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1. MUNI UNIVERSITY
FACULTY OF EDUCATION/SCIENCE
Introductory analytical chemistry ( Group 4)
Gravimetric analysis
1. Azabo kennedy
2. Ssempijja joel
3. Kisakye daniel
4. Muhumuza patrick
5. Eredu Alex

2. Definition
Gravimetry encompasses all techniques in which we measure
mass or a change in mass. When we step on a scale after
exercising, we be making a gravimetric determination of our
mass.

3. Gravimetric analysis is one of the commonly used quantitative
methods in analytical chemistry, the other being the
volumetric method. In gravimetric analysis, the amount of an
ion present in an analyte is estimated based on the parameter
of mass.

4. Principle of Gravimetric Analysis
The principle of gravimetric analysis is based on the
estimation of the mass percent of an ion in an impure
compound of known quantity by determining the mass of the
same ion in a pure compound. In order to determine the
mass, the ion of interest needs to be completely isolated. This
isolation of ions is done with the help of precipitation.

5. Common steps followed in the gravimetric
analysis are:
o Preparation of a solution by using a known weight of the
sample analyte.
o Separation of the desired ion/element/radical in pure forms
by various separation methods
o After the ion has been separated, weighing the amount of
the pure insoluble compound formed.
o Calculating the value of the individual component of
interest, based on the weight of the compound observed.

6. Steps to Perform Gravimetric Analysis
• Take the sample analyte and place it in a weighing bottle.
With no lid on, let the sample in the weighing bottle dry in
an oven and later cool it in a desiccator.
• After the sample has been completely dried and cooled, take
a known quantity of sample and dissolve it in water.

7. • Of the many methods available for the separation of the ion
from the sample, the most commonly applied method is
precipitation. Add a precipitating agent to the solution of the
sample such that it will form an insoluble complex with the
ion of interest and leave the rest in the soluble state.

8. • Optionally, the solution can also be heated to increase the size of the
insoluble complex for the ease of filtration. This process is called digestion.
• In order to test if the digestion has achieved completion, add a few drops
of the precipitating agent and check if the insoluble complex is still being
formed at the tip of the drop. The reason for performing this step is to
ensure that all the ions of interest present in the sample have been
precipitated and nothing is left behind.
• Using a Buchner’s funnel, filter the solution containing the insoluble
precipitate under a vacuum.

9. • Perform a couple of washings using the beaker containing
the solution to ensure that no residue is left on the walls of
the beaker.
• After all the precipitate has been collected onto the filter
paper, keep it for drying.
• Make sure to dry the product completely to get accurate
results.

10. • Once dried, weigh the product and, by using the
stoichiometry of the reaction, determine the mass of the ion
formed.
• This value of mass obtained by calculation can further be
computed to determine the mass of the same ion in the
unknown sample.

12. Volatilization gravimetry

13. Volatilization gravimetry
• This involves thermally or chemically decomposing a solid
sample containing the analyte and then,
 the volatile products of the decomposition reaction trapped
or
 the residue remaining when decomposition is complete
are weighed to provide quantitative information

14. Volatilization gravimetry can be ;
Direct …. The analyt its self is the one being weighed by
placing it on weighing balance and then recording mass
Indirect ….determine the analyte by a signal representing its
disappearance ..e.g.
determining water of crystallization by measure of change in
mass of a pre weighed sample after heating to a constant mass

15. Equipment
• Depending on method, equipment may be;
 simple
or complex

16. Using simple Equipment
• In simple , the weight of solid residue is determined following
thermal decomposition or combustion at a fixed temperature
using;
Bunsen burner,
Laboratory oven
Or Muffle furnace
with the volatile products being vented to the atmosphere
Analytical balance measures mass of both solid and residue

17. Using complex equipment
Constant temperature decomposition / combustion followed by
trapping and weighing the volatilized gases requires more
specialized equipment.
Decomposition is conducted in a closed container and the
volatilized products passed via one or more absorbent traps

18. .
• Whether analysis is direct or indirect volatilization gravimetry
requires that the products of decomposition are known

19. Measurements
• Mass of residue remaining after decomposition
• Mass of volatile product collected using a suitable trap
• Change in mass due to the loss of volatile material

20. Key steps
1) Preparation / isolation of the sample containing the analyte
2) Volatilization of the solution sample containing the analyte
3) Weighing of the isolated sample
4) Calculations

21. Qualitative applications of
volatilization gravimetry
Play great role in chemical analysis

22. Inorganic applications
• a) Determining the inorganic ash content of organic materials
like polymers , paper
A pre-weighed sample is combusted in a crucible
to remove the organic material
Residue heated at constant temperature
Remaining mass is the inorganic ash content

23. ………………..
b) Determination of dissolved solids in water and waste water
Water sample in pre-weighed dish dried to a constant weight
(103- 105oC).
The residue is ignited to constant weight at 500oC

24. • The loss in weight on ignition provides an indirect
measure of the amount of volatile solids in the sample
and the weight of the remaining residue gives the
amount of fixed solids

25. c) Determination of carbon in steels and other
metal alloys by heating the sample.
C is collected as CO2 in an appropriate
absorbent trap providing a direct measure of C in
the original sample

26. Organic analysis
i) Elemental analysis is the most important application
Once burnt in pure O2 , elements C and H are released as CO2 and
H2O
if passed via pre-weighed tubes with absorbents ,
increase in mass of these tubes indicate directly the mass % of C and
H in the organic material

27. ii) Determination of alkaline earth metals in organic materials
By adding H2SO4 to the sample before combustion.
the metal remains behind as a solid residue of metal
sulphate
iii) Silver , Gold, and platinum can be determined by
burning the organic sample leaving a metallic residue of the metal
oxide

28. Sample calculations
Qn. A sample of 2.94 grams of hydrated Calcium
chloride salt ( CaCl2.nH2O) was heated several times
till reaching to a constant mass of 2.22grams.
Find out the number of what molecules in hydrated
salt and hence molecular formular
( Ans 2moles, CaCl2.2H2O )

29. Mass H2O = (2.94 – 2.22) = 0.72grams
CaCl2 H2O
2.22
111
0.72
18
0.02
0.02
0.04
0.02
1 2 Hence CaCl2.2H2O

30. Qn 2
The amount of sodium hydrogen carbonate in a 1.00g
sample of baking soda is analysed by reacting with
excess sulfuric acid .the change in mass before and
after the reaction 0.260g. calculate %w/w NaHCO3
in the baking soda

31. NaHCO3(g) + H+
(aq) CO2(g) + H2O(l) + Na+
(aq)
Mass CO2 = 0.260g
moles CO2 =
0.260
44
= 5.91 x 10-3 moles
1 moles of CO2 are produce by 1moles of NaHCO3
5.91 x10-3moles are produced by
5.91 x 10−3 𝑥 1
1
moles
Mass NaHCO3
5.91 x 10-3 X ( 23+1+12+(3X16)) = 0.446g NaHCO3
%w/w =
0.446
1
X 100 = 44.6$%

32. Qn 3
• A sample of slag from a blast furnace is analyzed for
SiO2 by decomposing 0.5003g sample with HCl leaving a
residue with a mass of 0.1414g.
After treating with HF and H2SO4 and evaporating the
volatile SiF4 a residue with a mass of 0.0183g remains.
Determine the %w/w SiO2 in the sample.

33. The change in sample mass gives the mass of SiO2 in
the sample
Change in mass
=( 0.1414 – 0.0183 ) = 0.1231g
• %w/w =
𝟎.𝟏𝟐𝟑𝟏
𝟎.𝟓𝟎𝟎𝟑
X 1OO = 24.61%

34. Qn 4
• A 0.1013g sample of an organic compound known to contain
chlorine (Cl) was burnt in pure O2 and the combustion gases
collected in pre-weighed absorbent tubes.
CO2 tube increased in mass by 0.167g and that of water by
0.0137g .
A 2nd sample of 0.1218g was treated with conc. HNO3 acid
producing Cl2 which subsequently reacts with Ag+ forming
0.2627g of AgCl
(a) Determine the compounds composition as well as empirical
formula
( Ans C5H2Cl2 )

36. Precipitation or particulate
gravimetry

37. Precipitation gravimetry is an analytical technique that uses
the formation and mass of a precipitate to determine
the mass of an analyte.
The chemical that is added to cause the precipitation is called
the precipitant or precipitating agent
e.g. sodium chloride
A pprecipitate is a solid that forms out of solution
It is important for the precipitate to be a pure substance with a
definite and known composition.

38. A common example of precipitation reaction is that of the
mixing of two clear solutions:
Silver nitrate (AgNO3)- analyte
Sodium chloride (NaCl)- Precipitating agent
The precipitate forms because the solid (AgCl) is insoluble
in water. That is true for all precipitates

39. Setting Up and Performing Precipitation Gravimetry
1. The sample of interest is dissolved in a solvent, commonly
water, to give an aqueous solution.
2. An excess of the precipitation agent is then added to the
aqueous solution. A precipitate should form.
3. The solution is then filtered using ashless filter paper to
separate the precipitate from the solution.

40. 4. A few drops of the precipitating agent are then added to
the filtered solution to ensure all the analyte has
precipitated.
5.The solution is then refiltered using the same ashless filter
paper.
6.The precipitate is washed with some deionized water to
remove any impurities.
7.The precipitate and filter paper are then placed into a
crucible and ignited. This both dries the precipitate and
removes the filter paper.

41. 4.Once cool, the precipitate and the crucible are then
weighed. The final mass of the precipitate is then obtained
by subtracting the mass of the empty crucible from the
mass of the crucible and precipitate.

42. NB:Ashless filter paper is used so that during the heating and
drying phase, the precipitate is not contaminated with ash. If
needed, the precipitate can be left to dry further in a
desiccator.
It is essential for the precipitate to be completely dry as any
remaining solvent will give an incorrect mass reading. Any
error will propagate through the calculation and give an
inaccurate mass of the analyte.

43. Precipitation gravimetry can be used to determine the mass of
sodium sulphate in an aqueous solution

44. A good precipitating agent would be barium chloride, as
the sulphate and barium ions would react to form the
insoluble barium sulphate. The barium sulphate can then be
isolated and dried. Once the mass of the precipitate has been
determined, the first step is to write a balanced chemical
equation for the full reaction

45. The equation shows that the number of moles of barium
sulphate is equal to the number of moles of sodium sulphate in
the original solution. The number of moles of barium sulphate
can be calculated using the formula
𝑛=𝑚𝑀,
where 𝑛 is the amount in moles, 𝑚 is the mass in grams, and 𝑀
is the molar mass in grams per mole

50. Applications of precipitation
gravimetry
 Determination of the initial mass of the analyte
Used to determine the composition of the mixture sample
Used to identify the atoms forming the compound being
analyzed/identification of organic and in organic analytes

51. Particulate gravimetry
• This is the technique in which the analyte is already
present as in a particulate form that is easy to
separate from its liquid, gas, or solid matrix.
• When such a separation is possible, we can
determine the analyte’s mass without relying on a
chemical reaction

52. • A particulate is any tiny portion of matter, whether it is a
speck of dust, a globule of fat, or a molecule of ammonia.
For particulate gravimetry we simply need a method for
collecting the particles and a balance for measuring their
mass.
• There are two methods for separating a particulate analyte
from its matrix. The most common method is filtration, in
which we separate solid particulates from their gas, liquid, or
solid matrix. A second method, which is useful for gas
particles, solutes, and solids, is an extraction

53. Filtration
To separate solid particulates from their matrix we use gravity or
apply suction from a vacuum pump or aspirator to pull the
sample through a filter. The type of filter we use depends upon
the size of the solid particles and the sample’s matrix. Filters for
liquid samples are constructed from a variety of materials,
including cellulose fibres, glass fibres, cellulose nitrate, and
polytetrafluoroethylene (PTFE). Particle retention depends on
the size of the filter’s pores.

54. Extraction
Filtering limits particulate gravimetry to solid analytes that are
easily separated from their matrix. We can extend particulate
gravimetry to the analysis of gas phase analytes, solutes, and
poorly filterable solids by extracting them with a suitable
solvent. After the extraction, we evaporate the solvent before
determining the analyte’s mass. Alternatively, we can
determine the analyte indirectly by measuring the change in
the sample’s mass after extracting the analyte

55. Another method for extracting an analyte from its matrix is by
adsorption onto a solid substrate, by absorption into a thin
polymer or chemical film coated on a solid substrate, or by
chemically binding to a suitable receptor that is covalently
bound to a solid substrate Adsorption and binding occur at the
interface between the solution containing the analyte and the
substrate’s surface, the thin film, or the receptor

57. Applications of particulate gravimetry
Particulate gravimetry is commonly encountered in the
environmental analysis of water, air, and soil samples.
 Microbiological testing of water also is accomplished by
particulate gravimetry. For example, in the analysis for
coliform bacteria an appropriate volume of sample is passed
through a sterilized 0.45-μm membrane filter
Total airborne particulates are determined using a high-
volume air sampler equipped with either cellulose fibre or
glass fibre filters.

58. Grain size distributions for sediments and soils are used to
determine the amount of sand, silt, and clay present in a
sample. For example, a grain size of 2 mm serves as a
boundary between gravel and sand

60. Thermo gravimetric analysis
Definition
• A form of vitalization gravimetry in which the change in a
samples mas is monitored while it is heated.
• In thermogravimetry, the samples mass is continuously
monitored while the applied temperature is slowly increased.
• The product of such thermal decomposition is indicated by a
step in the thermogram, the change in mass at each step can be
used to identify both the volatized species and the solid residue.

61. Types of thermogravimetry:
61
1. Isothermal or static thermogravimetry: In this technique the sample weight is
recorded as function of time at constant temperature.
2. Quasistatic thermogravimetry: In this technique the sample is heated to constant
weight at each of series of increasing temperatures.
3. Dynamic thermogravimetry: In this technique the sample is heated in an
environment whose temperature is changing in a predetermined manner generally
at linear rate. This type is generally used.

62. Principle of TGA:
62
• In thermo-gravimetric analysis, the sample is heated in a given environment (air,
N2, CO2, He, Ar, etc.) at controlled rate. The change in the weight of the substance
is recorded as a function of temperature or time.
• The temperature is increased at a constant rate for a known initial weight of the
substance and the changes in weights are recorded as a function of temperature at
different time interval.
• This plot of weight change against temperature is called thermo-gravimetric
curve or thermo-gram, this is the basic principle of TGA.

63. TGAcurve:
• The instrument used for themo-
gravimetry is a programmed precision
balance for rise in temperature known as
Thermo-balance.
• Results are displayed by a plot of mass
change versus temperature or time and
are known as Thermogravimetric curves
or TG curves.
63

64. TGAcurve:
64
• TG curves are normally plotted with the mass change (Dm) in percentage on the y-
axis and temperature (T) or time (t) on the x-axis.
• There are two temperatures in the reaction, Ti(procedural decomposition temp.) and
Tf(final temp.) representing the lowest temperature at which the onset of a mass
change is seen and the lowest temperature at which the process has been completed
respectively.
• The reaction temperature and interval (Tf-Ti) depend on the experimental
condition; therefore, they do not have any fixed value.

65. TGA curve of AgNO3 :
65

66. Instrumentation of TGA:
66

67. Instrumentation of TGA:
• Recording balance
• Sample holder
• Furnace
• Temperature programmer /controller (thermocouple)
• Recorder
67

68. Factors affecting TGA:
68
Factors affecting the TG curve The factors which may affect the TG curves are
classified into two main groups.:
(1) Instrumental factors:
(a) Furnace heating rate
(b) Furnace atmosphere
(2) Sample characteristics includes :
(a) Weight of the sample
(b) Sample particle size

69. Factors affecting TGA:
69
1.Instrumental factors :
a. Furnace Heating rate: The temperature at which the compound (or sample)
decompose depends upon the heating rate. When the heating rate is high, the
decomposition temperature is also high. A heating rate of 3.5°C per minute is
usually recommended for reliable and reproducible TGA.
b. Furnace atmosphere: The atmosphere inside the furnace surrounding the sample
has a profound effect on the decomposition temperature of the sample. A pure N2
gas from a cylinder passed through the furnace which provides an inert atmosphere.

70. Factors affecting TGA:
70
2.Sample characteristics:
(a)Weight of the sample: A small weight of the sample is recommended using a small
weight eliminates the existence of temperature gradient throughout the sample.
(b)Particle size of the sample: The particle size of the sample should be small and
uniform. The use of large particle or crystal may result in apparent, very rapid
weight loss during heating.

71. Other factors affecting TGA curve:
71
• Sample holder
• Heat of reaction
• Compactness of sample
• Previous history of the sample

72. Advantages of TGA:
72
• A relatively small set of data is to be treated.
• Continuous recording of weight loss as a function of temperature ensures Equal
weightage to examination over the whole range of study.
• As a single sample is analyzed over the whole range of temperature, the
variation in the value of the kinetic parameters, if any, will be indicated.

73. Limitations of TGA:
73
• The Chemical or physical changes which are not accompanied by the change in
mass on heating are not indicated in thermo- gravimetric analysis.
• During TGA, Pure fusion reaction, crystalline transition, glass transition,
crystallization and solid state reaction with no volatile product would not be
indicated because they provide no change in mass of the specimen.

74. Applications of TGA:
74
• From TGA, we can determine the purity and thermal stability
of both primary and secondary standards.
• Determination of the composition of complex mixture and decomposition of
complex OR composition of complex systems.
• For studying the sublimation behavior of various substances.
• TGA is used to study the kinetics of the reaction rateconstant.

75. Applications of TGA:
75
• Used in the study of catalyst: The change in the chemical states of the catalyst
may be studied by TGA techniques.(Zn- ZnCrO4) Zinc-Zinc chromate is used as
the catalyst in the synthesis of methanol.
• Analysis of the dosage form in medicine.
• Oxidative stability of materials.
• Estimated lifetime of a product.

76. Applications of TGA:
76
• TGA is often used to measure residual solvents and moisture, but can also be used
to determine solubility of pharmaceutical materials in solvents.
• The effect of reactive or corrosive atmosphere on materials.
• Moisture and volatiles contents on materials.

78. Electrogravimetry
Presneted by Azabo Kennedy

79. Introduction
• Electrogravimetry is a method used to separate and quantify ion of a substances
usually a metal.
• In this process, the analyte solution is electrolysed and the electrochemical
reduction causes the analyte to be deposited on the cathode.
• The cathode is weighed before and after the experiment and the weighing by
difference is used to calculate the amount of the analyte in the original solution.
• Here controlling the potential of the electrode is very important to ensure only
the metal being analyzed will be deposited on the electrode.

80. Principle
The main principle involved in this method is the
deposition of the solid on an electrode from the
analyte solution.

81. Types of electrogravimetry
There are two types of electrogravimetry, namely:
• Constant current electrolysis
• Constant potential electrolysis

82. Constant current electrolysis
• In this process, the current is kept constant and the
potential is increased.
• Here no control of the potential of the working electrode is
exercised and the applied cell potential is held at a more or
less constant level but provides a large enough curent to
complete the electrolysis in a reasonable length of time.

83. Limitation of constant current electrolysis
• The limitation of constant current electrolysis is that; it
cannot be used for the separation of ion in solution
containing single species.

84. Constant potential electrolysis
• It is the simplest way of performing an analytical
electrolysis. It is more used in the separation of
compounds from a mixture in which the decomposition
potential are not widely separated.
• A fairly large voltage is applied to working electrode
inorder to force a constant relatively large current flow
throw the cell.

85. • CONTINUE
• Three electrode systems are used;
• Working electrode: used for the decomposition of the sample
• Counter electrode: used for the current sink
• Reference electrode: maintains the fixed potential despite the change
in solution compound.
• For example;
 The determination of copper from an acxidic solution at constant current
Suppose an EMF of 2.3V is applied, then the reaction taking place are given
below.
At cathode: Cu
2+
+ 2e → Cu
At the anode: 4OH → O2 + 2H2O + 4e

86. CONTINUE

87. Continue
• In electrogravimetry, the determination of metal is carried
out by above two methods, at a constant or with a
controlled potential procedure, but contact is limited.

88. Instrument
• The apparatus for an analytical electrodeposition without
cathode potential control consists of a suitable cell and direct
current supply.
• The voltage applied to the cell is controlled by the variable
resistance, R.
• A current metal and a volitmeter indicates the approximate
current and voltage. The voltage is then maintain at about the
initial level until the deposition is complete.

89. Application of electrogravimetry
• Used in the successive deposition of metals. For example, Cu,
Zn, and Sn
• Used in the electro synthesis
• Used in the purification process by removing the trace metal
from the sample.

90. FACTORS AFFECTING SOLUBILITY OF A
COMPLEX

91. i) Concentration
• The composition of the solution in which the complex is
influences the precipitate. For example; silver ions can be
precipitated by adding of chloride ions to form a precipitate of
silver chloride

92. • Addition of Cl- ion increases solubility of the precipitate. This is
because of formation of soluble chloro complexes

94. ii) Temperature
The solubility of a precipitate, in general increases with rise in
temperature.
• The dissociation of a solute is mostly an endothermic process,
so the solubility product constant increases as the temperature
is increased.
• This also increases the solubility of the.
• Whenever possible, it is advantageous to filter while the
solution is hot. The solubility of a foreign substance is more
than the solubility of the precipitate in hot solution.

95. iii) pH
• Hydroxide precipitates such as Fe(OH)2, are more soluble at
lower pH level are which the concentration of OH is small
The effect of pH on solubility is not limited to hydroxide of
precipitate but also basic or acidic ion

96. pH
• Depending on the solution pH the predominate phosphate
species is either PO4,HPO4 ,H2PO4 ,H3PO4.
• when the pH is greater than 12.4 the predominate species is
PO4 and the solubility of Ca3(PO4)2 will be minimum because of
the other react ion .as the solution become more acidic, the
solubility of Ca3(PO4)2 increases due to the contribution of
other reactions

98. iv) Solvents
• Solubility decreases when using non aqueous solvents.
• A precipitate solubility is generally greater in aqueous solution
because of water molecules to stabilize ion through solvation.
The poorer the solvating ability of aqueous solvent, even those
that are polar leads to smaller solubility product.

99. ADVANTAGES
• It is accurate and precise when using a modern analytical
balance
• Possible sources of error are readily checked since filtrate can
be tested for completeness of the precipitation and precipitate
may be examined for the presence of impurities
• Determination can be carried out with relatively inexpensive
apparatus,

100. • It is used to determine the atomic masses of elements
• It provides little room for instructional error and does not
require a series of standard for calculation of unknown

101. DISADVANTAGES
• Provides for analyzing of single element or limited group of
element at a time
• It requires the soil sample to be kept for one full day until it is
dried out completely at 105c and before it weighed
• Methods are often convoluted and a slight misstep in the
procedure can mean disaster for the analysis

102. References:
102
1. Gary D. Christian (1994). Analytical Chemistry. John Wiley and Sons Inc., New
York
2. David Harvey( 2000). Modern Analyitical Chemistry.DePauw University, The
McGraw-Hill Companies, Inc.
3. Skoog , Douglas A, F James holler and timothy Niemen, principles of instrumental
analysis, 5th edition New York 2001