How should hardness samples be preserved?

How should hardness samples be preserved?

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

Answer: (C)

Preserving hardness samples hinges on keeping the dissolved Ca and Mg in solution and preventing any changes that could alter their concentrations. Hardness can change if carbonates precipitate as the sample sits at higher pH or warmer temperatures, or if microbes alter the chemistry. Acidifying to a strongly acidic pH below 2 with a strong acid such as nitric acid or sulfuric acid stops carbonate precipitation and suppresses biological activity. Cooling to ≤6°C further slows chemical reactions and microbial growth, helping maintain the sample’s original composition for months. This combination provides stable preservation for up to about six months, which is why it’s the best approach. The other strategies would allow precipitation, biological activity, or changes over time, making them unsuitable for preserving hardness samples.

Preserving hardness samples hinges on keeping the dissolved Ca and Mg in solution and preventing any changes that could alter their concentrations. Hardness can change if carbonates precipitate as the sample sits at higher pH or warmer temperatures, or if microbes alter the chemistry. Acidifying to a strongly acidic pH below 2 with a strong acid such as nitric acid or sulfuric acid stops carbonate precipitation and suppresses biological activity. Cooling to ≤6°C further slows chemical reactions and microbial growth, helping maintain the sample’s original composition for months. This combination provides stable preservation for up to about six months, which is why it’s the best approach. The other strategies would allow precipitation, biological activity, or changes over time, making them unsuitable for preserving hardness samples.