How fast does light dissolve iodine and...

[yachtboy14]

How fast does a bright say 250watt mh hanging about 12in above the water dissolve chemicals such as iodine? What other chemicals are broken down by bright light in the home aquarium? Why does bright light break these chemicals down so much faster then others?

 

[Randy Holmes-Farley]

Dissolve them? I presume you mean degrade them? They are usually already dissolved before dosing.

If the question arises because someone told you that you need to dose more iodine because of the UV light, then I would disagree. I do not recommend that most aquarists ever add supplemental iodine as its benefit for any organisms that we keep is undemonstrated. The transformations of different forms of iodine is probably largely biological in aquaria (it is in the ocean) and not primarily driven by lighting unless you use a UV sterilizer. There is, however, the potential for lighting to have some effects on the iodine speciation in aquaria. UV light drives conversion of iodide to iodate. However, iodate is the primary species present in the ocean, so it is not unnatural, and is bioavailable to some algae and bacteria (although perhaps not as readily as iodide).

If the iodine supplement is Kent's time released compound (presumably an organoiodine compund), I do not know what impact the lights have on it, but I do not recommend using it anyway.

That said, there are many reactions that are driven by light in aquaria.
Many organic molecules absorb UV light, and can then more readily be oxidized. The same is also true for certain inorganic compounds,
including hydrogen sulfide.

So to be photodegraded, molecules first need to absorb the light, and then be prone to a breakdown reaction from the excited state that the UV absorption has put them into.

However, I would not conclude that such reactions are necessarily bad. Iron EDTA complexes are made bioavailable for organisms to use primarily after being photodegraded in an aquarium.

Discussion of photo reactions are scattered through my different articles. These articles have more:


Iodine in Marine Aquaria: Part I
http://www.advancedaquarist.com/issues/mar2003/chem.htm

Iodine in Reef Tanks 2: Effects on Macroalgae Growth
http://www.advancedaquarist.com/issues/april2003/chem.htm

Organic Compounds in the Reef Aquarium
http://reefkeeping.com/issues/2004-10/rhf/index.htm

Hydrogen Sulfide and the Reef Aquarium
http://reefkeeping.com/issues/2005-12/rhf/index.php

First Iron Article: Macroalgae and Dosing Recommendations
http://www.advancedaquarist.com/issues/aug2002/chem.htm

Second Iron Article: Iron: A Look at Organisms Other than Macroalgae
http://www.advancedaquarist.com/issues/oct2002/chem.htm

 

[yachtboy14]

Yeah, I wrote that late at night and noticed that I used disolved early this morning by mistake, lol at least you understood. But the main reason I was asking wasn't because I thought I was underdosing or thought I needed to add more. It was because I wanted to know why some chemicals in the home aquarium degrade under intense light and others don't.

 

[Randy Holmes-Farley]

It was because I wanted to know why some chemicals in the home aquarium degrade under intense light and others don't.[?It was because I wanted to know why some chemicals in the home aquarium degrade under intense light and others don't.B]

The short reason is that only some absorb UV light, and then only a portion of those will be likely to break down (usually by oxidation) after absorbing the UV light.

Calcium, for example, doesn't absorb any UV light.

Some ions do (like nitrate, IIRC), but are not very prone to any further reaction even after absorbing the light.