SHORT COMMUNICATION Journal of Radioanalytical and Nuclear Chemistry, Vol. 251, No. 2 (2002) 297–298
Concentration of iodine traces in solution by filtering through activated carbon M. Navarrete,1* A. Gaudry,2 L. Cabrera,1 T. Martínez1 1
Faculty of Chemistry, Bldg. D, CU, UNAM, 04510 Mexico D.F., Mexico 2 Laboratoire Pierre Süe, CE-Saclay, 91191 Gif-sur-Yvette, France (Received June 12, 2001)
The retention of iodine traces (ppb) was investigated in small activated carbon filters (50 mg) from solution at a yielding rate exclusively determined by pH. Retention is approximately 100% at pH values of 4–6, while no retention of iodine traces occurs after filtering them through an activated carbon filter from very acidic and basic solutions (pH values of 1 and 11, respectively) with 0% yielding rate. Since activated carbon is a very pure material, this procedure may be an alternative method of the activation analysis of iodine traces in foodstuffs, because the half-life of 128 I, formed by (n,γ) reaction, is only 25 minutes. It does not allow either the sample to be placed readily in solution or the radioisotope to be separated after irradiation with the purpose of attaining maximal accuracy and sensitivity in this type of analysis.
Preconcentration of selenium traces from organic samples by activated carbon filters has been successfully performed to measure concentrations as low as ppb by activation analysis.1–4 For iodine traces in foodstuffs dissolved by microwave digestion in HNO3, coprecipitation with Bi2S3 prior to irradiation, followed by the addition of an iodine carrier and the precipitation of PdI2 after irradiation, seems to be so far a very sensitive and reliable method to determine extremely low concentrations of iodine (5 ppb) by activation analysis.5,6 However, the preconcentration of iodine traces in solution by fixed pH and filtering through a small filter of activated carbon (50 mg) without any complexing agent is definitely simpler. It should also have comparable sensitivity and precision because activated carbon is a very pure material, whereas the content of iodine mass in 50 mg of carbon could be negligible. Moreover, testing this tiny mass by irradiation of carbon blanks is easier than testing it by irradiation of pure Bi2S3, measuring the iodine traces in foodstuffs.
a suction line. Each carbon layer was washed with 5 ml of deionized water. Once the alcohol and washing water were discarded from the Erlemmeyer flask, the filter was ready to be used. A stock solution was made by diluting 1 ml of radioactive 131I as iodide in a reducing agentfree NaOH solution to 100 ml with deionized water. The total activity of this product (Amersham IBS 3/) was 74 MBq (2 mCi). Solutions to be filtered were made by taking 0.1 ml of this carrier-free stock solution and diluting it to 50 ml with deionized water. Under our conditions, the pH of these solutions was 5. By adding drops of concentrated and diluted (10%) solutions of HNO3 and NH4OH, pH values were fixed in a range from 1 to 11 by means of a potentiometer. Before filtering each solution through a previously elaborated activated carbon filter, 1 ml of each was taken out with a pipette, deposited in a test tube and counted for 5 minutes (activity A). After filtering, 1 ml of each solution was removed in the same way, deposited in another test tube and counted for 5 minutes (activity B). The detection system used was a well type 3"×3" scintillation detector Bicron Canberra, connected to a PC running the Maestro Program for radioactive detection.
Experimental
Results
Filters were made by stirring 1.25 g of activated carbon (Merck No. 2186) in 25 ml of 2-propanol with a magnetic stirrer. One ml of this suspension was taken out with a pipette and deposited on a filter paper (Nuclepore 25 mm) inserted at the bottom of a glass microfiltration apparatus on an Erlenmeyer side arm flask plugged into
Iodine trace retention for each solution at various pH, can be given as a percentage of the activity retained by the filter:
Introduction
Y (%) = (A–B)×100/A
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M. NAVARRETE et al.: CONCENTRATION OF IODINE TRACES IN SOLUTION BY FILTERING THROUGH ACTIVATED CARBON
APDC, l-ascorbic acid, cupferron or oxine at a specific pH, before filtering. Trace elements retained in the charcoal by this method were Na, K, Br, P, Fe, Mn, Zn, Co, Sn, Se and Sb.7
Conclusions
Fig. 1. Iodine trace retention by activated carbon vs. pH of filtered solution
Figure 1 shows a plot of iodine trace retention by the carbon vs. pH values of the filtered solutions.
For pH values ranging from 4 to 6, radioactive iodine traces were retained at almost 100% by the activated carbon filters. Therefore, the retention of iodine traces in solution by activated carbon is merely a function of pH and should be useful as a pre-separation method to perform activation analysis of iodine in foodstuff or any other organic material.
Discussion In order to change the oxidation state of iodine traces, a few drops of 30% hydrogen peroxide (H2O2) were added to the solutions, the results yielding no variation. Additionally, in an attempt to find a reagent that might carry the iodine traces through the carbon filters, 20 mg of soluble starch were added to the solutions. However, the results remained the same. The time taken to perform the described experiment was approximately half an hour. The results were quite reproducible when the experiment was repeated three times. The retention of some other trace elements in solution by filtering through activated carbon seems to be very unlikely under these conditions, because they should be treated with some chelating agent such as
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