Determining the Accuracy of Varieties of Pipettes by Weighing and Finding the Density of a Liquid

Topics: Pipette, Density, Kilogram Pages: 4 (1327 words) Published: November 4, 2012
Determining the accuracy of varieties of pipettes by weighing and finding the density of a liquid

Different varieties of pipettes (P100, PR1000 pipettors and a serological pipette) along with a weighing balance was used in this investigation to check the density of an unknown liquid by first finding out what the liquid weighed and plotting a graph of the mass in grams against the volume of the liquid used in micro litres (µl) and finding the gradient of the graph (∆Y/∆X). Most of the standard deviation values were small showing that they were very close to the mean (average) hence more precise readings were obtained. The results from the graph also showed that the density of the unknown liquid A and B was denser that water and that is because it contains other dissolved substances which increased the overall weight.

Pipetting requires a lot of skill to acquire precision and accuracy. For example the pipette tip must be pre-wet about three times before it is dipped into the solution [1] because dry and pre-wet pipette tips reveal greatest discrepancies. Dry pipette tips consistently deliver significantly lower volumes than did the pre-wet tips [2-6]. This can influence the readings showing on the balance as lower volumes will record lower mass and higher volume will record a higher mass which will in turn affect the density which will be measured making the results of the investigation incorrect. Using the right pipette for the right volume increases its accuracy for example using a PR1000 to pipette out 50 µl of a solution will be less accurate than using a P100 to pipette out 50 µl of a solution.

Apparatus: P100 pipettor, PR1000 pipettor, serological pipette, pipette aid, weighing balance, weighing boat, sample solutions (pure water and an unknown solution).

The serological pipette sucks liquid via the pipette aid. It was first used alongside the PR1000 set to a 100 to pipette out 1000 µl volume of...

References: 1. Zeman GH, Mathewson NS. “Necessity of prerinsing disposable polypropylene pipet tips,” Clin Chem 1974; 20(4)497-8.
2. Sternberg JC. “Sampling with air-piston pipettes – a critical study,” Clin Chem 1975; 21(7):1037.
3. Ellis KJ. “Errors inherent in the use of piston activated pipettes,” Anal Biochem 1973; 55:609-14.
4. Wenk RE, Lustgarten JA. “Technology of manually operated sampler pipettes,” Clin Chem 1973; 20(3):320-3.
5. Joyce DN, Tyler JPP. “Accuracy, precision and temperature dependence of disposable tip pipettes,” Med Lab Technol 1973; 30:331-4.
6. Lochner KH, Ballwegt, Fahrenkrog HH. “Factors influencing the measuring accuracy of piston pipettes with air interface (German),” J. Lab Med 1996; 20(7/8)430-440.
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