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INDUSTRIAL AND ENGINEERING CHEMISTRY TABLE11. DETERMINATION OF STIBNITE SULFUR IN MIXTURES OF STIBNITE AND OTHERSULFIDES
Total Weight Stibnite of ----Foreign Sulfides0.i.N 0.1 N Sulfur Sulfur Sample SbzSa pbs CUS HgS FeSz Iodine NazSz08 Found PresentC Gramsn Grama Gram Gram Gram Gram Cc. Cc. % % 1.oooo 0.1986 0.1013 0.10oo 0.1004 0.1007 40.00 7.31 5.23 5.24 1.0000 0,2009 0.1008 0,1000 0.1002 0.0998 40.00 6.32 5.39 5.30 1.0000 0.1894 0.1002 0.0994 0.0996 0.1000 40.00 7.31 5.23 5.26 1.0000 0,2012 0,0998 0,1000 0.0981 0.1005 40.00 7.21 5.25 5.31 1.0000 0.0975 0.0972 0.0963 0.0968 0.1976 25.00 7.66 2.77 2.72 1.0000 0,1000 0.1000 0,0995 0.1004 0.1010 25.00 7.85 2.74 2.72 2.0000 0,0585 0.1003 0.0992 0,0983 1.0000 20.00 6.60 1.07 1.09 2.0000 0.0600 0.1018 0.1027 0.1006 0.9996 20.00 6.10 1.11 1.11 2.0000 0.0595 0.1020 0.1038 0.1000 1.0000 20.00 6.00 1.12 1.10 2.0000 0.0567 0.1015 0.10oo 0.1001 1.00oo 20.00 6.50 1.08 1.07 a Combined sulfides made up to weight indicated by addition of quartz sand. b Specimen contained 25.60 per cent of stibnite sulfur. Includes previously determined traces of sulfur preaent as arsenic and antimony sulfidea in the pyrite and cinnabar added to the samples.
9. Qualitatiue Tests of Hot Alkali Extraction of ArtiJicial Mixtures Stibnite and of stibnite Otherand the selected S u ~ dforeign e s *A mixture ofOf0.2 gram each sulfide was extracted first with 20 cc. and then again with 10 cc. of boiling 2 N sodium hydroxide, the residues being washed each time with hot 0.5 N sodium hydroxide and tested for antimony. The first extract was tested for presence of the foreign metal, and the second extract was tested also for antimony. To test for mercury there was used the Feigl spot-test (4) with aniline and stannous chloride. Lead, copper, and iron were sought by addition of sodium tartrate and sodium sulfide. To examine for antimony in the second extract there was employed the Clarke and Evans color test (.2) with potassium iodide, pyridine and gum arabic. These tests showed that antimony sulfide was completely dissolved by the first extraction, the second extract being free from antimony as were also the undissolved residues. Sulfides of mercury, copper, lead, and iron were unattacked in the first extraction, tests for these metals in the filtrates being negative. Only traces of lead, copper, and iron mere detected in the second
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extract from mixtures of stibnite with galena, copper glance, or pyrite, respectively, and cinnabar was wholly undissolved. 3. Determination of Stibnite Suljur in Artiricial Mixtures of Stibnite with ‘various Proportions of Foreign &&des. Mixtures of weighed amounts of stibnite! galena, copper glance, pyrite, and cinnabar were diluted with quartz sand to simulate lowgrade ores, and analyzed by the new method as described. Results are presented in Table 11.
The results in Table I1 indicate that the proposed method permits accurate determination Of the sulfur of stibnite in presence of sulfides of lead, copper, iron, and mercury. Free sulfur and arsenic sulfide interfere to whatever extent they are present, and Ordinarily introduce a n inconsiderable error. The new method is recommended for the rapid and accurate determination of stibnite sulfur in stibnite and in low-grade ores containing as impurities other sulfides such as galena, copper glance, cinnabar, and Pyrite- It will Probably yield the sulfur combined with antimony in certain mineral double-sulfides such as livingstonite, boulangerite, and zinkenite.
Literature Cited “Chemisoh-technisch Untersuchungsmethoden,” Band 11, Teil 2, p. 1677, Berlin, Julius Springer, 1932.
(1) Berl-Lunge,
(2) Clarke and Evans, Analvat, 54, 23 (1929). ENQ.CHEM.,10,376 (1918). (3) Cushman, J. IND.
(4)
Feigl, “Qualitative Analyse mit Hilfe von Tiipfelreactionen,” p. 132, Leipzig, Akademische Verlagsgesellschaft m. b. H., 1931.
(5) McNabb and Wagner, IND.ENG.CHEM.,Anal. (6) Ibid., 2, 254 (1930).
Ed., 1, 32
(1929).
RECEIVED July 16, 1934.
Zinc Cobaltinitrite for the Detection of Potassium JANE ADAMS, MARTHA HALL, AND W. F. BAILEY, MacMurray College, Jaclisonville, 111.
T
HE reagents commonly used for the qualitative detection of sodium and potassium are not completely satisfactory. Probably the most sensitive are sodium cobaltinitrite for potassium and zinc uranyl acetate for sodium as recommended by Kolthoff (1). If these are used, however, a separate portion must be used for sodium; and if a large amount of potassium is present it must be at least partially removed before use of the sodium reagent. For this purpose, the zinc perchlorate procedure of Reedy (2) is very satisfactory. The purpose of this investigation was to develop a reagent which would retain the sensitivity of the sodium cobaltinitrite reagent without introducing sodium ion and a t the same time leave the filtrate in a state best suited for the use of zinc uranyl acetate for the detection of sodium. Zinc cobaltinitrite should answer the purpose, since the added zinc ion should decrease the solubility of the sodium precipitate by common ion effect. Experimental Throughout the course of the investigation the chemicals used were Baker’s c. P. anal zed. Attempts to prepare zina nitrite were almost entirely witgout avail. Barium nitrite was made by metathesis of sodium nitrite and barium chloride. On dissolving this in a solution of cobalt nitrate and acidifying with acetic acid, a yellow precipitate of barium cobaltinitrite separated, and was converted to zinc cobaltinitrite by shaking with a solu-
tion of zinc sulfate. The resulting solution, however, was too unstable to be practical. Direct preparation of a cobaltinitrite solution was then tried by passing oxides of nitrogen through (1) a solution of cobalt nitrate containing suspended zinc hydroxide, (2) a suspension of zinc and cobalt hydroxides, and (3) a solution of zinc acetate and cobalt acetate. In all cases a darkening resulted and the solution gave a yellow precipitate with potassium nitrate. The third method gave by far the most consistent results. The oxides of nitrogen were made by the action of either dilute sulfuric acid on a solution of. potassium nitrite or concentrated nitric acid on copper, the latter being most convenient. The final method of preparation adopted was t o pass a rapid stream of oxides of nitrogen, made by the action of concentrated nitric acid on copper foil, through a solution saturated with both cobalt acetate and zinc acetate for from 45 to 60 minutes. The resulting dark brown solution was tightly stoppered, allowed to stand overnight, and then decanted from any precipitate. If kept tightly stoppered t o prevent loss of oxides of nitrogen, the solution was as sensitive as ever after 6 weeks; left open to the air, all cobaltinitrite disappeared within a week. T o test the sensitivity of the reagent, solutions containing varying amounts of potassium nitrate were mixed with an equal volume of the reagent. The sensitivity is given in milligrams of cation per milliliter of the resulting solution. Repeated tests showed that 1 mg. of potassium per milliliter gave an immediate yellow precipitate. After standing for 15
SEPTEMBER 15, 1935
ANALYTICAL EDITION
minutes, from 0.4 to 0.6 mg. per ml. was the smallest amount detectable. Sodium in amounts up to 50 mg. per ml. seemed without effect on the sensitivity. Solutions containing both sodium and potassium nitrates were treated with zinc cobaltinitrite until all the potassium was removed and then tested for sodium by zinc uranyl acetate reagent prepared according to Reedy (2). After removal of 50 mg. of potassium per milliliter 3 mg. of sodium per milliliter were detectable by the formation of a distinct yellow-green precipitate. When the amount of potasium was 10 mg. per milliliter, 1 mg. of sodium could be detected in the filtrate.
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Procedure To the solution from the alkaline earth group, which contains no barium, arsenate, phosphate, or ammonium, add an equal volume of zinc cobaltinitrite solution and let stand for 15 minutes. A yellow precipitate of potassium cobaltinitrite indicates potassium. Filter and to the filtrate add zinc uranyl acetate solution. Sodium is indicated by the formation of a crystalline
greenish yellow precipitate.
Literature Cited (1) Kolthoff, I. M., 2.anal. Chew., 70, 397 (1927). (2) Reedy, J. H., “Elementary Qualitative Analysis,” 2nd ed., New York, MoGraw-Hill Book Go., 1932. RECEIVED July 15, 1935.
The Estimation of Starch J. T. SULLIVAN, Purdue University Agricultural Experiment Station, Lafayette, Ind.
T
HE estimation of starch is of interest to those engaged in many industries, to those concerned with the enforcement of food laws, and to plant chemists and physiologists in the investigation of botanical, horticultural, and agronomic problems. The objects to be attained by the various analysts, the degree of accuracy required, the methods which are most suitable, and the particular technical difficulties involved, vary. I n the analysis of plant products for any constituent, a separate problem may arise for each type of material. This is true of starch analysis and each field of activity has its favored methods which are believed to be more suitable in their particular circumstances. Starch is usually accompanied by other carbohydrate substances, such as cellulose, hemiculluloses, pentosans, gums, mucilages, glucosides, pectins, and tannins, and its own hydrolytic products, sugars and dextrins. It is the problem, of course, to separate starch from these substances, if possible, and, if not, to hydrolyze it to sugars in their presence under such conditions as will cause the least interference by these other materials.
Acid Hydrolysis The simplest method of converting starch t o sugar is the acid hydrolysis method. Approximately 90 parts of starch yield 100 parts of glucose. I n practice, this yield, popularly considered theoretical, has not been obtained, What is the theoretical yield? To the starch manufacturer, “starch” is everything in the granule (17). It has been customary, however, with those in other fields to consider starch as only that which on hydrolysis yields simpler carbohydrates. With the latter viewpoint the theoretical yield may be calculated from the organic structure of starch. If starch consists of a chain of glucose units linked together with the loss of water a t each linkage, on hydrolysis there will be added to its weight one molecule of water for each glucose unit, less one. If the chain contains six, twelve, or twenty-four glucose units, the glucose starch conversion factors are, respectively, 0.917, 0.907, and 0.904. If the chain is longer, the conversion factor approaches more closely 0.9. If the chain is closed t o form a loop, the factor is exactly 0.9, regardless of the size of the molecule. Recent contributions of organic chemists (16, 18, dW, and others) indicate that the chain is very long and the correct factor is therefore very nearly 0.9. However, the work of Noyes (36) indicates that 100 per cent recovery cannot be obtained by acid hydrolysis, and he accordingly suggests the
factor 0.93. Others have adopted this and other factors. Noncarbohydrate constituents, moisture excepted, such as ash, protein, fat, and fiber, are rarely present in excess of 1 per cent in a good grade of starch, although these constituents and also moisture are not easily and accurately determined. It has been demonstrated that glucose is not injured by heating with acid unless excessive concentrations of acid or duration of heating are used (5,8). Nevertheless there have been formed from glucose by the action of acid and there also occur in the mother liquor of starch hydrolyzates in some industrial processes, disaccharides which are probably derived from glucose and have less reducing power than glucose. These factors probably account for the failure to obtain complete recovery of starch unless the larger factor, for which there is no theoretical basis, is used. Using the factor 0.9, the author has recovered, as measured by the reducing power of the glucose, 97 per cent of the starch. Because of the susceptibility of other carbohydrates to attack by boiling acids, the use of this method has limited application. It has been discontinued by workers in most plant fields. The only recent attempt to utilize this method is that of Fraps (15) who used a very dilute acid (0.02 N ) and completed the hydrolysis with stronger acid after the insoluble matter had been removed and corrected for pentosan dissolved.
Enzymatic Hydrolysis When carbohydrate substances other than starch are present, it has been customary to resort to enzymatic hydrolysis. Diastatic, as well as other enzymic preparations, are notoriously impure and contain other enzymes which attack many other substances. Moreover these preparations do not hydrolyze starch into a single product, but rather into mixtures of sugars, or of sugars and dextrins. The relative proportions of these products are not constant but depend upon every possible variation in the conditions of hydrolysis. As a result of this situation, the estimation of starch by diastatic means has been resolved into three types of methods : (1) the mixed products obtained are estimated by two different methods, and each product may be calculated by the use of simultaneous equations; (2) empirical methods, by which the products are obtained under rigid conditions of hydrolysis and are found to have certain definite reducing or rotational values; and (3) methods in which the diastatic action is supplemented by a subsequent hydrolysis with mineral acids to yield a single product, glucose.
