Exercise #3
Scientific Measurement and Analysis
In the previous exercise you were introduced to finite units
of measure.
measure
Ex: distance, volume, etc
Scientific data are often recorded as a finite value against some
standard value such as time or distance.
-For example, watching an ice cube melt may have several
conditions worth considerations; how long it took to melt,
etc.
Exercise #3
Scientific Measurement and Analysis
Graphic Analysis is used when collecting data as described in the
pprevious slide.
Graphic analysis allows one to demonstrate the
relationship of an experimental outcome to a standard value.
Using this type of analysis, we might see that the relative weight
change of our ice cube was not constant, but varied as the mass
of the cube decreased with time.
In this lab you will use graphic analysis to interpret your data.
Exercise #3
Scientific Measurement and Analysis
In this lab you will use a spectrophotometer to obtain measured
or finite data about a chemical compound,
compound methylene blue
blue.
From the data obtained you will be able to evaluate some of the
properties
i off this
hi compoundd at various
i
concentrations.
i
Furthermore,
F h
the information that you obtain will be plotted graphically, allowing
yyou to determine the concentration of an unknown methylene
y
blue
sample using your graph.
Exercise #3
Scientific Measurement and Analysis
The final goal of the lab is to acquaint the student with the concepts
off accuracy andd experimental
i
t l error.
Error is so common in experimental work that statistical methods
have been developed to establish the amount of error in a given set of
data.
During this lab you will calculate calculate your experimental error
and determine is this error falls within acceptable ranges.
This lab will allow you to calculate your deviation from an expected
result and to evaluate the significance of the deviation.
Exercise #3
Scientific Measurement and Analysis
VOLUMETRIC DETERMINATION AND ERROR
During
D
i this
hi portion
i off the
h lab
l b you will
ill be
b asked
k d to measure specific
ifi
volumes of water using a pipette (a glass cylinder used for volumetric
determination)) and pplace the solution into a small beaker.
Your lab instructor will demonstrate the correct use of the pipette.
When the liquid is introduced into the pipette the liquids in the tube
cling to the sides, creating a U-shaped appearance at the top of the
fluid column termed a meniscus.
meniscus
Exercise #3
Scientific Measurement and Analysis
The image below demonstrates the shape of the meniscus in the
pipette.
To read the volume uniformly, the measurement of volume is
always taken at the bottom of the meniscus.
Exercise #3
Scientific Measurement and Analysis
You will be given a list of volumes of distilled water to measure and
transfer to an empty beaker.
beaker
(Before you begin the experiment, make sure to weigh the
empty beaker and record its weight.)
After you have completed your pipette transfers you must again
weigh the beaker in order to determine your accuracy.
To determine
d
i your accuracy you will
ill measure the
h weight
i h off the
h
water and determine what the water should have weighed if you
had been 100% accurate.
Exercise #3
Scientific Measurement and Analysis
In science, accuracy is sometimes evaluated by percent error-- how
close you came to what you expected to get.
H close
How
l
you came to
t the
th expected
t d result
lt (the
(th theoretical
th
ti l value)
value
l )
can be determined using the total weight of your beaker and the water
that you transferred to it.
For example, assume that the total expected volume of
water transferred is 40 ml and that the actual weight of the
transferred water is 38.76 grams.
In other words, you should have transferred 40 ml
of water and the amount of water transferred is
38.76 grams.
Exercise #3
Scientific Measurement and Analysis
Since the theoretical amount transferred is in ml (40 ml) and the
actual weight of the water is 38
38.76
76 grams,
grams you must convert your
theoretical volume into a weight (comparative values must be in
the same unit).
In order to do this, you multiply the theoretical volume by
its density (0.9976 g/ml).
40 ml x 0.9976 g
1 ml
=
39.90 grams
Exercise #3
Scientific Measurement and Analysis
So, your water sample should have weighed 39.90 g, but it actually
weighs
i h 38.76.
38 76
To calculate % error,
error plug your numbers into the following
equation:
q
% error = experimental value - theoretical value
Theoretical value
What is your percent error?
X 100
Exercise #3
Scientific Measurement and Analysis
SPECTROPHOTOMETRY
Light is composed of photons of different wavelengths. When
visible light is passed through a prism it is broken up into its
constituent wavelengths which appear as different colors.
These colors can be seen in a rainbow: blue, indigo, violet, green
yellow orange and red.
yellow,
red
When these wavelength contact a biological pigment, the
pigment
p
g
can either absorb or reflect the wavelength.
g
We perceive reflected wavelengths.
Exercise #3
Scientific Measurement and Analysis
A spectrophotometer is a device that can break light down into
specific
p
wavelengths
g and determine how much of that light
g is
absorbed by a solution.
In this portion of the laboratory you will use the spectrophotometer
to determine the absorbance of various wavelengths of light
by a methylene blue solution.
The procedure for the use of the spectrophotometer is presented
on the next slide.
Exercise #3
Scientific Measurement and Analysis
1) Select absorbance by depressing the % T/A selector switch.
2) Select the correct wavelength using the knob on right top of
the machine near the clear glass window.
3) A clean glass cuvette filled 2/3 with deionized water should
be placed in the sample compartment.
4) Cl
Close the
th lid off the
th sample
l compartment
t
t andd adjust
dj t the
th digital
di it l
display on the absorbance scale to “zero” by adjusting the knob
on the right front of the machine.
5) Remove the “blank”
blank cuvette and insert the sample cuvette
filled with methylene blue into the sample compartment.
6) Read the digital display for the amount of absorbance and
record the value.
7) Remove the sample tube and repeat steps 4 - 6 for the remaining wavelengths.
Exercise #3
Scientific Measurement and Analysis
Click on the following image for a short video clip regarding the
pproper
p use of the spectrophotometer.
p
p
Exercise #3
Scientific Measurement and Analysis
Once you have collected your data you will be asked to present it
as a graph using the basic principles of graphic analysis.
analysis
Wavelength should be plotted on the x-axis and absorbance
on the y-axis.
-If necessary, refer to the discussion on graphic
analysis in Exercise #2 for a brief review of
ggraphing
p g techniques.
q
Exercise #3
Scientific Measurement and Analysis
ABSORBANCE CURVES, CONCENTRATION AND UNKNOWNS
During this portion of the laboratory you will be asked to create
a series of methylene blue solutions that vary in concentration.
The spectrophotometer will then be used to determine the
absorbance of each methylene blue solution at a wavelength
of 575 nm.
You will then construct a graph of absorbance vs.
vs
concentration and use this graph to determine the
experimental concentration of an unknown methylene
blue solution.
Exercise #3
Scientific Measurement and Analysis
To calculate the concentration of each methylene blue solution, you
take the amount of stock solution and multiply by the original
concentration of that solution. To find the final concentration you
divide this value by the total volume of the tube.
For example, in tube #3 you place 2 ml of dye and 6 ml of water.
Since the original concentration of the methylene blue stock
solution is 00.02
02 mg/ml
mg/ml, you multiply 2ml x 0.02
0 02 mg/ml and divide
by 8 (total volume of the tube).
2 ml x 0.02 mg/ml = 0.04 mg/8 ml = 0.005 mg/ml
Exercise #3
Scientific Measurement and Analysis
When all tubes are constructed they should resemble the image
depicted below:
Exercise #3
Scientific Measurement and Analysis
Use the procedure previously discussed and determine the absorbance
of each tube at 575 nm.
Don’t forget to determine the absorbance for the unknown
methylene blue sample in your tray as well (cork on top of
t be)
tube).
When all the data has been collected you must, once again, present
your data in graphic form.
In this instance, concentration will be plotted on the x-axis and
absorbance on the y-axis.
Be sure to carefully set the units so they are uniform
across the axis of the graph.
Exercise #3
Scientific Measurement and Analysis
Once you have plotted your points of absorbance for your graph,
use the concept of a best
best-fit
fit line to complete the graph.
Your graph should resemble the following:
Use a ruler and construct
a bestbest-fit line that
represents the data in
the graph.
graph
A
Wavelength (nm)
Exercise #3
Scientific Measurement and Analysis
Once the graph has been constructed take the absorbance value
for your unknown solution and find where this point intersects the
best-fit
best
fit line on the graph.
graph
Extrapolate down to the x-axis from this point and determine
the experimental concentration of your unknown.
Ex:
A
Wavelength (nm)
Experimental
concentration
Exercise #3
Scientific Measurement and Analysis
When you have determined the experimental concentration of the
unknown solution present it to your instructor and he/she will
inform you of the actual concentration.
Use these values and calculate your % error.
Exercise #3
Scientific Measurement and Analysis
This lab has demonstrated:
1) scientific
i tifi error and
d % error calculation;
l l ti
2) the appropriate use of the spectrophotometer and
the construction of an absorption spectrum for
methylene blue;
3) the concept of concentration and the utilization of
a graph to determine the experimental concentration
of an unknown methylene blue solution.