SP Presipitation

8/10/2019 SP Presipitation

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CHEMICAL PRESIPITATION

Presipitation could happen if multiplication of molarconcentration the ions more than solubility-product

constant (Ksp)

Presipitation stages :

- nucleation

-particle growth

ExampleCaCO3 Ca

2++ CO32-

If concentration of solid is considered as a constant Ks,

so ;

[Ca2+

][CO32-

] = K.Ks = Ksp = 5 x 10-9

(25 C)

Ksp : solubility-product constant- if multiplication of concentration molar ion Ksp

solution is super saturated, precipitate is formed

][

]][[

3

2

3

2

CaCO

COCaK


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Ksp is not enough, need to know factors that affect

chemical precipitation ;

1. Common ion effect;

repression of the solubility of a substance due to anexcess of one ionresult in a decrease in a solubility

Exp: solubility of CaCO3 in pure water is 13 mg/l; in

water containing 100 mg/l alkalinity of CO32-

the

solubility only 0,5 mg/l

2. Indifferent electrolyte effect ;Formation or dissolution of a precipitate always occur

in the presence of indifferent electrolyte. Although the

ions do not participate directly in the solubility

equilibrium reaction, they do affect the behaviour of

the precipitate.

concentration of indifferent electrolyte ionic strength

of solution value of activity coefficient value of

(Kc)eq solubility of the solution

2c

meqa

eqa

(aq)(aq)(s)

)(

)()(Kaswrittenbecan

[A][C])(Kor

(C)(A))(K

ACCA

m

eqaeq

m

K


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3. Other solutes effect;

a. Competing acid-base equilibria

If anion could form weak acid (CO3-, S2-, PO4

3-), exp ;

If acid added (H+)increase in a solubility

b. Effect of complex ion formationComplex formation increase in a solubility

c. Effect of hydrolysis;

- metal ion exist in aqueous solution as hydrated ions

(water-bound, exp Al(H2O)63+)

- in aqueous solution complex-forming reactions betweenhydrated metal ion with hydroxyl ion occurs lead to the

formation of hydroxocomplexes which contain one or more

metal ions (mono or polynuclear complexes)

CaCO3(s) Ca2+

+ CO3-

+

H+

HCO3-

+

H+

H2CO3


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EXAMPLE :

Complex formation :

ZnCO3 Zn2++ CO3

2+

+NH3

Zn(NH3)2+

+

NH3

Zn(NH3)22+

+

NH3

Zn(NH3)32+etc. untilZn(NH3)4

2+formed

Hydrolysis :

Following generalized equations is used to to describe hydroxocomplexes

formation for trivalent metals ion (when onlymononuclear complexes formed

1. Me3++ OH- MeOH2+

2. Me3++ 2OH- Me(OH)22+

3. Me3++ 3OH- Me(OH)32+

4. Me3++ 4OH- Me(OH)42+

Since hydrated metal cations are weak acid called cation acids (asam-2logam), the formation of metal hydroxocomplexes also considered as the

ionization of these cation acids.For trivalent metals ion that forms only

mononuclear complexes :


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1. Me(H2O)63+ Me(H2O)5(OH)

2+ + H+

or

Me3+ Me(OH)2+ + H+

2. Me3+ Me(OH)22+ + 2H+

3. Me3+ Me(OH)32+ + 3H+

4. Me3+ Me(OH)42+ + 4H+

These type of ionization occur whenever salts containing metal ions dissolved

in water, since H+

ions are produced resulting solution will be acidic.increase the solubility & should be considered in solubility calculations

Hydroxocomplexes formation constants for Al and Fe table

A log [spesies] vs pH diagram can be used to describe the effect of metal

hydroxocomplex formation on the solubility. Diagrams will be constructed by

using the plotting equations ;

1. Al(OH)3(s) Al3+

+ 3OH-

Ksp = [Al3+][OH-]3

Log Ksp = log [Al3+] + 3 log [OH-]

Log Ksp = -32,34

dari ionisasi air, diketahui log [OH-] =pH14

substitusi dua hal tsb di atas didapat :

-32,34 = log [Al3+] + 3pH42

atau

log [Al3+] = 9,66 3pH .1)


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2. Al3+ Al(OH)2+ + H+

-5,02 = log [Al(OH)2+] + log [H+] - log [Al3+]

log [Al(OH)2+] = 4,44 2pH .2)

Dengan cara yang sama didapatkan :

3. 2Al3+ Al2(OH)24+ + 2H+

log [Al2(OH)24+] = 13,05 4pH .3)

4. 6Al3+ Al6(OH)153+ + 15H+

log [Al6(OH)153+] = 10,96 3pH .4)

5. 8Al3+ Al8(OH)204+ + 20H+

log [Al8(OH)204+] = 8,58 4pH .5)

6. 13Al3+ Al13(OH)345+ + 34H+

log [Al13(OH)345+] = 28,19 5pH .6)

7. Al3+ Al(OH)4- + 4H+

log [Al(OH)4

-] = pH 13,91 .7)

3

2)(

Al

HOHAlK


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Log [spesies] vs pH diagram for the Al(OH)3(s)-H2O system

(equations 1 to 7) is presented bellow.

Fe(OH)3(s)

-H2

O system can be found in the same way


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APPLICATION OF CHEMICAL PRECIPITATION INENVIRONMENTAL ENGINEERING

A. Defluoridation

Defluoridation applicaions: - water treatment

- wastewater treatment

In wastewater treatment, fluoride is precipitate by addinglime

B. Metals removal

- Mostly metals can be precipitated as metal carbonate&

metal hydroxide, some in the form of metal sulfide

- Need to consider process condition, especially pH to

achieve minimum solubility

- Some metals is amphoter (amfoter), exp ; Cr & Zn

Ca(OH)2+ 2HF

CaF2(s) + 2H2O pH 11 - 12


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D. Water softening (removal of hardness)

- Precipitation reaction using lime soda ash

C. Penurunan kesadahan

Reaksi presipitasi dengan

kapursoda abu :

1. CO2 + Ca(OH)2 CaCO3(s)+ H2O

2. Ca(HCO3)2+ Ca(OH)2 2CaCO3(s)+ 2H2O Ca-C

3. Mg(HCO3)2 + Ca(OH)2 CaCO3(s)+ MgCO3+ H2O Mg-C

MgCO3 + Ca(OH)2 Mg(OH)2(s)+ CaCO3(s)

4. MgSO4 + Ca(OH)2 Mg(OH)2(s)+ CaSO4 Mg-NC

5. CaSO4 + Na2CO3 CaCO3(s)+ Na2SO4 Ca-NC

- Chemical needed:

(concentration of species in meq/l)

Discription Lime Soda Ash

CO2 1 x -

Cacarbonate 1 x -

Canon carbonate - 1 x

Mgcarbonate 2 x -

Mgnon carbonate 1 x 1 x


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Limesoda process ;

a. Excess lime treatment

To establish optimum precipitation, especially

Mg presipitation, excess lime is added in amount of

35 mg/l CaO (1,25 meq/l)

b. Split treatment

A fraction of raw water is treated in excess-lime

process, and neutralized the excess lime using (part of)

untreated water

SP Presipitation

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