A SEMI-AUTOMATED METHOD FOR THE DETERMINATION OF IODINE IN PLANTS

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1 A SEMI-AUTOMATED METHOD FOR THE DETERMINATION OF IODINE IN PLANTS J.R. Bellanger, J.C. Tressol, H.P. Piel To cite this version: J.R. Bellanger, J.C. Tressol, H.P. Piel. A SEMI-AUTOMATED METHOD FOR THE DETERMI- NATION OF IODINE IN PLANTS. Annales de Recherches Vétérinaires, INRA Editions, 1979, 10 (1), pp <hal > HAL Id: hal Submitted on 1 Jan 1979 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 A SEMI-AUTOMATED METHOD FOR THE DETERMINATION OF IODINE IN PLANTS J.R. BELLANGER J.C. TRESSOL H.P. PIEL I.N.R. A., Laboratoire des Maladies Nutritionnel%s, C.R.Z.V. de Theix, Beaumont, France Résumé METHODE SEMI-AUTOMATIQUE DE DOSAGE DE L IODE DANS LES PLANTES. - La matière organique est détruite par incinération alcaline. L iode libéré est déterminé automatiquement à l aide de la réaction de Sandell et Kolthoff. La moyenne de la récupération d 13 11, utilisé comme traceur, est de 93,4 % (écart-type 3,19 %1. Le coefficient de variation = pour différentes plantes varie entre 3 et 5 %. Simple endemic goitre in man and animals is thought to be caused generally by iodine deficiency, although other factors such goitrogens may be involved. Animals need sufficient iodine for optimal growth, reproduction and production (Underwood, 1977). Thus, determination of iodine in plants plays an important role in assessment of both human and animal nutrition. Methods for the determination of iodine in biological materials and plants have been reviewed (Binnerts and Das, 1974). In most cases, the iodide obtained after destruction of organic matter is determined by the colorimetric method of Sandell and Kolthoff (1937), which is based on the catalytic action of iodide on the reaction between arsenic (111) and cerium (IV) in acidic solutions. The method of Fioravanti and Halmi (1971) which is suitable for automation, was checked in this laboratory. But, when this method was applied to plant materials, we could not get reproducible results. The main source of errors encoutered in the determination of iodine was the loss during the destruction of organic matter. This technique was modified in order to obtain reproducible results during the incineration process and the colorimetric determination of liberated iodide was automated. Iodine losses were studied by incinerating plant samples with radioactive iodide ( as a tracer. The optimal conditions for the catalytic action of iodine in the Sandell and Kolthoff s reaction were determined. Material Technicon Auto Analyser I moduls with Gilson sampler, muffle furnace and drying oven were used. Reagents De-ionized water, zinc sulphate solution (10 %, w/v), potassium hydroxide solution (1N) and sulphuric acid (95-97 %) were used.

3 Arsenious solution (AslllJ Dissolve 3.2 g of di-arsenic trioxide resublimed in 50 ml of N potassium hydroxide. Dissolve 21.8 g of sodium chloride in 200 ml of water. Carefully add 60 ml of sulphuric acid to 500 ml of water. Mix these 3 solutions and dilute to 1 litre with water. Ceric sulphate solution (2 g 1-1) Carefully add 50 ml of sulphuric acid to 500 ml of water. Then, dissolve 2 g of cerium sulphate [Ce (S0 4)Z 4H 201 and dilute to 1 litre with water. Store the solution in darkness at 4 C. Stock iodine standard (1Ooo!g ml- ) Dissolve g of dried potassium iodide in water and dilute to 1 litre. Working iodide standard solutions With intermediate solution at 1!g ml- 1, prepare in 0.1N potassium hydroxide, immediately before use working solutions which contain 0, 2, 5, 10, 20, 30 and 50 ng ml- 1 of iodine. Procedure Calcination Weigh 1 g of sample in a porcelain crucible. Add 1 ml of Zn S04 and 2 ml of N (KOH). Prepare a blank of reagents. After each addition, MIX INTIMATELY with a glass or plastic rod. Wash, with few millilitres of water, the walls of the crucible and the rod. Dry overnight at 105 C. Cover the crucible and heat at 500C for 90 min. Close the chimney and the small hole in the furnace door. After cooling, add to each sample 1 ml of Zn S0< solution. Carefully mix with water and dry at 105 C. In the same way, heat at 500C for 90 min. Immediately, remove the crucible and allow to cool in a dessicator. Dissolve the ash in 20 ml of water and stir for 20 min. After centrifugation (10 min, 2500 g) transfer about 3 ml of supernatant into a tube for immediate determination. Automated determination Manifold The flow diagram is shown in figure 1. The reagent base line is between 10 and 20 % T. Measure standards and samples at 420 nm. Calculation Read off the peak height of the standards and construct the calibration curve. For concentrations between 20 and 50 ng ml-, the

4 peak height should be plotted against the logarithm of the iodine concentration. By using this graphs, determine after deduction of the blank, the iodine concentration of the samples. Results and discussion Destruction of organic matter. Recovery of iodine. To check for iodine losses during the incineration and recuperation process, grass and different plant material samples were labelled with radioactive iodine ( With the described method, the recovery was t 2.06 % (mean t -S.D.) for 18 determinations of a grass sample. For 40 different plant material samples the recovery was t 3.19 %. These recoveries were satisfactory, the highest recovery being 100 % (barley seeds, 0.02 mg kg- 1, DM) and the lowest 88 % (palm-tree leaves, 0.93 mg kg- 1, DM). According to a number of authors the disadvantage of this method is the variable recovery between the different forms of iodine. In previous investigations with the method of Fioravanti ei al. we obtained many low recoveries but a very good correlation (r 0.95) should be noted bet- = ween the recovery (p) and the concentration measured by colorimetry (c). The equation of regression was : Therefore, we think that this method is satisfactory. Admission of air. It is very important to work with the chimney and the hole in the door of the furnace closed and the crucibles covered. This is in agreement with Binnets and Das (19741, Oeischl5- ger and Feyler (1976) and Stabel-Taucher (1976) and in disagreement with Lauber (1975). The recovery of iodine was 95.7 % (SD = 0.89) when the furnace was closed and only 78.7 % (SD = 6.58) if the furnace was not airtightly closed (P < 0.01). ), Effect of the temperature. If the temperature was raised from 450 C to 600 C, the recovery of iodine was affected. Furthermore, for temperatures different from 500C the recovery decreased near the front

5 door and the bottom of the furnace as shown in table 1. These results partially agree with those of Fioravanti and Halmi (19761, Oelschl5- ger and Feyler (1976) and Matthes et al. (1973). But, we think that the conclusions concerning the recovery of iodine during the incineration process are strictly dependent on the conditions of incineration and cannot be generalized. Thus, at 500C, the crucible position did not influence the recovery whereas at 450 C, the recovery depended on the crucible position. This may partially explain the disagreement between authors. Time of calcination. Repeated calcinations were unfavourable. Recovery of iodine decreased from 95.5 % for 3 hr of calcination to 86.7 % for 4.30 hr (P < This is in agreement with Oelsch- Idger and Feyler (1976) and Matthes et al. (1973). Automated determination of iodine. In order to enhance the sensitivity, the reaction parameters were studied using the controlled concentration gradient method (Lebreton, 1960). Optimum concentration of cerium. The concentration of cerium reagent was adjusted to 2 g I- so that the base line fell between 10 and 20 % of transmission. An increase of temperature was found to be effective in improving the sensitivity (Truesdale and Smith, 1975). But, if the temperature was higher than 60 C, bubbles appeared, inducing an instability of the base-line. Therefore, the temperature of 50 C was selected. Influence of reagents on the discolouration of cerium solution alone. No fading of cerium solution occured when concentrations of sodium chloride solution, acid sulphuric solution, and N potassium hydroxide solution, increased from 0 to 33 g I- from 0 to 60 ml 1- and from 0 to 90 ml 1, I-res- pectively. A solution of arsenic (0 to 8 g I- I) discoloured the cerium reagent. The sodium chloride solution (0 to 30 g Iw) enhanced this discolouration. The addition of sulphuric acid (0 to 90 mil-i) decreased the effect of sodium chloride. Influence of reagent concentrations on the sensitivity. The main factor influencing the sensitivity was the composition of the arsenious solution (Cabello, 1973). With the concentration controlled gradient method we studied the opti-

6 mum concentration of arsenious trioxide and of sodium chloride. The concentration of sulphuric acid was set up to 60 mi The sensitivity was related to these concentrations and depended on the iodine concentration as shown in figure 2. In order to obtain a maximum sensitivity for low concentrations of iodine, the concentrations of arsenious trioxide and of sodium chloride were fixed to 3.2 and 21.8 g 1- respectively. Precision of the automated determination. For the measuring process alone the coefficients of variation of iodine standard solutions at 10 and 30 ng mi- I were 0.84 and 0.37 % respectively (N 20). = Comparison with the manual method. Ten samples of two grasses were incinerated with the method described here. Then, these 20 mineralized samples were analyzed with both manual and automated method already described. The variance of the 40 data obtained was analysed (table 21. As expected, there was a highly significant difference between samples but no significant difference between the means given by both methods. However, the variance of the automated method was significantly lower (variation coefficient 3.4 % vs 6.05 % for the manual). Precision. The described method was used to determine the iodine content in different plant materials (table 31. For repeated analyses (N = 10) of different samples, the coefficient of variation ranged from 3 to 5 % for concentrations range between 0.1 and 0.8 mg kg-, DM. Conclusion The described method is reliable and reproductible for the determination of iodine in plants in a routine laboratory. Compared to the manual procedure it has the advantage of preventing contamination and allowing a strict control of the time of discolouration. Accepted for publication December 8th Acknowledgements We are grateful to J.-C. FAYET for the statistical treatment of the data. Summary A method for the determination of iodine in plants is described. This method involves destruction of organic matter by alkaline incineration and automated spectrophotometric determination of iodide based on the Sandell and Kolthoff s reaction. The mean recovery of 131 was 93.4 % (S.D. = 3.19 %1. For replicate analyses of different plant material, the coefficient of variation is between 3 and 5 %. References BINNERTS W.T., DAS H.A., Determination of iodine in biological material. In Glick D.: Methods of biochemical analysis. 22, , Academic Press, New York. CABELLO G., lod!mie hormonale : mise au point d une m!thode analytique fiable, et dosage de ce paramètre chez quelques ruminants. Dipl6me d Etudes Sup6rieures n 747, Universit6 de Clermont- Ferrand, 101 p. FIORAVANTI P., HALMI M., Dosage de I iode dans les v6g6taux et dans les aliments pour bétail. Trav. Chim. Aliment. Hyg., 62, LAUBER K., Iodine determination in biological material. Kinetic measurement of the catalytic activity of iodide. Anal. Chem., 47, LEBRETON L., L analyse chromatographique avec gradient d 61ution. Bull. Soc. Chim. France, 2, MATTHES W., KISS T., STOEPPLER M., Zur Jodbestimmung in Nanogrammbereich nach der Sandell-Kolthoff-Reaktion. Z. Anal Chem., 267,

7 OELSCHLAGER W., FEYLER L., Gestimmung geringoter Mengen Jod in biologischem Material mittels der trockenen Veraschung. Landwirtsch. Forsch., 29, SANDELL E.B., KOLTHOFF I.M., Microdetermination of iodine by a catalytic method. Mikrochim. Acta, 1, 9. STABEL-TAUCHER R., Determination of iodine in milk. Finn. Chem. Lett., TRUESDALE V.W., SMITH P.J., The automatic determination of iodide or iodate in solution by catalytic spectrophotometry, with particular reference to river water. Analyst, 100, UNDERWOOD E.J., Trace elements in human and animal nutrition Academic Press, New York.