Which statement about pH is correct?

• A. True
• B. False
• C. Only at extreme temperatures
• D. Not defined

Answer: (A)

pH describes how acidic or basic a solution is by the activity of hydrogen ions present. It’s defined as the negative logarithm of hydrogen ion activity, pH = -log10(aH+). Because of this, more hydrogen ions mean a lower pH (more acidic), while fewer hydrogen ions mean a higher pH (more basic). This concept is central in wastewater work, since pH affects chemical reactions, metal solubility, disinfection, and many biological processes.

The statement that pH is a measure of hydrogen ion activity in solution is consistent with how pH is defined and used in practice. It isn’t something that only exists at extreme temperatures or something undefined, though the exact neutral point can shift slightly with temperature because the water dissociation constant Kw changes with temperature.