Concentration Units in Organics Article

[skearse]

Hi,

I was a bit confused about using the units µm to express concentration in your article in the October Reefkeeping (Organic Compounds in the Reef Aquarium). I am used to seeing µm refer to a spatial dimension, such as a 1-µm filter, etc. Did I miss something? Should these be µg/L? If so, in the Conc. of DOM in Seawater section, it seems that the conversion from µm (µg/L) to ppm is off by a factor of 10 [600µg/L would be equal to 600 ppb in freshwater or ~700 ppb in salt, or 0.7 ppm]?

[Randy Holmes-Farley]

In this context:

"materials that can pass through 0.2 - 1.0 um filters"


the um stands for micron, or micrometer (one millionth of a meter).


The concentration units should have read uM with a capital M, meaning one micromolar. I'll have to get that corrected.

One micromoilar is one millionth of a mole per liter. A mole is the amount of material that weighs the molecular weight of the material in question.

For example, carbon has a molecular weight of 12 grams per mole

1 M (molar) carbon = 1 mole/l= 12 g/L

So 1 uM = one micromolar = one millionth of a mole/L = 12 millionths of a g/L = 12 ug/l = 12 ppb

Hope that helps.

[Skipper]

Thanks for bringing this to our attention, Stacy. I'll get it fixed soon.

[Randy Holmes-Farley]

Thanks, Skipper. :)

[skearse]

No problem. More questions!

1) (Confirmattion) So the concentration is then expressed as '[DOM] as µmol/L C'?

2) Any idea how the concentrations in natural seawater compare to those in a reef system (I know that your conclusion states that the reef tank is still a mystery, but is there a starting point, yet?)

3) Would tests such as TOC or TKN (Total Kjeldahl N2) be better indicators of the state of a reef aquarium?

3) (More philospohical, possibly, than factual) I have used several organic 'supplements' in my tank (DNA ReefVital, for example), and have often wondered about the overall effect that this has on the ststem. Does the addition of such supplements have enough of a positive effect on the tank inhabitants to overcome the potential negatives (more rapid depletion of carbon media, additional skimmer loading, bio-availability of nutrients for nuisance algae, etc.)

Again, thanks in advance for your work and the article!

[Randy Holmes-Farley]

Any idea how the concentrations in natural seawater compare to those in a reef system (I know that your conclusion states that the reef tank is still a mystery, but is there a starting point, yet?)

I do not know of any good measurements of organics in reef aquaria.

Would tests such as TOC or TKN (Total Kjeldahl N2) be better indicators of the state of a reef aquarium?

Better beteween those two? I'm not sure. TOC, TKN, TOP, and others would all be interesting. I'd especially like to see how these varry relative to one another in aquaria with various C, N, and P export methods.

(More philospohical, possibly, than factual) I have used several organic 'supplements' in my tank (DNA ReefVital, for example), and have often wondered about the overall effect that this has on the ststem. Does the addition of such supplements have enough of a positive effect on the tank inhabitants to overcome the potential negatives (more rapid depletion of carbon media, additional skimmer loading, bio-availability of nutrients for nuisance algae, etc.)

I wouldn't add ones that I do not know what they are. I think the jury is out on whether additions of ethanol and certain other organics is beneficial or not. There are a few active threads on these sorts of additions in the chemistry forum right now.

1) (Confirmattion) So the concentration is then expressed as '[DOM] as µmol/L C'?

DOM can be expressed as DOC, DON, DOP, etc. Often that is in umole/L, but sometimes as ppm.

[skearse]

Would tests such as TOC or TKN (Total Kjeldahl N2) be better indicators of the state of a reef aquarium?

Better beteween those two? I'm not sure. TOC, TKN, TOP, and others would all be interesting. I'd especially like to see how these varry relative to one another in aquaria with various C, N, and P export methods.

Sorry, I meant using the TOP/TOC/TKN testing as opposed to the (inorganic) tests in use by aquarists.

1) (Confirmattion) So the concentration is then expressed as '[DOM] as µmol/L C'?

DOM can be expressed as DOC, DON, DOP, etc. Often that is in umole/L, but sometimes as ppm.

But the total/dissolved organic matter is reported in terms of molar carbon as opposed to moles of organics (or ppm, ppb, ppt)(like 1 mol BuOH would be 4 mol C).

Thanks again!

[Randy Holmes-Farley]

Sorry, I meant using the TOP/TOC/TKN testing as opposed to the (inorganic) tests in use by aquarists.

I'd be very interested in seeing them. However, without knowing how they change under different scenarios, I'd be hard pressed to say they'd be better.

But the total/dissolved organic matter is reported in terms of molar carbon as opposed to moles of organics (or ppm, ppb, ppt)(like 1 mol BuOH would be 4 mol C).

Yes, total organic matter has to be reported in terms of moles or weight of a particular atom (C, N, P, etc). One cannot measure the concentration of every individual organic compound, so one cannot give a total organic material concentration in moles/L (which is the sum of the concentrations of every organic compound). The reason is that organics vary widely in molecular weight, from about 16 grams per mole to more than a million grams per mole. So one can measure the amount of total carbon (in moles/L or ppb), but can't convert that into a moles of organic molecules unit of measure.

[skearse]

Thanks.

[Randy Holmes-Farley]

You're welcome.

Happy Reefing. :)