Download Chem 120 4 Gravimetric Analysis

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Chemistry Division
Department of Natural Sciences
College of Arts and Sciences
Caraga State University
A.Y. 2016-2017, 1st Semester
Gravimetric Analysis
• Quantitative methods that are based on
determining the mass of a pure compound to
which the analyte is chemically related
• Gravimetric methods of analysis are based on
mass measurements made with an analytical
balance
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Analytical Methods based on Mass Measurements
• Precipitation gravimetry
– 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
• Volatilization gravimetry
– The analyte is separated from other constituents of the sample by
conversion to a gas of known composition
• Electrogravimetry
– The analyte is separated by deposition on an electrode by an electric
current
• Gravimetric titrimetry
– The mass of a reagent, of known concentration, required to react
completely with the analyte is used to determine the analyte
concentration
• Atomic mass spectrometry
– Uses a mass spectrometer to separate the gaseous ions formed from the
elements making up a sample of matter
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Precipitation Gravimetry
• The analyte is converted to a sparingly soluble
precipitate
• The precipitate is then filtered, washed free of
impurities, converted to a product of known
composition by suitable heat treatment, and
weighed
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Determination of calcium in natural waters
• Excess oxalic acid is added to an aqueous
solution of the sample
• Ammonia is added to neutralize the acid and
causes all of the calcium in the sample to
precipitate as calcium oxalate
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Example 1
• The precipitate is filtered using a weighed
filtering crucible, then dried, and ignited
• This process converts the precipitate to
calcium oxide
• After cooling, the crucible and precipitate are
weighed
• The mass of calcium oxide is determined
• Calcium content is calculated
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Properties of precipitates and precipitating reagents
• Gravimetric precipitating reagents should:
– React specifically, or at least
– React selectively with the analyte
• Specific reagents (rare) react only with a single chemical
species
• Selective reagents (more common) react with a limited
number of chemical species
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Properties of precipitates and precipitating reagents
• An ideal precipitating reagent should react with the
analyte to give a product that is
– Easily filtered and washed free of contaminants
– Of sufficiently low solubility that no significant
loss of the analyte occurs during filtration and
washing
– Unreactive with constituents of the atmosphere
– Of known chemical composition after it is dried or
if necessary, ignited
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Particle size and filterability of precipitates
• Precipitates consisting of large particles are desirable
for gravimetric work
– Easy to filter and wash free of contaminants
– Purer compared to precipitates made up of fine particles
• Colloidal suspensions
– Solid particles with diameters <10-4 cm
– Little tendency to settle from solution
– Difficult to filter
• Crystalline suspension
– Tend to settle spontaneously
– Easy to filter
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Factors that determine the particle size of precipitates
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Precipitate solubility
Temperature
Reactant concentrations
Rate of mixing reactants
Relative supersaturation
– Q = concentration of the solute at any instant
– S = equilibrium solubility
• Particle size of a precipitate varies inversely with the average
relative supersaturation when reagent is being introduced
– When (Q-S/S) is large  the precipitate tends to be colloidal
– When (Q-S/S) is small  the precipitate tends to be crystalline
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Mechanism of precipitate formation
• Nucleation
• Particle growth
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Experimental control of particle size
• Elevate temperatures to increase solubility of the
precipitate
• Dilute solutions to minimize Q
• Slow addition of the precipitating reagent with good
stirring
• Control pH, if precipitate solubility is pH-dependent
– Large, easy to filter crystals of calcium oxalate are
obtained by forming the bulk of the precipitate in
a mildly acidic environment in which the salt is
moderately soluble
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Many precipitates cannot be formed as crystals under
practical laboratory conditions
• Hydrous oxides of iron(III), aluminum, and
chromium(III)
• Sulfides of most heavy metal ions
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Colloid Precipitate
• So small
• Retained by ordinary filter papers
• Brownian motion prevents their settling out
of solution under the influence of gravity
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Coagulation of Colloid
• Coagulation can be hastened by:
– Heating
– Stirring
– Adding an electrolyte to the medium
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Peptization of Colloids
• The process by which a coagulated colloid
reverts to its original dispersed state
• To prevent peptization of colloids, wash the
precipitate with a solution containing an
electrolyte that volatilizes when the
precipitate is dried or ignited
• Silver chloride (AgCl) is washed with a dilute
solution of nitric acid (HNO3)
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Practical treatment of colloidal precipitates
• Precipitate colloids from hot, stirred
solutions, containing sufficient electrolyte to
ensure coagulation
• Digestion is a process in which a precipitate is
heated for an hour or more in the solution
from which it was formed (the mother liquor)
• During digestion, weakly bound water is lost
from the precipitate resulting in a denser
mass that is easier to filter
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Crystalline Particles
• More easy to filter
• Purer than colloidal precipitates
• Size of individual crystalline particles and
filterability can be controlled to a degree
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Methods of improving particle size and filterability
• Minimize Q
– Use dilute solutions
– Add precipitating reagent slowly and with good
mixing
• Maximize S
– Precipitate from hot solution
– Adjust the pH of the precipitation medium
• Digest crystalline precipitates (without stirring)
– Improves purity and filterability due to dissolution
and recrystallization that occur continuously and
at an enhanced rate at elevated temperature
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Coprecipitation
• A process in which normally soluble compounds are
carried out solution by a precipitate
• Contamination of a precipitate by a second
substance whose solubility product has been
exceeded does not constitute coprecipitation
• Types of coprecipitation:
– Surface adsorption
– Mixed-crystal formation
– Occlusion
– Mechanical entrapment
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Types of coprecipitation
• Surface adsorption
– A normally soluble compound is carried out of
solution on the surface of a coagulated colloid
– This compound consists of the primarily adsorbed
ion and an ion of opposite charge from the
counter-ion layer
A coagulated colloid
continues to expose a
large surface area to the
solution from which it was
formed
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Types of coprecipitation
• Mixed-crystal formation
– A contaminant ion replaces an ion in the lattice of a crystal
• Occlusion
– A compound/impurities is trapped within a pocket formed during
rapid crystal growth
• Mechanical entrapment
– Occurs when crystals lie close together during growth
– Several crystals grow together and in so doing trap a portion of the
solution in a tiny pocket
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Minimizing adsorbed impurities on colloids
• Digestion
• Wash a coagulated colloid with solution
containing a volatile electrolyte
• Repreciptitation
– The filtered solid is redissolved and reprecipitated
– Increases time required for analysis
– Necessary for precipitates such as hydrous oxides
or iron (III) and aluminum, which have tendencies
to adsorb the hydroxides of heavy metal cations
such as zinc, cadmium, and manganese
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Coprecipitation errors
• Positive error results if the contaminant is not
a compound of the ion being determined
– Colloidal silver chloride adsorbs silver nitrate
• Either positive or negative error occurs when
the contaminant does not contain the ion
being determined
– Determination of barium as barium sulfate
– Positive error if the occluded contaminant is
barium nitrate
– Negative error if contaminant is barium chloride
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Precipitation from homogeneous solution
• A process in which a precipitate is formed by slow generation
of a precipitating reagent homogeneously throughout a
solution
• No localized reagent excesses
• Relative superunsaturation is kept low
• Example: urea is used for homogeneous precipitation of
hydroxide ion
Precipitation from homogeneous
solution is a technique in which a
precipitating agent is generated in a
solution of the analyte by a slowchemical reaction.
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Aluminum hydroxide formed by the direct addition of
ammonia (left) and the homogeneous production of
hydroxide (right)
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Methods for homogeneous generation of precipitating agents
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Drying and ignition of precipitates
• After filtration, a gravimetric precipitate is
heated until its mass become constant
– Removes solvent and any volatile species carried
down with the precipitate
– Decomposes the solid and forms a compound of
known composition (the weighing form)
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Effect of temperature on precipitate mass
• Moisture is completely removed
from AgCl at T>110C
• Dehydration of Al2O3 is not
complete until a T>1000C
• For CaC2O4:
– T<135C unbound H2O is
removed to give CaC2O4H2O
– T=225C, CaC2O4H2O is
converted to anhydrous CaC2O4
– T450C, CaC2O4 decomposed
to CaCO3 and CO
– T>800C conversion of the
carbonate to calcium oxide
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Calculation of results from gravimetric
data
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Example
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Sample Problems
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Sample Problems
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Sample Problems
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Sample Problems
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Sample Problems
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Sample Problems
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Applications of gravimetric methods
• Inorganic anions and cations
• Neutral species, like water, sulfur dioxide, carbon dioxide,
and iodine
• Organic substances
– Lactose in milk products
– Salicylates in drug preparations
– Phenolphthalein in laxatives
– Nicotine in pesticides
– Cholesterol in cereals
– Benzaldehyde in almond extracts
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Inorganic precipitating agents
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Reducing agents convert an analyte to its
elemental form for weighing
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Organic precipitating agents for the determination of
inorganic species
• Coordination compounds
– Form slightly soluble nonionic products
• Chelating agents
– Forms products in which the bonding between
the inorganic species and reagent is largely ionic
Heme is a part of
hemoglobin, the oxygencarrying molecule in
human blood
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Chelating reagents
• 8-hydroxyquinoline (oxine)
• Dimethylglyoxime precipitates only Ni(II)
from a weakly alkaline solution
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Organic precipitating reagent
• Sodium tetraphenylborate
– Near-specific precipitating agent
• NH4+
• K+
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Gravimetric methods for organic functional groups
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Volatilization gravimetry
• Determination of water
– Direct method
• Water vapor is collected on any of several solid
desiccants, and its mass is determined from the mass
gain of the desiccant
– Indirect method
• The amount of water is determined by the loss of mass
of the sample during heating
• Volatilization of carbon dioxide
– Determination of the sodium hydrogen carbonate
content of antacid tablets
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Apparatus for determining the NaHCO3 content of
antacid tablets by a gravimetric volatilization
procedure
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Assignment:
1. The aluminum in a 1.200-g sample of impure ammonium
aluminum sulfate was precipitated with aqueous ammonia as the
hydrous Al O .xH O. The precipitate was filtered and ignited at
1000°C to give anhydrous Al O , which weighed 0.2001 g. Express
the result of this analysis in terms of
(a) % NH4Al(SO ) .
(b) % Al2O3.
(c) % Al.
2
3
2
2
4
3
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2. A 0.6407-g sample containing chloride and iodide ions gave a silver halide
precipitate weighing 0.4430 g. This precipitate was then strongly heated in a
stream of Cl2 gas to convert the AgI to AgCl; on completion of this treatment,
the precipitate weighed 0.3181 g. Calculate the percentage of chloride and
iodide in the sample.
3. An aqueous solution contains NaNO3 and KBr. The bromide ion is
precipitated as AgBr by addition of AgNO3. After an excess of the precipitating
reagent has been added,
(a) what is the charge on the surface of the coagulated colloidal particles?
(b) what is the source of the charge?
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(c) what ions make up the counter-ion layer?