Question

A procedure to measure the alkalinity (Box 11-1) of home swimming pool water is to titrate a fixed volume of pool water by counting the number of drops of the standard ${\mathbf{H}}_{\mathbf{2}}{\mathbf{SO}}_{\mathbf{4}}$to reach the bromocresol green endpoint. Explain what is measured in this titration and why bromocresol green was chosen.

Short answer

The concentration of hydroxide, carbonate, bicarbonate ions and other ions present in the pool water is measured with the help of an indicator named Bromocresol green.

Here, Bromocresol green is chosen because while titrating with standard acid the pool water reacts with protons in acid and reaches a pH of 4.5. The endpoint is observed as green in color.

Step-by-step solution

The equivalence point and indicator.

The equivalence point ensures the titration reaction becomes completed and at this point, the number of moles of titrant and the number of moles of analyte remains equal.

A chemical compound called anindicator indicates the end of the reaction by color change. The endpoint refers to the color change and indicates the equivalence point has been reached.

 A procedure to measure the alkalinity

• One of the techniques to determine the alkalinity in pool water is titrating it with standardized acid.

• The reason for the alkaline nature of pool water is the presence of ions like hydroxide, carbonate, bicarbonate, etc. Here, the pH ofpool water remains above 7 (basic).

• The concentration of hydroxide, carbonate, bicarbonate ions and other ions present in the pool water is measured with the help of an indicator namedBromocresol green.

• The solution in presence of this indicator becomesblue in color above the pHvalue of5.4and becomesyellow in color at pH below 3.8.

While titrating with standard acid the pool water reacts with protons in acid and reaches a pH of 4.5 which is intermediate between 3.8 and 5.4. Thus, the endpoint is observed asgreen in color which isintermediate to that of yellow and blue.