Residual ozone in fresh water can be measured by the indigo colorimetric method, which is based on the decolorization
of indigo by ozone (American Public Health Association, 1989). The decrease in light absorbance is linear with
increasing concentration, and can be measured by a spectrophotometer at a wavelength of 600 nm. Because of the
rapid reaction of ozone with bromide in seawater, it is not possible to measure dissolved ozone directly. Methods
detecting the total residual oxidants (TRO) formed by ozone have been applied, including the DPD (N,
N-diethyl-p-phenylenediamine) method and the iodometric method. As a surrogate parameter, the redox-potential is
used for monitoring the oxidizing power of ozone and its oxidants in many practical applications. However, the absence
of a linear relationship between residual ozone concentration and redox-potential is a disadvantage of this method.