FWIW, magnesium tests work like the calcium test description above.
Ammonia:
http://reefkeeping.com/issues/2007-02/rhf/index.php
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.
