Measuring LR NO3 with Hanna ULR Phosphorous photometer

[zachfishman]

Agree. Diluting adds extra error but I do not know a way around it. Accuracy may be improved by using a 1ml syringe.

We must not exceed the range of a colorimeter. In case of Hanna P checker sample at 200ppb P (max) is just slightly blue (when measuring P with the standard Hanna reagents).

At least with the chlorine checker or regular phosphate checker I have a higher range. 3.50ppm for chlorine, 2.50ppm for phosphate on my checkers.

 

[dkeller_nc]

That would be a reasonable assumption, however zachfishman used standard amount of reagents for 10x the water volume. It is possible that it slows the reaction down.

Yeah, I read what he posted after my comment, and you're right, using 10 times less reagent than the standard kit would likely be a major issue.

Zach - keep in mind that tests like these are setup so the reagents are in large excess to the anticipated analyte concentration (in this case, nitrate). The tests are setup this way so that all, or nearly all, of the analyte in question gets turned into the colored end-product.

If you greatly reduce the amount of reagents in the test, there's a good possibility that you won't turn all of the analyte into the colored end-product. And as amebcia notes, reduced concentration of reagents will also slow down the reaction, though whether a slower reaction would matter depends on the specific chemistry involved.

In the case of the Salifert test, one way around this would be to blank the colorimeter on water. Then react your standards according to the instructions in the Salifert kit insert (using 1 mL samples). Then using syringes, simply add 0.5 mL or so of the reacted liquid into the Hanna test vial, and add 8.5 mL of RODI on top of it. Mix it, then do an "instant read" in the colorimeter zeroed with water.

You might get an offset doing it this way - that is, zero nitrates might not read zero on the colorimeter since it's been blanked with water, and the test reagents themselves might have some background absorbance even in the absence of nitrate. But since you're using known nitrate concentrations to develop your standard curve, and offset won't matter.

 

[zachfishman]

Yeah, I read what he posted after my comment, and you're right, using 10 times less reagent than the standard kit would likely be a major issue.

Zach - keep in mind that tests like these are setup so the reagents are in large excess to the anticipated analyte concentration (in this case, nitrate). The tests are setup this way so that all, or nearly all, of the analyte in question gets turned into the colored end-product.

If you greatly reduce the amount of reagents in the test, there's a good possibility that you won't turn all of the analyte into the colored end-product. And as amebcia notes, reduced concentration of reagents will also slow down the reaction, though whether a slower reaction would matter depends on the specific chemistry involved.

In the case of the Salifert test, one way around this would be to blank the colorimeter on water. Then react your standards according to the instructions in the Salifert kit insert (using 1 mL samples). Then using syringes, simply add 0.5 mL or so of the reacted liquid into the Hanna test vial, and add 8.5 mL of RODI on top of it. Mix it, then do an "instant read" in the colorimeter zeroed with water.

You might get an offset doing it this way - that is, zero nitrates might not read zero on the colorimeter since it's been blanked with water, and the test reagents themselves might have some background absorbance even in the absence of nitrate. But since you're using known nitrate concentrations to develop your standard curve, and offset won't matter.

I will try this. Have to order more standard solutions first...

 

[amebcia]

Then react your standards according to the instructions in the Salifert kit insert (using 1 mL samples). Then using syringes, simply add 0.5 mL or so of the reacted liquid into the Hanna test vial, and add 8.5 mL of RODI on top of it. Mix it, then do an "instant read" in the colorimeter zeroed with water.

That is a great suggestion for testing with Salifert Do you think that the dye produced during the standard Salifert test will remain stable after diluting with RO. I am a bit concerned that it might fully or partially break down, e.g. due to change in PH.

 

[amebcia]

Mix it, then do an "instant read" in the colorimeter zeroed with water.

Just an extra comment related to my concern of the dye breaking down after diluting with RO. I assume that if the dye started to break down indeed, it would happen over time. One way to check for such a thing would be to take few measurements. e.g. 1, 3 and 5min after diluting. A separate bottle with RO water could be used for zeroing during each test. If the dye remains stable all three tests should give the same results (+/- error).

 

[dkeller_nc]

Just an extra comment related to my concern of the dye breaking down after diluting with RO. I assume that if the dye started to break down indeed, it would happen over time. One way to check for such a thing would be to take few measurements. e.g. 1, 3 and 5min after diluting. A separate bottle with RO water could be used for zeroing during each test. If the dye remains stable all three tests should give the same results (+/- error).

Presuming that I've found the correct chemistry that Salifert uses (1-napthyl ethylenediamine, Wikipedia article here), then I would expect that the colored end product should be stable, at least long enough to work with it on the bench.

But experiments are always the "proof of the pudding", so I'd be interested in your observations.

 

[Myka]

Well this is interesting. I will give it a go. I have a HI736 UL Phosphorus checker. I want to check the checker for accuracy (with the Hanna HI736-11 calibration kit) though as it has been giving me "0.00" readings on tanks that I should be able to assume that they have phosphorus in them. Also, I do have both Salifert and API NO3 kits, but I was getting false high readings off my API kit when I decided to buy the Salifert kit. I wish API had expiry dates or at least manufacture dates on them.

I will post my results (as shady as they may be given the above info).

Has anyone done any more testing that's mentioned in the thread?

 

[Myka]

Ok, I lied. I have a HI713 phosphorus checker, and I just found out that it is for freshwater only. Though every vendor site I just checked list marine and freshwater, Hanna's website has no mention of marine water for the HI713 checker where the HI736 checker does mention marine. Ok, sorry I'm off track now...I guess I can't try this experiment out!

 

[zachfishman]

UPDATE: I tried testing NO3 with my Hanna Total Chlorine Checker (uses same 525nm light as their phosphate checkers). Unlike my previous attempts filling the Hanna cuvette 100% with salifert test water, adding just 1mL to the Hanna cuvette (blanked with 10mL of RO) seemed to work. I had relatively stable results if I let the reaction go for 10min before sampling. Values tested in the same cuvettes over 20min and 30min did not change much if at all (unlike last time).

It seems to work reliably for NO3 values up to ~50ppm - which is great for most reefs, although not for my needs. You could practically multiply the HI711 value by 100 and be dead on. This was with n=1 so multiple trials are necessary to validate this. This relationship fell apart at 100ppm and above, however...

I tried doubling the Salifert reagents for higher [NO3], in case there simply wasn't enough reagent to react with all the nitrate in my test samples (I tested up to 333ppm NO3), but I didn't get a reliable increase in the Hanna readout with increasing NO3 beyond 100ppm...

Anybody know if potassium interferes with the Salifert test? The standard solutions I used were of KNO3.

 

[amebcia]

Having to wait only 10min when using 1x of the reagents for 10x the amount of water is actually much better than I expected. Glad to hear you made it work

 

[Dan_P]

Just to pile on, I have just used the Hanna ULR phosphorous checker with Salifert phosphate kit (calibration curve), Salifert oxygen kit (relative reading) and to measure methylene blue concentration (calibration curve). It is a versatile photometer. I will try nitrate in the future.