Phase-Solubility Analysis

Phase-solubility analysis17 (sometimes referred to as phase equilibrium purification) is the quantitative determination of the purity of a substance through the application of precise solubility measurements. 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. There are examples of the use of this technique in HPLC methods development18 and in the characterization of reference standards,19 but the 

Peak Name Rt(min) Reference Solution Supernatant Enhancement Factor 

Of the impurity/degradant enhancement techniques available, the phase-solubility analysis technique requires minimal sample handling from the 

FIGURE 7 Citrate salt impurity enhancement example. 

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Several collaborating laboratories (usually five participating laboratories) test the proposed substance using a variety of techniques. The relative reactivity or relative absorbance of the impurities present in a substance must be checked when a nonspecific assay method is employed, e.g. by colorimetry or ultraviolet spectrophotometry. It is particularly important to quantify the impurities when a selective assay is employed. In such a case, it is best to examine the proposed substance by as many methods as practicable, including, where possible, absolute methods. For acidic and basic substances, titration with alkali or acid is simple but other reactions which are known to be stoichiometric may be used. Phase solubility analysis and differential scanning calorimetry may also be employed in certain cases. 

Phase solubility analysis is a technique to determine the purity of a substance based on a careful study of its solubility behavior [38,39]. The method has its theoretical basis in the phase mle, developed by Gibbs, in which the equilibrium existing in a system is defined by the relation between the number of coexisting phases and components. The equilibrium solubility of a material in a particular solvent, although a function of temperature and pressure, is nevertheless an intrinsic property of that material. Any deviation from the solubility exhibited by a pure sample arises from the presence of impurities and/or crystal defects, and so accurate solubility measurements can be used to deduce the purity of the sample. 

The experimental procedure for conducting phase solubility analysis is rather simple it consists of mixing increasing amounts of sample with a fixed volume of solvent and then determining the mass of sample that has dissolved after each addition. It is not necessary to exceed the solubility limit of the analyte species, but attainment of this condition makes it easier to recognize trend within the plots. An experimental protocol for phase solubility analyses is available [39]. The data are most commonly plotted with the system composition (total mass of sample added per gram solvent) on the x axis, and the solution composition (mass of solute actually dissolved per gram of solvent) on the y axis. 

Schirmer has succinctly summarized the strengths and limitations of phase solubility analysis [40]. The principal advantages are that (1) a reference standard known purity is not required, (2) the number and types of impurities in the sample need not be known, (3) all required solubility information is obtained from the analysis, (4) the technique can be applied to the analysis of any solute that can be dissolved in some solvent, (5) the deduced results are both precise and... 

A basic exposition of Gibbs phase rule is essential for understanding phase solubility analysis, and detailed presentations of theory are available [41,42]. In a system where none of the chemical species interact with each other, the number of independently variable factors (i.e., the number of degrees of freedom, F) in the system is given by... 

Table 4 Phase Solubility Analysis of Fluphenazine Dihydrochloride in Absolute Ethanol...

For phase solubility analysis, acetonitrile appears to be the most suitable solvent. A typical plot is given in figure 13 along with the experimental conditions. 

Figure 13. Phase Solubility Analysis Plot of Bromocriptine Mesilate, dried in High Vacuum for 15 Hours. Solvent dry Acetonitrile, Vibration for 24 hrs. in the Absence of Light. Slope 1.41 0.05 %.

Fig. 13 Broadening effect of the melting curve of P-hydroxy-propyltheophylUne due to impurities. PSA = phase solubility analysis. All batches have the same TLC purity results. (From Ref. )...

Phase-Solubility Analysis Phase-solubility analysis is the quantitative... 

William J. Mader. Phase Solubility Analysis, Critical Reviews in Analytical Chemistry, 2 193-215, 1970. 

United States Pharmacopoeia Rev. 25 and National Formulary Rev. 20. 2002 General Test (1171) Phase Solubility Analysis, p. 2122. 

USP 24 (1171) Phase-solubility analysis, The United States Pharmacopeia Convention, Inc... 

Grdinic, V., Jaksevac-Miksa, M., Bezjak, A., Radaic, A. and Briski, D. Importance of factors for ruggedness test in phase solubility analysis. Eur. J. Pharm. Sci. 2(4) 293-6, 1994. 

Smith, G. B. Downing, G. V, Phase Solubility Analysis as the Basis of a Separation Method. ... 

Phase Solubility Phase solubility analysis is carried out using... 

Phase solubility analysis is carried out using isopropanol as the solvent. A typical example is shown in Figure 4 which also lists the conditions under which the equilibration took place . However, the inherent instability of acidic solutions of chlordiazepoxide such as the hydrochlorides may result in some decomposition during phase analysis. 

In most cases, the purity of a pharmaceutical substance is determined by comparison of the reference standard with the known purity assigned. On the other hand, when no reference standard sample is available, the purity is determined by an absolute method in which the calculated result is based on theory and not by a comparative method. Purity established by analytical methods such as phase-solubility analysis or differential scanning calorimetry (DSC) is known as absolute purity (Figs. 3 and 4). 

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