SOLUTIONS
AND
SOLUBILITY
Solutions and Solubility

Solution: can be defined as a system in which
molecules of a solute are dissolved in a given solvent
vehicle. It is a mixture of 2 or more components
forming a Single Phase.
Solution
Solute
Solvent
Small proportion
e.g. ions- molecules
Largest proportion
EXAMPLE




"Simple syrup" consists of 66.67% (w/w) of
sucrose in water.
Mwt of sucrose=342
Mwt of water=18
The moles of sucrose=0.19 moles, and the
moles of water=1.85 moles
CALCULATION
Amount of sucrose = 66.7 g
Amount of water = 100 - 66.7
= 33.3 g
No. of moles of sucrose = 66.7/342
= 0.195 mole
No. of moles of water = 33.3/ 18
= 1.85 mole
HENCE, solute and solvent are
determined depending on molar basis
not weight basis
Classification of solutions
according to the amount of solute
Dissolved solute <
concentration needed for
complete saturation at a definite
temperature.
Equilibrium of dissolved with
excess undissolved solid at a
definite temperature.
Dissolved solute > concentration
needed for complete saturation at
a definite temperature.
e.g. sodium thiosulphate- sodium
acetate
Unsaturated
(Subsaturated)
Saturated
(Stable)
Supersaturated
(Metastable)
Supersaturated solutions
Stable solutions
1- Seeding with a small crystal of the solute.
2- Vigorous shaking.
3- Scratching the wall of container in contact to solution.

The maximum equilibrium solubility
(solubility-saturated solubility) of a drug
in a given medium is important because it
affects the rate of solution
(dissolution=amount dissolved/time).

The higher the solubility, the more rapid
is the rate of solution when no chemical
reaction is involved.
Solubility

IT is the concentration of solute in a saturated solution
at a certain temperature. (Quantitative Terms)
OR

IT is the spontaneous interaction of 2 or more
substances to form a homogeneous molecular dispersion
(single phase). (Qualitative Terms)
Expression of solubility
I) Quantitative
II) Qualitative
USP expression
Very soluble
Molar solution
Freely soluble
Molal solution
Soluble
Mole fraction
Sparingly soluble
Percent by weight
Slightly soluble
Percent weight/volume
Very slightly soluble
Percent by volume
Practically insoluble
Quantitative
USP
expression
Number of mls of solvent dissolving 1 gm of solute (gm/ml)
Molar solution
Number of moles of solute/1 liter of solution (mol/L)
Molal solution
Number of moles of solute/1000gm of solvent
Mole fraction
Mole fraction of (A)=Moles (A)/sum of moles of (A)+(B)
Percent by weight
Percent w/v
Percent by volume
Number of gms of solute/100 gms solution (%w/w)
Number of gms of solute/100 ml solution (%w/v)
Number of mls of solute/100 ml solution (%v/v)
Qualitative
Very soluble
Freely soluble
Soluble
< than 1 part of solvent
~1 - 10
~10 - 30
Sparingly soluble
~30 – 100
Slightly soluble
~100 – 1000
Very slightly soluble
~1000 – 10.000
Practically insoluble
> than 10.000

Example:
The solubility of a drug in water is 41.5 g per 1000 mL of
solution at 18oC. The density of the solution was 1.0375
g/mL and the molecular weight of the drug is 151.9.
Express the solubility of the drug in terms of molarity,
molality and percent by weight, calculate the mole
fraction and mole percent of both components of the
solution.
a. Molarity: (Number of moles of solute/1 liter of solution (mol/L))
No. of moles of the drug = 41.5 / 151.9 = 0.2732
Molarity of the drug = 0.2732 / 1liter of solution = 0.2732 M
b. Molality: (Number of moles of solute/1000gm of solvent)
Grams of solution = volume x density = 1000 x 1.0375
= 1037.5 g
Grams of solvent = 1037.5 - 41.5 = 996.0 g
Molality of the drug = 0.2732 x 1000 / 996.0 = 0.2743 m
c. Percent by weight: (Number of gms of solute/100 gms solution (%w/w)
(g of drug / g of solution) x 100 = (41.5/ 1037.5) x 100
= 4.00 % w/w
d. Mole fraction and Mole percent:
(Mole fraction of (A)=Moles (A)/sum of moles of (A)+(B))
No. of moles of water = 996 / 18.02 = 55.25 moles
Mole fraction of the drug (A) = 0.2732/(55.27 + 0.2732)
= 0.0049
Mole fraction of water (B) = 55.27 / (55.27 + 0.2732)
= 0.9951
Note that A drug + B water = 1
Mole percent of the drug = 0.0049 x 100 = 0.49%
Mole percent of water = 0.9951 x 100 = 99.51%
Steps of Determination of Solubility:
I) Preparation of a saturated solution of
substance in a given solvent at a particular
temperature by:
 Addition of excess powder in the solvent and
stirring till equilibrium is reached.
 Filtration is done to separate the undissolved
solid.
II) Analysis of the saturated solution.
Phase Solubility Analysis:
Importance:
1- Determination of solubility of pure substance
from non pure sample.
2- Determination of purity of sample.
3- Differentiate between optical isomers (unless 1:1)
Phase Solubility Analysis: (cont.)
Steps of determination:
shaken at
constant (T,P)
Equilibrium
1gm
2gm
3gm
4gm
Separate solid from solution
1gm



2gm
3gm
4gm
Determine amount dissolved
Plot Y axis (solution conc)
Plot X axis (system conc)
Gibbs phase Rule:
F= C – P + 2
F= Degree of freedom (T, P, C)
C= Number of components
P= number of phases
At constant T and P
(F= C- P)

Phase solubility curves:
A)For pure substance:
F=0
Saturation
F=1
A-
Phase solubility diagram for a pure substance
B: Conc is below saturation
 B- C: Conc is above saturation
 B – C: has no slope, indicating purity
Point D: Solubility of pure substance.
B) For non- pure substance: (one impurity)
F=0
F=
F=11
F=2
•Phase solubility curve for substance contain one impurity
A- B: Conc is below saturation for both (1 phase)
At B: Saturation with major component
From B to C: Conc is above saturation with major component and below
saturation for minor one (2 phases)
Section C – D: Saturation with both components (3 phases)
 Value of AE: Solubility of major component.
Value of EF: Solubility of minor component
At BC: Pure solid major