Predicting the Logarithmic Distribution Factors for Coprecipitation into an Organic Salt: Selection of Rare Earths into a Mixed Oxalate

Thermodynamic modelling of a leaching system that involves concurrent precipitation depends on an understanding of how the metals distribute into the precipitate before an assessment of solubility can be made. It has been suggested in the past that a pair of rare earths (A and B) in solution will separate from each other by oxalate precipitation according to a logarithmic distribution coefficient (λ), determined by the kinetics of the precipitation. By contrast, the present study hypothesises that λ may be approximated from thermodynamic terms, including the solubility product (KSp) of each rare earth oxalate and the stability constant (β1) for the mono-oxalato complex of each rare earth. The proposed model was used to calculate λ between pairs of rare earths. An experimental study was conducted to determine λ between selected pairs using homogenous precipitation through the hydrolysis of an oxalic acid ester, with fairly close agreement to the values under the proposed model. Though this model requires more thorough testing, as well as application to other organic salts, it may provide insight into distribution factors of a precipitate formed by a sequence of organic complexes.

Salts of Therapeutic Agents: Chemical, Physicochemical, and Biological Considerations

The physicochemical and biological properties of active pharmaceutical ingredients (APIs) are greatly affected by their salt forms. The choice of a particular salt formulation is based on numerous factors such as API chemistry, intended dosage form, pharmacokinetics, and pharmacodynamics. The appropriate salt can improve the overall therapeutic and pharmaceutical effects of an API. However, the incorrect salt form can have the opposite effect, and can be quite detrimental for overall drug development. This review summarizes several criteria for choosing the appropriate salt forms, along with the effects of salt forms on the pharmaceutical properties of APIs. In addition to a comprehensive review of the selection criteria, this review also gives a brief historic perspective of the salt selection processes.

Salts of Therapeutic Agents: Chemical, Physicochemical and Biological Considerations

Choice of the salts of therapeutic agents or active pharmaceutical ingredients (API) is based on the physicochemical properties of API and the dosage form considerations. The appropriate salt can have positive effect on overall therapeutic and pharmaceutical effects of API. However, the incorrect salt form can negatively affect the overall pharmaceutical outcomes of the API. This review addresses various criteria for choosing appropriate salt form along with the effect of salt forms on API’s pharmaceutical properties. In addition to comprehensive review of the criteria, this review also gives a brief historic perspective of the salt selection process.