Determination of Arsenic in Coal E. S. HERTZOG, Southern Experiment Station, U. S. Bureau of Mines, Tuscaloosa, Ala.
T
HE arsenic content of coal has received very little attention in the United States, and few of the procedures for the determination of arsenic in various materials are suited to the analysis of coal. The Bureau of Mines therefore undertook to develop a specific method for the determination of arsenic in coal. It seemed desirable to titrate the arsenic with some standard solution whose arsenic equivalent could be determined easily, rather than to depend on stains or mirrors that require considerable preliminary standardization before determinations on unknown samples can be made. As the arsenic content of coal is small, the usual titration methods applied to ores containing large amounts of arsenic would not be applicable without considerable modification. However, the method of Archbutt and Jackson (1) for the determination of arsenic in coke appeared to be well suited to the analysis of coal.
this assumption is incorrect. The amount lost would be of little consequence were it not for the fact that the arsenic content of coal is usually a few ten-thousandths of 1 per cent. It seemed desirable, therefore, to eliminate the use of nitric acid and avoid fuming.
A 5 mm. bore
Rubber tube: 5 mm. bore 10 mm.outride diameter
Method of Archbutt and Jackson Fifty grams of coke are digested with 100 cc. of hot concentrated nitric acid for at least 2 hours to extract the arsenic. Coal reacts so violently with nitric acid that the acid must be added in small portions and heated cautiously. The mixture is diluted and filtered and the residue discarded. The filtrate is evaporated to dryness, taken up with water and sulfuric acid, and evaporated to sulfuric acid fumes to drive off the nitric acid. The residue is then taken up with water and repeatedly evaporated t o sulfuric acid fumes to remove the last traces of nitric acid. The solution is placed in a distilling flask with ferrous sulfate and sodium chloride and the arsenic distilled off as arsenous chloride. The arsenous chloride and hydrogen chloride are caught in a small volume of water to which zinc sulfide is added to precipitate the arsenic, as arsenous sulfide. The bright-yellow precipitate is a good qualitative test for the presence of arsenic. It is seen more easily if hydrogen sulfide is used instead of zinc sulfide, but the zinc sulfide takes less time to precipitate the arsenic completely. This precipitate i s filtered off and dissolved by boiling in water. The resulting solution is concentrated, cooled, and titrated with 0.01 N iodine.
pit
Lead acetate cotton
Roll of lead acetate paper
FIQURE1. APPARATUS FOR GUTZEITTEST Nitric acid is also objectionable because of its noxious fumes and copious foaming, which make treatment of the mixture difficult in any vessel of reasonable size. Even after the coal is filtered off, the filtrate is likely to foam and cause trouble throughout the test. Foaming is especially undesirable in a Gutzeit bottle. A procedure in which the carbonaceous matter is burned off and foaming thereby eliminated is more practicable. Some investigators have ignited coal with lime, sodium carbonate, or similar substances, relying on these reagents to retain the arsenic during ignition. The ash is then extracted with nitric acid and the nitric acid eliminated by fuming with sulfuric acid. In the method finally adopted in this investigation the residue after ignition was treated direct with sulfuric acid, so that there was no nitric acid to remove.
The Archbutt and Jackson method is accurate to within 0.0005 per cent of arsenous oxide. This is satisfactory for high-arsenic coals, but virtually all the American coals investigated contained less than 0.002 per cent of arsenous oxide, for which a more sensitive method was necessary.
Method Based on Gutzeit Test The most accurate methods for estimating extremely small amounts of arsenic are the Gutzeit and the Marsh tests, both depending on the liberation of the arsenic as arsine. Of these the Gutzeit seems to be more practicable for use in the average chemical laboratory. It can easily detect 0.000001 gram of arsenous oxide, as this amount will produce a stain 0.3 cm. (0.125 inch) long, The methods of decomposing and preparing samples for the Gutzeit test are numerous, and several of those recommended for coal have been tested. One is similar to that used by Archbutt and Jackson-the sample of finely powdered coal is digested with concentrated nitric acid and the nitric acid expelled by fuming with sulfuric acid. I n other methods the coal is digested with both nitric and sulfuric acids, but in all of them the nitric acid is eliminated by fuming with sulfuric acid. It is assumed that virtually no arsenic is lost during this fuming process; however, the results of a number of the bureau’s experiments indicate that
Method Developed The procedure developed, although similar to many given in the literature, is an attempt to bring together the best features of all. APPARATUS.The apparatus used is almost the same as that described by Scott (2) but is a little easier to make. To bore a straight hole through a very small rubber stopper is rath& difficult, so the two tubes are joined with a small piece of rubber tubing. The constriction in the lower tube is not absolutely necessary and so has been eliminated. Figure 1 163
164
TABLEI. RECOVERY OF ARSEXIC Run Coal used, gram AsaOa added, per cent As208 found, per cent
-
VOL. 7, NO. 3
INDUSTRIAL AND ENGINEERIXG CHEMISTRY
1 1 None 0.0004
2 1 None 0.0004
3 1
0.0005 0.0009
4 1 0.0005 0.0009
~~
TABLE11. RECOVERY OF ARSENIC Run Coal ueed, ram AIOS added, per cent AszOs found, per oent
1
2
3
4
1 None 0.0005
1 None 0.0006
1 0.0005 0,0010
1 0.0005 0.0009
shows the apparatus and its dimensions. This apparatus can be made from materials available in almost any chemical laboratory and requires no special knowledge of glass-blowing technic. The constriction in the small tube is very important, as it concentrates the flow of gas and directs it a t the mercuric bromide paper. More important, the extreme lower end of the paper falls into this constriction and is held in the exact center of the tube. If the constriction were not there, even though the paper were perfectly straight the fumes probably would cause it to curl to one side and direct all the gas to that side of the paper, making a long stain on one side and a shorter one on the other. TESTPROCEDURE. The following procedure gave the most consistent results and is recommended as the most reliable: Weigh out a 1-gram sample of minus 100-mesh coal and mix thoroughly with 0.8 gram of a mixture of 5 parts of sodium carbonate 3 parts of magnesium oxide, and 1 part of potassium nitrate. kace the sample and mixture in a low porcelain or platinum crucible, set in a cold muffle, and raise the tem erature to between 700" and 750" C. in 1 hour. The ignition slould be continued about 1.5 hours longer to burn out all the chrbon. If the temperature does not exceed 750" C. there will be little trouble with sintering. Remove from muffle, cool and wet residue with a fine stream of water (2 or 3 cc.), washing down sides of crucible. Add concentrated arsenic-free sulfuric acid drop by drop slowly whiIe stirring until solution is acid to litmus. Wash the contents of the crucible into the Gutzeit bottle. Add 3.5 cc. of concentrated sulfuric acid and, in order, 2 cc. of ferric ammonium alum and 0.5 cc. of stannous chloride from a graduated pipet. Dilute to approximately 45 cc. with distilled water, stir until thoroughly mixed, and place bottle in pan of water at approximateIy 25" C. Load the lower tube with a roll of lead acetate paper and lead acetate cotton, connect apparatus, and place mercuric bromide paper in the upper tube. Place two pieces of stick zinc in the bottle and cork immediately with the upper part of the apparatus. After 50 to 60 minutes remove the test paper, dip in melted paraffin, and compare with standards. STANDARD STAINS. In preparing the standard stains add the arsenic solution, dilute with a little water, add 3 cc. of concentrated sulfuric acid, instead of 3.5 cc. as for the samples, and then add the other reagents as described. Standard stains should be made for 0.000, 0.002, 0.004, 0.006, 0.008, 0.010, 0.013, and 0.017 mg. of arsenic trioxides. The stains should be dipped in melted paraffin, not too hot, for a second and then cooled, They may be mounted on a piece of cardboard and kept in a calcium chloride desiccator in a dark place when not in use. The standard will fade gradually with time, but may be photographed to exact size and the print used in place of the stains, With a little practice the reading can be made as accurately as with standard stains, and the print will last indefinitely. Curves plotted through the tops of the stains can be used in place of a set of standards. The effect of ammonium hydroxide and hydrochloric acid in developing the stain was tested but was not thought to have any practical value.
Discussion of Results Duplicate determinations by this method checked very well, but failed to prove that no arsenic was lost by volatilization.
It seemed reasonable to suppose that any method capable of retaining arsenous oxide added in the form of a standard solution would be likely to hold arsenic in any other form in the coal. A sample of Black Creek coal analyzing 1.56 per cent ash, 0.64 per cent sulfur, and 0.25 per cent iron was selected for the test. As the coal was of high purity, the effect of a large amount of inorganic matter was eliminated. Duplicate determinations were made with and without addition of arsenic. The results are given in Table I, and show that all the arsenic added was obtained and recovered. The same experiment was repeated with a Montana coal analyzing 11.72 per cent ash, 1.22 per cent sulfur, and 1,02 per cent iron, to see if the results were equally satisfactory where the inorganic matter was relatively high. The results shown in Table I1 indicate that all the arsenic is retained and that all of it can be recovered by the Gutzeit test. Table I11 gives the results of duplicate determinations on a number of coals for fixed and total arsenic by the Gutzeit method and shows haw closely the results may be expected to check. Fixed arsenic is the arsenic found in the coal after it has been ashed without anything added to retain the arsenic. Duplicate runs by the method of Archbutt and Jackson are also shown and -check the Gutzeit method very well within the limits of their error, which is certainly not greater than 0.0005 per cent. TABLE111. COMPARISON OF DUPLICATE ANALYSESOBTAINED BY GUTZEIT AND ARCHBUTT-JACKSON METHODS --Method
Sample
Bed
State
of Analysis----
% Jefferson
2 3
I
4
5 6
7 8 9 10
11
12 13
14
TABLEIv. Sample 11 2 3 6 9 13 5 7 8 4 12 14 10 1
Ala.
0.0130 0.0120 0.0004 Black Creek Ala. 0,0004 Corona Ala. 0 0007 0.0007 0.0015 Mary Lee Ala. 0.0015 0.0007 Pittsburgh Pa 0.0010 0,0005 Alma W. Va. 0.0006 No. 6 111. 0.0007 0.0007 Weir-Pittsburgh Kans. 0.0007 0.0007 0,0004 Bear Creek Mont. 0.0004 No. 3 Iowa 0.0008 0.0012 Wadge Colo. Trace Trace No. 9 Ky. (western) 0.0008 0.0008 0.0006 Pittsburgh W. Va. 0.0007 0.0014 Lexington hlo. 0.0006
1
%
0.0140 0.0140 0.0004 0.0004 0.0007 0.0007 0,0015 0.0015 0.0014 0.0012 0.0008 0.0007 0,0013 0.0013 0.0014 0.0014 0.0009 0.0009 0 * 0020 0.0022 0.0001 0.0001 0.0016 0.0015 0.0011 0.0009 0.0016 0.0018
0.0143 0.0139 0,0002 0.0002 0.0005 0.0005 0.0011 0.0011 0.0009 0.0013 0.0006 0.0003 0.0010 0.0011 0.0010 0.0005 0.0005 0.0003 0.0026 0.0023 0.0002 0.0002 0.0012 0.0012 0.0003 0.0004 0.0021 0.0020
ANALYSES IN ORDER ARSENIC CONTENT^ State
Colo. Wadge Ala. BlackCreek Ala. Corona W. Va. Alma Mont. Bear Creek W. Va. Pitbeburgh Pa. Pittsburgh Ill. No.6 Weir-Pittsbm 'gh Kans. Ala. Mary Lee &I (western) No. 9 Lexington Iowa No. 3 Ala. . Jefferson
Fixed
SsnOa
%
O F INCREASMG
SUMM.4RY O F
Bed
Archbutt and Jackson Total
Modified Gutzeit Fixed Total AsaOa AszOs
Total
AsaOs
AszOp
%
%
Trace 0.0004 0.0007 0.0006 0.0004 0.0007 0.0009 0.0007 0.0007 0.0015 0.0008 0.0010 0.0010 0.0130
0.0001 0.0004 0.0007 0,0008 0.0009 0.0010 0.0013 0.0013 0.0014 0.0015 0.0015 0.0017 0.0021 0.0140
Iron Sulfur
% 0.19 0.25 0.91 0.86 1.02 1.23 0.82 2.64 2.10 1.23 2.82 2.95 5.02 2.37
Arsenic determmations by modified Gutzeit method.
% 0.45 0.64 1.98 1.70 1.22 2.76 1.55 4.47 3.29 1.36 4.44 4.76 7.17 3.35
Ash
% 7.29 1.56 8.54 5.62 11.72 7.35 6.37 10.45 7.28 18.12 14.50 19.46 19.12 6.79
MAY 15, 1935
ANALYTICAL EDITION
Table IV summarizes the analyses of all the samples tested in the order of increasing total arsenic. The arsenic does not increase in direct proportion to iron, sulfur, or ash, although there seems to be a general trend in that direction. However, as shown in Table V, in general the arsenic content increases as the other inorganic constituents increase. TABLEv. Number of Samples Averaged 2 4 4 4
AVER.4GE
ANALYSESO F GROUPSTAKEN FROM TABLE IV
Total AsiOa
Iron
Sulfur
%
%
%
%
0.0003 0.0009 0.0014 0.0048
0.22 1.01 1.70 3.29
0.55 1.92 2.67 4.93
4.43 5.31 9.81 14.97
A8 h
165
However, most of the analyses check within 0.0003 per cent, indicating that the arsenic is probably diffused in extremely h e particles throughout the coaly substance and is not concentrated in particles of arsenopyrite of any appreciable size.
Acknowledgments The author wishes to acknowledge his indebtedness to A. C. Fieldner, who suggested and directed the course of this investigation; to Alden H. Emery, B. W. Gandrud, and E. P. Barrett for many helpful suggestions; and to H. H. Schrenk for check determinations and careful criticism of the manuscript.
Literature Cited (1) Archbutt L., and Jackson, P. B., J . SOC.Chem. Znd., 20, 448-50
Good checks in arsenic analyses were obtained with 1-gram samples of minus 100-mesh coal. For the high-arsenic Jefferson coal 0.1-gram samples were used satisfactorily. One cube of arsenopyrite which would just pass the opening in a 100-mesh screen would add 0.001 per cent of arsenous oxide t o a 1-gram sample.
(1901). (2) Scott, W. W , “Standard Methods of Chemical Analysis,” 4th ed., PP. 46-52, New York, D. Van Nostrand Co., 1925. R E C ~ I ~ November ED 10, 1934. Presented before the Division of Gas emistry at the 89th Meeting of the American Chemical Society, N. Y , April 22 to 26, 1935. Published by permission of the Director, U. S. Bureau of Mines (Notsubject to copyright.)
Volumetric Determinations of Halides Use of Dichlorofluorescein as an -Adsorption Indica KARL BAJIBACH a n d T. H. RIDER, The Wm. S. Rlerrell Co., Cincinn
A
DSORPTION indicators for the argentometric determination of halides were suggested by Fajans and his collaborators (3, 4) ; and Kolthoff, Lauer, and Sunde (5) chose dichlorofluorescein as the most suitable substance for the purpose. They gave the results obtained with dichlorofluorescein in the argentometric titration of chlorides, and indicated that equally satisfactory results could be expected in the titration of bromides and iodides. Their work is mentioned in a recent textbook ( 7 ) on quantitative analysis, which stresses the value of their method only in the determination of chloride. Osterberg (8) used dichlorofluorescein as an adsorption indicator for the estimation of chlorides in the blood, carrying out the titrations in an acetone-water solution. During the past two years dichlorofluorescein has been used in these laboratories in many routine titrations with complete satisfaction. This paper will describe the use of the indicator for the analysis of organic hydrochlorides in alcohol solutions, and will give analytical data concerning the argentometric titration of bromides and iodides. The chlorine analysis of Diothane (piperidinopropanediol dipheriylurethan hydrochloride, 1, 9), a new local anesthetic, is an important control test. It is desirable to carry out such analytical work on Diothane with the sample in alcohol solution, and it was found that dichlorofluorescein is as satisfactory an indicator in 75 per cent alcohol solutions as in aqueous solutions. Other organic hydrochlorides prepared in the Merrell research laboratories and some anesthetics on the market have also been analyzed in this way. Halogen determinations on many inorganic compounds have been carried out with dichlorofluorescein as the indicator; among them may be mentioned ammonium chloride, ammonium bromide, ammonium iodide, sodium chloride, sodium bromide, sodium iodide, calcium bromide, potassium
bromide, potassium iodide, mercuric chloride (after previous removal of the mercury), and hydriodic acid. The analyses of these inorganic salts were all carried out in aqueous solution.
Titration of Organic Hydrochlorides The 0.05 N silver nitrate solution used was standardized against dried pure sodium chloride (Mallinckrodt analytical reagent) with the following procedure: Approximately 0.12 gram of the sodium chloride was weighed and dissolved in 80 cc. of 75 per cent alcohol, 8 drops of dichlorofluorescein solution (0.1 per cent Eastman indicator, catalog No 373, in 70 per cent alcohol) were added, and the solution was titrated with the silver nitrate until the coagulated silver chloride precipitate acquired a distinct pink color. This pink color on the silver chloride was taken as the end point; the addition of a few more drops of the 0.05 N silver nitrate caused a pink color through the entire solution. TABLEI. TYPICAL TITRATIONS NaCl Grams 0.1171 0.1195 0.1187 0.1184
AgNOa Solution cc. 40.40 41.22 40.95 40.93
Normality of AgNOa Solution 0.04959 0.04958
0 04959
0 04950
In Diothane analyses a sample of about 0.85 gram was dissolved in 80 cc. of 75 per cent alcohol and the titration was carried out with the 0.05 N silver nitrate solution in the manner described above. Some results are: 8.17 per cent chlorine, 4 batches (Nos. 67751, 67977, 68131, 68593); 8.18 per cent chlorine, 2 bgtches (NOS,67369, 68362); 8.16 per cent chlorine, 1 batch (Yo. 67934); theoretical, 8.17 per cent chlorine.
