Determination of Sulfur in Coal by Perchloric Acid Method G. FREDERICK SNITHAND A. GARRELL DEEM,University of Illinois, Urbana, Ill. A solution and digestion method f o r the oxidacatalysts, the most satisfactory HE Eschka bomb-washwere c e r i u m n i t r a t e , chromic ing and sodium peroxide tion of coal using strong perchloric acid preparaacid, and vanadic acid. Cemethods are commonly tory to the determination of sulfur by precipitarium nitrate was found to be employed in the determination tion as barium sulfate is described, and the influeffective to a s m a l l e x t e n t , of sulfur in coal. Check deence of several catalysts for use in shortening the but was erratic and sometimes terminations of sulfur in coal caused a violent reaction. Hot time necessary for the destruction of organic a n d coke were c a r r i e d o u t concentrated p e r c h l o r i c acid using s a m p l e s f r o m various matter in the presence of hot 70 per cent perchloric oxidizes chromic and v a n a d y l sources of supply t h r o u g h o u t acid shown. salts to c h r o m i c a n d vanadic the United States by committee The advantages of this new method consist acid. Both of the latter subD-5 of the A m e r i c a n Society mainly in a saving of time with simultaneous stances are in turn reduced to for Testing Materials, and the elimination of the need for special equipment. lower stages of valence by the results p u b l i s h e d by Selvig organic matter in the coal and and F i e l d n e r ( I ) . S i x t e e n The former advantage has previously been gained reoxidized by the perchloric samples were studied by variwith the sacrijice of the latter. The method was acid until the oxidation of the ous testing l a b o r a t o r i e s and examined by the analysis of six samples of coal coal is complete. V a n a d i u m corporations, and b y the with thirty individual determinations and the added in the form of ammonium Bureau of Mines. The results v a n a d a t e Droved to be the results compared with the Eschka method. reported showed a wide enough most satisfactory. A sample of v a r i a t i o n b e t ween methods and operators to justify the study of a new method for this coke requiring 9 hours for oxidation with 70 per cent perdetermination. Both the bomb-washing method and the chloric acid a t approximately 200" C., without a catalyst, sodium peroxide-fusion method are in use as substitutes for could be completely oxidized in approximately 70 minutes the Eschka method because of their speed. The perchloric using chromium and in approximately 10 minutes using acid method, like the latter two named above, is fast and has vanadium as a catalyst. The increase in the velocity of the the added advantage that it requires no special apparatus such reaction with increase in concentration of the catalyst added as a bomb or muffle furnace. Since a similar successful is shown in Figure 1. method has been developed for the determination of sulfur in PREVENTION OF FROTHING AND RETARDATION OF OXIDATION rubber using perchloric acid, by Wolesensky (S), it was PERIOD.Samples of coal with volatile matter higher than 35 thought worth while to extend the application with nec- per cent often catch fire when boiled with 70 per cent peressary modifications to the analysis of coal. chloric acid and, also, coal samples often froth excessively during the early oxidation period. Both of these difficulties GENERAL CONSIDERATIONS were overcome by the use of from 2 to 4 grams of monoThe general subject of the oxidation of coal using per- chloroacetic acid. This reagent is slowly volatilized before chloric acid was studied under the following headings: the oxidation period is complete. FACTORS INFLUENCING OXIDATIONOF LOWERVALENCE F A C T O R B AFFECTING O X I D A T I O N OF O R Q A N I C MATTER 1. Acid strength SULFUR TO SULFURIC ACID. Hot concentrated perchloric 2. Temperature of reaction acid only partially oxidizes the element sulfur to sulfuric 3. Influence of catalysts 4. Influence of highly volatile combustible matter acid. Volatile compounds of sulfur, oxygen, and chlorine, 5. Use of monochloroacetic acid to Drevent frothing - during- digestion of sample yet unidentified, are formed. Sulfur present in the coal in a FACTORS AFFECTINGOXIDA,TIONOF SULFVR form other than the sulfate would not be expected to oxidize to 1. Partial oxidation of sulfur by perchloric acid sulfuric acid, using perchloric acid alone as an oxidizing agent. 2. Complete oxidation of sulfur using nitric acid plus potassium nitrate 3. Effect of catalyst on precipitation of barium sulfate This was found to be true, and nitric acid or potassium nitrate or a mixture of both was added to bring about the oxidation of These factors will, therefore, be discussed in the order given. INFLUENCE OF STRENGTH AND TEMPERATURE OF PER- the lower valence sulfur compounds to sulfuric acid. The addition of both monochloroacetic acid to prevent frothing and CHLORIC ACID. Over the temperature range of 180" to 200°C. 70 per cent perchloric acid completely oxidized equal portions nitric acid to oxidize sulfur completely lowered the boiling point of the same sample of a given coal or coke a t a rate roughly of the mixture but did not add appreciably to the oxidation twice as fast for each increase of 10" in temperature. The period for any given coal (15 to 40 minutes). The presence of chromium in the form of a chromic salt or average sample of coke requires 9 hours a t approximately 200" C. for complete oxidation. The average sample of coal as chromic acid and the presence of vanadic acid would be requires 3 hours for complete oxidation under similar condi- expected to result in their occlusion by the precipitated tions. Increasing the concentration of perchloric acid from 70 barium sulfate. With ammonium vanadate as catalyst this to 73 per cent divides the time interval required for complete difficulty is eliminated by reducing the vanadium to a vanadyl oxidation roughly by three. The time required even when salt by use of a small excess of hydroxylamine hydrochloride. using the 7 3 per cent perchloric acid is thus seen to be exces- Test precipitations of barium sulfate from perchloric acid sive. A study was made, therefore, to select a suitable solution in the presence of a chromate or vanadate did not result in appreciable occlusions, as shown by qualitative catalyst for the reaction. SELECTIONOF CATALYST.From an extensive list of examination. The vanadium used as catalyst was always
.
221
ANALYTICAL EDITION
228
reduced to a vanadyl salt by addition of hydroxylamine hydrochloride before the sulfuric acid was precipitated. The influence of the presence of perchloric acid on this precipitation has been described by Wolesensky (9). Unlike hydrochloric acid, perchloric acid may be present in considerable quantities without appreciable effect. OUTLINEOF ANALYTICAL METHOD Place accurately weighed gram samples of the coal in 300-cc. Erlenmeyer flasks together with a gram of potassium nitrate, 5 cc. of concentrated nitric acid, and 0.16 gram of ammonium vanadate. Add also 2 to 4 grams of monochloroacetic acid and 15 cc. of 70 per cent perchloric acid. Place the flask and contents on the hot plate at 120" C. with the flask uncovered. Digest for 10 minutes, and then heat on the hot
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period required to oxidize the sample completely to the production of vanadic acid. The addition of 0.5 gram of potassium nitrate indicates the need for more potassium nitrate, or, in its place, nitric acid. Analyses 4 to 9, inclusive (sample 40236), show the improvement in results by adding 1.0 gram of potassium nitrate or 5 cc. of concentrated nitric acid, with the greatest improvement effected by the use of 1.5 grams of potassium nitrate in the absence of nitric acid and a longer digestion period. These results indicate a preference in the use of both potassium nitrate and nitric acid to ensure complete oxidation of sulfur. Analyses 10 to 14, inclusive (sample 40391), show the effect of the applications of the principles suggested in the preceding paragraph. The results obtained are persistently less than those obtained by the Eschka method, but are very concordant in duplicate determinations. The time of digestion in this case was purposely extended beyond that required for complete oxidation of the coal, as was likewise the time of the remaining analyses, for the reason given in the subsequent discussion. Analyses 15 to 22, inclusive (samples 40300, 40438, and 40210), further parallel the conclusions of the above paragraph on three additional samples of coal. The analyses by the Eschka method (determinations 1 to 22, inclusive, Table I) were carried out a t the Bureau of Mines Experiment Station, Pittsburgh, who kindly furnished samples for this investigation. The sulfur determination was calculated on the basis of the air-dry weight following the usual treatment. OF ESCHKA AND PERCHLORIC ACID TABLEI. COMPARISON METHODS
MONOTIME CHLOROOF -SULFURHNOa, ACETIC HC1, REAC-Present No. 70% KNOa 68% ACID 37% TION Esohks, Found Ce. Qrama Cc. arums Cc. Min. % yo 15 4.32 2.56 40280 15 None None None None 35 4.32 3.12 40280 15 None None None None 20 4.32 4.04 40280 15 0.5 None None None 30 3.14 2.98 40236 15 1.0 None 3.0 None 30 3.14 2.98 40236 15 1.0 None 3.0 None
ANALY-COALHClOh BIS
"-1 10 to 30 40 $0 60 70 8'0 90 100 110 120 130 Tirne,Min.
FIGURE1. INFLUENCE OF VANADIUM AND CHRO MIUM AS CATALYSTS IN OXIDATIONOF COKE plate a t 180" to 185" C. until the coal is oxidized and an orange-red precipitate of vanadium pentoxide appears. This requires an additional 5 to 25 minutes, depending upon the nature of the coal, but is generally less than 10 minutes, Remove the sample from the hot plate, cool somewhat, and add 5 to 10 cc. of concentrated hydrochloric acid. Return to the hot plate and heat until the orange-red precipitate of vanadium is again obtained. The addition of the hydrochloric acid speeds the removal of the nitric acid which is difficult to evaporate away, especially if the necessary digestion period for the oxidation of the sample is short. Remove the sample at this point from the hot plate and add 100 cc. of water and 0.2 gram of hydroxylamine hydrochloride. Heat to reduce the vanadic acid to vanadyl salt, filter insoluble matter, including the dehydrated silica of the sample (hot concentrated perchloric acid completely dehydrates silica, a), and dilute the filtrate and washings with water to a 400 cc. volume. Heat to boiling and precipitate with barium chloride in the usual manner. Typical results are found in Table I. The first three analyses of Table I (sample 40280) show the effect of omitting potassium nitrate or nitric acid to ensure complete oxidation of sulfur to sulfuric acid. The oxidation of sulfur by hot perchloric acid is not complete even though the Oxidation period is extended for more than twice the
1 2 3 4 5
6 7 8 9 10
40236 40236 40236 40236 40391
10 None 5.0 10 None 5.0 15 1.5 None 15 1.5 None 15 None 5.0
2.0 None 2.0 None 3.0 10 3.0 10 3.0 10
11 12 13 14 15
40391 40391 40391 40391 40300
15 None 5.0 15 1.0 None 15 1.0 None 15 1.0 5.0 15 1.5 None
3.0 4.0 4.0 3.0 3.0
16 17 19 20
18
40300 40438 40438 40210 40210
15 15 15 15 15
1.5 1.5 1.5 1.0 1.0
None None None None None
3.0 3.0 3.0 3.0 3.0
21 22 23 24 25
40210 40210 40212 40212 40224
15 15 15 15 15
None None None None None
5.0 5.0 5.0 5.0 5.0
5.0 None 5.0 None 3.0 10 3.0 10 3.0 10
26 27 28 29
40224 40224 40224 40224
15 15 12 12
None None None None
5.0 5.0 5.0 5.0
3.0 3.0 3.0 3.0
Diff.
Yo -1.76 -1.20 -0.28 -0.16 -0.16
30 30 45 45 35
3.14 3.14 3.14 3.14 6.11
3.08 3.07 3.12 3.17 5.85
-0.06 -0.07 -0.02 +0.03 -0.26
35 55 55 50 45
6.11
10 10 10
5.86 5.84 5.98 5.81 1.52
-0.25 -0.27 -0.13 -0.30 -0.06
10 4 4 5 5
45 45 45 45 45
1.58 1.50 -0.08 2.07 1.94 -0.13 2.07 1.94 -0.13 1.06 0.97 -0.09 1.06 0.96 -0.10
10 10
10
10 10
10
6.11 6.11 6.11 1.58
100 100 35 35 120
1.06 1.06 3.50 3.50 2.17
0.99 0.97 3.58 3.59 2.13
-0.07 -0.09 'rO.08 -I-0.09 -0.04
120 50 50 60
2.17 2.17 2.17 2.17
2.12 2.16 2.12 2.12
-0.05 -0.01 -0.05 -0.05
Samples 12 and 24 were supplied through the courtesy of the analytical division of the University of Michigan and were of the type having high volatile matter (approximately 45 per cent). The agreement between the results for the perchloric acid method and the Eschka method are more satisfactory in these cases. In many cases (for example, samples 40210 and 40224) the time of digestion of the coal was purposely greatly extended over that necessary for complete oxidation. The average time required for the oxidation of the organic matter fell in the range 5 to 30 minutes, or approximately 15 minutes, for all samples of Table I. The preliminary digestion period of 10 minutes a t a temperature of 120" C. is advisable if the volatile
April 15, 1932
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
matter in the coal is not known to be less than 35 per cent. Longer periods of digestion (60 to 120 minutes) were employed in search of results indicating the loss of sulfuric acid by volatilization. This might be expected but was not found at the temperatures of the boiling 70 per cent perchloric acid (200” C.) under ordinary barometric pressures. (Compare analyses 21 and 22 with 19 and 20, as well as analyses 25 and 26 with 27 and 28.)
229
method of this paper, complete precipitation of the barium sulfate does not result (3). This error is not alone sufficient to account, except in part, for the low results obtained.
APPLICATION OF METHODTO ANALYSIS OF COKE The method as described was applied to the analysis of coke and gave very satisfactory comparisons in values obtained for some samples and very poor results for others. The disCOMPARISON OF ESCHRA AND PERCHLORIC ACID METHODS crepancies were not explained, and, until this can be done and the remedy found, the method is not recommended for sulfur A comparison of the Eschka and perchloric acid methods determinations in coke. for the determination of sulfur in coal is hardly justified on the basis of the present work when compared with the work of ACKNOWLEDGMENT
insoluble matter before barium sulfate is precipitated. This probably is a much more complete removal of silica from the solution before barium sulfateis precipitated than by the methods investigated by Selvig and Fieldner. Second, because of a comparatively high concentration of perchloric acid a t the time of precipitation of barium sulfate in the
LITERATURE CITED (1) SelVk a n d Fieldner, I N D . CHEM.9 19, 729 (1927). (2) Willard and Lake, J. Am. Chem. Soc., 42, 2208 (1920). (3) Wolesensky, ENG, 20, 1234 (1928).
cHEM.,
RECEIVED October 16, 1931.
Carbonate Content of Volumetric Sodium Hydroxide Solutions JOHN E. S. HANAND T. Y. CHAO,Y-1065CNorth Szechuen Road, Shanghai, China
T
The minute amount of carbonate present in tions were filtered through Jena HE best reagent quality lye??and sodium hydroside solutions treated sintered-glass crucibles, G4 (7). of s o d i u m h y d r o x i d e c o n t a i n s m o r e than 1 Filtration and siphoning were with various precipitants has been determined by per cent of sodium carbonate. carried out in absence of carbon various methods have been degravimetric and volumetric methods. Barium dioxide, vised for removing c a r b o n a t e hydroxide and salts were found to be the most MILK OF LIME (6) AND CAL from s o d i u m hydroxide soheffective precipitants. The technic of Warder’s CIUM CHLORIDE METHODS.Onemethod of differential titration has been improved hundred twenty-five grams of sotions intended for volumetric analysis. T h e s e m e t h o d s indium hydroxide were dissolved in and the results obtained checked within 0.02 per clude (1) precipitation with the water and made up to 2.7 liters. hydroxides or soluble salts of cent against the gravimetric evolution method. To this 300 cc. of milk of lime, the alkaline earths; (2) utilizing prepared from 20 grams of marthe small solubility of sodium carbonate in a concentrated ble lime, or 300 cc. of a solution containing 16 grams of calsolution of sodium hydroxide which is known as “oil lye” (14) ; cium chloride (dry neutral granules, Merck’s reagent) were (3) using metallic sodium with ether vapor as a barrier against added. The mixture was vigorously shaken for an hour and atmospheric carbon dioxide (3); and (4) employing an electro- allowed to settle for at least 4 days. lytically prepared sodium amalgam ( 1 , 5 ) . OIL LYE METHOD(14). Five hundred grams of sodium For general use, the precipitation and oil lye methods are hydroxide were dissolved in 500 cc. of water in a stoppered preferred for their simplicity and the ease with which large measuring cylinder of Jena glass. The total volume amounted quantities of solution can be prepared. The authors deter- to about 646 cc. A rubber stopper carrying a soda lime tube mined the small amounts of sodium carbonate remaining in and a siphon was fitted into the neck of the cylinder. The lye such solutions prepared under practical conditions, by both was clarified by heating near the boiling point of water (10) gravimetric and volumetric methods. in a specially constructed water bath for several hours, and cooled slowly. The water bath was provided with a false PREPARATION OF VOLUMETRIC SODIUMHYDROXIDE bottom and the measuring cylinder was completely enclosed SOLUTIONS except for the neck. Carbon dioxide-free water was used in all the experiments. For a normal solution, about 240 cc. of the oil lye from the The methods used for removing carbon dioxide from water, center of the container and sufficient water for diluting to 4.5 for storing and standardizing volumetric solutions, and for liters were siphoned into a 5-liter bottle without access of filling burets were the same as those described by Han and carbon dioxide, and mixed. METHODS USINGHYDROXIDES OR SALTS OF STRONTIUM AND Chu (4). The sodium hydroxide was of reagent quality (Merck’s pure), and a sample of it was found to contain 1.40 BARIUMAS PRECIPITANTS. For a 4 N stock solution (IS), per cent of sodium carbonate. All sodium hydroxide solu- 170 grams of sodium hydroxide were dissolved in water and
