Ammonia testing with Hanna C99 photometer: Ca and Mg interference

[New Reefer]

Hi Everyone,

I recently purchased a Hanna C99 bench photometer, mainly to test for phosphates, but would obviously like to get as much "value for money" by doing other tests as well. One of the tests that should be possible with this unit is the test for ammonia using the Nessler method.

I did my first ammonia test a few days ago, and it did not work. After adding the required amount of reagents the water turned a milky white with (what I presume to be) calcium and magnesium precipitate. An hour or so on Google confirmed this being a problem when testing sea water with the Nessler method. Various articles recommend that one should pre-treat the sample with a "Mineral Stabilizer" to keep these elements in solution, but the Hanna manual did not say a word about this...

One publication recommended that one should use "Rochelle Salt" (Potassium sodium tartrate) as the Mineral Stabilizer. I have found the following links to making one's own Rochelle Salt

Rochelle Salt synthesis
Preparation of Rochelle Salt

Could anyone please comment on whether this would work, and perhaps elaborate a bit on this chemical?

Also, would adding this "stabilizer" have any influence on the accuracy of the test?

Any other hints or comments would be appreciated.

 

[Randy Holmes-Farley]

FWIW, I suspect that jiggering a method that may be designed for fresh water to try to make it work in seawater is not going top be especially satisfying, nor have known accuracy unless you make standard curves yourself, etc.

I discuss ammonia testing here:

Ammonia and the Reef Aquarium
http://reefkeeping.com/issues/2007-02/rhf/index.php

from it:

http://reefkeeping.com/issues/2007-02/rhf/index.php#14

Testing for Ammonia
There are several ways to test for ammonia in seawater. These include test kits based on both salicylate and Nessler chemistry.

Nessler Test Kits

The reaction of ammonia with Nessler's reagent, K2HgI4, forms a colored precipitate of (Hg2N)I·H2O. Low levels of ammonia are yellow, higher is orange and even higher levels can be brown. The overall reaction is:

NH3 + 2[HgI4]2− + 3OH− → HgO·Hg(NH2)I + 7I− + 2H2O

One significant concern with the Nessler method is the toxicity and hazardous nature of the waste that is generated by its use (it contains mercury).

Salicylate Test Kits

Ammonia's reaction with hypochlorite forms monochloramine, which then reacts with salicylate in the presence of sodium nitro-ferricyanide to form 5-aminosalicylate. That complex is yellow to green to dark green based on the level of ammonia present. In some versions of the test, calcium and magnesium can cause interference, so be sure such a kit is designed for marine systems.

The distinction between these methods can be important, as some combinations of ammonia binding products and test methods can lead to false ammonia test results (either causing apparent ammonia presence when it is bound, or simply causing a color that is not predicted by the test kit). For example, a Nessler type kit cannot read ammonia properly if the aquarist is using Amquel, Seachem Prime or related products to bind ammonia. The result is often an off-scale brown color.

So, it is particularly important that aquarists understand how the test kit that they are using, and the binder that they are using interact, and the manufacturers of each are the best place to find such information.

My suggestion is to always measure total ammonia. If a kit gives a choice of measuring free ammonia, don't bother. You can always use a table to convert total ammonia to free ammonia if there is a strong reason to do so. The reason to measure total ammonia is that the signal will be much larger, so the kit will be more capable of distinguishing a small reading of ammonia from no detectable ammonia.

 

[New Reefer]

Thanks again for your reply, Randy

FWIW, I suspect that jiggering a method that may be designed for fresh water to try to make it work in seawater is not going top be especially satisfying, nor have known accuracy unless you make standard curves yourself, etc.

Actually, the Hanna Instruction manual infers that it IS suitable for testing sea water. Page 26 of the manual states:

• Add 4 drops of the First reagent (6 drops in case of seawater analysis ). Replace the cap and mix the solution.
• Add 4 drops of the Second reagent (10 drops in case of seawater analysis). Replace the cap and mix the solution.

The problem though, is that if I just follow their instructions then the water turns totally milky, and it is impossible to see any color, and the instrument then just gives an error reading.

I have now made a (saturated, I think...) Rochelle Salt solution, as per the recipe in the link which I quoted in the first post. I then added 1ml of this solution to 20ml of tank water, and used 10ml of that mix in an "experimental" test - firstly by "zeroing" the instrument with this solution, and thereafter adding the 6 and 10 drops of the two test reagents supplied with the test kit. The solution remained clear (no visible precipitation), and developed a light yellow color after 3.5 minutes (the prescribed waiting time).

So far, so good... but the result was totally unbelievable - 1.24mg/l ammonia nitrogen :eek2: Now, I just know that this cannot be true, as the tank has been established for nearly 5 years, and my corals and fish are doing very well.

I repeated the test, using a dilution of 1ml of the Rochelle Salt solution in 50ml of tank water, but this resulted in a slight (but visible) white cloudiness after the addition of the reagents, and the instrument giving a reading of 1.85mg/l in this case. I will repeat the test again with a dilution of 1ml in 10ml tank water, just to see if there is any change in the result, but unless someone can help me to figure out what I'm doing wrong, I guess that this is just one more test that I will have to write off as being useless for sea water At least, this (and my other post on the same instrument ) will serve as a warning to other reefers who might otherwise just waste their money...