Monopotassium phosphate question?

TbyZ · Feb 17, 2017

I have some Monopotassium Phosphate powder (K2HPO4).

I mixed a few t-spoons into 150ml RODI water. Its very soluble.

When I tested the solution with my salifert PO4 test kit the reading was zero?

Why wouldn't I get a reading for phosphate?

Randy Holmes-Farley · Feb 17, 2017

What happened? It may not work in that solution, but it might also be so high that it doesn't show a recognizable color. The concentration will be on the order of 100,000 ppm.

TbyZ · Feb 17, 2017

Randy Holmes-Farley said:
What happened? It may not work in that solution, but it might also be so high that it doesn't show a recognizable color. The concentration will be on the order of 100,000 ppm.

I checked it after 1 t-spoon & there was no reading. No blue color at all. The same after 2 more spoons.

What do you mean by it not working in "that solution"?

Randy Holmes-Farley · Feb 17, 2017

The problem is likely the pH.

Chemical tests often require a certain pH for them to work. Many kits acidify the solution during the test. Phosphate is one of these.

For example, this is the standard ascorbic acid method for phosphate::

http://public.iorodeo.com/docs/phosphate/ascorbic_acid_method.html

"Ammonium molybdate and potassium antimonyl tartrate react in acid medium with orthophosphate to form a heteropoly acid - phosphomolybdic acid - that is reduced to intensely colored molybdenum blue by ascorbic acid. This method for measuring phosphate is based on APHA Standard Method 4500-P E"

That much phosphate is a very potent pH buffer. The kit likely cannot overcome the pH buffering of the phosphate to bring the ph low enough for the kit to work.

Randy Holmes-Farley · Feb 17, 2017

A better way to test, if you have the capability, is to spike a tank water sample with 0.1 or 0.5 ppm phosphate and detect that with the kit.

TbyZ · Feb 17, 2017

Thanks Randy for the info, very much appreciated.

Randy Holmes-Farley · Feb 17, 2017

You're welcome.

Good luck.

Alfrareef · Feb 18, 2017

Thks Randy for another important contribute for the better understanding about how our tests work.
What we can estimate to be the highest PH that a PO4 test can handle?

Randy Holmes-Farley · Feb 18, 2017

Alfrareef said:
Thks Randy for another important contribute for the better understanding about how our tests work.
What we can estimate to be the highest PH that a PO4 test can handle?

Thanks.

Its not necessarily the pH at the start, but the inability of the acid in the test kit to lower the pH enough. A phosphate buffer can hold the pH very strongly at pH 7, even without starting higher than tank water pH.

Alfrareef · Feb 19, 2017

Understand. Thanks Randy.

Randy Holmes-Farley · Feb 19, 2017

You're welcome.

Happy reefing.

TbyZ · Feb 19, 2017

Ok, so I added just a few specs of phosphate to 10ml of aquarium water, waited a few hours, & got some blue color on a test.
I estimate my PO4 to be somewhere below 0.01ppm.

Randy, you reckon the solution I made up is is in the order of 100,000 ppm.
If I wanted to raise PO4 by 0.04ppm in 180L (47gallons) how much solution would I add.
I get something like one drop, but that seems unlikely?

TbyZ · Feb 27, 2017

Randy Holmes-Farley said:
A better way to test, if you have the capability, is to spike a tank water sample with 0.1 or 0.5 ppm phosphate and detect that with the kit.

I'm starting to wonder if i was supplied K2HPO4 or maybe cocain?

I made a new solution using James' planted aquarium calc. I dosed an amount to my tank that should have raised PO4 by 0.14ppm, but measured no reading. I turned off the return pump before dosing & waited 5 minutes before measuring with salifert
?????????

Randy Holmes-Farley · Feb 28, 2017

TbyZ said:
I'm starting to wonder if i was supplied K2HPO4 or maybe cocain?

I made a new solution using James' planted aquarium calc. I dosed an amount to my tank that should have raised PO4 by 0.14ppm, but measured no reading. I turned off the return pump before dosing & waited 5 minutes before measuring with salifert
?????????

It may have rapidly bound to calcium carbonate surfaces (5 min seems too short, however, maybe it wasn't even mixed in well). Or the kit may not be adequately sensitive/accurate.

ChaseBurke · May 16, 2019

@Randy Holmes-Farley
Hey Randy sorry to bug you but wondering if you could answer a question for me. I am currently running a bare bottom triton method tank and am needing to dose phosphate and nitrate. No matter what I do I can’t get either above 0 and my corals are starting to show it. Am I safe to dose mono potassium phosphate and potassium nitrate where I am running triton method or will my potassium levels shoot through the roof? Also just wanted to say that you for all of the information you have shared And everything you have done for this hobby.