Specific Activity of Radioactive Iodine

From the specific activity of radioiodine in a reaction mixture, the counting rate, and the volume of the mixture, the concentration of total iodine can be determined. Since there is a correlation between concentration of iodine and anomalous behavior, the determination of specific activity in investigations of anomalous behavior is important. It should be pointed out, however, that in most of the studies of the behavior of carrier-free iodine no attempt was made to determine the specific activities of the reaction mixtures the total concentration of iodine was only estimated. 

The determination of the specific activity of an active sample involves two measurements a) the counting rate of the sample and b) the total iodine content of the sample. The counting rate is easily determined for a given geometry. For those solutions where the total iodine and iodide content are the same the total iodine content has been obtained in a variety of ways. In one procedure (25), a photocolorimetric microtechnique was developed which made use of the catalytic effect of iodide ion on the reduction of cerium (IV) to cerium (III) by arsenious acid in sulfuric acid solution. Under the conditions described it was possible to determine samples of iodine concentrations ranging from 20 to 160 ng/ml. 

Total iodine has also been determined with a radiation-insensitive iodide-sensitive membrane electrode (26). Potentiometric measurements on about 5 ml of 

Carrier-free iodine-125 for protein iodination is available from different manufacturers in an aqueous solution containing sodium hydroxide or sodium sulfite with the activity claimed to be in the form of iodide. This may be true of freshly prepared solutions but in the course of a few weeks significant amounts of this iodide activity is transformed into unidentified species. These species may be volatile and do not exchange with added iodide even after the addition of sodium sulfite or sodium thiosulfate. The uncertainty of the chemical properties of these species essentially eliminates the direct application of chemical methods for analysis (27). Inasmuch as iodine-125 is produced by neutron-irradiation of xenon the isotopic impurities are limited to iodine-126 and iodine-127. It turns out that the total amount of iodine in an iodine-125 solution is represented primarily by iodine-125 and iodine-127 with only a negligible amount contriuted by iodine-126. The nuclides iodine-125 and iodine-127 were determined by measuring the iodine-126 and iodine-128 formed by thermal neutron activation of a sample. 

Stability of Highly Concentrated Carrier-Free Iodide-131 Solutions. 

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