Testing phosphates in dry rock

[Tflow256]

So I got about 60lbs of dry rock soaking in ro/di water to see if it's leaching any phosphates. My question is, since it's in ro/di and not saltwater should I use a freshwater phosphate test kit or saltwater?? Freshwater kit shows 0 but saltwater kit shows .25. They both use the same reagents but have different color charts. Thanks in advance!!

 

[Ebone]

my phosphate kit measures both salt and fresh, so I would err on side of caution and believe the .25 number personally

 

[Tflow256]

The test kit im using does both as well, the only difference is the color chart. When I test just RO/DI it does the same, shows 0 on fresh chart and .25 on salt chart. Trying to find someone with a different test kit to compare to.

 

[Randy Holmes-Farley]

If the kit has two charts, use whichever one relates to the water you are using, but in general, I wouldn't expect much difference in the colors in a phosphate test.

 

[Tflow256]

Thanks Randy!

 

[Randy Holmes-Farley]

You're welcome.

Good luck!

 

[Nemo's Janitor]

I have been directed to believe that it is not a good idea to try and test parameters of RO water with our hobbyists test kits.. Apparently it is due to the ionic imbalance in RO water. Is this a fact?

 

[Randy Holmes-Farley]

Depends on the kit. Some are fine. Most phosphate kits will read in fresh water, and once rock has been soaking in it, it isn't RO/DI any more.

 

[Nemo's Janitor]

Thank you Randy. Makes sense. Would that apply just to PO4 test kits or other kits that test for N.

 

[Randy Holmes-Farley]

In general, if they kit doesn't specify on it, it is likely good for both fresh and salt, and if it is good in fresh it likely works OK in RO/DI.

 

[Breakthecycle2]

Not trying to hi-jack your thread, but if dry rock was leaching phosphates into your tank, after being in it for a few months, would you have a high phosphate reading?

 

[Randy Holmes-Farley]

Rock and phosphate is not a one way path (on or off).

Phosphate reversibly bonds to the calcium carbonate surface, and more binds at higher phosphate concentrations.

So if you soak it in high phosphate water, then reduce the phosphate in the water, it will come off.

So phosphate only really comes off if the concentration in the water is lower than what would be in equilibrium with the amount on the rock.

When phosphate is rising in the water, it can then be going onto the rock.

If you keep it in the same concentration for an extended period, it likely has reached a new equilibrium.