Basic Laboratory Techniques
Introduction
Scientists use measurements in describing objects and these measurements are based on
universally accepted standards. A measurement of height specifies exactly how tall
something is, whereas words such as tall or short are open to a wide range of
interpretations. In the scientific community, standardized measurements (length, weight
and volume) are fundamental to communication.
The metric system of measurement is used for all measurement in most countries of the
world. The scientific community of the entire world expresses data using the metric
system. Therefore, it is necessary for us to know and to be able to effectively use this
measurement system. It is also necessary to be able to convert English units into metric
units.
The metric system units are related to each other by a factor of ten, so interconversions
are done by simply moving the decimal point to the left or right.
The standard unit of length is the meter (m).
The standard unit of mass is the gram (g).
The standard unit of volume is the liter (L).
For measuring time, the second, minute and hour are units that are used.
Names of multiples or fractions of Metric units are formed by adding a prefix to meter,
gram or liter.
Prefix
Symbol
Meaning
Base Unit Multiplied by
teraT
trillion
1,000,000,000,000
gigag
billion
1,000,000,000
megaM
million
1,000,000
kilok
thousand
1,000
hectoh
hundred
100
decada
ten
10
single units, no prefix - Examples: meter, liter, gram 100
decid
tenth
0.1
centic
hundredth
0.01
millim
thousandth
0.001
microu
millionth
0.000001
nanon
billionth
0.000000001
picop
trillionth
0.00000000001
Factor
1012
109
106
103
102
101
10-1
10-2
10-3
10-6
10-9
10-12
VOLUME:WEIGHT CORRELATION: A milliliter is 1/1000 of a liter. This is
approximately equal to 1 cubic centimeter (1 cc or 1 cm3). In addition, 1 ml of pure water
weighs 1 gram at standard temperature and atmospheric pressure. This is a very
convenient conversion (volume:weight) that you should know (i.e. 1 ml water = 1 gram
water and 1 l water = 1 mg water).
Measuring the volume of a liquid with a graduated cylinder:
The surface of a liquid confined in a cylinder curves to form
what is known as a meniscus. The meniscus of most liquids
curves up the sides of the container, making the center of the
curve appear lower than the edges. Since reading the
meniscus at the top or at the bottom of the curve will make a
difference in the volume measured, it is generally agreed to
always read the bottom of the curve. The smaller the
container, the greater the curve of the meniscus. The figure to
the left is the meniscus in a 10 mL graduated cylinder.
Measuring the volume of a liquid with a pipet:
Pipets are much more accurate than graduated cylinders.
Reading the volume of liquid in a pipet is just like reading a
graduated cylinder, however there is one additional technique
needed with a pipet. The diameter does not allow a liquid to
be poured into a pipet - the liquid must be drawn into the
pipet. This picture shows standard pipet pumps used to draw
a liquid into a pipet. You may have to practice using the pump
with a pipet before you are able to accurately transfer a
measured volume of liquid.
How to use a pipet pump:
• Pour slightly more liquid than needed into a beaker using the "ballpark" graduations on
the beaker. Never pipet directly from a reagent bottle.
• Gently twist the pipet pump and press it firmly over the top of the pipet. Do not force
the pump onto the pipet! You may break the glass pipet.
• Place the tip of the pipet below the surface of the liquid in the beaker.
• Slowly, draw in more liquid than needed, but do not allow the liquid to enter the
pump.
Graduated pipets come in a variety of styles: glass, plastic, reusable, disposable, marked
for complete delivery and marked for delivery of a specific volume (i.e. you do not fully
expel the solution from the pipet).
Micropipettors:
A micropipettor is essentially a precision pump
fitted with a disposable tip. The volume of air
space in the barrel is adjusted by screwing the
plunger farther in or out of the piston and the
volume is displayed on the readout. Depressing
the plunger displaces the specified volume of air
from the piston; releasing the plunger creates a
vacuum, which draws an equal volume of fluid
into the tip. The withdrawn fluid is then
expelled by depressing the plunger again.
Please be sure to take the following precautions:
•Never rotate the volume adjustor beyond the
upper or lower range of the pipet.
•Never use the micropipettor without the tip in
place.
•Never invert or lay the micropipettor down
with a filled tip.
•Never immerse the barrel of the micropipettor
in fluid.
•Never reuse a tip that has been used to measure
a different reagent.
Measuring mass with an electronic balance:
The electronic balance has many advantages over other types of balance. The most
obvious is the ease with which a measurement is obtained. All that is needed is to place
an object on the balance pan and the
measurement can be read on the display to
hundredths of a gram. A second advantage, using
the Zero button on the front of the balance, is less
recognized by beginning science students.
Because one must never place a chemical
directly on the balance pan, some container
must be used. Place the container on the balance
and the mass of the container will be displayed.
By pressing the Zero button at this point, the
balance will reset to zero and ignore the mass of
the container. You may now place the substance
to be weighed into the container and the balance will show only the mass of the
substance. This saves calculation time and effort. However, when the container is
removed from the balance, the display will go into negative numbers until the Zero
button is pressed again.
Accuracy and Precision:
The accuracy of an analytical measurement is how close a result comes to the true value.
Accuracy refers to the agreement between a measurement and the true or correct value.
Determining the accuracy of a measurement usually requires calibration of the analytical
method with a known standard. Precision is the reproducibility of multiple measurements
and is usually described by the standard deviation, standard error, or confidence interval.
Precision refers to the repeatability of measurement.
Basic Laboratory Techniques Exercises
1. Complete the following conversions:
0.543 kilograms is equivalent to
________________ grams
1271.72 liters is equivalent to
________________ milliliters
4123.43 centimeters is equivalent to
________________ meters
72.3 milligrams is equivalent to
________________ micrograms
169.3 microliters is equivalent to
________________ milliliters
200 micrometers is equivalent to
________________ centimeters
2. What is the volume of water as seen in the figure for “Measuring the volume of a
liquid with a graduated cylinder” ?
3. Practice small volume transfer with adjustable micropipettors.
1) Obtain one of each size micropipettor, compare volume scales, note the maximum
volume given on top of each pipettor. Find the volume adjust knob, liquid
uptake/dispensing knob and tip eject button.
2) Practice adjusting volume.
DO NOT GO OVER THE RANGE OF THE PIPETTOR!
Set the 1000 l pipettor at 1000 l, 750 l and 275 l. Note that this pipettor has a
numerical readout for the 1000, 100 and 10 places. You may set the pipettor to
within 1l using tick marks at the bottom of the scale.
Set the 200 l pipettor at 200 l, 150 l and 25 l. Note that this pipettor has a
numerical readout for the 100, 10 and 1 places. You may set the pipettor to within
0.1l using tick marks at the bottom of the scale.
Set the 20 l pipettor at 20 l, 10 l and 4.5 l. Note that this pipettor has a
numerical readout for the 10, 1 and 0.1 places. You may set the pipettor to within
0.01l using tick marks at the bottom of the scale.
Large-volume micropipettor:
a) Label two microcentrifuge tubes 1 and 2.
b) Use the table below as a checklist while adding solutions to tubes 1 and 2.
Tube
1
2
Sol. I
100l
150l
Sol. II
200l
250l
Sol. III
150l
350l
Sol. IV
550l
250l
c) Set the micropipettor to add appropriate volumes of solutions I-IV to tubes 1
and 2.
d) A total of 1000l of colored solutions was added to each tube. To check the
accuracy of your measurements, set the micropipettor to 1000l and carefully
with draw the solution from each tube.
Small-volume micropipettor:
a) Label two microcentrifuge tubes 3, 4 and 5.
b) Use the table below as a checklist while adding solutions to tubes 3, 4 and 5.
Tube
3
4
5
Sol. I
4l
4l
4l
Sol. II
5l
5l
4l
Sol. III
1l
1l
Sol. IV
1l
1l
c) Set the micropipettor to add appropriate volumes of solutions I-IV to tubes 3, 4
and 5.
d) Sharply tap the tube bottoms on the bench top. Be certain that all of the drops
have pooled into on drop at the bottom of the tube.
or
Place tubes in the microcentrifuge and apply a short pulse of several seconds.
Make sure that the reaction tubes are placed in a balanced configuration in the
microfuge rotor. Spinning in an unbalanced position will damage the microfuge
motor.
e) A total of 10l of colored solutions was added to each tube. To check the
accuracy of your measurements, set the micropipettor to 10l and carefully with
draw the solution from each tube.
3) Practice accurately transferring specific volumes of water with the 1000l and 200l
pipettors.
1 ml of DI water weighs 1 g; 1 l of water weighs 1 mg
Transfer 1000l of water into a tared beaker on a balance to practice liquid
transfer. Repeat four more times. Record the volume (mass) transferred into your
notebook. Follow this same procedure with the 200 l pipettor and 200 l of
water.
Calculate the average volume dispensed for each pipettor. Calculate your %
accuracy.
% error = (volume wanted – volume dispensed) X 100
volume wanted
% accuracy = 100 - % error
You should aim for + 1% error.
4. Practice large volume transfer with graduated pipettes.
Transfer 10 ml of water with a 10 ml graduated pipette, into a tarred beaker on a
balance to practice liquid transfer. Repeat four more times. Record the volume
(mass) transferred into your notebook. Calculate the average volume dispensed
for the 10 ml pipette. Calculate your % accuracy.