I. Essential Difference Between ppm and mg/L
In industrial instrumentation and laboratory work, ppm and mg/L are two common ways of expressing concentration. For dilute aqueous solutions, 1 ppm ≈ 1 mg/L, and the values can be used interchangeably; however, strictly speaking, their physical meanings differ.
ppm is a dimensionless ratio—1 ppm means the solute mass is one millionth of the total solution mass, and its unit is essentially mg/kg. mg/L is mass concentration—how many milligrams of solute are contained in 1 liter of solution.
The reason they are equated in dilute aqueous solutions is that the density of water is about 1 kg/L, so 1 liter of water is approximately equal to 1 kilogram. When the solution density is 1, mg/kg ≈ mg/L. In low-concentration aqueous solutions ranging from tens to thousands of ppm, the two can be regarded as equivalent.
II. Four Common Concentrations in Laboratories
There are four commonly used concentration expression methods in laboratories, each suitable for different scenarios and reagent types.
1. Mass Fraction
This is the concentration commonly seen on reagent labels, such as 95% industrial alcohol and 25% ammonia water; these all refer to mass fraction. Mass fraction is the percentage of solute mass in the total solution mass.
- Commonly expressed as percentage content: W = m(analyte) ÷ m(sample) × 100%
- Unit: % (percentage), dimensionless
- Formula: solute mass / total solution mass × 100%
Common reagents (standard in laboratories):
| Reagent | Mass Fraction |
|---|---|
| Concentrated hydrochloric acid | 37% |
| Concentrated sulfuric acid | 98% |
| Concentrated nitric acid | 65%~68% |
| Ammonia water | 25%~28% |
| Sodium hydroxide solid solution | Commonly 10%, 20% |
2. Mass Concentration
This is the concentration commonly used in wastewater testing. Mass concentration refers to the mass of solute contained in a unit volume of solution. For example, COD, ammonia nitrogen, and total phosphorus testing in wastewater commonly use mg/L, and the unit for preparing flocculant stock solutions is g/L.
- Common conversion: 1g/L = 1000mg/L
- Wastewater example: Take 0.5 g of solid reagent, dilute to volume in a 500 ml volumetric flask, ρ = 0.5g ÷ 0.5L = 1g/L = 1000mg/L
3. Amount-of-Substance Concentration
Commonly used in titration analysis, standard solutions, and physicochemical testing. Amount-of-substance concentration refers to the amount of solute contained in a unit volume of solution, with the unit mol/L. 1 mol/L means 1 liter of solution contains 1 mole of solute.
- Unit: mol/L (mole/liter)
- Formula: c = n/V
4. ppm, ppb
These are proportional units, not concentrations in the strict sense. ppm is "parts per million" and ppb is "parts per billion." They are not concentration units themselves but proportional relationships. For dilute aqueous solutions, 1 ppm ≈ 1 mg/L, and the values can be used interchangeably.
III. Simple Mnemonic for Differentiation
To quickly select the appropriate concentration unit, refer to the following mnemonic:
- Preparing standard solutions, titration → use mol/L
- Concentrated acids and bases, solid reagents → use mass fraction %
- Alcohol, mutually soluble liquids → use volume fraction %
- Wastewater, water samples, trace ions → use mg/L (ppm)
- Temporary dilute acids and bases → use volume ratio 1+1, 1+4