Experiment 2:          Accuracy in Measuring Samples

 

 

            In the laboratory, accurate measurement of samples is essential for getting valid, reproducible results.  Taking the time to carefully weigh out a sample could be the difference between an experiment that works and one that doesn’t.  The following laboratory exercise will familiarize you with the many different ways to measure your sample and the advantages and limitations of each method.

 

Measurement of a Solid:

 

            The most common way to measure a solid is by using a balance.  The major drawback here is there are several different types of balances that can be used.  The first type is the triple beam balance.  This balances uses a series of counter weights to determine the weight of the sample that is placed on a weighing pan.  This is the least accurate of the balances that are available for sample weighing, because a triple beam balance can only measure to the ones place (no decimal measurement).  This can give you a very rough determination of the weight of your sample. Triple beam balances are also useful for measuring heavy objects because they often can measure up to several hundred pounds.

            The second type of balance is the digital top loading balance.  These are more accurate than the triple beam balance because they can usually measure to several decimal places.  The top loading balances in this chemistry laboratory measures to two decimal places.  Top loading balances are useful for measuring chemicals for large quantity solutions.  Top loading balances can usually measure to 400 – 600 grams, but anything heavier would max out the balance and you wouldn’t be able to get a measurement.

            The third type of balance that can be used to measure a solid is the analytical balance.  This is the most accurate balance available in our chemistry lab.  Most analytical balances can measure to four or more decimal places.  Analytical balances are used to measure small quantities for solutions and for research experiments where the high degree of accuracy that can be achieved by this balance is important. 

            When weighing a solid sample, you should use a weighing bottle to get the most accurate measurement.  Occasionally, weighing paper or a weigh boat may be used, but both of these require additional handling that may skew your sample weight.  When you weighing a sample, the weighing bottle should be clean and dry.  You should never handle the weighing bottle with your hands once it has been cleaned and dried – the oils from your skin could add to the weight of the sample.  You should handle the weighing bottle with tongs or a test tube holder (depending on the size of the weighing bottle).  You can also use the paper method for handling a weighing bottle.

            In the paper method, take a piece of weighing paper and fold it lengthwise several times until you have a long, thin strap of paper.  Wrap the paper around the weighing bottle and use the two overlapping pieces as a handle.  When carrying a weighing bottle to and from your workstation, you should also rest the weighing bottle on a wire gauze to prevent contact with your skin.  Between wire gauze and the tongs / paper, you should be able to safely carry the weighing bottle to your workstation from the balance.  (See Figure 1).

 

 

 

 

 

 

 

 

 

Figure 1: Paper Method

 

Measurement of a Liquid:

 

            There are many different ways to measure a liquid also.  Here to, the degree of accuracy varies based on what method you use.  The least accurate measurement is achieved by using the marked gradations on a beaker or flask.  These marks are approximate and usually have a 5% error margin.  This is fine if you need 300 ml of water to rinse a buret – accuracy here isn’t important to the experiment.

            Next, there is the graduated cylinder.  Graduated cylinders are accurate for large whole number measurements.  They are not useful for decimal measurements, as the markings are not that detailed.  Graduated cylinders are fine if you need to add 255 ml of acetone to your reaction vessel, as most graduated cylinders are divided in whole number gradations.  When measuring with a sample with a graduated cylinder, you should choose a cylinder that is no larger than 10 times the volume you want to measure.  For example, if you need to measure 1 ml of liquid, you should use a 10 ml graduated cylinder instead of a 1 liter graduated cylinder.  Look at your graduated cylinder.  At the top of most graduated cylinders, there is a small TC20O stamped on the glass.  This tells you that the graduated cylinder is manufactured to measure a liquid accurately at 20O C.  If you are measuring a liquid that is hotter or colder than this temperature, your measurement may not be completely accurate.

            Another type of liquid measuring device is the volumetric flask.  This type of flask comes in varying sizes, but they only have one gradation.  They are specifically designed to make solutions of a particular quantity.  For example, if you wanted to make one liter of a 0.9% salt solution, you would add 9 grams of sodium chloride to enough water to fill the one liter volumetric flask to the gradation.  This would give you an accurate measure of the solution you just made.  However, you can only use these flasks to make solutions in the amounts that the flasks are manufactured for – usually 1 liter, 500 ml, 250 ml, 100 ml, 50 ml, and 25 ml.  You couldn’t use them to make 750 ml of solution.

            Pipettes can also be used to accurately measure liquids.  There are two types of pipettes that can be used to accurately measure a liquid.  The first is the standard pipette or a Mohr pipet.  They generally come is varying sizes (from 1 ml to 50 ml) with different size gradations marked on them.  They are stamped at the top with their accuracy (usually +/- so many milliliters) and TD20O, which means total delivery at 20O Celcius.  When using a pipette, some type of additional device is necessary to draw the liquid up into the chamber.  There are several different types of pipetting devices available.  Some are shown in Figure 2.

            The other type of pipette is the volumetric pipette.  It is made the same way the volumetric flask is made – there is only one gradation marked on the pipette.  This type of pipette is used to measure a specific amount of liquid only.  They usually come in varying sizes – from 1 ml to 50 ml.  As long as you measure accurately to the line, you will have the marked amount of liquid.  Other pipettes are used to transfer liquids, but can’t be used to accurately measure how much liquid you are transferring (unless you just need to count drops).

            The most accurate method to measure liquids is by using a micropipettor.  Micropipettors are generally used to measure liquids in units smaller than the milliliter, although there is a micropipettor that can measure up to 1 milliliter.  These devices generally measure in the micrometer range and are used in biotechnology labs to measure very small quantities.  Micropipettors require special tips that are placed on the end of the pipette before liquid dispersal.  The tips are usually sterilized prior to use and are disposed of after one use.

 

 

 

 

 

 

 

 

 

 

 

 

Figure 2: Pipettes and Pipettors

 

            The last liquid measurement device is the buret.  Burets are useful for dispensing liquids while performing an experiment.  Burets usually consist of a small diameter graduated tube with a capillary tip and a flow device called a stopcock.  This allows for the dispensing of varying amounts of liquid into a reaction tube.  The scale is commonly 0 – 50 ml with 0.1 ml divisions.  We will discuss the use of a buret in later experiments.

            When a liquid is placed into any glass container, the surface of the liquid appears curved.  This curve is called the meniscus and requires special attention when reading the volume of the liquid.  When the curve is concave, the bottom of the meniscus is read.  Reading the meniscus is done by placing the glass container on a flat surface and bringing the eyes level with the gradations.  The volume can accurately be read from this angle.  If the liquid is clear, it sometimes helps to put a lined paper behind the container to use as a reference point.  When the curve is convex, the top of the meniscus is read in the same manner as the concave liquid.

 


Experimental Procedure:

 

Equipment:                                                           Chemicals:

 

Triple beam balance                                                             Lead Shot

Digital top loading balance                                                 Steel Shot

Analytical balance                                                             Water

Beaker (50 ml, 100 ml)                                                           

Volumetric Flask (50 ml)

Graduated cylinder (100 ml)

Volumetric Pipette (10 ml)

Pipette (10 ml)

Mechanical Pipettors (green)

Rubber bulb (blue)

Micropipettor

Micropipette Tips

Transfer Pipette

Pasteur Pipette

Weighing Bottle

Crucible Tongs

Test Tube Holder

 

Procedure:

 

Measurement of a Solid Sample:

 

  1. Using the triple beam balance, make sure that when the weighing pan is empty, the scale is balanced.  Remember to push all of the weights to the left at zero.
  2. Weigh 5 pieces of steel shot.  The steel shot may be placed directly on the weighing pan.  Record your data on the data sheet.  Remember to record the mass in the correct number of significant figures.
  3. Repeat the procedure with the lead shot and record your data on the data sheet in the correct number of significant figures.
  4. Using the digital top loading balance, weigh the steel shot.  For this balance you must use a weighing bottle.  Place the bottle on the balance and wait for the digital read-out to stabilize.  Record this mass on your data sheet in the correct number of significant figures..
  5. Hit the tare or zero button to subtract the weight of the bottle.  The digital read-out should read all zeros.
  6. Add the shot and record the mass on your data sheet in the correct number of significant figures.
  7. Remove the weighing bottle and hit the tare or zero button again to re-zero the digital readout.
  8. Repeat the procedure with the lead shot.
  9. Using the analytical balance, weigh the steel shot.  For this balance, you must use a weighing bottle and the doors of the balance must be closed when recording your reading.
  10. Place the weighing bottle on the balance and close the doors.  Record the mass of the bottle on your data sheet in the correct number of significant figures.
  11. Hit the tare or zero button to reset the digital read-out to zero.  Add the steel shot, close the door, and record your mass on the data sheet in the correct number of significant figures.
  12. Remove the weighing bottle and hit the tare or zero button.
  13. Repeat the procedure with the lead shot.
  14. Return the steel and lead shot to their appropriate containers.

 

Measurement of a Liquid Sample:

 

  1. Using a 100 ml beaker, measure out 50 ml of water.
  2. Pour that 50 ml of water into the 100 ml graduated cylinder.  Is the volume actually 50 ml?  If not, what is the actual volume?  Record this volume on your data sheet.
  3. If there was less than 50 ml of water in the graduated cylinder, add more water until there is exactly 50 ml.  If there was more than 50 ml of water in the graduated cylinder, pour out the excess water.
  4. Pour the 50 ml of water from the graduated cylinder into the 50 ml volumetric flask.  Is the volume of water more or less than 50 ml?  Why can’t you record an accurate volume when there is more or less than 50 ml in the flask?
  5. Pour the water back into the 100 ml beaker.
  6. With a standard 10 ml pipette and a green pipettor, transfer 10 ml of water from the 100 ml beaker to a 50 ml beaker.
  7. With a 10 ml volumetric pipette, remove the water from the 50 ml beaker.  Was there exactly 10 ml of water in the beaker?  Was there too much water or not enough?  Why can’t you tell exactly how much water was present?
  8. Put the water back into the 100 ml beaker.
  9. With the micropipettor, transfer 1 ml of water from the 100 ml beaker to the 50 ml beaker.  Use the following procedure:
    1. Check the micropipettor to make sure that the scale reads 1000.  Your instructor will demonstrate how to adjust the micropipettor if it is not set at 1000.
    2. Seat the tip firmly on the end of the micropipettor.  Do not touch the tip with your hands.
    3. Push the plunger down to the first stop.  There are actually 3 stops on the micropipettor.  The first stop draws up the required amount of fluid.  The second stop will blow out any bubbles that remain in the tip after dispensing the liquid.  The third stop pushes the tip off of the micropipettor.  Some micropipettors have a separate button for tip removal.
    4. Once the plunger has been depressed to the first stop, hold the micropipettor at the stop, and put the tip of the micropipettor into the water.  Slowly let the plunger up until it is completely back to its original starting point.
    5. Remove the micropipettor from the water.
    6. Put the micropipettor tip into the 50 ml beaker, not touching the sides or bottom of the beaker.
    7. Depress the plunger to the second stop, removing all the liquid from the tip.
    8. Remove the micropipettor from the beaker and remove the tip by pushing the plunger to the third stop.  Throw the tip in the trash.
  10. With a plastic transfer pipette, pick up the 1 ml of water from the 50 ml beaker.  Can you tell if you have all of the 1 ml?  Why isn’t this type of pipette good for volumetric measurements?
  11. Put a blue rubber bulb on the end of a Pasteur pipette.  Draw up into the pipette water from the 100 ml beaker.  Can you tell how much liquid you have just collected?

 

Data Sheet:

 

Measurement of a Solid Sample:

 

  1. Mass of steel shot from triple beam balance:                                                                      
  2. Mass of lead shot from triple beam balance:                                                                      
  3. Mass of weighing bottle from top loading balance:                                                          
  4. Mass of steel shot from top loading balance:                                                                      
  5. Mass of lead shot from top loading balance:                                                                      
  6. Mass of the weighing bottle from analytical balance:                                              
  7. Mass of the steel shot from analytical balance:                                                          
  8. Mass of lead shot from analytical balance:                                                                      

 

Measurement of a Liquid Sample:

 

  1. Volume in the graduated cylinder:                                                                                  
  2. Answer any other questions in the space below: