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Transcript of CDB 3093 Gravimetry May2016 1
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Gravimetric analysis
CDB 3093
Dr Asna Mohd ZainRSci AMIChemE
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Outline
• Overview of gravimetric methods
• Gravimetric calculation
• Precipitation gravimetry
• Solubility consideration
• Impurities – coprecipitation (inclusion, occlusion and adsorption)
• Particle size control, S & Q
• Filtration, rinsing, drying and igniting
• Gravimetry results
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Intro to gravimetric analysis
• Mass based analysis rely on weighing products
and/or reactant before and after some chemical
reaction
• A product of known composition is precipitated as an
insoluble product from a reaction mixture
• The precipitate is filtered from the solution, dried and
weighted to find the mass of product.
• 1900s T.W. Richard and colleagues winning Nobel
prize for gravimetric analysis by measuring atomic
mass of Ag, Cl and N to six figure accuracy.
Titration is a tried-and-true technique, capable of high precision and accuracy.
©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)
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Gravimetric calculation
• The fundamental of gravimetric analysis is by understand all the chemical reactions its
balance equation to perform stoichiometric calculations by knowing their:
• The concentration, ppm, ppb
• Moles of compound, mol
• Molarity of compound, M
• Weight percent, wt %
• Molecular mass/ formula mass (FM)
• Concentration in ppm or ppb
• ppm = g/mL =mg/L
[wt solute (g)/vol sample (mL)]x106
• ppb = ng/mL = g/L
[Wt solute (g) / vol sample (mL)]x109
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Gravimetric calculation• Moles/molarity
• Ag+ + Cl- AgCl(s)
• Unknown Cl- is titrated with Ag+, equilibrium constant, k of 5.6 x 109
• Rapid reaction forming AgCl in solid form.
•If, 10 mL of chloride solution (measured by tranfer pipet) requires 22.97 mL of0.052 74 M AgNO
3(delivered by a buret) for a complete reaction.
• What are concentration of Cl- in the unknown?
• Mol Ag+ = vol x molarity = (0.02297 L)(0.05274 mol/L)
• = 0.001211 mol
• I mol of Cl- react with I mol of Ag+, if 0.001211 mol of Ag+ is required,
• then 0.001211 mol of Cl- must be in 10 mL of unknown, therefore:
• Cl- unknown = mol Cl- /L of unknown = 0.001211 mol/ 0.01 L = 0.1211 M
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Gravimetric calculation
• The solution titrated was made by dissolving 1.004 g of unknown solid in a
total volume of 100 mL.
• What is the weight percent of chloride (Cl-) in the solid?
• 10 mL of unknown contain 0.001211 mol of Cl- Therefore, 100 mL must
contain 10 times as much, or 0.01211 mol of Cl-
•Cl
-
weight = (0.01211 mol Cl-
)(35.4527 g/mol Cl-
) = 0.4294 g Cl-
• Weight percent of Cl- in the unknown:
• Wt % [Cl-]
• = (g Cl-)/(g unknown) x 100
• = (0.4297 g Cl-)/(1.004 g unknown) x 100
• = 42.78 wt %
• For the diluted concentration
• [Ag]conc x Vconc = [Ag]diluted x Vdiluted
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Gravimetric calculation• Normality is equivalent of material per liter of solution
• N = eq/L or meq/mL
• Eq = mol x no. of reacting units per molecule
• E.g.: eq = mol x 1 reacting unit of Hydrogen per molecule of HCl
• eq. wt. is the weight of substance (g) to furnish one mole of reacting unit.
• Example:
• Calculate the equivalent weight of the following substances
• (a) NH3
and (b) H2C
2O
4(in reaction with NaOH).
• Answer:
• (a) Eq wt = (NH3
g/mol)/(1 eq/mol) = 17.03/1 = 17.03 g/eq
• (b) Eq wt = (H2C
2O
4g/mol )/ (2 eq/mol) = 90.04/2 = 45.02 g/eq
• Note: reacting unit is the H in acid or OH- in base
• eq is applicable in oxidation and reduction reaction
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Direct and indirect gravimeticy
analysis• Direct analysis
• the total suspended solids in the water (filtration)
• If the analyte is not a solid (Pb2+)? convert it into a solid form
• If we add HgCl2
in excess, each mole of PO3
3-produces one mole of
Hg2Cl
2. The precipitate’s mass, therefore, provides an indirect
measurement of the amount of PO3
3 –in the original sample.
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Precipitation gravimetry•
In precipitation gravimetry, an insoluble compound forms, when we add aprecipitating reagent, or precipitant, to a solution containing our analyte.
• Add precipitating reagent to sample solution
• Reacts with analyte to form insoluble material
• Precipitate has known composition or can be converted to known
composition
• Precipitate handling involves
–Quantitative collection (no losses)
– Isolation of pure product
– Measure mass of precipitate
– Calculation of original analyte content (concentration)
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Solubility consideration
• An accurate precipitation gravimetric method requires that
the precipitate’s solubility be minimal
• Solubility losses are minimized by carefully controlling the
composition of the solution in which the precipitate forms.
• Requires an understanding of the relevant equilibrium
reactions affecting the precipitate’s solubility
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Solubility consideration
•Solubility losses may be minimized by adding a large excess of Cl
–
• Ag+ also form a series of soluble chloro-complexes, therefore:
• Increasing the concentration of chloride, however, leads to an increase in the solubility
of AgCl due to the soluble chloro-complexes large excess of chloride must be
avoided
• (Note Ksp is solubility product constant, Ko
spfor AgCl = 1x10-10)
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Solubility consideration• ph Effect
• When the pH is greater than 12.4, the predominate phosphate species is PO4
3 –, and the
solubility of Ca3(PO
4)2
will be at its minimum because only the first reaction occurs.
• As the solution becomes more acidic, the solubility of Ca3(PO
4)2increases due to the
contributions of the 3 other reactions.
• Q1: What must be the concentration of added Ag+ to just start precipitation of AgCl in a 1.0 x
10
-3
M solution of NaCl? Given the Ksp of AgCl at 25C is 1.0 x 10
-10
.
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Impurities
• Precipitation gravimetry is based on a known stoichiometry
between the analyte’s mass and the mass of a precipitate.
• Any impurities present in the precipitate’s matrix must be
removed before obtaining its weight.
• the precipitate must be free from impurities
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Impurities – coprecipitation
• Three main mechanisms of coprecipitation
• Inclusion Occlusion
• Adsorption
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Impurities
• Inclusions are difficult to remove since the included material is chemically part of the
crystal lattice. The only way to remove included material is through reprecipitation.
• Reprecipitate occur by small particle tend to dissolve and attached to the surface of larger
particle or individual particles agglomerate to share common counter ions layer
• Occlusions are minimized by maintaining the precipitate in equilibrium with its supernatant
solution for an extended time. This process is called digestion.
• One benefit of digestion (Ostwald ripening) is that it also increases the average size of
precipitate particles or crystal
• Surface adsorption is minimized by decreasing the precipitate’s available surface area.
• But colloidal particles (size 1-100 m) promote surface adsorption
Ostwalt ripening
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Impurities
• Silver chloride colloidal particle and adsorptive layers when
Cl- is in excess
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Impurities
• The formation of these additional precipitates can usually be minimized by
carefully controlling solution conditions
• Interferents forming precipitates that are less soluble than the analyte
may be precipitated and removed by filtration, leaving the analyte behind insolution.
• Either the analyte or the interferent can be masked using a suitable reagent,
preventing its precipitation.
• Post precipitation occurs when the precipitate allow to stand in contact with
mother liquor producing second substance e.g. calcium oxalate precipitated
in the presence of magnesium ions forming magnesium oxalate.
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Particle size control
• By carefully controlling the precipitation reaction we can significantly increase a
precipitate’s average particle size.
• Precipitation consists of two distinct events:
• Nucleation: the initial formation of smaller stable
• particles of precipitate (Nuclei).
• Particle growth: Larger particles form combination of ions/molecules with the
existing nuclei (when the rate of particle growth exceeds the rate of nucleation).
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Particle size control• Supersaturation, where more dissolved salt in solution than in equilibrium, a metastable condition, is
driving force for equilibrium before nucleation occur
• Solute’s relative supersaturation, RSS, expressed as:
• Q = solute’s actual concentration, S = solute’s expected concentration at equilibrium (solubility). RSS is
the von Weimarn ratio.
• When RSS is small, precipitation is more likely to occur by particle growth than by nucleation. High RSS
promote small crystal but low RSS lead to large crystal.
• Temperature and pH are useful ways to increase the value of S, precipitate from hot solution or used
low pH value where acid medium reduce rate of precipitation.
• Conducting the precipitation in a dilute solution of analyte, or adding the precipitant slowly and with
vigorous stirring are ways to decrease the value of Q.
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Particle size
control
•Surface adsorption of excess lattice ions, however, provides theprecipitate’s particles with a net positive or negative surface charge.
Electrostatic repulsion between the particles prevents them from
coagulating into larger particles.
• Coagulation can be induced by increasing the concentration of the ions
responsible for the secondary adsorption layer or by heating the solution.
• One way to induce coagulation is to add an inert electrolyte, which
increases the concentration of ions in the secondary adsorption layer
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Filtration of precipitate
• Using a filter paper,
crucible and a
vacuum pump,
• Separate solid
precipitate and
solution by filtration
to produce a solid
products andfiltrate in flask
Preparation of solution
Precipitation
Digestion
Filtration
Washing
Drying or igniting
Weighing
Calculation
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Rinsing the precipitate
• Involves the use of cold solvents or rinse solutions containing organic
solvents such as ethanol. Precipitates containing acidic or basic ions
may experience solubility losses if the rinse solution’s pH is not
appropriately adjusted .
• A volatile inert electrolyte is often added to the rinse water to prevent
the precipitate from reverting into smaller particles that may not be
retained by the filtering device.
• This process of reverting to smaller particles is called peptization.
The volatile electrolyte is removed when drying the precipitate.
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Drying or igniting the precipitate
• After filtration, a gravimetric precipitate is heated until its mass becomes constant.
• Ignition decomposes the solid and form a compound of known composition.
• A temperature of 110 °C is usually sufficient when removing water and other easily
volatilized impurities.
• https://youtu.be/rZMzGxjvklo
• https://youtu.be/dERZhN-01f8
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Cations based precipitation
•
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Anions based precipitation
•
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Application• Gravimetric methods have been developed for most inorganic anions
and cations as well as neutral species such as water, sulfur dioxide,
carbon dioxide, and iodine.
• A variety of organic substances can also be easily determined
gravimetrically. Examples: lactose in milk products, salicylates in drug
formulations, nicotine in pesticides etc.
• Organic precipitant
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Gravimetric result•
Gravimetry resulted in the following :• Percent of analyte, % A
• = wt of analyte /wt of sample x 100
• GF is gram of analyte per gram of ppt
• GF = gravimetry factor = [FM analyte/ FM ppt] x a/b
Here, a is mol of analyte
b is mol of precipitate (ppt)
FM is formula mass
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Gravimetry factor
•
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Summary
• Gravimetry calculation
• Advantages of gravimetry
• Precipitation gravimerty
• Solubility consideration
• Impurities
• Particle size control
• Completing steps
• Outcome result
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Example calculation
• Q2: An ore is analyzed for the manganese content by converting
the manganese to Mn3O
4and weighing it. If a 1.52 g sample
yields Mn3O
4weighing 0.126 g, what would be the percent Mn
2O
3
in the sample? The percent Mn? (3 mol Mn2O
3reactant and
product of 2 mol Mn3O
4)
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References
• Gary D. Christian, 2003 Analytical Chemistry, 6th Ed., Wiley, QD101.2
C57 2003
• Daniel C Harris, Exploring Chemical Analysis Second Ed., W.H
Freeman and Company, 2000 QD 75.2. H368.
• Seamus P.J. Higson, Analytical chemistry, Oxford University Press,
2004 QD 101.2.H54