Activities in Real Solutions

Real undergroxmd liquids and gases by conditions and properties of their state are noticeably different from ideal gases and infinitely diluted solutions. This difference becomes greater with the growth of pressure or salinity and due to the increased role of inter-molecular interaction forces. Under such conditions, dissolved components turn out ever closer to one another, and their interaction increases influence on the values of relative activities. 

In order to use the laws of ideal gas for real solutions, Lewis in 1901 introduced a notion of fugacity (volatility). He proposed not to change equations of state for ideal gases but only to modify the values of partial pressures in them. Fugacity of a component i is its modified partial pressure, which under real conditions does not contradict equations of state of the ideal gas. Fugacity in its substance is partial pressure, which accoxmts for the effect of the forces of intermolecular interaction and is measured in units of pressure. 

For this reason activity in real gas solutions a. = f-/, where is fugacity of component i. As under the standard conditions p° of gas components is equal 1 bar, their activity under real conditions is also a dimensionless value numerically equal to fugacity in bars  

Relative activities of dissolved components imder real conditions are in direct relation with their concentration or partial pressure. Proportionality coefficient of such interrelation is called fugacity coefficient for a case of non-polar components and activity coefficient for polar ones. These coefficients enable the calculation of components activities from values of their partial pressures or concentrations  

In these equations addends J rdnic, and i T-lny. characterize deviation of the solutions from ideal and the work, which is necessary to expend in order to squeeze 1 mole of component i of the ideal solution into real solution. Activity coefficients can be greater or smaller than 1. When pressure of a gas solution or concentration of dissolved substances tends to 0, fugacity coefficients or activities coefficients approach 1. Even in diluted real solutions charged ions and dipole molecules experience electrostatic interaction, which shows up in a decrease of activities coefficient. Only in very diluted solutions this interaction becomes minuscule, and fugacity and activities values tend to values of partial pressure and concentration, respectively. Table 1.3 summarizes calculation formulae for activities values of groxmd water components under ideal and real conditions. 

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