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Volumetric and Gravimetric Analysis 57
PART 2
GRAVIMETRIC ANALYSIS
2.28 PRINCIPLE OF GRAVIMETRIC ANALYSIS
Gravimetric analysis is a method used in analytical chemistry to determine the quantity of
analyte based on the mass of a solid. By measuring the solids suspended in the water sample after
filtration of a known volume of water, the collected solids are weighed. The theory behind
gravimetric analysis is that the mass of an ion in a pure compound may be determined and then
used to calculate the mass per cent of the same ion in a known quantity of an impure compound.
2.29 TYPES OF GRAVIMETRIC ANALYSIS
There are four techniques of gravimetric analysis; the two most common types involve
changing the phase of the analyte to separate it from the rest of a mixture, resulting in a change in
mass.
➢ Volatilization gravimetry
➢ Precipitation gravimetry
➢ Electrogravimetry
➢ Thermogravimetry
Advantages of Gravimetric Analysis
➢ Gravimetric analysis provides exact and accurate results when a modern analytical scale
is used.
➢ Filtrates can be examined for perfect precipitation and precipitates can be tested for the
presence of contaminants. The possible sources of errors and the presence of
contaminants in precipitates may be easily verified.
➢ Gravimetric analysis is an absolute approach that requires no calibration and involves
direct measurement.
➢ Analysis can be performed with relatively inexpensive equipment.
➢ It is also used to calculate the atomic masses of various elements in the periodic table
with an accuracy of six figures.
➢ It has minimal instrumental error and does not need of a series of standards to determine
the unknown substance.
Disadvantages of Gravimetric Analysis
➢ This method of analysis is a very time-consuming process.
➢ Gravimetric analysis is confined to analyze a single element or limited group of
elements, at a time.
➢ This method is often complicated and the slightest error in procedure will mean disaster
in analysis.
58 Text Book of Pharmaceutical Chemistry
2.30 STEPS INVOLVED IN PRECIPITATION GRAVIMETRY
After the sample has been dissolved, the main steps required in gravimetric analysis are
summarized as follows:
1. Preparation of the sample solution
2. Precipitation process
3. Digestion or Ostwald ripening
4. Filtration
5. Washing
6. Drying
7. Igniting
8. Weighing
9. Calculation
Precipitation Gravimetry
It is an analytical technique that uses a precipitation reaction to separate ions from a solution.
The analyte is separated from a solution of the sample as a precipitate and is converted to a
compound of known composition that can be weighed.
2.30.1 Step 1: Precipitation by Means of Precipitating Agents
In precipitation gravimetry, a chemical is added to a solution containing analyte that causes
the precipitation. These chemicals are called the precipitant or precipitating agent.
Analyte is converted to a sparingly soluble precipitate that can be separated from the liquid
components using filtration process. After filtration, the solid mass can be used along with the
balanced chemical equation to calculate the amount or concentration of ionic compounds in
solution.
Mechanism of Precipitation:
When a solution of precipitating agent is introduced to the test solution, precipitate is formed
such as the precipitation of AgCl from a solution of AgNO
3
using NaCl as a precipitant.
Super Saturation
Test solution contains a number of dissolved ions more than it can accommodate. The system
will be driven to reach equilibrium (saturation), and precipitation will occur.
Nucleation
It is the initial phase of precipitation in which a few ions, atoms, or molecules come together
to form a nucleus of precipitate or stable solid (nucleation occur on the surface of solid
contaminants, such as dust particles). Higher degree of precipitation then involves a competition
between additional nucleation and growth on existing nuclei (particle growth). Nucleation
involves the formation of ion pairs, and finally, a group of ions is formed. Nucleation is of two
types – (1) Spontaneous and (2) Induced.
Volumetric and Gravimetric Analysis 59
Particle Growth
Nuclei will grow by joining together to form a crystal of a certain geometric shape. It
involves two steps – diffusion of ion to surface of nucleus and deposition on surface.
Organic Precipitants
Organic precipitating agents have the advantages of giving precipitates with very low
solubility in water and a favorable gravimetric factor. Most of them are chelating agents that form
slightly insoluble, uncharged chelates with the metal ions.
Name
Ion Precipitated
Salicylaldoxime
Cu
2+
, Pb
2+
, Bi
3+
, Zn
2+
, Ni
2+
, Pd
2+
Dimethyl glyoxime
Ni
2+
, Pd
2+
, Pt
2+
Cupferron
Fe
3+
, Ce
4+
, Sn
4+
Zr
4+
8-Hydroxyquinoline
Mg
2+
, Zn
2+
, Cu
2+
, Cd
2+
, Pb
2+
, Al
3+
1-nitroso-2-naphthol
Fe
3+
, Pd
2+
, Zr
4+
Nitron
NO
3
–
, ClO
4
–
, BF
4
–
Properties of Precipitates and Precipitating Reagents
A gravimetric precipitating agent should react specifically or at least selectively with the
analyte and give precipitates that is:
➢ Easily filtered and washed free of contaminants
➢ Have sufficient particle size for retaining on filter paper
➢ Must have small solubility that no significant loss of the analyte occurs during filtration
and washing
➢ Unreactive with components of the atmosphere air
➢ Known chemical composition after it is dried, or if necessary, ignited
Types of Precipitates
There are two types of precipitates:
1. Colloidal precipitates
2. Crystalline precipitates
Colloidal Precipitates
Colloidal precipitates are formed in colloidal suspensions, whose tiny particles are invisible
to the naked eye (cm in diameter). The colloidal suspension consists of particles having diameters
ranging from 10
–7
to 10
–4
cm. These particles do not tend to settle down at the bottom of the
container. Because these particles are very tiny, it is not filtered easily. By addition of suitable
coagulating agent, we can form large particles or a precipitate that is easy to filter. The
precipitation or the coagulation of colloidal particles can be increased by heating, by stirring, or
by adding an electrolyte to the suspension.
60 Text Book of Pharmaceutical Chemistry
Crystalline Precipitates
A crystalline precipitate consists of particles having large diameters about a tenth of a
millimetre or greater. These are the solid masses formed in a crystalline suspension. The effect of
gravity on these large particles is much higher than that of colloidal particles. Therefore, the
particles of the crystalline suspensions settle down automatically and are easily filtered and
purified.
2.30.2 Step 2: Digestion of the Precipitate
It is the process in which a precipitate is heated (without stirring) for a long time in the
solution from which it was formed (the mother liquor). During this process, small particles tend to
dissolve and reprecipitate on the surface of larger particles/crystals causing the particles to grow
even larger. This process is normally done at high temperature to speed the process and called
digestion or Ostwald ripening. By heating the number of adsorbed particles and the effective
charge in the adsorbed layer tends to decreases, thereby aiding coagulation.
2.30.3 Step 3: Filtration of the Precipitate
After completion of precipitation and digestion the precipitate, it is separated from solution
by the process of filtration using filter paper.
Filter Types
Filters can be classified according to the nature of the driving force that causes filtration:
➢ Gravity filtration
➢ Vacuum filtration
➢ Centrifugal filtration
➢ Hot filtration
➢ Cold filtration
➢ Granular media filtration
➢ Mechanical filtration
Gravity Filtration
This process requires placing a filter paper in the funnel which is fixed on an iron ring and
liquid is poured into the funnel. As the liquid passes through the filter paper, it gets collected in a
beaker placed beneath.
Vacuum Filtration
A vacuum filtration technique is similar to gravity filtration. The difference is that a
‘Buchner’ flask is needed to collect the filtrate. The flask, in turn, is connected to an aspirator or a
vacuum line with the help of a tube.
Volumetric and Gravimetric Analysis 61
Centrifugal Filtration
The centrifugal filtration contains a rotating drum onto which the fluid pumped. By the force
created by rapid rotations of the drum, the suspended solids travel away from the centre and get
trapped onto the walls of the equipment and the liquid is forced out.
Hot Filtration
This technique of filtration is needed when certain elements in the fluid are likely to get
crystallized. Fluted filter paper and stemless filter funnel are recommended for this process.
Cold Filtration
In this process of filtration, the fluid is chilled rapidly to accelerate the formation of the
crystals which can then be separated.
Granular Media Filtration
In this process of filtration, the fluid is made to pass through the beds of granular media like
sand, gravel, or activated carbon, e.g., water treatment and wastewater treatment.
Mechanical Filtration
This process of filtration involves forcing the fluid through a filter element in order to remove
or separate suspended solids from the fluid (Fig. 2.7).
Fig. 2.7: Mechanical Filtration
Filter Paper
The most frequently filtering medium is cellulose-based filter paper, which is classified
according to its filtering speed, its size, and its ash content on ignition. Generally, filter papers are
used in laboratory techniques such as gravity or vacuum filtration. Whatman filter paper is a
particular type of filter paper used for qualitative analysis. It is used for qualitative chemical
analysis. Furthermore, the ash content of these filter papers is about 0.13%. Normal filter paper is
a semi-permeable barrier that we use to separate components in a liquid flow or an air flow.
62 Text Book of Pharmaceutical Chemistry
Qualitative Filter Paper
Qualitative filter paper is a type of filter paper that is used to determine materials in
qualitative analytical procedures. This type of filter paper, also known as ash-free filter paper, is
utilized in quantitative and gravimetric analysis. In quantitative filter papers, ash content is less
than 0.007%, and its impurity dissolution rate is very low. Hawach filter paper, which is utilized
in gravimetric analysis, is especially suitable for routine quantitative analysis.
The precipitate is separated by filtration either through crucibles or Whatman’s No. 40, 41, or
42 filter papers.
Ashless Filter Paper
There are mainly 5 grades of ashless qualitative filter paper from Grades 40 to 44 have
0.007% ash maximum, and the variations of grade 589 have 0.01% ash maximum, making both
suitable for critical analytical uses, summarized below:
Grade
Filtration Speed
Ash Content (%)
Basis Weight (g/m
2
)
F20 40
Medium-slow
< 0.007
84
F20 41
Fast
< 0.007
84
F20 42
Very slow
< 0.007
84
F20 43
Medium
< 0.007
84
F20 44
Slow
< 0.007
84
F20 45
Very fast
< 0.007
84
Applications for High-purity Grade 40 Filters
High-purity filters support accurate gravimetric analysis. Whatman’s Grade 40 ashless paper
is suitable for:
➢ Analysis of soil for separating solid matter from aqueous extracts.
➢ It is used as a clean-up filter for solutions prior to atomic absorption (AA) spectrometry.
➢ Quantitative determination of milk sediment.
➢ Separating components in cements, clay, iron, and steel products.
➢ Collection of trace elements and radionuclides from the atmosphere.
2.30.4 Step 4: Washing of Precipitates
On the surface of precipitate, several ions and impurities get adsorbed. Consecutively, these
coprecipitated impurities especially on surface are removed by washing the precipitate.
A precipitate can be washed immediately on the filter, or it also washed slightly by
decantation and partly on the filter. In the decantation process, precipitate is allowed to settle, and
the supernatant liquid is allowed to pour on the filter. Distilled water is added to the precipitate,
and after settling, the decantation is repeated, and finally, the precipitate is transferred to the paper
or Gooch crucible.
Volumetric and Gravimetric Analysis 63
Some precipitate cannot be washed with pure water. Therefore, there is a need to add
electrolyte to the wash liquid. Add the wash liquid to the precipitate, stir well, centrifuge, and
decant the wash liquid. The wash liquid is usually discarded. Precipitates should be washed twice
for best results (Fig. 2.8).
Wash liquids may be divided into three classes:
1. Wash liquids that prevent the precipitate from becoming colloidal and passing through
the filter. These types of wash liquids should contain an electrolyte. The electrolyte
should not provide its action upon the precipitate either during washing or ignition, e.g.,
Ammonium salts are widely used. Thus, dilute ammonium nitrate solution is employed
for washing iron (III) hydroxide [hydrated iron (III) oxide], and 1% nitric acid for
washing silver chloride.
2. Wash liquids or solutions which decrease the solubility of the precipitate. The solution
may contain balanced concentration of a compound with one ion in common with the
precipitate use being made of the fact that substances tend to be less soluble in the
presence of a slight excess of a common ion. Organic solvents like ethanol and water or
a dilute electrolyte is effective in reducing the solubility to negligible proportions.
Fig. 2.8: Washing of Precipitate
3. Wash solutions that minimize the hydrolysis of salts of weak acids and bases. When the
precipitate is a salt of weak acid and is significantly soluble, it may exhibit a tendency to
hydrolyze, and the soluble content of hydrolysis will be a base. Therefore, wash liquid
must be basic. Hence, (Mg(NH
4
)PO
4
may hydrolyze appreciably to give the hydrogen
phosphate ion HPO
4
2–
and hydroxide ion, and should accordingly be washed with dilute
aqueous ammonia.
Properties of Ideal Washing Liquids
Precipitates are washed with liquid to eliminate all soluble impurities sticking with the
precipitates.
64 Text Book of Pharmaceutical Chemistry
➢ Wash liquid have no action on precipitates but must dissolve and remove all foreign
impurities.
➢ It should not form any volatile product with precipitates. Wash liquid should be volatile
at the temperature of drying of precipitate.
➢ It should have no dispersive action on the precipitates.
➢ Wash liquid should not interfere with precipitates.
2.30.5 Step 5: Drying or Ignition
Filtered precipitate after washing generally heated or ignited to produce a constant weight.
Precipitates are dried or ignited depending on the nature of precipitates and filtering medium.
At the time of drying, the solvent (s) and any volatile species carried down with the
precipitate get removed and precipitate collected on filter paper, porcelain filtering crucibles or
silica filtering crucibles. The drying can be done by heating the precipitate at 110 to 120°C for
1 to 2 hours (for the precipitate which is in a form suitable for weighing).
The temperature at which precipitates are dried at definite temperature can be determined by
the thermogravimetry. It is a technique in which a change in the weight of a substance is recorded
as a function of temperature or time. The electric drying ovens are used for the purpose of drying
fitted with a thermoregulator, which maintains the appropriate temperature automatically.
Types of Crucibles
Different types of crucibles are used for drying and weighing of the precipitates. They are:
1. Silica glass crucible is made of porcelain or silica is used when the precipitates are
dried in an oven or muffle furnace (Fig. 2.9).
Fig. 2.9: Silica Glass Crucible
2. Sintered crucibles are used for precipitates which are weighed after drying in air-oven.
Normally, it is useful in directly collecting a precipitate that requires drying. These
types of crucibles contain various degrees of porosity in order to suit precipitates of
different sizes (Fig. 2.10).
Volumetric and Gravimetric Analysis 65
Fig. 2.10: Sintered Glass Crucibles
3. Gooch crucible is a type of filtration device that can resist very high temperatures. It is
useful for collecting a precipitate directly within the vessel in which we can let the
precipitate dry, produce its ash and measure the weight of the analyte sample
through gravimetric analysis (Fig. 2.11).
Fig. 2.11: Gooch crucible
Procedure for Drying
➢ The crucible used for drying should be clean and dry.
➢ Before the ignition, the weight of the empty crucible must be taken.
➢ Place the sample precipitates in the clean and dry crucible.
➢ The crucible is heated in the oven.
➢ The precipitates are cooled to room temperature in a desiccator before weighing.
➢ Repeat the process of heating, cooling and weighing until a constant weight is achieved.
Ignition of the Precipitate
Ignition converts the precipitates to compounds that are more suitable for weighing. Ignition
at a much higher temperature converts hygroscopic compound to non-hygroscopic compound.
While paper is used for the filtration, precipitate must be incinerated. The important apparatus
consists of a Bunsen burner, an iron tripod about 20 cm high, a pipe clay triangle for porcelain
crucibles, a glass stirring rod with rounded end, platinum wire triangle for the platinum crucible,
etc. (Fig. 2.12).
66 Text Book of Pharmaceutical Chemistry
Procedure:
➢ Take the empty weight of the clean and dry crucible.
➢ Filter paper and precipitate are first dried and then are placed in the crucible.
➢ Crucible is set on the triangle-shaped tripod.
➢ Tilted the crucible at an angle of about 45° and the Bunsen flame is gently applied.
➢ First, the filter paper takes fire and burns, leaving a black mass.
➢ The burner is now set under the crucible so that the flame strikes the bottom, and the
heat is increased and the ignition continued, until all black particles disappear.
➢ The crucible is then removed and placed in the desiccators.
➢ Repeat the process of heating, cooling and weighing until a constant weight is achieved.
Fig. 2.12: Ignition Process
2.30.6 Step-6: Weighing of the Precipitate
After heating, the precipitate is allowed to cool at room temperature in desiccators before
weighing (to keep it free from absorbing moisture). It is weighed in the crucible and the process
of heating, cooling and weighing is repeated until a constant weight is achieved or the difference
between two consecutive weighing is not more than 0.0002g.
Calculating the Results
In gravimetric analysis, the result is usually reported as a percentage of analyte. Generally,
two values are required: the weight of the analyte and the weight of the sample. For calculating
the weight of analyte from the weight of the precipitate, a gravimetric factor (GF) is used.