Chemical analysis allows determination of the chemical composition of collected samples and, therefore, to identify specific compounds in the chosen environment. Each chemical compound has one or more analytical methods, from the many different methods available, that are more suitable for obtaining an accurate determination of concentration. This section of this chapter presents a brief summary of the analytical methods most used at present for chemical composition identification. It includes coverage of toxicity tests that properly supplement chemical analyses when used to assess the possible impact on living organisms.
7.6.2.2 Selective Ion Measurement
Ion selective electrodes (ISE) are membrane electrodes that respond selectively to specified ions in the presence of other ions. ISE include probes that measure specific ions and gasses in solution. ISE are most commonly used to determine cations and anions. An ISE (with its internal reference electrode, Fig. 7.11) is immersed in an aqueous solution containing the ions to be measured, together with a separate, external reference electrode.
Fig. 7.11 Ion selective electrode — main constituent parts
|
Ion |
Concentration range (mol. l-1) |
|
Ag+/S2- |
10-7 ^ 1 |
|
Ca2+ |
5 x 10-7 ^ 1 |
|
Cd2+ |
10-7 ^ 1 |
|
Cl- |
5 x 10-5 ^ 1 |
|
CN- |
10-6 ^ 10-2 |
|
Cu2+ |
10-8 ^1 |
|
F- |
10-6 ^ 1 |
|
H+ |
0 < pH < 14 |
|
K+ |
10-6 ^ 1 |
|
NH3 |
10-6 ^ 1 |
|
NO- |
6 x 10-6 ^ 1 |
|
Pb2+ |
10-7 ^ 1 |
|
Table 7.2 Examples of ion-selective electrodes and measurement ranges |
The most commonly used ISE is the pH probe (see Section 7.5.2). Other commonly used ISEs measure electrical conductivity, metals (see Table 7.2) and gases in solution such as ammonia, carbon dioxide, nitrogen oxide and oxygen.
The principle of the measurement is ion exchange between the ion which is dissolved in the solution being monitored and the ions behind the membrane Fig. 7.11. The electro-chemical membrane permits the desired ions to cross it, resulting in a charge on the fluid inside the membrane. At the same time the same amount of charge is passed from the reference electrode to the sample solution, thus maintaining electrical equilibrium. ISEs are normally available as pen-sized probes that can be lowered into the fluid to be assessed — see Fig. 7.12. An excellent guide to ISEs
and their use is available on-line (Rundle, 2000). Ions commonly analysed using ISEs are listed in Table 7.2.
Chemical analyses are able to give more precise figures but ISEs can be useful to give an approximate value and also indicate a need for more advanced analyses.
