Concentration and activity

When environmental chemists measure the amounts of chemical substances, for example in a water sample, they are usually measuring the concentration of that substance, for example the concentration of calcium (Ca) in the water. It is very easy to assume that the analysis has measured all of the free Ca2+ ions in the sample, but in fact it will almost certainly have measured all of the calcium-bearing dissolved species, called ion pairs, as well. Ions in solution are often sufficiently close to one another for electrostatic interactions to occur between oppositely charged species. These interactions reduce the availability of the free ion to participate in reactions, thereby reducing the effective concentration of the free ion. Collisions between oppositely charged ions also allow the transient formation of ion pairs, for example:

The formation of these ion pairs (see Box 6.4) further reduces the effective concentration, and the frequency of collisions increases as the total amounts of chemical species in the solution increase. The effective concentration of an ion therefore becomes an important consideration in concentrated and complex solutions like seawater.

In order to predict accurately chemical reactions in a concentrated solution, we need to account for this reduction in effective concentration. This is done using a concentration term known as 'activity' that is independent of electrostatic interactions. Activity is the formal thermodynamic representation of concentration and it describes the component of concentration that is free to take part in chemical reactions. Activity is related to concentration by an activity coefficient (g).

activity= concentrationxg eqn. 2.9

Equation 2.9 shows that units of activity and concentration are proportional; in other words g can be regarded as a proportionality constant. These constants, which vary between 0 and 1, can be calculated experimentally or theoretically and are quite well known for some natural solutions. Having said this, measuring g in complex solutions like seawater has proved very difficult. Most importantly for our purposes, as solution strength approaches zero, g approaches 1. In other words, in very dilute solutions (e.g. rainwater), activity and concentration are effectively the same.

In this book activity is expressed in units of mol l-1 in the same way as concentration, but activity is denoted by the prefix 'a' in equations.

We should also note that all thermodynamic terms (e.g. equilibrium constants, see Box 3.2) are expressed as activity. Thus, measured concentrations of any chemical species should usually be converted to activities before comparison with thermodynamic data.

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