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Appendix 1 - Precision of Measuring Devices

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    216827
  • General Information

    Analytical instruments are calibrated to a specified precision and if you were told to transfer 5 ml of water you would consider that value to have one significant digit, but if you were told to use a 5 ml pipet to transfer water the number of significant digits would depend on the calibration value of that instrument.  In our teaching lab a 5 ml pipet is calibrated to the centiliter and so the value would have 3 significant digits, but if you were working in an analytical lab that need a higher precision, you would be using more expensive equipment and there would be more significant digits.  So always look at your instruments and determine their calibration value and then record those values to the proper number of significant digits.

    There are two ways to calibrate volumetric instruments depending on if you want the instrument to contain the volume or transfer it.

    TC Volumetric Calibration

    TC stands for To Contain.  These are devices that contain the stated volume.  A volumetric flask is a TC device and when it is properly filled it contains the stated volume of a fluid.  If you then pour that fluid and any remains behind, the amount you transfer is less than the stated.  A graduated cylinder may be TC, but it also may be TD, and you must look at it.  The graduated cylinders in the general chemistry labs are usually TC, but you must look.  Note when empty the scale reads zero (it contains nothing) and when full it contains the quantity it is calibrated to contain (this may not be true for graduated cylinders).

    TD Volumetric Calibration

    TD stands for To Deliver.  These devices deliver the stated volume.  A burette and pipet are TD as they are designed to transfer fluid, while a graduated cylinder may or may not be TD.  A pipet has no scale and is designed to deliver only one volumetric quantity of liquid and you must read the calibration value on the pipet to determine the number of significant digits. A burette has a scale and the number of significant digits is determined by the scale (report one more unit than the number of certain units, as you can guess how far between the lowest unit of the scale the value is).  Because the burette has a scale, it can transfer multiple values, you simply calculate the difference between the initial and final values.  But the scale of a burette is to deliver, so when it is full, the value is zero, and the numbers go up as the volume in the burette goes down.  Typically, you do not blow out the last bit of fluid in a TD device, but you must always look at the manufacturer's instructions, as sometimes you do. (In this class we do not blow out the last bit of fluid).

     

    Precision in Chemistry Lab

    The following values you will need to know, and these are the values that will be associated with these instruments in this class.  Realize these are not universal values and you may find other instruments that are calibrated to a different precision.

     

    Burette

     

    image002.gif
    Figure \(\PageIndex{1}\): Precision Based on Equipment

    Pipet

     

    image004.jpg
    Figure \(\PageIndex{2}\): Precision Based on Equipment

     

    Value Precision
    5mL 5.00mL
    10mL 10.00mL
    25mL 25.00mL
    50mL 50.00mL

    Volumetric Flask

     

    image006.jpg
    Figure \(\PageIndex{3}\): Precision Based on Equipment
    Value Precision
    25mL 25.00mL
    50mL 50.00mL
    100mL 100.0mL
    250mL 250.0mL
    500mL 500.0mL

     

    All other devices we will use have a precision based on the scale

     

    Note, the actual value for the precision of volumetric apparatus depends on the calibration of the equipment, and in general, the more expensive equipment will be more precise.  We will use these values in this class, but upper level courses may have more precise equipment.

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