The commonly used indicator for measuring iron concentration in water is 1,10-phenanthroline. This indicator is widely used because of its high sensitivity and specificity towards iron ions. It forms a complex with iron ions, resulting in a color change that can be easily measured and quantified.

1. Sensitivity and specificity:
- 1,10-phenanthroline has a high sensitivity towards iron ions, meaning it can detect even low concentrations of iron in water. This is important for accurate measurement and monitoring of iron levels.
- It also has high specificity, meaning it specifically reacts with iron ions and does not interfere with other substances in the water. This ensures accurate and reliable results.

2. Complex formation:
- 1,10-phenanthroline forms a complex with iron ions, resulting in a color change. This complex is highly stable, allowing for accurate measurement and analysis.
- The color change is typically from pale yellow to orange-red, depending on the concentration of iron ions present. The intensity of the color is directly proportional to the iron concentration, allowing for quantitative analysis.

3. Measurement and quantification:
- The color change can be measured using a spectrophotometer, which measures the absorbance of light by the colored complex. The absorbance is directly related to the iron concentration, allowing for precise quantification.
- A calibration curve can be established by measuring the absorbance of known concentrations of iron solutions. This curve can then be used to determine the iron concentration in water samples based on their absorbance values.

4. Wide applicability:
- 1,10-phenanthroline can be used to measure iron concentrations in various types of water, including drinking water, wastewater, and natural water sources.
- It is also suitable for laboratory analysis as well as field-testing, making it a versatile indicator for iron measurement.

In conclusion, 1,10-phenanthroline is the commonly used indicator for measuring iron concentration in water due to its high sensitivity, specificity, and ability to form a stable complex with iron ions. Its color change can be easily measured and quantified, allowing for accurate analysis of iron levels in water samples.