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(1) Concentration of the solution: Solution concentration can be divided into three categories: mass concentration (such as mass percentage concentration), volume concentration (such as molar concentration, equivalent concentration), and other forms of volume concentration. 1. Mass percent concentration: The concentration of a solution is expressed as the percentage of the mass of the solute relative to the total mass of the solution, and it is denoted by the symbol %. For example, 25% glucose injection means that 100 milliliters of the injection contains 25 grams of glucose. Mass percent concentration (%) = Mass of solute / Mass of solution × 100%. 2. Volume concentration: (1) Molar concentration – The concentration of a solution is expressed as the number of moles of solute per liter of solution; this is denoted by the symbol mol. For example, if 1 liter of concentrated sulfuric acid contains 18.4 moles of sulfuric acid, then its concentration is 18.4 mol. Molar concentration (mol) = number of moles of solute / volume of solution (liters). (2) Equivalent concentration (N): The concentration of a solution is expressed as the number of equivalent grams of solute per liter of solution, and this is denoted by the symbol N. For example, 1 liter of concentrated hydrochloric acid contains 12.0 equivalents of HCl, so its concentration is 12.0 N. Equivalent concentration = number of gram-equivalents of solute / volume of solution (in liters). 3. Mass-volume concentration: The concentration expressed as the mass of solute per unit volume (1 cubic meter or 1 liter) of solution is called mass-volume concentration, and it is denoted by the symbols g/m3 or mg/L. For example, if 1 liter of chromium-containing wastewater contains 2 milligrams of hexavalent chromium, then the concentration of hexavalent chromium is 2 milligrams per liter (mg/L). Mass-volume concentration = mass of solute (in grams or milligrams) / volume of solution (in cubic meters or liters). 4. Formulas for converting concentration units: 1) Equivalency concentration = 1000 × mass percent concentration / E; 2) Mass percent concentration = equivalency concentration / 1000 × E; 3) Molar concentration = 1000 × mass percent concentration / M; 4) Mass percent concentration = mass-volume concentration (mg/L) / 1000; 5) Mass-volume concentration (mg/L) = 1000 × mass percent concentration. 5. ppm represents a percentage by weight; ppm = mg/kg = mg/L. That is, 1 ppm = 1000 ug/L, and 1 ppb = 1 ug/L = 0.001 mg. Here, E represents the gram-equivalent of the solute ； d—Specific gravity of the solution ； M—molar mass of the solute ； (II) Gas concentration: For pollutants in the atmosphere, their concentration there is commonly expressed in terms of volume concentration and mass-volume concentration. 1. Volume concentration: Volume concentration is expressed as the volume of pollutants per cubic meter of air, in cubic centimeters or ml/m3. The commonly used unit for expressing this is ppm, where 1 ppm = 1 cubic centimeter per cubic meter = 10-6. In addition to ppm, there are also ppb and ppt. The relationships between them are as follows: 1 ppm = 10-6, which means one part in a million; 1 ppb = 10-9, which means one part in a billion; 1 ppt = 10-12, which means one part in a trillion. Additionally, 1 ppm = 10^3 ppb = 10^6 ppt. 2. Mass-volume concentration: This type of concentration is expressed as the mass of pollutants per cubic meter of air, and its units are milligrams per cubic meter or grams per cubic meter. Its conversion relationship to ppm is as follows: X = M.C/22.4; C = 22.4X/M. Here, X represents the concentration of the pollutant in milligrams per cubic meter ； C—Concentration value of pollutants expressed in ppm ； M—the molecular weight of the pollutant. From the above formula, the following relationship can be obtained: 1 ppm = M/22.4 (mg/m3) = 1000·M/22.4 ug/m3. Example 1: Determine the ppm concentration of hydrogen fluoride at 30 mg per standard cubic meter under standard conditions. Solution: The molecular weight of hydrogen fluoride is 20; therefore, C = 30.22 × 4/20 = 33.6 ppm. Example 2: Given that the concentration of sulfur dioxide in the atmosphere is 5 ppm, determine its concentration in mg/Nm3. Solution: The molecular weight of sulfur dioxide is 64. X = 5.64/22.4 mg/m3 = 14.3 mg/m3. 3. Conversions between ppm, ppb, and mass concentration in soil, animals, plants, and solid waste: 1 ppm = 1 mg/kg = 1000 ug/kg; 1 ppb = 1 ug/kg = 10-3 mg/kg; 1 mg/kg = 1 ppm = 1000 ug/kg; 1 ug/kg = 1 ppb = 10-3 ppm