Concentration measures how much solute is dissolved in a solution. Mass concentration = mass of solute ÷ volume of solution. 100 mg salt in 1 L water = 100 mg/L concentration. Higher values = stronger solution.

• Concentration = mass/volume
• g/L = grams per liter (base)
• mg/L = milligrams per liter
• Higher number = more solute

Mass concentration: mass of solute per volume. Units: g/L, mg/L, µg/L. Direct and unambiguous. 1 g/L = 1000 mg/L = 1,000,000 µg/L. Used in water quality, clinical chemistry, environmental monitoring.

• g/L = grams per liter
• mg/L = milligrams per liter
• µg/L = micrograms per liter
• Direct measurement, no ambiguity

ppm (parts per million) ≈ mg/L for water. ppb (parts per billion) ≈ µg/L. Percent w/v: 10% = 100 g/L. Easy to understand but context-dependent. Common in water quality testing.

• 1 ppm ≈ 1 mg/L (water)
• 1 ppb ≈ 1 µg/L (water)
• 10% w/v = 100 g/L
• Context: aqueous solutions

Standard units: g/L, mg/L, µg/L, ng/L. Clear and unambiguous. Each prefix = ×1000 scale. Universal across chemistry, environmental science, clinical testing.

• g/L = base unit
• mg/L = milligrams per liter
• µg/L = micrograms per liter
• ng/L, pg/L for trace analysis

ppm, ppb, ppt commonly used. For dilute aqueous solutions: 1 ppm ≈ 1 mg/L, 1 ppb ≈ 1 µg/L. EPA uses mg/L and µg/L for standards. WHO uses ppm for simplicity.

• ppm = parts per million
• ppb = parts per billion
• Valid for dilute water solutions
• EPA standards in mg/L, µg/L

Expressed as CaCO₃ equivalent. Units: gpg (grains per gallon), °fH (French), °dH (German), °e (English). All convert to mg/L as CaCO₃. Standard for water treatment.

• gpg: US water hardness
• °fH: French degrees
• °dH: German degrees
• All as CaCO₃ equivalent

Concentration = mass/volume. C = m/V. Units: g/L = kg/m³. Conversion: multiply by 1000 for mg/L, by 1,000,000 for µg/L. ppm ≈ mg/L for water (density ≈ 1 kg/L).

• C = m/V (concentration)
• 1 g/L = 1000 mg/L
• 1 mg/L ≈ 1 ppm (water)
• %w/v: mass% = (g/100mL)

Dilution formula: C1V1 = C2V2. Initial concentration x volume = final concentration x volume. 10 mL of 100 mg/L diluted to 100 mL = 10 mg/L. Conservation of mass.

• C1V1 = C2V2 (dilution)
• Mass conserved in dilution
• Example: 10x100 = 1x1000
• Useful for lab preparation

Solubility = maximum concentration. Temperature dependent. NaCl: 360 g/L at 20°C. Sugar: 2000 g/L at 20°C. Exceeding solubility → precipitation.

• Solubility = max concentration
• Temperature dependent
• Supersaturation possible
• Exceeding → precipitate

Drinking water standards: EPA limits for contaminants. Lead: 15 µg/L (15 ppb) action level. Arsenic: 10 µg/L (10 ppb) maximum. Nitrate: 10 mg/L (10 ppm) maximum. Critical for public health.

• Lead: <15 µg/L (EPA)
• Arsenic: <10 µg/L (WHO)
• Nitrate: <10 mg/L
• Chlorine: 0.2-2 mg/L (treatment)

Blood tests in g/dL or mg/dL. Glucose: 70-100 mg/dL normal. Cholesterol: <200 mg/dL desirable. Hemoglobin: 12-16 g/dL. Medical diagnosis relies on concentration ranges.

• Glucose: 70-100 mg/dL
• Cholesterol: <200 mg/dL
• Hemoglobin: 12-16 g/dL
• Units: g/dL, mg/dL common

Air quality: PM2.5 in µg/m³. Soil contamination: mg/kg. Surface water: ng/L for trace organics. ppb and ppt levels for pesticides, pharmaceuticals. Ultra-sensitive detection required.

• PM2.5: <12 µg/m³ (WHO)
• Pesticides: ng/L to µg/L
• Heavy metals: µg/L range
• Trace organics: ng/L to pg/L

g/L × 1000 = mg/L. mg/L × 1000 = µg/L. Quick: each prefix = ×1000 scale. 5 mg/L = 5000 µg/L.

• g/L → mg/L: ×1000
• mg/L → µg/L: ×1000
• µg/L → ng/L: ×1000
• Simple ×1000 steps

For water: 1 ppm = 1 mg/L. 1% w/v = 10 g/L = 10,000 ppm. 100 ppm = 0.01%. Quick percentage!

• 1 ppm = 1 mg/L (water)
• 1% = 10,000 ppm
• 0.1% = 1,000 ppm
• 0.01% = 100 ppm

C1V1 = C2V2. To dilute 10x, final volume 10x larger. 100 mg/L diluted 10x = 10 mg/L. Easy!

• C1V1 = C2V2
• Dilute 10x: V2 = 10V1
• C2 = C1/10
• Example: 100 mg/L to 10 mg/L

Water sample has 12 µg/L lead. Is it safe (EPA action level: 15 µg/L)?

12 µg/L < 15 µg/L. Yes, below EPA action level. Also expressed as 12 ppb < 15 ppb. Safe!

Dilute 50 mL of 200 mg/L to 500 mL. Final concentration?

C1V1 = C2V2. (200)(50) = C2(500). C2 = 10,000/500 = 20 mg/L. 10x dilution!

Make 0.9% saline. How many grams of NaCl per liter?

0.9% w/v = 0.9 g per 100 mL = 9 g per 1000 mL = 9 g/L. Physiological saline!

Seawater contains ~35 g/L of dissolved salts (3.5% salinity). Mostly NaCl, but also Mg, Ca, K, SO4. Dead Sea: 280 g/L (28%) so salty you float! Great Salt Lake: 50-270 g/L depending on water level.

ppm (parts per million) popularized in 1950s for air pollution and water quality. Before that, used % or g/L. Now standard for trace contaminants. Easy to understand: 1 ppm = 1 drop in 50 liters!

Fasting blood glucose: 70-100 mg/dL (700-1000 mg/L). That's only 0.07-0.1% blood weight! Diabetes diagnosed at >126 mg/dL. Small changes matter—tight regulation by insulin/glucagon.

Pool chlorine: 1-3 mg/L (ppm) for sanitation. Higher = burning eyes. Lower = bacteria growth. Hot tubs: 3-5 ppm (warmer = more bacteria). Small concentration, big effect!

Soft: <60 mg/L CaCO3. Moderate: 60-120. Hard: 120-180. Very hard: >180 mg/L. Hard water causes scale buildup, uses more soap. Soft water better for washing, but can corrode pipes!

EPA lead action level: 15 µg/L (15 ppb) in drinking water. Lowered from 50 ppb in 1991. No safe level of lead! Flint, Michigan crisis: levels reached 4000 ppb in worst cases. Tragic.

The 1858 Great Stink of London—when Thames sewage odors shut down Parliament—catalyzed the first systematic water quality studies. Cities began crude chemical tests for contamination.

Early methods were qualitative or semi-quantitative: color, smell, and rough precipitation tests. The germ theory revolution (Pasteur, Koch) drove demand for better water standards.

• 1858: Great Stink forces London to build modern sewers
• 1890s: First chemical tests for hardness, alkalinity, and chloride
• Units: grains per gallon (gpg), parts per 10,000

Water chlorination (first US plant: Jersey City, 1914) required precise dosing—too little failed to disinfect, too much was toxic. This drove adoption of mg/L (parts per million) as the standard unit.

Spectrophotometry and titrimetric methods enabled accurate concentration measurement. Public health agencies set drinking water limits in mg/L.

• 1914: Chlorine dosed at 0.5-2 mg/L for disinfection
• 1925: US Public Health Service sets first water standards
• mg/L and ppm become interchangeable for dilute aqueous solutions

Silent Spring (1962) and environmental crises (Cuyahoga River fire, Love Canal) spurred regulation of pesticides, heavy metals, and industrial pollutants at µg/L (ppb) levels.

Atomic absorption spectroscopy (AAS) and gas chromatography (GC) enabled detection below 1 µg/L. EPA's Safe Drinking Water Act (1974) mandated Maximum Contaminant Levels (MCLs) in µg/L.

• 1974: Safe Drinking Water Act creates national MCL standards
• 1986: Lead ban; action level set at 15 µg/L (15 ppb)
• 1996: Arsenic limit lowered from 50 to 10 µg/L

Modern LC-MS/MS and ICP-MS instruments detect pharmaceuticals, PFAS, and endocrine disruptors at ng/L (ppt) and even pg/L (ppq) levels.

Flint water crisis (2014-2016) exposed failures: lead reached 4000 ppb (267× EPA limit). WHO and EPA continuously update guidelines as analytical sensitivity improves.

• 2000s: PFAS 'forever chemicals' detected at ng/L levels
• 2011: WHO updates guidelines for >100 contaminants
• 2020s: Routine detection at pg/L; new challenges in microplastics, nanomaterials

For dilute aqueous solutions (like drinking water), 1 ppm ≈ 1 mg/L. This assumes solution density = 1 kg/L (like pure water). For other solvents or concentrated solutions, ppm and mg/L differ because density ≠ 1. ppm is mass/mass or volume/volume ratio; mg/L is mass/volume. Always use mg/L for precision!

g/L (mass concentration) and mol/L (molar concentration) are different quantities. Conversion requires molecular weight: mol/L = (g/L) / (MW in g/mol). Example: 58.44 g/L NaCl = 1 mol/L. But 58.44 g/L glucose = 0.324 mol/L (different MW). Need to know the substance!

%w/v = percent weight/volume = grams per 100 mL. 10% w/v = 10 g per 100 mL = 100 g per 1000 mL = 100 g/L. Direct conversion! Different from %w/w (weight/weight, needs density) and %v/v (volume/volume, needs both densities). Always specify which % you mean!

Use C1V1 = C2V2. C1 = initial concentration, V1 = initial volume, C2 = final concentration, V2 = final volume. Example: dilute 100 mg/L by 10x. C2 = 10 mg/L. Need V1 = 10 mL, V2 = 100 mL. Add 90 mL solvent to 10 mL concentrate.

Water hardness comes from Ca2+ and Mg2+ ions, but different atomic weights make direct comparison difficult. Converting to CaCO3 equivalent provides standard scale. 1 mmol/L Ca2+ = 100 mg/L as CaCO3. 1 mmol/L Mg2+ = 100 mg/L as CaCO3. Fair comparison despite different actual ions!

Depends on context. Water quality: µg/L (ppb) to ng/L (ppt) range. Environmental: ng/L to pg/L. Clinical: often ng/mL to µg/mL. 'Trace' generally means <1 mg/L. Ultra-trace: <1 µg/L. Modern instruments detect femtograms (fg) in research!