Hi All,

I thought some might be interested to know what is in and the science behind their test kits.

Let's start with:

pH: Bromothymol Blue (https://en.wikipedia.org/wiki/Bromothymol_blue)
Different protonation state affects the color of the molecule.

High range pH: Metacresol purple (http://www.sigmaaldrich.com/catalog/...g=en&region=US, https://en.wikipedia.org/wiki/Metacresol_purple)
Different protonation state affects the color of the molecule.

Ammonia: This test works by detecting chloramines, likely if you are super-dosing with prime or another dechlorinator then you might be getting a false reading for ammonia.
API bottle 1 contains sodium salicylate. API bottle 2 contains bleach and sodium hydroxide.
The reaction is below:
The product 4-aminosalicylate is blue allowing a readout of the ammonia concentration.

Nitrite: This test works by reacting nitrite with an aryl amine to form a diazonium ion. Diazoniums can react in many different ways but form diazo dyes with electron rich aromatics. It is called the Greiss test (https://en.wikipedia.org/wiki/Griess_test) Diazo-dyes are some of the most common dyes used today.

Nitrate: Nitrate is less reactive than either ammonia or nitrite and common tests rely on the reduction of nitrate to nitrite or ammonia. Based on the SDS of API it is reduced to nitrite to react with sulfanilamide in the same way as the nitrite test kit. I'm not 100% sure on this one but that's my best guess.

General Hardness: This is based on EDTA addition until no free Ca/Mg ions are available at which point the EDTA pulls Mg from Eriochrome Black T which turns back to a blue color.

Carbonate hardness: Again a guess, but this is simply the Bromothymol Blue indicator mixed with an acid. This would be sensitive to the pH range at which carbonate/hydrogen carbonate/carbonic acid is an effective buffer.

Obviously this is just an intellectual curiosity and not a suggestion to go out and try to prepare your own test kits/reagents.