Polythermal Measurements

Polythermal methods have in common that a suspension containing known amounts of solvent and solute in excess is heated and the temperature where last particles dissolve is detected. For detection, visual observation (e.g., under a microscope), turbidity measurements, particle-detecting inline probes (e.g., FBRM probe (Lasentec , Mettler Toledo GmbH)), or calorimetry may be used. Since it is a dynamic method, the results depend on dissolution kinetics of the particular system. In general, polythermal measurements are easier to automate since just a temperature has to be followed and no special analytical technique is required. The above-mentioned Crystall6 multiple reactor system can also be used to perform such kind of measurements. To detect both the dissolution process for derivation of saturation temperatures (clear points) and the formation of particles (cloud points) for determination of the metastable zone width, the 

Due to equilibrium constraints, polythermal methods typically do not reach the accuracy achievable with isothermal measurements, but provide first information about the general dissolution and recrystallization behavior of a substance. In addition, the identity of the soUd phase for which the solubiUty is measured cannot 

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For polythermal measurements, different heating rates must be checked for the particular system in order to determine the optimal one. The lower the heating rate. 

C, polythermic method, measured range 20-79.5°C, Udovenko Aleksandrova 1963)... 

Polythermal methods. The temperature of an alloy sample is progressively changed, generally at a well-defined and controlled heating or cooling rate, as some properties of the sample itself are continuously measured. 

The method of autoprotolysis constant value determination based on electromotive force measurement in galvanic element composed from Pt,H2(l atm) KOH(m), KBr(m), solvent AgBr,Ag was proposed elsewhere. The method was used for polythermal study of autoprotolysis constant of binary mixed solvent 2-methoxyetanol - water. On the basis of ionization constants, polytherms data, and the autoprotolysis process, thermodynamic data were calculated. 

A visual polythermic method (1) was used as well as an isothermal method which involved conductivity measurements. Standard methods of analysis were used but no specific details are given. 

Nine solubility polytherms were studied. They differed from each other in the composition of the solvent. The pH of each solvent was measured. For each polytherm, 10-14 samples were prepared by precise weighing. The saturation temperature of each sample was determined by using an apparatus constructed for measuring crystal growth rate (1). 

The KoHPOA-K COi-HyO system. Solubility measurements have been made over the temperature interval of 253 to 353 K (11). Later these same investigators published the solubility polytherm of the quaternary system K2HP0 -K2C03-C0(NH2)2-H20 (12). 

Figure 3.23 Determination of the saturation concentration Csat and the corresponding saturation temperature Tsa. in isothermal (1) and polythermal (2) measurements.

In the following sections, examples of isothermal and polythermal techniques for solubility measurement are presented. Here, advantages and weaknesses of the particular methods as well as the question, how to guaranty equilibrium conditions, are addressed. 

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