The document describes a method for estimating the amount of free CO2 present in a water sample. Free CO2 in water reacts with water to form carbonic acid, which can then be titrated with a strong base like sodium hydroxide (NaOH) or a weak base like sodium carbonate (Na2CO3) while using a phenolphthalein indicator. The method involves collecting a water sample, adding an indicator, and titrating with 0.05 N NaOH solution until a pink color is reached. The volume of NaOH used is then used to calculate the concentration of free CO2 in mg/L.