Which statement describes the direct method for osmometry?

Which statement describes the direct method for osmometry?

• A. Direct/Measured uses colligative properties; freezing point depression is the most common direct method
• B. Direct/Measured uses calculated formulas based on sodium, glucose, and BUN
• C. Indirect/Calculated uses freezing point depression
• D. Indirect/Calculated measures osmolality by density only

Answer: (A)

Direct osmometry determines osmolality by measuring a physical property that changes with the number of dissolved particles—the colligative properties. The most common direct method is freezing point depression: as solute particles increase, the solution’s freezing point lowers in a predictable way, so the instrument reports osmolality from the observed freezing point. This approach doesn’t depend on knowing which solutes are present, only how many particles there are. By contrast, calculating osmolality from measured substances like sodium, glucose, and BUN is an indirect method, and using density alone isn’t how indirect osmolality is determined. So the statement that direct/Measured uses colligative properties and that freezing point depression is the most common direct method is the best description.

Direct osmometry determines osmolality by measuring a physical property that changes with the number of dissolved particles—the colligative properties. The most common direct method is freezing point depression: as solute particles increase, the solution’s freezing point lowers in a predictable way, so the instrument reports osmolality from the observed freezing point. This approach doesn’t depend on knowing which solutes are present, only how many particles there are. By contrast, calculating osmolality from measured substances like sodium, glucose, and BUN is an indirect method, and using density alone isn’t how indirect osmolality is determined. So the statement that direct/Measured uses colligative properties and that freezing point depression is the most common direct method is the best description.