Does ppm can express in volume, pressure and concentration?
I always see ppm in gas is expressed in 1 volume to 10^6 volume, if it can also be expressed in volume, this can make me very annoyed that I don’t know how to express the ppm in which term.
The Correct Answer and Explanation is:
Does ppm express volume, pressure, and concentration?
Answer:
PPM is a dimensionless ratio that can express concentration in terms of volume or mass but is generally not used to express pressure directly. It depends on the context and what is being measured.
Detailed Explanation:
1. What is ppm?
PPM stands for parts per million, which means “one part of a substance in one million parts of the total mixture.” It is a way to express very dilute concentrations. Importantly, ppm is a ratio, so it is dimensionless — it compares one quantity to another.
2. PPM in Gas Mixtures:
When dealing with gases, ppm is commonly expressed as:
- Volume/Volume (v/v) — For example, 1 ppm means 1 volume unit of gas per 1,000,000 volume units of the gas mixture. This is very common for measuring trace gases in air (e.g., CO, CO₂, or pollutants).
- This is valid because gases behave ideally under many conditions, and volume ratios correspond closely to mole ratios due to Avogadro’s law (equal volumes of gases at the same temperature and pressure contain equal numbers of molecules).
So, in gases, ppm typically expresses concentration in terms of volume fraction.
3. PPM in Liquids or Solutions:
- For liquids or solids dissolved in liquids, ppm usually means mass of solute per mass of solution (mg per kg), or sometimes mg per liter (assuming water density ~1 kg/L). So it expresses concentration in mass/mass or mass/volume terms.
4. Can ppm express pressure?
- No, ppm is not commonly used to express pressure because pressure is an absolute physical quantity with units (e.g., atm, Pa, mmHg).
- However, sometimes partial pressure of a trace gas can be converted into ppm if you divide the partial pressure by the total pressure and multiply by 10⁶. This gives ppm as a fraction of total pressure, which is conceptually the same as volume ppm because of the ideal gas law (partial pressure ratios = mole fraction = volume fraction).
But this is an indirect use. ppm itself is not a unit of pressure.
Why is this important?
Because ppm can represent different physical quantities (volume fraction, mass fraction), it’s critical to know the context:
- In gases: ppm usually = volume ratio (v/v).
- In liquids/solids: ppm usually = mass ratio (mg/kg or mg/L).
- In pressure: ppm not directly used, but partial pressures can be converted to ppm based on total pressure.
Summary:
- PPM is a dimensionless ratio expressing parts per million parts.
- For gases, ppm is usually volume/volume ratio (1 ppm = 1 part gas per 10⁶ parts total volume).
- For liquids, ppm often means mass per mass or mass per volume.
- PPM is not a direct unit of pressure, but can relate indirectly via partial pressures.
- Always clarify the basis of ppm measurement in your context to avoid confusion.
