Pipetting accuracy not only depends on the conditions of the lab pipettes, but also on our pipetting technique. The way to know whether our pipetting is accurate is repeat the steps for many times. Then, the standard deviation is calculated and this is our pipetting errors. In this practical, since our standard deviations are zero, this means our pipetting was accurate.
The accuracy can also be determined by comparing the expected mass with the mean mass that gained in practical. For the volume of 738.5 μL (0.7385ml), since the density should be one, by deriving the formula of density = mass/volume, the expected mass is 0.7385g. By comparing the expected mass (0.7385g) with the mean mass (0.7100) that gained from the practical, the difference of the values is small (0.0285g), so
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By comparing the expected mass (0.0785g) with the mean mass (0.0700) that gained from the practical, the difference of the values is small (0.0085g), so this means that the accuracy is high.
For the volume of 7.8 μL (0.0078ml), since the density should be one, by deriving the formula of density = mass/volume, the expected mass is 0.0078g. By comparing the expected mass (0.0078g) with the mean mass (0.0100) that gained from the practical, the difference of the values is small (0.0022g), so this means that the accuracy is high.
When 790μL of absolute ethanol is pipetted, the pipette drips. This may because absolute ethanol is the liquid with high vapour pressure force air cushion to expand. Besides, ethanol has lower density. This will increase the size of air cushion. Techniques such as reverse pipetting and pre-wetting of the tip can be used to prevent this condition, so that may be why the pipette does not drip for the second time during the practical. However, these techniques can only be used to obtain better accuracy, dripping still