Thread: Converting ppm to percentages, and to molarity

If the atmosphere is roughly 20% oxygen, how would one convert this number to parts per million (is it 200000 ppm?) Likewise, does it make sense to talk about the molarity of oxygen in the atmosphere.

Thank you for any help with these basic conversions.

Yes, 20% is 200 000ppm. Think of it as converting parts per hundred into parts per million (adding four zeroes). You ought to be able to talk about the molarity of a gas, since it's a measure of moles per liter of solution. Don't know if people actually do that, though

since it's a measure of moles per liter of solution. Don't know if people actually do that, though

They might use it when it is concidered that gas at higher pressure would have more moles per litre (elevation).

CHeers -

I wonder if there is another answer. I am currently reading a set of papers, and some use a headspace methane concentration of 500 ppm , whereas others state their headspace methane concentration to be 40%. These papers have comparable results and it seems likely that that the headspace methane concentrations must be somewhat more similar than a simple conversion of ppm to percentage, as this would yield 0.5% methane in the former set of papers (intuitively, not enough to see the reported effects when 40% is required elsewhere.)

Originally Posted by free radical

CHeers -

I wonder if there is another answer. I am currently reading a set of papers, and some use a headspace methane concentration of 500 ppm , whereas others state their headspace methane concentration to be 40%. These papers have comparable results and it seems likely that that the headspace methane concentrations must be somewhat more similar than a simple conversion of ppm to percentage, as this would yield 0.5% methane in the former set of papers (intuitively, not enough to see the reported effects when 40% is required elsewhere.)

Maybe they mean 40% saturation that would equate to 0.5% of total gas volume?

Originally Posted by KALSTER

Maybe they mean 40% saturation that would equate to 0.5% of total gas volume?

I don't know this conversion - Can you walk me through it?

Originally Posted by free radical

Originally Posted by KALSTER

Maybe they mean 40% saturation that would equate to 0.5% of total gas volume?

I don't know this conversion - Can you walk me through it?

I'd love to, but unfortunately I have no idea! I am thinking that atmospheric air would be able to hold a certain percentage of methane in the same way it can hold a certain percentage of water vapour. This percentage would be subject to temperature, elevation maybe and possibly solar radiation. So with quick maths, and assuming your two sources are talking about the same amount of methane: if 500ppm is 0.5% of total gas volume and 40% saturation, it would come out to maximum saturation equating to 1250ppm or 1.25% total volume. This probably all just nonsense though. :wink:

Also, possibly worth double-checking that they haven't made a mistake - saying ppm when they mean parts per thousand (which would make your two concentrations nearly equal), or, instead of 40% whether they meant something like 40 'proof' (as Kalster suggested - a measure of saturation) or 40 ^o/_oo (40 per mille).

I know, I know - granny knows how to suck eggs and stuff but sometimes you just need to check that the computer is plugged in! :P

In engineering ppm can refer to volume or weight. It is usual to use the terms ppmv or ppmw to distinguish the two uses, and while ppmv is directly convertible to mole fraction, ppmw is not so the molecular weight of the various species has to be factored in to get to mole fraction. If air is 20% oxygen by volume this would be 20000 ppmv. The ppmw would be slightly different. Where the constituent gases have widely different molecular weights, like CO2 and methane, the difference between ppmv and ppmw can be quite large.

Although this couldn't account for 500 ppm vs. 40%.

Upon further review of the literature it appears that some of the discrepancy is down to experimental design. In soils, methane-consuming bacteria can subsist on atmospheric methane (1.7 ppm). In pure culture, they evidently require ridiculously high levels (40% or perhaps 400000 ppm).

So, wildly different levels of methane might be used to study the bugs, depending on growth conditions (in situ v in pure culture.). But I'm not convinced yet.

FR

Whoa, that's pretty interesting. Do they give an explanation why the little buggers need such high concentration of methane in culture?

Compounds in the soil, such as methylated amines, may up-regulate the process, although the mechansim hasn't been worked out. The discrepancy is so large that I remain sceptical that this is the entire basis of the difference between protocols. If it were, it seems that the relevant cofactor in soils would have been identified long ago in order to grow the organisms more efficiently in culture.

You can download the article by surfing from the link:

http://aem.asm.org/cgi/reprint/64/3/1091