ppm of the phosphate ion itself. But keep in mind that phosphate we are talking about is orthophosphate, also known as “reactive phosphorus” but is generally used synonymously with “phosphate”

The write up is below, if you want to pull some nuance out of it that might be helpful for your usage. I might also have one like this for the ammonia test kit as well, if it would be useful.

The Phosphate #1 bottle contains sulfuric acid and ammonium heptamolybdate

(information found here: https://www.apifishcare.com/pdf/API_PHOSPHATE_TEST_SOLUTION_1-1.pdf)

The Phosphate #2 bottle contains glycerol and stannous chloride, anhydrous

(information found here: https://www.apifishcare.com/pdf/API_PHOSPHATE_TEST_SOLUTION_2.pdf)

Ammonium heptamolybdate looks like this:  (NH­­₄)₆Mo₇O₂₄:4H₂0

Stannous Chloride looks like this:  SnCl₂:2H₂0

In an acidic solution (which is why there is sulfuric acid), phosphate reacts with ammonium molybdate to form molybdophosphoric acid, which is then reduced by stannous chloride to the intensely colored molybdenum blue. The resulting blue color is directly proportional to the phosphate concentration. Results are expressed in ppm (mg/L) phosphate as PO₄.

The reaction looks roughly like this:

12MoO₄^2- + 3NH₄⁺ + PO₄^3- + 24H⁺ → (NH₄)₃[P(Mo₁₂O₄₀)] + 12H₂O

(Source: http://www.wiredchemist.com/chemistry/instructional/laboratory-tutorials/qualitative-analysis)

Molybdenum blue: https://pubchem.ncbi.nlm.nih.gov/compound/170447

If you are looking to do experiments or to compare ways of testing phosphate, please do make sure the equation is balanced properly and that you understand what the different concentrations of phosphate will mean for it.

Depending on depth you wish to understand this topic, this article may also be useful: https://pearl.plymouth.ac.uk/bitstream/handle/10026.1/4352/eddie%20molyb%20blue%20ACA%20PEARL.pdf?sequence=1

We hope that we have helped!