Nitrate colorimeter. - The Planted Tank Forum
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post #1 of 22 (permalink) Old 10-28-2018, 02:35 AM Thread Starter
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Nitrate colorimeter.

Well started a new project destined to not be completed (it's a gift)...


Was researching methods to do a more accurate Nitrate reading.
found this article:
http://www.qualiteitems.com/images/nitrate.pdf


And the "key" was this:
Quote:
We selected LaMotte Company reagents for the Nitrate colorimetric test because: (a) The
recommended wavelength, 540 nm, is very close to the 565-nm wavelength setting on the
Vernier Colorimeter,
That's green btw.
initial photoshop checks of the Salifert Nitrate test revealed indeed green was selectively removed from the spectrum.
Couple that w/ cheap Lux meters that are heavily weighted (or should be) to measure green.. well it all sort of came together..


Couple of catches: Obviously the Nitrate test needs to produce a magenta sample (Cadmium reduction types which also work by turning nitrate to nitrite and actually sampling that.. so really all these "nitrate" tests are really Nitrate+Nitrite (which should be zero (Nitrite). Note: when cycling this actually could be an issue but just think about it for now ))


Well tried to buy an already built unit (like the Vernier, which btw is pretty cheap on the bay, BUT that is only one PART of a chain making a complete "meter" costly.

I'm sure a "box" could be played with but why? All it would be is buying 4 LED diodes and box and switch)




Anyways more to results:
Made a housing to cover the sensor of a LX-200 Lux meter.
drilled a 1/4" hole as an entry point
took a sample jar (about 1" round) for the "cuvette".
Put it on top w/ 1ml water
Shined an LED flashlight though it, resting on the lip and centered

Recorded reading 1000Lux
took out water added a sample of about 50ppm Nitrate Salifert test
measured reading 830Lux


Now this is w/ out using a more selective diode.
Ideally a 540nm green should be used BUT not in my stable (actually don't have any high power greens atm..


Now for fun I cheated:
Had a small minibar bottle of Tanqueray which, is a nice green color.
Sadly bottle was almost empty..


will see how that effects readings along w/ th next step of creating a Nitrate sample and diluting it to plot..


Sooo for the more ambitious.. That is the groundwork.


Seems solid, simple and accurate..
Oh I plotted the G of the rgb components of the salifert sample chart and it was pretty darn linear..May mean something, may not..
Oh using the LUX meter I'm not sure the light needs to be filtered, just consistent..
either way should work..

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post #2 of 22 (permalink) Old 10-28-2018, 01:49 PM
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Interesting take on reading Nitrate test samples - remove the human eye and replace it with an electronic eye.
Now the trick will be getting the exact same amount of tank water each time, then adding the exact same (or close enough) amount of reagent each time.

Sounds like an interesting project to follow along on.


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post #3 of 22 (permalink) Old 10-29-2018, 06:45 AM Thread Starter
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Naaah that's the esaiest part.. Salifert test kit gives me a 1ml sample.
OK firts rough test..
Consider the x axis "relative" values and forgive the sloppyness.

good thing is that this will be geared more to the precision of the "high end" of the Nitrates.
Not going to be concerned about 2mm or 10ppm..for now.
Most "commercial" units aren't geared to that end considering the health standards are concerned w/ say 0 to 20ppm..
so they need the most accuracy and ease of use in that range.




I'm working on a more "dedicated" light atm using a cyan diode.. It's high range so mostly green blue than blue green.
Should be in the 505-520 range so should have enough spectrum close to 540..
We shall see.

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post #4 of 22 (permalink) Old 10-30-2018, 04:19 PM
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Check this out:

https://iorodeo.com/collections/open-source-colorimeter

Additionally, Hach colorimeters can sometimes be found fairly cheap on eBay. A DR/900 or DR/890 will allow you to use the chromotropic acid method which is an improvement over the traditional cadmium reduction method.

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post #5 of 22 (permalink) Old 10-30-2018, 04:47 PM Thread Starter
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Quote:
Originally Posted by DiscusStu View Post
Check this out:

https://iorodeo.com/collections/open-source-colorimeter

Additionally, Hach colorimeters can sometimes be found fairly cheap on eBay. A DR/900 or DR/890 will allow you to use the chromotropic acid method which is an improvement over the traditional cadmium reduction method.

Yea finding cheap Hach's is fairly easy.. Finding a nitrate one isn't..


Those kits look fun but still too pricey..


Getting a photodiode or complete "analog" unit like a LUX meter is a bit more ideal..and cheap..
almost any photodiode w/ a resistor can be "read" by a VOM..


For a few bucks (currently $5 driver, scavenged power supply, and $1 diode (pick a color) gives me a fairly stable light source.. fairly stable..


I now have both a "cyan" and a red puck set up to use..


Just need to refine the light path..and of course calibrate it.

Cadmium reduction is used by both Salifert and Ocean life (afaict) and they list the ingredients in the pdf above..
Not arguing one way or another but the "template" is set for Cadmium..

https://aquaponics.com/store2/reagen...-nitrate-test/
https://aquaponics.com/store2/reagen...te-refill-kit/

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Last edited by jeffkrol; 10-30-2018 at 05:12 PM. Reason: edit
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post #6 of 22 (permalink) Old 10-30-2018, 05:15 PM
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Quote:
Originally Posted by jeffkrol View Post
For a few bucks (currently $5 driver, scavenged power supply, and $1 diode (pick a color) gives me a fairly stable light source.. fairly stable..

Cadmium reduction is used by both Salifert and Ocean life (afaict) and they list the ingredients in the pdf above..
Not arguing one way or another but the "template" is set for Cadmium..

I am really curious how this will turn out. I am a photochemist, so if you don't mind I will give you few tips. First of all, you definitely want to setup the calibration in terms of absorbance/transmittance, not absolute values of flux. Check this out, if you are not sure what I mean:


https://en.wikipedia.org/wiki/Beer%E2%80%93Lambert_law


It basically means that you will be measuring % of light loss upon passing the sample. That's how professional spectrophotometers do it. There are many reasons why you want to do it - aging of diode, varying photon flux etc etc.


The second point I would like to make is that cadmium reduction method results in lots of mess, i.e. solid particles dispersed in the sample manifesting as turbidity. The absorbance/transmittance I was talking about cannot really distinguish between loss of the light due the true absorption by the compounds and due to scattering of the light due to turbidity. To make sure your results are not very skewed, I would think about involving a short filtering set prior to the measurement by the colorimeter (a piece of cotton in a small pipette should be enough).
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post #7 of 22 (permalink) Old 10-30-2018, 05:47 PM Thread Starter
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Noted....

Hey can you find me the absorption spectra of resultant color??
Recommended wavelength was 540.
565 was useable..

Pretty sure my cyans are 505nm..
min 490 max 520

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post #8 of 22 (permalink) Old 10-30-2018, 06:19 PM
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Quote:
Originally Posted by jeffkrol View Post
Yea finding cheap Hach's is fairly easy.. Finding a nitrate one isn't..
The Hach DR/890 and DR/900 can perform both cadmium reduction and chromotropic acid methods. Not to mention chlorine, ammonia, and nitrite...

The cadmium reduction method is technique-sensitive and uses toxic chemicals hence all the research to develop alternatives.
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post #9 of 22 (permalink) Old 10-30-2018, 06:31 PM
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Quote:
Originally Posted by jeffkrol View Post
Noted....

Hey can you find me the absorption spectra of resultant color??
Recommended wavelength was 540.
565 was useable..

Pretty sure my cyans are 505nm..
min 490 max 520

There are two versions of the test, I am not sure which one is your test using.


I was only able to find reliable absorption spectrum for the chromotropic derivative (see the bottom product in the attachment), which has a maximum between 550-560 nm. The other one, should have a maximum at 420 nm based on this patent:


Method, reagent and kit for the determination of nitrate ions



What you can do though is to test different colored LEDs you have and the one which has highest % loss of light upon passing the sample is the "best" or most sensitive one.
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post #10 of 22 (permalink) Old 10-30-2018, 07:06 PM Thread Starter
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Quote:
Originally Posted by DiscusStu View Post
The Hach DR/890 and DR/900 can perform both cadmium reduction and chromotropic acid methods. Not to mention chlorine, ammonia, and nitrite...

The cadmium reduction method is technique-sensitive and uses toxic chemicals hence all the research to develop alternatives.

Yea for $1300 used..

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post #11 of 22 (permalink) Old 10-30-2018, 07:20 PM Thread Starter
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Quote:
Originally Posted by acino View Post
There are two versions of the test, I am not sure which one is your test using.


I was only able to find reliable absorption spectrum for the chromotropic derivative (see the bottom product in the attachment), which has a maximum between 550-560 nm. The other one, should have a maximum at 420 nm based on this patent:


Method, reagent and kit for the determination of nitrate ions



What you can do though is to test different colored LEDs you have and the one which has highest % loss of light upon passing the sample is the "best" or most sensitive one.

Whatever the indicator solution is it's the same as the Nitrite ones..since that's all you do here nitrate to nitrite.. measure nitrites..
most indicator solutions are this:
Quote:
1,10-phenanthroline
7782-63-0
1 - 2.5
FERROUS SULFATE HEPTAHYDRATE


But I believe that is "brownish"..
Which leads to this... maybe..

Quote:
Nitrite reacts with 3-nitroaniline in the presence of hydrochloric acid to form a diazonium cation, which is subsequently coupled with N-(1-naphtyl)ethylenediammonium chloride to form a stable purple azo dye. The method is suitable for the determination of 0.01–0.80 μg ml−1 nitrite. The reactions are very fast and require no control of temperature. The observed molar absorptivity and Sandell's sensitivity of the azo dye are 4.9 104 l mol−1 cm−1 and 9.4 10−4 μg cm−2, respectively. The method is free from most interferences. The method has been applied successfully to polluted river water.
pg 9 broad spectrum..
https://www.google.com/url?sa=i&rct=...41014163162469


Hmmm .how one gets from Nitrate to Nitrite seems of lesser importance..

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Last edited by jeffkrol; 10-30-2018 at 07:35 PM. Reason: edit
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post #12 of 22 (permalink) Old 10-30-2018, 07:44 PM
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Quote:
Originally Posted by jeffkrol View Post
pg 9 broad spectrum..
https://www.google.com/url?sa=i&rct=...41014163162469
Hmmm .how one gets from Nitrate to Nitrite seems of lesser importance..

You are absolutely right, at the end of the day, you go from nitrate to nitrite which is then used to perform azo-coupling with. Nitrates are reduced to nitrites with cadmium, hence it's called "cadmium reduction". There are endless possibilities what you can couple it to, I showed two, which I believe, are the two most common options. The one you showed is also very viable and I would not be surprised if there were others (all you really need is an electron rich aromatic core) for the sake of circumventing patents etc. Any basic chemical laboratory could measure what is the absorption spectrum of the compound produced by the kit you're using, but if that's out of reach, you can try the stuff with different LEDs that I have mentioned.
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post #13 of 22 (permalink) Old 10-30-2018, 08:14 PM Thread Starter
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Rough approx of Salifert color chart reveals:

RGB
225,209,220 @ 25ppm
207,150,198 @ 50ppm
185,37,160 @ 100ppm

Switching to CMYK:
C,M,Y,K(black)
9,18,4,0 @25ppm
16,47,0,0 @50ppm
34,98,0,0, @100ppm

One question is can you mix /match reduction compounds/dyes?
No matter , this will currently be geared to whatever Salifert uses.. IF one wants to get creative.....

cyan will work w/ the Salifert dye..
going deeper. , what wavelength involves the less possible interference by "other stuff".
Cyan/green pretty "transparent" to most aquarium aqueous things..

Most of the dye curves I saw were pretty broad. Bandwidth looks to be more by "convention"..well obviously shooting for max.

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post #14 of 22 (permalink) Old 10-30-2018, 08:19 PM
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Quote:
Originally Posted by jeffkrol View Post
Rough approx of Salifert color chart reveals:

RGB
225,209,220 @ 25ppm
207,150,198 @ 50ppm
185,37,160 @ 100ppm

Switching to CMYK:
C,M,Y,K(black)
9,18,4,0 @25ppm
16,47,0,0 @50ppm
34,98,0,0, @100ppm

One question is can you mix /match reduction compounds/dyes?
No matter , this will currently be geared to whatever Salifert uses.. IF one wants to get creative.....

cyan will work w/ the Salifert dye..

Ok, I just googled their chart. Cyan will definitely work. But the pink color suggest absorption maximum at around 540-560 nm so a green diode would give you better senstivity AND smaller error. The further away from the absorption maximum you're measuring, the greater error due to possible difference in peak shape etc (since you're measuring a point on a slope).
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post #15 of 22 (permalink) Old 10-30-2018, 08:29 PM Thread Starter
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not shooting for less than 10% error....20,25,30,35,40 ect would be fine..

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