Pure water must contain equimolar amounts of H ⫹ and OH ⫺ , so [H ⫹ ] ⫽ [OH ⫺ ] ⫽ (K w ) 1兾2 ⫽ 10 ⫺7 M. Since [H ⫹ ] and [OH ⫺ ] are reciprocally related by Eq. 2-2, when [H ⫹ ] is greater than 10 ⫺7 M, [OH ⫺ ] must be cor- respondingly less and vice versa. Solutions with [H ⫹ ] ⫽ 10 ⫺7 M are said to be neutral, those with [H ⫹ ] ⬎ 10 ⫺7 M are said to be acidic, and those with [H ⫹ ] ⬍ 10 ⫺7 M are said to be basic. Most physiological solutions have hydrogen ion concentrations near neutrality. For example, human blood is normally slightly basic with [H ⫹ ] ⫽ 4.0 ⫻ 10 ⫺8 M.