CHEM352 UNIT 1 analytical chemistry .pdf

Analytical Chemistry Notes

1. Analytical Chemistry is the study of the separation, identification and quantification
of the chemical components of natural and artificial materials.
CHEM 352
ANALYTICAL CHEMISTRY
Qualitative Analysis = Identification
Quantitative analysis = Amount
Classical methods (wet chemistry) use separation such as precipitation, extraction and distillation,
gravimetric and titrimetric measurementsand qualitative analysis by color, odour, or melting point
boiling,.
Instrumental methods use an apparatus to measure physical quantities of analytesuch as light
absorption or emission, fluorescence, conductivity, mass-to-charge ratio, etc.The separation of
materials is accomplished by using chromatography or electrophoresis methods.
1

2. Types of Instrumental Methods
1. Spectroscopic methods:a.Atomic spectroscopy
b.Molecular spectroscopy
2. Chromatographic methods (separations):
3. Electrochemistry:

3. 3
Examples Of uses
•The concentrations of oxygen and of carbon dioxide are determined in millions of blood
samples every day and used to diagnose and treat illnesses.
•Smog-control is done by the measurement of quantities of hydrocarbons, nitrogen
oxides, and carbon monoxide in automobile exhaust.
•Analytical chemistry helps diagnose parathyroid diseases in humans measurements of
ionized calcium in blood serum.
•Determination of nitrogen in foods establishes their protein content and thus their
nutritional value.
•Analysis of steel during its production permits adjustment in the concentrations of such
elements as carbon, nickel, and chromium to achieve a desired strength, hardness,
corrosion resistance, and ductility.
Analytical chemistry plays a vital role in the development of science.

4. ELECTRONIC WEIGHING BALANCE
Principle of Operation:
When an object is placed on an electronic weighing
balance, it exerts pressure and pushes the pan down with a
forceequal to masstimes accelerationof gravity.
F = m.a
Balances do not directly measure mass; they measure the force
(weight) that acts downward on the balance pan and uses an
electromagnetic force to restore the pan to its original position. The
electric current required to generate the force is proportional to the
mass of the object being weighed, which is displayed on a digital
readout.
4

5. 5

6. Preventing Weighing Errors:
1.Do not touch the weighing vessel/glasswaresdirectly with your hand.
Always use gloves, tweezers, paper towel or tissues
3. A sample that has been dried in an oven should be cooled down to room temperature
in order to avoid errors due to convective air currents.
2. Place the balance away from radiators
6

7. Burettes:
a graduated glass tube with a tap at one end, for measuring the volume of a liquid delivered,
especially in titrations
Volumetric Flasks:
A volumetric flask is calibrated to contain a particular volume of
water at 20
o
C.
It is used to prepare a solution of a known volume.
7

8. Pipettes:
A pipette is a laboratory tool to dispense a measured volume of liquid.
8

9. Calibration of volumetric glasswares
Effect of Temperature
Example:
A dilute aqueous solution with a molarity of 0.03146M was prepared at 17
o
C/ What will the
molarity of that solution be at 25
o
C?
The density of water 25
o
C = 0.99705 g/mL
The density of water 17
o
C = 0.99878 g/mL
9
??????
1
??????
1
=
??????
2
??????
2
C
1
at 25
o
C
0.99705 g/mL
=
0.03146 M at 17
o
C

0.99878 g/mL
C
1
at 25
o
C = 0.03141 M
Difference: 0.03146 –0.03141 = 0.00005
i.e. the concentration has decreased by 0.16%

10. 10
SAMPLING
LOT
BULK SAMPLE
HOMOGENOUS LAB SAMPLES
ALIQUOT ALIQUOTALIQUOT

11. -The most important step is the collection of the sample of the
material to be analyzed
-Sample should be representative of the material
-Sample should be properly taken to provide reliable
characterization of the material
-Sufficient amount must be taken for all analysis
Representative Sample
-Reflects the true value and distribution of analytein the
original material
SAMPLING

12. Steps in Sampling Process
-Gross representative sample is collected from the lot
-Portions of gross sample is taken from various parts of material
Aliquot
-Quantitative amount of a test portion of sample solution
SAMPLING

13. -Care must be taken since collection tools and storage
containers can contaminate samples
-Make room for multiple test portions of sample for replicate
analysis or analysis by more than one technique
Samples may undergo
-grinding
-chopping
-milling
-cutting
SAMPLING

14. Liquid Samples
-May be collected as grab samples or composite samples
-Adequate stirring is necessary to obtain representative sample
-Stirring may not be desired under certain conditions
(analysis of oily layer on water)
-Undesired solid materials are removed by filtration
or centrifugation
-Layers of immiscible liquids may be separated with the
separatoryfunnel
SAMPLING

15. Solid Samples
-The most difficult to sample since least homogeneous
compared to gases and liquids
-Large amounts are difficult to stir
-Must undergo size reduction (milling, drilling, crushing, etc.)
to homogenize sample
-Adsorbed water is often removed by oven drying
SAMPLING

16. 16
SIGNIFICANT FIGURES
The number of significant figures is the minimum number of digits needed to write a
given value in scientific notation without loss of accuracy.
Significant Figure Rules:
1.All non-zero digits are significant.
For example, 123.45 has five significant digits (1, 2, 3, 4 and 5)
2. All zeros between other significant figures are significant.
e.g. In 20405, both the zeroes are significant figures.
3. The number of significant figures is determined starting with the leftmost non-zero digit.
The leftmost non-zero digit is sometimes called the most significant figure.
e.g. in the number 0.004205, the digit “4” is the most significant figure.
The left-hand “0”s are not significant.
4. The rightmost digit of a decimal number is the least significant figure but it is still significant.
5. If no decimal point is present, the rightmost non-zero digit is the least significant figure.
e.g. In the number 5800, the least significant figure is ‘8’

17. ROUNDING OFF
17
ROUNDING UP
ROUNDING DOWN

18. 18
Accuracy & Precision
Accuracy indicates how close a measured value is to true value
i.e. agreement between experimental mean and true value
Precision is a measure of the reproducibility of a result.
High accuracy, low precision High accuracy, high precision
Low accuracy, low precision
Low accuracy, high precision
Illustrating the difference between “accuracy” and “precision”

19. ERRORS
-Two principal types of errors
-Determinate(systematic) and Indeterminate (random)
Determinate (Systematic) Errors
-Caused by faults in procedure or instrument
-Fault can be found out and corrected
-Results in good precision but poor accuracy
May be
-constant (incorrect calibration of pH meter or mass balance)
-variable (change in volume due to temperature changes)
-additive or multiplicative
Examples of Determinate (Systematic) Errors
-Uncalibratedor improperly calibrated mass balances
-Improperly calibrated volumetric flasks and pipettes
-Analyst error (misreading or inexperience)
-Incorrect technique
-Contaminated or impure or decomposed reagents
-Interferences
To Identify Determinate (Systematic) Errors
-Use of standard methods with known accuracy and precision
to analyze samples
-Run several analysis of a reference analytewhose
concentration
is known and accepted

20. Indeterminate (Random) Errors
-Sources cannot be identified, avoided, or corrected
-Not constant
Examples
-Limitations of reading mass balances
-Electrical noise in instruments
-Random errors are always associated with measurements
-No conclusion can be drawn with complete certainty

21. MEAN
Themeanisthe averageofthenumbers.
Add upall the numbers, thendivide by how
manynumbers there are.
In other words it is thesumdivided by thecount.
Example 1: What is the Mean of these numbers?
6, 11, 7
•Add the numbers:6 + 11 + 7 = 24
•Divide byhow manynumbers (there are 3 numbers):24 / 3 = 8
The Mean is 8
21

22. How do you handle negative numbers? Adding a negative number is the same as
subtracting the number (without the negative). For example 3 + (−2) = 3−2 = 1.
Findthemeanofthesenumbers:3,−7,5,13,−2
•Thesumofthesenumbersis3−7+5+13−2=12
•Thereare5numbers.
•Themeanisequalto12÷5=2.4
Themeanoftheabovenumbersis2.4
Hereishowtodoitoneline:
22

23. MEDIAN
23
themedianis the value separating the higher half from
the lower half of a data sample.
For example, in the data set, 1, 2, 2,3, 4, 7, 9, the median is 3.
If there is an even number of observations, the there is no single middle value;
the median is then usually defined to be the mean of the two middle values.
For example, in the data set 1,2,3,4,5,6,7,8,9,
The median is the mean of the middle two number: 4+5/2 = 4.5

24. STANDARD DEVIATION
24
It is a measure of how closely the data are clustered about the mean value.
x
i
= each value from the population
= Mean of the individual measurements
x
i
n = number of measurements
n-1 = degree of freedom
Degree of Freedom:
Maximum permissisblenumber of independent values
that have the freedom to vary.

25. STUDENT’S T
25
Student’s t A statistical tool used to express confidence intervals and to compare results from different experiments.

26. 26
Confidence limitsare the numbers at the upper and lower end of aconfidence interval;
for example, if your meanis7.4 withconfidence limitsof 5.4 and 9.4, yourconfidence
interval is5.4 to 9.4.
Most people use 95%confidence limits, although you could use other values.
Confidence limitsand confidence interval
The confidence interval is an expression stating that the true mean is
likely to lie within a certain distance from the measured mean.
The confidence interval is computed from the equation given below:
where t = Student’s t, taken from Table for a desired level of confidence, such as 95%.

27. 27

28. PROPAGATION OF ERRORS
28
Propagation of Error (or Propagation of Uncertainty) is defined as the effects on a
function by a variable's uncertainty.
It is a calculus derived statistical calculation designed to combineuncertaintiesfrom
multiple variables, in order to provide an accurate measurement of uncertainty.
Let us assume two measurements with values a and B resulting to
If a + b = or a –b = , then
If a.b= or a/b = , then
=

29. 29
Example:
+1.76 ( 0.03)
+1.89 ( 0.02)
-1.59 ( 0.02)
2.06 ( )
6
2 2
 
= 0.04
Thus, we can write the answer as 3.06 0.04

30. 30
Test of Significance
1.Null Hypothesis
2.Alternative hypothesis

31. REJECTION OF A RESULT
31
a = difference between the suspect number and its nearest neighbor
w = difference between the highest and lowest numbers
??????=
??????
??????
Q test is not applied to three data points if two are identical
because ‘a’ equals ‘w’, thus Q value is always One

32. 32