[SuRje1976]

Well, I got a bit tired of fussing with the LaMotte kits, and was in an impulsive mood the other night. So I ordered these:

Hanna Nitrate Photometer

Hanna Low Range Phosphate Photometer

I also picked up 2 of these, because I'm goofy:

Shockproof Boot


They came today! I'm not going to bother posting pictures, they don't look any different from the ones in the links above.

The Nitrate Photometer reads Nitrate-Nitrogen (not the same as Nitrate). It has a range of 0.0 to 30.0 mg/L. This equates to about 0-130 mg/L Nitrate. It reads in increments of 0.1 mg/L and is accurate to 0.5 mg/L, or +-10% (probably due to the fact that the reagents come in unit-dose packets, and an a somewhat unpredictable amout adheres to the inside of the packet itself). This is probably still more accurate than my interpretation of the colorimetric test results I get with the LaMotte kits. Conversion of the Nitrate-Nitrogen result reported by the device itself to Nitrate is done by multiplying the result by 4.43. Not too difficult.

The LR Phosphate Photometer has a range of 0.00 to 2.50 mg/L of Phosphate. It reports results with a resolution of 0.01 mg/L, and is accurate to 0.04mg/L, or +- 4%. No conversion is necessary.

They are simple to operate. A sample is added to the vial and is used to "zero" the device. The reagent (content of one single-dose packet) is added, shaken for a minute and put back into the machine. There is a "Read Timed" button that can be pressed and the machine will count down the appropriate amount of time before it takes the reading (analagous to "wait 5 minutes, compare to colorimeter" with traditional kits). The alternative is to wait the appropriate amount of time "manually," and use the "Read Direct" button. I'll probably never use that, because if I'm not impatient and read the result prematurely, I'll forget about it, and leave it much too long. Both are considered to make the result less accurate.

I used them both today, to test the water on the 120P. I got 12.7ppm NO3 and 1.17ppm PO4. A little more NO3 than I would have liked, so I'll change some water tonight!

 

[SuRje1976]

Ok, couple preliminary observations. The cuvets that the sample is to be placed into are marked at 10mL. This is OK(ish) for the PO4 test, but the NO3 test requires a 6mL sample (cuvette is not marked at 6mL) SO, you'd need a means of measuring this out. I am using 10mL Class A Mohr Pipettes for this. The 10mL mark on the cuvette appears to be slightly mis-marked as well. Sample would probably be 10.2mL if filled to the mark.

Another issue is - variability in readings. I know the "test kits are no good" crew is going to jump on this, but I really think I know where it's coming from. It's a bit difficult to tell when you've completely emptied the pre-filled unit-of-use reagent packets into the sample cuvet. If you aren't extremely careful, you'll leave quite a bit of reagent in the corners of the packet itself, but it will appear that you've emptied the packet completely. I've emailed Hanna regarding this and asked them if they could provide the actual WEIGHT of reagent that I should be able to produce from each packet. I also intend to ask if the reagents can be purchased in a bottle, as opposed to the packets, but I'll wait for a response on the initial inquiry first.

 

[SuRje1976]

Ok, this is where I'm at. I did receive a response (a couple actually) from Hanna. They asked that I looked over the manual to ensure that none of the interfering substances are present in my water. I unforunately do not have a means to PROVE that my water contains none of them, it likely does actually. BUT, I would expect that it contains a fairly consistent amount of them from one sample to the next, so logically I would presume the results should be consistent as well.

What I intend to do is to run multiple tests on a sample made with RO/DI water and enough KNO3 to result in 15mg/L NO3-N and see if I can get consistent results with a consistent amount of reagent. This will rule out pretty much anything other than a defective machine/design. I MAY even be able to calculate how much reagent I SHOULD be using to produce an accurate result (assuming that I actually GET consistent results).

I haven't been GIVEN the weight of reagent that I should be getting (yet), but I have a dialogue going with a representative that seems to be helpful. He actually asked me to keep him posted with results, but I don't really think he wants to divulge any proprietary information to me.

All of my test samples are taken right before lights-on. SO, today I began weighing the reagents before the tests. I got:

212mg (0.212g) of Nitrate reagent out of its packet. This gave me a test result of 2.4mg/L NO3-N (10.6mg/L NO3) I will not add NO3 today, I hope it is less tomorrow, I have a light fish load!

170mg (0.170g) of Phosphate reagent from its packet. This gave 1.89mg/L PO4. I will also not add anything today, and retest tomorrow. This kit SEEMS to be a bit more accurate so far. The results I've been getting from it make sense.


I will report my results here (reagent weight & test result) daily (ideally). I will also report any useful info I get from Hanna as well.

 

[plantbrain]

Sergio,

Good job on a purchase.
You might have wanted to simply get a spect or a multi meter rather than single parameters unless these are the only ones of interest. I got a nice Hach and an Orion meter for about 200$ ea used.

If the nm range is printed/given in the manual, (most all of them are), you can make and use your own DIY reagents as you have learned are not cheap to buy pre done ones!

So say the nm max range for NO3 is 525nm for Absorbic acid reduction, then you can use those absorbic acid reduction chemicals to do so rather than paying primo pricing for the Hanna reagents. They are convenient.

Likewise, you should calibrate the colorimeter as well against a know standard. Maybe a 30, 10 and 3 ppm N-NO3 standard solution series.

Yes, nothing will get you out of calibration if you want accuracy and confirmation:tongue:

Here's a listing of the various reagents for various test and their nm ranges:

http://www.hach.com/fmmimghach?/CODE:4487900H-N175|1

With the reagents, a good scale, etc, you can do a lot with a 200-400$ spect.

Reduce them light and CO2, then add more, see how that affects the PO4/NO3. Try limiting PO4 and see how the rate of NO3 is affected.

The other thing, most methods test for total, they do not distingush between organic and non bioavailable and the bioavailable inorganic forms of N, P etc. :icon_idea

They do have special methods for addressing that however.
Still, lots better than a color test kit like Lamotte and much better than the API test kits.

Also, if you take a little bit of the sample water and wash out the powder from the packet, that will help and then save that back into the test vial curvette.

That way you get it all.
I think some companies also sell the reagents in bulk and all you do is weight them for each test amount. Another way around it: use a larger volume(pretty wasteful, but adds more accuracy) and take a subsample for the test.

Basically use 100mls or 1000mls instead of 10mls add X amount of grams of reagent of the 1000mls and you have better accuracy for the weight to volume of liquid ratio. But you have to use a lot of the reagents.

Print that ref out for the NO3 and PO4 test procedures and methods.
It might save you hundred's of $ in cost very quickly if you plan on using these often.


Regards,
Tom Barr

 

[KevinC]

Don't worry too much about the amount of reagent getting into the vial:

The reagent reacts with the sample/analyte to produce a color dependent on the amount of analyte. So there should be enough reagent in the packet to completely react with the analyte at the highest concentration. Since your samples do not have the highest levels of nitrate or phosphate, there will be excess reagent anyway.

Is your reproducibility poorer than stated on the kits? How bad is it? Light sources (esp LED's) are noted for changing with time as they warm up - let the meter warm up for a few minutes before making any readings (including the blank) and see if that helps.

Kevin

 

[SuRje1976]

Tom & Kevin thanks for the responses! The issue that I'm seeing is (primarily) with the Nitrate meter. I'm getting results that don't seem to make sense to me. I have a VERY light fish load in the tank that I've been using them for. I get higher nitrate readings at night (so far). I have not been adding ANY NO3. My results have fluctuated between 1.1 & 2.5ppm NO3-N, (5-11ppm NO3), but not at a steady DECLINE, which I would expect, being that for the purposes of "testing" the meter, I'm not dosing nitrogen??? This is MUCH poorer reproducability than listed in the spec sheet 0.5ppm, or +-10% reading).

 

[KevinC]

Well, as a chemist I prefer to remove the biology from the equation, so here is what I would suggest (including letting the LED warm up):

Make 1L of a 2.0ppm NO3-N solution using a standard and ion-free water. Make a reagent blank - use the ion free water, add a reagent packet, and use that to zero the instrument (you may already do this - just had to say it). Then make repeated measurements of the 2.0ppm solution. Be sure to use the same cuvette for the reagent blank AND each sample - there can be significant differences between cuvettes.

I also don't know what they mean by "0.0" as the lower end of the scale. You can't measure "0.0" - analytically you can only say the sample is "below the limit of detection (LOD)" where the LOD is sometimes defined as the concentration that gives a signal 3x the noise level of the blank. In other words, at the lower end of the scale, how accurate is the instrument? In the range of 0.0-30.0, a 2.0ppm sample is at the very low end.

I suspect you will find that a prepared 2.0ppm NO3-N solution gives you just as much variability in readings as your aquarium samples. If that is the case, make a 10.0ppm NO3-N solution and try the same thing - the variability should be much less when you are higher in the calibration range.

Kevin

 

[SuRje1976]

Wow. I will do this Kevin, thanks for the advice. I suspect you're correct as well. If that is the case, these photometers (well, the Nitrate one at least) will be essentially useless for my purposes. I'll keep you posted.

 

[SuRje1976]

Well, final verdict - they're going back. The posphate photometer proved to be quite accurate, probably because as Kevin suggested, my water tests in the middle of the device's capable range. The Nitrate photometer would give extremely incosistent (not to mention inaccurate) results at the levels I was using it for. I've started looking at this as a potential alternative.