Gravimetri - Kimia Analitik

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1

gravimetric =

measuring

mass



Silica gel goes from blue to

pink as it absorbs moisture

Can be

regenerated

in

oven

Anhydrous sodium sulfate

gets clumpy as it absorbs

water

More Information about desiccants including common interferentsand regeneration temperature can be found at:

http://www.jtbaker.com/techlib/documents/3045.html





Coverage Types of gravimetry

Steps in gravimetric analysis

Precipitation

gravimetry

Properties

of

precipitates

and

precipitants

Solubility consideration & Impurity of

precipitates Mechanism of precipitate formation

Calculations with gravimetry & applications

3



Gravimetric

Method

of

Analysis

Quantitative methods that are based on determining the

mass of

a

pure

element

or

compound

to

which

the

analyte

is

chemically related.

Based on mass measurements made with an analytical

balance

that

produces

highly

accurate

and

precise

data. Any method in which the signal is mass or change in

mass.

There

are

several

types

of

gravimetric

analysis;

precipitation gravimetry, volatilization gravimetry, and

electrogravimetry . The other two methods are gravimetric

titrimetry and atomic mass spectrometry.

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Types

of

Gravimetry

Precipitation gravimetry: method in which the signal is the mass of precipitate. E.g.. Direct determination of Cl‐ by precipitation as AgCl

Electrogravimetry: method in which the signal is the

mass of an electrodeposit on the cathode or anode in

an electrochemical cell. E.g.. Oxidation of Pb2+ and its

deposition as

PbO2 on

the

anode.

Reduction

of

Cu2+ as

Cu deposits on the cathode.

Volatilization gravimetry: method in which the loss of a

volatile species gives rise to the signal (mass). Thermal or

chemical

energy

is

used

to

remove

such

volatile

compounds. E.g Determine C in organic cmpd by CO2using chemical energy and measuring mass difference.

5



Precipitation

Gravimetry

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Cl- + Ag+ AgCl ppt

Mass AgCl precipitate is directly related to

%Cl- in the water sample



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General

Steps

in

Gravimetry

Dry and weigh sample

Dissolve sample

Add precipitating reagent in excess

Coagulate precipitate usually by heating

Filtration-separate ppt from mother liquor

Wash precipitate (peptization) Dry and weigh to constant weight

7



Precipitation

Gravimetry

The analyte is converted to a stable and pure precipitate of known

composition. For example: Ca determination method in natural waters by

Association of

Official

Analytical

Chemist.

Ca2+ in natural water sample directly related to CaO

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NH3

+H2C2O4

2NH4

+

+C2O4

2-

Ca2+

+ C2O42-

CaC2O4 (s)

CaC2O4 (s) CaO(s)+CO(g)+ CO2(g)

Ca2+

CaO



1. Properties of Precipitates

Low solubility

‐ No significant loss of the analyte occurs during

filtration and

washing

High purity

Stable under atmospheric conditions

Known composition

After drying and ignition, known chemical composition

Easy to

separate

from

reaction

mixture

and

filtered

and

washed free of impurities

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2.

Precipitating

Reagents:

precipitants Gravimetric precipitating agent should react specifically

or at least selectively with the analyte.

Specific reagents react only with a single chemical species. E.g.. Dimethylglyoxime, is a specific reagent that only precipitates Ni2+ ions from alkaline solutions.

Selective reagents are more common and reacts with a

limited number of species. E.g.. AgNO3, the common

ions

that

it

precipitates

from

acidic

solutions

are

Cl‐

,

Br‐

, I‐ and SCN‐.

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Table

1.0

Selected

Gravimetric

Method

for

Inorganic

Cations

Based

on

Precipitation

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Table

1.0

Selected

Gravimetric

Method

for

Inorganic

Anions

Based

on

Precipitation

12



Organic precipitating agents are chelating agents.

They form insoluble metal chelates.

©Gary Christian,

Anal yt ical Chemistry,6th Ed. (Wiley)



3.

Solubility

considerations

Factors responsible for solubility loss of precipitates

a.. Composition of

solution

in

which

ppt

forms

b.. pH of solution

c.. Solvent type (aqueous and non‐aqueous)

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a. Composition of Solution

● Add only enough precipitating reagent till the precipitation is in

dynamic equilibrium, any extra addition of precipitants will increase

solubility of ppt’s.

● Ag+ can be determined gravimetrically by adding Cl‐ as a precipitant forming AgCl precipitate.

Consider the equilibrium:

Ag+

(aq) + Cl

‐

(aq)

↔

AgCl

(s) One might conclude that addition of Cl‐ at equilibrium would

increase precipitation

however, it has been

observed that an increase in

the equilibrium

concentration

of

Cl‐

forms more soluble chlorides and increases solubility of the AgCl ppt (fig..)

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b.

pH

2..pH of solution in which the ppt forms. For example

hydroxides ppts as Fe(OH)3 are more soluble at low

pH’s

were

[OH

‐

]

is

small.

Why? At lower pH’s [H+] is greater therefore more

electrostatic attraction of H+ to ppt OH‐ occurs, breaking the ppt OH bond and solvation occurs. Like

wise ppt’s

containing

acidic

ions

are

more

soluble

at

higher pH’s were [H+] is low. Explain.

What is solvation?

Electrostatic attraction

of

+ and

– charged

ends

to

the

ppt ions and dissolving the ppt.

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c.

Solvent Type

..solubility can be affected by solvent type (aqueous or non‐aqueous solvents)

Aqueous‐ a

solution

with

water

as

the

solvent,

ppt

solubility is greater in this solution because the water molecules stabilize the ions by solvation process.

Non‐aqueous

‐ solution

with

no

water

mainly

organic

solvents. Organic solvents have poorer solvating ability leads to smaller solubility product.

List some

examples

of

aqueous

and

non

‐aqueous

solvents and explain how solvent type effects solubility of precipitates.

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4. Mechanism of Precipitate Formation

Precipitates form by nucleation and by particle growth. If nucleationpredominates, a large number of very fine particles results; if particlegrowth predominates, a smaller number of larger particles is obtained.

In nucleation, a few ions, atoms, or molecules come together to form stable solid . Often these nuclei form on the surface of suspendedsolid contaminants, such as dust particles.

Further precipitation then involves a competition between additionalnucleation and growth on existing nuclei.

If nucleation predominates, a precipitate containing a large number of small particles results; if growth predominates, a smaller number of

larger particles is produced. The rate of nucleation is believed to increase vastly with increasing

relative super saturation. On the contrary, the rate of particle growth isonly moderately enhanced by high relative super saturations.

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5. Experimental Control of Particle Size

a,. Super‐saturation

Problems faced

during

Precipitation

ProcessDuring the precipitation process, super saturation occurs

(this should be minimized) followed by nucleation and

precipitation.

Supersaturated solution is an unstable solution that contains a high

solute

concentration.

With

time

super

saturation

is

relieved by precipitation of the excess solute.

Experimental variables that minimize supersaturation

and thus produce crystalline precipitates include◦Precipitate from hot solutions (increase Temperature)◦Precipitate from dilute solutions◦Slow addition of precipitating agent with good stirring

◦Controlling pH

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b.

Colloidal

Precipitates

Colloids are so small to hold on a filter and they can

settle down in solution as well due to Brwonian motion

(stochastic process,

random

fluctuation).

In

order

to

have the colloids filterable, we can coagulate or agglomerate.

Coagulation of Colloids: It can be hastened by heating, stirring and adding an electrolyte.

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6. Impurity of precipitates: Coprecipitation

Precipitation gravimetry is based on knownstoichiometry between the analyte mass and ppt mass ,

therefore precipitate must be free of impurities. Greatest source of impurities results from chemical orphysical interactions occurring at the ppt surface.

A ppt is generally crystalline with a well defined latticestructure of cations and anions.

Those cations or anions at the surface carry respectively

a (+) or (–) ve charge as a result of incompletecoordination. Example AgCl Any impurity in the ppt must be removed before taking

its weight.

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Coprecipitation:

Inclusion

and

Occlusion

A phenomenon in which otherwise soluble compounds areremoved from solution during precipitate formation.

There are four types of co‐precipitation: Inclusion, surfaceadsorption and occlusion.

Inclusion: chemical species similar to precipitating ions may substitute into the chemical lattice via chemical adsorption. Thisis very difficult to remove but recrystallization process often

minimizes. Surface adsorption: soluble compound is carried out of solution on

the surface of coagulated colloid particle. Such easily bound watercan be removed via digestion in mother liquor.

Occlusion: a compound is trapped within a pocket formed duringrapid crystal growth.

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8. Applications of Gravimetric Methods

Gravimetric methods have been developed for mostinorganic anions and cations as well as neutral speciessuch as water, sulfur dioxide, carbon dioxide, andiodine.

A variety of organic substances can also be easily determined gravimetrically. Examples: lactose in milkproducts, salicylates in drug formulations, nicotine in

pesticides etc.

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Inorganic Precipitating Agents: Table 12‐2 in the

textbook.

Reducing Agents: Table 12‐3 shows the reagents can

convert the an analyte to its elemental form. Organic Precipitating Agents: There are numerous

examples of this type. In one form of organic reagents,

they produce sparingly soluble non‐ionic coordinationcompounds; In the other forms products with largely

ionic bonds.

Organic Functional Group Analysis: See Table 12

‐

4in the textbook

Volatilization Gravimetry: Based on volatilization

such as water and carbon dioxide determination.26



Organic precipitants

27

N

OH

+ Mg2+

N

O

N

O

Mg

2 2H+

8-Hydroxquinoline (oxine)

Dimethylglyoxime



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What Do We Get Out of Gravimetry?

• % of analyte, % A

• %A = weight of analyte x 100weight of sample

• weight of ppt directly obtained ->?A



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How Do We Get %A?

• % A = weight of ppt x gravimetric factor (G.F.) x 100

weight of sample

• G.F. = Molar mass of analyte X a (mol analyte/mol ppt)

Molar mass of precipitate b

• a: mole analyte; b. mole precipitate

• Hence, a/b give mole ration fraction

• G.F. = # grams of analyte per 1 gram

precipitate



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The Gravimetric Factor

• G.F. = Mr. analyte X a (mol analyte/mol ppt)

Mr. precipitate b

• Analyte ppt G.F.?

CaO CaCO3

FeS BaSO4

PbCl2 PbCrO4

UO2(NO3)2.6H2O U3O8

Cr2O3 Ag2CrO4



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Gravimetric Factor

• Analyte ppt G.F.CaO CaCO3 CaO/CaCO3 x 1/1

FeS BaSO4 FeS/BaSO4 x 1/1

PbCl2 PbCrO4 PbCl2/PbCrO4 x 1/1UO2(NO3)2 U3O8 3UO2(NO3)2/U3O8 x 3/1

Cr 2O3 Ag2Cr O4 Cr 2O3/2Ag2CrO4 x 1/2



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Example 10.1:

An ore is analyzed for the manganese

content by converting the manganese to

Mn3O4 and weighing it. If 1.52-g sample

yields Mn3O4 weighing 0.126-g, what

would be the percent Mn2O3 in thesample? The percent Mn?

Molar mass:

Mn2O3=157.9; Mn3O4= 228.8; Mn=54.94

Ref: Gary D Christian 6th ed p-323



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Example 10.2:

Orthophosphate (PO33-) is determined by

weighing as ammonium phosphomolybdate,

(NH4)PO4.12MoO3. Calculate the

percentage P and P2O5 if 1.1682-g

precipitate ere obtained from a 0.2711-gsample.

Molar mass:

(NH4)PO4.12MoO3 =1876.5; P2O5= 141.95;

P=30.97

Ref: Gary D Christian 6th ed p-323



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Example 10.3:

The Aluminum in a 0.910g sample of impure

ammonium aluminum sulfate was precipitated

with aqueous ammonia as the hydrous

Al2O3.xH2O. The precipitate was filtered and

ignited at 1000oC to give anhydrous Al2O3,which weighed 0.2001g.

Express the result of this analysis in terms of:

a. % Al2O3 b. % Al in the sample.



Example 10.4 An iron ore was analyzed by dissolving a

1.1324-g sample in concentrated HCl. The

resulting solution was diluted with waterand iron (III) was precipitated as the

hydrous oxide Fe2O3.xH2O by the additionof NH3. After filtration and washing the

precipitate was ignited to give 0.5394-g of

pure Fe2O3 (159.69 g/mol). Calculate the % Fe (55.847 g/mol), %

Fe3O

4(231.54 g/mol) in the sample.

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Example: 10.5

The calcium in a 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.



Example 10.6

An iron ore was 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.



Example 10.7

A 0.2356 g sample containing only NaCl (fwt 58.44

g/mol) and BaCl2 (fwt 208.23 g/mol) yielded 0.4637

g of

dried

AgCl (fwt 143.32

g/mol).

Calculate

the

percent of each halogen compound in the sample.





More

Review

Questions1. Treatment of a 0.2500‐g sample of impure potassium chloride with an excess of AgNO3

resulted in the formation of 0.2912‐g of AgCl. Calculate the percentage of KCl in the

sample. (Molar mass AgCl 143.321 g/mol, KCl 74.551 g/mol)

2. An

iron

ore

was

analyzed

by

dissolving

a 1.1324

‐g sample

in

concentrated

HCl.

The

resulting solution was diluted with water and iron (III) was precipitated as the hydrous oxide Fe2O3.xH2O by the addition of NH3. After filtration and washing the precipitate was ignited to give 0.5394‐g of pure Fe2O3 (159.69 g/mol).

3. Calculate the % Fe (55.847 g/mol), % Fe‐3O4 (231.54 g/mol) in the sample.

The aluminum content of an alloy is determined gravimetrically by precipitating it to

Al(C9H6ON)3. If a 1.021‐g sample yielded 0.1862‐g of precipitate, what is the percent aluminum in the alloy?

4. State the problem often faced during precipitation process. How can you as chemists minimize it?

5. What are the steps involved in gravimetric analysis?

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Gravimetri - Kimia Analitik