World Research Journal of Pharmaceutical Research
Volume 1, Issue 1, 2013, pp.-05-11.
Available online at http://www.bioinfopublication.org/jouarchive.php?opt=&jouid=BPJ0000294
PHASE SOLUBILITY ANALYSIS: A TECHNIQUE OF PURITY DETERMINATION
JADHAV P.B.* AND PANDEY P.S.
Ideal College of Pharmacy & Research, Bhal, Thane- 421306, MS, India.
*Corresponding Author: Email- [email protected]
Received: September 09, 2012; Accepted: July 04, 2013
Abstract- Phase solubility analysis is a simple and elegant technique whereby absolute purity of a crystalline material can be determined.
Phase solubility analysis is the quantitative determination of the purity of a substance through the application of precise solubility measurements. Constancy of solubility, like constancy of melting temperature or other physical properties, indicate that the material is pure or is free
from foreign admixture except in the unique case in which percentage composition of the substance under test is in direct ratio to solubility of
respective components. Phase solubility analysis is applicable to all species of compound that are crystalline solid and forms stable solutions.
It is not readily applicable for compounds that form solid solution with impurities.
Keywords- Solubility measurement, Gibb’s rule, impurity determination, quantitative determination
Introduction
When we talk about the mixing of two or more substances together
in solution we must consider solubility. Solubility may be defined as
the maximum concentration of a substance that may be completely
dissolved in a given solvent at a given temperature and pressure.
When both solute and solvent are liquids, the term miscibility rather
than solubility may be used to describe the affinity between the
liquids. The solubility of a substance may be described in a variety
of ways. The USP/NF generally expresses the solubility in terms of
the volume of solvent required to dissolve 1 gram of the drug at a
specified temperature (e.g. 1 g ASA in 300 ml H2O, 5 ml ethanol at
25°C). Other references may use more subjective terms to describe
solubility, such as those given in the [Table-1] from Remington's [1].
Table 1- Subjective Terms To Describe Solubility
Descriptive terms
Parts of solvent needed for 1 part solute
Very soluble
<1
Freely soluble
01-10
Soluble
10-30
Sparingly soluble
30-100
Slightly soluble
100-1000
Very slightly soluble
1000-10,000
Practically insoluble or insoluble
>10,000
behaviour. As the temperature is raised gases usually become less
soluble in water, but more soluble in organic solvents [2].
The chart shows solubility curves for some typical inorganic salts
(all solids) [3]. Many salts behave like barium nitrate and disodium
hydrogen arsenate, and show a large increase in solubility with
temperature. Some solutes (e.g. NaCl in water) are fairly independent of temperature. A few, such as cerium (III) sulfate, become less
soluble in hot water. Occasionally, a more complex pattern is observed, as with sodium sulfate, where the less soluble decahydrate
crystal loses water of crystallization at 32°C to form a more soluble
anhydrous phase. Organic compounds nearly always become soluble as the temperature is raised, in most solvents. The technique of
recrystallization, used for purification of solids, depends on this
difference in solubility in hot and cold solvent. There are a few exceptions, such as certain cyclodextrins [4].
Liquids which form a homogenous system when mixed in any proportion are said to be miscible (e.g. Water and ethanol). Those in
which only certain volume ratios produce homogenous mixtures are
said to be miscible in certain proportions (e.g. Water and chloroform). Immiscible liquids will not produce a homogenous solution in
any proportions (e.g. water and olive oil) [1].
Factors Influencing Solubility
Temperature
The solubility of a given solute in a given solvent often depends on
temperature. For around 95% of solid solutes, the solubility increases with temperature [2], but gaseous solutes exhibit more complex
Fig. 1- Chart of Solubility Vs. Temperature non-polar CompoundS
There is essentially no detectable heat effect in non-polar substances. The forces holding the particles together are small, and any
interaction between solute and solvent is small.
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Jadhav P.B. and Pandey P.S. (2013) Phase Solubility Analysis: A Technique of Purity Determination.
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Polar Substances
In polar substances, it takes energy to separate the molecule from
surrounding molecules. This energy is supplied in the form of heat,
producing a cooling effect. On the other hand, there is the possibility of interaction between the solute and solvent with formation of a
dipole-dipole type bond, and this interaction will tend to give off
heat. Depending on which of the two interactions is greatest you
can get an increase or decrease in temperature.
Presence of Multiple Solutes
The aqueous solubility of non-electrolytes is nearly always affected
in some way by the addition of an electrolyte. Salting-out is the
precipitation of organic solutes from aqueous solution by the addition of an electrolyte or salt. This is attributed to competition between solute molecules for the solvent and is dependent upon the
size and valence of the ion. Salting-in is the increase in solubility of
an organic solute upon addition of an electrolyte. The mechanism of
this phenomenon is poorly understood and it is rarely encountered.
An example is with the group of proteins called globulins which are
more soluble in dilute salt solutions than in water. Complex ion formation occurs when an insoluble solute reacts with a soluble substance to form a soluble complex. An example is the addition of the
soluble potassium iodide (KI) to the insoluble iodine molecule (I2) to
form a soluble tri-iodide complex (KI3) [1].
Solute pKa, Solvent pH and Solubility
According to the Henderson-Hasselbach equation, the relationship
between pH, pka, and relative concentrations of an acid and its salt
is as follows [1]:
Non-ionizable Substances
Similar to the lack of effect of heat on the solubility of non-polar
substances, there is little effect of pH on nonionizable substances.
Nonionizable, hydrophobic substances can have improved solubility
by changing the dielectric constant of the solvent by the use of cosolvents rather than the pH of the solvent.
Ionizable Substances
For substances that have an ionizable like a carboxylic acid (HA),
solubility is a function of pH. As you remember from general chemistry, pH of a substance is related to its pKa and the concentration
of the ionized and unionized forms of the substance by the equation:
If the substance is brought outside its pKa, the pH value where half
the substance is ionized and half in not, than solubility will be
changed because you are introducing new intermolecular forces,
mainly ionic attraction.
Where [A-] is the molar concentration of the salt (dissociated species) and [HA] is the concentration of the undissociated acid. When
the concentrations of salt and acid are equal, the pH of the system
equals the pka of the acid. As the pH decreases, the concentration
of the molecular acid increases and that of the salt decreases. This
has some interesting implications regarding the aqueous solubility
of the acid, since the undissociated form is much less soluble than
its salt. Of further interest, therapeutically, is the fact that it is the
undissociated acid (HA) that more readily penetrates biological
tissues to exert a therapeutic effect. Thus, in formulating the prod-
uct, some balance must be struck between the more soluble salt
form and the biologically active acid and factors other than pka and
pH must be considered (e.g. safety and comfort) [1].
Fig. 2- Effect of pH on weak acid (A) and weak base (B) Solute and
solvent structure/polarity
Solute molecules are held together by certain intermolecular forces
(dipole-dipole, induced dipole-induced dipole, ion-ion, etc.), as are
molecules of solvent. In order for dissolution to occur, these cohesive forces of like molecules must be broken and adhesive forces
between solute and solvent must be formed [1].
The solubility of a drug in a given solvent is largely a function of the
polarity of the solvent. Solvents may be considered polar, semipolar or non-polar. Polar solvents will dissolve ionic and other polar
solutes (i.e. those with an asymmetric charge distribution [like dissolves like]), whereas, non-polar solvents will dissolve non-polar
molecules [1]. For a substance to dissolve the cohesive energy of
the bonds holding the solid or liquid solute together, and the energy
cost of disrupting the solvent-to-solvent bonds must be overcome
by the cohesive energy released by the formation of the solute-tosolvent bonds. Thus there are two energy “costs” (one solute/solute
and one solvent/solvent) and two energy “gains” (two solute/solvent
bonds). If these energies are approximately equal, which occurs
when the solvent and solute molecules are structurally similar, then
the substance will dissolve in the solvent. Hence the saying: "Likes
dissolve Likes” [Fig-3].
Fig. 3- Like Dissolve Likes
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Jadhav P.B. and Pandey P.S. (2013) Phase Solubility Analysis: A Technique of Purity Determination.
World Research Journal of Pharmaceutical Research, Volume 1, Issue 1, pp.-05-11.
Semi-polar solvents (e.g. Alcohols and ketones) may induce a certain degree of polarity in non-polar molecules and may thus act to
improve the miscibility of polar and non-polar liquids. The relationship between polarity and solubility may be used in practice to alter
the solubility of a drug in a pharmaceutical solution.
One approach is to alter the polarity of the solute by shifting it between its molecular (undissociated) and ionic (dissociated) states. A
shift toward the ionic form improves solubility of the solute in water
and other polar solvents. A shift toward the molecular species improves solute solubility in non-polar solvents. Such shifts may be
produced by altering the pH of the solution (or using the salt form of
the compound) [1].
Another approach is to mix solvents of different polarities to form a
solvent system of optimum polarity to dissolve the solute. Such
solvents must, obviously, be miscible. This method is referred to as
solvent blending or co-solvency and uses the dielectric constant as
a guide to developing the co-solvent system [1]. If we look at the
structure of water, we see it is highly ordered due to hydrogen
bonding caused by the dipole nature of the molecule. Due to this
bonding shown in [Fig-4], water is a good solvent for polar molecules and has a high dielectric constant.
Fig. 4- Structure of Water
The dielectric constant is a measure of the effect a substance has
on the energy needed to separate two oppositely charged bodies. A
vacuum is arbitrarily given a dielectric constant of 1. If you put two
oppositely charged bodies into any medium, the medium should
tend to separate or make it more difficult for the two oppositely
charged bodies to unite. The energy required to separate two oppositely charged bodies is inversely proportional to the dielectric constant of the medium.
The dielectric constant is also a measure of the degree of polarization in both an induced and permanent dipole. The dipole moment
is a function of the charge and the distance between the charges.
Associated molecules such as water and alcohol have high dipole
moments and therefore high dielectric constants because of the
long-chain pseudo-molecules.
Non-polar compounds like benzene do not have a sufficiently high
dielectric constant to separate polar molecules. These compounds
can only dissolve those molecules held together by very weak intermolecular forces (induced dipole-induced dipole), such as naphthalene. Because there are very weak interactions (i.e., London forces)
between solute-solute, solute-solvent, and solvent-solvent, these
type of non-polar solutions behave near ideally. Remember that
ideal solutions imply there is no interaction between solvent and
solute.
The dielectric constant of a compound is an index of its polarity. A
series of solvents of increasing polarity shown in [Table-2] will show
a similar increase in dielectric constant.
Table 2- Dielectric Constant of Different Solvents
Compound
Water
Sorbitol Solution USP (70% w/w)
Syrup USP
Glycerol (glycerin)
Methanol
Propylene glycol
Ethanol
n-Propyl alcohol
Acetone
Dielectric constant at 20°C
80
62
56
46
33
32.1
25
22
21
Solvents may be classified according to their dielectric constants as
polar (ε > 50), semi-polar (ε = 20 - 50), or non-polar (ε = 1 - 20) [1].
Water, on the other hand, cannot dissolve things like naphthalene
because the attraction of water for naphthalene is much less than
that of water for water. The classification of solvents on the basis of
polarity is often referred to as the rule of “like dissolves like”. In
other words, if you want to dissolve a highly polar or ionic compound you should use a solvent that is also highly polar or has a
high dielectric constant. If you want to dissolve a compound that is
non-polar, you should use a solvent that is relatively non-polar, or,
in other words, as a low dielectric constant.
Effect of Pressure
Liquids and solids exhibit practically no change of solubility with
changes in pressure. Gases as might be expected increase in solubility with an increase in pressure. Henry's Law states that: The
solubility of a gas in a liquid is directly proportional to the pressure
of that gas above the surface of the solution.
If the pressure is increased, the gas molecules are "forced" into the
solution since this will best relieve the pressure that has been applied.
The number of gas molecules is decreased. The number of gas
molecules dissolved in solution has increased as shown in the
graphic. For example : carbon dioxide is filled in cold drink bottle
(such as coca cola, Pepsi, 7up) under pressure.
Effect of Polymorphs
A solid has a rigid form and a definite shape. The shape or habit of
a crystal of a given substance may vary but the angles between the
faces are always constant. A crystal is made up of atoms, ions, or
molecules in a regular geometric arrangement or lattice constantly
repeated in three dimensions. This repeating pattern is known as
the unit cell. The capacity for a substance to crystallize in more than
one crystalline form is polymorphism. It is possible that all crystals
can crystallize in different forms or polymorphs. The color, hardness, solubility, melting point, and other properties of a compound
depend on its polymorphic form. If the change from one polymorph
to another is reversible, the process is called enantiotropic. If the
system is monotonic, there is a transition point above the melting
points of both polymorphs. The two polymorphs cannot be converted from one another without undergoing a phase transition. As
mentioned, polymorphs can vary in melting point. Since the melting
point of the solid is related to solubility, than polymorphs will most
likely have different solubility’s. If the wrong polymorph is chosen
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Jadhav P.B. and Pandey P.S. (2013) Phase Solubility Analysis: A Technique of Purity Determination.
World Research Journal of Pharmaceutical Research, Volume 1, Issue 1, pp.-05-11.
during the formulation process, the detestable (i.e., thermodynamically unstable form) form can convert to the stable form which can
result in changes in solubility.
Effect of Molecular Size
Molecular Size will affect the solubility. The larger the molecule or
the higher its molecular weight the less soluble the substance will
be. Larger molecules are more difficult to surround with solvent
molecules in order to solvate the substance. In the case of organic
compounds the amount of carbon "BRANCHING" will increase the
solubility since more branching will reduce the size (or volume) of
the molecule and make it easier to solvate the molecules with solvent.
Principle
The equilibrium solubility of a pure substance in a given solvent, at
constant temperature, is a quantity characteristic of the substance,
and may therefore be utilized as a criterion of identity and purity. If a
sample exhibits a solubility in excess of that expected for the pure
compound, then the additional quantity of solute may be ascribed to
the presence of a second component (that is impurity). The refinement of this basic idea for the determination of purity is known as
phase solubility analysis [5].
A method that does not require a specific assay. It need a balance
and can be used to determine the purity of a sample even in the
absence of an assay. It is based on the Gibbs Phase Rule [6].
Gibbs' phase rule describes the possible number of degree of freedom (F) in a closed system at equilibrium, in terms of the number of
separate phases (P) and the number of chemical components (C) in
the system. It was deduced from thermodynamic principles by Josiah Willard Gibbs in the 1870s.
The variables needed to describe the system are Pressure, Temperature and the Chemical Potential (as may be related to the relative mole fractions X) of the components in each phase, i.e. PC + 2
- P in total.
The key thermodynamics result is that at equilibrium the Gibbs free
energy change for small transfers of mass between phases is zero.
This requires the chemical potentials for a component to be the
same in every phase.
Gibbs’ rule then follows, as:
F = C − P + 2.
Where F is the number of degrees of freedom, C the number of
chemical components, and P is the number of phases that cannot
be shared [6].
Phase solubility analysis is applicable to all species of compounds
that are crystalline solids and that form stable solutions. It is not
readily applicable to compounds that form solid solutions with impurities.
Theory
Phase solubility analysis for purity involves measurement of the
solution concentration at several system compositions after equilibration at constant temperature (system composition is the amount
of solid sample per unit weight of solvent) [7].
Case 1
When the original solid is composed of one pure compound, the
diagram is represented by ABC. The slope of the line AB is 45 de-
gree, and extrapolation of the line BC to the y-axis give the solubility
of the solid in that solvent. As more solid is being added, all of it
dissolve in the solvent along line AB, where there is only one
phase, the solution phase, and the two components. According to
Gibbs phase rule, since temperature and pressure are to be constant, there is one degree of freedom, that of solution composition.
Thus the system can be defined completely by the solution composition in appropriate units. At point B, there is a sharp break in the
phase diagram. This is because the solubility of the solid is reached
at B; therefore, no more solid dissolves into the solution, so that
more solid added remains in solid phase. Equilibrium between the
solid and the solution phase is reached. Note that there are two
phases, solid and solution, and two components, so the system is
invariant.
Case 2
When the solid phase consist of a mixture of two compounds, a
curve such as ABCD is obtained. In the first part of the graph along
line AB, as solid is being added, all of it is dissolved into the solution. There is only one phase, the solution phase, and three components therefore according to the phase rule, the system has two
degree of freedom. In order to define the system completely, the
solution composition with respect to both the solids must be expressed. At point B, thus is a sharp break in the line and as more
solid is added, the less soluble solid is precipitated out of the solution. All along line BC, the solid phase obtained is a pure substance
having solubility S (1), as obtained by extrapolation of line BC to the
y-axis. Since there are two phases and three components, the system has one degree of freedom. At point C, there is another sharp
break and, as more solid is added, none of it will dissolve along line
C since the solubility of the more soluble component is reached.
Extrapolation of line CD to the y-axis gives the solubility of the less
soluble component which is S (11) [Fig-5] [Fig-6].
Fig. 5- Phase solubility diagram of a mixture of two solids (ABCD).
Extrapolation of line BC gives the solubility of the less soluble component of the mixture in the solvent studied. Note that both the system are invariant in the region beyond C.
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Jadhav P.B. and Pandey P.S. (2013) Phase Solubility Analysis: A Technique of Purity Determination.
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Steps to Determine the Solubility
 Control temperature and pressure
 Add a quantity of solid to the solvent in the excess of what will
dissolve
 Let the system come to the equilibrium
 Use the specific assay to determine how much of the substance
is in solution (i.e., concentration of the saturated solution)
One of the most important application of the property of solubility
lies in the fact that conclusions regarding the purity and identity of a
substance can be drawn by sophisticated solubility techniques without ever having to know the chemical structure of the solid(s) i.e.
phase solubility analysis.
The standard solubility method consists of six distinct steps:
a
 using, in a series of separate system increasing quantities or
material with measured, filled amounts of a solvent;
 establishment of equilibrium for each system at identical constant temperature and pressures;
 separation of the solid phase from the solution;
 determination of the concentration of the material dissolved in
the various solution;
 plotting the concentration of the dissolved material per unit of
solvent (y-axis or solution composition) against the weight of
material per unit of solvent (x-axis or system composition); and
b
 extrapolation and calculation.
Phase solubility analysis is the quantitative determination of the
purity. At a given temperature, a definite amount of a pure substance is soluble in a definite quantity of solvent. The resulting solution is saturated with respect to the particular substance, but the
solution remains unsaturated with respect to other substances,
even though such substances may be closely related in chemical
structure and physical properties to the particular substance being
tested. Conversely variability indicates the presence of an impurity
or impurities.
c
d
Fig. 7- Experimental System
e
Solvents
Prior to conducting the phase solubility experiment, various solvents
or solvent systems are evaluated in order to select a solvent or
solvent system that is suited for the phase solubility analysis. The
proper solvent or solvent system has following characteristics:
Fig. 6- Points Plotted For Different Concentration of Substance in
Solution
 sufficient volatility to be evaporated under vacuum, but not so
volatile that it cannot be accurately transferred or accurately
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Jadhav P.B. and Pandey P.S. (2013) Phase Solubility Analysis: A Technique of Purity Determination.
World Research Journal of Pharmaceutical Research, Volume 1, Issue 1, pp.-05-11.
weighed. In general, suitable solvents for phase solubility have
boiling points between 60°C and 150 °C.
 does not adversely affect the test, compound, i.e., causing degradation or precipitate formation.
 has known purity and composition.
 the test compound has solubility of about 10-20 mg/ml in the
solvent or solvent system. However, solvents that solubilize the
drug substance at concentration greater than 20 mg/ml can be
used.
Apparatus
Constant Temperature Bath
Use a constant temperature bath that is capable of maintaining the
temperature within ± 0.1˚ and that is equipped with horizontal shaft
capable of rotating at approximately 25 rpm. The shaft is equipped
with clamps to hold the Ampoules. Alternatively, the bath may contain suitable vibrator, capable of agitating the ampoules at 100 to
120 vibrations per second, and equipped with the shaft and suitable
clamps to hold the ampoules.
Ampoules
Use 15 ml ampoules of the type shown in the accompanying illustration. Other containers may be used provided that they are leak
proof and otherwise suitable.
Ampoule (left) and Solubility Flask (right) used in Phase Solubility
Analysis.
Solubility Flasks
Use solubility flasks of the type shown in the accompanying illustration [Fig-8].
stance is selected so that the first ample contain slightly less material than will go into solution in 5 ml of the selected solvent, the second ampoule contain slightly more material, and each subsequent
ampoule contains increasingly more material than meets the indicated solubility. Transfer 5.0 ml of the solvent to each of the ampoules, cool in a dry ice-acetone mixture, and seal, using the double-jet air-gas burner and taking care to save all the glass. Allow the
ampoules and their content to come to room temperature, and
weigh the individual sealed ampoules with the corresponding glass
fragment. Calculate the system composition, in mg per g, for each
ample by the formula:
Csystem (mg/g) = 100 x (W2 – W1) / (W3 – W2)
In which W1 is the weight of the ample plus test substance, W2 is
the weight of the empty ample, and W3 is the weight of ample plus
test substance, solvent, and separated glass.
Equilibration
The time required for equilibration varies with the substance, the
method of mixing (rotation or vibration), and the temperature. Normally, equilibrium is obtained more rapidly by the vibration method
(1 to 7 days) than by the rotational method (7 to 14 days). In order
to determine whether equilibration has being effected, one ample,
that is, the next to the last in the series, may be warmed to 40˚ to
produce a super-saturated solution. Equilibration is assured if the
solubility obtained on the super-saturated solution falls in line with
the test specimens that approach equilibrium from an under saturated solution.
Solution Composition
After equilibration, place the ampoules vertically in a rack in the
constant temperature bath, with the necks of the ampoules above
the water level, and allow the contents to settle. Open the ampoules, and remove a portion greater than to 2 ml from each by
means of a pipette equipped with a small pledged of cotton membrane or other suitable filter. Transfer a 20 ml aliquot of clear solution from each ample to a marked, tarred solubility flask, and weigh
each flask plus its solution to obtain the weigh of the solution. Cool
the flasks in a dry-ice acetone bath, and then evaporate the solvent
in vacuum.
Gradually increase the temperature to a temperature consistent
with the stability of the compound, and dry the residue to constant
weight. Calculate the solution composition, in mg per g, by the formula:
Csolution (mg/g) =1000 x (F3-F1)/F2-F3)
In which F1 is the weight of the flask plus residue, F2 is the weight of
the solubility flask, and F3 is the weight of the flask plus solution.
Note: All the weightings within ± 10 µg.
Fig. 8- Ampoule and Solubility Flask
Procedure
System Composition
Weigh accurately, in g, not less than 7 scrupulously cleaned 15 - ml
ampoules. Weigh accurately, in g, increasingly larger amount of test
substance into each of the ampoules. The weight of the test sub-
Calculation
For each portion of the test substance taken, plot the solution composition as the ordinate and the system composition as the abscissa. As shown in the accompanying diagram, the points for those
containers, frequently only one, that represent a true solution fall on
a straight line (AB) with a slope of 1, passing through the origin; the
points corresponding to saturated solution fall on another straight
line (BC), the slope, S, of which represent weight fraction of impurity
or impurities present in the test substance. Failure of points to fall
on a straight line indicates that equilibrium has not been achieved.
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Jadhav P.B. and Pandey P.S. (2013) Phase Solubility Analysis: A Technique of Purity Determination.
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A curve indicates the material under test may be a solid solution.
Calculate the percentage purity of the test substance by the formula:
Purity (%) =100-100S
The slope, S, may be calculate graphically or by least-squares
treatment for best fit of the experimental values to a straight line.
Limitation
A mixture of solids that exist in solid solutions, or one solid existing
in different polymorphic form, cannot be easily distinguished by
phase solubility analysis. In such cases, analysis in additional solvents reveals the existence of such a condition, such in both cases,
a phase solubility graph to that of pure substance can be obtained,
whenever phase solubility analysis is carried out on an unknown
substance it is advisable to isolate the initial precipitate and reexamine its solubility in that solvent, and other properties. If identical
solubility is obtained, it is strong evidence of presence of a pure
solid [7].
Analysis in different solvent is also helpful in distinguishing a pure
solid and solid solution of two compounds. Components of a racemic mixture cannot be distinguished by phase solubility analysis
techniques, but where isomer ratio is not equal, fractional crystallization is realized.
Caution must be exercised in assuming that the solid phase is a
single component, identical to the original material. Mc Donald and
North (1974) found that in the case of slightly soluble salts such as
calcium carbonate and strontium sulfate, under some high pressure
conditions, the solid phase after equilibrium is a mixture of the original anhydrous form and a hydrate form. Some solutes accept guest
molecules of solvent to form catharses which are revealed only by
analysis.
Purification Technique
Since the solvent phase in all combination of solvent and solute that
are used to construct segment BC of a phase solubility diagram
contains essentially all of the impurities originally present in the
substance under analysis, while the solid phase is essentially free
from impurities, phase solubility analysis can be used to prepare
pure reference specimens of desired compounds as well as concentrates of impurities from substances otherwise consider pure. A
simple modification technique can be used to accomplished these
purposes with considerably less effort than is usually required for
rigorous phase solubility analysis.
In practice, a weighed amount of test specimen is suspended in a
non-reactive solvent of suitable composition and amount so that
about 10% of the material is dissolved at equilibrium. The suspension is sealed (a screw cap vial is usually adequate) and shaken at
room temperature until equilibrium is attained (usually 24 hrs. is
sufficient for this purpose). The mother liquor is drawn off and evaporated at or near room temperature to dryness. Since mother liquor
contains all of the impurities that were present in the specimen, the
residue has been concentrated with respect to impurities roughly in
proportion to the ratio of the weight of the specimen taken to the
weight of the solids dissolved in the volume of solvent used. The un
-dissolved crystal remaining after withdrawal of the mother liquor
are usually sufficiently pure to be used as a reference standard
after appropriate rinsing and drying.
Applications
 Employed routinely for purity determination, particularly in the
pharmaceutical industry.
 Earliest application, Landsteiner and Heidelberger (1923) used
this solubility technique to distinguish between hemoglobin obtained from very closely related biological species.
 Garrett et al. (1963) have used this technique to determine the
solubility of steroids in mixtures of organic solvents.
 It is an absolute method of analysis. Since the slope of the second segment is equal to the fraction of impurities in the sample
and the accuracy of the slope determination is approximately
0.5%, evidently a sample that contains about 0.5% impurity is
indistinguishable from a pure compound. Despite this insensitivity the method is extremely valuable.
 The solubility technique can be modified to study the extent of
non-covalent interaction between two solutes.
 The solution phase along segment BC is enriched in the impurity (relative to the original sample), so detection of the impurity is
more sensitive in this solution than in the original sample.
 Many proteins are purified by this technique. Advantage of using this technique for purifying proteins is that it does not involve
any drastic treatment of the protein sample.
 Smith and Downing (1979) used “Swish Purification Technique”
to enrich impurities in solution phase, which helps in identifying
them.
 Purity of a substance can be determined even in absence of an
assay.
Conclusion
Thus, phase solubility analysis is an important tool not only for determining solubility of a substance but can also be well applied as
an absolute method to determine the purity of a substance.
References
[1] Sokoloski T.D. (2006) Remington: The Science and Practice of
Pharmacy, 19th ed., 194-200.
[2] Hill J.W., Retrucci R.H. (1999) General Chemistry, 2nd ed., 524
-527.
[3] Veazey W.R., Charles David Hodgman C.D. (1943) Handbook
of Chemistry and Physics, 27th ed., Chemical Rubber Publishing Co., Cleveland, Ohio.
[4] Filippone S., Heimann F., Rassat A. (2002 ) Chem. Commun.,
(14), 1508-1509.
[5] Connors K.A. (2007) Textbook of Pharmaceutical Analysis, 3rd
ed., 328.
[6] Gibbs J.W. (1906) Scientific Papers of J. Willard Gibbs, 1.
[7] Gruenwedel D.W, Whitaker J.R. (1987) Food Analysis: Principle
and Techniques, 4, 51.
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