For tracers composed of single atoms, such as 125Г, 22Na+, etc., tests are simpler than for tracer compounds based on molecular complexes. This is exemplified here with the tracer compound cobalthexacyanide ([Co(CN)6]3-). This molecule may be labelled with 56Co, 57Co, 58Co, 60Co or 14C, and it may be used as the unlabelled complex. The latter requires a sensitive analytical method for Co, such as thermal neutron activation analysis. One single molecular carrier may then give rise to six different tracers. The total complex constant of this molecule (P6) is reported to be very high (1038-1064) [9]. This may wrongly be interpreted as indicating that the (CN)6 ligand molecule is very stable and that it will exist in this molecular form more or less regardless of the chemical environment. This argument has led to extensive and somewhat uncritical field use of radiolabelled versions of these molecules. In many cases, good results have been obtained, while in others the tracer has never been produced back. A thorough investigation of radiolabelled [Co(CN)6]3- has already been conducted and a few results of this study and some complementary experiments are discussed.

The 60Co labelled hexacyanide was purchased from one of the largest commercial producers of radiochemicals as a ready-to-inject solution. One of the quality control methods used is the electrophoresis technique. Batches purchased at different times showed different results, indicating a radiochemically impure product. A new synthesis was carried out using procedures provided by the company. Results from this experiment are shown in Fig. 44. The electrochromatogram shows a relatively broad distribution with two distinct peaks. This indicates that the 60Co label exists in, at least, two different anionic forms. These forms are not identified; they may have different stability and chemical behaviours. The compound was then synthesized by a modified method. The modified procedure produced chromatograms similar to that shown in Fig. 45.

Cobalthexacyanide from the commercial company was then subject to thermal stability and sorption investigations. There was fast sorption onto corroded steels already at ambient temperatures, but the sorption became even more pronounced at elevated temperatures. Liquid solutions, after heating to 120°C for 24 h, were again investigated by electrophoresis. The results are shown in Figs 46 and 47.

ID 2D ЗО 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190

POSITION (mm)

A substantial fraction is non-charged and does not move away from the application point. On the positive potential side is a relatively low and broad, nearly constant, distribution indicating different 60Co labelled anionic forms. On the negative potential side is a substantial and broad distribution indicating various positively charged complexes where heating leads to breakdown of the

POSITION (mm)

hexacyanide complex into a range of different complexes with varying masses and charges.

The CN ligands may be exchanged to some degree with Cl-, OH — or even H2O to saturate the coordination number of 6. This leads to complexes of different charges and different chemical properties. The referred experimental investigations showed that the quality control on the radiochemical purity is very
important, and the purity of [Co(CN)6]3 should be ensured at the start of any field application. This tracer compound should not be used at reservoir temperatures >90°C.