How can you verify the accuracy of a pipette?

1 Gravimetric method

The gravimetric method is a way of measuring the mass of water dispensed by a pipette at a given volume and temperature. This method is suitable for pipettes that deliver volumes between 1 and 1000 microliters. To complete the gravimetric method, you will need a balance, a container, distilled water, and a thermometer. The steps to follow include placing the container on the balance and taring it to zero, filling the pipette with distilled water and adjusting the meniscus to the desired volume, dispensing the water into the container and recording the mass displayed on the balance, repeating this process at least 10 times for each volume setting and calculating the average mass, measuring the temperature of the water and using a table or formula to find its density at that temperature, dividing the average mass by this density to obtain an average volume, comparing this average volume with the nominal volume of the pipette to calculate error and coefficient of variation, and finally calibrating the pipette or contacting its manufacturer if these values are outside of acceptable ranges.

2 Photometric method

The photometric method is a process used to measure the absorbance of a colored solution that is dispensed by a pipette at a known concentration and volume. This method is suitable for pipettes that deliver volumes below 10 microliters and requires a spectrophotometer, a cuvette, a standard solution, and a diluent. The steps to follow include preparing a standard solution of a colored compound, such as potassium permanganate, at a known concentration and filling the pipette with the solution. Additionally, one must dispense the solution into the cuvette and fill it up with the diluent to a fixed volume before measuring the absorbance at a specific wavelength. After repeating this process at least 10 times for each volume setting, calculate the average absorbance and use the Beer-Lambert law to find the average volume from the average absorbance, concentration, and path length. Then compare the average volume with the nominal volume of the pipette to determine any errors or coefficient of variation. If either are outside an acceptable range, calibrate or contact the manufacturer. By using this method, one can verify their pipettes' accuracy and ensure laboratory work is consistent and reliable. It's important to regularly perform these checks as well as follow manufacturer instructions for cleaning, storing, and maintaining your pipettes.

3 Here’s what else to consider

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