What pH measures
The letters pH stand for a measure of how many hydrogen ions are present in a water-based solution. In water, some molecules split slightly into a hydrogen ion (H+) and a hydroxide ion (OH-). An acid, in the simple view used in most classes, increases the number of hydrogen ions. A base lowers it, usually by supplying hydroxide ions or by taking up hydrogen ions.
The pH scale turns that concentration into a convenient number, usually between 0 and 14. At about room temperature, pure water sits at 7, which is called neutral. Lower numbers are acidic and higher numbers are basic, also called alkaline.
Why the scale is logarithmic
Each step on the scale is a tenfold change in hydrogen ion concentration, not a fixed amount. A solution at pH 3 has ten times as many hydrogen ions as one at pH 4, and a hundred times as many as one at pH 5. This is why a change of even one or two units can be a big deal, for a swimming pool, a fish tank or a lake.
A quick check of the arithmetic: pH 2 to pH 5 is three steps, so the difference is 10 x 10 x 10, or a factor of 1,000.
Typical examples
Exact values vary with the product, its age and how it was made, so treat these as rough ranges:
| Substance | Approximate pH |
|---|---|
| Stomach acid | about 1.5 to 3.5 |
| Lemon juice | about 2 |
| Vinegar | about 2.5 to 3 |
| Black coffee | about 5 |
| Pure water | 7 |
| Seawater | about 8 |
| Baking soda dissolved in water | about 8 to 9 |
| Soap | about 9 to 10 |
| Household ammonia cleaner | about 11 to 12 |
| Household bleach | about 12 to 13 |
Taste is an old clue: acids tend to taste sour, which is why lemons and vinegar do. Bases tend to taste bitter and feel slippery, but tasting unknown chemicals is never a safe way to test them.
Strong versus weak
Strength and concentration are different ideas. A strong acid, such as hydrochloric acid, splits almost completely into ions in water. A weak acid, such as the acetic acid in vinegar, splits only partly. A weak acid can still be quite acidic if it is concentrated, and a strong acid can be fairly mild if it is very dilute. That is also why safety advice depends on both the substance and how much of it is present.
How pH is measured
In a lab, a pH meter uses an electrode that produces a small voltage depending on the hydrogen ion concentration. Cheaper options use indicators, substances that change color at certain pH values. Litmus paper turns red in acid and blue in base. Many gardeners and cooks know that red cabbage juice works as a home indicator, shifting from reddish through purple to blue and green as it moves from acid to base. Universal indicator mixes several dyes to give a rainbow of colors across the scale.
Neutralization and buffers
When an acid and a base are mixed, they can neutralize each other, forming water and a salt. The classic example is hydrochloric acid with sodium hydroxide, which gives water and sodium chloride. A baking soda paste on a vinegar spill works on the same principle, and the fizzing is carbon dioxide released along the way.
Buffers are mixtures that resist changes in pH. They matter a great deal in living things. Blood, for instance, is kept within a narrow range, and the body has several systems to hold it there. This article is general information only, not medical advice, and individual cases vary.
Where pH matters in the world
In the digestive system, the stomach is strongly acidic, which helps break down food and deals with many microbes, while the small intestine is closer to neutral. Soil pH affects which plants thrive, which is why some gardeners test it before planting. Rain is naturally slightly acidic because carbon dioxide from the air dissolves in it, an everyday piece of the water cycle. Limestone and other carbonate rocks can neutralize acid, a link to the long-term behavior described in the rock cycle.
The oceans are mildly basic, but as they absorb more carbon dioxide their pH drifts slightly lower. Scientists often call this ocean acidification, even though the water is still on the basic side of 7. Shell-building organisms are among those that may be affected, though the details vary by species and place.
Safety in the kitchen and lab
Never mix household cleaners, particularly bleach with products containing ammonia or with acids, because dangerous gases can form. Wear eye protection when handling any strong acid or base in a lab or workshop, and follow the label. Both strong acids and strong bases can burn skin and eyes, and strong bases are often underestimated because they do not always feel painful right away.
Why the idea is useful
The pH scale is a compact way to describe a property that affects cooking, cleaning, farming, medicine and environmental science. Knowing that it is logarithmic and that the middle is neutral gets you most of the way to reading a label or a lab report with confidence.