Head Scratcher.. Nitrates But No Phosphates

[HighestOlive]

I wouldn't place too much emphasis on the results of the tests. It's unlikely that they can be very precise at that cost. There are some articles on Triton testing accuracy, if you want some data on the subject.

I man not placing to much stock in the exact numbers reported out. While I am sure they are in the right ball park I have no expectation on the accuracy of test method. I had this analysis performed many to confirm that I didn't have some sort of high levels of a toxin that I wouldn't expect to be there.

 

[HighestOlive]

Just a further thought. Roughly speaking, for a precision of +/- 0.04 ppm, a meter reading of 0 ppm means 0 +/- 0.04 ppm. This is the doubt or range of the exact level. If the vendor is being generous and the stated precision represents only 1 standard deviation (67% certainty), a better estimate is 3 standard deviations (99% certainty that the reading isn't chance or noise) or 0 +/- 0.12 ppm.

I think even if the meter is working properly, there is reasonable doubt about exactly how low the phosphate is.

So I did a bit of experimentation with my meter this evening. I made a series of dilution from a stock of Phosphate with a known concentration. I was able to get predictable results down to about 0.1 mg/L. At what should have been a concentration of 0.05 mg/L, I obtained a test result of 0.01 mg/L (there is your +/- 0.04 mg/L accuracy right there). At 0.03, 0.02, and 0.01 mg/L I was unable to obtain a result above 0.00. I am sure that part of this error has to do with the inability to recover the some portion of the regent from the packaging. You could probably increase you accuracy a small bit if you rinsed the remaining reagent out of the packaging with the test sample. There is also obviously going to be some level of error in the meter as well as some minimum value that the meter will be able to detect (as previously discussed). So ultimately I would conclude from this that trying to test levels below 0.05 would not be feasible, with this (or most consumer) meters.

But back to the more important question at hand. Based on a reading on 0.00 on my tank water I can probably make a fairly safe assumption that the actual phosphate level is less that 0.05 mg/L. If I take the ICP testing results at face value, the phosphate level in my tank should be roughly 0.24 mg/L (again as previously discussed). Assuming that the LOQ of the assay is 0.05 mg/L, a minimum of 75% of the phosphate is present at organic molecules or polyphosphate in the water or be part of microorganisms in the water sample. This could be significantly high but that can't be determined accurately.

What's this all ultimately mean... who knows. Am I actually deficient in phosphate? I would say it is impossible to tell of sure. I could have 0.05 or I could have 0.00. Or it could be somewhere inbetween. My only thought is that I could bring my levels up to 0.05 so that it can be detected and see how things respond.

This raises a larger question for me... How do most people who are targeting a phosphate concentration of 0.03 mg/L measure that. I would say it would probably be quite difficult with a standard colorimetric assay, and the Hanna Phosphate checkers has the same stated accuracy. A reading of 0.03 could really be any where between 0.00 and 0.07 if you do the assay just right (the stated accuracy is mostly much tighter that the standard consumer and hopeful with out some lab grade equipment or a micropipetter at the very least). Are most people using the Phosphorus checker and then converting? And if so I they just assuming the 3.066:1 ratio?

 

[tmz]

Fair enough. Either way I have ordered new filters for my RODI system and they will be changes out soon.

FWIW, using two di canisiters with a dual tds meter measuring the output for each is an efficient way to insure zero tds/ unexhausted. When the tds leaving the first shows 1ppm or more , I change the resin in that container and rotate the second into the lead position .This way the output form the second is always zero and late or premature resin changes are avoided.

 

[HighestOlive]

FWIW, using two di canisiters with a dual tds meter measuring the output for each is an efficient way to insure zero tds/ unexhausted. When the tds leaving the first shows 1ppm or more , I change the resin in that container and rotate the second into the lead position .This way the output form the second is always zero and late or premature resin changes are avoided.

Great suggestion TMZ. I actual had just purchased an additional TDS meter so I can monitor in 4 places not. So many options

 

[tmz]

Good luck.