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ANALYTICAL EDITION
have come from the soil. This effect is probably small, however, because samples of leaves taken from a nearby check plot during the fumigation period of plot 3-5 showed no appreciable change of sulfur content. The absorption values have been checked by analysis of the leaves for total sulfur using the Burgess-Parr sulfur bomb. The leaf samples are taken in large rubber-stoppered test tubes, weighed fresh, then frozen in salt and ice, and allowed to dry in air for a4bout 24 hours before grinding. Analyses are run on 0.7-gram air-dried samples. The procedure is carefully standardized, particularly as to the amounts of reagents and the volume and acidity of the liquid before precipitation with barium chloride. Ten milliliters of 0.1 N sulfuric acid are added to each solution as a true blank and the precipitate is allowed to stand 2 to 3 days, with occasional agitation, before filtration. Table I1 shows a fairly satisfactory concordance between the sulfur absorbed, as indicated by the analysis of the gas
Vol. 4, No. 3
and the analysis of the leaves. The latter value is in most cases somewhat higher than the former, probably because the absorption took place unevenly on the plants, and there was doubtless a tendency to take as the leaf sample more of the active leaves near the top of the plants, which probably had absorbed more gas than the less active leaves lower down. The uncertainty in the sulfur analysis of the leaves is of the order of several hundred parts per million, owing principally to sampling errors, but also partly to analytical errors. These errors were both minimized by taking at least two samples before and after fumigation and running a number of duplicate analyses on each sample.
LITERATURE CITED (1) Thomas and AbersoId, IND. ENO.CHEM.,Anal. Ed., 1, 14 (1929). (2) Thomas and Cross, IND.ENG.C H ~ M20, . , 645 (1928).
RECEIVED December 21, 1931.
Methods for Microanalysis of Extremely Hygroscopic Substances DOUGLAS F. HAYMAN, Merck & Co., Inc., Rahway, N. J.
D
URING the regular course of analysis, certain series of compounds were found to be so extremely hygroscopic that the existing methods of keeping the material free from water were found to be inadequate. New methods were therefore developed for the analysis of these compounds.
MICRO-DUMAS METHOD While investigating some extremely hygroscopic derivatives of choline chloride, it was found that a solid micro sample would become liquid in an interval from 10 to 100 seconds. According to the regular method of Pregl ( I ) , the material was weighed in a small weighing tube about 3 mm. in diameter, fitted with a ground-glass stopper. The material, however, became sticky from absorbed water and could not be shaken from the tube properly. The material which was shaken into a shaking bottle became liquid and could not be quantitatively transferred to the combustion tube. The nitrogen values on these compounds ran consistently low and were never within the 0.3 per cent limit of error. Very good results were obtained, however, by a modification of the Pregl method. A glass-stoppered weighing bottle or “piggie” as described by Pregl, having a diameter of 10 mm. and a length of 40 mm., was cut down to a length of 15 mm. The sample was easily placed in the bottle with great speed. The bottle was then placed in a vacuum desiccator for about 20 minutes. After it had been dried in this way it was stoppered, placed in the balance, and weighed within 5 minutes. Then the stopper was removed and the sample was transferred quickly to the shaking bottle which contained dry powdered copper oxide. The stoppered weighing bottle was weighed at once. The sample in the shaking bottle was mixed with the copper oxide immediately so that the material would not become sticky and adhere to the glass. The vacuum desiccator was situated near the balance so that the weighing tube was kept a t the same temperature as the balance. The air which was admitted to the desiccator was passed slowly over magnesium perchlorate trihydrate. Results obtained by this method were quite constant and were only slightly below theory.
ANALYSISFOR CARBONAND HYDROGEN Certain compounds of the type B.3Hz0 gave off the exact amount of water (3H20) as determined by drying experiments on a macro scale. The analysis of B, however, always indicated that the material still held a t least 1 HzO. The material B apparently picked up water very rapidly, as all methods of keeping the sample dry were found to b e inadequate. The analyses for carbon in B were seldom within 1 per cent of theory, and the analyses for hydrogen were always high. The material appeared to be very hygroscopic and could not be exposed to the air even for a very few seconds without absorption of moisture. Therefore, the material was dried and analyzed in the following manner: The airdried material which had been previously analyzed as B.3Hz0 was weighed in a platinum boat and placed in the combustion tube in the same manner as for regular carbon and hydrogen determination. Previously weighed absorption tubes were placed in the usual position to determine the amount of water driven off and to show any decomposition which might take place. Nitrogen was then passed through the tube by way of the regular preheater and drying train at a rate of 5 cc. per minute. The material was heated to 100” to 105” C. by means of a Pregl heating block. The nitrogen was passed over the material for a period of about 90 minutes. The apparatus was then swept out with air for 20minutes, and the absorption tubes were removed and weighed to determine the results of the drying. When the amount of water was very small it was found necessary to correct for the dehydrite absorption tube which picked up water over the 2-hour period. The weights of the ascarite absorption tubes were very constant, showing little or no decomposition. The absorption tubes were then attached to the combustion tube, and the carbon and hydrogen analysis was carried out in the usual manner. The results were very satisfactory for the material B when entirely free from water. LITERATURE CITED (I) Pregl, “Quantitative Organic Microanalysis,” translated by
Fyleman, Blakiston, 1930. RECEIVED January 18, 1932.
