Conductivity Detection

Conductivity detection is an important example of bulk property detection and is used commonly when the eluted analytes are ionic, for example acids and bases. However, the major use of this form of detection is for inorganic anions and cations after their separation by ion exchange chromatography. Conductivity detection is universal in response for such analytes, and the detectors themselves are relatively simple to construct and operate.

A solution of an electrolyte will conduct an electrical current if two electrodes are inserted into the solution and a potential is applied across the electrodes. It is relatively straightforward to show that the conductance of a solution, G (having the units of microsiemens, represented by the symbol |S) is given by:

1000AC AC

where A is the limiting equivalent conductance of the electrolyte (with units S cm2 equiv-1), C is the concentration of the electrolyte, expressed as equivalents per litre of solution (equiv L-1), and K is the cell constant (with units of cm-1) determined by the geometry of the electrodes. The conductance can be seen to be proportional to the equivalent conductance of the electrolyte and its concentration. In addition, the lower the cell constant, the higher the conductance. This occurs for cells with large surface area electrodes which are close together.

Since the conductance of the solution results from both the anions and cations of the electrolyte, we must therefore calculate conductance using values for the limiting equivalent ionic conductances (X) of the individual anions and cations in solution. Equation [1] can now be rewritten as:

of the electrolyte, respectively. Limiting equivalent ionic conductances for some common ionic species are listed in Table 1.

The operating principles of conductivity detection can be illustrated by considering the conductance of a typical eluent prior to and during the elution of an analyte ion. The conductance change, AG, produced when an anionic analyte S_ is eluted by an anionic eluent E_, is given by:

where X + and X- are the limiting equivalent ionic conductances of the cationic and anionic components where CS is the concentration of the analyte and Is is the fraction of the analyte present in the ionic form. Equation [3] shows that the detector response depends on analyte concentration, the difference in the limiting equivalent ionic conductances of the eluent and analyte anions, and the degree of ionization of analyte. The last of these parameters is generally governed by the eluent pH.

Sensitive conductivity detection can result as long as there is a considerable difference in the limiting equivalent ionic conductances of the analyte and elu-ent ions. This difference can be positive or negative, depending on whether the eluent ion is strongly or weakly conducting. If the limiting equivalent ionic conductance of the eluent ion is low, then an increase in conductance occurs when the analyte enters the

Table 1 Limiting equivalent ionic conductances of some ions in aqueous solution at 25°C

Anion

X 3 (S cm2 equiv )

Cation

X + (S cm2 equiv )

OH-

198

H3O#

350

Fe(CN)6-

111

Rb#

78

Fe(CN)6-

101

Cs#

77

CrO4-

85

K#

74

CN-

82

NH#+

73

so4-

80

Pb2 +

71

Br-

78

Fe3 +

68

I-

77

Ba2 +

64

ci-

76

Al3 +

61

C2O4-

74

Ca2 +

60

co3-

72

Sr2 +

59

no3-

71

CH3NH3+

58

PO4-

69

Cu2 +

55

ClO3

67

Cd2 +

54

SCN-

66

Fe2 +

54

CIO3-

65

Mg2 +

53

Citrate3-

56

Co2 +

53

HCOO-

55

Zn2 +

53

F-

54

Na +

50

HCO3-

45

Phenylethylammonium +

40

CH3COO-

41

Li +

39

Phthalate2-

38

N(C2H5)++

33

c2h5coo-

36

Benzylammonium#

32

Benzoate-

32

Methylpyridinium#

Solute Us)

Eluent 2 UE)

Figure 6 Schematic illustration of the principles of direct and indirect conductivity detection.

detection cell. This detection mode is direct, since the analyte has a higher value of the measured property than does the eluent ion. Alternatively, an eluent ion with a high limiting equivalent ionic conductance can be employed and a decrease in conductance would occur when the analyte enters the detection cell. This type of detection is indirect, where the analyte has a lower value of the measured property than does the eluent ion. These detection modes are shown schematically in Figure 6 and practical examples of direct and indirect conductivity detection are illustrated in

Figures 7 and 8. In Figure 7A, the weakly conducting borate-gluconate complex is used as eluent, whilst in Figure 7B a suppressed carbonate-bicarbonate eluent is used (see below). In contrast, highly conducting eluent ions are used in Figure 8, namely hydroxide (Figure 8A) and hydronium (Figure 8B) ions. Examination of Table 1 shows that these are the most strongly conducting ions and should therefore lead to sensitive indirect conductivity detection.

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