Reef Chemistry Question of the Day #243 Iodine in Seawater

[Randy Holmes-Farley]

Reef Chemistry Question of the Day #243

Iodine can exist in many forms, and a variety of them are found in reef aquaria in various situations. In addition to the many and varied organic forms, some of the inorganic forms are:

Iodate (IO3-)
Iodide (I-)
DiIodine (I2)
Hypoiodate (IO-)
Iodite (IO2-)
Periodate (IO4-)
Triiodide (I3-)

Which of these chemicals is most abundant in normal seawater?

A. Iodate (IO3-)
B. Iodide (I-)
C. Hypoiodate (IO-)
D. Iodite (IO2-)

Good luck!


































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[MnFish1]

A - Quoting a world-renown expert 'In surface seawater, iodate usually is the dominant form, with typical values in the 0.04 to 0.06 ppm iodine. Likewise, iodide is usually present at lower concentrations, typically 0.01 to 0.02 ppm iodine.'

 

[coral-boss]

will also agree with A.

 

[Bob Lauson]

tough call but I think I will go with A

 

[MnFish1]

A - Quoting a world-renown expert 'In surface seawater, iodate usually is the dominant form, with typical values in the 0.04 to 0.06 ppm iodine. Likewise, iodide is usually present at lower concentrations, typically 0.01 to 0.02 ppm iodine.'

PS the 'world renowned expert' is @Randy Holmes-Farley (from one of his articles)

 

[Randy Holmes-Farley]

PS the 'world renowned expert' is @Randy Holmes-Farley (from one of his articles)

Hey, I didn't claim that title.

 

[Randy Holmes-Farley]

And the answer is... Usually A..

Which of these chemicals is most abundant in normal seawater?

A. Iodate (IO3-)
B. Iodide (I-)
C. Hypoiodate (IO-)
D. Iodite (IO2-)

Chemistry And The Aquarium: Iodine in Marine Aquaria: Part I ? Advanced Aquarist | Aquarist Magazine and Blog
http://www.advancedaquarist.com/2003/3/chemistry

Iodine in the Ocean
Iodine in the ocean takes a wide variety of forms, both organic and inorganic, and the iodine cycles between these various compounds are very complex and are still an active area of research. The nature of inorganic iodine in the oceans has been generally known for decades. The two predominate forms are iodate (IO3-, with the central iodine and three attached oxygen atoms; Figure 1) and iodide (I-). As a curiosity, note the huge size of the iodine atom compared to the oxygen atoms in iodate in Figure 1. Together these two iodine species usually add up to about 0.06 ppm total iodine (~0.5 mM), but the reported values vary over about a factor of 2. In surface seawater, iodate usually is the dominant form with typical iodate values in the 0.04 to 0.06 ppm iodine (0.3 – 0.5 mM).2,3 Likewise, iodide is usually present at lower concentrations, typically 0.01 to 0.02 ppm iodine (0.07 – 0.18 mM).2,3

Organic forms of iodine are any in which the iodine atom is covalently attached to a carbon atom, such as methyl iodide, CH3I. The concentrations of the organic forms (of which there are many different molecules) are only now becoming recognized by oceanographers. In some coastal areas, organic forms can comprise up to 40% of the total iodine, and many previous reports of organoiodine compounds being negligible may be incorrect.4 Later in this article, some of the organoiodine species found in the ocean are discussed in the context of those organisms that produce them.

All of these various forms can be interconverted in the oceans. Phytoplankton, for example, take up iodate and convert it into iodide, which is mostly, but not completely, released.5 One research group has suggested that iodate, looking chemically like nitrate, is taken up by the same pathways, and is internally converted to iodide before being released.6 This process is fast enough that in one local studied, the phytoplankton can convert all of the iodate present to iodide in a month. Iodate is also converted to iodide by bacteria in low oxygen environments of the oceans.7

Marine algae can also take up iodide directly, and apparently do so preferentially over iodate.8-10 This process may, in fact, be a primary way that iodide is depleted from aquaria, but that’s getting ahead of things.