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INDUSTRIAT, AND EXGINEERIYG CHEMISTRY
immersed in a cooling liquid a t any desired temperature for crytallization. Folloiving crystallization the assembly is inverted and quickly centrifuged. When smaller amounts of material are to be handled, the simpler apparatus shown on the right of Figure 2 has been found more desirable. The crystallizing vessel, &, is a small-sized test tube 6 mm. or less in diameter, with a perfectly symmetrical opening. R, the filter rod, is an ordinary glass rod heated in the flame until its upper end forms a symmgtrical round ball of such size that the test tube will not be able to slip over it. These two parts are placed inside a larger test tube, 8, which is in turn placed inside a still larger test tube closed with a rubber stopper and of such
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size that it fits the centrifuge cup. The mouth of Q fits the ball of the filter rod well enough without’ grinding, so that the passage of any but the finest crystals will be prevented during centrifuging. The crystalline material will thus be caught at the mouth of Q. After centrifuging, Q and R can be lifted out together with a narrow pair of curved forceps and the mother liquor left in S. This apparatus has the advantage of affording small surfaces without the danger of fritted glass or shreds of filter paper.
Literature Cited (1) Emich, F., Hausler, H., Rasin, and Schally, Monats., 5 3 / 5 4 , 312 (1929). (2) Ing, H. R., and Bergmann, &I., J . B i d . Chem., 129, 603 11938). (3) Skau, E. L., J . P h y s . Chem.. 33, 951 (1929).
A New Oxidation-Reduction Reaction Catalyzed bv Iodine J
J
Application to the Detection of Iodide D.4VID HART AND ROBERT LMEYROWITZ, Brooklyn College, Brooklyn, N. Y .
A new oxidation-reduction reaction catalyzed by iodine is described, by means of which as little as 0.2 microgram of iodide can be detected. The effect of 74 different anions and cations has been determined. A procedure for the detection of 0.03 mg. of iodide per ml. of solution in the presence of 50 mg. of interfering anion or cation has been developed. The nitrite-arsenite reaction is catalyzed by other substances, but iodine is the most effective catalyst.
D
URIPJG the development of new methods for the systematic detection of the halides without the use of silver ion as a group reagent, and using lead and mercuric ions t o
remove interferences (4, a new oxidation-reduction reacbion catalyzed by iodine was discovered. When nitrite was added to a dilute nitric acid solution containing lead arsenite in the presence of iodide ion, a heavy precipitate of lead arsenate formed; in the absence of iodide, no precipitate was obtained.
Bailey ( 1 ) reported that “dilute nitrous acid and arsenious acid react, either not, at all or with extreme slowness in the presence of highly dilute sulphuric acid a t ordinary temperatures”. Klemenc (6) developed a method for the determination of nitrous and arsenious acids in the presence of one andher. Arsenious acid is titrated with iodine in the presence of an excess of sodium bicarbonate and then both the arsenite and nitrite are titrated in strong sulfuric acid with potassium permanganate. Shilov and Perzner ( 9 ) , in their attempt to oxidize salt,? of arsenious acid by the oxygen of the air, reported that neit,her nitrogen tetroxide nor iodine is effective as a catalyst. I-Iovever, they found that a combination of hydrogen iodide and nitrogen oxides as catalysts effected complete oxidation of arsenious acid by air, Baines ( 2 ) described a method for “the determination of small quantities of iodide in mixtures of halides depending on the pseudo-catalysis by iodide of the oxidation of thiosulfate by nitrous acid”.
This new nitrite-arsenite reaction appears to be somewhat similar t o the ceric-arsenite reaction ( 3 , 6, 7 ) used as a basis for a method of detecting and determining small amounts of iodine. The ionic partials for the half cells in this reaction may be formulated as follows: 2H90 e€€NO2+ H30+ + e ++4HzO == HSAsO4 + 2H30c + 2e
SO HAs02
Although the molal oxidation potentials of nitrite and arsenite for the above half cells are -0.99 and -0.559, respectively (8),no evidence of any reaction between t,liese two in acid medium was found after a search of the literature. According to Swift (11) arsenite can be oxidized to arsenate by boiling with sodium nitrite in a solution alkaline with ammonia. However, repeated attempts by the authors to effect this oxidation under the conditions described by Swift (IO) were unsuccessful. Attempts to effect oxidation in acid medium were also unsuccessful, except in the presence of cat’alyst’sas described below. The most effective catalyst, is iodine. By this new nitrite-arsenite reaction, as little RS 0.2 microgram of iodide (as potassium iodide) can be detected, using the following procedure in which the limit of concentration is 1 part per 500,000. Treat 2 drops of 1 Jf sodium arsenite in a 5-ml. beaker n-ith 7 drops of 1 -1.I nitric acid and add 1 drop of 1 JI lead nit,ratP. Llix thoroughly and add the iodide (0.1 ml.). S o w add 1 drop of 6 +lf sodium nitrite and let stand for 5 minutes, stirring occasionally. Add another drop of 6 AMsodium nitrite and allox to stand for 10 minutes with occasional ;tiwing. ii xliite prccipitate shows the presence of iodide. When less than 0.5 mirrogram of iodide is present, it is necessary to run a control without iodide, for on long standing a slight precipitate forms. Howvcr, this precipitate i 4 exceedingly small xhen compared to that, ivhich is obtained x-hen 0.2 microgram of iodide is present. -4pplying this reaction on a macro scale, it was found necessary t o modify the procedure as follow: In a test tube treat 1 ml. of solution containing the iodide n-ith 1 ml. of 1 .M lead nitrate. Add 1 ml. of 6 J!I nitric arid, 1 ml. of 1 JI sodium arsenite, and shake thoroughly. Then add 0.5 ml. of 6 J!I sodium nitrite, let stand for 1 minute, add another 0.5 ml. of nitrite, and let stand for 3 minutes. The formation of a heavy white crystalline precipitate shows the presence of
DECEMBER 15, 1940
AR’ALYTICAL EDITIOK
iodide. To ascertain the chemical composition of this precipitate, the lead was determined as lead sulfate. The percentage of lead was found to be the same as in PbHAsOl. This procedure is capable of detecting 0.004 mg. or 4 micrograms of iodide. Free iodine in aqueous solution also catalyzes this reaction. Although the above procedures involving the formation of
a white precipitate of lead arsenate are accurate and effective, i t was deemed advisable to devise a procedure in which a characteristic colored precipitate for arsenate was obtained. Consequently, the following method involving the formation of silver arsenate was developed. Treat 1 ml. of solution containing the iodide in a test tube with 1 ml. of water and 1 ml. of 6 M nitric acid. Add 1 ml. of 1 M sodium arsenite and mix thoroughly. Now add 1 ml. of 6 M sodium nitrite in 0.5-ml. portions a t 1-minute intervals and after 3 minutes render the solution alkaline to litmus by the dropwise addition of 3 M sodium carbonate. Add 1 ml. of 0.5 M silver nitrate; the formation of a red-brown precipitate proves the presence of iodide. The lower limit of identification of this procedure is 0.01 mg. or 10 micrograms of iodide. This procedure may be modified by substituting 2 ml. of 1.5 M sulfuric acid for 1 ml. of water and 1 ml. of 6 iM nitric acid.
Effect of Other Anions and Cations The effect of other anions and cations upon the arsenitenitrite reaction was determined by means of the following procedure : Treat 1 ml. of solution containing 0.03 mg. of iodide in a 15ml. centrifuge tube with 1 ml. of solution containing 50 mg. of anion or cation. Acidify the solution with 3 M nitric acid, add 1 nil. of Y lead nitrate, and remove by centrifuging any precipitate that forms. Transfer the supernatant liquid to another centrifuge tube, add 1 ml. of 6 M nitric acid and 1 ml. of 1 M sodium arsenite, and thoroughly mix the solution. If any precipitate forms, remove it by centrifuging. To the supernatant liquid add 0.5 ml. of 6 M sodium nitrite. Let stand for 1minute, add anot,her 0.5 ml. of nitrite, and let stand for 3 minutes. The formation of a heavy vhite crystalline precipitate indicates the presence of iodide. The following anions and cations cause no interference : bromide, chloride, ferricyanide, ferrocyanide, sulfite, oxalate, tartrate, phosphate, borate, sulfate, arsenate, chromate, perchlorate, cobalt, cadmium, manganese, nickel, magnesium, barium, strontium, zinc, calcium, tungsten, zirconium, uranium, bismuth, ,potassium, copper, ammonium, aluminum, lanthanum, erbium, antimony, rubidium, cesium, platinum, osmium, thorium, rhodium, tellurium, vanadium, titanium, cerium (ous and ic), selenium (ous and ic), molybdenum, lithium, yttrium, and praseodymium. I n the presence of any of the following ions; thiocyanate, tin (ous and ic), chlorate, permanganate, iodate, bromate, ferrous, periodate, thiosulfate, gold, vanadium, and hydrogen peroxide, a precipitate is obtained even though iodide is absent. A gelatinous precipitate is obtained in the presence of stannous or stannic tin. K i t h gold a chocolate-brown precipitate forms and with vanadium a yellow one. I n the presence of iodide, stannous and stannic tin and gold yield the same type of precipitate as was mentioned above. With vanadium, however, the precipitate is white and crystalline as usual. When thiocyanate or thiosulfate are present together with iodide, the precipitate is not so heavy as is ordinarily obtained. The least amount of hydrogen peroxide t h a t will catalyze this reaction is about 21 mg. or 0.7 ml. of 3 per cent hydrogen peroxide. The lower limit of iodate and periodate is 200 and 300 micrograms, respectively. With cyanide, sulfide, chromic, silver, fluoride, persulfate, mercury (ous and ic), ferric, beryllium, iridium, and ruthenium ions, no precipitate is obtained in either the presence or absence of iodide. With stannous and stannic, ferrous and ferric, mercurous and mercuric, and chromic ions, the procedure was applied t o
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a sodium carbonate extract prepared by boiling the sample with 1.5 M sodium carbonate. Where iron or chromium was present, 50 mg. of tartrate were added. Positive results were obtained when iodide was present and negative results when iodide was absent with stannic, ferric and ferrous (gelatinous precipitate), and chromium. With stannous ion a gelatinous precipitate formed in the absence of iodide, but with iodide the precipitate was white and crystalline. Mercury continued to inhibit the reaction under these conditions. The interferences caused by fluoride, beryllium, thiocyanate, cyanide, sulfide, thiosulfate, chlorate, and bromate ions are removed by methods described below.
Procedure On the basis of these experiments the authors have developed a procedure which is capable of detecting 0.03 mg. of iodide in 1 ml. of solution in the presence of 50 mg. of interfering anion or cation. This procedure does not apply in the presence of mercury, silver, iodate, periodate, hydrogen peroxide, permanganate, persulfate, gold, iridium, and ruthenium. Treat 1 ml. of prepared sodium carbonate extract in a 15ml. centrifuge tube with 3 M nitric acid until no more carbon dioxide bubbles are given off. Remove any precipitate that forms, add 1 ml. of 1 M lead nitrate, and centrifuge. T o the supernatant liquid add 1 ml. of 6 M nitric acid and 1 ml. of 1 M sodium arsenite. Remove any precipitate that forms, add 0.5 ml. of 6 M sodium nitrite, and let stand for 1 minute. Add another 0.5 ml. of nitrite and let stand 3 minutes. A heavy white crystalline precipitate proves the presence of iodide. NOTES. If fluoride IS present add 3 ml. instead of 1 ml. of 1 M lead nitrate. If beryllium is present add 1 ml. of 6 Jf sodium nitrite 1 minute after the addition of the second 0.5 ml. of nitrite and let stand 3 minutes. If ferrous ion is present the precipitate although heavy will be gelatinous. In the presence of thiocyanate 1 ml. of 0.3 M ferric nitrate must be added before the addition of 1ml. of 6 iM nitric acid. If cyanide and sulfide are present treat 1 ml. of prepared solution in a 50-ml. beaker with 6 +Macetic acid until no more carbon dioxide bubbles are given off. Add 0.5 ml. in excess and dilute to 6 ml. Evaporate over a small flame to 2 ml., cool, and apply procedure. Use 6 M acetic acid instead of 3 M nitric acid if thiosulfate is present. If chlorate and bromate are present, add 1 ml. of 6 M sodium nitrite before adding the arsenite and then add the nitrite in 0.5-ml. portions as usual.
Literature Cited Bailey, T. L., Ann. Rept. Alkali Works, 62, 14-16 (1926). Baines, H., J . SOC.Chem. Ind., 49, 481T (1930). Feigl, F., “Qualitative Analysis by Spot Tests”, p. 168, New York, Nordemann Publishing Co., 1939. Hart, David, and Meyrowitz, Robert, IXD.ENG.CHEM.,Anal. Ed., 12, 318 (1940).
Klemenc, A., 2 . anal. Chem., 61, 448-64 (1922). Kolthoff, I. A I . , and Sandell, E. B., J . Am. Chem. Soc., 56. 1426 (1934). Kolthoff, I. M . , and Sandell, E. B., Mikrochem. Acta, 7, 9-25 (1937). Latimer, W. & , “Oxidation I Potentials”, pp. 296-7, New York, Prentice-Hall, 1938. Shilov, and Perzner, J . Chem. I n d . (Moscow), 7, 759-60 (1930). Swift, E. H., “System of Chemical Analysis”, pp. 473, 476, Ken. York, Prentice-Hall, 1939. I bid., p. 475.
Correction In the article entitled “Removal of Static Charges from Glassware by Ultraviolet Light” [IND. ENG.CHEM.,Anal. Ed., 12, 693 (1940)],the temperature in Table I was erroneously given as C. whereas it should have been ’ F. CLEMENT J. RODDEN
