484
ANALYTICAL CHEMISTRY
active material-eg., the addition of sufficient gelatin to an alkuline tartrate solution (pH 14) of copper shifts the half-wave potential of the copper by 1.3 volts while the wave still retains its reversible character. The effect of surface-active materials in producing two-step waves was also considered. A theory was outlined which attempts to explain certain observed changes in half-wave potential and diffusion current on the basis of the formation of highly hydrated micelles whose concentration can be correlated with the addition of surface-active agent and the vapor pressure of water above the solution. The rate of micelle formation and difference in rate of diffusion of micelles of varying degree of hydration may account for some types of two-step waves. I t was suggested that rather than to try to suppress maxima by the addition of a substance such as gelatin, m-hich might result in unpredictable changes in polarographic behavior of the substance under study, it would be preferable to remove the maxima by increasing the dilution so that the concentration of the active species was not more than a few hundredths millimolar. It was brought out in subsequent discussion that readily interpietable waves are very difficult to obtain for organic (*ompourids
when present in concentrations below 0.1 to 0.5 millimolar. I t was also indicated that pH will alter the effect of varying the concentration of gelatin in a solution. Arguments were advanced for and against the explanation of the effect of gelatin as being due to adsorption of the gelatin on the surface of the drop. In a discussion of the coulometric measurement of n values, microcoulometric cells using dropping mercury were considered as a possible way of evading the assumption usually made that the behavior at a large stirred surface is comparable to that which occurs at the mercury drop. I t was pointed out that one could not always depend on stirring by the falling mercury in the microcoulometric cell to avoid local depletion of the solution. Among those taking extensive part in the discussions were J. K. Taylor, Sational Bureau of Standards; G. A. Crowe, Hercules Powder Company; Louis Lykken, Shell Development Company; W. W. Davis, Eli Lilly & Company; K. L. Metcalf, E. I. du Pont de Nemours & Company; and P. A. Geary, Smith, Kline & French Laboratories. RECWVED December 19, 1949.
Karl Fischer Reagent for Use in the Determination of Water 3Ioderator: JOHN MITCHELL, JR., E. I . d u Pont de Nemours 6% Company, Inc., Polychemicals DepartmentChemicals, Wilmington, Del.
Panel: L. R. KhNGAS, Hercules Powder Company, Wilmington, Del.
WILLIAV S E i l I i N , Calco Chemical Dicision, American Cyanamid Company, Bound Brook, N . J .
T
1IE Karl Fischer reagent is a solution of iodine, sulfur dioyide, and pyridine in methanol. Each of these components enters into the basic two-step reaction for water. Normally, sulfur dioxide, pyridine, and methanol are present in excess. Therefore, the strength of any preparation is dependent on the iodine concentration. For general laboratory use the reagent may be prepared to contain the components in the molar ratios, 112:3S02: 10CbHJ ill methanol, a t a concentration equivalent to 3 to 4 mg. of water per nil. of reagent. This composition gives a reagent suitable for the titration of samples of extremely low to high water concentrations, minimizes some types of interference, is a good general solvent, does not degrade to darkly colored end products, and assures an excess of components other than iodine. However, for specialized purposes other ratios may be better suited-for example, a reagent containing a higher concentration of pyridine (20) is better suited for the determination of water in acetone. Of course, the same effect can be obtained by employing pyridine as an inert diluent for the sample. A preparation containing less pyridine, such as 4 to 5 moles per mole of iodine, is satisfactory for analyses for water in alcohol. A reagent of about one third the water ryuivalence is useful for titrations of samples containing trace quantities of water (8). Contrary to some reports, the Fischer reagent may be employed for the titration of samples containing amounts varying from minute traces of moisture to pure water. Obviously, as the concentration of water becomes high more care must be exercised i n weighing. Normally, titrations are made by delivering the complete reagent from a desiccant-protected buret into the sample to be analyzed. Because the reagent is subject to parasitic side reactions, which effect a gradual reduction in strength, daily standardizations are necessary. These can be made by titrations of a metha-
nol or ethanol solution containiiig a known quantity of water, water-saturated alcohols, a weighed amount of water, or a stable hydrate. Incidentally, the reagents prepared to be equivalent to only 1 to 2 mg. of water per ml. are more stable than preparations of higher iodine concentrations (8). Thus, Wiberley reported that a reagent equivalent to 1 mg. of water per milliliter was stable. By employing desiccant tubes packed with phosphorus pentoxide on asbestos, he found that usually only weekly standardizations were necessary (21). Reagent left in the buret appears to degrade more rapidly than that in the reservoir, possibly because of the increased exposure to light (18). Fischer reagent is an extremely aative desiccant. Adequate protection against exposure to atmospheric moisture must be maintained for both the stored reagent and the reagent being delivered from a buret into the sample to be analyzed. In areas of high humidity titration in a closed system might be beneficial. A rubber sheet or polythene film can be used to cover the titration flask. Bdequate protection is maintained by inserting the buret tip through a hole in the sheet (6, 16). -4modification of the titrimetric technique was suggested by Johannson (S), who proposed a divided reagent. A solution of pyridine and sulfur dioxide in methanol was added to the sample flask, and a second solution of iodine in methanol was delivered from a buret. On contact with the contents of the sample flask active Fischer reagent was formed which reacted immediately with the water in the sample. In this way a stable reagent was obtained. Seaman and his co-workers (16) made a more thorough study of this method, and found that the stable iodine in methanol solution could be standardized against sodium thiosulfate. No evidence of significant degradative side reactions wm observed, provided no great excess of iodine wm added over that required t o react with all the water in the flask. Several investigators have mentioned some difficultyin finding
V O L U M E 22, NO. 3, M A R C H 1 9 5 0 a stopcock grease suitable for burets containing Fischer reagent.
Cello-grease, Celvacene, and Cenco lubricant have been used successfully. In practice, usually sufficient pyridine hydroiodide, one of the Fischer reagent end products, diffuses around the stopcock plug to act as an excellent lubricant. A well-ground stopcock is essential. Automatic burets with a vertical stopcock plug prevent slippage. The titration may be made visually or electrometrically. The former requires only simple apparatus and is less timevoiisuming; the latter is more accurate for any given quantity of water. Most of the difficulties with the visual end point are associated with the choice of a suitable color. During direct titration the color changes from canary yellow to chromate yellow and, finally, a t the end point to the marked brown color of unused iodine. Often darkly colored solutions can be titrated by using a second sample as a color standard.(7, 2 1 ) . At the proper end point, the addition of 0.1 to 0.2 ml. of Fischer reagent results in a considerable deepening of the color. Most chemists unfamiliar with the end point tend to undertitrate. Unskilled operators can be trained easily for routine visual titrations. Most of the electrometric titrations are made using the deadstop end point in which excess Fischer reagent is added and the solution is back-titrated with standard water in methanol. Karl Fischer reagent is composed of toxic materials, but when accepted laboratory practice is followed no harmful effects should result. The reagent may be used in any well ventilated laboratory (a hood is not necessary). Spent reagent should be disposed of in a hooded area; otherwise, significant concentrations of sulfur dioxide might be passed into the atmosphere. The srnsitivity of the reagent is equivalent to about 0.3 mg. of water. However, the precision and accuracy of the titration are dependent on the quantity of water in the samples and the amounts of sample employed. Those containing 20 to 40 p.p.m. of .moisture require large quantities-eg., a t least 100 grams. The technique of Gester (1) might prove applicable in many cases. By employing ethylene glycol as an extractant, this investigator determined a few parts per million of water in hydrocarbonsfor example, 250 ml. of glycol were used to extract moisture from a gallon of hexane. APPLICATIONS
Lafferty ( 6 )found that water in uranyl acetate could be determined satisfactorily. However, some uranyl phosphates, which nere only slightly soluble in alcohol, did not appear to react. Direct extraction might be feasible for insoluble compoundsfor example, addition of excess Fischer reagent, agitation, and back-titration with standard water-in-methanol often has proved satisfactory. In other cases inert solvents often can be found, such as acetic acid, boron fluoride complexes, and sulfuric acid (later treated with pyridine and dioxane, fd). Some classes of inorganic compounds, such as metal oxides, hydroxides, carbonates (except calcium), sulfites, and bisulfites, interfere with the titration for water. Usually, the interfering reaction is stoichiometric, and, therefore, the moles of apparent mater found are equal to the equivalents of interfering compound. Suter (18) suggested that samples containing small amounts of alkali and relatively large amounts of water are conveniently titrated directly and the quantitative correction for interference applied. However, for the reverse situation Suter recommended preliminary distillation with xylene and titration of the distillate ( 1 7 ) . In many cases a homogeneous azeotrope might be better suited for removal of water, provided a sufficiently high temperature could be attained to remove all the water from the sample. Csually free water plus water of hydration is determined by rischer reagent titration. Where only free water is desired a preliminary separation must be made. Often an extraction technique is feasible. In some r:iSes azeotropic distillation or
485 oven drying separates only free water. Thus both forms of water in monosodium glutamate monohydrate might be determined by Fischer reagent titration of a sample before and after vacuum oven drying. Investigations should be continued on the application of the Fischer reagent to the determination of moisture in foodstuffs. Many important uses already have been reported. However, because of the lack of an absolute standard method, the value of the reagent in the titration of some foods has not been definitely ascertained. Morel1 (10, 14) applied the Fischer method successfully to the determination of moisture in malt, malt sirups, and miscellaneous food materials. He employed a Waring Bleiidor to facilitate extraction of moisture from the insoluble food into the methanol in which the sample was dispersed. Other investigators (4, 19) extracted moisture quickly by using methanol or excess Fischer reagent and gently refluxing for short periods of time. A4scorbicacid is the only compound present in foods which has been shown to interfere in the Fischer titration. This material is oxidized to dehydroascorbic acid by the iodine of the reagent. However, Johnson ( 4 ) has reported that the quantity of ascorbic acid present in foods would indicate no more than 0.03% apparent moisture. The Fischer reagent titration should find many uses for the determination of watet in whole milk and milk products. One of the publications covering this field (6) indicated relatively poor precision for this titrimetric method, particularly for whole milk. Other data (11) demonstrated that with sufficient precautions the usual high precision of +0.2% can be expected. Before investigating the potential application of the Fischer reagent to titrations of commercial materials, the chemist should learn the natures of the compounds involved. This will permit him to predict likely interferences. In the organic field active aldehydes and ketones (exceptions include formaldehyde, chloral, and sugars) interfere because of slow condensation with the methanol of Fischer reagent. Diacyl peroxides are reduced by the hydrogen iodide always present in Fischer reagent. However, dialkyl and hydroperoxides do not interfere. Mercaptans (thiols) are oxidized by iodine. Methods for eliminating most of these class interferences have been described in the literature (9). Several procedures for the determination of compounds other than water have been proposed which employ the Fischer reagent. Those inorganic materials which react quantitatively can be determined by direct titration if the sample is anhydrous or if an independent method is available for ascertaining the original water content. Many organic functional groups can be determined by use of quantitative reactions in xhich water is eliminated or absorbed. Because these procedures involve titrations for water, direct analyses often can be made of samples containing materials which interfere when analyzed by acidimetric methods. Thus, the esterification procedure for alcoholic hydroxyl is unaffected by the presence of organic or inorganic acids, easily hydrolyzed esters, or high concentrations of water. Furthermore, tertiary hydroxyl is determined quantitatively. Other analytical methods am available for carboxylic acids, anhydrides, carbonyl compounds, amines, nitriles, dialkyl peroxides, mercaptans, etc. (13). Because of its nearly specific nature, Karl Fischer reagent should be considered for all analytical problems involving the determination of water. Often simple, rapid routine methods ran be developed which may be used readily by nonskilled laboratory personnel. LITERATURE CITED (1) (2) (3) (4)
Gester, G. C., Jr., Chem. Eng. Progress, Trans., 1, 117-22 (1947). Heinemann, B . , J.D a i r y Sci., 28,845-51 (1945). Johannson, A . , Scensk Papperstidn., 50, No. 11B, 124-6 (1947). Johnson, C. >I IND. .,EKG.CHEM., .4NAL. ED.,17, 312-6 (1945).
ANALYTICAL CHEMISTRY
486 ( 5 ) Lafferty, Carbide & Carbons Chemicals Corp., Oak Ridge,
Tenn., report during discussion. (6) Levy, G. B.,Murtough, J. J., and Rosenblatt, M., IND. ENG. CHEM.,ANAL.ED., 17, 193-5 (1945). (7) Mitchell, J., Jr., and Smith, D. M.,“Aquametry,” p. 75, New York, Interscience Publishers, 1948. (8) Ibid., p. 98. (9) Ibid., p. 103 ff. (10)Ibid., pp. 162, 184, 198, 210, 213. (11) Ibid., p. 186. (12) Ibid., pp, 263 ff. (13) Ibid., pp. 267 ff. (14) Morell, S.A.,Pabst Brewing Co., Milwaukee, \Tis., report during discussion.
(15) Seaman, W., MoComas, W. H., Jr., and Allen, G. A . , ANAL. CHEM.,21, 51&2 (1949). (16) Suman, F. T.,Jr., General Electric Co., Pittsfield, Maes., report. during discussion. (17) Suter, H. R.,ANAL.CHEM.,19,326-9 (1947). (18) Suter, H. R., Wyandotte Chemical Corp., report during discus sion. (19) Warren, A. T.,E. R. Squibb & Sons, Brooklyn, N. Y., report during discussion. (20) Wernimont, Grant, and Hopkinson, F. J., IND. ENG. CEEM., ANAL.ED.,12, 392-6 (1940). (21) Wiberley, J. L., Socony-Vacuum Oil Po., Brooklyn, N. Y . , report during discussion. RECEIVED January 24, 1950
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Determination of Carbon in Ferrous Alloys Moderator: ROBERT M. FOWLER, Carbide & Carbon Research Laboratories, .Viaguru Falls, N . Y .
Panel: W. G. GULDNER, Bell Telephone Laboratories, Murray Hill, N . J. T. C . BRY SON, Westinghouse Electric Corporation, Pittsburgh, Pa.
JOHN L. HAGUE, National Bureau of Standards, Vashington, D. C. H. J. SCHI\.IITT, Laboratory Equipment Corporation, St. Joseph, Mich.
I
N THE round-table discussion of the determination of carbon in ferrous alloys each speaker discussed the apparatus used in his laboratory for determining carbon in low-carbon ferrous materials, which were defined for the purpose of the discussion as metals with carbon contents
