**Core Concept**
The estimation of free chlorine in water is crucial for ensuring its safety and quality, particularly in medical and laboratory settings. Free chlorine refers to the amount of chlorine that is available to disinfect and is not bound to organic matter or other substances.

**Why the Correct Answer is Right**
The correct method for estimating free chlorine in water is the DPD (N,N-diethyl-p-phenylenediamine) titration method. This method involves adding a DPD reagent to the water sample, which reacts with the free chlorine to form a pink-colored complex. The intensity of the color is directly proportional to the concentration of free chlorine, allowing for accurate measurement. The DPD reagent specifically reacts with hypochlorous acid (HOCl), the active form of chlorine responsible for disinfection.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the orthotolidine method is used to estimate chlorine, but not specifically free chlorine. It reacts with both free and combined chlorine.
**Option B:** This option is incorrect because the iodometry method is used to estimate chlorine, but not specifically free chlorine. It is based on the reaction between chlorine and iodide ions.
**Option C:** This option is incorrect because the amperometric method is not a commonly used technique for estimating free chlorine in water.

**Clinical Pearl / High-Yield Fact**
In water treatment and disinfection, the free chlorine level must be carefully controlled to ensure effective disinfection without causing damage to equipment or harm to individuals.

**Correct Answer:** D. DPD titration method.