LCS Laboratory Inc. can measure both parameters: vapour density, and the density of the air saturated with the vapour of a solvent. The most common question our clients ask us about this test is: “What is the difference?”In Health and Safety, vapour density is commonly used to demonstrate the property of flammable vapours to accumulate along the floor and in a confined space. It seems obvious that the heavier the vapour is, the more likely it accumulates near the floor. So what is vapour density?
Vapour Density is a theoretical value which shows whether the vapour is heavier than air. It was originally used for gases. If a gas follows the Ideal Gas Law, its density is proportional to its molecular mass. Therefore, if you need to compare the densities of two gases, the simplest way to do so is to measure and compare their molecular masses. It is a common practice to compare vapour density vs. air density by simply dividing the molecular mass of the gas (or vapour) by the “molecular mass” of air (estimated at 29). This allows us to calculate the Vapor Density relative to Air. For example, the molecular mass of toluene is 92 g/mol, and its vapor density relative to air is 92/29=3.2. In other words, toluene vapour is approximately 3 times heavier than air.
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If your material is not a pure compound, you need to know the effective molecular weight, and then calculate the vapour density specific to your product.
The problem, however, is that the vapour density value is misleading because it doesn’t take into account the concentration of saturated vapour in the air.
Saturated air density is the density of the air saturated with the vapour of a solvent. We saturate the air with the solvent vapour and measure its density relative to that of dry clean air.
Vapour Density and Saturated Air Density look very similar to each other, but the difference becomes very significant for semi-volatile compounds. For example:
– Vapour Density of mercury is 6.9, which should mean that the mercury vapour is much heavier than air and toxic vapours should accumulate along the floor.
– Density of air saturated with mercury vapor is 1.00 because its concentration is low. Therefore, there will be no excessive mercury vapour accumulation along the floor.
If you need to test your product for vapor density or perform any other tests, please do not hesitate to email us. To learn more about other tests from LCS Laboratory for SDS preparation please visit our SDS testing page.
