Hanna nitrate checker reading ~2.5x on KNO3 standard

wccbr1

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I was doubting my Hanna High Range Nitrate checker so we ran a few tests in the lab at work. Would the Hanna checker fail to properly detect a known potassium nitrate standard (20ppm)? I won't bore everyone with details at this point but I used a lab grade 1000ppm KNO3 stock solution to create a 20ppm internal standard in newly mixed salt water, zeroed my Hanna checker with the same salt water and then tested the sample containing the 20ppm KNO3 internal standard. The Hanna checker read 72.6ppm!
I also did similar tests with a few other levels of the KNO3 standard created in a 4N KCl solution which is what we normally use to analyze some of our lab samples on a continuous flow injection analyzer. In the KCl solution a 5ppm KNO3 standard reads 12.3ppm on the Hanna checker, 20ppm KNO3 standard reads 49.5ppm on the Hanna checker.
I started down this rabbit hole because my Hanna checker was telling me that my display tank sample was over 75ppm NO3-N and so we ran the same tank sample on the continuous flow injection analyzer and it gave a result of about 33ppm NO3-N granted that the method for the analyzer was set up for samples (that are not seawater) in the KCl solution but I wouldn't expect it to be wildly wrong.
I've ordered a Salifert Nitrate test to cross check the Hanna but thought I'd pose the question to see what others had to say before I reach out to Hanna to see if I have a defective unit.
Before the obvious replies come through with "take it to your LFS to check", I'll just say that there are no LFS within several hours of driving and I don't know of any local reefers or a local reefing club in my area.
I'd be interested to hear what @Randy Holmes-Farley has to say as well. Perhaps my assumptions about the checker are way off since it uses a far simpler detection method than what we use in the lab.
Thanks in advance!
 

Randy Holmes-Farley

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I was doubting my Hanna High Range Nitrate checker so we ran a few tests in the lab at work. Would the Hanna checker fail to properly detect a known potassium nitrate standard (20ppm)? I won't bore everyone with details at this point but I used a lab grade 1000ppm KNO3 stock solution to create a 20ppm internal standard in newly mixed salt water, zeroed my Hanna checker with the same salt water and then tested the sample containing the 20ppm KNO3 internal standard. The Hanna checker read 72.6ppm!
I also did similar tests with a few other levels of the KNO3 standard created in a 4N KCl solution which is what we normally use to analyze some of our lab samples on a continuous flow injection analyzer. In the KCl solution a 5ppm KNO3 standard reads 12.3ppm on the Hanna checker, 20ppm KNO3 standard reads 49.5ppm on the Hanna checker.
I started down this rabbit hole because my Hanna checker was telling me that my display tank sample was over 75ppm NO3-N and so we ran the same tank sample on the continuous flow injection analyzer and it gave a result of about 33ppm NO3-N granted that the method for the analyzer was set up for samples (that are not seawater) in the KCl solution but I wouldn't expect it to be wildly wrong.
I've ordered a Salifert Nitrate test to cross check the Hanna but thought I'd pose the question to see what others had to say before I reach out to Hanna to see if I have a defective unit.
Before the obvious replies come through with "take it to your LFS to check", I'll just say that there are no LFS within several hours of driving and I don't know of any local reefers or a local reefing club in my area.
I'd be interested to hear what @Randy Holmes-Farley has to say as well. Perhaps my assumptions about the checker are way off since it uses a far simpler detection method than what we use in the lab.
Thanks in advance!

I’d think a good test would be to spike new seawater or tank water with, say, 20 ppm nitrate, and see if that is properly detected.

Some methods need the pH or buffering or other aspects of seawater to be correct to work right.
 
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wccbr1

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I’d think a good test would be to spike new seawater or tank water with, say, 20 ppm nitrate, and see if that is properly detected.

Some methods need the pH or buffering or other aspects of seawater to be correct to work right.
Thanks Randy! That’s actually what I did. Spiked new seawater to be 20ppm nitrate and this is what the Hanna checker read as 72.6ppm.
And after typing this I realize that you may have meant to add a 20ppm standard to a volume of “seawater”, perhaps 10ml for a diluted concentration. I have also done something like this already and got readings ~2.5x the expected result.
The method for our CFIA actually includes buffering and this is the one that resulted in 33ppm nitrate where the Hanna went offscale.
 

Randy Holmes-Farley

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Thanks Randy! That’s actually what I did. Spiked new seawater to be 20ppm nitrate and this is what the Hanna checker read as 72.6ppm.
And after typing this I realize that you may have meant to add a 20ppm standard to a volume of “seawater”, perhaps 10ml for a diluted concentration. I have also done something like this already and got readings ~2.5x the expected result.
The method for our CFIA actually includes buffering and this is the one that resulted in 33ppm nitrate where the Hanna went offscale.

Ok, I misunderstood. Not sure why it is so far off, but that certainly isn’t good.

Did you make the potassium nitrate stock yourself?
 
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Ok, I misunderstood. Not sure why it is so far off, but that certainly isn’t good.

Did you make the potassium nitrate stock yourself?
The potassium nitrate stock solution was a 1000ppm solution provided by Sigma Aldritch or Avantor. I don’t remember which. Dilutions performed with volumetric flasks and digital pipettes and appropriate cleaning between steps.
 

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The potassium nitrate stock solution was a 1000ppm solution provided by Sigma Aldritch or Avantor. I don’t remember which. Dilutions performed with volumetric flasks and digital pipettes and appropriate cleaning between steps.

You sure it was 1000 ppm nitrate, not 1000 ppm potassium nitrate?
 
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It is a potassium nitrate stock solution. 1000 ppm KNO3
 

Randy Holmes-Farley

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It is a potassium nitrate stock solution. 1000 ppm KNO3

So then it is about 614 ppm nitrate? Did you use that value in determining the spike? If not, the error is significant.
 
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That’s an excellent point. I need to double check the stock solution label now as you’re making me doubt my memory. It was a coworker that prepared some of the standards and I believe he switched suppliers recently.
In either case, the readings are still very wrong. Time for more investigation!
 

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I was doubting my Hanna High Range Nitrate checker so we ran a few tests in the lab at work. Would the Hanna checker fail to properly detect a known potassium nitrate standard (20ppm)? I won't bore everyone with details at this point but I used a lab grade 1000ppm KNO3 stock solution to create a 20ppm internal standard in newly mixed salt water, zeroed my Hanna checker with the same salt water and then tested the sample containing the 20ppm KNO3 internal standard. The Hanna checker read 72.6ppm!
I also did similar tests with a few other levels of the KNO3 standard created in a 4N KCl solution which is what we normally use to analyze some of our lab samples on a continuous flow injection analyzer. In the KCl solution a 5ppm KNO3 standard reads 12.3ppm on the Hanna checker, 20ppm KNO3 standard reads 49.5ppm on the Hanna checker.
I started down this rabbit hole because my Hanna checker was telling me that my display tank sample was over 75ppm NO3-N and so we ran the same tank sample on the continuous flow injection analyzer and it gave a result of about 33ppm NO3-N granted that the method for the analyzer was set up for samples (that are not seawater) in the KCl solution but I wouldn't expect it to be wildly wrong.
I've ordered a Salifert Nitrate test to cross check the Hanna but thought I'd pose the question to see what others had to say before I reach out to Hanna to see if I have a defective unit.
Before the obvious replies come through with "take it to your LFS to check", I'll just say that there are no LFS within several hours of driving and I don't know of any local reefers or a local reefing club in my area.
I'd be interested to hear what @Randy Holmes-Farley has to say as well. Perhaps my assumptions about the checker are way off since it uses a far simpler detection method than what we use in the lab.
Thanks in advance!

Was your standard in saltwater? Chloride is known to interfere with this test, giving lower values in saltwater than in freshwater. The Hanna marine nitrate test is probably calibrated for the saltwater effect, and hence, you should expect a higher value in freshwater. Also, is the nitrate standard nitrite free?.
 
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Yes, the standard is nitrite free. We actually have separate nitrite standards. I ran the standard in new seawater and also in KCl. I was going to check it in lab grade RODI water too but I was getting tired of burning through all my Hanna checker reagents and free time. I'm going to check a few more things tomorrow and then wait for the Salifert test kit to arrive.
Thanks!
 

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Yes, the standard is nitrite free. We actually have separate nitrite standards. I ran the standard in new seawater and also in KCl. I was going to check it in lab grade RODI water too but I was getting tired of burning through all my Hanna checker reagents and free time. I'm going to check a few more things tomorrow and then wait for the Salifert test kit to arrive.
Thanks!
You might just go ahead and contact the Hanna tech about your concerns.
 
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Checked the standard. Nitrate standard, 1000ppm (as Nitrogen) so 1mL=4.43mg NO3. That explains why it was reading ~72ppm NO3 on the checker as the diluted standard should have been 20mg/L NO3-N (88.6 mg/L NO3).
Considering some reagent is always stuck in the envelope and the fact that the accuracy of the checker is +\- 2ppm AND +\- 5% of reading, per Hanna’s website, it isn’t unreasonable. Unfortunately that also means that my tank nitrates are on the order of 150ppm NO3!
Thanks @Randy Holmes-Farley and @Dan_P for pointing out what was embarassingly obvious!
 

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Checked the standard. Nitrate standard, 1000ppm (as Nitrogen) so 1mL=4.43mg NO3. That explains why it was reading ~72ppm NO3 on the checker as the diluted standard should have been 20mg/L NO3-N (88.6 mg/L NO3).
Considering some reagent is always stuck in the envelope and the fact that the accuracy of the checker is +\- 2ppm AND +\- 5% of reading, per Hanna’s website, it isn’t unreasonable. Unfortunately that also means that my tank nitrates are on the order of 150ppm NO3!
Thanks @Randy Holmes-Farley and @Dan_P for pointing out what was embarassingly obvious!
Good to hear that the test method issue is resolved.
 

Randy Holmes-Farley

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Checked the standard. Nitrate standard, 1000ppm (as Nitrogen) so 1mL=4.43mg NO3. That explains why it was reading ~72ppm NO3 on the checker as the diluted standard should have been 20mg/L NO3-N (88.6 mg/L NO3).
Considering some reagent is always stuck in the envelope and the fact that the accuracy of the checker is +\- 2ppm AND +\- 5% of reading, per Hanna’s website, it isn’t unreasonable. Unfortunately that also means that my tank nitrates are on the order of 150ppm NO3!
Thanks @Randy Holmes-Farley and @Dan_P for pointing out what was embarassingly obvious!

Glad all is resolved with respect to the testing. :)
 

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