Read the text. Put the steps of a comparator usage in the right order.
There is a wide variety of methods for improving the quality of drinking-water. Disinfection is the process of destroying various organisms to prevent infection. There are a number of methods of disinfecting water, but chlorination is the most common.
Chlorine takes time to kill organisms. At temperatures of 18OC and above, the chlorine should be in contact with the water for at least 30 minutes. If the water is colder then the contact time must be increased.
The quickest and simplest method for testing for chlorine residual is the dpd \(diethyl paraphenylene diamine\) indicator test, using a comparator.
Chlorine residual should be checked regularly. If the system is new or has been rehabilitated then check daily until you are sure that the chlorination process is working properly. After that, check at least once a week.
\(The stronger the colour, the higher the concentration of chlorine in the water\.\)

                                                                                                                                                                            \(World Health Organisation, Geneva\)

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