In the alkalinity laboratory test, indicator changes occur at which two pH values?

• A. pH 8.3 and about pH 4.5
• B. pH 7 and about pH 9
• C. pH 3 and about pH 11
• D. pH 6 and about pH 8

Answer: (A)

Two indicator endpoints define the alkalinity titration because the carbonate system changes in two distinct pH regions, and we use indicators that shift color at those points. One indicator flips color around pH 8.3, so as acid is added the pink color of phenolphthalein disappears near 8.3, marking the endpoint for carbonate alkalinity. The other indicator flips around pH 4.5, so at this lower pH the methyl orange endpoint is reached, marking the endpoint for noncarbonate alkalinity. In short, the indicator color changes occur near pH 8.3 and pH 4.5.