Which parameter's storage time is 24 hours or 48 hours, requiring cooling?

Which parameter's storage time is 24 hours or 48 hours, requiring cooling?

• A. Color
• B. Alkalinity
• C. BOD
• D. Conductivity

Answer: (A)

The key idea here is how holding times and preservation affect sample integrity. Some water-quality parameters need cooling or other preservation to stay representative between sampling and analysis. For color, the measurement can change if the sample sits around—due to light exposure, microbial activity, or chemical reactions in the bottle. Cooling the sample to about 4°C slows these changes, extending the time you can hold the sample before measuring. That’s why color is associated with a 24-hour (and sometimes up to 48-hour) holding window when refrigerated. Other options don’t fit this 24/48-hour cooling hold pattern as neatly. Alkalinity is relatively stable and is often managed by chemical preservation rather than cooling. BOD requires a 5-day incubation to develop the biological oxygen demand, so 24 or 48 hours wouldn’t be the standard holding time. Conductivity is also quite stable and typically doesn’t require a strict cooling hold window.

The key idea here is how holding times and preservation affect sample integrity. Some water-quality parameters need cooling or other preservation to stay representative between sampling and analysis. For color, the measurement can change if the sample sits around—due to light exposure, microbial activity, or chemical reactions in the bottle. Cooling the sample to about 4°C slows these changes, extending the time you can hold the sample before measuring. That’s why color is associated with a 24-hour (and sometimes up to 48-hour) holding window when refrigerated.

Other options don’t fit this 24/48-hour cooling hold pattern as neatly. Alkalinity is relatively stable and is often managed by chemical preservation rather than cooling. BOD requires a 5-day incubation to develop the biological oxygen demand, so 24 or 48 hours wouldn’t be the standard holding time. Conductivity is also quite stable and typically doesn’t require a strict cooling hold window.