Reef Chemistry Puzzle #17 I'm very hefty

[Randy Holmes-Farley Verified Community Expert]

Reef Chemistry Puzzle 17

1. I am a very hefty ion in seawater
2. I come in two primary ionic forms in seawater
3. Quite a few reef aquarists dose me
4. In my elemental state, I am so darkly colored as to appear black

What am I?

Good luck!


Previous Reef Chemistry Puzzle:

https://www.reef2reef.com/threads/reef-chemistry-puzzle-16-i-shine.1031570/

 

[drolmaeye]

Iodine is my guess . . .

 

[rhitee93]

I'd say phosphate. It's close to 100g/mol. There is a black form (allotrope?) of elemental phosphorus.

 

[CBonito]

carbon

 

[vabben]

Iron

 

[RichReef]

Boron

 

[rhitee93]

Boron

Hmm, might need to change my answer...

 

[PeterEde]

Iodine

 

[Dburr1014]

Magnesium... Nope it's not black.

I think I'll say boron too.

 

[TokenReefer]

Manganese

 

[dennis romano]

Boron

 

[DCR]

I would vote for Iodine which is a black solid with a specific gravity of 4.93 at room temperature and exists as Iodide or Iodate ion.

 

[musel101]

Potassium

 

[Koty]

Iodine:
1. It is a very hefty ion in seawater (I worked on an iodine channel in the thyroid, which also transports carbonate)." iodine has the largest electron cloud..."
2. I come intwo primary ionic forms in seawater
3. Quite a few reef aquarists dose I.
4. In elemental state, I is darkly colored as to appear black

 

[PeterEde]

Do we win a prize with iodine?

 

[Miami Reef]

For number 4) Lugol’s Iodine (I2)

 

[Randy Holmes-Farley Verified Community Expert]

And the answer is...

1. I am a very hefty ion in seawater
2. I come in two primary ionic forms in seawater
3. Quite a few reef aquarists dose me
4. In my elemental state, I am so darkly colored as to appear black

What am I?

Iodine. Good job, folks!

Iodine is among the heaviest ions we discuss. Much heavier than iron, for example. In the table below from my trace element dosing recommendation article shown below, the further down the table an element is, the heavier it is. Iodine is the lowest/heaviest of the ions typically dosed.

Iodine is most prevalent in seawater as iodide (I-; 0.01 to 0.02 ppm) and Iodate, (IO3-; 0.04 to 0.06 ppm). There are smaller levels of other forms such as organoiodine compounds, although these can comprise fairly high levels in some coastal locations (up to 40% of the iodine total). Diatomic iodine, I2, can also be present in low levels due to reaction of ozone and iodide at the ocean surface.

FWIW, I'm curious to know how much organioiodine is present in many reef aquaria that dose iodine. It might be quite high and be a substantial fraction of the total I detected by ICP.

Solid elemental iodine (I2) is a very dark solid. In the gas phase, it is purplish.

https://www.chemkits.eu/others/94-iodine-crystals-7553-56-2.htmlgas phase iodine picture by: Matias Molnar - Laboratorio Quimica Inorganica II - UBA, Argentina