When measuring total chlorine, which chemical must be added to the sample during analysis?

When measuring total chlorine in a water sample, potassium iodide (KI) must be added during the analysis. This is because the presence of potassium iodide reacts with free chlorine in the sample, forming a complex that enables the chlorine to be quantified. The reaction typically results in the liberation of iodine, which can then be titrated or measured using various spectrophotometric methods, allowing for an accurate evaluation of the total chlorine concentration.

The choice of potassium iodide is particularly important as it effectively facilitates the detection process. The iodine liberated from the reaction with chlorine can be visually observed or quantified via colorimetric analysis, which is a common method in laboratory settings. This method is crucial since total chlorine includes both free chlorine and combined chlorine species.

In contrast, the other chemicals listed do not play a role in the measurement of total chlorine. Sodium bicarbonate is generally used to buffer solutions, calcium chloride is used to supply calcium ions, and ferrous ammonium sulfate is typically used in different types of titration and redox chemistry but does not serve a function in the total chlorine analysis. Thus, potassium iodide is the correct choice for this analytical procedure.