Which analyte has preservation that requires acidification to pH < 2 and can be stored up to 28 days?

Which analyte has preservation that requires acidification to pH < 2 and can be stored up to 28 days?

• A. Phosphate
• B. Oil and Grease
• C. Nitrite
• D. Dissolved Oxygen

Answer: (D)

Preserving dissolved oxygen samples relies on acidification to a very low pH and careful storage to keep the sample stable until analysis. In the Winkler method, dissolved oxygen is captured and then stabilized by acidifying the sample (typically to pH below 2). This strong acidity inhibits biological activity that would otherwise consume oxygen and also helps ensure the chemical reactions needed for the titration proceed correctly when you measure the oxygen later. Because of this stabilization, acidified samples can be stored for a relatively extended period—up to about 28 days in the dark at the correct temperature—before analysis without significant loss or alteration of the measured oxygen concentration. Other analytes have different preservation requirements and do not use this same low-pH, long-hold approach, which is why this specific preservation protocol uniquely fits dissolved oxygen.

Preserving dissolved oxygen samples relies on acidification to a very low pH and careful storage to keep the sample stable until analysis. In the Winkler method, dissolved oxygen is captured and then stabilized by acidifying the sample (typically to pH below 2). This strong acidity inhibits biological activity that would otherwise consume oxygen and also helps ensure the chemical reactions needed for the titration proceed correctly when you measure the oxygen later.

Because of this stabilization, acidified samples can be stored for a relatively extended period—up to about 28 days in the dark at the correct temperature—before analysis without significant loss or alteration of the measured oxygen concentration.

Other analytes have different preservation requirements and do not use this same low-pH, long-hold approach, which is why this specific preservation protocol uniquely fits dissolved oxygen.