Transcript Fundamental Measurements

Fundamental Techniques and
Measurements
 Mass
Measurements
 Volume Measurements
 Preparation of a solution of known
concentration
 UV-Visible Spectrophotometer
Electronic Balance
 What
does an electronic balance measure?
_____
force
 If you took an electronic balance with a
capacity of 100 g to the moon what would
its range be? _____
600 g
Mass: Electronic Balance

Accuracy


4 to 6 significant
digits
Calibration
Use known mass
 Check weekly or
when balance is
moved


Sources of error
Balance must be calibrated and
maintained in same orientation in
_________
field
gravity
 hydroscopic chemicals: dry to
constant mass first (will increase in
mass rapidly as they reabsorb water
on the balance!)
 When preparing a solution of a given
concentration it may be difficult to get
the exact mass desired
 evaporation of wet samples

Electronic Balance
M o d el
D I-100
D I-800
D I-5000
 For
Cap acity Reso lutio n
100 g
0.0001 g
800 g
0.01 g
5000 g
0.1 g
maximum accuracy use balance with
lowest
_______
capacity possible!
 Don’t forget to clean the balance if you spill
any chemicals!!!!!!
Volume
 Volumetric
 accuracy
 Graduated
 accuracy
flask
0.16 mL
of ______/100
mL
cylinder
0.6 mL
of ______/100
mL
 Beaker
 accuracy
5 mL
of _____/100
mL
What will accuracy of
solution be if you use
pipette, volumetric
flask, and electronic
1%
balance? ________
What controls the
Pipette
accuracy? _______
 Pipette
 accuracy
0.6% for 100-1000 µL
of ± _____
0.8% for 10-100 µl
 accuracy of ± _____
Digital Pipettes
 Air
displacement
 Do not directly contact fluid volume
 avoids
contamination of pipette
 avoids sample carryover
 Require
body
air tight connection between tip and
Pipette Workings
piston
cylinder
Pipette tip
Pipettes: Sources of Error
 Jetting
 Incorrect
transfer technique (getting too
much sample) Wipe tip on container to remove droplets
 Contamination from previous samples
 Viscous fluids
 Hot or cold fluids
 Fluids with high vapor pressure
Preparation of Solutions
Example: Prepare 100 mL of a 30 mM solution of
methylene blue.
 The molecular weight of methylene blue
(C16H18N3SCl) is 319.87 g.

CV M
30 x 10-3 mole MB 319.87 g MB
mole MB
L
concentration
conversion
100 x 10-3 L = 0.9596 g MB
volume
mass
Preparation of Dilutions
 Prepare
100 mL of a 300 µM solution from
the 30 mM solution
 Conservation
mass
of _____
M dilute  M concentrat e
CdiluteVdilute  Cconcentrat eVconcentrat e
Vconcentrate
Vconcentrate
CdiluteVdilute

Cconcentrate
300 M fa
100 mLf
a

a30 mMf
= 1 mL
UV-Visible Spectrophotometer
 Theory
 Instrument
 Sample
requirements
 Software
Light Attenuation by an Aqueous
Solution
P0
dP
P
 kP
dx
P0
dP
P
dx
P is light intensity
(photons/s)
P
dP
P0
p

x
   kdx
0
P
ln    kx
P 
 0
k C
Theory: Light Attenuation = f(?)
 For
a given excitation process, a molecule
absorbs only one discrete amount of energy:
expect very narrow absorption lines.
 Different vibrational and rotational states
yield _______
broad absorption lines.
 Exponential decay with distance
A  log
Po
P
= bc
A=bc
 Po
- _________
incident light intensity
 P light intensity after passing through
sample
length
 b - path
______________
 c - ______________
concentration
  - ___________
extinction coefficient (function of
wavelength and molecule)
Absorption Spectra

Absorption Spectra for Methylene Blue
Broad peaks
blue
red looks ______
 Absorbs _____,

Instrument Light Path
Diode Array
Spectrograph Lens
Grat ing
Slit
Sam ple Cell
Shut ter
Source Lens
Deut erium Lamp
Absorbance Measurement
Limitations
lamp
is a function of the _____.
 If absorbance is high what is P? ______
small
 Suppose A = 3, what is Po/P? _____
1000
 Suppose I create samples of higher and
higher concentration. What will happen to
the absorbance measurements?
 Po
minimum (non zero) P that
There is a _________
A  log = bc can be measured by an instrument.
P
A _______
doesn’t keep increasing! Amax 3
Po
Sample Requirements
 Sipper
cell
 peristaltic
pump draws sample into sipper cell
 requires a few mL to displace previous cell
contents
sample
Light source
pump
detector
Software

Reference (single sample)



Standards (multiple samples)


subtracts absorbance of sample
cell and reference solution
usually distilled water or reagent
blank
calibration
used to create a __________
curve
Samples (multiple samples)

after sampling standards can be
used to estimate the
concentration of samples
Maximum Absorbance

Max absorbance f()
lamp intensity
 ________________
detector sensitivity
 ________________
cell absorbance
 ________________
reference absorbance
 ________________
acceptable error
 ________________

absorbance readings
that exceed this value
will not be used in
analysis
Standards
your name
general description
rinse time
sample time
sample concentrations
select number of
samples by moving
this control
Samples
enter sample
descriptions here
select number of samples
by moving this control