Avogadro and all that  (Page 2/3)

  3. Measuring the volume of liquids

When working with liquids, we usually describe the quantity of the liquid in terms of volume, with the usual units being milliliters (mL). We use three types of glassware to measure volume – (1) burette, (2) bulb pipette, and (3) graduated cylinder. A volumetric flask will also allow for a high degree of accuracy and precision in the measurements of any liquids, so a 100 mL volumetric flask will contain precisely 100.0 mL of solution when filled to the line marked on the neck of the flask.

• Examine each piece of equipment. Note that the sides of each are graduated for the graduated cylinder and the burette. The bulb pipette delivers a specific volume, 10.00 mL. The burette will be used to deliver a variable volume of solution and will also be precise to two decimal places.
• 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. Since volumes in graduated cylinders are only precise to one decimal place, a graduated cylinder is generally only used when a high degree of precision is not required .

1. Using your 10mL graduated cylinder, add water up to the 10 mL line as accurately as possible.
2. Dry a small beaker and weigh it (2).
3. Pour the 10 mL of water from the cylinder into the beaker. Reweigh (1).
4. Subtract the appropriate values to get the weight of the water (3). 

Bulb Pipette

1. Half-fill a beaker with water.
2. 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.
3. When the bottom of the meniscus is above the line on the pipette, remove the pipette from the water.
4. Squeeze the bulb to run the extra water into a waste container until the bottom of the meniscus is level with the line on the pipette.
5. Add 10 mL of water to a pre-weighed dry beaker (5).
6. Weigh (4).
7. Subtract to get the weight of the water (6). 

Burette

1. Examine the graduations. Note that 0 is at the top. Note that the stopcock is horizontal to close the burette and vertical to open it.
2. First, lower the burette so that the top is easy to reach and make sure the burette is closed. Using a funnel, add about 10 mL of water.
3. Open the burette and run a little water into a waste container. Then turn the burette upside down and allow the rest of the water to run into the container.
4. 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.
5. Fill the burette to any convenient level (half-way is fine). It is a good technique to add more liquid than you need, and allow some 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.
6. Dry a beaker and weigh (8).
7. Add 10 mL of water to a pre-weighed dry beaker (7).
8. Subtract to get the weight of the water (9).