Unit conversion problems
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Dear experts, regarding the conversion between mg/l and mg/m3: taking CH3OH in low-carbon CO2 exhaust as an example, 25 mg/m3 is approximately equal to 17 ppm； But some sources say that 1 ppm = mg/l – what’s going on?
This post was last edited by ylb913 on 2016-11-29 at 12:15. mg/l = ppm refers to aqueous solutions; the density of such solutions is 1 g/cm3, and 1 liter equals 1 kg. mg/kg is mass ppm.
PPM stands for one part per million, and it is a unit of mass, while mg/L is a unit of concentration; the two are not the same concept. In the case of an aqueous solution with a density of 1, the numerical values of the two are identical, but their meanings are different
The original poster has incorrect basic concepts. . . PPM stands for one part per million; it is a dimensionless unit. For example, if there is 1 gram of salt in 1 ton of water, then the salt concentration can be expressed as 1 g/1 t = 10^-6 = 1 ppm. Similarly, if there is 1 ml of SO2 in 1 cubic meter of air, the SO2 concentration can be said to be 1 ppm. As long as the units of the numerator and denominator are the same, it’s fine – they can all be units of mass, volume, or even moles. But ppm simply denotes one part per million. . Whether it is mg/l or mg/m3, both are units of mass concentration (that is, density), which is mass per volume; they cannot be expressed in ppm!
This post was last edited by ylb913 on 2016-12-2 07:18. I have quite a lot of experience with the use of ppm. 1. First, regarding the use of mg/l as a unit equivalent to ppm – this is something that exists, for example, in terms of the sulfur content in sulfur-containing wastewater. As mentioned earlier, the density of the liquid must be close to 1000 kg/m3, or 1 kg/l; this usually refers to aqueous solutions. MG divided by KG equals one millionth, which is mass in ppm. 2. Using the original question’s example: “Taking CH3OH in CO2 exhaust as an example: 25mg/m3 ≈ 17ppm”. To calculate mass ppm, ignore the 25mg of methanol; the molecular weight of CO2 is 44, so the amount of CO2 per m3 is 1*44/22.4 = 1.96 kg/m3. Therefore, 25/1.96 = 12.8 ppm (mass ppm). To calculate volume ppm, since CO2 is already measured in the volume unit of m3, it can be ignored; the molecular weight of methanol is 32, so 25mg corresponds to 25/32*22.4 = 17.5 milliliters, which is 17.5 ppm (volume ppm). Conclusion: 1. For liquids, mass ppm is a more commonly used unit. 2. For gases, mass ppm or volume ppm are both very common. 3. Typically, ppm means one part per million; it has no unit of its own. Whether it refers to one part per million by mass or one part per million by volume depends on common sense or industry conventions. 4. Finally, regarding the ppm of gases, there are cases where mg/m3 is actually treated as ppm as well. On the one hand, regarding density, the weight unit corresponding to cubic meters is kilograms, and there is a relationship of one millionth between milligrams and kilograms. On the other hand, there is the issue of gas composition; for example, total sulfur refers to the weight of sulfur elements. Since sulfur can exist in various molecular forms, it is difficult to convert this weight into the volume of the corresponding sulfur-containing gases, and thus it is not possible to determine the ppm value based on volume ； Also, the gas itself can be multi-component, and its composition is constantly changing. The unit for total sulfur is mg, but it’s difficult to convert the volume of the gas (in m3) into kilograms. Firstly, it’s inconvenient to carry out such calculations every time; secondly, even if an average molecular weight is used, this value also changes over time. So when should it be adjusted? This situation is also quite troublesome (this is because we have been using the density data of one fixed gas for many years, and it has even led to jokes in our laboratory). It’s better to use the most basic value, which is mg/m3. ——Calling mg/m3 ppm is certainly not precise, but it is possible that some companies or industries have such a convention; for example, our unit uses it, and it is also used in the entire liquefied gas desulfurization industry.
It seems to be the case that first, the mass of methanol needs to be calculated: n = V/Vm = m/M. Using this formula, the mass of methanol can be found by multiplying V/Vm by M; in other words, 1 cubic meter divided by 22.4 L/mol multiplied by 32 g/mol. The resulting mass of methanol is 1428.87 g. Dividing 25mg by 1428.87g again gives a result of approximately 17ppm. I’m not sure if it’s correct. The statement in the material that 1 ppm = mg/L applies when the density of the solution is 1 g/cubic centimeter, right?
Ah, I’ve suffered a huge loss this time – what can Haichuan coins be used for? It seems you have a lot of coins:D