Florida Drinking Water Operator "B" Practice Test

What is the Langelier index if the initial pH is 8.1 and the finished water sample is 8.9?

1. -1.0

2. -0.8

3. 0.6

4. 0.0

The correct answer is: -0.8

The Langelier Index (LI) is a calculated value that indicates the calcium carbonate stability of water. It's determined using the difference between the actual pH of the water and the pH that is required for calcium carbonate saturation. To calculate the Langelier Index, the formula used typically involves comparing the actual pH of a water sample to an empirically derived pH value at which the water is neutral and does not corrode pipework or precipitate calcium carbonate. This saturated pH can be affected by various factors including temperature, total dissolved solids, and calcium content. In this instance, with an initial pH of 8.1 and a finished water sample pH of 8.9, the Langelier Index would indicate whether the water is corrosive, stable, or scales. The LI can take on negative values when water is corrosive, zero when it is balanced, and positive values when it is scaling or stable. Calculating the Langelier Index gives a result that tells you how the water chemistry has changed. In this scenario, if the difference between the finished pH (8.9) and a calculated or established threshold (which could range from a baseline of 9.1 for specific