Potential hydrogen, abbreviated as pH, is a measure of how acidic or alkaline a solution is. The scale runs from roughly 0 to 14, where lower numbers mean more acidic, higher numbers mean more alkaline (or basic), and 7 is the neutral midpoint for pure water at room temperature. The concept was introduced in 1909 by the Danish chemist S.P.L. Sørensen, and it remains one of the most widely used measurements in chemistry, medicine, and everyday life.
What the “p” and “H” Actually Mean
The “H” in pH stands for hydrogen, specifically hydrogen ions floating in a solution. The “p” represents a mathematical operation: the negative logarithm of the hydrogen ion concentration. In practical terms, pH tells you how many free hydrogen ions are packed into a liquid. More hydrogen ions means more acidic. Fewer hydrogen ions means more alkaline.
Sørensen’s original notation was a subscript formula that looked different from what we use today. Over the following decades, scientific societies simplified it to the clean two-letter “pH” that became standard worldwide.
Why the Scale Is Logarithmic
The pH scale is not linear. Each whole number represents a tenfold change in hydrogen ion concentration. A solution with a pH of 3 is ten times more acidic than one with a pH of 4, and a hundred times more acidic than one at pH 5. This logarithmic design compresses an enormous range of acidity into a compact 0-to-14 scale, which is why small shifts in pH can have outsized effects in biology and chemistry.
The formula itself is straightforward: pH equals the negative logarithm of the hydrogen ion concentration. If a solution has a hydrogen ion concentration of 0.001 (one thousandth of a mole per liter), the pH is 3. If the concentration drops to 0.0001, the pH rises to 4.
Common Substances on the pH Scale
Putting real substances on the scale helps make it concrete:
- Battery acid: around pH 1
- Lemon juice: around pH 2
- Black coffee: around pH 4.5
- Pure water (at 25°C): pH 7
- Baking soda solution: around pH 8.5
- Household bleach: around pH 12–13
The extremes of the scale are corrosive in different ways. Strong acids dissolve metals and burn tissue. Strong bases feel slippery and can cause chemical burns just as severe as acids.
Neutral pH Isn’t Always 7
One detail that surprises most people: a pH of 7 is only neutral at 25°C (77°F). As water temperature rises, more hydrogen ions break free from water molecules, which shifts the neutral point downward. At 0°C, pure water has a pH of 7.47. At 50°C, it drops to 6.63. At boiling (100°C), pure water sits at a pH of 6.14, yet it’s still perfectly neutral because the concentration of hydrogen ions and hydroxide ions remains equal. A solution at pH 7 in boiling water is actually slightly alkaline.
How pH Is Measured
The simplest method is litmus paper or pH indicator strips, which change color when dipped in a solution. These give rough estimates but aren’t precise enough for scientific or medical work.
Electronic pH meters are far more accurate. They use a glass electrode that generates a tiny voltage when it contacts a solution. The voltage changes depending on the hydrogen ion concentration at the glass surface. A sealed reference solution inside the electrode stays at a constant pH, so any shift in the voltage reading comes entirely from the solution being tested. The meter converts that voltage into a pH number, typically accurate to two decimal places.
pH in Your Body
Your body maintains different pH levels in different compartments, each tuned to a specific function.
Human blood stays in a remarkably tight range of 7.35 to 7.45, averaging 7.40. When blood pH drops below 7.35, the condition is called acidemia. When it rises above 7.45, it’s called alkalemia. Either direction can become life-threatening because enzymes and cellular processes depend on that narrow window to function properly. Your lungs and kidneys work constantly to keep blood pH stable by adjusting how much carbon dioxide you exhale and how much acid your kidneys excrete.
Your stomach is a completely different environment. Parietal cells in the stomach lining produce hydrochloric acid, creating a pH of 2 to 3. This extreme acidity activates digestive enzymes that break down proteins, helps your body absorb certain nutrients, and kills many bacteria and pathogens before they can reach the intestines. When stomach pH rises too high (becomes less acidic) due to medications or disease, bacteria can overgrow in the stomach and upper gut.
Your skin sits at about pH 5.5 on its surface, forming what dermatologists call the “acid mantle.” This mildly acidic layer helps defend against microbial colonization and supports the skin’s barrier function. When the acid mantle is disrupted, whether by harsh soaps, overwashing, or skin conditions, it can contribute to problems like contact dermatitis, eczema, acne, and fungal infections. Cleansers formulated near pH 5.5 are less likely to disturb this protective layer.
pH in the Environment
Soil pH determines which nutrients plants can actually absorb. Most nutrients reach their peak availability when soil pH falls between 6 and 7. Outside that range, certain minerals become locked into chemical forms that roots can’t take up, even if the soil contains plenty of them. This is why gardeners and farmers test soil pH before adding fertilizer. An acidic soil might need lime to raise the pH, while an alkaline soil might need sulfur to lower it.
Ocean pH is a growing concern. Surface ocean water currently averages about 8.1, making it mildly alkaline. But since the Industrial Revolution, that number has dropped by 0.1 pH units. On a logarithmic scale, that small-sounding shift represents roughly a 30 percent increase in acidity. The cause is straightforward: the ocean absorbs carbon dioxide from the atmosphere, and dissolved CO₂ reacts with seawater to produce carbonic acid. If carbon emissions continue on their current trajectory, ocean surface pH could fall to around 7.8 by the end of this century, putting severe stress on coral reefs, shellfish, and other organisms that build calcium carbonate structures.
Why a Simple Number Matters So Much
pH governs chemical reactions everywhere, from industrial processes to the inside of a living cell. Enzymes only work within specific pH ranges. Metals corrode faster in acidic water. Swimming pools become unsafe when pH drifts too far in either direction. Water treatment plants monitor pH continuously because it affects how well disinfectants work. In food production, pH controls everything from the tang of yogurt to the safety of canned goods, since the bacteria responsible for botulism cannot grow below pH 4.6.
What Sørensen created in 1909 as a laboratory convenience turned out to be one of the most practical measurements in science. Whether you’re testing your aquarium water, choosing a face wash, or trying to understand why coral reefs are shrinking, you’re working with potential hydrogen.