INDUSTRIAL AND ENGINEERING CHEMISTRY
294
Aclrnowledgments
TABLE I. TYPICAL ANALYSES Calcd. Knownor
I Mercuric chloride I1 Tolylmercuric. iodide I11 Chloromerouriphenol
IV p-Mercury ditolyl V Phenylmercuric chloride V I Phenylmercuric nitrate
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Found by Spacu Spacu Method and
%
%
73.88 47.93 60.66 52.07 56.14 62.80
73.90 47.90 60.50 52.04 56.03 62.87
reason, the mercury content was determined by a sulfide precipitation or by a thiocyanate titration. Compound 11 was the only one to contain iodine, and therefore, 0.2 to 0.3 gram of diiodofluoresceinwas added to all the others.
The writers wish to thank N. M. Stover of this laboratory for his valuable suggestions, and the Carnegie Corporation Research Fund for a grant which enabled the purchase of certain reagents.
Literature Cited Dunning and Farinholt, J. Am. Chem. SOC., 51, 804 (1929). Fenimore and Wagner, Ibid., 53, 2468 (1931). Kharasch and Flenner, Ibid.,54,674 (1932). Spacu and Spaou, Z. and. Chem., 89,188 (1932). ( 5 ) Tabern and Shelberg, IND.ENG.CHEM.,Anal. Ed., 4, 401 (1932). (6) Whitmore and SobatZki, J *Am. Chem. SOC., 55, 1128 (1933). (1) (2) (3) (4)
RECEIVED June 13, 1935.
Microchemical Analysis of Solid Fuels J
W. R. KIRNER, Coal Research Laboratory, Carnegie Institute of Technology, Pittsburgh, Pa.
T
HE history of the development of Pregl's methods of quantitative organic microanalysis is familiar to everyone interested in this field. These methods were developed by carrying out analyses on pure organic compounds of known composition. Only after Pregl had demonstrated that his methods were able greatly to reduce the expenditure of time, energy, material, and hence money, and that the results obtained were as accurate and precise as the older macromethods, were the methods accepted by other chemists. Their introduction into industrial laboratories was a t first viewed with a good deal of skepticism by industrial chemists, their main argument being that it was impossible for a microsample to be actually representative of the total substance whose analysis was desired. Certain difficulties are encountered in the elementary macroanalysis of solid fuels, as well as of many other natural substances, such as agricultural and biological products, alkaloids, etc., which are almost entirely absent in dealing with pure organic compounds. In the first place, solid fuels have a complex composition, and all samples, besides containing carbon, hydrogen, and oxygen, also have present nitrogen, sulfur, and mineral matter and sometimes phosphorus and chlorine. Some of these constituents are present in very small amounts, which makes their determination difficult by any method of analysis. In the combustion the mineral matter is converted into ash which may cause certain complications in the analysis. The combustion of solid fuels often leads to the formation of exceedingly combustion-resistant cokes which, a t times, makes complete oxidation of the sample extremely difficult. During the expulsion of the volatile matter by the action of heat, large volumes of gas are evolved, consisting primarily of methane, hydrogen, and carbon monoxide, and special precautions must be taken during this part of the combustion to insure their complete oxidation. Another difficulty involved in the analysis of solid fuels concerns those methods which require solution of the sample. Solid fuels are relatively insoluble and there is no solvent known which dissolves them completely a t a moderate temperature and without reaction. The difficulties connected with the elementary analysis of solid fuels by the usual macromethods have been known for some time and certain modifications of standard methods, as developed on pure compounds, were made by fuel analysts to render the results more certain, consistent, and accurate. In the light of the development of micromethods applied to
pure substances, it is now recognized that the conversion of a macro- to a micromethod involves much more than the mere diminution of the size of the sample taken for analysis, Errors which are of no significance in macromethods often assume gigantic proportions in micromethods and much tedious work is necessary for their elimination. It is this rationalization, however, which gives micromethods their accuracy and precision. If such difficulties are encountered in converting macro- to micromethods applied to pure substances, it is only reasonable to expect that the difficulties will be still greater when dealing with complex, heterogeneous substances such as solid fuels. For the purpose of a systematic discussion, the individuai determinations necessary for the complete microanalysis of a solid fuel will be considered.
Sampling Solid Fuels for Microanalysis Since Pregl, in his development of the methods of quantitative organic microanalysis, was primarily interested in the application of these methods to pure substances, he did not mention in his book the problem of sampling heterogeneous materials for microanalysis. In dealing with pure substances, the microsample taken for analysis is directly a true, diminished representative of the whole. The problem of sampling solid fuels for microanalysis has been fully discussed elsewhere (85). In order to obtain homogeneous, representative microsamples, solid fuek must be subjected to extremely fine grinding and thorough mixing. One difficulty in the fine grinding of solid fuels concerns the extremely troublesome appearance of electrostatic charges induced on the finely ground particles. Great care must be exercised in grinding samples which exhibit this phenomenon, since rather large amounts of the finer particles may be lost from the agate mortar. Various attempts have been made to overcome this difficulty with but moderate success. Such charges can also seriously affect the accurate weighing o f microsamples.
Determination of Moisture The method generally used in this laboratory for determining moisture consists in drying the sample in a microdesiccator at a temperature of 110" to 115" C. for 15minutes in an atmosphere of nitrogen. The microdesiccator is fitted with a bubble counter which in turn is connected with a Pregl
SEPTEMBER 15, 1935
ANALYTICAL EDITION
pressure regulator, both containing concentrated sulfuric acid; the nitrogen is taken directly from a high pressure cylinder. By calibrating the velocity of the bubble rate against the passage of nitrogen, the flow of gas over the sample is known and can never become so violent that particles of the sample are blown out of the boat. In case of hygroscopic samples, all weighings are made in a Pregl stoppered weighing tube. Vetter (39) has devised an apparatus for the direct microdetermination of the moisture content of fuels and other solid substances. The sample is placed in a tube surrounded by a vapor bath-. g., toluene, b. p. 111’ C. A stream of dry nitrogen passes through a ressure regulator and bubble counter, then over the sample, ani! the moisture is collected in a weighed calcium chloride microabsorption tube, which, in turn, is connected with a Mariotte bottle. By slight modification the apparatus can also be used for vacuum-drying the sample.
Determination of Carbon and Hydrogen COMBUSTION TUBEFILLINQ.The Pregl Universal filling was used by all earlier investigators in determining the applicability of micromethods to coal analysis, and is used in all routine analyses for carbon and hydrogen in this labora-
tory. Kopfer (28) was the first to point out the advantages of using a platinum-asbestos filling for the combustion of organic substances. For retention of halogens he added a silver spiral and first suggested the use of lead peroxide to retain oxides of nitrogen and sulfur. Haber and Grinberg (21) adopted certain of Kopfer’s ideas. Their tube filling consisted of a 6-cm. layer of lead chromate held in place by two small copper gauze spirals and a IO-cm. layer of 10-20 per cent platinum-asbestos. The particular advantage of the platinum-asbestos is that the combustion of methane takes place quantitatively at temperatures as low as 410’ C., whereas with copper oxide a much higher tem erature is necessary, the specifications reaching to a bright red \eat and just below the softening point of the hard-glass combustion tube. It was primarily the advantages obtained by this reduction in temperature which led Dennstedt to develop his contact method. The Dennstedt method has also been extended to microsamples, particularly by Funk (19) and Friederich (16). There are numerous cases cited in the literature where the elementary analysis of pure substances containing a number of inethoxyl or methyl groups has caused considerable difficulty, due to the evolution of methane or carbon monoxide which escape complete combustion, thus yielding low results for carbon and hydrogen. For such difficultly combustible substances, Pregl has used (22) and recommended (SI) a modified Universal filling, removing 6 cm. of the copper oxidelead chromate portion and substituting for it a 6-cm. platinum star. Pregl, therefore, appeared to favor the presence of a platinum contact in his tube filling when dealing with substances difficult to burn. If the first portion of the combustion analysis of coal is not conducted carefully, gases, especially methane, may pass through the combustion tube without being burned quantitatively. If the Universal filling is used, the copper oxidelead chromate layer must be heated to such a high temperature that the tube is likely to be permanently injured after a few combustions, whereas, if only the platinum contact filling is used, great care must be taken to insure an excess of gaseous oxygen always being present. A tube filled with 40 per cent palladium-asbestos (0.75 gram occupied 15 cm. in an ordinary Supremax microcombustion tube), in place of the copper oxide-lead chromate portion of the Universal filling and using sufficient care during the combustion has been found in this laboratory t o yield excellent results for carbon and hydrogen, in the presence or absence of other elements such as halogens, nitro en, or sulfur (26, 27). Niederl (3.4) has criticized the use of t f e contact filling. He made calculations of the volume of carbon dioxide and water (steam), which would be formed in the combustion and the volume of oxygen necessary to burn 4 mg. of naphthalene. However,
295
these volume relationships have no significance other than to illustrate the condition which would exist if the entire sample was completely vaporized at a single instant. Naturally, the combustion must be conducted sufficiently slowly so that an excess of oxygen is always present. Dennstedt and also Funk (19) insured this condition by having both primary and secondary oxygen inlets while Friederich (16) ufied a countercurrent pyrolysis of the sample in order to achieve a stepwise combustion of the sample. In this laboratory the sample was merely vaporized very slowly and regularly. From the typical analyses cited by Friederich and Funk it is apparent that the contact filling is capable of yielding satisfactory results in the microdetermination of carbon and hydrogen. The use of the palladium-asbestos contact filling in this laboratory also yields satisfactory results. This is indicated by the results in Table I, obtained on fairly large samples of naphthalene. The samples were dried over phosphorus pentoxide, drawn into capillaries while molten, sealed, and weighed. The analyses were made on a hot, humid, August day. TABLEI. ANALYSESON NAPHTHALENE Weight of Sample
C
Found
Mo.
%
H %
6.385 7.916 8.638 7.102
93.48 93.59 93.55 93.69
6.28 6.40 6.38 6.35
Difference from Theory C H
%
%
-0.22 -0.11 -0.15 -0.01
+O.lO
-0.02
+0.08 f0.05
It would appear that a compromise between the Universal and contact filling, as used by Pregl and Haber, would be a logical way in which to avoid the disadvantages of each. With such a filling the platinum contact will easily effect the oxidation of methane and there still will be an oxygen “reservoir” present, in the form of the copper oxide-lead chromate, to oxidize any incompletely oxidized decomposition products which might persist due to an insufficient amount of gaseous oxygen to complete the oxidation. This modified tube filling is therefore suggested for the carbon-hydrogen determination in fuels or for compounds which are known to evolve methane during combustion. ABSORPTION TUBEFILLINGS. In the Coal Research Laboratory only Ascarite is used for the absorption of carbon dioxide. It has been found that the finer mesh (20 to 30) material is extremely well suited for microanalytical purposes. Having a much larger surface for a given weight than the 4- to 8-mesh material, many more analyses can be made without the necessity of refilling the tube. Thus, a 3.5- to 4.0-cm. layer in a 7- to 8-mm. outside diameter absorption tube absorbed over 400 mg. of carbon dioxide from 21 analyses made on various fuel samples weighing from 6 to 10 mg. before the tube filling was exhausted to a point where further use might have entailed loss. No cases of stoppage have so far been encountered. For absorption of water either Dehydrite (magnesium perchlorate trihydrate) is used, or phosphorus pentoxide deposited on pumice, as recommended by Boetius (4). Dehydrite should never be placed in direct contact with cotton, as suggested by Pregl when using calcium chloride. Any mineral acid which might reach the desiccant could react, forming perchloric acid, which in turn could explosively oxidize the cotton or any other organic material present. A layer of glass wool is used to separate either of the above drying agents from the small layer of cotton placed in the end of the absorption tube. When absorption tubes filled with phosphorus pentoxide-pumice are used, the tubes are weighed filled with oxygen, the capillaries being stoppered with tightfitting pins, also as suggested by Boetius. The advantages of using oxygen only in microdeterminations of carbon and hydrogen in industrial laboratories are discussed by Vetter (58), who points out that air, free of or-
296
INDUSTRIAL AND ENGINEERING CHEMISTRY
ganic and absorbable inorganic vapors, is not likely to exist in the vicinity of an industrial plant in which the microchemical laboratory is located. This air, if used, contains sufficient harmful materials to influence the accuracy of the carbonhydrogen determination. He recommends using oxygen only and weighs the absorption tubes filled with oxygen, unstoppered. A concomitant advantage is that the combustion requires less time, since the final sweeping can be done with 50 cc. of oxygen, a complete carbon-hydrogen determination requiring but 40 minutes.
Determination of Ash The determination of ash is generally made in connection with the carbon-hydrogen or sulfur determination, the boat merely being weighed after combustion. Precautions must be taken during the combustion so that no ash is lost due to spattering. In burning dry samples this is most likely to occur during the time the volatile matter of the fuel is being actively evolved. In independent ash analyses the Pregl micromuffle is used. A useful modification of this apparatus has been suggested by Coombs (9). The disadvantages of the rather crude Pregl apparatus are pointed out and in place of the bent tube it is suggested that a straight silica tube be used which can be fitted with a bubble counter and pressure regulator, so that the velocity of the air, or where necessary, oxygen, which passes over the sample can be accurately regulated. The use of oxygen in the determination of ash in solid fuels is a great advantage, as combustion with air is a very slow process, particularly when dealing with coke, anthracite, or graphite.
Determination of Nitrogen The Kjeldahl or Dumas method is generally used for the determination of nitrogen in solid fuels. There appears to be a considerable difference of opinion among fuel analysts as to which method gives the most reliable results. Pregl developed microprocedures for both these methods, being primarily interested, however, in their application to pure organic compounds. In both the macro- and micromethods certain difficulties are encountered in the analysis of solid fuels if they are used in their “standard” form. Other methods for determination of nitrogen in solid fuels have been suggested, but to date have not been developed on microsamples. THEKJELDAHLMETHOD. As pointed out by Terres (37), Kjeldahl published his method in 1883 and applied it to the determination of nitrogen in natural substances such as cereals. The advantages of the method in simplicity, convenience, economy of time, and adaptability to series analyses caused it to be immediately recognized and it was rapidly adopted for general analytical practice and used in the analysis of all types of substances. After a few years a great deal of experience was gained and numerous modifications, such as those of Wilfarth, Gunning, Arnold, and others, were made in order to speed up the method still further. It is now well known that certain types of nitrogen linkages in organic dompounds prevent the successfuluse of the Kjeldahl method, but with a knowledge of the structure of the substance being analyzed, one can predict with a good deal of certainty whether or not the method or one of its modifications can be expected to yield quantitative results. However, very little is known regarding the type of nitrogen linkage in solid fuels, so that it would be impossible to predict whether or not the Kjeldahl method could be expected to yield quantitative results. Bunte and Schilling (6) applied the Kjeldahl method to the analysis of solid fuels shortly after it was published and found it yielded very consistent results. The method was retained
VOL. 7 , NO. 5
for fuel analyses, largely on the basis of Bunte’s results, and even a t present it is the method mainly used for routine nitrogen determinations on solid fuels. The question as to whether this method actually determines the total nitrogen in solid fuels was really never further tested until 1919, when Terres made an exhaustive study of the problem. As a result of his investigation, Terres concluded that the “standard” Kjeldahl method (he actually used the Kjeldahl-Gunning method, adding potassium sulfate to the concentrated sulfuric acid), applied to solid fuels, always gave low results. The amount of nitrogen found in the sample was a function of the manner in which the digestion was carried out: the highest values were obtained when the digestion was done a t relatively low temperatures-that is, with prolonged low-temperature digestion. The values obtained by the Kjeldahl and Dumas methods (the latter including a subsequent combustion with oxygen) may differ by as much as 46 per cent for coal and 32 per cent for coke. These facts are indicated in Table 11. TABLE 11. EFFECTOF DIGESTION TEMPERATURE ON KJELDAHL DETERMINATION OF NITROGEN IN VARIOUS FUELS --Temperature, O ( 2 . 7 250 275 300 325 Dumas5 1.70 1.56 1.46 1.36 1.72 221 50 14 3 ,
Peat, % nitrogen Hours digested. Brown coal, % nitrogen 0.80 0.76 0.62 Hours digested 282 77 35 Sam coal, % nitrogen 1.31 1 . 2 3 1 . 1 5 Houra digested, 408 118 44 Ruhr coal, % nitrogen 1.33 1.26 1.20 Hours digested 491 142 74 Anthracite, % nitrogen 1.35 1.34 1.24 Hours digested 623 213 59 Peat coke, % nitrogen .. 0.82 0.73 Hours dieested . 50 22 Brown coke, % nitrogen 0.55 0.52 Hours digeated 96 69 S a m coke, % ni1.11 1.01 trogen Hours digested 117 63 Ruhr coke, % .. nitrogen 1.53 1.48 Hours digested 45 24 Anthracite coke, % nitrogen 1.49 1.27 Hours digested 48 30 Subsequent combustion in oxygen.
.
....
....
.*.. ... .
0.58 4.5 1.00 4.5 1.07 6 1.13 15.5 0.71 12 0.44 28 0.98 48 1.34 18 1.22 24
. .. 1.73 ,. 1.55 ..
1.08
Diff. (Dumas and Kjeldahl) High Low - 1.2 -20.4
....
....
-25.9
-46.3
-24.4
....
-42.1
-14.2
....
-31.0
-21.5
-34.3
....
...,
1.72
....
....
.. .... - 7.9 .. .... 0.62 -11.3 .. .... -22.4 1.43 .... .. 1.59 - 3.8 .. .... 1.79 .. -16.8 ....
0.89
....
-20.2
....
-29.0
....
-31.5
....
-15.7
....
-31.9
....
As a result of this work Terres maintained that, in order to get correct results by this method, the gases evolved during digestion must be collected and analyzed for their elementary nitrogen content since a considerable portion (as much as 50 per cent) of the total nitrogen present may be evolved in this form and so is not converted into ammonia. This is shown in Table 111. TABLE111. PROPORTIONATION OF NITROGENIN KJELDHAL DETERMINATION ON SOLID FUELS
--
As NHI
Fuel Peat
NitrogenAs Nr
%
1.32 1.36 0.57 Brown coal 0.64 Sasr coal 1.33 1.13 Ruhr coal 1.19 1.09 Anthracite 1.20 1.14 a Subsequent combustion in
% 0.42 0.43 0.61 0.58 0.42 0.65 0.46 0.42 0.62 0.72 oxygen.
Total
NI NHs
%
%
%
1.74 1.79 1.18 1.22 1.75 1.78 1.65 1.51 1.82 1.86
24 24 52 48 24 37 28 28 34 39
1.72
Dumas”
1.08 1.73 1.55 1.72
This work of Terres casts grave suspicion on the use of the “standard” Kjeldahl method for the determination of nitrogen in solid fuels. His results have recently been confirmed by Bornstein and Petrick (6).
SEPTEMBER 15, 1935
ANALYTICAL EDITION
According to Carlile ( 7 ) ,the elementary nitrogen found by Rornstein and Petrick may have come from air which was incompletely removed from the apparatus, from occluded and adsorbed nitrogen in the coal and in the reagents used, or from cumulative traces of air in the carbon dioxide used to sweep the apparatus. According to Carlile's results, the Kjeldahl-Wilfarth-Gunningmethod is adequate for the nitrogen determination in solid fuels. Also, Fieldner and Taylor (11) and Baranov and Mott (2) claim that if the KjeJdahl-Wilfarth-Gunning method (use of sulfuric acid, potassium sulfate, and mercury or mercuric oxide in digestion) is applied to solid fuels, satisfactory results are obtained which agree with modified Dumas determinations. Beet (3) added selenium to the above list of catalysts used during digestion and claimed to obtain more rapid, but still satisfactory, results. If considerable quantities of elementary nitrogen are lost during the macrodigestion of solid fuels, i t is very probable that similar losses will occur during microdigestion. MicroKjeldahl determinations on coal and coke samples made in the Coal Research Laboratory have yielded practically identical results to those obtained by other laboratories using the macro-Kjeldahl method on the same samples. If an error is present in the macromethod it is also present to approximately the same extent in the micromethod. This whole question of validity can be settled satisfactorily only by further careful research on the two methods. I n this laboratory the ammonia, on distillation, is absorbed directly in boric acid solution and titrated with standard acid using methyl orange as indicator, as first suggested by Winkler (4.2) for the macrodetermination and applied to the microdetermination by Allen ( I ) , Stover and Sandin (36), and Meeker and Wagner (32). The advantage of this procedure is that but one standard solution is necessary. THE DUMASMETHOD. The Dumas method is also not free of suspicion when applied to the analysis of solid fuels, nor even when applied to certain pure nitrogen-containing compounds. It frequently yields high results on substances containing a large number of methyl groups, presumably because of the formation of methane or carbon monoxide, which escape complete oxidation in the presence of copper oxide in an atmosphere of carbon dioxide, and hence are measured along with the nitrogen. Still other substances form combustion-resistant cokes under these conditions, making it very difficult or impossible to obtain complete combustion in an atmosphere of carbon dioxide, even when intimately mixed with fine copper oxide, low results then being obtained. Incomplete combustion can, therefore, lead to high or low results, depending upon whether the combustion of the coke or the gaseous thermal decomposition products is incomplete. Terres (37) made a careful study of the Dumas method applied to solid fuels and found that complete combustion of the fuel could be obtained only provided a subsequent oxidation was conducted using gaseous oxygen. However, it is again very important to make an exact analysis of the resulting gas collected in the azotometer, since only about onethird of this gas is nitrogen. The method is definitely complicated by this procedure, but Terres claims that only with this modification of the Dumas method can accurate and reliable results be obtained for the nitrogen content of solid fuels. This has been confirmed by a number of investigators (10, 13, 18, I S , 30). The difficulties mentioned in connection with the macroDumas method are also involved in the micromethod. In his book, Pregl stated that occasionally one meets a substance, which on heating with copper oxide in an atmosphere of carbon dioxide forms a nearly incombustible, nitrogen-containing coke. In such cases he recommended that the sample be mixed with potassium chlorate and fine copper oxide. It
297
has since been pointed out, however, that such a procedure is purposeless, since the potassium chlorate is completely decomposed and has furnished all of its oxygen (at 350" to 400" C.) long before the portion of the tube containing the sample is strongly heated and even before the combustion-resistant coke is actually formed. Lead chromate or potassium dichromate is probably better suited for this purpose since they evolve oxygen only when heated to high temperatures. According to Friederich (14), none of these subterfuges solve the difficulty, the nitrogen values only being raised by a few tenths of a per cent. If larger amounts of oxidizing agents are used, higher values for nitrogen are obtained but duplicate analyses often vary by several per cent. Addition of these materials to compounds which yield satisfactory results with copper oxide alone also give results which are too high, owing to the passage of either excess oxygen or carbon monoxide into the azotometer. TABLEIV. COMPARATIVE DETERMINATIONS Micro-Dumas Coal A Coke A Edenborn coal
-~j~ld~hlMicro-Dumas Micro Macro Micro-Kjeldahl
%
%
%
%
0.48 0.14
1.21 0.86 1.52
1.24 0.86 1.60
40
1.61
16
106.9
Fleischer (12) also pointed out that the micro-Dumas method, developed on pure compounds, cannot be applied to coal and coke without modification of the method and the apparatus, because these substances have a very low nitrogen content and are difficult to burn, all of the nitrogen escaping only after complete combustion of the sample. From 2 to 4 mg. of a coal sample containing 1 per cent of nitrogen only 0.016 to 0.032 cc. of gas will be obtained. These amounts are too small for accurate measurement even by the microazotometer. Fleischer, therefore, recommends a somewhat larger apparatus than that used by Pregl and burns samples weighing from 50 to 100 mg. mixed with about 20 grams of lead chromate so that from 0.3 to 1.00 cc. of nitrogen are obtained. This amounts essentially to a semi-micromethod, Terres also included the micro-Dumas method in his abovementioned investigation. He used samples weighing about 35 mg. and carried out a subsequent combustion of the residual coke with gaseous oxygen. He found that the subsequent oxidation yielded only insignificant amounts of nitrogen and the final values obtained were considerably lower than even the values obtained by the Kjeldahl method. I n the experience of the Coal Research Laboratory the micro-Dumas and Kjeldahl methods yield practically identical values for nitrogen when applied to Edenborn coal (the coal being intensively investigated in this laboratory), while other coals yield values by the micro-Dumas method which are very much lower than either the micro- or macro-Kjeldah1 values. This is illustrated in Table IV; the microanalyses were made by F. C. Silbert. If a weighed amount of potassium chlorate was mixed with the copper oxide and sample, larger amounts of nitrogen were obtained, but if the amount of gas collected in a blank determination with the same weight of potassium chlorate was deducted, the increase in nitrogen was insignificant. It was also found that if the fine copper oxide, containing the residue from the Dumas combustion, was removed and digested with sulfuric acid as in a micro-Kjeldahl determination, an appreciable amount of ammonia was obtained on distillation with alkali. Vetter (40) has devised what might be called an ultramicroazotometer for use in micro-Dumas determinations on substances with very low nitrogen content, such as solid and liquid fuels. The azotometer is made of 1-mm. capillary so that a length of 1 mm. contains but 0.8 cu. mm.; 1 mm. in Pregl's azotometer contains about 10 cu. mm. However, if
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INDUSTRIAL AND ENGINEERING CHEMISTRY
the combustion of the sample is incomplete, it is futile to refine the azotometer to this extent. The microdetermination of nitrogen in solid fuels, using existing procedures, is inadequate and should be modified to meet the very special requirements imposed upon it by these substances. Work is now under way in this laboratory on this problem. The aim of this investigation is to develop a method which will result in complete, visual combustion of the sample and yield only pure nitrogen as the gas collected in the azotometer.
Determination of Sulfur The main difficulty encountered in the microdetermination of sulfur in solid fuels, whether by the combustion or the Carius method, is due to the low sulfur content of these materials. These methods are conducted under such conditions that complete visual destruction of the sample is insured and no difficulty is met regarding incomplete oxidation. However, using Pregl’s standard procedures, these methods will not yield the same value for sulfur on most solid fuels, since the microCarius method determines the total sulfur of the sample and the microcombustion method determines only the volatile sulfur. Fortunately, Edenborn coal has a negligible amount of nonvolatile sulfur, so that the microcombustion method yields essentially the same values as the macro-Eschka method. The micro-Carius method is not convenient for routine series analyses and Merten (33) of the I. G. Farbenindustrie Laboratory modified Pregl’s microcombustion method so as to determine the total sulfur in solid fuels. Fifty- to 150-mg. samples are weighed into a large platinum boat and mixed with twice their weight of Eschka mixture. The combustion is conducted in oxygen and oxides of sulfur are absorbed in perhydrol present on the beads or the spiral in the combustion tube. The boat and its contents are removed and boiled with water and the perhydrol solution from the combustion tube is also added, the solution filtered, acidified, and the barium sulfate precipitated as usual. The filtration of minute amounts of barium sulfate through the micro-Neubauer crucible is a rather inconvenient procedure, even with the Wintersteiner modification, and in this laboratory the filter stick and crucible devised by Emich are now used exclusively. Guillemet (90) recommends the use of the benzidine-hydrochloride method for the microdetermination of sulfur in substances of low sulfur content. The sulfuric acid resulting from the combustion of the sample is precipitated as benzidine sulfate and collected and weighed in a Jena fritted-glass filter. The filtration is said to be more rapid and simple than with barium sulfate. An investigation is now under way in this laboratory to develop a rapid, volumetric method for determination of total or volatile sulfur in solid fuels. The presence of nitrogen in the sample is a complicating factor, since nitrogen-containing acids are formed during the oxidation and must be eliminated before an acidimetric titration can be made. Friederich and Watzlaweck (17) have developed a microvolumetric method for just such cases, but the procedure is somewhat complicated for routine purposes.
Determination of Methoxyl In the determination of the methoxyl content of various derivatives obtained from coal in this laboratory, the microvolumetric method of Viebock and Brecher (41) is exclusively used. This method has been found to be extremely rapid and very well suited t o series analyses. A possible difficulty has been pointed out by Kuhn and Roth (99) in connection with the methylimide group determination, and might also affect the methoxyl determination. They found that in
VOL. 7, NO. 5
dealing with certain substances, low results were often obtained which were traced to the insolubility of the samples in the reagents used in the determination. Methylated derivatives of fuel products, such as humic acids, or other high molecular weight compounds, are also insoluble in most organic solvents and a similar difficulty may be encountered here. In the absence of a standard for comparison it is difficult to know whether or not correct analytical results are obtained on such samples. The volumetric method, however, yields good checks on duplicate determinations and also agrees with the results obtained by the semi-microvolumetric method of Clark (8).
Determination of Molecular Weight Kone of the methods described by Pregl appear suitable for the microdetermination of the molecular weight of solid fuels or of high molecular weight products derived from them. The Pregl ebullioscopic method is unsatisfactory for these substances, first, because of their insolubility in most solvents, and, second, because the change in boiling point caused by these high molecular weight substances is extremely small and, because of frequent and large fluctuations in the atmospheric pressure in Pittsburgh, is difficult to determine accurately. The micro-Rast method is also unsuitable for these substances because they form such deeply colored melts with camphor and other, more recently discovered, cryoscopic solvents possessing high constants, that i t is practically impossible to determine the melting point depression. Solubility is also low in these solvents. The semi-microcryoscopic method, developed by Smith and Howard (35) in the Coal Research Laboratory for use with complex fuel products, utilizes catechol or biphenyl as the solvent. Catechol is the best solvent known a t present for substances of this type. A microcryoscopic method has been described by Iwamoto (24, using the Pregl-Beckmann thermometer. Pure samples were used weighing 7 to 30 mg. and with molecular weights of 180 to 480, quite accurate results being obtained. Using catechol as solvent, this method could probably be adapted for use with solid fuels and their products.
Direct Determination of Oxygen The apparatus developed in this laboratory for the direct microdetermination of oxygen in solid fuels (16)is essentially a modification of Pregl’s apparatus for the microdetermination of carbon and hydrogen. The most important features of the modification consist of the Sprengel pump for circulating oxygen over the sample during combustion, and the thermostating of the entire apparatus for the purpose of getting constant temperatures from which to calculate the volume of oxygen consumed during the combustion. The principle of the method consists in burning the sample in a closed system of known volume, filled with oxygen, and determining the weight of the combustion products and the weight of the oxygen consumed during the combustion. The oxygen in the sample is then equal to the difference between the sum of the weight of the oxygen content of the combustion products and the weight of oxygen consumed. The method was developed on pure compounds, starting first with substances containing only carbon, hydrogen, and oxygen, then adding halogens, nitrogen, and finally sulfur; then, after the various details had been worked out, the method was applied to solid fuels (97). The presence of halogens in the compounds caused no difficulties, In the case of sulfur compounds (97), the data indicate that the sulfur is completely oxidized to sulfur trioxide under the specific conditions used in this work. In the case of nitrogen-containing substances (a?‘),knowledge of the
ANALYTICAL EDITION
SEPTEMBER 15, 1935
’
fate of the nitrogen after combustion was necessary. The hypothesis is advanced that the form in which the nitrogen appears after combustion is a function of the manner in which it is linked in the compound. It is suggested that, under the specific conditions of the combustion method used, all nitrogen compounds yield their nitrogen as nitric oxide and nitrogen, the ratio of these products, in case of amines and amides, being different from that obtained from nitriles, nitro-, and nitrogen-heterocyclic compounds. Experiments are now being carried out in order to get further evidence in support of this hypothesis as well as to get some definite information as to the mechanism by which lead peroxide absorbs oxides of nitrogen which are formed during the combustion of nitrogen-containing compounds.
Summary The difficulties involved in the application of Pregl’s micromethods to the analysis of solid fuels are pointed out and the advantages of certain modifications, at present in use, are discussed. These modifications are necessitated by the unique physical and chemical characteristics of solid fuels which involve (1) their complex composition, (2) the low percentage of several of the constituents present, (3) the possibility of incomplete combustion of gaseous decomposition products, (4) the formation of highly combustion-resistant cokes, (5) their limited solubility in all solvents, and (6) the lack of a definite standard to which analytical results can be compared. Still other modifications to existing methods are suggested, some of which are in process of investigation in the Coal Research Laboratory.
(30)
299
Lambris, G., Brennstpff-Chem., 6, 1-6 (1925); 8, 69-73, 89-93
(1927). (31) Lunde, G., Biochem. Z., 176, 157-64 (1926). (32) Meeker, E., and Wagner, E., IND.ENQ.CHEM.,Anal. Ed., 5, 396-8 (1933). (33) Merten, H., Mikrochemie, 6, 122-3 (1928). (34) Niederl, J. B., Z . anal. Chem., 89, 62 (1932). (35) Smith, R. C., and Howard, H. C., J . A m . Chem. Soc., 57,512-16 (1935). (36) Stover, N., and Sandin, R., IND. ENG.CHEY., Anal. Ed., 3, 240-2 (1931). (37) Terres, E.,J. Gusbeleucht., 62, 173-7, 192-200 (1919). (38) Vetter, F., Mikrochemie, 10,109-13 (1931). (39) Ibid., 10, 407-8 (1931). (40) Ibid., 12, 102-8 (1932). (41) Viebock, F., and Brecher, C., Bey., 63, 3207-10 (1930). (42) Winkler, L., 2. angew. Chem., 26, 231-2 (1913); 27, 630-2 (1914). RECEIVED June 15, 1935. Presented before t h e Division of Physical and Inorganic Chemistry, Symposium on Recent Advances in Microchemicel Analysis, at the 80th Meeting of the American Chemical Society, New York, N. Y., April 22 t o 26, 1935.
Titration of Fluorine in Biological Materials EUGENE W. SCOTT AND ALBERT L. HENNE Kettering Laboratory of Applied Physiology, University of Cincinnati, Cincinnati, Ohio
Literature Cited (1) 411en, W., IND.ENQ.CHEM.,Anal. Ed., 3, 239-40 (1931). (2) Baranov, A,, and Mott, R., Fuel, 3,31-4, 49-52 (1924). (3) Beet, A. E., Iiid., 13, 343-5 (1934). (4) Boetius, M., “Uber die Fehlerquellen bei der mikroanalytischen Bestimmung des Kohlen- und Wasserstoffes,” pp. 78-9, Berlin, Verlag Chemie, 1931. (5) Bornstein, E., and Petrick, A. J., Brennst0.f-Chem., 13, 41-5 (1932). (6) Bunte, H., and Schilling, E., J. Gusbeleucht., 30,707-15 (1887). (7) Carlile, J. H. G., J. Soc. Chem. Ind., 52,306T-8T (1933). (8) Clark, E. P., J. Assoc. Oficiul Agr. Chem., 15,136-40 (1932). (9) Coombs, H., J . Soc. Chem. Ind., 53,311 (1934). (LO) Coufalik, F., Mitt. Kohlenforschungsinst. Prag, 3, 163-9 (1932). (11) Fieldner, A. C., and Taylor, C. A,, Bur. Mines, Tech. Paper 64 (1915). (12) Fleischer, H. C., “Die Stickstoffbestimmung in Kohle und Koks,” Sonderdruck aus dem Jahrbuch des Halleschen Verbandes fur die Erforschung der mitteldeutschen Bodenschntze und ihrer Verwertung, Erstes Heft, Halle (Saale), Wilhelm Knapp, 1919. (13) Foerster, F., Brennstof-Chem., 2, 33-4 (1921). (14) Friederich, A., “Die Praxis der quantitativen organischen Mikroanalyse,” pp. 75, Leipzig und Wien, Franz Deuticke, 1933. (15) Friederich, A., Ibid., pp. 20-53; iwikrochemie, 10, 329-54 (1931). (16) Friederich, A., 2. angew. Chem., 45, 476-8 (1932). (17) Friederich, A., and Watzlaweck, O., Z.anal. Chem., 89, 401-11 (1932). (18) Fritsche, W., Brennstof-Chem., 2, 365-7 (1921). (19) Funk, C., “Mikroanalyse nach der Micro-Dennstedt Methode,” Munchen, J. F. Bergmann, 1925. (20) Guillemet, R., Bull. soc. chim., 51,1611-15 (1932). (21) Haber, F., and Grinberg, A,, Z. anal. Chem., 36,557-67 (1897). (22) Herzig, J., and Faltis, F., Monatsh., 35,997-1020 (1914). (23) Hiinerbein, R., Brennstof-Chem., 4,337-8 (1923), (24) Iwamoto, K., Science Repts. Tohoku I m p . Univ., 17, 719-22 (1928). (25) Kirner, W. R., IXD. ENG.CHEM., Anal. Ed., 5, 363-9 (1933). (26) Kirner, W. R., Ibid., 6,358-63 (1934). (27) Kirner, W. R., unpublished paper. (28) Kopfer, F., 2. anal. Chem., 17, 1-53 (1878). (29) Kuhn, R., and Roth, H., Ber., 67,1458 (1934).
F
LUORINE can be isolated from inorganic samples by distillation as hydrofluosilicic acid ( 3 ) , and titrated in the distillate with cerium nitrate (1) in the presence of a mixed indicator (8). I n organic materials, the problem is somewhat complicated by the necessity of ashing the samples without losing fluorine, and of obtaining the ash in such condition that the subsequent isolation and titration of the fluorine will not be interfered with. Fluorine in organic substances has been titrated before; a variety of elaborate and unreliable methods have been used and the results have always been inconsistent. As there are only minute amounts of fluorine in plant or animal tissues, the details of the chemical procedures become all-important. Therefore, standardized conditions for calcination, distillation, and titration are now presented, not on a basis of new principles but as simple and correct procedures which combine the desirable features of the various methods selected.
Preparation of the Samples The tissues obtained after necropsy are cleaned, washed, weighed, placed in individual containers, and kept in a refrigerator until ready for analysis. They are transferred to silica dishes, the containers are washed with hot water, the washings are transferred to the dish, and in the case of small samples, such as the heart, lungs, spleen, and kidneys, 50 to 100 cc. of a saturated solution of lime are added. To the large samples 2 to 15 grams of finely powdered calcium oxide are added, the amount depending on the size of the sample; to bones or teeth, no lime is added; to the liver of a guinea pig, 0.5 gram of lime is added; this is needed to prevent fusion on ashing, as well as loss of fluorine by volatilization. The samples are dried by keeping the dishes for several days on an electric heater at medium heat. The major part of fat-containing samples is burned by inserting s wick and igniting while on the hot plate. The dried samples are burned out in an electric muffle furnace at 600’ C. Teeth and bones require a higher temperature, 650’ to 700’ C. Liver samples fuse readily, unless a fairly large amount of lime has been added. Other samples do not fuse a t
