How is a basic pipette accuracy check performed using the gravimetric method?

• A. Weigh distilled water at a set volume with the pipette and compare to a known mass for a single trial.
• B. Weigh distilled water at a set volume with the pipette, repeat several times, calculate mean and percent error, and verify within specifications.
• C. Check the pipette by measuring dry air displacement and comparing to the advertised volume.
• D. Use a color change indicator to confirm volume delivery.

Answer: (B)

Gravimetric pipette accuracy relies on measuring the actual mass of water delivered at the set volume and converting that mass to volume using water density. In practice you tare a receiving vessel, fill the pipette to the calibration mark, deliver into the tared vessel, and weigh the collected water. Repeat several times to obtain a set of masses. From those masses you calculate the delivered volumes (mass divided by density) and then compute the mean volume and the percent error relative to the nominal volume, verifying that all results stay within specified limits. Repeating helps reduce random error and flags any systematic issues like leaks or incomplete delivery. This method is preferred because mass is a direct proxy for volume for liquids like water, and density is well-known at the calibration temperature.

A single trial doesn’t capture variability, so it’s less reliable. Measuring dry air displacement isn’t the gravimetric approach, and using a color-indicator has no relation to precise volume delivery.