Reef Chemistry Question of the Day #154 Ion Abundance

Randy Holmes-Farley

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Reef Chemistry Question of the Day [HASHTAG]#154[/HASHTAG]

Without looking it up, what is the fifth most abundant ion (by weight) in natural seawater?

A. Potassium
B. Sulfate
C. Magnesium
D. Calcium
E. Bicarbonate

Good luck!























.
 

jason2459

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I was going to say D but then noticed you said by weight and I think that's by percentage.
 
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Randy Holmes-Farley

Randy Holmes-Farley

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FWIW, ppm is a weight based measure (parts per million).

The alternative is a molar measure (numbers of ions) such as 55 millimolar (50 mM).
 

jason2459

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<=== :D It's fifth by molarity, too.


lol, I kept looking at your post wondering if you mistyped something and got a smiley face. But I've been viewing from my phone/tapatalk all day and your avatar doesn't quite line up. Now I'm seeing it on a browser and it clicked.
 

Cory

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Im guessing potassium becaise of chloride, sulphate, sodium then magnesium then potassium or calcium.
 
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Randy Holmes-Farley

Randy Holmes-Farley

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And the answer is...D. Calcium

By weight in seawater, the rank order of the ions present is:

chloride 19,400 ppm
sodium 10,500 ppm
sulfate 2,700 ppm
magnesium 1280 ppm
calcium 420 ppm
potassium 398 ppm

Happy Reefing!
 

JimWelsh

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By weight in seawater, the rank order of the ions present is:

chloride 19,400 ppm
sodium 10,500 ppm
sulfate 2,700 ppm
magnesium 1280 ppm
calcium 420 ppm
potassium 398 ppm

You seem to be using "ppm" to mean "mg/kg" in some cases, and "mg/L" in other cases. For example, Pilson says that in NSW at S=35.000%, there are 1284 mg/kg of Mg++, and 411.9 mg/kg of Ca++, but when converted to mg/L at 25C, then these become 1314 and 421.5 respectively. So, the magnesium number you cite appears to be mg/kg, but the Ca number looks more like it's mg/L. (This whole w/w vs. w/v issue is a pet peeve.)
 
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Randy Holmes-Farley

Randy Holmes-Farley

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Yes, I was sloppy. I meant mg/L for all of them. :)

These molar numbers are from Millero. Others may have slightly different values.

0.546 M chloride = 19.355 mg/L
0.468 M = 10,760 mg/L
28.2 mM sulfate = 2710 mg/L
53.2 mM magnesium = 1290 mg/L
10.3 mM calcium = 413 mg/L
10.2 mM potassium = 399 mg/L
 

JimWelsh

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I respectfully submit that those numbers still look a whole lot like the correct numbers for 1 kg of seawater, and not 1 liter, assuming a salinity of 35. I cite as my references Table 2.5 on page 67 of Millero, Frank J. Chemical Oceanography, 1996, and also Table 4.1 on page 59 of Pilson, Michael EQ, An Introduction to the Chemistry of the Sea, 1998 (which I can link to here: http://www.ocean.washington.edu/courses/oc400/Lecture_Notes/CHPT4.pdf).
 
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Randy Holmes-Farley

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I took the molar (moles/L and mmoles/L) numbers from "Chemical Oceanography" Second Edition by Millero, Table 3.1, page 98. He says they are from Bruland (Chemical Oceanography, Vol 8, 1983).

I do not guarantee them beyond that. :D

I converted them myself to mg/L by multiplying by the molecular weight of each ion :)
 
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Randy Holmes-Farley

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Randy Holmes-Farley

Randy Holmes-Farley

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So if we correct for Millero's transcription error in the units, the numbers from Millero/Bruland do not change, but we are back to ppm:

chloride = 19.355 mg/kg = ppm
sodium = 10,760 ppm
sulfate = 2710 ppm
magnesium = 1290 ppm
calcium = 413 ppm
potassium = 399 ppm
 

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