LAB EXERCISE: 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). Furthermore, 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).
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.
Statistical Significance:
Tests of Significance allow biologists to estimate the probability that any differences
between their experimental results and the predictions of the null hypothesis are due to
chance alone. This probability is abbreviated as P. Usually, biologists accept a level of
uncertainty of 5% or less (P < 0.05). A P = 0.05 means that at most (estimated) a 5%
chance that you will be wrong if you reject the null hypothesis (the hypothesized value of
the parameter is called the null hypothesis). A difference between the observed results
and those predicted under the null hypothesis that is large enough to produce such a small
P value is termed a significant difference.
Confidence Intervals:
A confidence interval gives an estimated range of values which is likely to include an
unknown population parameter, the estimated range being calculated from a given set of
sample data. If independent samples are taken repeatedly from the same population, and a
confidence interval calculated for each sample, then a certain percentage (confidence
level) of the intervals will include the unknown population parameter . Confidence
intervals are usually calculated so that this percentage is 95%, but we can produce 90%,
99%, 99.9%, confidence intervals for the unknown parameter. The width of the
confidence interval gives us some idea about how uncertain we are about the unknown
parameter (see precision). A very wide interval may indicate that more data should be
collected before anything very definite can be said about the parameter. Confidence
intervals can be more informative than the simple results of hypothesis tests (where we
decide "reject H0" or "don't reject H0") since they provide a range of plausible values for
the unknown parameter.
Graphing:
Scientists always want to present their results with the least amount of error. There are
many ways to help reduce error. One way is to average your results. This means to take
all of your results and add them together and then divide by the number of things that you
added; if you measured something five times, then add the five measurements together
and divide by 5.
Another way to help reduce the error is by plotting the results on a graph and then
drawing a best fit line or curve. When drawing a best fit line, plot the data points and then
place your ruler along them. Move your ruler around until you get about the same number
of points above and below the ruler edge remembering to guide by the first and last
points. Once you have positioned the ruler, use it to draw your line. Some of the points
may land on the line while others may be scattered far away. You may not get any on the
line at all. This line shows where the points would have been if there had been no error.
The closer the points are to the line, the more accurate they are.
Best Fit Line
--- = Correct
--- = Incorrect
Not everything can be plotted in a linear graph. Many measurements in astronomy have
graphs that curve up and down. In these cases you need to draw a best fit curve. Plot your
data points and notice how the dots tend to suggest a direction for the curve. DO NOT
connect-the-dots when drawing the curve! Remember, we are trying to get rid of the
error. Instead, draw a freehand smooth curve that follows the general trend of the points.
Best Fit Curve
--- = Correct
--- = Incorrect
Name: __________________________
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. Each group has been provided five pieces of string.
a) Measure the length of each piece.
b) Convert measurement into metric values.
c) Weigh each piece.
d) Graph length (cm) vs. weight (g)
e) Graph length (m) vs. weight (mg) How do the two graphs (graph for d and
graph for e) compare to one another?
Table 1: Measured values of individual pieces of string.
Piece #
Length
(in)
Length
(cm)
Length
(m)
Weight
(g)
Weight
(mg)
1
2
3
4
5
d) Graph your results using a line graph. Properly label the x and y axis.
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1
0
0
0.5
1
1.5
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2.5
Figure 1. _______________________________________________________________
________________________________________________________________________
e) Graph your results using a line graph. Label the x and y axis.
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0
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Figure 2. _______________________________________________________________
________________________________________________________________________
4.
a) Practice pipeting 10ml of water, using both a 5ml pipet and a 10ml pipet.
b) Weigh out 10ml of water using 10ml pipet, five times. Calculate mean,
deviation and squared deviation. Record data in the table below.
10ml pipet
Trial #1 Trial #2 Trial #3 Trial #4 Trial #5
(g)
(g)
(g)
(g)
(g)
Sum
Water Weight
Deviation
(Data- mean = d)
Squared Deviation
(d 2)
Record your mean (sum of water weight for all trials/number of trials):
_____________________________
c) Weigh out 10ml of water using 5ml pipet, five times. Calculate mean, deviation and
squared deviation. Record data in the table below.
5ml pipet
Trial #1 Trial #2 Trial #3 Trial #4 Trial #5
(g)
(g)
(g)
(g)
(g)
Water Weight
Deviation
(Data- mean = d)
Squared Deviation
(d 2)
Record your mean (sum of water weight for all trials/number of trials):
_____________________________
Sum
d) Calculate standard deviation, standard error (SE), confidence interval (CI), upper and
lower confidence limits for both 10ml and 5ml trials. Record your calculations in
the table below.
10ml pipet
5ml pipet
Stdev = sqrt (sum d2 / ( n-1))
SE = sd / sqrt ( n)
CI = 2*SE
UCL = mean + CI
LCL = mean - CI
e) Graph your results. Label the x and y axis.
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0
0
0.5
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2.5
Figure 3. _______________________________________________________________
________________________________________________________________________