Accuracy = of a measurement is how close that measurement is to the true or “exact” value EX: Standard weight = 5.00g 4.98g more accurate than 5.12 g Accuracy is also subject to the reliability of the measuring instrument. 1
Precision Precision = making reproducible or repetitive measurements of the same quantity How fine the divisions are There will always be some uncertainty because of the limits in the accuracy of your instruments 2
Determining the % Accuracy of a Balance All balances DO NOT measure items with the same precision. Accuracy can be expressed in two ways: 1. % accuracy = indicates how close the experimental value is to the standard or reference value 2. % error = describes hour far off the experimental value is from the stander or reference value. 3
Determining the % Accuracy of a Balance To calculate the % accuracy of the balance you are using % Accuracy = experimental value X 100% reference or standard value The larger this value, the greater the degree of accuracy. 4
Determining the % ERROR of a Balance To calculate the % error of the balance you are using % error = standard mass – experimental mass X 100% standard mass The smaller this value, the greater the degree of accuracy. 5
Absolute Value = considered the amount of mass difference between zero and the determined mass - you are determining how far “off” the standard the mass is - recorded as a positive answer even if the value resulted in a negative number 6
Comparing % Accuracy of Multi Beam Balance and Digital Balances % Accuracy = experimental value X 100% reference or standard value 7
Precision of Repeated Measurements By Pipetting • Pipetting involves drawing a liquid into a pipet and allowing liquid to drain from the pipet in a controlled manner. • Pipetting is used to quantitatively transfer exact volumes of a liquid from one container to another.
Mohr pipet Has graduations that allow it to deliver a range of volumes.
Serological pipets have no base mark (the graduations continue onto the tip) and are graduated to deliver. That means you let all the (measured) liquid in the pipet exit. Graduated to the tip
The volumetric pipet has a single graduation that allows it to deliver one specific volume accurately.
Reading a Pipette Smallest Unit Increment Total Volume
Reading the Volume • Determine the volume of solution in a pipet by reading the bottom of the meniscus at eye level. • Record the volume using all certain digits and one uncertain digit. Certain digits are obtained from calibration marks. • Uncertain digits (the last digit in the number) are estimated between calibration marks. • What are the volumes in these pipets?
Dispensing Liquid • Accurately transferring a specified volume of liquid from one container to another requires careful attention to detail. • Pay special attention to the level of liquid in the pipet and carefully monitor the meniscus.
Using a 3-Way Safety Bulb • Make sure there is enough solution in your beaker to completely fill the pipet. • Never pipet a reagent directly from a reagent bottle. • Carefully place the attachment of the three-way bulb over the mouth of the pipet.
Squeeze the air valve (A) and the bulb simultaneously to empty the bulb of air. • Place the tip of the pipet below the solution's surface in the beaker. Gradually squeeze the suction valve (S) to draw liquid into the pipet. • If the level of the solution is not high enough, squeeze the air valve (A) and the bulb again to expel the air from the bulb. • Draw up more liquid by squeezing the suction valve (S).
When the liquid is above the specified volume, stop squeezing the suction valve (S). • Do not remove the bulb from the pipet. • Do not allow liquid to enter the pipet bulb. • Use the expel (E) valve to release the solution.