Chlorine is very unstable in aqueous solutions, especially at low concentrations where its content rapidly decreases. Exposure to sunlight and other strong light or agitation can accelerate the reduction rate of chlorine. Therefore, after sampling, it cannot be stored and chlorine measurement must be immediately started, while avoiding light exposure and agitation of the water sample.
All operations in the measurement process should avoid direct sunlight exposure, preferably at the lowest possible temperature and in soft light. Moreover, all colorimetric methods require the use of color and turbidity blanks to compensate for the color and chromaticity of the raw water, especially when turbidity and chromaticity are high, blank values must be measured.
When using the ortho toluidine visual colorimetric method to determine residual chlorine, if the water sample is mixed evenly with a standard ortho toluidine solution and immediately colorimetric, the measured result is free residual chlorine. If left in the dark for 10 minutes to produce high chromaticity and then colorimetric, the obtained result is total residual chlorine. The total residual chlorine minus the free residual chlorine is the combined residual chlorine. When using the visual colorimetric method of ortho toluidine for measurement, if there is a large amount of residual chlorine, it will produce orange yellow; If the alkalinity of the water sample is too high and the residual chlorine content is small, it will produce light green or light blue. At this point, an additional 1mL of ortho toluidine standard solution can be added to produce a normal light yellow color.
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