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I) Qualitative (as discussed) Applying UV-vis Spectrophotometry
fromqualitative information as to structure bothmolecular structure, therefore it providesThe UV-vis absorption spectrum is related to
(^) ε (^) and
(^) λ
. In order to take advantage of
computer interface.instrument or a single beam unit with(either a double beam, direct readinginstrument to measure the entire spectrumthis it is desirable to have a scanning
By itself an electronic spectrum is not sufficient to
identify a compound (while an IR spectrum is)
II) Quantitative
1 chemical & spectroscopic reasons):analysis for several reasons (bothparticularly well suited to quantitativeThe UV-vis region of the spectrum is )
Large values for
ε make it
- possible to measure low concentrations
Many compounds (both organic &
is a limiting factor in IR.but most common solvents do not. Thisinorganic) absorb in the UV-vis region,
Spectrophotometric Titrations
Example where titrant absorbs
Sample + Titrant
Product
MnO
4
2+
MnO
2
3+
Volume of Titrant Added
A
Endpoint
at the endpointReaction incomplete
absorbs
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Can calculate
ε ML
(^) from A at excess [L]
Can calculate equilibrium constant (K) fromGet n from extrapolated curve
[ML
n ]
and mass balances
K = -----------
[M] [L]
n
e.g. C
M = [M] + [ML
n ]
Method or the Method of Continuous Variationprocedures, e.g. Job’scomputer curve fittinginvolving graphical ormethods can be usedOther calculation
A
Mole ratio
Study Kinetics
X + Y
applications, weFor quantitativeafter mixingMeasure A vs timeZ
usually want to measure at one particular
λ .
most suitableMust first scan spectrum to determine the
λ
for measurements.
A
time
absorbs
imperfections in glass (square cells too)each measurement because ofshould be positioned the same way forRound cells, e.g. Spectronic 20 cells
Must: a)Source drift – in single beam instruments.
warm upAllow source sufficient time to
b)
Check 0 and
00 % T settings
frequently
going through sampleStray Radiation – strikes detector without
oI
A
true
= log ------
I
I
o
s
where I
s is intensity
A
measured
= log -----------
I + I
s
of stray radiation
characteristic of monochromatorsStray Light Specification – important
readouts this usually involves:reading A value; today with digitalReading Error – random error involved in
1 ) taking the readingNot allowing instrument to stabilize before
than the instrument is capable ofAssuming greater precision (or accuracy)
Biasing the result
e.g.
1 9 + 0.
and
1 (^) + 0.
Answer is NO! Are these 2 readings different from one another?
What about if s.d. is is 0.
1 ?
Scale Expansion
can be improved upon by expanding scale
Normally use a blank solution to set
00% T
& set 0% T with shutter closed (i.e. no light)
Expand scale by using a standard that has 1 (for example) 50% T & set instrument to 00% T
this expands lower end of scale
accuracy & precisionT to 50% T range to be made with greaterby a factor of 2 & allows readings in the 0%
