http://www.endmemo.com/sconvert/ppmul_l.php

Hope this will help !

The straight forward way is to calculate the ratio with common units. Then extrapolate it to a million parts which is 10^6.

But a volatile compound like ethanol also adds to some complications. What temperature are you doing the sensing? Then you need to know the vapor pressure at that temperature and incorporate it accordingly (from ideal gas law) to know the volume. Then calculate the volumetric ratio as above. Hope this helps.

Thank you Rahul !

Sir Arun Kumar.Prasad . I didn't get it.. could you please explain with an example by taking random values.

Temp. Is around 60 C .

Thanku!

Okay. There are some guidelines and conventions. I am stating generally so as to not venture into dangerous waters (in the subject). Each point mentioned below can be dealt separately by experts in respective segment. This will help you make meaningful conclusions to do your sensing experiments without getting bogged down.

First, let us look at your own case 2 L and 40 microlitre. I am assuming 2 L of "air" at NTP (normal atmospheric pressure and temperature) since the ethanol has to be diluted "in" something to get its concentration. Otherwise, it is just 100% ethanol. Hope this is clear. If you put 10 ml of something into vacuum, you are not in principle diluting it. Hence its concentration is 100%. Sameway as acids are diluted (volume fraction). If you spill concentrated acid on your hand with you sitting inside a 1000 litre by volume chemistry lab, it is still going to burn you !

Second, in a simple case of gas dilution, 40 microlitre of gas (solute) in 2 L air (solvent) leads to 20 uL/L or 20 ppm. Hence in order to get 1 ppm, you need further dilution (you can play with either solute or solvent).

Third, but in case of volatile compounds like ethanol, which prefers to be liquid at normal conditions, it is tricky. Since you mentioned closed container, there will be some vapour above the ethanol whose pressure depends upon temperature. Ethanol boils around 78°C. Meaning, its vapor pressure becomes equal to atmospheric pressure at that temperature. Lower than this temperature, the vapor pressure is lower than atmospheric pressure. Meaning there is mixture of vapour and liquid phases. Or in other words, some amount of liquid remains in the closed container while the remaining amount is present in form of vapor. The vapor pressure chart (pressure vs temperature) of ethanol is easily available in literature.(Clasius Clayperon equation or Antoine equation in this case where it is only liquid-vapor equilibrium)

Fourth, since your operating temperature is 60°C, some amount of liquid will remain which your sensor will not see immediately (depending on sensing setup and geometry). What you need to measure is amount of vapour at that temperature (volume) which you can get from vapor pressure data. And use this volume in your calculations. Think the vapor pressure of ethanol is around 0.47 atm (from charts online!). This is for 1 mole of gas. 40 microlitre would correspond to 0.00145 moles (taking into account density and molecular mass). This would exert a pressure of 0.0006815 atm. (0.47 x 0.00145) (based on Raoult's Law)

Fifth, now you can also calculate the concentration of the gaseous phase in parts per million (ppm) directly from pressure instead of converting it to volume.

Sixth, Divide the vapor pressure of the gas by the total pressure of the gas and liquid combined (operating pressure inside your container). The result is known as the mole fraction of the gas and is a measure of gas concentration. Your value is 0.0006815 atm. Considering your operating pressure at 1 atm, your mole fraction in terms of pressure would be 0.006815 (fraction, no unit)

Seventh: Multiply by 10^6 to get ppm - 681.5 ppm.

As you can see, if you consider ethanol as non-volatile, you will get 20 ppm. But if you account for its volatile nature, you get a true value of 681.5 ppm (with assumptions). This is because, a vapour will occupy larger volume than a liquid. And higher the temperature, this volume increases.

Or if you can spray ethanol through atomizer, and know the exact volume of vapour, the calculations would be much simpler.

Researchers working with VOCs for sensing may be able to enlighten you further.

Thank you so much sir. It is really benefitial. I will look further for more details.