444 SHORT COMMUNICATIONS

I’extraction du bismuth (et du plomb) sous forme de dithizonate et mesure sClective du bismuth par l’absorption atomique. Une strie de 4 & 6 dosages peut Ctre effectuke en une demi-journke par un chimiste entrain&

Les auteurs remercient la Direction du Centre de Recherches de Pont-&- Mousson pour les avoir autorisCs & publier les r&ultats de leurs travaux.

Ce~re de Recherches de Pont-h-Mousson, G. Kisfaludi 54 Ponr-d-Mousson (Frrmce) M. Lenhof

1 G. KISPALUDI ET M. LENHOF, And. Chim. Add, 54 (1971) 83. 2 R. C. ROONEY, Andyst, 83 (1958) 83. 3 J. C. MARGERW ET M. DROUZY. M~I. Sci. Rev. Met., LVI (1959) 72. 4 Z. VEVERA LT B. BIEBEI~, Hutnicka Lisfy, 16 (1961) 667. 5 N. LEONTOVIT~H, Chim. Anal. (Pwis), 47 (1965) 458. 6 D. CHR. FILIPOV ET I. R. NATCHEV, Compf. Rmd. Acd. Bulpre Sci., 20 (1967) 109. 7 C. G. CARLSTROM in' W. PALVARINNE, Jerrrkorrrr/rets Am., 146 (1962) 453. 8 I-1. FISCHBR, Angerv. Chm., 46 (1933) 442.

(Requ le 18 mars 1971)

Anal. Chirn. Actu, 55 (1971) 442-444

Determination of iodine in sea water by an improved Sugawara method

The method of Sugawara et al.’ for the determination of iodine in natural water has been applied to geochemical problems2. When the method is used to determine iodine in sea water3, however, the results for the content of iodine in sea water showed large fluctuation from sample to sample and systematically low values as compared with those of Barkley and Thompson4.

Recently, Matthews and Riley5 critically discussed the method and developed a modified procedure. Tsunogai and Sase6 suggested the mechanism of iodide forma- tion in sea water, in which iodate was proved to be fairly easily reduced to iodide by biological activity. Therefore, it is necessary to analyze or separate iodide from iodate without delay after sea water samples are collected. Miyake and Tsunogai’, and Tsunogai and Henmi’ have obtained good results for iodide and iodate in sea water by using the improved Sugawara method, which is reported here.

In the method of Sugawara et al., iodide is coprecipitated with silver chloride from a large volume of water sample. Iodide in the precipitate is oxidized to iodate by acetate solution saturated with bromine at. pH 2.8. After the precipitate is filtered off, the iodate in the solution is reduced by sulfite and reoxidized by the bromine solution. The reaction between the iodate and the excess of added iodide renders iodine as triiodide, which is spectrophotometrically determined as the starch-iodine complex. The large errors observed are probably derived from coprecipitation of iodide from sea water, incomplete removal of bromine compounds in higher oxidation states than bromide which are formed in the oxidation treatment_

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Coprecipitatitin of iodide Matthews and Riley reported that even with vigorous agitation and slow

addition of silver nitrate the coprecipitation of iodide never exceeded 90%, even if sufficient silver nitrate was added to precipitate 4% of the chloride present. According to Oweng, ,the solubility product of silver iodide is 1.2. lo- I6 at 25” and is 2.1. lo6 times smaller than that of silver chloride. If a pure solid phase of silver iodide is made from iodide jn sea water, about 20 pg l- ’ of iodide remains in the solution on the addition of silver nitrate equivalent to 1% of the chloride present. However, if iodide can freely replace chloride in the solid silver halide, almost all the iodide in sea water precipitates as a eutectic mixture under the condition that a univalent ion in sea water has an activity coefficient of 0.65 and the activity coefficients of halides in the solid are approximately equal to one another.

This situation was experimentally examined ‘by using radioactive 13iI as an iodide tracer. To 200 ml of sea water, stirred with a magnetic stirrer, 4 ml of 0.1 N silver nitrate solution was added. After stirring for 10 min, it was stored in a dark place. When the silver halide precipitate was filtered within an hour, a consider- able fraction of iodide remained in the solution, as shown in Table I. After standing for about one day, however, more than 98% of the iodide was found in the precipitate.

TABLE I

COPRECIPITATION 01; IODIIX FROM SEA WATER”

Reager~ts added ml Tim? (h) No. OJ I- pptd. detns. (‘%)

0.1 N AgNO, 4 0.5-l 5 53-80 0.1 N AgNO, 4 18-24 8

(

99f 1 0.1 N NaBr 2 Ammonium buff& 16 0.5-l 5 97f2 0.1 N AgNO, 8

a Sampled in the western North Pacific; contained 10 ~(8 of i--iodine and 38 118 of IO;-iodine per liter;

200 ml was used. ’ 1.4 M NH&l and 8.0 M NH,OH. pH = 10.2.

Another method for the complete precipitation of iodide is to coprecipitate iodide with silver bromide while chloride remains in solution by the addition of ammonium hydroxide. In this case, the time of analysis is shorter, but the precipitate needs to be separated by centrifuging because the silver bromide precipitate does not easily coagulate.

Interference by bromine compounds On oxidation of iodide in the precipitate with bromine, some interferences

caused serious errors in the analysis. Sugawara et al. considered that metallic silver is formed photochemically in the silver halide precipitate and reacts with bromine to produce hypobromous acid. Thus, Sugawara et al. reduced it with sulfite and repeated the oxidation treatment with bromine.

It was found, as shown in Table II, that the interferences are completely re- moved at pH 2.8 when the solution is evaporated to S-10 ml on a boiling water bath for 2 h and stood for 18 h before the calorimetric determination. The results did not change even though part of the silver in the precipitate was reduced before treatment

Anal. Chint. ,4CfU, 55 (1971) 444447

446 SHORT COMMUNICATIONS

with bromine. Besides hypobromous acid, bromate is likely to form in the solution at a

higher pH . lo These ions are more stable in the more alkaline solution and could not be removed by standing at pH 4. On the other hand, below pH.2, free iodine is formed

TABLE II

lNI’l?HFii’RBNCI! IJY UROMINE IN HIGHER OXIDATION STATES

TintS (II) No. of Mean absorbance detns. und std. deo.’

1 0.2-0.5 6 2 0.2-0.5 6 1 12-18 11 2 12-18 6 1” 12-18 5

u Time elapsed before the color development.

0.44 1 + 0.043 0.426 -f 0.034 0.370 & 0.006 0.377 + 0.008 0.374 f 0.007

b Before treatment, the precipitate wns exposed to sunlight for 2 h. c Trial iodine in 250 ml of the same sea water as that in Table I.

and escapes together with bromine. Therefore, it is necessary to expel bromine from the solution at pH ca. 3, but it is not necessary to reduce the solution with sulfite and reoxidize with bromine. It is preferable; if possible, to avoid the photochemical de- composition of silver halide because it consumes bromine and the oxidation of the precipitates requires more time.

Thus, the difficulties in Sugawara’s method are easily resolved by letting the solutions stand for a sufficiently long time in both cases in question and the analytical procedure is simplified. The following procedure is recommended.

Procedure Place a 200-300-ml portion of sea water in a beaker. While stirring with a

magnetic stirrer, add 20 ml of 0.1 N silver nitrate solution. Allow the solution to stand in the dark more than 20 h. Filter the precipitate through a coarse filter paper (No. 5A, Toyo Filter Paper Co. Ltd.) and reserve the solution for the determination of iodate. Transfer the precipitate to a small beaker with 15 ml of water, add 10 ml of acetate buffer saturated with bromine and let it stand for an hour with occasional stirring. Warm the beaker on a boiling water bath until the solution is colorless. Filter off the precipitate through a fine filter paper (No. 5C) and reduce the filtrate again on a water bat11 to 5-10 ml. Transfer the solution to a 25-ml graduated flask with 5 ml of acetate buffer and water. After letting it stand for more than 12 h, add 1 ml of 6.5% starch solution and 1 ml of 5% cadmium iodide. Measure the absorbance of the solu- tion at 570 nm after 20 min.

Iodate in the reserved filtrate is determined by the same procedure as mentioned above af$er the reduction of iodate to iodide by 3 rnl of 1 o/o sodium sulfite and 1 n$ of 3 N sulfuric acid.

Reagents used are the same as those of Sugawara et al. Acetate buffer contains 30 ml of acetic acid and 0.50 g of sodium acetate anhydride in 1 1.

Conclusions The method of Sugawara et al. for the determination of iodide in natural water

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is applied to the determination of iodide in sea water with some simplifications. The error of the method does not exceed &- 3% or _L 1 pg I I- ’ (Table II).

In the actual ocean, the concentration of iodide is often much smaller than that of iodate. Therefore, it is preferable to use more than 500 ml of sea water, to determine the iodide accurately by using the same procedure, but the concentration of iodate is so large that the solution must be diluted twice before the color development.

Matthews and Riley reported that a considerable proportion of iodide in the precipitates of silver halide remained in the precipitate during oxidation by bromine. However, that tendency was not observed unless the precipitate consisted of large aggregates in the solution, which can be easily broken down by the mechanical stirring.

The starch solution greatly influences the sensitivity of this method. High sensitivity can be attained with a starch solution of high amylose content. For this purpose, the commercially available soluble starch reagent is not always appropriate, because the degree of hydrolysis is often too high. We used soluble starch of high amylose content which was obtained by gel filtration’ 1 or by the following hydrolysis. Wheat starch was immersed in twice the amount of 2 M hydrochloric acid at room temperature with occasional stirring. After a week, the starch was washed with water, dried, and used for the analysis.

The author is ,greatly indebted to Prof. M. Nishimura for his valuable discus- sions.

Department of Chemistry, Faculty of Fisheries, Hokkaido University, Hhkodate (Japan)

Shizuo Tsunogai

I K. SUGAWARA, T. KOYAMA AND K. TIXADA, BUN. Chern. Sot. Japan. 28 (1955) 494. 2 K. TERADA. Thesis, Nagoya University, 1958. 3 K. SVGAWARA AND K. TERADA, J. Earth Sci., Nagoya Univ., 5 (1957) 81. 4 R. A. BARKLEY AND T. G. THOMPSON, Deep-Sea Res., 7 (1960) 24. 5 A. D. MATTHEWS AND J. P. RILEY, And. Chirn. Ada, 51 (1970) 295. 6 S. TSUNOGAI AND T. SASE, Deep-Sea Res. Oceunogr. Ahsrr., I6 (I 969) 489. 7 Y. MIYAKE AND S. TSUNOGAI, L.a hfer, Bull. Sot. Fr.+p. Oceanogr., 4 (1966) 65. 8 S. TSUNOGAI AND T. HWNMI, Clrikyu Kaguku, 3 (1969) 14. 9 B. B. OWEN, J. Amer. Chem. Sot., 60 (1938) 2229.

10 W. M. LATIM~K, Oxidation Potentials, 2nd Edn., Prentice-Hall, 1952, p. 392. 1 I N. HANDA, personal communication.

(Received 26th January 1971)

Ancr/. Chint. Acta, 55 (1971) 444-447