Hi all,
But could be mistaken or informed falsly, always thought chlorine only excists in gass form and eventualy will degass from the water especialy rather faster if it's moving water.
It is the difference between chlorine gas (Cl2) (and the
<"hypochlorite ion"> (ClO−)) and the chloride ion (Cl-).
Dissolved chlorine gas (Cl2) will equilibrate with the level of atmospheric chlorine, and because the level of chlorine in the atmosphere is really low (it is measured in parts per trillion) all the chlorine gas will disappear from the water.
However the chloride ion (Cl-) will just remain in solution, you know it doesn't go anywhere because the sea is salt with NaCl (as Na+ Cl-) that has accumulated over millennia.
It is the chloride ion (Cl-) that interferes with the testing of other monovalent anions (like NO3-). In sea water we have a known amount of chloride ions (19.5 part per thousand) and we can factor that in to our equation, in fresh water we don't the level of Cl- ions. Most chloride compounds are soluble, but silver chloride (AgCl) is insoluble, meaning that you can precipitate out any chlorides from your tank water sample before testing by adding an excess of a soluble silver compound, usually silver sulphate ((Ag2)SO4).
Nearly all nitrate compounds are soluble as well, so to use a colorimetric method (like in the API test kit) you need to reduce the NO3- to nitrite (NO2-).
Nitrate is reduced almost quantitatively to nitrite in the presence of Cd. The nitrite thus produced is determined by diazotizing with sulfanilamide and coupling with N-(1-napthyl)-ethylenediamine dihydrochloride to form a highly colored azo dye that is measured colorimetrically.
You can get accurate NO3- levels for water, but none of the methods are very straight forward. In the lab we use
<"ion selective electrodes">, but you still needs to make up standards etc. If you want accurate results (like a water company would want) you need to use high performance liquid chromatography (HPLC).
There is a much easier way of estimating the nitrate content of your tank water, that is to observe the growth rate and colour of a non-CO2 limited plant in your aquarium.
If your plant grows quickly, and is dark green, you have lots of nitrate.They use this approach for
<"crop plants">
Plants with access to the atmosphere (400ppm CO2) are non-CO2 limited so a floating plant is perfect, and this was how the
<"Duckweed Index"> was born.
cheers Darrel