Ciencia

EXPERIMENT 01 Precision and Accuracy of volumetric pipettes
Pipettes are used to dispense a given volume of liquid

Precision: how reproducible are the volumes dispensed? Accuracy: how close thevolumes dispensed are to the nominal volume?

Type of Errors
Systematic Errors (e.g. flaws in the experimental design) Accuracy = nearness to the true value

Random Error (e.g. fluctuations dueto instrumental noise) Precision = reproducibility of the results

Absolute Uncertainty ( g (e.g. ±0.02ml for a calibrated buret) ) Relative Uncertainty = = Absolute Uncertainity / Magnitude ofMeasurement

Statistics: Gaussian Distribution
Number of measu urements

x

(precision)

(accuracy) label
9.7 9.8

volume (mL)

9.9

10.0 10.1 10.2

Average Standard deviation

x

1n

n

xi
i 1

1 n 1i

n 1

xi

x

2

Evaluation of Precision
x 1 n
n i 1

xi
(%) 100

1 n 1i

n 1

xi

x

2

x

Evaluation of Accuracy
xtrue (%) x xtrue xlabelxtrue

100

Background Tools for measuring mass and volume of liquids
• Pipettes • Volumetric Flask • Buret • Analytical balance measures volume measures volume measures volume measures mass[volume] = [mass] / [density]

Pipettes
Used to measure small amounts of solution very accurately

Squeeze the bulb in your hand / Place the bulb on the pipet flat end / Place pipet tip in thesolution / Release your grip on the bulb / Draw solution in above the mark / Remove the pipet bulb and put your index finger / Gently release the seal made by your finger until the level of the solutionmeniscus exactly lines up with the mark on the pipet.

Volumetric Flask
Used to make up a solution of fixed volume very accurately

• Dissolve the solid material completely, in less water thanrequired and carefully fill the flask • Add water a d t drop at a ti t time until th b tt til the bottom of f the meniscus lines up exactly with the mark (the circle around the neck should look like...