In chemistry, pH is a scale used to specify how acidic or basic a water-based solution is. Acidic solutions have a lower pH, while basic solutions have a higher pH. At room temperature (25°C or 77°F), pure water is neither acidic nor basic(alkaline) and has a pH of 7.
The pH scale is logarithmic and inversely indicates the concentration of hydrogen ions in the solution (a lower pH indicates a higher concentration of hydrogen ions). This is because the formula used to calculate pH approximates the negative of the base 10 logarithm of the molar concentration of hydrogen ions in the solution. More precisely, pH is the negative of the base 10 logarithm of the activity of the hydrogen ion.
The exact meaning of the p in pH is disputed, as Sorensen did not explain why he used it. He describes a way of measuring it using potential differences, and it represents the negative power of 10 in the concentration of hydrogen ions.
pH is defined as the decimal logarithm of the reciprocal of the hydrogen ion activity, aH+, in a solution: