Diatom powder

Cory · Jan 1, 2016

So i just got my diatom filter. I hooked it up and let it ran. it go very dirty as expexted. However i then tested po4 via hanna. .38ppm!

This is strange to me, so i went on testing my ro/di with a tds of 003. The po4 came out as .04ppm po4. I thought, thats too small to be .38ppm in the tank. I add about 1 liter a day of this ro/di water.

Then i tested some water with the diatom powder in it. .64ppm! Hanna says any particles will cause a faulty reading but im not sure now.

So my question is, would diatom powder contain po4?

Is .04ppm po4 added 1 liter per day enough to cause .38ppm in the display?

And is this just a testing error?

Randy Holmes-Farley · Jan 1, 2016

I suspect the problem may be what they mentioned in the way of interference. The Hanna cannot distinguish absorbance by phosphate from scattering of the light by particles. That said, it may also contain some phosphate and will release dissolved dissolved silicate, and perhaps a high silicate level even interferes with the chemistry.

Cory · Jan 1, 2016

It would be good to know if silicate would interfere.

Im also wondering if that diatom powder would interfere with the alk test by hanna.

Randy Holmes-Farley · Jan 2, 2016

Most likely the solids are a bigger problem than the silicate. I could not find the list of interferences for the checker, but for one of their other phosphate devices they say silicate interferes, so if you want a definitive answer, I'd ask Hanna.

http://www.hannainst.com/manuals/manHI_93713.pdf
from it:
Interference may be caused by:
Iron above 50 mg/L
Silica above 50 mg/L
Silicate above 10 mg/L

Cory · Jan 3, 2016

Thats good to know! Do you think diatom powder would have substantial po4 in it?

Randy Holmes-Farley · Jan 3, 2016

Cory said:
Thats good to know! Do you think diatom powder would have substantial po4 in it?

I don't know. Sorry.

robert · Jan 3, 2016

I highly doubt it - remember the most common use of DE is in swimming pools where increased phosphate would be seen as a significant problem and would have been reported decades ago....

Since you mixed the DE powder into the water, rather than testing the water after it exited the filter - its certainly optical interference...

Cory · Jan 3, 2016

No problem Randy. How could i filter the sample water down past say .5 microns to be sure there is nothing to cause optical interference?

Robert, its what im thinking too. But its also in the tank too.

robert · Jan 3, 2016

Try filtering through a rinsed paper coffee filter - just remember to test a control...

Cory · Jan 3, 2016

I tried with a paper towl but the powder passed through the paper towel.

robert · Jan 3, 2016

then you need something finer....depending on grade of DE used, 5-100 micron, 20-25 shoudl be fine enough for most product.

Habib(Salifert) · Jan 3, 2016

If you want to use diatomaceous earth it is IMO best to have both the in and outflow in a small bucket with aquarium water. This will allow to either filter out the very fine fraction or at least to get rid off most of it.

If the aquarium water is still turbid at the moment then you could do the above to attempt to get clearer water for testing purposes.

But it could also be silicate, some phosphate kits are sensitive to silicate by virtue of their chemistry or procedure. FWIW the Salifert is not sensitive for it.

I highly expect that if you find with the Hanna 0.38 or even o.1 then it would have a blue tinge if viewed through the top of the vial against a white piece of paper. If so then it could be phosphate or silicate interference.
If not then it could be interference by particles.

HTH

Cory · Jan 3, 2016

Yeah it needs something finer to filter them out.

Habib, thanks for the info. Ill try what you suggested. I had some Salifert po4 test on hand, and it was a slight blue tinge to it. Good to know its not effected by high silicate.

Fwiw i tested my parameters and what resulted scared me lol:

Po4: .38ppm
Alk: 7.7 dkh
Calcium: 300ppm
Potassium: 360ppm

All test except po4, and alk were salifert. My algae turf scrubber seems to be sucking out potassium. Previously (two months ago) i bumped potassium up to 410ppm.