Final answer:
Performing the first chlorine check after the RO has been running for 10 minutes involves determining the total chlorine residual using a titration method.
Step-by-step explanation:
When performing the first chlorine check of the day after the RO (Reverse Osmosis) has been running for 10 minutes, the total chlorine residual in the water sample can be determined. The method involves acidifying the sample and adding potassium iodide (KI) to form a brown solution of iodine (I3). Then, titration with sodium thiosulfate (Na2S2O3) is done, which converts iodine (I3) to iodine (I) using starch as an indicator. The amount of Na2S2O3 needed to reach the end point is proportional to the total chlorine residual in the water sample.