9th meeting - analisis gravimetri - 060914.pptx

8/11/2019 9th meeting - Analisis Gravimetri - 060914.pptx

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Analisis GravimetriUniversitas Islam Negeri Sunan Kalijaga

Yogyakarta

ANALISIS GRAVIMETRI


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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


(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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(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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(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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(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

-

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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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

9th meeting - analisis gravimetri - 060914.pptx

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