HI782 Reagent Issue

[JinWu]

I wanted to see if my HI782 was accurate, so I conducted a small experiment, and the results were quite interesting.
I used 9.5 mL of the ASTM D1141 standard solution and added 0.5 mL of a 100 mg/L NO3-N standard solution; the theoretical NO3 concentration of the resulting mixture should be 22.13 ppm.However, the HI782 yielded a reading of only 16.3. Is this reading normal? It looks like it far exceeds the margin of error allowed for the HI782.

 

[Randy Holmes-Farley Verified Community Expert]

I think perhaps your expectations of accuracy exceed what we usually get. lol

The Hanna estimates of accuracy assume, IMO, everything is perfect.

 

[JinWu]

I conducted another experiment afterwards, replacing 9.5ML of artificial seawater with RO water, and the reading soared directly to 33.5ppm. This reagent seems to be significantly affected by chloride ions in water

 

[Randy Holmes-Farley Verified Community Expert]

I conducted another experiment afterwards, replacing 9.5ML of artificial seawater with RO water, and the reading soared directly to 33.5ppm. This reagent seems to be significantly affected by chloride ions in water

Why think it is chloride and not things like alkalinity impacting the pH? Chloride should not react with the reducing agents in a nitrate test.

 

[JinWu]

See next post

 

[JinWu]

Why think it is chloride and not things like alkalinity impacting the pH? Chloride should not react with the reducing agents in a nitrate test.

I believe that the testing principle of HANNA for NO3 is essentially the same as that of HACH. HACH explicitly states, "Chloride concentrations above 100 mg/L cause low results."

 

[JonasRoman]

I wanted to see if my HI782 was accurate, so I conducted a small experiment, and the results were quite interesting.
I used 9.5 mL of the ASTM D1141 standard solution and added 0.5 mL of a 100 mg/L NO3-N standard solution; the theoretical NO3 concentration of the resulting mixture should be 22.13 ppm.However, the HI782 yielded a reading of only 16.3. Is this reading normal? It looks like it far exceeds the margin of error allowed for the HI782.

Its different colour development, weaker, in marine water and Hanna is factory calibrated for marine water. When I tested this the factor was approx 30% between marine and fresh water, meaning if you do a test on fresh water the colour is 30% stronger (absorbance). I checked the ASTM and as thats marine water, the ref fluid is correct for the situation, meaning the accuracy is not better than this for the Hanna. AMO its good enough in this quite high range.

 

[JonasRoman]

I conducted another experiment afterwards, replacing 9.5ML of artificial seawater with RO water, and the reading soared directly to 33.5ppm. This reagent seems to be significantly affected by chloride ions in water

Thats is logic, fresh water gives higher absorbance, Hanna is factory calibrated based on marine water.

 

[JinWu]

Its different colour development, weaker, in marine water and Hanna is factory calibrated for marine water. When I tested this the factor was approx 30% between marine and fresh water, meaning if you do a test on fresh water the colour is 30% stronger (absorbance). I checked the ASTM and as thats marine water, the ref fluid is correct for the situation, meaning the accuracy is not better than this for the Hanna. AMO its good enough in this quite high range.

ASTM D1141 is the standard practice for preparing solutions that simulate the composition of natural ocean water for laboratory testing.

 

[JonasRoman]

ASTM D1141 is the standard practice for preparing solutions that simulate the composition of natural ocean water for laboratory testing.

As I said, then ref sol is correct for this situation

 

[Randy Holmes-Farley Verified Community Expert]

I agree that chloride is not a concern with the Hanna nitrate tests in seawater, and Hanna specifically says so:

The HI-782 uses the zinc reduction method for testing, specifically development for use with salt/seawater and with no requirement to filter the sample before testing,

 

[JonasRoman]

To be clear: chloride and mg/Ca is affecting the ability for zink to reduce nitrate to nitrit, (which is what the nitrate test do.) So its harder for zink to act as a reducing agent in environment with a lot of chloride ions.
So less nitrit is formed. Second reason for weaker colour is that nitrit itself also react some different with he colour developing agent and make less colour

This is why you get more colour when you did the test with your fresh water based solution.
This is though not a problem because all nitrate measuring photometers are factory calibrated to the marine water situation(or fresh) . You as user you must know this so you understand its not possible to measure both marine and fresh water IF not the machine has a function for that, which in practice/behind the scenes in that case is just 2 different absorption constants for marine and fresh water.

 

[Dan_P]

I conducted another experiment afterwards, replacing 9.5ML of artificial seawater with RO water, and the reading soared directly to 33.5ppm. This reagent seems to be significantly affected by chloride ions in water

Chloride interferes with the metal reduction of nitrate to nitrite, hence the greater nitrate reading in RODI. That is normal..

The expected Hanna results should be within 2 ppm + 0.8 ppm, say 3 ppm which makes your observed 16 ppm to far off. The reagent quality might be an issue. A second possibility for the low reading is insufficient mixing during the two minute shaking. I would guess reagent quality is the issue.

 

[Dan_P]

Its different colour development, weaker, in marine water and Hanna is factory calibrated for marine water. When I tested this the factor was approx 30% between marine and fresh water, meaning if you do a test on fresh water the colour is 30% stronger (absorbance). I checked the ASTM and as thats marine water, the ref fluid is correct for the situation, meaning the accuracy is not better than this for the Hanna. AMO its good enough in this quite high range.

It seems that I repeated your answer. I acknowledge your precedence and will not pursue the prize money :-)

 

[JonasRoman]

Oh, no problem. it's good to have explanations in different ways. Repetition is the mother of pedagogy.