combined available (residual) chlorine is obtained as the difference between total and free chlorine. Standard laboratory methods for determining aqueous chlorine concentrations involve iodometry, in which free iodine liberated from potassium iodide is titrated with sodium thiosulfate, using starch as an indicator. All active forms of chlorine will liberate free iodine, and therefore, the iodometric methods of analysis are limited to the determination of total active chlorine. For an automatic total residual chlorine analyzer with a 4-20 mADC logarithmic output, refer to Figure 8.11a. Other titrimetric procedures employing indicators other than starch have also been developed for the determination and differentiation of the active chlorine forms. The amperometric titration method is based on polarographic principles and allows determination of the various forms of active chlorine. Various colorimetric procedures are available in which a colorless indicator is oxidized to a colored product, the color intensity of which is proportional to the chlorine concentration. Many of the colorimetric procedures allow the determination of the various forms of chlorine. Colorimetric procedures are adaptable to field determination by the use of kits containing the necessary reagents, color standards for visual estimation, and portable, battery-operated colorimeters. The amperometric titration and colorimetric methods have also been adapted to automated continuous analyzers.Automatic free or total chlorine analyzers can control in the on–off mode by stopping and starting a feed pump and sounding alarms, or can provide throttling control by modulating the pumping rate of the chlorine feed pump. The output signals can be to computers or printers through an RS232 interface, or can be analog selected from among the choices of 0 to 10 mV, 0 to 100 mV, 0 to 1 V, and 4 to 20 mA. The measurement range is usually 0 to 5 mg/l with a 0.01 resolution, but other ranges from 0–1 to 0–10 mg/l are also available.