Что такое findslide.org?

FindSlide.org - это сайт презентаций, докладов, шаблонов в формате PowerPoint.


Для правообладателей

Обратная связь

Email: Нажмите что бы посмотреть 

Яндекс.Метрика

Презентация на тему Gravimetric Analysis

Содержание

How to Perform a Successful Gravimetric AnalysisWhat steps are needed?Sampled dried, triplicate portions weighedPreparation of the solutionPrecipitationDigestionFiltrationWashingDrying or ignitingWeighingCalculation
Section 09Gravimetric Analysis and Precipitation Equilibria How to Perform a Successful Gravimetric AnalysisWhat steps are needed?Sampled dried, triplicate Gravimetric Analysis Gravimetric Analysis – one of the most accurate and precise Gravimetric AnalysisHow?Quantitative collection of material of known compositionPrecipitation of analyte with selective Gravimetric AnalysisPrecipitation TechniquesAdd precipitating reagent to sample solutionReacts with analyte to form Gravimetric AnalysisDesirable properties of analytical precipitates:Readily filtered and purifiedLow solubility, preventing losses Gravimetric AnalysisPrecipitating reagents:Selective Ag+ + Halides (X-) ? AgX(s) Ag+ + CNS- Gravimetric AnalysisFilterability of Precipitates:Colloidal suspensions10-7 to 10-4 cm diameterNormally remain suspendedVery difficult Gravimetric AnalysisFilterability of Precipitates:Precipitate formation affected by RELATIVE SUPERSATURATION(R.S.)R.S. = (Q-S)/SS = Important Factors for Gravimetric AnalysisNucleationIndividual ions/atoms/molecules coalesce to form “nuclei”Particle GrowthCondensation Important Factors for Gravimetric AnalysisCoagulation, agglomerationSuspended colloidal particles coalesce Important Factors for Gravimetric AnalysisCo-precipitationNormally soluble compounds carried down with insoluble Fig. 10.1. Ostwald ripening. During digestion at elevated temperature:Small particles tend to Fig. 10.2. Representation of silver chloride colloidal particleand adsorptive layers when Cl- Organic precipitating agents are chelating agents. They form insoluble metal chelates.©Gary Christian, Gravimetric AnalysisCalculations of analyte content requires knowledge of :ChemistryStoichiometryComposition of precipitate Gravimetry and Solution EquiliriaThermal Conversion to Measurable FormRemoval of volatile reagents & Gravimetric CalculationsGravimetric Factor (GF):GF = (fwt analyte (g/mol)/fwt precipitate(g/mol))x(a(moles analyte/b(moles precipitate))GF Gravimetric ErrorsUnknown Stoichiometry:Consider Cl- determination with AgNO3Ag+ + Cl- ? AgClAg+ + Gravimetric Errors Gravimetric Errors Co-precipitation: (w/AgCl) Alternative Gravimetry TechniqueHomogeneous PrecipitationWhat? Precipitating agent generated slowly by chemical reaction in Precipitation Equilibria: The Solubility ProductSolubility of Slightly Soluble Salts:AgCl(s)??(AgCl)(aq)?? Ag+ + Cl-Solubility The molar solubility depends on the stoichiometry of the salt. A 1:1 Precipitation Equilibria: The Common Ion EffectCommon Ion EffectWill decrease the solubility of a slightly soluble salt. Fig. 10.3. Predicted effect of excess barium ion on solubility of BaSO4. Diverse Ion Effect on Solubility:Presence of diverse ions will increase the solubility Predicted effect of increased ionic strength on solubility ofBaSO4. Solubility at zero Gravimetric calculation using spreadsheet.Cell B3 calculates %Fe from g. Fe2O3 (Cell D2) Using Exel Solver to calculate solubility.Enter the formula (=s2/Ksp) in Cell E4 Calculating Results from Gravimetric DataThe calcium in a 200.0 mL sample of Calculating Results from Gravimetric DataAn iron ore was analyzed by dissolving a Calculating Results from Gravimetric Data A 0.2356 g sample containing only NaCl
Слайды презентации

Слайд 2 How to Perform a Successful Gravimetric Analysis
What steps

How to Perform a Successful Gravimetric AnalysisWhat steps are needed?Sampled dried,

are needed?
Sampled dried, triplicate portions weighed
Preparation of the solution
Precipitation
Digestion
Filtration
Washing
Drying

or igniting
Weighing
Calculation

Слайд 3 Gravimetric Analysis
Gravimetric Analysis – one of the

Gravimetric Analysis Gravimetric Analysis – one of the most accurate and

most accurate and precise methods of macro-quantitative analysis.
Analyte selectively

converted to an insoluble form.
Measurement of mass of material
Correlate with chemical composition
Why?
Simple
Often required for high precision


Слайд 4 Gravimetric Analysis
How?
Quantitative collection of material of known composition
Precipitation

Gravimetric AnalysisHow?Quantitative collection of material of known compositionPrecipitation of analyte with

of analyte with selective agent
Volitization and collection of analyte
w/o

loss of material in handling/processing
Free from solvent, impurities
Determination of mass
Direct or
By difference

Слайд 5 Gravimetric Analysis
Precipitation Techniques
Add precipitating reagent to sample solution
Reacts

Gravimetric AnalysisPrecipitation TechniquesAdd precipitating reagent to sample solutionReacts with analyte to

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)


Слайд 6 Gravimetric Analysis
Desirable properties of analytical precipitates:
Readily filtered and

Gravimetric AnalysisDesirable properties of analytical precipitates:Readily filtered and purifiedLow solubility, preventing

purified
Low solubility, preventing losses during filtration and washing
Stable final

form (unreactive)
Known composition after drying or ignition


Слайд 7 Gravimetric Analysis
Precipitating reagents:
Selective
Ag+ + Halides (X-) ?

Gravimetric AnalysisPrecipitating reagents:Selective Ag+ + Halides (X-) ? AgX(s) Ag+ +

AgX(s)
Ag+ + CNS- ? AgCNS(s)
Specific
Dimethylglyoxime (DMG)
2 DMG +

Ni2+ ? Ni(DMG)2(s) + 2 H+

Слайд 8 Gravimetric Analysis
Filterability of Precipitates:
Colloidal suspensions
10-7 to 10-4 cm

Gravimetric AnalysisFilterability of Precipitates:Colloidal suspensions10-7 to 10-4 cm diameterNormally remain suspendedVery

diameter
Normally remain suspended
Very difficult to filter
Crystalline suspensions
> tenths of

mm diameter
Normally settle out spontaneously
Readily filterable

Слайд 9 Gravimetric Analysis
Filterability of Precipitates:
Precipitate formation affected by
RELATIVE

Gravimetric AnalysisFilterability of Precipitates:Precipitate formation affected by RELATIVE SUPERSATURATION(R.S.)R.S. = (Q-S)/SS

SUPERSATURATION(R.S.)
R.S. = (Q-S)/S
S = Equilibrium Solubilty of Precipitate
Q =

Instantaneous Concentration
Larger Q leads to colloidal precipitates.

Слайд 10 Important Factors for Gravimetric Analysis
Nucleation
Individual ions/atoms/molecules coalesce

Important Factors for Gravimetric AnalysisNucleationIndividual ions/atoms/molecules coalesce to form “nuclei”Particle

to form “nuclei”
Particle Growth
Condensation of ions/atoms/molecules with existing “nuclei”

forming larger particles which settle out
Colloidal Suspension
Colloidal particles remain suspended due to adsorbed ions giving a net + or - charge

Слайд 11 Important Factors for Gravimetric Analysis
Coagulation, agglomeration
Suspended

Important Factors for Gravimetric AnalysisCoagulation, agglomerationSuspended colloidal particles coalesce

colloidal particles coalesce to form larger filterable particles (inert

electrolyte allows closer approach)


Peptization
Re-dissolution of coagulated colloids by washing and removing inert electrolyte

Слайд 12 Important Factors for Gravimetric Analysis
Co-precipitation
Normally soluble compounds

Important Factors for Gravimetric AnalysisCo-precipitationNormally soluble compounds carried down with

carried down with insoluble precipitate (surface adsorption, occlusion, mixed

crystals, entrapment)


Digestion
Precipitation heated for hour(s) in contact with solution form which it was formed

Слайд 13 Fig. 10.1. Ostwald ripening.
During digestion at elevated

Fig. 10.1. Ostwald ripening. During digestion at elevated temperature:Small particles tend

temperature:
Small particles tend to dissolve and reprecipitate on larger

ones.
Individual particles agglomerate.
Adsorbed impurities tend to go into solution.

©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)


Слайд 14 Fig. 10.2. Representation of silver chloride colloidal particle
and

Fig. 10.2. Representation of silver chloride colloidal particleand adsorptive layers when

adsorptive layers when Cl- is in excess.
Cl- adsorbs

on the particles when in excess (primary layer).
A counter layer of cations forms. The neutral double layer causes the colloidal particles to coagulate.
Washing with water will dilute the counter layer and the primary layer charge causes the particles to revert to the colloidal state (peptization). So we wash with an electrolyte that can be volatilized on heating (HNO3).

©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)


Слайд 15 Organic precipitating agents are chelating agents.
They form

Organic precipitating agents are chelating agents. They form insoluble metal chelates.©Gary

insoluble metal chelates.

©Gary Christian, Analytical Chemistry, 6th Ed.

(Wiley)


Слайд 16 Gravimetric Analysis
Calculations of analyte content requires knowledge of

Gravimetric AnalysisCalculations of analyte content requires knowledge of :ChemistryStoichiometryComposition of precipitate

:
Chemistry
Stoichiometry
Composition of precipitate


Слайд 17 Gravimetry and Solution Equiliria
Thermal Conversion to Measurable Form
Removal

Gravimetry and Solution EquiliriaThermal Conversion to Measurable FormRemoval of volatile reagents

of volatile reagents & solvent
Extended heating at 110 to

115 OC
Chemical conversion to known stable form
CaC2O4(s)? CaO(s) + CO(g) + CO2(g)
Volatilization & trapping of component
NaHCO3(aq)+ H2SO4(aq) ? CO2(g)+ H2O + NaHSO4(aq)

Слайд 18 Gravimetric Calculations
Gravimetric Factor (GF):
GF = (fwt analyte

Gravimetric CalculationsGravimetric Factor (GF):GF = (fwt analyte (g/mol)/fwt precipitate(g/mol))x(a(moles analyte/b(moles

(g/mol)/fwt precipitate(g/mol))x(a(moles analyte/b(moles precipitate))
GF = g analyte/g precipitate
% analyte

= (weight analyte (g)/ weight sample (g)) x 100%
% (w/w) analyte (g) = ((wt ppt (g) x GF)/wt sample) x 100%

Слайд 19 Gravimetric Errors
Unknown Stoichiometry:
Consider Cl- determination with AgNO3
Ag+ +

Gravimetric ErrorsUnknown Stoichiometry:Consider Cl- determination with AgNO3Ag+ + Cl- ? AgClAg+

Cl- ? AgCl
Ag+ + 2 Cl- ? AgCl2
Gravimetric Factor:


GF = fwt analyte/fwt precipitate x moles analyte/moles precipitate
Calculation for Cl- = wt. Ppt * GF

Слайд 20 Gravimetric Errors

Gravimetric Errors

Слайд 21 Gravimetric Errors Co-precipitation: (w/AgCl)

Gravimetric Errors Co-precipitation: (w/AgCl)

Слайд 22 Alternative Gravimetry Technique
Homogeneous Precipitation
What?
Precipitating agent generated slowly

Alternative Gravimetry TechniqueHomogeneous PrecipitationWhat? Precipitating agent generated slowly by chemical reaction

by chemical reaction in analyte solution
Why?
Precipitant appears gradually throughout
Keeps

relative supersaturation low
Larger, less-contaminated particles
How?
(OH-) by urea decomposition
(NH2)2CO ? 2 OH- + CO2 + 2 NH4+
(S=) by thioacetamide decomposition
CH3CSNH2? H2S + CH3CONH2
(DMG) from biacetyl + hydroxylamine
CH3C(=0)-C(=0)CH3 + 2 H2NOH ?DMG + 2 H2O




Слайд 24 Precipitation Equilibria: The Solubility Product
Solubility of Slightly Soluble Salts:
AgCl(s)??(AgCl)(aq)??

Precipitation Equilibria: The Solubility ProductSolubility of Slightly Soluble Salts:AgCl(s)??(AgCl)(aq)?? Ag+ +

Ag+ + Cl-
Solubility Product KSP = ion product
KSP =

[Ag+][Cl-]
Ag2CrO4(s) ?? 2 Ag+ + CrO42-
KSP = [Ag+]2[CrO42-]


Слайд 25 The molar solubility depends on the stoichiometry of

The molar solubility depends on the stoichiometry of the salt. A

the salt.
A 1:1 salt is less soluble than

a nonsymmetric salt with the same Ksp.

©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)


Слайд 26 Precipitation Equilibria: The Common Ion Effect
Common Ion Effect
Will decrease

Precipitation Equilibria: The Common Ion EffectCommon Ion EffectWill decrease the solubility of a slightly soluble salt.

the solubility of a slightly soluble salt.


Слайд 27 Fig. 10.3. Predicted effect of excess barium ion

Fig. 10.3. Predicted effect of excess barium ion on solubility of

on solubility of BaSO4.
The common ion effect is

used to decrease the solubility.
Sulfate concentration is the amount in equilibrium and is equal to the BaSO4 solubility.
In absence of excess barium ion, solubility is 10-5 M.

©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)


Слайд 28 Diverse Ion Effect on Solubility:
Presence of diverse ions

Diverse Ion Effect on Solubility:Presence of diverse ions will increase the

will increase the solubility of precipitates due to shielding

of dissociated ion species.
KSPo and Activity Coefficients
AgCl(s)??(AgCl)(aq)?? Ag+ + Cl-
Thermodynamic solubility product KSPo
KSPo = aAg+ . aCl- = [Ag+]ƒAg+. [Cl-]ƒCl-
KSPo = KSP ƒAg+. ƒCl-
KSP = KSPo/(ƒAg+. ƒCl)

Слайд 29 Predicted effect of increased ionic strength on solubility

Predicted effect of increased ionic strength on solubility ofBaSO4. Solubility at

of
BaSO4. Solubility at zero ionic strength is 1.0 x

10-5 M.

Ksp = Ksp0/fAg+fSO42-
Solubility increases with increasing ionic strength as activity coefficients decrease.

©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)


Слайд 30 Gravimetric calculation using spreadsheet.
Cell B3 calculates %Fe from

Gravimetric calculation using spreadsheet.Cell B3 calculates %Fe from g. Fe2O3 (Cell

g. Fe2O3 (Cell D2) and g. sample (Cell B2).



©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)


Слайд 31 Using Exel Solver to calculate solubility.
Enter the formula

Using Exel Solver to calculate solubility.Enter the formula (=s2/Ksp) in Cell

(=s2/Ksp) in Cell E4 (don’t enter =1; that goes

in Solver).
The value of s (Cell C4) is changed iteratively until the formula equals 1.

©Gary Christian, Analytical Chemistry, 6th Ed. (Wiley)


Слайд 32 Calculating Results from Gravimetric Data
The calcium in a

Calculating Results from Gravimetric DataThe calcium in a 200.0 mL sample

200.0 mL sample of a natural water was determined

by precipitating the cation as CaC2O4. The precipitate was filtered, washed, and ignited in a crucible with an empty mass of 26.6002 g. The mass of the crucible plus CaO (fwt 56.077 g/mol) was 26.7134 g. Calculate the concentration of Ca (fwt 40.078 g/mol) in the water in units of grams per 100 mL.

Слайд 33 Calculating Results from Gravimetric Data
An iron ore was

Calculating Results from Gravimetric DataAn iron ore was analyzed by dissolving

analyzed by dissolving a 1.1324 g sample in concentrated

HCl. The resulting solution was diluted with water, and the iron(III) was precipitated as the hydrous oxide Fe2O3.xH2O by addition of NH3. After filtration and washing, the residue was ignited at high temperature to give 0.5394 g pure Fe2O3 (fwt 159.69 g/mol). Calculate (a) the % Fe (fwt 55.847 g/mol) and (b) % Fe3O4 (fwt 231.54 g/mol) in the sample.

  • Имя файла: prezentatsiya-gravimetric-analysis.pptx
  • Количество просмотров: 372
  • Количество скачиваний: 0