pH is the negative log of hydrogen ion concentration in a water-based solution. pH is an abbreviation for "power of hydrogen" where "p" is short for the German word for power, potenz and H is the element symbol for hydrogen. The H is capitalized because it is standard to capitalize element symbols.
pH is a logarithmic measure of hydrogen ion concentration.
It is a number between 0 and 14 indicating the degree of acidity or alkalinity. The pH scale resembles a thermometer scale, but the pH scale indicates intensity of acidity or alkalinity. The midpoint of the pH scale is 7, and a solution with this pH I neutral. Numbers below 7 denote acidity; those above, alkalinity.
For example, a pH of 3 is ten times more acidic than a pH of 4 and 100 times (10 times 10) more acidic than a pH value of 5. So, a strong acid may have a pH of 1-2, while a strong base may have a pH of 13-14. A pH near 7 is considered to be neutral.
Laboratory pH meters are often made with a glass electrode consisting of a silver wire coated with silver chloride immersed in dilute hydrochloric acid. The electrode solution is separated from the solution to be measured by a thin glass membrane.
The potential which develops across that glass membrane can be shown to be proportional to the hydrogen ion concentrations on the two surfaces. In the measurement instrument, a cell is made with the other electrode commonly being a mercury-mercury chloride electrode. The cell potential is then linearly proportional to the pH and the meter can then be calibrated to read directly in pH.