LAB ACT # 2 COMMON LABORATORY PROCEDURES

LAB ACT # 2

COMMON LABORATORY PROCEDURES

Using the Balance

A balance is used to measure the mass of an object. Turn the balance on by pressing the on/off button down. The electronic readout should then be lit. Open one of the sliding doors and be sure the balance pan and surrounding area is clean. You can clean it with a balance brush. Next shut the doors and press the tare bar to set the balance at zero. Now simply place the object to be weighed on the balance and measure the mass to 0.0001 grams.

Always use weighing paper when weighing solids to protect the balance. To do this simply place the weighing paper on the balance pan and be sure it is not touching the side. Press the tare bar on the right side and the balance will then read 0.0000 g. Now add the desired mass of solid and record the mass. Always clean the balance carefully after use. At the end of the period, turn off the balance by pressing the on/off button. Always use the balance with extreme care as it is very expensive.

Handling Solids

Use a clean spatula to transfer solid from bottles. Never use a contaminated spatula. Also, never return unused solid to the reagent bottle. Simply discard it. To avoid waste, never remove more solid from a bottle than is necessary.

Handling Liquids

When transferring liquids from a reagent bottle, always remove the cap/stopper and hold it in your hand. Never place the cap/stopper on the bench or contamination could result. Pour the liquid slowly and carefully to avoid spillage. You may find the use of a glass rod helpful, as shown below.

Capping a Flask

During many experiments you will have to cap a flask to protect the contents from contamination. The figure below illustrates the proper method using Parafilm.

CAPPING A FLASK:

Cut the piece of parafilm and cover the opening.

Place one thumb at one corner and pull gently on the other end to stretch the parafilm.

Pull the long end around the circumference of the opening to form a tight fit.

Measuring Liquid Volumes

Many glassware items have volume marks printed on them. Before using a piece of glassware to make a volume measurement, you should take a moment to study its calibrations to insure that you know how to read them properly. A beaker or Erlenmeyer flask can be used for rather rough measurements. A graduated cylinder of the appropriate size can be used for measurements of moderate accuracy. A pipet is commonly used to transfer an accurately known volume of a liquid from one container to another. However, the accuracy of such a transfer is only as good as the technique of the operator will allow.

In making any volume measurement, the liquid level should always be the same as your eye level. Erlenmeyer flasks and graduated cylinders are usually filled/read by raising them to your eye rather than by squatting down to bring your eye level to the bench top. The liquid level in a pipet is always lowered to the mark while the mark is held steady at eye level.

Burettes: With practice, the position of the meniscus of a liquid in the 50 mL burettes can be estimated to within 0.1 mL. The figure on the right shows the use of a card with a dark strip on it to sharpen the image of the meniscus. You will find by experiment that if the top of the strip is positioned slightly below the level of the liquid in the burette, the bottom of the meniscus will be very easy to see.

You should always use the following procedure when changing the solution in a burette. First, empty the burette out the top and half–fill it with distilled water. Open the stopcock and drain about 5 mL out of the tip. Over the sink, empty the burette out the top by inverting it swiftly, and then repeat the water washing, this time also opening the stopcock when the burette is inverted to allow most of the water to drain back out of the tip. Wait about 30 seconds for drainage and then close the stopcock. While it is still upside down, blot/wipe off the top of the burette with tissue. Then turn it upright, and using a clean beaker for the transfer, add enough of the new solution to bring the liquid level up to about the 48 mL mark. Next, drain part of the liquid out of the tip into a waste receiver, close the stopcock, and wipe off the tip with a laboratory tissue. Then, at the sink, cradle the top of the burette between the thumb and index finger of one hand. While holding it by the tip with your other hand, turn the burette horizontal. While twirling the burette by the tip, slowly empty it through the top, being careful to wet the entire interior wall with the new solution. Repeat this operation two more times. Finally, fill the burette above the zero mark and drain the excess out the tip until the meniscus is within the calibrated portion of the burette. Be sure that no air bubbles are trapped in the tip. Do not attempt to bring the meniscus to 0.00. This method is both time consuming and unwise, since the 0.00 line may not be in precisely the right place.

Pipets: Students often experience some initial difficulty in using a pipet. The following instructions, the illustrations in the figure below and some hands–on practice using distilled water should help you to become proficient fairly quickly. In what follows, we assume that the pipet has been pre–rinsed with the solution you want to transfer following essentially the same procedure as that described above for burettes, except that you must use a bulb to suck the small doses of water or the new liquid into the pipet rather than pouring them in from a beaker.

Using a Pipets:

Drawing the Liquid

Use a bulb with an Eppendorf tip to draw the solution above the mark on the pipet.

Bottoming the pipet makes it easier to remove the bulb and place your finger over the hole before the liquid level falls below the mark.

Adjusting to the Mark

Hold at eye level to bring the meniscus exactly to the pipet mark.

Touch the pipet tip to the back wall of a beaker or receiver while doing this.

Do not let liquid drain back into the original solution bottle to prevent contamination.

Delivering the Liquid

Wipe the pipet barrel using a tissue after lowering the beaker.

Touch the flask wall of the final receiver with the pipet tip, then remove your finger to let the solution drain.

Wait 30 seconds with the tip touching the wall after the pipet looks empty.

Do not blow out the remaining last drop.

To begin a pipetting operation hold the pipet vertical and rest the pointed end on the bottom of the container from which you want to transfer a sample. With your least–dexterous hand, use a rubber bulb fitted with an Eppendorf tip to draw the liquid a few centimeters above the mark on the pipet. If you keep the pipet bottomed, you can then remove the bulb and quickly seal the pipet mouth with the index finger of your "better" hand before the liquid level falls below the mark. You might try conditioning your index fingertip first by rubbing it gently in the palm of the other hand.

Raise the over filled pipet vertically out of the vessel from which you are taking the measured sample and quickly put a beaker or some other waste receiver under it. Raise the mark on the pipet to your eye level, tilt the receiver slightly, and touch the pointed tip of the pipet to a dry spot on its sidewall.

If you now slightly rock your index finger you can open and close a tiny crack at the mouth of the pipet and thereby allow the liquid level in the pipet to fall exactly to the mark on its shaft. Be patient because if you overshoot the mark you must begin the whole process again.

Remove the accurately filled pipet from its container and while still tightly sealing its top with your finger, quickly dry the lower portion of the shaft with a single downward stroke of a tissue. Tilt the final receiver slightly and while holding the pipet vertical, place its tip against the receiver wall so that when take your finger off of the pipet mouth, liquid will flow smoothly down to the bottom of the vessel. You want to avoid splashing as much as possible. Keep the tip of the pipet in contact with the flask sidewall for at least 30 seconds after it looks empty, and then remove it from the receiver.

The pipets in the laboratory are calibrated "to deliver" the specified quantity of liquid rather than "to contain" it. What this really means is that you should never blow the last drops out of them.

Filtration

You will often need to separate a liquid from a solid. At times you will simply decant, that is, you will carefully pour out the liquid, leaving the solid behind. At other times you will need to filter the solution. To do this you will use filter paper and a funnel. You must first flute the paper in order to accelerate the process; this is shown on the figure on the right.

FLUTING THE FILTER PAPER:

11cm filter paper

Fold in half, in fourth, in eighth, and in sixteenth

-fold behind, fold in front

-Unfold to a half circle.

-Bend at arrows 1 and 2 and fold one end back and the other end forward.

-Obtain a quarter of a circle.

-Open inner slit to get a funnel

You will then set the paper in the funnel using your wash bottle. To do this simply place the paper into the funnel and add a small amount of water to the bottom of the filter. Slowly add water to the sides with a circular motion to avoid air bubbles between the paper and the funnel. Once the paper has set, transfer the solution to be filtered. If the solid has settled, decant the liquid through the filter first in order to save time. Never overwhelm the filter; don't add the solution too quickly and never come to within one centimeter of the top of the paper. Transfer the solid using a wash bottle and rubber policeman, and then wash the solid as directed by the experimental procedure.

Heating

You will use both a hot plate and a Bunsen burner to heat solids and solutions. Always be careful to avoid burns and never heat a material too quickly or explosive "bumping" can occur. When using a hot plate always begin at the setting indicated in the manual. However, this setting may vary depending on the hot plate so you will have to experiment. In using a Bunsen burner, always use a tight blue flame as shown in the figure below. Control the heat transfer by adjusting the distance from the burner to the object. Note that the distances suggested in the manual are measured from the hottest part of the flame to the object.

Barometric Readings and Unit Conversions

There are barometers placed in each laboratory room that give the barometric pressure readings in inches of Hg. This measurement must be converted to mmHg. The conversion factor is 1.00 inch = 25.4 mm.

pH Meter Operating Instructions

1. Prepare Electrode

Remove cap

2. 1-Point Calibration

pH Buffer 7

Stir then leave

3. Rinse Electrode

4. 2 or 3-Point Calibration

Repeat step 2 when carrying out a 2 or 3-point calibration, then rinse

5. Measure sample

Sample, Stir, then leave

6. Rinse Electrode

7. Store Electrode