High Silica interference with PO4 measurement

[taricha]

Tip to @Jason mack who brought it up earlier in a dino thread.
I was skeptical, but it seems to be true, and in some cases, it might change how we choose to dose and test etc.

This is a combo of data from me and @Rick Mathew

Saltwater spiked with different levels of Silica was measured for PO4 with the Hanna ULR P meter.
What is plotted is the difference in the PO4 measured value from its original (zero SiO2) value.
That Rick's (red) and my (blue) data are so closely in agreement given different Si sources etc makes the effect look likely to be real.

Is this a big effect? Not really. Hanna's Low Range Si checker maxes out at 2.00ppm SiO2. I know of no reason why anyone would want more than 2ppm SiO2. There is no evidence of significant interference at or below that range. But it looks like the interference starts around 2ppm SiO2 and becomes significant compared to the uncertainty in the PO4 test by 4ppm SiO2.
The interference is known about, but may be understated...

info in the Standard Methods - PO4 by ascorbic acid...
"b. Interference: Arsenates react with the molybdate reagent to produce a blue color similar to that formed with phosphate. Concentrations as low as 0.1 mg As/L interfere with the phosphate determination. Hexavalent chromium and NO2 interfere to give results about 3% low at concentrations of 1 mg/L and 10 to 15% low at 10 mg/L. Sulfide (Na2S) and silicate do not interfere at concentrations of 1.0 and 10 mg/L."

I'm not sure why we are seeing interference at a lower range than we ought to, but it seems to be the case. @Dan_P pointed out this is not a hanna issue, it's underlying chemistry - and other test kits will probably not avoid it either.

Under what case could it matter? Jason earlier pointed out the fact that Si is dosed sometimes by those trying to encourage diatoms to compete with dinos, and many don't want to test for Si, or get a faulty kit. Then there are certainly some out there that are shooting for PO4 in the 0.05-0.10 range and may only have half or less of that if they are dosing Si without a reliable Si kit.

 

[Randy Holmes-Farley]

Silicate is a known interferent. See Figure one in this paper. It looks like your graph:

https://aslopubs.onlinelibrary.wiley.com/doi/pdfdirect/10.4319/lom.2014.12.223

 

[taricha]

In my system, silicate disappeared rather quickly. If this is true for all systems, we might not have to worry about interference because it might be difficult to maintain a high level of silicate, unless as you point out, some is winging the dose.

yep! I went back and looked at old data. I added anywhere from 0.2 to 1ppm SiO2 per day for over a month to my tank and by the next day, I was always back under 2.0ppm SiO2.
Organisms have quite an appetite for Si - not sure if there's significant abiotic factors depleting Si as well.

 

[Dan_P]

yep! I went back and looked at old data. I added anywhere from 0.2 to 1ppm SiO2 per day for over a month to my tank and by the next day, I was always back under 2.0ppm SiO2.
Organisms have quite an appetite for Si - not sure if there's significant abiotic factors depleting Si as well.

There is something bothering me about this discussion. Unless you are running controls, there does not seem to be a way you can detect silicate interference with phosphate readings. How did this narrative get started?

 

[taricha]

There is something bothering me about this discussion. Unless you are running controls, there does not seem to be a way you can detect silicate interference with phosphate readings. How did this narrative get started?

I'd never heard it until a few weeks ago. So I dunno to what extent it's a narrative.
But @Jason mack apparently collected a bunch of ICP tests and hobbyist measurements. He noticed that the largest over-measurements of hobby kit PO4 vs ICP happened in tanks with big Si values in ICP as well. There are some ways you can get a big Si value unintentionally (not dosing). Siporax, Silica sand, maybe others.

 

[Dan_P]

I'd never heard it until a few weeks ago. So I dunno to what extent it's a narrative.
But @Jason mack apparently collected a bunch of ICP tests and hobbyist measurements. He noticed that the largest over-measurements of hobby kit PO4 vs ICP happened in tanks with big Si values in ICP as well. There are some ways you can get a big Si value unintentionally (not dosing). Siporax, Silica sand, maybe others.

OK thanks.

This makes sense. Even though ICP PO4 measurements are generally biased low, they would appear even more off when the hobby PO4 measurement was biased in the high direction. Confusion cleared up!

 

[Jason mack]

I'd never heard it until a few weeks ago. So I dunno to what extent it's a narrative.
But @Jason mack apparently collected a bunch of ICP tests and hobbyist measurements. He noticed that the largest over-measurements of hobby kit PO4 vs ICP happened in tanks with big Si values in ICP as well. There are some ways you can get a big Si value unintentionally (not dosing). Siporax, Silica sand, maybe others.

Yes . I had been getting comments about people on my fb page from people who had been reporting high phosphate readings when dosing si for dinos ..it started when one member sent an icp test of ..he took a hanna reading the same day ...0.15ppm ..icp test came back showing 0.02 ...since then a few more have done the same ...and there seems too be some truth in it ...

 

[AlbertGF]

Yes . I had been getting comments about people on my fb page from people who had been reporting high phosphate readings when dosing si for dinos ..it started when one member sent an icp test of ..he took a hanna reading the same day ...0.15ppm ..icp test came back showing 0.02 ...since then a few more have done the same ...and there seems too be some truth in it ...

Hi I see this is and old post.
Precisely I come from your FB page, I am dosing Silica to combat dinos. I have a Mastertronic with Red Sea Phosphate Pro reagent which give me low PO4 readings (+ or - 0.02mg/l) but my Hanna Phosphorus ULR checker is giving me 65 PPB which is + or - 0.2mg/l.
I was lost and didn't know what to do. Now I that found this post and I know Silica interfere with P measurements I will rely on RedSea tests and relax a little bit.

 

[Randy Holmes-Farley]

Hi I see this is and old post.
Precisely I come from your FB page, I am dosing Silica to combat dinos. I have a Mastertronic with Red Sea Phosphate Pro reagent which give me low PO4 readings (+ or - 0.02mg/l) but my Hanna Phosphorus ULR checker is giving me 65 PPB which is + or - 0.2mg/l.
I was lost and didn't know what to do. Now I that found this post and I know Silica interfere with P measurements I will rely on RedSea tests and relax a little bit.

I'm confused by your post. How much silicate did you dose? 2 ppm is more than enough to drive diatoms and does not significantly interfere with phosphate measurements shown in the first post.

 

[AlbertGF]

I'm confused by your post. How much silicate did you dose? 2 ppm is more than enough to drive diatoms and does not significantly interfere with phosphate measurements shown in the first post.

Yes, you are right. I had a problem mistaking silica for silicates so I dosed too much waterglass. I read that silicates will deplete if I do not add more so I am waiting to see if they get lower. Everything is OK except for the dinos and HANNA readings so now that I know that silicates can interfere with HANNA and the rest is OK I will wait to see what happens.