How to make gaseous ppm solution?

Dear Bhargav,

Basically ppm refers to molar fraction of one gas in mixture of gases. So you should count the number of molecules. Assuming the ideal gas law – when the pressure and temperature are constant, number of molecules is proportional to the volume, but assuming all is in gaseous state!

You didn’t specify the temperature of mixing but I assume it is the room temperature.

In that temperature toluene is a liquid so, if you take 5.7microliters than you have 54 micromole of toluene ( in 20deg Celsius it has a density of 0.87g/ml and molar mas of 94.115g/mol what gives 9.44 micromole/microliter).

Nitrogen is a gas and in normal condition its quite ideal and 0.5l makes 22.3 mmol = 22300 micromoles (one mole takes the 22.4dm3 volume).

If you mix these amount it will give you 2455ppm of toluene in nitrogen.

To check if toluene ha reached the equilibrium pressure lets look at its vapour pressure in 20deg C. The vapour pressure of toluene in 20deg C is 2.8kPa , for nitrogen it is 101kPa than equilibrated mixture will be close to 2.7% (if enough toluene to vaporise to this pressure volume and temp of the gases are equal), so definitely 2500ppm is below this value. If you provide 5.7 microliters to 500ml volume of tedlar bag – all of it will vaporise giving you 2455ppm gas mixture.

To get 10ppm use 0.024 microliters of liquid toluene and put it to 500ml (0.5l) of nitrogen. But due to surface adsorption effects it is better to keep it hot it or at least warm.

I’m not sure if you should add the correction of Van der Waals for molecular volume in case of toluene – check it.

Best regards,

Jarek

Thank you so much, Dr. Jaroslaw M Necki

Jaroslaw M Necki very helpful, however, how do you control the pressure inside the Tedlar bag?

You do not. You count the volume. You can use syringes or flow controller.

Then we can't actually know the number of moles of N2 inside the Teldar bag, can we? So we can't calculate the concentration in ppm...

, yes we can. In Tedlar bag you will have exactly what you will fill in. If you fill small amount of liquid or gas and than fill a lot of nitrogen than you exactly know what is the mole fraction inside.

Now, important part. You will use large syringe (like 500ml SGE), you will just calculate the amount of gas in reference to atmospheric pressure and lab temperature. If You want to be very precise, use precise measurement pressure gauge and try to keep everything in thermal equilibrium. Hope it is clear now.

Alternatively, you can use precise mass flow controller (MFC) to fill the bag with requested amount of nitrogen, helium or any other gas like zero air. MFC are offered widely. Be careful with any dead or unknown volumes behind MFC !