acidity
Meaning of acidity: A property rather than a substance, measured on a scale where each single step stands for a tenfold change.
Definition of acidity
Acidity is the degree to which a solution behaves as an acid, which comes down to how many free hydrogen ions it carries. It is reported as pH, a scale where low numbers mean strongly acidic and high numbers mean the reverse. An acid gives up hydrogen ions; a base accepts them, and the two neutralize one another and form salts.
Why is the scale so easily misread?
Because it is compressed. Each whole step on the pH scale stands for a tenfold change in the number of free hydrogen ions, so two fluids a few points apart are not slightly different but different by many thousands of times. Stomach juice and blood illustrate it: the gap between them sounds modest written down and is enormous in fact. The same compression means that a small movement in the blood, of a few tenths of a point, represents a large chemical shift and is enough to disturb how enzymes fold and how the heart conducts. A chemical described as strong is one that gives up its hydrogen ions completely, which is a separate matter from how concentrated it happens to be.
How acidic is each part of the body?
| Fluid | Character |
|---|---|
| Stomach juice | Strongly acidic, activating digestive enzymes and killing swallowed organisms |
| Blood | Very slightly alkaline, held within a narrow band |
| Small intestine | Alkaline, since bile and pancreatic fluid neutralize the stomach contents |
| Urine | Swings widely with diet and with what the kidney is disposing of |
| Skin surface | Mildly acidic, which discourages many organisms |
Can what a person eats change the blood?
Not meaningfully, and the belief that it can underpins a good deal of marketing. Three systems defend the blood: chemical buffers that mop up hydrogen ions instantly, the lungs, which alter acidity within minutes by changing how much carbon dioxide is blown off, and the kidneys, which adjust it over hours to days. Diet shifts the acidity of urine, which is the kidney doing its work, and leaves the blood where it was. Genuine departures from the band come from illness rather than food, among them lung disease, kidney failure, uncontrolled diabetes and shock, where poor blood flow leaves tissues producing lactic acid faster than the liver clears it.
Also known as
Used in a sentence
Increased acidity of the tumor microenvironment was noted in the report.
Good to know
The pH scale is logarithmic, so a change of one unit is a tenfold change in acidity, and the blood is held within a narrow band by buffers, breathing and the kidneys rather than by what is eaten.
Word origin
Where “acidity” comes from
From the Latin acidus, meaning sour, which comes in turn from acere, to be sour.