*2.2.3 Colorimetric spectrometry*

Several different colorimetric methods are used for measuring ozone residuals. But most of them are sensitive to significant interference from secondary oxidants [14].

The first reagents for measuring ozone in air and exhaust gases are indigo and its water-soluble derivatives, the sulfonated indigo compounds like indigo disulfonate, indigo carmine, and indigo trisulfonate. Water-soluble derivatives of indigo, indigo disulfonate, and indigo trisulfonate are pH or redox indicators. Purity and age of the indigo trisulfonate are very important for the method because they affect the stoichiometry of the reaction [17, 19].

The indigo molecule contains only one double bond (C═C). It reacts with ozone in order to produce sulfonated isatin and similar substance (**Figure 3**). Maximum absorbance of indigo is at 600 nm [17, 26]. If the pH value is below 4, sensitivity of the method does not vary with ozone concentration, small changes of temperature of reaction, or the chemical composition of the water. The advantage of the method is applicable for lake water, too hard groundwaters, and biologically treated domestic wastewater. Indigo trisulfonate method is quantitative, selective, fast, and simple. Classical instrumentation of water work laboratories is enough for measurement. The method is based on the decolorization of the dye by ozone. The loss of color is directly proportional to the concentration of ozone, and pH value of the sample is adjusted to about 2 in order to minimize destruction of the ozone by hydroxide ions [17, 19, 21]. The concentration is the difference in absorbance between sample and blank [16]. Mn+2 ion is a problem in this method. Because, oxidation products from the reaction of Mn+2 ions with ozone can demolish indigo trisulfonate. So, glycine is added to the sample in order to demolish the ozone

**Figure 3.** *Ozonation of potassium indigo trisulfonate [26].*

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selectively. Then, indigo reagent is added to the sample to measure the seeming ozone concentration because of the reaction with manganous ion oxidation products. This value is subtracted from the value of the sample without glycine [19].

Indigo method is applied to "AccuVac Ampul" ozone measurement. For the measurement, all the chemicals are packaged under vacuum into an ampule. The ampule is like a cuvette, and it is used precisely for the spectrophotometric determination of the dissolved ozone [25].
