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EH40 and the conversion of PPM to Mg/m3 for Hydrocarbons
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Posted By Dave Morris
I have recently measured an airborne contaminant involving hydrocarbons, specifically Alkanes. My measurement is in PPM but EH40 (page 52) suggests a guidance value of 1200mg/m3 for alkanes. My problem therefore is the need to convert mg/m3 to ppm. For my online conversion calculator I require the molecular weight of Alkanes, for this I require the chemical formula for Alkanes so I can add the atomic weights of all atoms present in the chemical formula. The manufacturer has not specified this chemical formula on the safety data sheet. Does anyone know of another way (other than sampling in mg/m3) that I can find out these molecular weights?
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Posted By Paul Leadbetter
Dave
You do need to know the molecular weight but I guess you are dealing with a mixture; does the CAS number, or the boiling point range, enable you to make an educated guess at the main constituents?
Paul
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Posted By Adrian Watson
Dave,
Unfortunately, you cannot convert between the two unless you have either a full and complete analysis of the material sampled by GCMS or know the average molecular weight. The Molecular Weights can be obtained from a chemistry database or textbook.
Regards Adrian
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Posted By Robert K Lewis
If you consider the alkane series they have the same general formula:
C(X)H(2X+2)where X is the number of carbon atoms
Below 4 the alkanes are generally gases eg Methane (CH4), and above 4 they are liquid eg Pentane,hexane, heptane, octane. Above 16 they become solids.
The most likely to be used are 5, 6 and 7 carbon atoms if this is vaporised liquid so you could work on a molecular weight of 86 as a first approximation.
If this is from a gaseous source at room temperature then use a figure of 44 as a first approximation
Bob
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Posted By Dave Morris
Paul,
the CAS no is 90622-57-4 with a 50-100% concentration. The boiling point is 173C. How does this help me?
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Posted By Jane Blunt
That CAS no. appears to belong to a mixture C9 to C12, which if my arithmetic is correct gives a molecular weight in the range 128 to 170.
Jane
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Posted By Paul Leadbetter
Dave
From the CAS number, your solvent mixture is otherwise known as Isopar G which, if memory serves me correctly, is a mixture of C10 - C11 alkanes and cycloalkanes. The boiling point of n-decane is 174ºC which fits closely with the bp you quote. The methylnonanes boil at 165 - 168, the dimethyloctanes at 155 - 161 and undecane at 196. In the absence of compositional data (although you may be able to find that on the internet), you can get a rough conversion by assuming that the solvent is n-decane.
Paul
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Posted By Robert K Lewis
Shows the importance of getting all available information
Nonane, decane and dodecane are a little esoteric though for me. I think I will stick with teh aromatics as I am more at home in their presence.
Dave
What process is this that you are emitting measurable vapours from relatively non-volatile materials?
Bob
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Posted By Dave Morris
Bob
It's a close loop solvent degreasing process where the hydrocarbon is heated in an ultrasonic bath. I believe the chiller unit is malfunctioning and I wanted to ensure we didn't exceed any WEL's.
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Posted By Adrian Watson
You have also added another complication. The formula in EH40 is for 20 C, so will need correcting.
Regards Adrian
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EH40 and the conversion of PPM to Mg/m3 for Hydrocarbons
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