4. 4
• Course Contents :
• Introduction and data handling
• Stoichiometric calculations
• General equilibrium concepts
• Gravimetric analysis
• Acid Base Equilibria
• Acid Base Titrations
• Complexometric titrations
• Precipitation titrations
• Oxidation reduction reactions and titrations
• Electrochemical cells and Electrode potentials
•
5. المساق تدريس أسلوب
:
األحيان بعض في والنقاش المحاضرة
المساق مراجع
:
أ
.
الرئيس المرجع
:
Analytical Chemistry, 2004, Gary D. Christian, 6th Ed.
ب
.
اإلضافية المراجع
:
1. Analytical Chemistry, Principles and Techniques, Larry Hargis,
Prentice Hall.
2. Arts and Science of Chemical Analysis, Christie Enke, Wiley
and Sons.
3. Principles of Analytical Chemistry, Miguel Valcarcel, Springer.
4. Fundamentals of Analytical Chemistry, Skoog, Holler, and
West, Brooks/Cole
5
6. 6
6
Areas of Chemical Analysis
and Questions They Answer
Quantitation:
How much of substance X is in the sample?
Detection:
Does the sample contain substance X?
Identification:
What is the identity of the substance in the sample?
Separation:
How can the species of interest be separated from
the sample matrix for better quantitation and
identification?
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7
Chapter 1: Introduction
This course is a quantitative course
where you have seen some qualitative
analysis in general Chemistry Lab and
will also encounter the topic in other
classes.
In addition, Analytical Chemistry can be
classified as Instrumental or Classical
(wet Chemistry).
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8
In this course, we will cover the classical
methods of Chemical Analysis. However, it
should not be implied that the term
classical means something old, which is
studied like history,
but rather the term means understanding the
basics of Chemical Analysis that were
eventually laid down long time ago.
Some of the classical methods still serve as
the standard methods of analysis, till now.
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9
The analytical process involves a sequence of
logical events including:
1. Defining the problem
This means that the analyst should know what
information is required, the type and
amount of sample, the sensitivity and
accuracy of the results, the analytical
method which can be used to achieve these
results, etc...
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2. Obtaining a representative sample
It is very important to collect a
representative sample for analysis. This
could be appreciated if, for example, an
ore is to be analyzed to decide whether
the ore concentration in a mine or
mountain can be economically produced.
One should take several samples from
different locations and depths, mix them
well and then take a sample for analysis.
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11
3. Preparing the sample for analysis
Most analytical methods require a solution of the
sample rather than the solid. Therefore, samples
should be dissolved quantitatively and may be
diluted to the concentration range of the method.
4. Chemical separations
The sample may contain solutes which interfere with
the determination of the analyte. If this is the case,
analytes should be separated from the sample
matrix by an accepted procedure.
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12
5. Performing the measurement
This implies conducting the analytical
procedure and collecting the required data.
6. Calculations
The final event in the analytical process is to
perform the calculations and present the
results in an acceptable manner.
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RANGE
The size of the sample can be used to describe
the class of a method where:
A method can be described as meso if the sample
size is above 100 mg or 100 microliters.
A semimicro method describes a sample size from
10 to 100 mg or 50 to 100 microliters.
When the sample size is in the 1 to 10 mg or less
than 50 microliters, the method is said to be a
micro method
A sample size less than 1 mg denotes an
ultramicro method.
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An analyte in a sample can be classified as a
major constituent if it constitutes more than 1%
of the sample or a minor in the range from 0.1 to
1.0 %. It is classified as trace if it constitutes less
than 0.1%.
Analyze versus Determine
These terms are sometimes misused. Always use
the term ‘analyze’ with the sample while use the
term ‘determine’ with specific analyte. Therefore,
a sample is “analyzed”, while an analyte is
“determined”.
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16
Accuracy and Precision
Accuracy can be defined as the degree of
agreement between a measured value and
the true or accepted value. As the two values
become closer, the measured value is said to
be more accurate.
Precision is defined as the degree of
agreement between replicate measurements
of the same quantity.
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17
Assuming the correct or accepted value is
represented by the center of the circles below;
If all values occurred close together within, for
example, the red or blue circles, results are
precise but not accurate.
If all values occurred within the yellow circle,
results are both accurate and precise.
If results were scattered randomly in one direction
of the center, results are neither precise nor
accurate.
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19
Example
The weight of a person was measured five times
using a scale. The reported weights were 84, 83,
84, 85, and 84 kg. If the weight of the person is
76 kg weighed on a standard scale , then we
know that the results obtained using the first
scale is definitely not accurate.
However, the values of the weights for the five
replicate measurements are very close and
reproducible. Therefore measurements are
precise. Therefore, the measurement is precise
but definitely not accurate.
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20
Math With Significant Figures
Addition and Subtraction
In mathematical operations involving significant figures, the
answer is reported in such a way that it reflects the
reliability of the least precise operation. Let's state that
another way: a chain is no stronger than its weakest link.
An answer is no more precise that the least precise
number used to get the answer. Let's do it one more
time: imagine a team race where you and your team
must finish together. Who dictates the speed of the
team? Of course, the slowest member of the team.
Your answer cannot be MORE precise than the least
precise measurement.
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21
For addition and subtraction, look at the decimal
portion (i.e., to the right of the decimal point) of
the numbers ONLY. Here is what to do:
1) Count the number of significant figures in the
decimal portion of each number in the problem.
(The digits to the left of the decimal place are not
used to determine the number of decimal places
in the final answer.)
2) Add or subtract in the normal fashion.
3) Round the answer to the LEAST number of
places in the decimal portion of any number in
the problem.
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22
Find the formula weight for Ag2MoO4 given the
following atomic weights: Ag = 107.870, Mo =
95.94, O = 15.9994.
The number with the least number of digits after
the decimal point is 95.94 which has two digits
for expression of precision. Also, it is the number
with the highest uncertainty. The atomic weights
for Ag and O have 3 and 4 digits after the
decimal point. Therefore if we calculate the
formula weight we will get 375.6776. However,
the answer should be reported as 375.68 ( i.e. to
the same uncertainty of the least precise value.
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23
Multiplication and Division
The number having the least number of
significant figures is called the KEY
NUMBER. The LEAST number of significant
figures in any number of the problem
determines the number of significant figures
in the answer.
In case where two or more numbers have the
same number of significant figures, the key
number is determined as the number of the
lowest value regardless of decimal point.
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Note that: When the uncertainty of a number is not
known, the uncertainty is assumed to be +-1 of the
last digit to the right
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25
2.5 x 3.42 = ?
The answer to this problem would be 8.6
(which was rounded from the calculator
reading of 8.55). Why?
2.5 is the key number which has two
significant figures while 3.42 has three.
Two significant figures is less precise
than three, so the answer has two
significant figures.
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Relative uncertainty in key number = ( +1/25) = +0.04
Now find the absolute uncertainty in answer:
(8.55/25) = 0.342
Therefore, the uncertainty in the answer should be
known to one decimal point. The answer can be
written as 8.6 + 0.3. The relative uncertainty in
answer can now be calculated:
Srel = (+0.342/8.6) = +0.04
The relative uncertainty in answer is the same as
that of the key number, which should be the
case.
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28
2.33 x 6.085 x 2.1= ? How many significant
figures should be in the answer?
Answer - two.
Which is the key number?
Answer - the 2.1
Why?
It has the least number of significant figures in
the problem. It is, therefore, the least precise
measurement.
Answer = 30.
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Relative uncertainty in key number = ( +1/21) = +0.048
Now find the absolute uncertainty in answer:
(29.77/21) = 1.4
Therefore, the absolute uncertainty in the answer
should be known to integers.
The answer can be written as 30. + 1. The relative
uncertainty in answer can now be calculated:
Srel = (+1/30.) = +0.033
The relative uncertainty in answer is close to that of
the key number, which should be the case.
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30
How many significant figures will the
answer to 3.10 x 4.520 = (Calculator
gives 14.012) have?
3.10 is the key number which has three
significant figures.
Three is supposed to be the correct
answer. 14.0 has three significant
figures. Note that the zero in the tenth's
place is considered significant. All
trailing zeros in the decimal portion are
considered significant.
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31
Another common error is for the student to
think that 14 and 14.0 are the same thing.
THEY ARE NOT. 14.0 is ten times more
precise than 14. The two numbers have the
same value, but they convey different
meanings about how trustworthy they are.
However, the correct answer should be
reported as 14.01. Note that an additional
significant figure is included in the answer.
This is because the answer is less than the
key number.
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Relative uncertainty in key number = ( +1/310) =
+3.2*10-3
Now find the absolute uncertainty in answer:
(14.012/310) = 0.0452
Therefore, the absolute uncertainty in the answer
should be known to one hundredth. The answer can
be written as 14.01 + 0.05. The relative uncertainty
in answer can now be calculated:
Srel = (+0.05/14.01) = +3.5*10-3
The relative uncertainty in answer is very close to that
of the key number, which should be the case.
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33
Why do we add an additional significant
figure in the answer when the answer is
less than the key number?
The answer to this question simply is to
reduce the uncertainty associated with
the answer. When the answer is less
than the key number, the uncertainty
associated with the answer is
unjustifiably large.
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34
(4.52 x 10-4) ÷ (3.980 x 10-6).
How many significant figures in the answer?
Answer - three.
Which is the key number?
Answer - the 4.52 x 10-4.
Why?
It has the least number of significant figures in the
problem. It is, therefore, the least precise
measurement. Notice it is the 4.52 portion that plays
the role of determining significant figures; the
exponential portion plays no role. However, since
the answer is less than the key number, an
additional significant figure is used and written as a
subscript.
Answer = 113.6
35. 35
Relative uncertainty in key number = ( +1/452) =
+2.2*10-3
Now find the absolute uncertainty in answer:
(113.568/452) = 0.251
Therefore, the uncertainty in the answer should be
known to one decimal point. The absolute
uncertainty in the answer is therefore +0.3. The
answer can be written as 113.6 + 0.3. The relative
uncertainty in answer can now be calculated:
Srel = (+0.3/113.6) = +2.6*10-3
The relative uncertainty in answer is very close to that
of the key number, which should be the case.
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36
4.20x3.52 =
Which is the key number?
Both have 3 significant figures. In this case,
the number with smaller value, regardless of
the decimal point, is the key number (3.52).
The correct answer should be reported as
14.78
Once again, we have added an additional
significant figure as a subscript since the
answer is less than the key number
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Uncertainty in key number = ( +1/352) = +2.84*10-3
Now find the absolute uncertainty in answer :
(14.784/352) = 0.042
Therefore, the absolute uncertainty in the answer
should be known to one hundredth. The absolute
uncertainty in the answer is therefore +0.042. The
answer can be written as 14.78 + 0.04. The
relative uncertainty in answer can now be
calculated:
Srel = (+0.042/14.78) = +2.84*10-3
The relative uncertainty in answer is the same as that
of the key number, which should be the case.
