What pH and storage condition are specified for hardness preservation?

What pH and storage condition are specified for hardness preservation?

• A. Add HNO3 or H2SO4 to pH < 2 and Cool <= 6C; 6 Months
• B. Add NaOH to pH 9-11 and store at room temperature for 1 week
• C. No acidification required; store at -20C for 1 year
• D. Keep at pH 7 and store at 25C for 2 months

Answer: (A)

Preserving hardness relies on keeping calcium and magnesium ions in solution and preventing chemical changes that would remove them or alter their concentration before analysis. The proven approach is to acidify with a strong mineral acid to well below pH 2 and keep the sample cold, at or below 6°C, for up to about six months. This acidic, cool storage slows biological activity and prevents precipitation of hardness-causing ions as carbonates or hydroxides, so the measured hardness reflects the original sample. This is why the option specifying adding a strong acid to reach pH < 2 and storing cool for up to six months is the best choice. Alternatives fall short: raising the pH with NaOH would precipitate hardness ions, not preserve them; room-temperature storage for only a week is insufficient to maintain stability; no acidification with freezing or neutral storage does not prevent changes and can alter the dissolved ion concentrations.

Preserving hardness relies on keeping calcium and magnesium ions in solution and preventing chemical changes that would remove them or alter their concentration before analysis. The proven approach is to acidify with a strong mineral acid to well below pH 2 and keep the sample cold, at or below 6°C, for up to about six months. This acidic, cool storage slows biological activity and prevents precipitation of hardness-causing ions as carbonates or hydroxides, so the measured hardness reflects the original sample.

This is why the option specifying adding a strong acid to reach pH < 2 and storing cool for up to six months is the best choice. Alternatives fall short: raising the pH with NaOH would precipitate hardness ions, not preserve them; room-temperature storage for only a week is insufficient to maintain stability; no acidification with freezing or neutral storage does not prevent changes and can alter the dissolved ion concentrations.