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A wet-combustion method for the determination of chlorine in coals
IG
AXAr..YTICA
cmslIcr\
ACTA
VOL.
8
(1953)
The standard method for the determination of chlorine in coals depends on incineration of the sample with Eschka misttrrc at 500” -& zg”C for 24 hi. The chloride ion is finally determined by the Volhard titration. Several years ago Panel I (SamlJlin~, Analysis and Testing) of the British Coke Research Association initiated an investigation with the ohjcct of reducing considerably the titnc involved in this process. The same organisation had already recommended an alternative methods* :i, requiring a total time of about 20 min for a single sample, but since this method necessitated the USCof a high temperature furnace, it was considered desirable to recommend an alternative method for which more readily available apparatus could be used. As a result of this investigation, which was syrcad over four years, and in which many laboratories cooperated, B.C.R.A. Panel4 finally recommended a modified Eschka method which required an incineration period of only f 1~. During the course of this work it was considered desirable to check the accuracy of the method not only against the standard method, but against independent methods of analysis. Only two such methods for the determination of chlorine in coal have been described: the high temperature combustion method3 and the bomb method”. It was not convenient to use the latter method, because the bomb is generally in constant USC for the determination of calorific values. Accordingly, the present author was asked to explore the possibility of applying one of the many classical methods available for the determination of halogens in organic compounds to coals. The most attractive of these appeared to be the method of wet combustion, and after suitable modification this was applied successfully to the determination of chlorine in coals. The wet-combustion method was originally developed as a means of obtaining an independent check on the chlorine content of coals, but it has certain features which may make it useful as a routine method of analysis. J?it&-ences p. 2x.
VOL. 8 (1953)
l>BTElZMIN/\TIOS
OF
CHLORINE
IN
CO.\LS
=7
(I) It requires only simple apparatus. (2) It is rapid. (3) No filtration is necessary. (4) It is suitable for batch analysis. (5) The results arc unaffected by a contaminated laboratory atmosphere. Although a large amount of potassium dichrofnate is consumed per cletermination, the actual cost works out about the same as the 13.C.R.A. method and considerably less than the 13.S. method.
Although the amount of sample to bc taken was necessarily restricted to the macro range (to avoid sampling errors) the amount of chlorine to be dctcrminecl (rarely as much as I per cent.) lay in the micro range. Accordingly, only micro methods for the dctcrmination of chlorine were considered. Of these the VIEB~CK proccdurc’; was chosen for adaption bccausc some considerable experience with it had been gained in the microanalysis of organic compounds. The apparatus consists essentially of a decomposition flask and an absorption system. This general form was retained with some modification. Although the amount of chlorine to 1~ absorbed was of the same order as in the microanalysis of organic compounds, the amount of sample to be decomposed was at least one hundred times as great. A much larger decomposition flask was therefore used. In the initial cxpcrimcnts the original VIEB&I; absorption system was used, was incorporated. but later, il more compact system described by NUTTEN~ l’hc clcctrically-heated aluminium block described by the same author could probably bc adapted for batch analysis. It was not tried in the present investigatio n since the determinations*lw&e done singly. The method is based on decomposition with sulphuric acid and potassium dichromate in the prcsencc of silver sulphate. The products of decomposition arc swept into neutral hydrogen peroxide by means of a current of air. The hydrochloric acid formed is determined alkalimetrically. In certain cases other acid-forming gases arc retained by the absorbent, e.g. nitrogen oxides when nitro-groups arc present, and a control titration is recommended which is specific for halide ions. A solution of mercuric oxycyanide is added to the neutralized solution Hg(OH)CN
+ NaCl
--b HgCl.CN
+
NaOW
and the increase in alkalinity is ccluivalent to the original amount of chloride ion. This liberated alkali is titrated with standard sulp!luric acid. When coal was decomposed, it was found that fumes of sulphuric acid or sulphur dioxide ;,assed into the absorber and the mercuric oxycyanide method l?~fctwtce.s p.
2x.
I8
I<. HELCHER
\'O'
f? (rc)53)
had to be used in eve%+ c&c. Since the amount of sample taken (0.5 g) was over one hundred times tf;at used by VIEBiiCK it became necessary to establish the conditions under which chlorine could be liberated quantitatively from coal. Without describing in detail the many trials made, it will suffice to say that the amounts of sulphuric acid and potassium dichromatc and their ratios, the time of digestion and the temperature of reaction were varied. Potassium sulphate was also added in an attempt to overcome the passage of acid fumes into the absorbent. Some of thcsc conditions gave low results, but it was established that the most important factors were the minimum amounts of sulphuric acid and potassium dichromatc which could lx used and the tempcraturc of dccomposition. Tfic prcscncc of silver ions arc csscntial in tlic process. Silver chloride is formed intermedia tcly and when dccomposcd by sulphuric acid and dichrornate dots not form chromyl chloriclc. Accordingly, the ratio of silver sulphatc to sulphuric acid sl~oulcl not lx important providecl that there is sufficient silver ion nvailable to combine with the total amount of chloride prcscnt. Tests wcrc carried. out in which the silver sulphate was rcduccd to the tcmo~rnl present in the original Vr~sBcxt method although the sulphuric acid had been incrcnsccl almost fiftecnfold. No chromyl chloride formed under thcsc conditions i\nd the results we& unaffected. This modification cffccted considerable saving in silver sulphatc although as a matter of routine it was recovered from the residue. In the original V~enijcrc method digestion is continued until the compound This was not found to be ncccssary in the cnsc of is decomposed completely. coals, probably because the chlorine is prcscnt completely in an inorganic form. When 0.5 g of coal was decomposed with 40 ml of sulphuric acid containing 0.15 g of silver sulphate and 25 g of potassium dichromatc, at 150 “C no further quantities of chlorine were evolved after digesting for about I h. The total time for the cligcstion was standardised for a period of go min. Appreciable quantities of undecomposcd organic material remain at this stage but need cause no alarm for the reasons already stated. The above conditions were established by working with a coal supplied by the Midland Coke Research Station, the chlorine content of which (0.48 per cent.) had been established by several laboratories. This coal is represented as Coal g in Table I and is the mean of about 20 determinations carried out after the final conditions had been cstablishcd. Cosls I to 6 in the Table wcrc determined as unknowns, and the results by other methods were only disclosed after the figures obtained by the wet-combustion method had been submitted to B.C.R.A. Panel I. Only single determinations were purposely carried out on these coals to avoid selection or the inclusion of mean values. Several dctcrminations were carried out using Coal 8 to ascertain whether or not difficulties were likely to arise with a coal containing an abnormal amount of chlorine. These results are summarised in Table I. Rcfcremes
p.
zr.
COMPARISON
01:
__._.
RESULTS
l3Y
.-
I
Coal
__..-
R.C.R.A.
Method
TIIE
(expressed ..-- _. --*
O.OG
High Combustion Temperature Method**o.o5
\VET-COMBUSCION
as
percentage
2
_.. 3
._
M15’CH013
on
the
dry
\t+l’rlI
O’lliliR
hll
coal)
.-_ _
-
5
4
6
7
8
-:j .._
0.57
o-47
0.67
0.39
0.r4
0.04
0.W
0.4s
0.59
-
O.GG
0.30
0.15
c>.o5
o.SS
--
\Vct-Combustion Method
0. 10 0.40 0.37 0.00 o.o.+ u.<+s 0.65 0.55 ___._. _ ._ ..__ -.--__.__ 0.44 .._._._ ____-_. .-. * Incineration with I’Xschka mixture followed by IFolhar~l tttratioll. ** Combustion in a tube :rt CL high tcmperaturc ;rncl ab,orptlon of chlorillc 111
hydrogen
pcroxidc.
A ppnrntzrs
The final form of the apparatus used and its dimensions wllicll arc not critical are shown in Fig. I. The discs in the absorber have a hole in tllc ccrrtrc through which passes the capillary delivery tube. The h&s arc of such :L size that an ;ljrlock is formed and maintained when the absorption vcs~~~l is filled and ail is passed through. Filling is cffectcd through ttlc vertical sldc-arm. This is stoppered during a determination. The large-sized flask is esscnt!al hecausc the mixture froths during i tile decomposition.
Fig.
I.
Apparatus
for
&t-combustion.
20
1..j g of silver sulpll:Ite l>cr 400 nil Conccntratcd sulphuric: acid containing Potassium dichromrttt: (rccq*st:~lliscd) Hydrogen peroxide (3’%,) Saturated mercuric osycyuriidc sohttion 0.7 N sodium hyclroxidc fi.1 lx’r cent.. Cl 011 o.5 g sam~~lc) 0.0141 N sulphuric acid (1 ml Screened indicator (0,125 g m&l;\:! rctl and o.083 g mt~thylene 13luc dissolved in 10~ ml of qs’i{, etlimol).
‘l’rxnsfcr 10 ml of .; I)cr cent. llydrogcn lx!roxiclc to the rtl~sorbcr and distrihutc it equally throughout the three compartments. Transfer 0.5 g of coal and 25 g of potassium dichromatc (which tnrty be measured from a graduated tube) to the digestion flask and mix well. Add 40 ml of concentrated sulphuric acid containing silver sulphatc, attach a soda-lime guard tube to the side-arm of the flask and draw air through the apparatus at a rate of z to 3 bubbles per sec. Allow the reaction to proceed for 20 min. The heat of reaction is sufficient to maintain the required tcmpcrnture during this period. Immerse the flask in an oil bath maintained at x45”-155°C and allow the decomposition to proceed for it further 7~’ min. Disconnect the absorber and pour the contents into a IOO ml conical flask through the side-arm. Wash by introducing water through the vcrtical filling tube and through the larger ground joint. Add 3 or 4 drops of indicator xnd add 0.x N sodium hydroxide until the indicator just turns green. Adjust the solution to the neutral point by adding 0.0x4x N sulphuric acid. Add IO ml of saturated mercuric osycyanidc solution neutralised just before use with the standard sulphuric acid. Titrate the libcratccl alkali with the standard sulphuric acid using a mocroburet tc. Deduct a blank value obtained under the same conditions of digestion. 3 ml of o.oi;(r H,SO, -: 0.x per cent. Cl.
‘l’ransfcr 20 ml of 0.01 N sodium hydrosidc (standardised against potassium hi-iodate) to ZL100 ml conical flask. Add xc) ml of neutralisecl saturated mercuric * oxycynnidc solution and a few drops of screened indicator. Titrutc with the sulphuric acid to the neutral point. Adjust the strength of the stock solution to 0.0141 N according to the titrc obtained and re-check the standardisation. Thanks arc due to the Uritish Coke Research Association for permission to quote the analyses given in Table I. Ic’~jcrt!wCcSp.
2f.
SUMMARY Chlorine in coals can be determined by wet-combustion with sulphuric acid in the presence of potassium dichromate and silver sulphate. The liberated chlorine is absorb& in hydrogen peroxide and after treatment with mercuric oxycyanide is determined acidimetrically. The method is simple, fairly rapid and should be The results agree well with thoEc given by two suitable for multiplicatc analysis. i.e. incineration with Eschka mlx?urc folio\\ cd independent methods of analysis, and the high tcmpcrature combustion methc~cl. by the \rOLl~ARD titration,
Lc chlorc t1an.s Its charbons peut Gtre dose par combustion par voic humitle au otassium ct de sulfate moycn d’aciclc sulfurique en prdscnce de dichromatc dc aprOs traitcd’argent. Ix chlorc lib&d cst absorbc5 clans le pcroxytlc d *Eytlrogtinc: il cst dos6 par acidimdtrie. ment avcc l’osycyatiure de mercure, La m&hodc est simple, rclativement rapide et convicnt A dcs dosages cn s3lics. Ces rdsultats correspondent bicn h ceux obtcnus par z m&hoclcs tliffdrcntcs: I) incin&rntion avcc Ic mdlangc d’Eschka et titration par la mtithodc~ tie \‘OI.IIAI ; -, -) m&hotlc do combustion & hnutc tempdraturc.
%C’SAhIhIEI%~~ASSUI%G ClA,r 111 I
AXAr..YTICA
cmslIcr\
ACTA
VOL.
8
(1953)
The standard method for the determination of chlorine in coals depends on incineration of the sample with Eschka misttrrc at 500” -& zg”C for 24 hi. The chloride ion is finally determined by the Volhard titration. Several years ago Panel I (SamlJlin~, Analysis and Testing) of the British Coke Research Association initiated an investigation with the ohjcct of reducing considerably the titnc involved in this process. The same organisation had already recommended an alternative methods* :i, requiring a total time of about 20 min for a single sample, but since this method necessitated the USCof a high temperature furnace, it was considered desirable to recommend an alternative method for which more readily available apparatus could be used. As a result of this investigation, which was syrcad over four years, and in which many laboratories cooperated, B.C.R.A. Panel4 finally recommended a modified Eschka method which required an incineration period of only f 1~. During the course of this work it was considered desirable to check the accuracy of the method not only against the standard method, but against independent methods of analysis. Only two such methods for the determination of chlorine in coal have been described: the high temperature combustion method3 and the bomb method”. It was not convenient to use the latter method, because the bomb is generally in constant USC for the determination of calorific values. Accordingly, the present author was asked to explore the possibility of applying one of the many classical methods available for the determination of halogens in organic compounds to coals. The most attractive of these appeared to be the method of wet combustion, and after suitable modification this was applied successfully to the determination of chlorine in coals. The wet-combustion method was originally developed as a means of obtaining an independent check on the chlorine content of coals, but it has certain features which may make it useful as a routine method of analysis. J?it&-ences p. 2x.
VOL. 8 (1953)
l>BTElZMIN/\TIOS
OF
CHLORINE
IN
CO.\LS
=7
(I) It requires only simple apparatus. (2) It is rapid. (3) No filtration is necessary. (4) It is suitable for batch analysis. (5) The results arc unaffected by a contaminated laboratory atmosphere. Although a large amount of potassium dichrofnate is consumed per cletermination, the actual cost works out about the same as the 13.C.R.A. method and considerably less than the 13.S. method.
Although the amount of sample to bc taken was necessarily restricted to the macro range (to avoid sampling errors) the amount of chlorine to be dctcrminecl (rarely as much as I per cent.) lay in the micro range. Accordingly, only micro methods for the dctcrmination of chlorine were considered. Of these the VIEB~CK proccdurc’; was chosen for adaption bccausc some considerable experience with it had been gained in the microanalysis of organic compounds. The apparatus consists essentially of a decomposition flask and an absorption system. This general form was retained with some modification. Although the amount of chlorine to 1~ absorbed was of the same order as in the microanalysis of organic compounds, the amount of sample to be decomposed was at least one hundred times as great. A much larger decomposition flask was therefore used. In the initial cxpcrimcnts the original VIEB&I; absorption system was used, was incorporated. but later, il more compact system described by NUTTEN~ l’hc clcctrically-heated aluminium block described by the same author could probably bc adapted for batch analysis. It was not tried in the present investigatio n since the determinations*lw&e done singly. The method is based on decomposition with sulphuric acid and potassium dichromate in the prcsencc of silver sulphate. The products of decomposition arc swept into neutral hydrogen peroxide by means of a current of air. The hydrochloric acid formed is determined alkalimetrically. In certain cases other acid-forming gases arc retained by the absorbent, e.g. nitrogen oxides when nitro-groups arc present, and a control titration is recommended which is specific for halide ions. A solution of mercuric oxycyanide is added to the neutralized solution Hg(OH)CN
+ NaCl
--b HgCl.CN
+
NaOW
and the increase in alkalinity is ccluivalent to the original amount of chloride ion. This liberated alkali is titrated with standard sulp!luric acid. When coal was decomposed, it was found that fumes of sulphuric acid or sulphur dioxide ;,assed into the absorber and the mercuric oxycyanide method l?~fctwtce.s p.
2x.
I8
I<. HELCHER
\'O'
f? (rc)53)
had to be used in eve%+ c&c. Since the amount of sample taken (0.5 g) was over one hundred times tf;at used by VIEBiiCK it became necessary to establish the conditions under which chlorine could be liberated quantitatively from coal. Without describing in detail the many trials made, it will suffice to say that the amounts of sulphuric acid and potassium dichromatc and their ratios, the time of digestion and the temperature of reaction were varied. Potassium sulphate was also added in an attempt to overcome the passage of acid fumes into the absorbent. Some of thcsc conditions gave low results, but it was established that the most important factors were the minimum amounts of sulphuric acid and potassium dichromatc which could lx used and the tempcraturc of dccomposition. Tfic prcscncc of silver ions arc csscntial in tlic process. Silver chloride is formed intermedia tcly and when dccomposcd by sulphuric acid and dichrornate dots not form chromyl chloriclc. Accordingly, the ratio of silver sulphatc to sulphuric acid sl~oulcl not lx important providecl that there is sufficient silver ion nvailable to combine with the total amount of chloride prcscnt. Tests wcrc carried. out in which the silver sulphate was rcduccd to the tcmo~rnl present in the original Vr~sBcxt method although the sulphuric acid had been incrcnsccl almost fiftecnfold. No chromyl chloride formed under thcsc conditions i\nd the results we& unaffected. This modification cffccted considerable saving in silver sulphatc although as a matter of routine it was recovered from the residue. In the original V~enijcrc method digestion is continued until the compound This was not found to be ncccssary in the cnsc of is decomposed completely. coals, probably because the chlorine is prcscnt completely in an inorganic form. When 0.5 g of coal was decomposed with 40 ml of sulphuric acid containing 0.15 g of silver sulphate and 25 g of potassium dichromatc, at 150 “C no further quantities of chlorine were evolved after digesting for about I h. The total time for the cligcstion was standardised for a period of go min. Appreciable quantities of undecomposcd organic material remain at this stage but need cause no alarm for the reasons already stated. The above conditions were established by working with a coal supplied by the Midland Coke Research Station, the chlorine content of which (0.48 per cent.) had been established by several laboratories. This coal is represented as Coal g in Table I and is the mean of about 20 determinations carried out after the final conditions had been cstablishcd. Cosls I to 6 in the Table wcrc determined as unknowns, and the results by other methods were only disclosed after the figures obtained by the wet-combustion method had been submitted to B.C.R.A. Panel I. Only single determinations were purposely carried out on these coals to avoid selection or the inclusion of mean values. Several dctcrminations were carried out using Coal 8 to ascertain whether or not difficulties were likely to arise with a coal containing an abnormal amount of chlorine. These results are summarised in Table I. Rcfcremes
p.
zr.
COMPARISON
01:
__._.
RESULTS
l3Y
.-
I
Coal
__..-
R.C.R.A.
Method
TIIE
(expressed ..-- _. --*
O.OG
High Combustion Temperature Method**o.o5
\VET-COMBUSCION
as
percentage
2
_.. 3
._
M15’CH013
on
the
dry
\t+l’rlI
O’lliliR
hll
coal)
.-_ _
-
5
4
6
7
8
-:j .._
0.57
o-47
0.67
0.39
0.r4
0.04
0.W
0.4s
0.59
-
O.GG
0.30
0.15
c>.o5
o.SS
--
\Vct-Combustion Method
0. 10 0.40 0.37 0.00 o.o.+ u.<+s 0.65 0.55 ___._. _ ._ ..__ -.--__.__ 0.44 .._._._ ____-_. .-. * Incineration with I’Xschka mixture followed by IFolhar~l tttratioll. ** Combustion in a tube :rt CL high tcmperaturc ;rncl ab,orptlon of chlorillc 111
hydrogen
pcroxidc.
A ppnrntzrs
The final form of the apparatus used and its dimensions wllicll arc not critical are shown in Fig. I. The discs in the absorber have a hole in tllc ccrrtrc through which passes the capillary delivery tube. The h&s arc of such :L size that an ;ljrlock is formed and maintained when the absorption vcs~~~l is filled and ail is passed through. Filling is cffectcd through ttlc vertical sldc-arm. This is stoppered during a determination. The large-sized flask is esscnt!al hecausc the mixture froths during i tile decomposition.
Fig.
I.
Apparatus
for
&t-combustion.
20
1..j g of silver sulpll:Ite l>cr 400 nil Conccntratcd sulphuric: acid containing Potassium dichromrttt: (rccq*st:~lliscd) Hydrogen peroxide (3’%,) Saturated mercuric osycyuriidc sohttion 0.7 N sodium hyclroxidc fi.1 lx’r cent.. Cl 011 o.5 g sam~~lc) 0.0141 N sulphuric acid (1 ml Screened indicator (0,125 g m&l;\:! rctl and o.083 g mt~thylene 13luc dissolved in 10~ ml of qs’i{, etlimol).
‘l’rxnsfcr 10 ml of .; I)cr cent. llydrogcn lx!roxiclc to the rtl~sorbcr and distrihutc it equally throughout the three compartments. Transfer 0.5 g of coal and 25 g of potassium dichromatc (which tnrty be measured from a graduated tube) to the digestion flask and mix well. Add 40 ml of concentrated sulphuric acid containing silver sulphatc, attach a soda-lime guard tube to the side-arm of the flask and draw air through the apparatus at a rate of z to 3 bubbles per sec. Allow the reaction to proceed for 20 min. The heat of reaction is sufficient to maintain the required tcmpcrnture during this period. Immerse the flask in an oil bath maintained at x45”-155°C and allow the decomposition to proceed for it further 7~’ min. Disconnect the absorber and pour the contents into a IOO ml conical flask through the side-arm. Wash by introducing water through the vcrtical filling tube and through the larger ground joint. Add 3 or 4 drops of indicator xnd add 0.x N sodium hydroxide until the indicator just turns green. Adjust the solution to the neutral point by adding 0.0x4x N sulphuric acid. Add IO ml of saturated mercuric osycyanidc solution neutralised just before use with the standard sulphuric acid. Titrate the libcratccl alkali with the standard sulphuric acid using a mocroburet tc. Deduct a blank value obtained under the same conditions of digestion. 3 ml of o.oi;(r H,SO, -: 0.x per cent. Cl.
‘l’ransfcr 20 ml of 0.01 N sodium hydrosidc (standardised against potassium hi-iodate) to ZL100 ml conical flask. Add xc) ml of neutralisecl saturated mercuric * oxycynnidc solution and a few drops of screened indicator. Titrutc with the sulphuric acid to the neutral point. Adjust the strength of the stock solution to 0.0141 N according to the titrc obtained and re-check the standardisation. Thanks arc due to the Uritish Coke Research Association for permission to quote the analyses given in Table I. Ic’~jcrt!wCcSp.
2f.
SUMMARY Chlorine in coals can be determined by wet-combustion with sulphuric acid in the presence of potassium dichromate and silver sulphate. The liberated chlorine is absorb& in hydrogen peroxide and after treatment with mercuric oxycyanide is determined acidimetrically. The method is simple, fairly rapid and should be The results agree well with thoEc given by two suitable for multiplicatc analysis. i.e. incineration with Eschka mlx?urc folio\\ cd independent methods of analysis, and the high tcmpcrature combustion methc~cl. by the \rOLl~ARD titration,
Lc chlorc t1an.s Its charbons peut Gtre dose par combustion par voic humitle au otassium ct de sulfate moycn d’aciclc sulfurique en prdscnce de dichromatc dc aprOs traitcd’argent. Ix chlorc lib&d cst absorbc5 clans le pcroxytlc d *Eytlrogtinc: il cst dos6 par acidimdtrie. ment avcc l’osycyatiure de mercure, La m&hodc est simple, rclativement rapide et convicnt A dcs dosages cn s3lics. Ces rdsultats correspondent bicn h ceux obtcnus par z m&hoclcs tliffdrcntcs: I) incin&rntion avcc Ic mdlangc d’Eschka et titration par la mtithodc~ tie \‘OI.IIAI ; -, -) m&hotlc do combustion & hnutc tempdraturc.
%C’SAhIhIEI%~~ASSUI%G ClA,r 111 I