The pH Differential Method
One way of determining the concentration of anthocyanin pigment in samples is through the use of visible-light spectrophotometry. The spectrophotometric method of determining anthocyanin pigment concentration is called the pH differential method. This method is based on the known fact that anthocyanin pigmentation is uniquely pH dependent. At a pH of 1.0, anthocyanins are colored, they transmit various wavelengths from orange to purple light, depending on the specific anthocyanin. At a pH of 4.5, however, all anthocyanins are colorless. This change in pigmentation is due to a change in the ring C structure of anthocynins (see figure below).
This relatively unique pH-dependent change in pigmentation can be measured spectrophotometrically and can used to determine anthocyanin concentration in fruits and wines at the exclusion of other sources of similar pigmentation, such as artifical colorants.
Continue to the next section: Spectrophotometry.