cate. Only 2 to 3 mm. of bead need be introduced into a gas sample, and contamination of or by the cement is avoided. Bnalvses by means of the combustion coil are stated to be unsatisfactory for hydrocarbons higher than methane. This is indeed the case in determining hydrogen content, which necessitates a knowledge of the oxygen used. Carbon content, however, may be determined very accurately for higher hydrocarbons (and presumably other volatile compounds) by determining the carbon dioxide formed by combustion in the usual manner; water is first removed with a phosphorus pentoxide bead and then carbon dioxide is absorbed with a potassium hydroxide bead.
A Device to Prevent Bumping in Micro-Kjeldahl Digestions. John C. Henniker, Stanford Research Institute, Stanfold, Calif. the determination of nitrogen in asphalts, bumping D was- often so severe as to break the sturdy Pyrex flasks ~RIUG
ordinarily used. Seither glass beads nor Carborunduni chips were effective; they often came to rest after a period of quiet boiling and allowed the acid to be superheated. The difficulty was overcome by attaching a vibrator to the metal stand on n-hich the digestions are done. The first vibrator used, a commercial alternating current hummer made for agitating small photographic developing tanks, was effective but was unnecessarily powerful and noisy. A very satisfactory vibrator 11-as then improvised from the solenoid of a magnetic switch. The main U-shaped core was bolted to the metal digestion frame. The T-shaped plunger was free to move, but n-as prevented from touching the core by sheet rubber spacers. The thickness of the spacers was so adjusted that enough vibration was transmitted to the flasks to stop bumping, but not enough to jar the flasks from their sockets. The vibrator is used in conjunction with glass balls in the flasks. I t is no more noisy than many stirring motors. Blacet-Leighton Method of Gas Microanalysis. University of Rochester, Rochester, S. Y. URISG
WORKsupported by Contract N6onr-211, Task I. with the Ofice of Naval Research, United States S a v y .
Preparation of Barium Carbonate for Assay of Radioactive Carbon 14. R. B. Regier, Phillips Petroleum Company, Bartlesville. Okla. assays for C14,the carbon is frequently converted to barI ium .making carbonate, because this form is conveniently handled and N
Robert Gomer,
photochemical studies the Blacet-Leighton apparatus
D [Smith, R. N., and Leighton, P. A., IXD. ENG. CHEY., ANAL.ED., 14,758 (1942)l was used extensively, and several very minor but useful modifications were gradually evolved. The accuracy of leveling the microburet and mercury reservoir can be greatly increased if a reference mark on the inner telescoping brass support tube is lined up with a similar mark situated on a slot cut in the outer tube, or directly with the top of the latter. This requires only that the mercury level in the large reservoir be kept constant, a very simple matter. This procedure avoids the necessity of lining up visually two mercury levels several centimeters apart, and thus eliminates parallax effects. The useful life of potassium hydroyide beads may be prolonged almost indefinitely by periodically wiping them with a piece of filter paper moistened with distilled water. This removes the coatings of potassium carbonate, potassium sulfate, etc., which normally terminate the absorbing power of these beads, and also ensures the moist surface necessary for speedy absorption. Phosphorus beads may be prepared very simply and safely.
A small glass thimble (such as is used for containing BlacetLeighton samples) is filled with water. A small piece of yellow phosphorus, not necessarily clean, is introduced, the thimble is held under the warm water faucet, and a moderately intense stream of water is played on it. The phosphorus soon melts, and impurities are washed away. A straight platinum wire, attached in the usual manner to a reagent holder, is introduced into the molten phosphorus, and the system is held in a stream of cold water until the brad has solidified. I t is then quickly removed from the water and submerged in the mercury reservoir. Excess moisture is easily removed by raising the bead a few times into a half-empty gas thimble. This method avoids the danger of violent spontaneous combustion, inherent in the “dry” method, and minimizes the obstructive phosphoric acid coating on the bead. Difficulty with cementing silver oxide beads onto platinum wires was avoided by making the beads cylindrical in shape, about 3 mm. in diameter, and 5 mm. in height. Ordinary Duco cement was then used, instead of the troublesome sodium sili-
may be deposited in a uniform and reproducible manner if certain precautions are observed. The procedure described by Dauben, Reid, and Tankwich [ANAL.CHEW, 19, 828 (1947)] is not completely satisfactory. Careful evaporation of the alcohol from the barium carbonate in alcohol slurry sometimes produces very smooth and adherent layers, whereas a t other times the barium carbonate is grossly marked with cracks, with the result that the deposit is flaky and easily lost from the aluminum disk. In an effort to establish conditions that would consistently yield good barium carbonate layers, an investigation has been made of the relation between crystal size and the nature of the resulting layer when barium carbonate is deposited from an alcohol slurry by evaporation of the alcohol. I t was found that the individual crystal size is the principal factor in determining the nature of the deposited layer. A series of barium carbonate precipitates was prepared under varying conditions of temperature and alkalinity, using partially carbonated aqueous sodium hydroxide, ammonium chloride, and barium chloride as reagents, and examined under an optical microscope. The individual barium carbonate needles varied in length from 1 or 2 microns to approximately 150 microns. Sufficient quantities of the preparation comprising the smallest crystals, and of the preparation comprising thelargest crystals, were mounted on aluminum disks to give a thick layer-i.e., about 25 mg. per sq. em. The small-crystal barium carbonate exhibited most of the properties that are considered undesirable. It wm difficult to manipulate into a layer of uniform thickness, and the resulting deposit mas badly cracked and adhered loosely to the disk. In contrast, the large-crystal barium carbonate deposited as a layer with a uniformly smooth surface and was not loosened even when the disk was inverted and tapped hghtly. It was concluded that barium carbonate behaves best when precipitated under conditions that cause slow crystal growth. These include using dilute reagents, elevated temperature, and lowered alkalinity (obtained by using ammonium chloride). The procedure outlined by Dauben et al. has been modified to give consistently satisfactory results in preparing barium carbonate samples. An approximately 2070 excess of ammonium chloride is used, instead of a quantity equivalent to the sodium hydroxide in the absorber. The resulting solution is then diluted to about 100 ml. and heated to near boiling before precipitating with barium chloride. A4ftercooling, the precipitate is filtered off and treated in the usual way.
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