In hard waters, copper sulfate has poor control because

• A. It precipitates and settles to sediment
• B. It remains dissolved
• C. It evaporates
• D. It becomes more toxic

Answer: (A)

Hard water reduces copper sulfate’s effectiveness because the copper ions react with components of hardness (calcium and magnesium, along with carbonate) to form insoluble compounds that precipitate and settle out of the water. When Cu2+ encounters carbonate or hydroxide groups in hard water, it forms copper carbonate or basic copper carbonates, and copper hydroxide as pH rises. These solids remove copper from the dissolved phase, so there isn’t enough active copper ions present to control algae. That’s why copper sulfate works poorly in hard water: the active ingredient is pulled out of solution as a sediment, not remaining available to do the job.