Potential scale physical

Modular scale-up involves the scale-up of individual components or unit operations of a manufacturing process. The interactions among these individual operations comprise the potential scale-up problem, i.e., the inability to achieve sameness when the process is conducted on a different scale. When the physical or physicochemical properties of system components are known, the scalability of some unit operations may be predictable. 

Some questions a formulator needs to address up front are What are the marketing plans What are the potential obstacles to uniformity Is the active raw material physically and chemically consistent What are the physical plant constraints Addressing these and other questions in the early stages of development could aid in avoiding many scale-up nightmares. Additionally, the identification of potential scale-up issues forces the formulator to consider commercialization of the drug delivery system. Too often, formulation scientists develop tablet formulations in a bubble, only to be later handed off to some poor process development person who has to make it work. 

The work function for ions is defined with respect to the process in which one mole of ions in the ideal standard state of unit concentration in a given solvent are transferred to charge-free infinity in vacuum as unsolvated ions under conditions that the solution bears no net charge. This process differs considerably from that for electrons in metals, which involves removal of electrons from the Fermi level in the metal to charge-free infinity. An important aspect of the latter process is that it involves the definition of zero on the physical potential scale. Thus, an electron at rest in vacuum is defined to have zero potential energy on that scale. 

The input for VisiMix calculations includes all the geometric information related to the reactor (its diameter, height), the agitator (type, size, position in the reactor), and the baffles (type, size, position in the reactor). In addition, the physical properties of the media (e.g., density, viscosity) are included in the input. Some of the VisiMix calculated process parameters are potential scale-up factors. 

Any cell reaction can be considered to be an electron transfer between two coupled half-cells. The measured potential corresponds to the difference of the electron energy. The arbitrary definition of a reference electrode raises the question of whether the electrochemical potential scale can be correlated with energy scales of electrons in surface physics. If measuring work functions or electron affinities, the reference value is the free electron in vacuum. Mehl and Lohmann calculated for the electron affinity of a hydrogen electrode —4.5 eV using the following Bom-Haber process... 

In non-aqueous electrolytes, the different properties of the solvated metal ions lead to different equilibrium and standard potentials. For comparing standard potentials, electrode reactions should be defined as reference systems with similar values in different solvents. Koepp, Wendt, and Strehlow suggested ferrocene/ferrocinium and cobaltocene/ cobaltocenium redox systems. The redox systems are bis-pentadienyl complexes of Fe +/Fe + and Co /Co , respectively. Gritzner and Kuta recommended ferrocene/ferrocinium and bis(biphenyl)Cr(l)/bis(biphenyl)Cr(0). Salt bridges with conventional cells should be avoided. Similar to aqueous electrolytes a reference to the physical potential scale is possible. Similar considerations hold for ionic melts and molten and solid electrolytes. 

Relation between electrochemical and physical potential scales... 

If the flat band potential U-, is known, one can relate the energy terms E of the semiconductor to the energy scale U of an electrochemical cell versus a suitable reference electrode. In solid state physics, the reference state of electron energies is usually the vacuum level, if one wants to compare different solids with each other. The electrochemical potential scale versus a reference electrode is fully in parallel with this absolute scale having only the opposite sign because of the negative charge of the electron. 

It is in principle impossible to measure the potential drop at only one electrode/ electrolyte interface. Two electrodes always have to be combined in an electrochemical cell. To normalize all potential measurements, the standard hydrogen electrode is used as a reference with [H" ] = 1.0 M (pH = 0) and p(H2) = 1.013 bar at an inert metal electrode such as platinum. Its potential is °=0 V by definition. This definition is directly linked to the definition in thermodynamics of AGf (H" ) = 0 for the formation of hydrogen ions under standard conditions. The cell voltage between an electrode and the standard hydrogen electrode is called the electrode potential E. Work function measurements have related this hydro-gen-based potential scale to the vacuum scale used in physics. The standard hydro-... 

Fig. 13.11. A schematic drawing of the potential energy surfaces for the photochemical reactions of stilbene. Approximate branching ratios and quantum yields for the important processes are indicated. In this figure, the ground- and excited-state barrier heights are drawn to scale representing the best available values, as are the relative energies of the ground states of Z- and E -stilbene 4a,4b-dihydrophenanthrene (DHP). [Reproduced from R. J. Sension, S. T. Repinec, A. Z. Szarka, and R. M. Hochstrasser, J. Chem. Phys. 98 6291 (1993) by permission of the American Institute of Physics.]...

---