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The rapid determination of sulfur in coal
480
((:I Elscvicr Scicntik
SHORT
JACKSON (Rcceivcd
Analyricu
Clzirniro Actu. 68 ( 1974) 380483 Amsterdam - Printed in The Netherlands
COMMUNICATION
The rapid determination
Scrc:ircs
Publishing Company.
E. HICKS.
Anrrlytirctl
of sulfur in coal
JAMES
Ldwroror?:
E. FLEENOR
and HAROLD
Tcr~rw.s.wc~Easrr~rt~ Cottpu~~:
R. SMITH Kimgsport.
Tcwr.
37662 (U.S.A.)
39th April 1973)
The emission of sulfur oxides into the atmosphere from the combustion of fossil fuels is now regulated by both Federal and State legislation; coals are classified not only according to B.T.U. and ash value but also according to the sulfur content. The Eschka method’ has long been the recognized reference method for the determination of sulfur in coal. but this tedious procedure is unsuitable for routine control. Many combustion techniques have been used to replace the Eschka method; Selvig and Fieldner2 showed that the bomb-washing and sodium peroxiqe fusion methods gave results which agreed closely enough with the Eschka method for routine purposes. Laboratories making calorimetric determinations on coal find it convenient to determine sulfur in the washings from the bomb calorimeter; commonly, the time-consuming and error-prone3 gravimetric barium sulfate determination is applied, and a faster precise analytical finish is needed. Ross and Frant3 and Selig’ have described the determination of sulfate by potentiometric titration with lead perchlorate and a lead ion-selective electrode. This communication discusses the application of this titration to the determination of sulfur in coal, after the sample has been burned in an oxygen bomb. The procedure is more rapid and precise than the standard gravimetric technique. Apparatus. A Metrohm Potentiograph E-436 Recording Titrator equipped with an E-436-D Multi-titration Stand was used. An Orion model 94-82 solid-state lead-selective electrode was used in conjunction with an Orion double-junction reference electrode, model 90-02. The salt bridge compartment of the reference electrode was filled with 1 M sodium nitrate. All samples were burned in a Parr 1241 automatic calorimeter with the Parr 1108 stainless steel bomb. Sotiitrnz sulfhtr, 0.01 M. Dissolve eiactly 1.4205 g of analytical reagentgrade sodium sulfate, which has been dried for 2 h at 1 lo”, in distilled water and dilute to 1 1. Flo~er.s oJ’ sulji~r (99.9 + purity) and cystitze (N.B.S. Standard Reference Material 143A) were also used. Procedure. Thoroughly mix the sample of coal and dry for 1 h at 105”. Accurately weigh ( +O.l mg) cu. 1 g of dried coal into the metal capsule used
SHORT
COMMUNICATION
481
in the Parr bomb. Add 1 ml of distilled water to the bomb. Set up the bomb and pressurize to 25 atmospheres with oxygen. Burn the sample using the normal practices involved in determining the calorific value of coal. Remove the bomb and loosen the pressure release valve to exhaust the gas over a period not to exceed 1 min. Flush the release valve with distilled water and wash the electrodes, capsule and bomb head into the bomb. Wash the contents of the bomb into a loo-ml beaker with about 50 ml of wash water. Add 2 drops of 0.5% (w/v) phenolphthalein in methanol to the combined washings and neutralize with 1 M potassium hydroxide solution. Add dilute perchloric acid ( 15 ml of 85% acid diluted to 1 1) dropwise until the pink color fades, and add one drop in excess. Add 50 ml of reagent-grade 1,4-dioxane and place the electrodes in the beaker. Start the titration with 0.05 M lead perchlorate, automatically record the titration curve, and determine the equivalence point. Standardize the titrant against 0.01 M sodium sulfate solution. Inasmuch as most people who are interested in the sulfur content of coal are also interested in the calorific value, a determination of the bomb washings is attractive. The use of a recording titrator and an ion-selective electrode provides the rapidity and precision desired. It is important that the air should not be flushed from the bomb during pressurization with oxygen. The. oxides of nitrogen formed during combustion act as a catalyst in converting sulfur completely to sulfur trioxide and prevent the formation of sulfur dioxide”. The lead-selective electrode develops a potential proportional to the logarithm of the activity of the free lead ion in the titrated solution. It is reasonably selective but is poisoned by copper, mercury and silver ions4. The following ions interfere4*’ in the titration of sulfate with lead perchlorate: 50-fold amounts of nitrate or chloride, and IO-fold amounts of hydrogencarbonate at pH 6 (lOO-fold amounts at pH 4); phosphate must be absent. It was shown that the titration of sodium sulfate in 50 ‘z, dioxane was virtually unaffected by pH in the apparent pH range 4-9. The recommended procedure provides a pH of 6-7. It is important that a double-junction reference electrode be used. The outer chamber is filled with 1 M sodium nitrate. With a single-junction electrode, chloride ion from the filling solution may precipitate lead or form lead-chloro complexes. Although the potential of the Orion lead electrode varies with temperature, the only requirement for the present purpose is that the temperature should not calibration of the electrode is ‘not change much during the actual titration; required. The concentration of sulfur in most coal samples is such that no problems were caused by insufficiently fast response of the electrode. The automatic titrator used slowed down the titration rate near the inflection portion of the curve. As recommended by Ross and Frant 4, dioxane was used to reduce the solubility of lead sulfate so that sharp end-points would be obtained; 50 ‘x, dioxane was satisfactory whereas more than 70 “A,caused alkali metal sulfates to precipitate. After combustion of the sample, the sulfur content can be determined in less than 5 min. The potential jump at the end-point of a typical titration curve is CU. 150 mV. The electrode can be stored in air or in a dilute lead solution. After lo-15 titrations. the electrode surface usually requires cleaning with the abrasive
482
SHORT
COMMUNICATION
plastic supplied or with Buehler 4/O Emery polishing paper. The response of the electrode becomes sluggish, reproducibility becomes poor, and the break in the titration curves becomes smaller as the surface of the sensing element becomes coated. Initially, when the pressure of the bomb was released after combustion, the gas was Lented through a tube immersed in water to prevent losses. It was soon found that this step was not required. The most crucial step of the manipulations is the washing of the pressure release valve and bomb electrodes. The pressure release valve espedially must be carefully back-flushed into the bomb to prevent losses. For a series of analyses of sodium sulfate solution, the proposed titrimetric method gave an average value of 0.80 “/, S with a standard deviation of 0.017 “/, (10 determin8tions); the gravimetric determination with barium chloride, which took about 4 h, gave an average value of 0.87(x, S, with a standard deviation of 0.054 X,. Ten determinations of sulfur in a typical coal by the proposed method gave an average value of 0.79 ‘x, S with a standard deviation of 0.027 7;. Known amounts of sulfur as flowers of sulfur or cystine were added to samples of coal and activated carbon which were ‘then analyzed by the proposed method. Sample mixtures were usually prepared in a ball mill for 5-6 h, but TABLE
i
RECOVERY CARBON
Flowrs
OF
of .sul/irr
Flowers
Cysritw 0.14 0.14 0.14 0.14 0.14 0.14 0.14
~cldetl
IO
OF
oj’ d&w
SULFUR
AND
CYSTINE
ADDED
TO
COAL
OR
ACTIVATED
cod
0 0.10 0.24 0.4 I 0.61
3.62 0.62 0.62 0.62 0.62
0.14 0.14 0.14 0.14 0.14
FLOWERS
0.62 0.72 0.86 I .03 1.23
0.62 0.70 0.87 0.99 1.15
0 - 0.02 +0.01 - 0.04 - 0.08
0.37 0.68 1.02 1.19 1.95 2.27
0.36 0.68 0.98 1.32 1.85 2.27
-0.01 0 -0.04 +0.13 -0.10 0
0.72 0.97 1.46 2.19 2.26 2.3 1 5.54
0.80 1.05 1.41 2.32 ’ 36 _._ 2.16 5.32
+ 0.08 + 0.08 - 0.05 +0.13 0 -0.15 - 0.22
ctdtletl 10 ccrrlwn 0.23 0.54 0.88 1.05 1.81 0.142.13
trtlckd to wrhori 0.58 0.83 1.32 2.05 2.12 2.17 5.40
.-
SHORT
COMMUNICATION
483
’
cystine did not mix well and so was weighed directly into the bomb capsule. Table I shows the results obtained, which indicate a satisfactory recovery of sulfur over the range O.36-5.54”/0 sulfur. The sulfur contents of three standard coal samples (N.B.S., S.R.M. 163 1) were determined by the proposed method. The results (Table II) agreed quite closely, though sample B consistently gave results lower than the certified value, possibly because of its higher ash content. Some insoluble sulfate may have formed during combustion resulting in slightly lower values. TABLE
II
DETERMINATION
Cod
scll,lplc
OF
SULFUR
IN COAL
Sitljicr by titratiim”
(NBS
STANDARD
SIdfIll
REFERENCE
MATERIAL
1631)
A.4
Certi$iicrl vcrliie.\h A B C
0.549 1.92 2.99
” Avcragc of 12 determinations. b For these, each participating laboratory _
0.546 + 0.003 2.016&0.014 3.020 _C0.008
rcportcd
5.00 & 0.02 14.59 -t_0.09 6.1720.02
the avcragc of 12 determinations.
The data reported here indicate that the determination of sulfur in coal by combustion, followed by automatic titration of the bomb-washings with lead perchlorate solution and a lead-selective electrode is satisfactory. As previously stated2, the bomb-washing approach provides a satisfactory recovery of sulfur from coal samples. With respect to speed, precision, recovery and ease of operation the proposed method is excellent. The authors are grateful to Ruth S. Jessee who prepared and carbon mixtures.
most of the sulfur, coal
REFERENCES ASTM D-271. Sonrplhg uml A~1~se.s u/’ Cod untl Coke. Sections 21 to 24. 197 I. W. A. Selvig and A. C. Ficldncr. It~cl. EII~. C/rem, 19 (1927) 729. I. M. Kolthoff, E. B. Snndell, E. J. Mcchan nnd S. Bruckcnstcin. QItrrrlfitcltioc MacMillan. New York, 4th Ed., 1969. pp. 603-610. J. W. Ross, Jr. and M. S. Frant, Ard. Clrerrr.. 41 (1969) 967. W. Sulig. Mikrochim. Acta. (1970) 168. S. H. Regcster, In{/. EM{/. Chrn.. 6 ( 1914) 812.
Chenricd
Afu/ysi.s.
((:I Elscvicr Scicntik
SHORT
JACKSON (Rcceivcd
Analyricu
Clzirniro Actu. 68 ( 1974) 380483 Amsterdam - Printed in The Netherlands
COMMUNICATION
The rapid determination
Scrc:ircs
Publishing Company.
E. HICKS.
Anrrlytirctl
of sulfur in coal
JAMES
Ldwroror?:
E. FLEENOR
and HAROLD
Tcr~rw.s.wc~Easrr~rt~ Cottpu~~:
R. SMITH Kimgsport.
Tcwr.
37662 (U.S.A.)
39th April 1973)
The emission of sulfur oxides into the atmosphere from the combustion of fossil fuels is now regulated by both Federal and State legislation; coals are classified not only according to B.T.U. and ash value but also according to the sulfur content. The Eschka method’ has long been the recognized reference method for the determination of sulfur in coal. but this tedious procedure is unsuitable for routine control. Many combustion techniques have been used to replace the Eschka method; Selvig and Fieldner2 showed that the bomb-washing and sodium peroxiqe fusion methods gave results which agreed closely enough with the Eschka method for routine purposes. Laboratories making calorimetric determinations on coal find it convenient to determine sulfur in the washings from the bomb calorimeter; commonly, the time-consuming and error-prone3 gravimetric barium sulfate determination is applied, and a faster precise analytical finish is needed. Ross and Frant3 and Selig’ have described the determination of sulfate by potentiometric titration with lead perchlorate and a lead ion-selective electrode. This communication discusses the application of this titration to the determination of sulfur in coal, after the sample has been burned in an oxygen bomb. The procedure is more rapid and precise than the standard gravimetric technique. Apparatus. A Metrohm Potentiograph E-436 Recording Titrator equipped with an E-436-D Multi-titration Stand was used. An Orion model 94-82 solid-state lead-selective electrode was used in conjunction with an Orion double-junction reference electrode, model 90-02. The salt bridge compartment of the reference electrode was filled with 1 M sodium nitrate. All samples were burned in a Parr 1241 automatic calorimeter with the Parr 1108 stainless steel bomb. Sotiitrnz sulfhtr, 0.01 M. Dissolve eiactly 1.4205 g of analytical reagentgrade sodium sulfate, which has been dried for 2 h at 1 lo”, in distilled water and dilute to 1 1. Flo~er.s oJ’ sulji~r (99.9 + purity) and cystitze (N.B.S. Standard Reference Material 143A) were also used. Procedure. Thoroughly mix the sample of coal and dry for 1 h at 105”. Accurately weigh ( +O.l mg) cu. 1 g of dried coal into the metal capsule used
SHORT
COMMUNICATION
481
in the Parr bomb. Add 1 ml of distilled water to the bomb. Set up the bomb and pressurize to 25 atmospheres with oxygen. Burn the sample using the normal practices involved in determining the calorific value of coal. Remove the bomb and loosen the pressure release valve to exhaust the gas over a period not to exceed 1 min. Flush the release valve with distilled water and wash the electrodes, capsule and bomb head into the bomb. Wash the contents of the bomb into a loo-ml beaker with about 50 ml of wash water. Add 2 drops of 0.5% (w/v) phenolphthalein in methanol to the combined washings and neutralize with 1 M potassium hydroxide solution. Add dilute perchloric acid ( 15 ml of 85% acid diluted to 1 1) dropwise until the pink color fades, and add one drop in excess. Add 50 ml of reagent-grade 1,4-dioxane and place the electrodes in the beaker. Start the titration with 0.05 M lead perchlorate, automatically record the titration curve, and determine the equivalence point. Standardize the titrant against 0.01 M sodium sulfate solution. Inasmuch as most people who are interested in the sulfur content of coal are also interested in the calorific value, a determination of the bomb washings is attractive. The use of a recording titrator and an ion-selective electrode provides the rapidity and precision desired. It is important that the air should not be flushed from the bomb during pressurization with oxygen. The. oxides of nitrogen formed during combustion act as a catalyst in converting sulfur completely to sulfur trioxide and prevent the formation of sulfur dioxide”. The lead-selective electrode develops a potential proportional to the logarithm of the activity of the free lead ion in the titrated solution. It is reasonably selective but is poisoned by copper, mercury and silver ions4. The following ions interfere4*’ in the titration of sulfate with lead perchlorate: 50-fold amounts of nitrate or chloride, and IO-fold amounts of hydrogencarbonate at pH 6 (lOO-fold amounts at pH 4); phosphate must be absent. It was shown that the titration of sodium sulfate in 50 ‘z, dioxane was virtually unaffected by pH in the apparent pH range 4-9. The recommended procedure provides a pH of 6-7. It is important that a double-junction reference electrode be used. The outer chamber is filled with 1 M sodium nitrate. With a single-junction electrode, chloride ion from the filling solution may precipitate lead or form lead-chloro complexes. Although the potential of the Orion lead electrode varies with temperature, the only requirement for the present purpose is that the temperature should not calibration of the electrode is ‘not change much during the actual titration; required. The concentration of sulfur in most coal samples is such that no problems were caused by insufficiently fast response of the electrode. The automatic titrator used slowed down the titration rate near the inflection portion of the curve. As recommended by Ross and Frant 4, dioxane was used to reduce the solubility of lead sulfate so that sharp end-points would be obtained; 50 ‘x, dioxane was satisfactory whereas more than 70 “A,caused alkali metal sulfates to precipitate. After combustion of the sample, the sulfur content can be determined in less than 5 min. The potential jump at the end-point of a typical titration curve is CU. 150 mV. The electrode can be stored in air or in a dilute lead solution. After lo-15 titrations. the electrode surface usually requires cleaning with the abrasive
482
SHORT
COMMUNICATION
plastic supplied or with Buehler 4/O Emery polishing paper. The response of the electrode becomes sluggish, reproducibility becomes poor, and the break in the titration curves becomes smaller as the surface of the sensing element becomes coated. Initially, when the pressure of the bomb was released after combustion, the gas was Lented through a tube immersed in water to prevent losses. It was soon found that this step was not required. The most crucial step of the manipulations is the washing of the pressure release valve and bomb electrodes. The pressure release valve espedially must be carefully back-flushed into the bomb to prevent losses. For a series of analyses of sodium sulfate solution, the proposed titrimetric method gave an average value of 0.80 “/, S with a standard deviation of 0.017 “/, (10 determin8tions); the gravimetric determination with barium chloride, which took about 4 h, gave an average value of 0.87(x, S, with a standard deviation of 0.054 X,. Ten determinations of sulfur in a typical coal by the proposed method gave an average value of 0.79 ‘x, S with a standard deviation of 0.027 7;. Known amounts of sulfur as flowers of sulfur or cystine were added to samples of coal and activated carbon which were ‘then analyzed by the proposed method. Sample mixtures were usually prepared in a ball mill for 5-6 h, but TABLE
i
RECOVERY CARBON
Flowrs
OF
of .sul/irr
Flowers
Cysritw 0.14 0.14 0.14 0.14 0.14 0.14 0.14
~cldetl
IO
OF
oj’ d&w
SULFUR
AND
CYSTINE
ADDED
TO
COAL
OR
ACTIVATED
cod
0 0.10 0.24 0.4 I 0.61
3.62 0.62 0.62 0.62 0.62
0.14 0.14 0.14 0.14 0.14
FLOWERS
0.62 0.72 0.86 I .03 1.23
0.62 0.70 0.87 0.99 1.15
0 - 0.02 +0.01 - 0.04 - 0.08
0.37 0.68 1.02 1.19 1.95 2.27
0.36 0.68 0.98 1.32 1.85 2.27
-0.01 0 -0.04 +0.13 -0.10 0
0.72 0.97 1.46 2.19 2.26 2.3 1 5.54
0.80 1.05 1.41 2.32 ’ 36 _._ 2.16 5.32
+ 0.08 + 0.08 - 0.05 +0.13 0 -0.15 - 0.22
ctdtletl 10 ccrrlwn 0.23 0.54 0.88 1.05 1.81 0.142.13
trtlckd to wrhori 0.58 0.83 1.32 2.05 2.12 2.17 5.40
.-
SHORT
COMMUNICATION
483
’
cystine did not mix well and so was weighed directly into the bomb capsule. Table I shows the results obtained, which indicate a satisfactory recovery of sulfur over the range O.36-5.54”/0 sulfur. The sulfur contents of three standard coal samples (N.B.S., S.R.M. 163 1) were determined by the proposed method. The results (Table II) agreed quite closely, though sample B consistently gave results lower than the certified value, possibly because of its higher ash content. Some insoluble sulfate may have formed during combustion resulting in slightly lower values. TABLE
II
DETERMINATION
Cod
scll,lplc
OF
SULFUR
IN COAL
Sitljicr by titratiim”
(NBS
STANDARD
SIdfIll
REFERENCE
MATERIAL
1631)
A.4
Certi$iicrl vcrliie.\h A B C
0.549 1.92 2.99
” Avcragc of 12 determinations. b For these, each participating laboratory _
0.546 + 0.003 2.016&0.014 3.020 _C0.008
rcportcd
5.00 & 0.02 14.59 -t_0.09 6.1720.02
the avcragc of 12 determinations.
The data reported here indicate that the determination of sulfur in coal by combustion, followed by automatic titration of the bomb-washings with lead perchlorate solution and a lead-selective electrode is satisfactory. As previously stated2, the bomb-washing approach provides a satisfactory recovery of sulfur from coal samples. With respect to speed, precision, recovery and ease of operation the proposed method is excellent. The authors are grateful to Ruth S. Jessee who prepared and carbon mixtures.
most of the sulfur, coal
REFERENCES ASTM D-271. Sonrplhg uml A~1~se.s u/’ Cod untl Coke. Sections 21 to 24. 197 I. W. A. Selvig and A. C. Ficldncr. It~cl. EII~. C/rem, 19 (1927) 729. I. M. Kolthoff, E. B. Snndell, E. J. Mcchan nnd S. Bruckcnstcin. QItrrrlfitcltioc MacMillan. New York, 4th Ed., 1969. pp. 603-610. J. W. Ross, Jr. and M. S. Frant, Ard. Clrerrr.. 41 (1969) 967. W. Sulig. Mikrochim. Acta. (1970) 168. S. H. Regcster, In{/. EM{/. Chrn.. 6 ( 1914) 812.
Chenricd
Afu/ysi.s.