Activity is defined as the effective concentration in solution. When a strong electrolyte (e.g. NaCl) dissolves it completely dissociates into its ions (Na+and Cl-). At relatively high concentrations these ions interact with each other. Oppositely charged ions attract each other to form ion associations (e.g. Na+Cl- , Na+Cl-Na+). These ions are engaged in association such that they become not effectively affecting the properties of the solution. Solution properties affected by ion associations include: electrical conductance, freezing point depression, pH, and others. Activity, a, of an ion in solution is calculated using the following equation:
Where, γ is the activity coefficient of the ion in molarity scale, C is the concentration of the ion in molarity. The activity coefficient is a unitless number that is usually between zero and one. Usually the activity coefficient decreases as the concentration of ions in solution increase. On the other hand, it approaches unity when the solution is diluted. In some cases (CaCl2 ) the activity coefficient can be larger than unity at relatively high concentrations.
The activity coefficient of a given ion in solution is usually affected by:
1. Temperature.
2. Dielectric constant of the solvent.
3. Ionic strength of the solution (related to total concentration of ions in solution).
4. Valence of the ion.
Some equations are used to make calculations of the activity coefficient of a given ion.
To calculate the activity (mean ionic activity) of a dissolved salt, a±, the following equation is used:
Where, a+ is the activity of the positively charged ion, m is the stoichiometry of the positively charged ion, a- is the activity of the negatively charged ion, n is the stoichiometry of the negatively charged ion. For example, if we need to calculate the activity of FeCl3 in solution then we use the following equation:
Where, γ is the activity coefficient of the ion in molarity scale, C is the concentration of the ion in molarity. The activity coefficient is a unitless number that is usually between zero and one. Usually the activity coefficient decreases as the concentration of ions in solution increase. On the other hand, it approaches unity when the solution is diluted. In some cases (CaCl2 ) the activity coefficient can be larger than unity at relatively high concentrations.
The activity coefficient of a given ion in solution is usually affected by:
1. Temperature.
2. Dielectric constant of the solvent.
3. Ionic strength of the solution (related to total concentration of ions in solution).
4. Valence of the ion.
Some equations are used to make calculations of the activity coefficient of a given ion.
To calculate the activity (mean ionic activity) of a dissolved salt, a±, the following equation is used:
Where, a+ is the activity of the positively charged ion, m is the stoichiometry of the positively charged ion, a- is the activity of the negatively charged ion, n is the stoichiometry of the negatively charged ion. For example, if we need to calculate the activity of FeCl3 in solution then we use the following equation:
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