Initial lab: avogradro and all that  (Page 2/4)

 3. Measuring the volume of liquids

When working with liquids, we usually describe the quantity of the liquid in terms of volume, usual units being milliliters (mL). We use three types of glassware to measure volume – (1) burette, (2) volumetric pipette, and (3) graduated cylinder.

• Examine each piece of equipment. Note that the sides of each are graduated for the graduated cylinder and the burette. You can read each to the accuracy of half a division.
• Put some water into the graduated cylinder. Bend down and examine the side of the water level. Note that it has a "curved shape." This is due to the water clinging to the glass sides and is called the meniscus. When reading any liquid level, use the center of the meniscus as your reference point. 

Graduated cylinder

• Look at the graduations on the side of the cylinder. Note that they go from 0 on the bottom and increase upwards. Thus, to get the mass of 10 mL of a liquid from a graduated cylinder, do the following:
• Add water up to the 10 mL line as accurately as possible.
• Dry a small beaker and weigh it (2).
• Pour the 10 mL of water from the cylinder into the beaker. Reweigh (1).
• Subtract the appropriate values to get the weight of the water (3). 

Pipette

• You may find either that 0 is at the spout end or at the top of the pipette. You should be aware of how these graduations go when using each pipette. Thus, to get the mass of 10 mL of a liquid from a pipette, do the following:
• Half-fill a beaker with water.
• Squeeze the pipette bulb and attach to the top of the pipette. Put the spout of the pipette under water and release the bulb. It should expand, drawing the water into the pipette, do not let the water be drawn into the bulb.
• When the water level is past the last graduation, remove the bulb, replace with your finger, and then remove the pipette from the water.
• Removal of your finger will allow liquid to leave the pipette. Always run some liquid into a waste container in order to leave the level at an easy-to-read mark.
• Add 10 mL of water to a pre-weighed dry beaker (5).
• Weigh (4).
• Subtract to get the weight of the water (6). 

Burette

• Examine the graduations. Note that 0 is at the top.
• Using a funnel, add about 10 mL of water. To do this, first lower the burette so that the top is easy to reach.
• Run a little water from the burette into a waste container. Then turn the burette upside down and allow the rest of the water to run into the container (you will have to open the top to equalize the pressure).
• You have just "rinsed your burette." This should be done every time before using a burette – first rinse with water, then repeat the process using whatever liquid is needed in the experiment.
• Fill the burette to any convenient level (half-way is fine). It is a good technique to "overfill" and then allow liquid to run into a waste container until you reach the appropriate level so that you fill the space from the top to the tip of the burette.
• Dry a beaker and weigh (8).
• Add 10 mL of water to a pre-weighed dry beaker (7).
• Subtract to get the weight of the water (9).