Phosphorus ULR and high silicates

[taricha]

oops. After I posted this I realized it's not in the Reef Chemistry Forum.... gonna cross-post where I thought it was in the first place.

Edit: like I said, this isn't a hanna issue, it's probably the underlying chemistry but if they want to respond, that would be good too.

 

[Dan_P]

Was going to make a new thread on hanna PO4 chemistry showing interference with high Silicates, but a search found this one already.

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 don't 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.

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.

 

[ReefPig]

I've had the issue I reported on a few occasions, even at relatively low levels of silicates, which is represented as massive phosphorous level.

I reached out to Hanna here and emailed them twice, they never replied.

IMO it's a waste of time using it, Elos High Res is far better, just as easy, maybe not quite as accurate as the Hanna could be, but it works in all situations, which I guess means it more accurate.

 

[Dan_P]

I've had the issue I reported on a few occasions, even at relatively low levels of silicates, which is represented as massive phosphorous level.

I reached out to Hanna here and emailed them twice, they never replied.

IMO it's a waste of time using it, Elos High Res is far better, just as easy, maybe not quite as accurate as the Hanna could be, but it works in all situations, which I guess means it more accurate.

What were the silicate levels when you experienced interference? How did you determine the PO4 reading was incorrect?

 

[Stephers]

Im glad you liked into this, Taricha. We've been advising people not to use hanna while dosing silicates for a while now. Those using red sea do not appear to be having false increases.