9th meeting - analisis gravimetri - 060914.pptx
TRANSCRIPT
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Analisis GravimetriUniversitas Islam Negeri Sunan Kalijaga
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ANALISIS GRAVIMETRI
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Analisis GravimetriUniversitas Islam Negeri Sunan Kalijaga
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A Gravimetric analysis is based upon the
measurement of the weight of a substancethat has a KNOWN composition AND IS
chemically related to the analyte.
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Introduction
1.) Gravimetric Analysis:
(i) A technique in which the amount of an analyte in a sample is
determined by converting the analyte to some product
Mass of product can be easily measured
(ii) Analyte: the compound or species to be analyzed in a sample
(iii) Overall, gravimetry sounds simple.
Advantages- when done correctly is highly accurate (most
accurate of all time); requires minimal equipment
Disadvantage- requires skilled operator, slow.
Convert analyte into a solid, filter, weigh, calculate via a mole map
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Introduction
1.) Gravimetric Analysis:
(iii) Example:
Determination of lead (Pb+2) in water
Pb+2 + 2Cl- PbCl2(s)
By adding excess Cl-
to the sample, essentially all of thePb+2will precipitate as PbCl2.
Mass of PbCl2is then determined.
- used to calculate the amount of Pb+2in original
solution
ReagentAnalyte Solid Product
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Type of Gravimetric Analysis
Precipitation methods.
Volatilisation methods.
Electrogravimetry.
Particulate gravimetry.
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Type of Gravimetric Analysis Precipitation methods : a gravimetric method in which the signal isthe
mass of the precipitateex. Determination of Cl-by precipitating AgCl
Volatilisation methods : a gravimetric method in which the loss ofvolatile speciesgives rise to the signal ex. Determination of moisturecontent of food, thermal energy vaporizes the H2O. The amount ofcarbon in an aorganic compound by converting C tp CO2.
Electrogravimetry : a gravimetric method in which the s signal is themass of an electrodeposit on the cathode or anodein an electrochemicalcell ex. The electrodeposition of Cu on a Pt cathode
Particulate gravimetry : a gravimetric method in which the mass of aparticulate analyteis determined following is separation from its matrixex. The determination of suspended solids .
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Conditions:
1. Must be a stoichiometric reaction.
2. A stable product; no oxidation, dehydration
or gelatinous precipitates.
3. Must avoid side reactions which result in
co-precipitates.
Gravimetric Analysis
The stoichiometry indicates how the precipitate relates to the analyteMoles Pb2+= moles PbO2(precipitation)
g suspended solid = filters final mass filters initial mass (suspended solid)
g H2O = samples initial mass samples final mass (moisture content)
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In precipitation methods, the species to
be determined is precipitated by a reagent
that yields a sparingly soluble product thathas a known composition or can be
converted to such a substance.
Precipitation gravimetry is based on the formation of an insolublecompoundfollowing the addition of a precipitating reagent, or precipitant,to a solution of the analyte
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Requirement of Gravimetric Analysis
1. The analyte precipitate must be low solubility,
high purity, and of known composition if itsmass is to accurately reflect to analytes mass
Zat yg ditentukan hrs dpt diendapkan secara terhitung (99%)
2. The precipitate must be in a form that is easy
to separate from the reaction mixture
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Precipitation methods
Analyte (or chemically related species) isolated as a
sparingly soluble precipitate of known composition.
Analyte (or chemically related species) isolated as a
sparingly soluble precipitate that can be converted by
heat to species of known composition.
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Precipitation Analysis
1.) Principals:
(i) Reagent + Analyte Solid Product(collect and measure mass)
(ii) Desired Properties of Solid Product
Should be very insoluble
Easily filterable (i.e., large crystals)
Very Pure
Known and constant composition
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Precipitation Analysis2.) Solubility:
(i)The solubility of a precipitate can be decreased by:
Decreasing temperature of solution
Using a different solvent- usually a less polar or organic solvent (like
dissolves like)Solubility vs. pH Solubility vs. TemperatureSolubility vs. Common Ion Effect
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Precipitation Analysis
3.) Gravimetric Analysis:
(i)Governed by equilibrium: AgCl Ksp= 1.8 x 10-10
Solubility of AgCl = [Ag+] + [AgCl] + [AgCl2-]
Cl- + Ag+ AgCl(ag) ion pair formation
AgCl(aq) AgCl(s) intrinsic solubility
AgCl +Cl- AgCl2- complex ion formation
ClAg
AgClK
o
AgClKi
ClAgCl
AgClKf
2
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Precipitation Analysis4.) Filterability:
(i)Want product to be large enough to collect on filter: Doesnt clog filter
Doesnt pass through filter
(ii) Best Case: Pure Crystals
(iii)Worst Case: Colloidal suspension Difficult to filter due to small size
Tend to stay in solution indefinitelysuspended by Brownian motion
- usually 1-100 nm in sizeWhether crystals or colloids are obtained depends on condi tions used in the precipi tation
Brownian Motion
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Precipitation Analysis5.) Process of Crystal Growth:
(i)Two Phases in Crystal Growth
Nucleationmolecules in solution come together randomly and form
small aggregates
Particle growthaddition of molecules to a nucleus to form a crystal
Crystal Growth
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Precipitation Analysis5.) Process of Crystal Growth:
(ii)Nucleation and Particle growth always compete for
molecules/ions being precipitated.
If nucleation is faster than particle growth:
- a large number of small aggregates occur givingcolloidal suspensions
If particle growth is faster than nucleation:
- only a few, large particles form giving pure
crystals
Colloidal suspension Crystal formation
Want to
Convert to
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Precipitation Analysis5.) Process of Crystal Growth:
(iii)Methods for Maximizing Crystal Growth (avoid colloids)
Increase temperature of solution
- increase amount of solute that can be in solution at equilibrium
Add precipitating reagent slowly while vigorously mixing solution
- avoids local high concentrations of solutionKeep volume of solution large
- keep concentration of analyte and precipitating reagent low
Precipitate Ionic compounds in presence of electrolyte (0.1 M
HNO3)
- overcome charge repulsion and promotes particle growth
Control solubility of solute through chemical means- by adjusting pH
- adding complexing agents
- example: precipitation of Ca2+with C2O42-
C2O42- + H+ HC2O4
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Ca2+ + C2O42- CaC2O4(s)
Ksp
Note: As pH ([H+])
changes, the solubility
of CaC2O4changes
Avoid ColloidalParticle
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Particle Size
Ions in solution = 10-3 cm
Coloidal particle = 10-710-4cm
Crystaline particle = 10-4
cm
A solutes relative supersaturation, RSS can be expressed as:
RSS = (Q-S)/S
With,Q = the solutes actual concentration
S = the solutess expected concentration at equilibrium
Q-S = a measure of the solutes supersaturation when precipitation
begins
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A solutes relative supersaturation, RSS can be expressed as:
RSS = (Q-S)/S
With,
Q = the solutes actual concentration
S = the solutess expected concentration at equilibrium
Q-S = a measure of the solutes supersaturation when precipitation
begins
Large number of RSS = solution is highly supersaturated, unstable solution,high rates of nucleation, small particles production (coloidal particles),
while
Small RSS = precipitation is more likely to occur by particle growth thanby nucleation (crystaline particles)
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Precipitation Analysis6.) Crystal Impurities:(i)Impurities are undesirable (known as co-precipitation)
Change the chemical composition of the precipitate
Creates errors in gravimetric analysis
(iii) Types of Impurities
Adsorption, Occlusion, Inclusion
Adsorbed to
crystalsurface
Absorbed or
trapped within
crystal pockets
Impurity placed in
crystal instead of
analyte
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Precipitation Analysis6.) Crystal Impurities:
(iii) Ways to Minimize Impurities
Maximize crystal growth
large pure crystals decrease occlusions and adsorbed impurities
Digestion: allowing precipitate to stand in mother liquor
(precipitating solution), usually while being heated-promotes removal of impurities from crystal
- increases size of crystals
Wash precipitate, re-dissolve in fresh solvent and re-precipitate
- helps decrease all types of impurities
Add a masking agent to solution
- keeps impurities from precipitating, but not analyte
Mg2+ + CPCH Solid
product
Mn2+ + 6CN- Mn(CN)64-
Masking
agentprecipitant
ColorImpurity
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Precipitation Analysis
7.) Final Preparation of Precipitates:
(vi) Washing Precipitates
Precipitates from ionic compounds
- need electrolyte in wash solution
- keep precipitate from breaking up and redissolving (peptization)
Electrolyte should be volatile
- removed by drying
- HNO3, HCl, NH4, NO3, etc.
Illustration:
- AgCl(s) should not be washed with H2O, instead wash with dilute
HNO3(vii) Drying/Igniting Precipitates
Many precipitates contain varying amounts of H2O
- adsorbed from the air (i.e. hygroscopic)
Precipitates are dried for accurate, stable mass measurements
Precipitates are also ignited to convert to a given chemical form
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Representative Gravimetric Analyses
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Berapa jumlah sampel yang mengandung 12 %
klorin (Cl) harus diambil untuk analisis jika
kimiawan tersebut ingin memperoleh suatuendapan AgCl yang beratnya 0,5 g?
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Penyelesaian:
Reaksi pengendapannya adalah :
Ag++ Cl- AgCl(s)
dan mol Cl-= mol AgCl
Jika w = gram sampel, maka:
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EXAMPLE
An ore containing magnetite, Fe3O4, wasanlyzed by dissolving a 1.5419 g sample in
concentrated HCl, giving a mixture of Fe2+
andFe3+. After adding HNO3 to oxidize any Fe2+to Fe3+, the resulting solution was diluted withwater and the Fe3+ precipitated as Fe(OH)3
by adding NH3. After filtering and rinsing, theresidue was ignited, giving 0.8525 g of pureFe2O3. Calculate the % w/w Fe3O4 in the
sample!
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EXAMPLE
An ore yang mengandung magnetite, Fe3O4, dianalisisdengan melarutkan 1,5419 g sampel dalam HCl pekat,menghasilkan campuran Fe2+dan Fe3+. Selanjutnya,
Fe2+dioksidasi menjadi Fe3+, dengan menambahkanHNO3. Hasilnya dilarutkannya dalam air dan denganmenambahkan NH3, Fe
3+diendapkan sebagai Fe(OH)3.Setelah penyaringan dan pencucian selesai, didapatkanresidu sebesar 0,8525 g Fe2O3 murni.
Hitunglah % (w/w) Fe3O4dalam sampel!
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Penyelesaian,
Kasus ini merupakan salah satu contoh analysis langsung karena besi dalam analit,Fe3O4 merupakan bagian dari endapan yang diisolasi, Fe2O3. Dengan, mengacupada persamaan reaksi sempurna, maka:
3 mol Fe3O
4= 2 x mol Fe
2O
3
Menggunakan rumus berat, dengan mengkonversikan mol ke gram didapat:
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Penyelesaian,
3 mol Fe3O4= 2 x mol Fe2O3
Menggunakan rumus berat, dengan mengkonversikan mol ke gram didapat:
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Penyelesaian,
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Volatilisation GravimetryThe changes in a samples mass is monitored as an increasing of temperature
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Dry substance (ds) is the mass portion of solid matter in sample whichobtained by the drying process, after the water has been removed and itis given in percentage (%).
Dry substance (ds)
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Dry substance (ds)
8 g homogeneous samples(duplicates)
Placed on the weighing dish andInitial weight was noted
Infrared heater wasadjusted;
output weight was noted
ma= initial weight [g]mb= output weight [g]
a
b
m
mds %100
Dry substance content in manure :Liquid : up to 10%Semi-solid : 10 to 20%
Solid : 20% or more
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Example:
The thermogram shows the change in mass for a sample of calcium oxalatemonohydrate, CaC2O4.H2O. The original sample weighed 24.60 mg and washeated from room temperature to 1000 oC at rate of 5 oC per min. Thefollowing changes in mass and corresponding temperature ranges were
observed:
Loss of 3.03 mg from 100 250 oCLoss of 4.72 mg from 400 500 oCLoss of 7.41 mg from 700 850 oC
Determine the identities of the volatilization products and the solidresidue at each step of the thermal decomposition.
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Solution:a) The loss of 3.03 mg from 100 250 oC corresponds to a 12.32%decrease in the original samples mass
(30.3 mg/24.60 mg) x 100 = 12.32%
In term of CaC2O4.H2O this corresponds to a loss of :
0.1232 x 146.11 g/mol = 18 g/mol
The prducts molar mass, coupled with the temperature range, suggests
that this represents the loss of H2O. The residue is CaC2O4
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Solution:b) The loss of 4.72 mg from 400 500 oC corresponds to a 19.19%decrease in the original samples mass
(4.72 mg/24.60 mg) x 100 = 19.19%
In term of CaC2O4this corresponds to a loss of :
0.1919 x 146.11 g/mol = 28.04 g/mol
The products molar mass, coupled with the temperature range, suggests
that this represents the loss of CO. The residue is CaCO3
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Solution:c) The loss of 7.41 mg from 700 850 oC corresponds to a 30.12%decrease in the original samples mass
(7.41 mg/24.60 mg) x 100 = 30.12%
In term of CaC2O4this corresponds to a loss of :
0.3012 x 146.11 g/mol = 44.01 g/mol
The products molar mass, coupled with the temperature range, suggests
that this represents the loss of CO2. The residue is CaO
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Particulate GravimetryBased on precipitation or volatilization reactions require that the analyte,or some species in the sample, participate in a chemical reaction producinga change in physical state.
Example:
A 200-mL sample of water was filtered through a preweighed glass fiberfilter. After drying to constant weight at 105 oC, the filter was found tohave increased in mass by 48,2 mg. Determine the total suspended solidsfor the sample in parts per million.
Solution:Part per million is the same as milligrams of anlyte oer liter of solution;thus the total suspended solids for the sample is
48.2 mg/0.2000 L = 241 ppm
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GRAVIMETRY Procedures
Sample preparation
Precipitation Dillution Filtration Rinsing
Drying/combusting Weighing Calculation
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Applications
Extensive numbers of inorganic ions are
determined with excellent precision andaccuracy.
Routine assays of metallurgical samples.
Relative precision 0.1 to 1%.Good accuracy
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Thank you for your attention