Express ppm and ppb on weight/weight, weight/volume, and volume/volume bases.
The correct answer and explanation is:
Correct Answer:
- ppm (parts per million) and ppb (parts per billion) can be expressed in different ways depending on the basis:
1. Weight/Weight (w/w):
- ppm = mg of solute / kg of solution
Example: 1 ppm = 1 mg solute / 1 kg solution - ppb = µg of solute / kg of solution
Example: 1 ppb = 1 µg solute / 1 kg solution
2. Weight/Volume (w/v):
- ppm = mg of solute / liter of solution
Example: 1 ppm = 1 mg/L - ppb = µg of solute / liter of solution
Example: 1 ppb = 1 µg/L
3. Volume/Volume (v/v):
- ppm = µL of solute / liter of solution
Example: 1 ppm = 1 µL/L - ppb = nL of solute / liter of solution
Example: 1 ppb = 1 nL/L
Explanation (300 words):
Parts per million (ppm) and parts per billion (ppb) are units of concentration commonly used in environmental science, chemistry, and biology. These units describe very small quantities of a substance present in a mixture. The interpretation depends on the nature of the system—solid, liquid, or gas—and the desired basis: weight/weight, weight/volume, or volume/volume.
On a weight/weight (w/w) basis, ppm represents milligrams of solute per kilogram of total mixture, which is convenient when both the solute and solvent are solids or when dealing with soils and powders. For ppb, it means micrograms per kilogram.
On a weight/volume (w/v) basis, which is commonly used in aqueous solutions, ppm corresponds to milligrams of solute per liter of solution. Since 1 liter of water weighs approximately 1 kilogram, this often matches the w/w ppm for dilute solutions. ppb on a w/v basis would be micrograms per liter, typically used when measuring trace pollutants in water or blood.
On a volume/volume (v/v) basis, used mostly for gases and some liquid mixtures, ppm equals microliters of solute per liter of solution. For example, 1 ppm of ethanol in air means 1 µL of ethanol vapor per liter of air. Similarly, ppb would be nanoliters per liter.
Understanding the basis of ppm or ppb is critical for correct interpretation and conversion between units, especially in environmental regulations, pharmacology, and chemical analysis.