Method for measuring acidity

An acidity measuring apparatus for determining free fatty acids in a sample comprises a measuring container (7) accommodating an electrolyte solution consisting of a 1,2-benzoquinone derivative or a 1,4-benzoquinone derivative, an organic solvent, an electrolyte and the acid-containing sample; a working electrode (9), a counter electrode (8) and a reference electrode section (10) provided in the measuring container and immersed in the electrolyte solution; and a control section (26) for sweeping the potential of the working electrode within a prescribed range and for detecting the value of the current flowing between the working electrode and the counter electrode. The peak corresponds to oxidation of the product formed by reaction of the acid with the quinone. Preferred quinones are 3,5-ditertiary-butyl-1,2-benzoquinone Ä3383-21-9Ü and 2,6-dimethyl-1,4-benzoquinone Ä527-61-7Ü.