8. Gravimetri1 New

AW-UII

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2Definisi

Jenis

Tahapan gravimetri

Stoikiometri Faktor gravimetri

Terbentuknya endapan

Kopresipitasi

Agen pengendap

Gravimetric Analysis- What is It?

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Definisi (umum):

Metode penentuan bobot analit

(dlm bntuk endapan tidak larut)

yang komposisinya diketahui

dan secara kimia berhubungandengan analit.

Direct methods Penentuan bobot analit itu sendiri Ni(DMG)2Indirect methods Melibatkan perubahan bobot analit (selisih) sbg hasil dari menghilangkan bagian analit dan weighing

by difference contoh: Moisture analysis

Terjadinya perubahan unsur menjadi senyawa murni, stabil, dan dapat ditimbang dengan teliti. Berat unsur (analit) dapat dihitung berdasarkan rumus senyawa dan berat atom-atom di dalamnya.

Determination of mass:Direct orBy difference


Reaction:aA + rR AaRr pptwhere:

a: moles of analyte A

r : moles of reagent R

ppt = precipitate = endapan4

AaRr produk (endapan) murni, tidak larut, yang dapat dikeringkan dan ditimbang atau

dibakar untuk dirubah menjadi senyawa yang dapat ditimbang


Hitunglah berat Na+ (23 g/mol) yang terkandung dalam 25,0 g Na2SO4 (142,0 g/mol)!

Berapa gram perak nitrat (169,9 g/mol) yang dibutuhkan untuk mengubah 2,33 g natrium karbonat (106,0 g/mol) menjadi perak karbonat? Berapa gram pula perak karbonat (275,7 g/mol) yang dihasilkan?

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Types of Gravimetric Methods

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Precipitation Methods (pengendapan)Analit diubah menjadi endapan dengan kelarutan yang rendah.

Electrogravimetric Methods Analit diendapkan pada elektroda

Volatilization Methods (penguapan)Analit atau produk dekomposisi diuapkan. Lalu Produk yang menguap dikumpulkan dan ditimbang atau massa produk ditentukan dari penurunan berat massa sampel, contoh:NaHCO3(aq)+ H2SO4(aq) CO2(g)+ H2O + NaHSO4(aq)(gas CO2 yg dihasilkan, dikumpulkan dlm tabung khusus, lalu ditimbang)

Kadar air dalam ekstrak maupun dalam bahan pangan.

Particulate GravimetryMassa analit ditentukan setelah dilakukan pemisahan (penyaringan) dari matriks (ex. Total solid suspend)

Sampel disiapkan

(dikeringkan; ditimbang)

Sampel dilarutkan

Ditambahkan senyawa

pengendap (boleh berlebih)

Endapan dikoagulasi (by heating; digesti)

Endapan Disaring

(dipisahkan dari larutan asal)

Endapan Dicuci

Endapan Dikeringkan

Ditimbang hingga diperoleh

bobot tetap7

Steps in Gravimetric Analysis

8precipitating agent

sample

dissolvedcomponents

Ukuran partikel endapan

Colloidal suspensions

Tidak mudah mengendap

(Normally remain suspended)

Sulit disaring

Ukuran (diameter)

10-7 s/d 10-4 cm

Crystallin suspension

Cenderung mengendap scr

spontan

Dapat langsung disaring

Ukuran

> 10 mm

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Faktor yg mempengaruhi:

Kelarutan endapan

Suhu

Konsentrasi reaktan

Kecepatan reaksi

Pengontrolan pH

Relative Supersaturation

(RS)

more solute than

should be present

in solution (lewat

jenuh)

Persamaan Van Weimarn:

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Relative Supersaturation (RS) = (Q-S)/S

S = Equilibrium Solubilty of Precipitate/ kelarutan endapan saat setimbangQ = Instantaneous Concentration/ actual solute concentration/ konsentrasi zat terlarut pd waktu tertentu

Endapan dalam ukuran koloidalRS >>>

Endapan dalam ukuran kristalRS

Bgaimana cara menurunkan nilai RS??

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Meningkatkan suhu (meningkatkan

kelarutan endapan, S )

Dilute solution (endapan dibentuk dari larutan encer)

(Q)

Good stirring in heated solutions

(Q).

Penambahan agen pengendap scr

perlahan

Careful pH control (jika kelarutan

dipengaruhi oleh pH)

mencegah

kopresipitasi

mencegah

mixed cristal

dan reaksi

berjalan

sempurna

Mekanisme pembentukan endapan

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Ukuran endapan dihasilkan dari kompetisi proses:

Nucleation

Individu ion/ atom / molekul bergabung membentuk "inti/

nuclei

Meningkat secara eksponensial

dengan meningkatnya nilai

RS.

Jika nukleasi mendominasi,

endapan tersusun atas sejumlah

besar partikel kecil endapan koloid

Particle growth

Kondensasi ion / atom / molekul dengan "inti/

nuclei" yang ada, membentuk partikel

yang lebih besar, lalu mengendap

Terjadi lebih lambat, pada nilai RS yang

kecil.

Jika pertumbuhan partikel mendominasi, endapan terdiri dari

sejumlah kecil partikel yang lebih besar

endapan kristal

Induction period

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A B

RS RS

Senyawa/ agen pengendap

React only with single chemical species

Dimethylglyoxime (DMG)

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

Spesifik

More common, react with a limited number of species

Ag+ + Halides (X-) AgX(s) Ag+ + CNS- AgCNS(s)

Selektif 14

Senyawa/ agen pengendap (organik) merupakan agen pengkelat.

Senyawa ini membentuk kelat-logam yang tidak larut.

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

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Senyawa/ agen pengendap (anorganik).

pH control of precipitation

Ca2+ + C2O42- CaC2O4 (s)

H2C2O4 2 H+ + C2O4

2-

Feeder Reaction:

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Sampel disiapkan


Sampel dilarutkan

Tambahkan senyawa



Endapan Disaring

(separate pptfrom mother

liquor)

Endapan Dicuci

Endapan Dikeringkan


bobot tetap19


Koagulasi

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Proses perubahan colloidal suspensions filterable solid

Coagulation, agglomeration

Suspensi partikel koloid bergabung membentuk partikel filterable yang lebih besar (merusak kestabilan koloid mudah mengendap filterable)

Dengan cara:By heating; by stirring; by adding an elctrolyte

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Mengapa koloid

cenderung bersifat stabil?

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Figure 8.6 Two methods

for coagulating a

precipitate of AgCl.

(a) Coagulation does not

happen due to the

electrostatic

repulsion between

the two positively

charged particles.

(b) Decreasing the

charge within the

primary adsorption

layer, by adding

additional NaCl,

decreases the

electrostatic

repulsion, allowing

the particles to

coagulate.

(c) Adding additional

inert ions decreases

the thickness of the

secondary

adsorption layer.

Because the particles

can approach each

other more closely,

they are able to

coagulate.

Digestion

Digesti endapan dipanaskan dalam larutan asal mula terbentuknya endapan tsb (mother liquor)

selama waktu ttt, shg tjd pemadatan inti.

Padatan kembali melarut dan mengendap, bersamaan dengan meningkatnya pertumbuhan

kristal

Padatan yang dihasilkan biasanya lebih filterable dan lebih murni

Meminimalkan kontaminasikopresipitasi23

Membantu menghilangkan suspensi koloid

yang sulit atau tidak mungkin untuk disaring

DT

Fig. 10.1. Ostwald ripening.

During digestion at elevated temperature:

Small particles tend to dissolve and reprecipitate on larger ones.

Individual particles agglomerate.

Adsorbed impurities tend to go into solution.


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Sampel disiapkan


Sampel dilarutkan

Tambahkan senyawa



Endapan Disaring


liquor)

Endapan Dicuci

Endapan Dikeringkan


bobot tetap25


Filtration Usually accomplished with filter

paper or fritted glass crucibles Pore size should be chosen carefully to

retain analyte particles but not clog/tersumbat

Filter paper must usually be removed by ignition but this requires a low ash paper (< 0.010% w/w)

Proses penyaringan dimulai dengan men-dekantasi cairan supernatan melalui media saring, diikuti oleh endapan padat sembari dicuci

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Sampel disiapkan


Sampel dilarutkan

Tambahkan senyawa



Endapan Disaring


liquor)

Endapan Dicuci

Endapan Dikeringkan


bobot tetap27


Rinsing

Removes the remaining supernatant

Avoid solubility losses

Use of cold solvent often accomplishes this goal

pH adjustment may be necessary for acids and bases

A volatile inert electrolyte may be added to the wash to avoid peptization

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Peptization:

Coagulated colloidal Colloidal suspension

Peptidization

A procedure where the precipitate is washed and filtered, but part of the precipitate reverts to the colloidal form because supporting electrolyte is gone.

Cooling the system with an ice-water bath minimizes loss of precipitate due to dissolution

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AgCl (s) AgCl (colloid)

Fig. 10.2. Representation of silver chloride colloidal particle

and 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)

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Kehadiran elektrolit dapat membantu tjd

endapan

Excess charge on

colloid creates ionic

atmosphere around

particle

D.C. Harris, Quantitative Chemical Analysis, 6th Ed., p686

Silver nitrate is added very slowly to an

acidic solution containing chloride. Silver

chloride nuclei form with a surface layer of

ions. The charged AgCl particles (or colloidal particles) repel each other.

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Mechanism of Precipitation

In addition to the primary adsorbed silver ions, some nitrate ions form an electrostatic layer around the nucleus.

These counter ions tend to aggregate around the [AgCl:Ag]+ center because these centers have a net positive charge (excess Ag+) and additional negative charge is required to maintain electrical neutrality.

Counter ions are less tightly held than the primary adsorbed ions and the counter ion layer is somewhat diffuse and contains ions other than those of the counter ions.

These layers of charged ions associated with the surface of the nuclei are known as the electric double layer.

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Mechanism of Precipitation

Sampel disiapkan


Sampel dilarutkan

Tambahkan senyawa



Endapan Disaring


liquor)

Endapan Dicuci

Endapan Dikeringkan


bobot tetap34


Pengeringan dan Pemanasan Endapan

The temperature required to produce asuitable weighing form varies from precipitate toprecipitate.

Setelah disaring endapan dipanaskan hingga diperoleh bobot konstan.

Pemanasan akan menghilangkan pelarut maupun

senyawa menguap lain yang masih terdapat di

dalam endapan.

Endapan dibakar to decompose the solid form form a compound of know composition or called weighing form.

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(g)(g)s(s) OH CO)(OFeOH )Fe(HCO 22321hr C850

23

-

2 o

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Meningkatkan kemurnian endapan?

1. Re-precipitation

a procedure including washing away the mother liquor, redissolving the precipitate, and precipitating the product again

2. Drying the solid

Generally the solids are dried at~120 oC, but conditions fordrying can vary considerably. Todetermine the correct drying regime, a thermogravimetric (TGA) balance may be used.

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Meningkatkan kemurnian endapan?

3. Precipitation in the presence of electrolyte Coulombic repulsion is diminished in the presence of

electrolyte because of a compression of the volume of the ionic atmosphere

4. Digestion Raising the temperature will increase the collision

energy for colloidal particles and overcome Coulombicrepulsion, leading to formation of larger particles (coalescence)

Karakteristik endapan yang diharapkan:

1. Dapat disaring dan dicuci shg bebas dari kontaminan (murni)

2. Memiliki kelarutan yang rendah (analit tidak mudah hilang selama proses penyaringan dan pencucian).

3. Tidak bereaksi dengan udara/ tidak mudah teroksidasi.

4. Diketahui komposisinya setelah dikeringkan/ dipanaskan).

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Criteria for Gravimetric Analysis

1. The desired substance must completely precipitate from solution

In most determinations the precipitate is of such low solubility that dissolution of the analyte is negligible

An additional factor is the "common ion" effect, further reducing the solubility of the precipitate

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Criteria for Gravimetric Analysis

When Ag+ is precipitated from solution through the addition of Cl-

the (low) solubility of AgCl is further reduced by the excess of Cl- that is added, pushing the equilibrium to the right (Le Chateliers Principle).

Precipitation occurs when the value of [Ag+][Cl-] exceeds the solubility product Ksp of AgCl (1.810

-

10).

)(sAgClClAg

Why AgCl?

Reaction is highly selective - no interferents 2AgCl ----> 2Ag + Cl2(g)

AgCl is insoluble in water, i.e., only slightly soluble in water-losses negligible 1.4 mg/L at 200C 22 mg/L at 1000C

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Why AgCl is a Good Precipitate?

Small mass of analyte yields large mass of precipitate-sensitive technique

AgCl precipitates in curds/lumps (gumpalan) that can be easily collected,

dried, and weighed

Precipitate (ppt) is not hygroscopic

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Precipitation Equilibria:The Solubility Product

Solubility of Slightly Soluble Salts:

AgCl(s)(AgCl)(aq) Ag+ + Cl-

Solubility Product KSP = ion product

KSP = [Ag+][Cl-]

Ag2CrO4(s) 2 Ag+ + CrO4

2-

KSP = [Ag+]2[CrO4

2-]

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The molar solubility depends on the stoichiometry of the salt.

A 1:1 salt is less soluble than a nonsymmetric salt with the same Ksp.


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Precipitation Equilibria:The Common Ion Effect

Common Ion Effect

Will decrease the solubility of a slightly soluble salt.

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Fig. 10.3. Predicted effect of excess barium ion 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,


6th Ed. (Wiley)

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Diverse Ion Effect on Solubility:

Presence of diverse ions 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+]Ag+

. [Cl-]Cl- KSP

o = KSP Ag+. Cl-

KSP = KSPo/(Ag+

. Cl)

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Predicted effect of increased ionic strength on solubility 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,


6th Ed. (Wiley)

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8. Gravimetri1 New