Emerson TCL instruction manual Calibrating Total Chlorine, Determination of Total Chlorine

MODEL TCL	SECTION 9
	CALIBRATION

9.3 CALIBRATING TOTAL CHLORINE

9.3.1 Purpose

The continuous determination of total chlorine requires two steps. See Figure 9-1. First, the sample flows into a conditioning system (the Model TCL) where it is treated with acetic acid (vinegar) and potassium iodide. The acid lowers the pH, which allows total chlorine in the sample to quantitatively oxidize the iodide to iodine. The treated sample then flows to the sensor. The sensor is a membrane-covered amperometric sensor, whose output is pro- portional to the concentration of iodine. Because the concentration of iodine is also proportional to the concentra- tion of total chlorine, the analyzer can be calibrated to read total chlorine.

Figure 9-2 shows a typical calibration curve for a total chlorine sensor. Because the sensor really measures iodine, calibrating the sensor requires exposing it to a solution containing no iodine (zero standard) and to a solution con- taining a known amount of iodine (full-scale standard).

The zero standard is necessary because the sensor, even when no iodine is present, generates a small current called the residual or zero current. The analyzer compensates for the residual current by subtracting it from the measured current before converting the result to a total chlorine value. New sensors require zeroing before being placed in service, and sensors should be zeroed whenever the electrolyte solution is replaced. Deionized water is a good zero standard.

Do not zero the sensor by leaving it in the TCL flow cell and turning off reagent injec- tion. Even though no iodine is present, the current measured under these conditions is not the zero current. Instead, it is the slight response of the sensor to total chlorine in the sample. Always use deionized water for zeroing the sensor.

The purpose of the full-scale standard is to establish the slope of the calibration curve. Because stable total chlorine standards do not exist, the sensor must be calibrated against a test run on a grab sample of the process liquid. Several manufacturers offer portable test kits for this purpose. Observe the following pre- cautions when taking and testing the grab sample.

•Take the grab sample from a point as close as possible to the inlet of the TCL sample conditioning system.

•Total chlorine solutions are unstable. Run the test immediately after taking the sam- ple. Try to calibrate the sensor when the chlorine concentration is at the upper end of the normal operating range.

FIGURE 9-1. Determination of Total Chlorine

FIGURE 9-2. Sensor Current as a Function of Total

Chlorine Concentration

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