Problem 14-53

pH = -log[H⁺]
A solution is neutral when [H⁺] = [OH^-]
At 25 °C, both concentrations of [H⁺] and [OH^-] are 1.0x10^-7 therefore pH = 7.00
At 25 °C, a pH of 7.40 means that [H⁺] < [OH^-] which is basic.

However, at the temperature of the human body (37 °C) [H⁺] = [OH^-] = 1.56x10^-7
Therefore, the neutral pH in the human body is pH = -log(1.56x10^-7) = 6.81.