Solubility Lecture 1, 2 & 3

7/30/2019 Solubility Lecture 1, 2 & 3

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Solubility and SolubilityPhenomenon

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Solvent-solute interactions

Solubility of gases in liquids

Henrys law

Solubility of liquids in liquids

Ideal solutions

- Raoults law

Real solutions

Distillation of

Binary mixtures

Azeotropic mixtures

Fractional distillation

Solubility of solids in liquids

Definition, Determination, Factors

influencing solubility

Colligative properties

Molecular weight determination

Partition coefficient

Contents

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Solution

A solution is a homogeneous mixture of a solute dissolved in a

solvent.

In a soln, the subs that is present in large proportion is termed as

solvent.

Component that is present in small proportion is known as solute.

Based on proportion of solute present insolvent, solns are

classified as Dilute or Concentrated.

E.g. Solution of salt/sugar (Solute) in water (solvent).

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Solubility

Solubility is defined in quantitative terms as the conc of solute in a

saturated soln at a certain temp.

In qualitative terms, it may be defined as spontaneous interaction

of two or more subs to form a homogeneous molecular dispersion.

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Solution

A saturated soln is one which has reached its equilibrium

solubility at definite temp.

An unsaturated or subsaturated soln is one containing dissolved

solute in a conc below that necessary for complete saturation at a

definite temp. (i.e. not reached its equilibrium solubility)

A supersaturated soln is one that contains more of the dissolved

solute than it would normally contain at definite temp. (i.e.equilibrium solubility has been temporarily exceeded)

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At saturation the system is at equilibrium where the rate of dissolution is

equal to rate of crystallization.

the blue "molecules" escape into solution from the ordered crystal. At the

same time, molecules are coming out of solution and depositing on the

solid.

Since this is a continual process and the concentrations do not change, it

is called dynamic equilibrium.

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Concentration

Weight percent(%w/w)

Wt % of solute = (Wt of solute/ Total wt of soln)*100

Volume percent(%v/v)

Vol % of solute = (Vol of solute/ Total Vol of system)*100

NormalityNo. of gram equivalents of solute present in 1 litre of soln.

N = Gram-equiv wt/ No. of litres of soln

MolarityNo. of moles of solute per litre of soln.

M = No. of moles of solute/ No. of litres of soln

No. of moles = Wt. of subs in soln/ molecular mass of subs

Unitmol/l

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MolalityNo. of moles of solute present in 1 Kg of solvent.

m = No. of moles of solute/ No. of Kg of solvent

Unitmol/kg

Mole fractionRatio of no. of moles of component to total no. of

moles (all components) present in soln.

X = No. of solute present in soln/ Total no. of moles of solute plus

solvent

Mole percentMole fraction expressed as percent mole fraction

Mole percent = Mole fraction * 100

Equivalent weight = Molecular mass (g/mol)/ equivalent per mol

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Term Parts of solvent required for 1 part ofsolute

Very soluble < 1 partFreely soluble 1 10 parts

Soluble 10 30 parts

Sparingly soluble 30 100 parts

Slightly soluble 100 1000 parts

Very slightly soluble 1000 10,000 parts

Practically insoluble or

insoluble

> 10,000 parts

Solubility ExpressionsUSP-NF gives the solubility of drugs as no. of millilitres of solvent in

which 1 gram of solute will dissolve

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Solubility of Gases in Liquids

Solubility of gas in liquid is the conc of dissolved gas when it is in

equilibrium with some of the pure gas above the solution.

The solubility depends on the:

Pressure

Temperature

Presence of salts

Chemical Reaction

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Effect of Pressure

The pressure of gas above the soln is an imp consideration in

gaseous solns becoz it changes the solubility of dissolved gas in

eqm with it.

The effect of the pressure on the solubility of a gas in liquid is

expressed by Henrys Law.

The law is named after William Henry (17741836), English

chemist who first reported the relationship.

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Henrys Law states that, in a very dilute soln at constant temp, the

conc of dissolved gas is proportional to the partial pressure of the

gas above the soln at eqm.

(provided no chemical reaction takes place b/w soln & gas).

At P2, more gas dissolves into the15


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The partial pressure of the gas is obtained by subtracting the vapor

pressure of solvent from total pressure above soln.

If C is conc of dissolved gas in g/litre of solvent

p is partial pressure in ml of undissolved gas above soln,

Henrys relationship can be written as:

C = p

where = inverse ofHenrys law constant. Also termed as Solubility

coefficient.

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gn cance o enry s aw o e


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gn cance o enry s aw o ePharmacist

Solubility of a gas increases directly with the pressure on the gas,

& conversely, solubility of gas decreases, so that the gas escapes

with violence when the pressure above the solution is released.

This phenomenon is commonly recognized in effervescent

solutions when the stopper of the container is removed.

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Henrys Law & Soft Drinks

Soft drinks contain carbonated water

water with dissolved carbon dioxide gas.

Drinks are bottled with CO2 pressure

greater than 1 atm.

When the bottle is opened, pressure of

CO2 decreases & solubility of CO2 also

decreases, according to Henrys Law.

Therefore, bubbles of CO2 escape from

solution.

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Limitations of Henrys law

Law applies closely to gases with nearly ideal behavior.

Law applies at moderate temperature and pressure.

Law applies if the solubility of gas in solvent is low.

Law applies if the gas doesnt react with the solvent to form a newspecies. Thus ammonia (or HCl) which reacts with water does not

obey Henrys law.

NH3 + H2O NH4+ + OH-

Law applies if the gas doesnt associate or dissociate on dissolving

in the solvent.

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Effect of Temperature

As the temp increases, solubility of most gases decreases, owing to

greater tendency of gas to expand.

Pharmacist must be careful in opening containers of gaseous

solutions in warm climates & other conds of elevated temp.

A vessel containing a gaseous solution or a liquid with a high

vapor pressure, such as ethyl nitrate, ammonia soln, liq bromine or

chlorine water, should be immersed in ice or cold water for some

time to reduce the temp and pressure of gas before opening the

container.

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Effect of Temperature

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Applications

Dissolved gases are removed by heating the soln.

Distilled water is maintained at 80C in order to make it

convenient for parenteral use, becoz gases cannot dissolve in water

at that temp.

Dissolved air influences boiling ofliqs.

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Salting Out

Gases are often liberated from solutions in which they are dissolved by the

introduction of an electrolyte such as NaCl and sometimes by a non-electrolyte

such as sucrose.

This phenomenon is known as salting-out.

This effect is of great indirect importance to stabilize aqueous solns of Vit A.

High sugar conc decreases solubility of oxygen so that subs liable to oxidation

are better protected.

Salting out effect may be demonstrated by adding a small amount of salt to a

carbonated solution.

The resultant escape of gas is due to the attraction of the salt ions or the highly

polar non electrolyte for the water molecules, which reduces the density of the

aqueous environment adjacent to the gas molecules.23


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Effect of Chemical Reaction

Henry's law applies strictly to gases

which are only slightly soluble in solution &

which do not react in any way in the solvent

Gases such as hydrogen chloride, ammonia and CO2 show deviations as

a result of chemical reaction between the gas and solvent, usually with a

resultant increase in solubility.

Accordingly, hydrogen chloride is about 10,000 times more soluble in

water than is oxygen.

Applications: These principles are used for prep of reagents s.a. conc

solns namely HCl, H2SO4 & HNO3.

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Solubility Calculations

Solubility of a gas in liq can be expressed either by inverse Henrys Law

constant , or Bunsen absorption or Ostwalds solubility coefficient, .

Bunsen coefficient is defined as the vol of gas in liters (reduced to std conds of

0C & 760mm Hg pressure) that dissolves in 1 liter of solvent under a partial

pressure of 1 atm of gas at definite temp:

Vgas = Vol of gas at STP dissolved in as Volsoln of soln at a partial gas pressure p

Henrys law constant k can be represented as:

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Applications of Solubility of Gases in Liquids Preparation of reagents

E.g. HCl, H2SO4, HNO3 & Liq ammonia.

Preparation of carbonated beverages

Solubility of oxygen in blood Major respiratory function of lungs is to

add O2 to blood & remove CO2 from it.

Solubility of O2 depends on conc of Hb.

Transportation of anesthetic gas through blood

Oil solubility of anesthetic gasesAnesthetic property of gas & its oil

solubility is imp.

Anesthetic gas with high oil solubility is effective at low alveolar conc &

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Solubility of Liquids in Liquids

The solutions of liquids in liquids may be divided into 3 classes:

Liquids that are Completely miscible

E.g. Alcohol and water

Liquids that are Partially miscible

E.g. Ether and water

Liquids that are Practically immiscible

E.g. Benzene and water

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It is defined as the one in which there is no change in the properties of

components other than dilution, when they are mixed to form a soln.

Heat is neither absorbed nor evolved during mixing.

Final vol of soln represents an additive property of individual constituents.

There is no shrinkage or expansion when liqs are mixed.

Examples:

Liquid mixture

Methanol-ethanol

Benzene-toluene

n-hexane & n-heptane

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These liqs have similar properties, i.e. attractive forces are in complete

uniformity.

This means, forces b/w unlike molecules are of same type as those presentb/w like molecules.

Ideal soln theory provides a model system to which real or non-ideal solns

can be compared.

They can be characterized by physicochemical property of a liq, namely

vapor pressure.

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VP of a soln serves as a quantitative expression of escaping tendency.

Raoults law states that Partial vapor pressure of each volatile constituent is

equal to the vapor pressure of the pure constituent multiplied by its mole

fraction in the solution, at a given temperature

Consider a mixture of miscible liquids A and B

Let Partial VP exerted by liq A = pA (kPa)

Let Partial VP exerted by liq B = pB

Let VP exerted by pure liq A= pA0

Let VP exerted by pure liq B= pB0

Let mole fraction concentration of A in liq = XA

Let mole fraction concentration of B in liq = XB30


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As per Raoults law:

pA = pA0

XA ----------------------(1)

pB = pB0 XB ----------------------(2)

When 2 liqs are mixed, the VP of each one is reduced by the presence of other

by the extent of dilution of each phase.

Ideal solns are the ones that obey Raoults law.

Raoults law is obeyed by few solns having similar structures.

E.g. Benzene & Toluene

n-hexane & n-heptane

Ethyl bromide & Ethyl iodide

The individual components do not have interaction of any kind or complete

uniformity of attractive forces is observed.31


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It states that, The total vapor pressure exerted by a mixture of

ideal gases may be considered as sum of the partial vapor

pressure exerted by each gas, if alone were present & occupied

the total vol.

Total vapor pressure = Partial VP of A + Partial VP of B

P = pA + pB--------------------------------(3)

P = (pA0 XA) + (pB

0 XB)

These properties are additive.

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In an ideal soln, when liq A is mixed with liq B, the VP of A is reduced by

dilution with B in a manner depending on mole fractions of A & B present in

final soln.

This will diminish the escaping tendency of each constituent, leading to a

reduction in the rate of escape of molecules of A & B from the surface of the

liq.

If additional volatile components are present in soln, each will produce a

partial pressure above the soln, which can be calculated from Raoults law.

The total pressure is the sum of partial pressures of all constituents.

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Vapor pressure-composition curve for an ideal binary

system

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Applicable only to mixtures in which the components are very similar

chemically and the two components do not interact in any way.

Works only for ideal solutions.

Raoult's Law only works for solutes which don't change their nature when

they dissolve. For example, they mustn't ionize or associate.

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Ideality in solns presupposes complete uniformity of attractive forces.

Many examples of soln pairs are known, in which, cohesive attraction of A

for A exceeds the adhesive attraction existing b/w A & B.

Similarly, attractive forces b/w A & B may be greater than those b/w A & A

or B & B.

This may occur even though liqs are miscible in all proportions.

Such mixtures are real or non-ideal; i.e. they do not adhere to Raoults law

throughout the entire range of composition.

Two types of deviations from Raoults law are recognized.

Deviations may be observed if solute-solute, solute-solvent & solvent-

solvent interactions are unequal.

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In non-ideal solns, the Raoults law is modified by replacing the term

concentrationby a term effectiveconcentration indicating activity.

Thus eq. can be modified as

pA = pA0 aA

pB = pB0 aB

Where aA

& aB

are activities of components A & B, respectively.

In case of ideal soln: a = X

In case of non-ideal soln: a X

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When the interaction b/w A & B molecules is less than that b/w molecules of

pure constituents, the presence of B molecules reduces the interaction of A

molecules, & A molecules correspondingly reduce B-B interaction.

Accordingly, dissimilarity of polarities or internal pressures of constituents

result in greater escaping tendency of both A & B molecules.

The partial VP of constituents is greater than that expected from Raoults law

& the system is said to exhibit positive deviation.

The total VP shows a maximum at one particular composition.

E.g. Benzene & ethyl alcohol; Carbon disulfide & acetone; Chloroform &

Ethyl alcohol.

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A-A > A-B

aA

> XA

& aB

> XB

They are known as maximum VP & minimum boiling point solutions

Occurs when the components differ in their polarity, length of hydrocarbon

chain and degree of association.

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When adhesive attractions b/w molecules of different species exceed the

cohesive attractions b/w like molecules, the VP of the soln is less than that

expected from Raoults ideal soln law, & negative deviation occurs.

The total VP curve shows a minimum.

E.g. Chloroform & acetone; Pyridine & acetic acid; Water & nitric acid.

Chloroform & acetone manifest an attraction for one another through

formation of hydrogen bond, thus reducing the escaping tendency of each

constituent.

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A-A < A-B

aA

< XA

& aB

< XB

They are known as minimum VP & maximum boiling point solutions

Occurs due to interactions such as H-bonding, salt formation & hydration

between components.

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Raoults law does not apply over the entire conc range in non-ideal soln.

It describes the behavior of either component of a real liq pair only when the

subs is present in high conc & thus is considered to be solvent.

Raoults law may be expressed as:

psolvent = psolvent0 Xsolvent

This situation is valid for solvent of nonideal soln & cannot hold for

component in low conc, i.e. solute.

This is clear when we observe the curves in phase diagram.

The actual VP curve of a component approaches ideal curve defined by

Raoults law as soln composition approaches pure component.

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VP curve for both components as solutes is observed to lie below the VP of

an ideal mixture.

The molecules of solute, being in relatively small no. in 2 regions of diagram,

are completely surrounded by molecules of solvent & so reside in uniform

environment.

Thus, Partial pressure or escaping tendency of a component at low conc is

proportional to its mole fraction, but its proportionality constant is not equal to

VP of pure subs.

Thus VP-comp relationship of solute cannot be expressed by Raoults law, but

by an eq known as Henrys law as:

psolute = ksoluteXsolvent

Where k < p

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Henrys law applies to solute & Raoults law applies to dilute solns of real liq

pairs.

Raoults law also applies over entire conc range when both constituents are

sufficiently similar to form an ideal soln.

When partial VP of both constituents is directly proportional to mole fractions

over entire range, the soln is said to be ideal; Henrys law becomes identical

with Raoults law & k becomes equal to p.

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Relationship b/w VP & composition of binary liq phases is the underlying

principle in distillation.

Depends on relative volatilities of the components of the liquid mixture.

In case of miscible liqs, instead of plotting VP v/s comp; it is more useful to

plot boiling point of various mixtures, determined at atmospheric pressure,

against comp.

Higher the VP of a liq, the more volatile it is & lower the BP.

Since the vapor of a binary mixture is always richer in more volatile

constituent, the process of distillation can be used to separate more volatile

from less volatile constituent.

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S l d ll


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Simple distillation Separating a volatile liquid from non volatile component of the mixture.

A process of converting a single constituent from a liquid into its vapors

and then condensing the vapors at other place.

Based on the difference between the volatility of two components.

Examples:

1. Simple distillation for preparation of distilled water and Water for injection.

2. Separation of Non volatile solids from volatile liquids.

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F ti l Di till ti / R tifi ti


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Fractional Distillation/ Rectification It is a process in which vaporization of liq mixture gives a mixture of constituents

from which desired one is separated in pure form.

Used to separate miscible volatile liquids with close boiling points.

It is a mass transfer process involving counter current diffusion of components at

each equilibrium stage.

A fractionating column introduced between still & condenser.

Partial condensation of vapor is allowed to occur in a fractionating column

through which the vapor must pass before reaching the condenser.

In the column, ascending vapor from the still is allowed to come in contact with

the condensing vapor returning to the still.

This results in enrichment of vapor with more volatile component.49


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By condensing the vapor & reheating the liq repeatedly, eqm b/w liq & vapor is

set up at each stage, which ultimately results in the separation of more volatile

component.

Condensate can be re-circulated to achieve further separation.

It is a mass transfer process involving counter current diffusion of components at

each equilibrium stage.

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Consider a mixture of high boiling liq A & low boiling liq B


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Consider a mixture of high boiling liq A & low boiling liq B.

A mixture of these subs having comp a is distilled at boiling point b.

Comp of vapor v1 in eqm with liq at this temp is c; which is also the comp of

distillate when it is condensed.

Vapor is thus richer in B than the liq from which it was distilled.

If a fractionating column is used, A & B can be completely separated.

The vapor rising in the column is met by condensed vapor or downward-flowing

liq.

As the rising vapor is cooled by contact with the liq, some of the lower-boiling

fraction condenses, & the vapor contains more of the volatile component than itdid when it left the retort.

Thus, as the vapor proceeds up the fractionating column, it becomes progressively

richer in more volatile component B, & the liq returning to distilling retort

becomes richer in less volatile component A.51

Construction of BOILING POINT composition


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Construction of BOILING POINT composition

curve for Type I Miscible liquids

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Fractional distillation is suitable for a system when the BP of the mixture is always


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Fractional distillation is suitable for a system when the BP of the mixture is always

intermediate b/w those of pure components.

There is neither a maximum or minimum in vapor composition curves.

These systems are known as Zeotropic mixtures.

E.g. Benzene-Toluene;

Carbon tetrachloride-Cyclohexane &

Water-Methanol

Application: Fractional distillation is used for separation of miscible liquids.

Disadvantage: It cannot be used to separate miscible liquids, which form

Azeotropic mixtures.

Documents

Solubility Lecture 1, 2 & 3