Organic Microanalysis 11. Drying and Analysis of Hygroscopic Substances R. T.MILNER AND MILDRED S. SHERMAN Bureau of Chemistry and Soils, U. S. Department of Agriculture, Washington, D. C.
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YGROSCOPICITY is especially important to the microanalyst where the small size of sample and relatively large area exposed may lead to difficulties that vanish when working on a larger scale. The methods of Pregl (5) are in general probably satisfactory for drying substances which only adsorb water on their surface. I n localities where the temperature and relative humidity are low throughout the year such materials should give little trouble. The case is quite different with compounds which form hydrates or solvates and those, such as proteins and nucleic acids, where a large molecule of great surface area, usually noncrystalline, is present without very definite composition. As a typical example, 3 mg. of a substance with a molecular weight of 480 can form its monohydrate with the amount of water contained in 5 ml. of air (partial pressure of water 22.0 mm.). Even though the drying has been successful in Pregl’s microdesiccator or other apparatus, the time of exposure in removing it to the “pig” for weighing and introducing it, into the combustion tube may be sufficient for it to hydrate completely. The apparatus and procedure described below for handling compounds tending to absorb water rapidly upon the unavoidable exposure to atmospheric moisture during analytical manipulations are an improvement over the old type of Abderhalden drier, in that the pig containing the dried sample can be closed inside the apparatus. The importance of this point is discussed more fully by Booth and McIntyre (I) as it affects desiccators. Advantages of this apparatus are well illustrated by the authors’ experience in analyzing chrysanthemin chloride and its derivatives (6). This substance was not hygroscopic as received for analysis and lost little or no weight upon drying in the old type of Abderhalden a t temperatures up to 140’ C.
The analyses were always low in carbon and high in hydrogen even on drying a t 140’ C. and a pressure of less than 10-6mm. of mercur;y, usually by amounts which showed the presence of from one t o one and a half molecules of water per molecule of chloride. After the present apparatus was developed and the pig capped while in the drier, excellent analyses were secured on drying a t 100’. Apparently chrysanthemin chloride is a compound forming 8 hydrate with a low partial pressure of water a t room temperature. Experiment showed that the anhydrous form completely hydrates on exposure to the room air in as little as 10 seconds. The apparatus also proved of great valule in studying the unusual water and alcohol solvates of 1-d-glucosidocytosine which were discussed by Hilbert and Jansea (4). The details of the apparatus are shown in Figures 1 and 2. A vapor bath maintains a constant temperature as in the ordinary Abderhalden. The two-way stopcock can be turned to permit evacuation or the drying of incoming air, the latter entering through a very fine capillary cemented in the plug of the cock and then through the U-tube containing the desiccant. Colored Anhydrite (anhydrous calcium sulfate) has proved very satisfact0r.yas a drying agent. The other end of the tube carriea two ground joints, a large one for introducing the plate holdin the pig and a small one to hold the brass rod used t o open and close the pig while the sample is in the drier. A closed-end mercury manometer is used to seal the small ground joint and to indicate leeks if present when the apparatus is evacuated overnight.
-.b ‘r;
]FIGURE 2. DETAIL OF
FIGURE1. DRIERWITH PIGAND BOATIN PLACE 427
PIG
HELDON BASE
The Pyrex pigs used are shorter than in the Pregl type and do not have the long handles recommended by Pregl (6). Stability is much greater without these handles, and with tweezers curved t o fit the pig and chamois over the fingers the glass is not unduly heated by the hand and the pig is easily handled. No difficulty has been encountered in securing constant weights, although the glass is heavier and the pig more substantial than in the Pregl type. A phosphor-bronze spring clip holds the pig firmly in place on an aluminum base with two depressionsfor the front legs. The pig is easily transported while on this plate. The base, whose edges are curved to fit the drier, is held fixed within the drier by another spring clip as shown in Figure 1. The solid glass stop er of the pig has a small handle with projecting kno%which is engaged by a brass sleeve comprising a bayonet joint
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and enables the stopper to be withdrawn or firmly seated while the pig is in the drier. The effectiveness of this apparatus was tested on cupric sulfate pentahydrate. A few clear crystals were selected and crushed; 9.783 mg. weighed in a previously dried boat and pig were heated in the apparatus 0.5 hour a t 100” C. and pumped during only half this time. The loss in weight was 3.102 mg., corresponding to 4.4 moles of water per mole of sulfate. This may have been all the water originally present in the sulfate; for the present purpose it does not matter. The efficiency of the apparatus and procedure is shown by the fact that two more treatments for the same time gave no further gain or loss, in spite of the fact that the vapor pressure of this substance is approximately 2.7 mm. (9) and the average room temperature was 26.67’ C. (80” F.), with relative humidity of 65 to 80 per cent. In the analysis of organic compounds the following procedure is used. It eliminates much of the work in the method proposed by Hayman (3). The method has been generally applied in this laboratory for the last several years in organic
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microanalytical determinations on many hundreds of compounds. Before the sample is weighed out, the pig and empty boat aSe dried and capped in the apparatus. The sample is weighed and dried and the result recorded. Then the pig is opened and the sample allowed to equilibrate with the moisture in the room. Usually after 30 t o 60 minutes equilibrium is so nearly approached that any further gain in weight while transferring the sam le to the combustion tube is negligible. The pig is then closet! and reweighed. The gain in weight upon exposure to air is subtracted from the weight of water found before the percentage of hydrogen is calculated.
Literature Cited (1) Booth and McIntyre, IND. ENQ.CHEN,,Anal. Ed., 8, 148 (1936). (2) Bower, Bur. Standapds J . Reseawh, 12, 241 (1934). (3) Hayman, IND.ENG.CHEM., Anal. Ed., 4, 256 (1932). (4) Hilbert and Jansen, J . Am. Chem. Soc., 58, 60 (1936). ( 5 ) Pregl, “Quantitative Organic Microanalysis,” 2nd ed., London, J. and A. Churohill, 1930. (6) Sando, Milner, and Sherman, J. Bid. Chem., 109, 203 (1935). RE~CEIVED July 18, 1936. Preaented before the Microchemical Section at the 92nd Meeting of the American Chemical Society, Pittsburgh, Pa., September 7 to 11, 1936.
Microscopical Qualitative Analysis of Antimony and Bismuth Tetraethyl Ammonium Iodide as a Reagent FRANCIS T. JONES’ AND CLYDE W. MASON, Cornel1 University, Ithaoa, N. Y.
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F T H E existing microscopical tests for antimony and bismuth, none is sufficiently free from interferences to be satisfactory. Since these two elements often occur together, especially in alloy samples, it is desirable that characteristic reactions should be developed rather than that separation methods should be elaborated, for in rapid analysis a series of preliminary treatments or the precise control of conditions is usually impractical. A brief review (I) of the best microscopical reactions for antimony and bismuth is in order, since in an analysis it is often desirable to employ several tests as checks, or conditions may necessitate the use of one which is not of the highest sensitivity. Cesium chloride gives double salts, 3CsCl.2SbC13, colorless hexagonal plates or rosettes, and 3CsCl.BiCl8, colorless rhombshaped plates, with pure salts. However, 3CsC1.2BiCl8, isomorphous with the antimony double salt, will form if cesium chloride is not in excess, or if antimony is resent. Ag, Pb, Hg.+, Cd, Sn, Tlf, and Cu may also yield crysta?line precipitates of distinctive appearance, but in complicated mixtures or for the detection of relatively small amounts of the elements sought cesium chloride is not specific or sensitive enough. Cesium sulfate yields, with bismuth as sulfate, hexagonal plates; no reaction is given with antimony. Sodium sulfate gives rods or short prismatic crystals. Insoluble sulfates and alum-forming elements interfere. Potassium binoxalate gives tiny tetragonal “octahedra” with bismuth, and trichites with antimony, but so many other metals yield insoluble oxalates that this reaction is useful only for relatively pure salts. Antimonyl tartrates of potassium or barium, possessing characteristic crystal forms, are subject to this same objection. Stibine, generated on a micro scale, may be used as a means of identifying antimony, since bismuth forms no analogous hydride. 1
Present address: Department of Chemistry, Pacific University, ForOre.
est Grove,
Metallic bismuth, obtained by the reduction of bismuth salts by sodium stannite, is usefuI confirmatory evidence, since antimony is not reduced under similar treatment. Bismuth cobalticyanide gives a crystalline precipitate, but is not suitable for mixtures containing much antimony. Numerous organic reagents (alkaloids, etc.) have been suggested, most of which yield “amorphous” precipitates, and are subject to interferences.
Reactions with Tetraethylammonium Iodide When a solid fragment of tetraethylammonium chloride is added to a fairly concentrated hydrochloric acid solution of antimony trichloride, colorless hexagonal plates or short prisms are formed. If an excess of potassium bromide is present, colorless hexagonal plates and tablets or short hexagonal prisms will be formed. Both compounds are fairly soluble, and no precipitate will be obtained from dilute solutions of antimony. Both compounds give a positive, uniaxial interference figure. Pentavalent antimony yields a colored and much less soluble precipitate of purple hexagonal plates (Figure 1, left) if iodides are present in a test drop containing even a very dilute solution of antimony. When the concentration of antimony is high, the crystals are likely to be imperfect (fragments, ribbed plates, skeletal stars), and so thick as to appear black. Attempts to prepare this compound in sufficient quantity for analysis failed because of its instability. It may be analogous to the bromo compound reported by Petzold (5) which forms “red pyramids” of the formula [ ( C B W I [SbBrsI. Trivalent antimony gives yellow anisotropic crystals which are usually too small to be reliable as an indentifying form. These crystals generally appear in clusters of three or four, but some individuals may become large enough to be identi-
