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

Determining the accuracy of varieties of pipettes by weighing and finding the density of a liquid Abstract 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. Introduction 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.

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

Procedure 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 water. The serological pipette had to be filled with water to the last reading which is 10 to get a 1000 µl volume into the weighing boat and then weighed on the balance. A P100 was then used with the serological pipette and the PR1000 mechanical pipette to draw out and weigh100 µl of water into the weighing boat and weighed. The P100 mechanical pipette was calibrated to 100 and the PR1000 mechanical pipette was calibrated to 010 which are both equal to a 100 µl volume.

For the serological pipette, moving down from for example 3 to 4 was equal to a 100 µl volume. The P100 was then set to 050 and 100 to draw out a volume of 50 µl and 100 µl respectively and the PR100 was set to 020 and 100 to draw out a volume of 200 µl and 1000 µl respectively of water and of an unknown liquid for it to be weighed and recorded to help determine the density. Results Table 1| | My data (pure water)| | | Pipettor| Volume| Mass 1| Mass 2| Mass 3| AverageMass| Standard Deviation| P100| 50 µl| 0. 050 g| 0. 050 g| 0. 050 g| 0. 050 g| | 0| | P100| 100 µl| 0. 99 g| 0. 098 g| 0. 100 g| 0. 099 g| 0. 001| PR1000| 200 µl| 0. 199 g| 0. 199 g| 0. 200 g| 0. 1993 g| 0. 00057| PR1000| 1000 µl| 0. 998 g| 1. 002 g| 1. 002 g| 1. 0007 g| 0. 00230| Table 1 shows the weight of the water (grams) in each volume (Micro Litres) and the graph below (Fig. 3) shows the mean/average mass against the volume. Density of pure water = ? Y/? X =1. 007-0. 099 g/1000-100 µl =0. 908 g/900 µl =0. 001g/ µl =1g/ml Table 2| | My data (unknown Sample B)| | | Pipettor| Volume| Mass 1| Mass 2| Mass 3| AverageMass| Standard Deviation| P100| 50 µl| 0. 50 g| 0. 050 g| 0. 052 g| 0. 0507 g| 0. 001155| P100| 100 µl| 0. 099 g| 0. 098 g| 0. 101 g| 0. 0993 g| 0. 001528| PR1000| 200 µl| 0. 202 g| 0. 201 g| 0. 200 g| 0. 201 g| 0. 001| PR1000| 1000 µl| 1. 112 g| 1. 112 g| 1. 114 g| 1. 1127 g| 0. 001155| Table 2 shows the weight (grams) of the unknown sample B in each volume (Micro Litres) and the graph below (Fig. 1) shows the mean/average mass against the volume. Density of pure water = ? Y/? X =1. 1127-0. 0993 g/1000-100 µl =1. 0134 g/900 µl =0. 001126g/ µl =1. 13g/ml Table 3| | Group 71 (unknown sample A)| | |

Pipettor| Volume| 1| 2| 3| Average| Standard Deviation| P100| 50 µl| 0. 050 g| 0. 045 g| 0. 046 g| 0. 047 g| 0. 002646| P100| 100 µl| 0. 096 g| 0. 101 g| 0. 105 g| 0. 1007 g| 0. 004509| PR1000| 200 µl| 0. 216 g| 0. 198 g| 0. 200 g| 0. 2047 g| 0. 009866| PR1000| 1000 µl| 1. 030 g| 1. 016 g| 1. 030 g| 1. 0253 g| 0. 008083| Table 3 shows the weight (grams) of the unknown sample A from group 71in each volume (Micro Litres) and the graph below (Fig. 2) shows the mean/average mass against the volume. Density of pure water = ? Y/? X =1. 0253-0. 007 g/1000-100 µl =0. 9246 g/900 µl =0. 001027g/ µl =1. 03g/ml Discussion The density of water was 1. 00 g/ml which was accurate because that is the exact density of water however the density of unknown liquid A was found out to be 1. 13 g/ml much denser than that of water and the density of unknown liquid A from group 71 was found to be 1. 03 g/ml slightly denser than that of water. This is because of dissolved substances it contained which increased the weight of the liquid hence increasing the density since density = mass/volume.

However the standard deviation values for group 71 was much higher showing that there were a lot of fluctuations in their readings (low precision) and this may be due to systematic error. The balance was very sensitive to pressure changes and so gave fluctuating values so in order to obtain a much more constant value the working table had to be left steady without any vibrations. This may have caused fluctuations in the readings of the weight for group 71 causing them to have such a low value of density. So the density of sample A from Group 17 I would say was inaccurate and not precise.

On the other hand there were fewer fluctuations in my group’s readings of the weight and so the standard deviation was less meaning more precision and accuracy was obtained. The first two parts of the practical helped me gain some experience and techniques for example the pipettes for each volume was used because smaller values like 50 and 100 µl were best measured using the P100 rather than the PR1000 and volumes like 200 and 1000 µl were best measured with the PR1000 as the P100 cannot measure such huge values. References 1. Zeman GH, Mathewson NS. “Necessity of prerinsing disposable polypropylene pipet tips,” Clin Chem 1974; 20(4)497-8. . 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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