Task: Text Categorization
Input (English): One analogue of this quantity, designed for use in concentrated solutions, is equal to -19 for a certain mixture that contains antimony pentafluoride. One equation for this quantity, which may be derived by rearranging the defining equation for "K sub a", is called the Henderson- Hasselbalch equation. Under standard conditions, this quantity is half the self-ionization constant pKw. This quantity is defined as the negative logarithm of the concentration of hydronium ions in a solution. For 10 points, name this quantity which can be crudely measured using indicators like phenolphthalein, and which is 7 for neutral solutions and less for acids.
Output (English): Science