Look at the diagram below. A buret filled with a base of known concentration is set up on a stand with a stopcock to drip into a flask containing acid of an unknown concentration. A small container of phenolphthalein is nearby. Which best describes what is taking place in the diagram? The calibration of an acid is being completed with a base. A direct measurement of pH is taking place. An indirect measurement of pH is taking place. The final volume of an acid or base is being measured.