Hanna Phosphorus ULR question

Righteous · Jun 17, 2015

So I've been checking my phosphates using the Hanna ULR Phosphorus meter (the 736). I've been having readings ranging from 40 - 15ppb P (0.12 - 0.05 ppm PO4). I've got a small amount of GHA, but nothing serious, and I have started running GFO in a phosban reactor. I've since noticed the GHA starting to brown, but I did not notice a corresponding decrease in the PO4 numbers. Instead it seemed like the numbers just bounced around from test to test.

So I began assuming there was an issue with how I was running the test. After some googling, I found a video from Hanna showing how they cut the packet on two perpendicular sides to make sure none of the reagent gets stuck in the corners.

After doing that, I got two back to back readings of 9, and 7 ppb P! (the preceding reading was 26ppb)

My question is this. Why would the numbers go down, assuming my initial problem was leaving reagent in the pouch? What's the chemistry behind the reaction (my reagents are marked potassium disulfate)? My instinct would be that increasing the reagent added would increase the color change, which I would think would have raised the readings instead of lowering them.

Randy Holmes-Farley · Jun 17, 2015

Hanna says they use a modified standard ascorbic acid method. I'm not sure exactly how they modified it, but this is the standard method:

Ammonium molybdate and antimony potassium tartrate react in an acid medium with dilute solutions of phosphorus to form an antimony-phospho-molybdate complex. This complex is reduced to an intensely blue-colored complex by ascorbic acid. The color is proportional to the phosphorus concentration.

There is normally a significant excess of potential reagent in such test to allow for values with high levels to react, so missing a little shouldn't generally cause a issue.

The bisulfate is used n the standard method in case of high levels of iron or arsenate to avoid interferences. Not sure why Hanna has it in this method, but if it is eliminating interference, reducing it could lead to false high values.

Or, your lower result may just be coincidence.

Righteous · Jun 17, 2015

I like the idea of something eliminating interference, and not so much that it could be just a coincidence (which is exactly what I'm worried about!!

) If it's just coincidence, then the range I got on a single day of 0.08 - 0.02 ppm PO4 would mean that the test isn't very helpful.

I'm thinking of maybe doing an experiment where I run the test blind, once with a purposefully reduced amount of reagent.

The only other thing I could think of would be sending off for a Triton test. Do you know if a the Triton test for Phosphorus would have a low enough limit of detection to compare/verify a reading of 7ppb on the Hanna?

Thanks Randy for you time and help!!