You see "pH balanced" on shampoo bottles, "pH 6.5" on kombucha, and "alkaline water" advertised. Hidden behind every "pH" reference is the same simple scale, and once you understand it, you'll see it shaping food, body, and home chemistry every day.
The pH scale
pH ranges from 0 (very acidic) to 14 (very basic, also called alkaline), with 7 as neutral.
- 0–3: very acidic (battery acid, lemon juice)
- 3–6: mildly acidic (vinegar, coffee, tomatoes)
- 6.5–7.5: roughly neutral (pure water, blood)
- 7.5–10: mildly basic (baking soda, soap)
- 10–14: very basic (bleach, drain cleaner)
Each whole number is a 10× change in concentration of hydrogen ions. pH 4 is 100× more acidic than pH 6.
pH of common substances
| Substance | pH |
|---|---|
| Battery acid | 0.5 |
| Stomach acid | 1.5–3.5 |
| Lemon juice | 2.0–2.5 |
| Vinegar | 2.4–3.0 |
| Coca-Cola | 2.5 |
| Orange juice | 3.5 |
| Coffee | 5.0 |
| Milk | 6.5 |
| Pure water | 7.0 |
| Human blood | 7.35–7.45 |
| Seawater | 8.1 |
| Baking soda solution | 8.5 |
| Soap (typical) | 9–10 |
| Ammonia | 11.5 |
| Bleach (household) | 12.5 |
| Drain cleaner | 13–14 |
Body pH: tightly regulated
Human blood pH is rigidly maintained between 7.35 and 7.45. Outside this narrow range, enzymes stop functioning and you die.
The body has multiple buffer systems (bicarbonate, phosphate, proteins) that absorb pH changes. Even drinking lemon juice doesn't acidify your blood — it gets buffered before it reaches the bloodstream.
Stomach: pH 1.5–3.5. Strongly acidic to digest proteins and kill pathogens. The mucus lining protects the stomach wall.
Saliva: pH 6.5–7.5. Slightly acidic to slightly alkaline. Drops after eating sugary foods (cavity-forming bacteria love this) and recovers slowly.
Urine: pH 4.5–8. Varies with diet and metabolism. Athletes and people on high-protein diets have more acidic urine.
Food and drink
Why coffee tastes acidic: pH around 5. The acidity comes from chlorogenic, citric, and quinic acids. Brewing methods that extract more of these (espresso, drip) taste sharper than slower methods (cold brew, which is around pH 5.5).
Why citrus tastes sour: pH 2-3. Citric acid, malic acid, ascorbic acid (vitamin C) all contribute.
Why baking soda neutralizes acid: pH 8.5. Reacts with acidic compounds to form CO₂ and water. Used in cooking, antacids, and cleaning.
Why pickled foods taste tart: vinegar (pH 2.4) brings food pH down. The acidic environment also inhibits bacterial growth, which is the original purpose of pickling.
Soil and plants
Most plants prefer slightly acidic soil (pH 6.0–7.0). Some are picky:
- Blueberries, azaleas, rhododendrons: need acidic soil (pH 4.5–5.5).
- Lavender, lilac: tolerate alkaline soil (pH 7.0–8.0).
- Most vegetables: 6.0–7.0.
- Lawns: 6.0–7.0.
Soil pH affects nutrient availability. At wrong pH, plants can't absorb iron, magnesium, or other essential elements even when present.
Fixing pH: lime (calcium carbonate) raises pH; sulfur or aluminum sulfate lowers it. Soil tests show current pH.
Pool and spa chemistry
Swimming pools are kept at pH 7.4–7.6. Why so close to body pH?
- Eyes burn outside this range.
- Chlorine works most efficiently here.
- Avoids corrosion of pool equipment.
Pool chemicals adjust pH:
- Sodium bicarbonate raises pH.
- Sodium bisulfate or muriatic acid lowers pH.
Test pool pH weekly with strips or kits. Drift happens from rain, swimmer body chemistry, evaporation.
Cleaning products
Different cleaners use different pH:
- Acidic cleaners (pH 1-3): dissolve mineral deposits, rust, soap scum. Examples: CLR, Lime-A-Way, vinegar.
- Neutral cleaners (pH 6-8): general-purpose, gentle. Most dish soap.
- Basic cleaners (pH 9-12): dissolve grease, oil, organic matter. Examples: ammonia, oven cleaner, dishwasher soap.
Mixing acidic and basic cleaners is dangerous (can produce toxic gases, especially mixing ammonia and bleach).
Beauty products and pH
Skin pH: 4.5–5.5 (slightly acidic). The "acid mantle" protects against bacteria.
"pH balanced" shampoos: aim for pH 5–7 to match skin/scalp.
Soap (traditional): pH 9–10 (basic). Strips natural skin oils. Modern "syndet" body washes are pH 5–7 (gentler).
Conditioners: pH 4–5 (acidic). Smooths hair cuticle.
Hair coloring: often basic to open the cuticle so dye can penetrate.
Drinking water and "alkaline water"
Tap water in the U.S. is typically pH 6.5–8.5 (varies by source). Well water can be more variable.
"Alkaline water" sold in stores is often pH 8–9. Marketing claims about health benefits are largely unsupported by evidence — the stomach quickly acidifies anything you drink.
Bottled water varies: Evian is around 7.2; Dasani 5.5; Smart Water 7.0; Fiji 7.7.
Acid rain
Normal rain is slightly acidic (pH ~5.6) due to dissolved CO₂. "Acid rain" (pH below 5) results from sulfur and nitrogen oxide emissions reacting with water in the atmosphere. Damages plants, fish populations, and limestone buildings.
U.S. acid rain has decreased dramatically since 1990s due to emissions controls.
Ocean acidification
Atmospheric CO₂ dissolving into seawater lowers ocean pH. Pre-industrial ocean: pH 8.2. Today: ~8.1. Doesn't sound like much, but it's a 30% increase in hydrogen ions on the log scale.
Affects coral, mollusks, and any creature using calcium carbonate (their shells dissolve faster in more acidic water).
Calculate pH
Our pH calculator handles conversions between pH and hydrogen ion concentration. Useful for understanding any of these contexts mathematically, or for chemistry homework.