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An apparatus for the wet ashing of organic matter
VOL.
10 (1954)
AN;APPARATUS
ANALYTICA
FOR
--f 74/26
-.-
Q!!E 0.0.
Fig. I. Semi-micro apparatus wet combustion. G.
FREDEIZJCK
THE
CHIMICA ACTA
WET
ASI-IING
3x7
OF
ORGANIC
MATTER
During the past year some rcscarch work has been done in this laboratory on methods for the dctcrmination of sulfur in wood, treated by the sulfite process. The sulfur content of the samples was rather low, of the order of one per cent., and dry combustion mcthocls proved to be inapplicable. Wet ashing methods, where the substance was digested with misturcs of pcrchloric and nitric acids, were found to be satisfactory. \Vet ashing proccdurcs of this type have become more popular in the past few years and recently have been invcstigatcd thoroughly by Snrrrrrl. In our first cxperimcnts, which wcrc carried out in Kjcldahl flasks and flasks with reflus condensers of various types, only about 75 per cent. of the sulfur was retaincd. The investigation was then continued using pure, well clcfincd sulfur-containing organic substances. the results of which will bc For thcsc experiments, presented later, an apparatus was constructed, which has proved to be so valuable for all types of invcstigations on ashing that it was consiclercd justified publishing its construction scparatcly. A semi-micro apparatus was first constructed of Pyrex glass using standard taper joints. Later a macro-apparatus of the same general design was built. The construction and the dimensions of the scmimicro apparatus arc shown in Fig. I. It consists of four parts, a digestion flask A, a reflux collector, 13, a rcflux condenser C, and a top part D. The digestion flask is drawn as an ordinary round-bottomed flask, but it may be changed at will KOa flat-bottomed flask,
SMITH, And.
Chim.
Ada,
8 (1953)
397.
313
P. 0. BETHGE
VOL. 10 (1954)
which is convcnicnt if heating on a hot plate is desired. A two necked flask may be used if reagents are to be added during the digestion, the second neck being fitted with a dropping funnel. The reflux collector is provided with an oblique bored stopcock, F. When this stopcock is open the apparatus functions as a reflux condenser but if it is closed, the rcflux from the condenser is collected and may be analyscd scparatcly, or after completing the ashing it may be added through the stopcock’ to the contents in the digestion flask. The reflux condenser is an ordinary Allihn condcnscr, and the top part makes it possible to wash the escaping gases with the desired reagent and to judge if gases are evolved during the reaction. (In an earlier model of the apparatus the escaping fumes wcrc washed in washbottles of the ordinary type, and a carrier-gas, such as nitrogen, had to be introduced in the digestion flask to prevent sucking back.) A method of combustion often applied is performed as follows. The substance, up to I gram, is wcighcd into the digestion flask. The top part is filled with washing liquid to such a level that gas may pass in both directions without the liquid splashing over. The acid mixture, e.g. 40 per cent. pcrchloric acid and Go per cent. nitric acid, is poured directly into the digestion flask, or, if the substance reacts in the cold, into the reflux collector. The cligcstion flask is conncctcd to the apparatus and the stopcock F is opened. The flask is heated until the nitric acid has reacted, that is, when the liquid in the top part indicates that no more gas is cvolvcd. The stopcock is then closed and the rest of the nitric acid is collected in the reflux collector. During this operation the tcmpcraturc of the acid mixture rises slowly, and when it reaches x50” C, the pcrchloric acid begins to osidize the substance. This is indicated by white fumes, which appear in the flask. The heating is now regulntcd so that these fumes fill the rcflus collector and dissolve in the nitric acid thcrc, but they should not bc allowed to ascend through the r&flux condcnscr and the top part. This is clone bccausc it is suspcctccl, but not that these fumes mechanically carry away some of the conyet proved, stituents lcfl. in the acid mixture. When the substance has hccn cligcstcd some time with boiling pcrchloric acid, the tcmpcraturc rises to about XN~ C. In most casts the substance is complctcly oxidized when no more gases cscapc through the top part. In doubtful cases, a little chromic acid is added to the acid misturc. Complete oxidation is then inclicatcd by the color of the acid turning orange, the apparatus is as dcscribccl by SMITH l. When the digestion is complctcd, left to cool, and the liquids in the top part, rcflus collector and the cligcstion flask are analyzed separately or togcthcr. This method for wet ashing is given only as an csamplc. It may bc modified in numerous ways to fulfil special rcquiremcnts. It must bc unclcrstood that this apparatus is constructed for rcscarch work on analytical methods involving wet ashing, and, when a final method has been workccl out, it may bc modified in some ways. When traces of elements arc to bc dctcrmincd, it is oftcu Ilcccssary to use
VOL.
10
WET
(1954)
ASIIING
OF
ORGANIC
319
MATTER
up to five or ten grams of substance. For this reason the bigger macro apparatus was made, which has the same construction as the semimicro one, with the exception of a modified top part as shown in Fig. 2.
I;1g.
2.
Modificcl
top part.
Fig.
3.
Mochficd
rcflllx
collector.
In one apparatus the stopcock F was exchanged for ZL three-way stopcock in Fig. 3. This makes it possible to empty the receiver when, for example, nqueous solutions arc ashed. Both sets of apparatus arc obtainable from the firm Wcmcr-Glas, Slijjdgatan I, Stockholm. The main advantage of this apparatus lies in the possibility of analysing the escaping gases. After a wet ashing of some sulfur-containing substances, sulfate has been found in the top part as well as in the rcflux collector and the digestion flask. The sum of these sulfate residues 11avc been estimated and it was concluded that all the sulfur was quantitatively retained in the apparatus. This is true for a11 substances so far analyzccl. An other advantage is that it is not necessary to operate the apparatus in a hood. The semimicro type can be operated anywhere LLS it is, and if the macro npl>aratus is provided with a funnel, mounted upside down over the top part and conncctecl to a water suction pump, all escaping gases can be rcmovcd safely. ZLS shown
320
P.
0.
VOL.
BETHGE
10
(1954)
SUMMARY An apparatus for the wet ashing of organic material is dcscribccl. It makes possible the ashmg of lar e quantities of organic matter with pure! ash-free reagents and the determination oB various constituents in the samples. This procedure is most useful for the determination of trace elements. It has mamly been used so far for the detcrmination of small amounts of sulfur. RtiSUMX? de substances organiqucs par voie humidc cst Un a parcil de min&ralisation 1 crmet la min&ralisation dc grandes quantitbs dc mat&es organiques d&rite pas de r6sidu par 6vaporation et avcc dcs% r, actifs purs, sans cendres, ne la&ant Ce proc&li: est dcs plus utilcs pour le dosage de divers 6ldments dc l’dchautillon. utllis6 pour ddtermincr dc falbles le dosage dcs tracts. 11 a 6th principalemcnt quantitds dc soufrc. ZUSAMMENFRSSUNG von organischcm Material wird bcschricben. Ein A parat fiir Nassverbrennung ltijnncn lnlt aschcfreien Reagcnzicn verbrannt werden Grosse Z ubstanzmengcn und es ist m6iglich, verschicdcnc l3cstsndtcile zu bcstimmen. Der Apparat ist besonderes ftir die Bestimmung von Spurenelemcnten gccignet. Bis jctzt ist cr haupts%chlich ftir die Bestimmung von lcleinen Mengen Schwcfcl bcnlitzt worden. ICcccivcd
October
zdth,
1953
10 (1954)
AN;APPARATUS
ANALYTICA
FOR
--f 74/26
-.-
Q!!E 0.0.
Fig. I. Semi-micro apparatus wet combustion. G.
FREDEIZJCK
THE
CHIMICA ACTA
WET
ASI-IING
3x7
OF
ORGANIC
MATTER
During the past year some rcscarch work has been done in this laboratory on methods for the dctcrmination of sulfur in wood, treated by the sulfite process. The sulfur content of the samples was rather low, of the order of one per cent., and dry combustion mcthocls proved to be inapplicable. Wet ashing methods, where the substance was digested with misturcs of pcrchloric and nitric acids, were found to be satisfactory. \Vet ashing proccdurcs of this type have become more popular in the past few years and recently have been invcstigatcd thoroughly by Snrrrrrl. In our first cxperimcnts, which wcrc carried out in Kjcldahl flasks and flasks with reflus condensers of various types, only about 75 per cent. of the sulfur was retaincd. The investigation was then continued using pure, well clcfincd sulfur-containing organic substances. the results of which will bc For thcsc experiments, presented later, an apparatus was constructed, which has proved to be so valuable for all types of invcstigations on ashing that it was consiclercd justified publishing its construction scparatcly. A semi-micro apparatus was first constructed of Pyrex glass using standard taper joints. Later a macro-apparatus of the same general design was built. The construction and the dimensions of the scmimicro apparatus arc shown in Fig. I. It consists of four parts, a digestion flask A, a reflux collector, 13, a rcflux condenser C, and a top part D. The digestion flask is drawn as an ordinary round-bottomed flask, but it may be changed at will KOa flat-bottomed flask,
SMITH, And.
Chim.
Ada,
8 (1953)
397.
313
P. 0. BETHGE
VOL. 10 (1954)
which is convcnicnt if heating on a hot plate is desired. A two necked flask may be used if reagents are to be added during the digestion, the second neck being fitted with a dropping funnel. The reflux collector is provided with an oblique bored stopcock, F. When this stopcock is open the apparatus functions as a reflux condenser but if it is closed, the rcflux from the condenser is collected and may be analyscd scparatcly, or after completing the ashing it may be added through the stopcock’ to the contents in the digestion flask. The reflux condenser is an ordinary Allihn condcnscr, and the top part makes it possible to wash the escaping gases with the desired reagent and to judge if gases are evolved during the reaction. (In an earlier model of the apparatus the escaping fumes wcrc washed in washbottles of the ordinary type, and a carrier-gas, such as nitrogen, had to be introduced in the digestion flask to prevent sucking back.) A method of combustion often applied is performed as follows. The substance, up to I gram, is wcighcd into the digestion flask. The top part is filled with washing liquid to such a level that gas may pass in both directions without the liquid splashing over. The acid mixture, e.g. 40 per cent. pcrchloric acid and Go per cent. nitric acid, is poured directly into the digestion flask, or, if the substance reacts in the cold, into the reflux collector. The cligcstion flask is conncctcd to the apparatus and the stopcock F is opened. The flask is heated until the nitric acid has reacted, that is, when the liquid in the top part indicates that no more gas is cvolvcd. The stopcock is then closed and the rest of the nitric acid is collected in the reflux collector. During this operation the tcmpcraturc of the acid mixture rises slowly, and when it reaches x50” C, the pcrchloric acid begins to osidize the substance. This is indicated by white fumes, which appear in the flask. The heating is now regulntcd so that these fumes fill the rcflus collector and dissolve in the nitric acid thcrc, but they should not bc allowed to ascend through the r&flux condcnscr and the top part. This is clone bccausc it is suspcctccl, but not that these fumes mechanically carry away some of the conyet proved, stituents lcfl. in the acid mixture. When the substance has hccn cligcstcd some time with boiling pcrchloric acid, the tcmpcraturc rises to about XN~ C. In most casts the substance is complctcly oxidized when no more gases cscapc through the top part. In doubtful cases, a little chromic acid is added to the acid misturc. Complete oxidation is then inclicatcd by the color of the acid turning orange, the apparatus is as dcscribccl by SMITH l. When the digestion is complctcd, left to cool, and the liquids in the top part, rcflus collector and the cligcstion flask are analyzed separately or togcthcr. This method for wet ashing is given only as an csamplc. It may bc modified in numerous ways to fulfil special rcquiremcnts. It must bc unclcrstood that this apparatus is constructed for rcscarch work on analytical methods involving wet ashing, and, when a final method has been workccl out, it may bc modified in some ways. When traces of elements arc to bc dctcrmincd, it is oftcu Ilcccssary to use
VOL.
10
WET
(1954)
ASIIING
OF
ORGANIC
319
MATTER
up to five or ten grams of substance. For this reason the bigger macro apparatus was made, which has the same construction as the semimicro one, with the exception of a modified top part as shown in Fig. 2.
I;1g.
2.
Modificcl
top part.
Fig.
3.
Mochficd
rcflllx
collector.
In one apparatus the stopcock F was exchanged for ZL three-way stopcock in Fig. 3. This makes it possible to empty the receiver when, for example, nqueous solutions arc ashed. Both sets of apparatus arc obtainable from the firm Wcmcr-Glas, Slijjdgatan I, Stockholm. The main advantage of this apparatus lies in the possibility of analysing the escaping gases. After a wet ashing of some sulfur-containing substances, sulfate has been found in the top part as well as in the rcflux collector and the digestion flask. The sum of these sulfate residues 11avc been estimated and it was concluded that all the sulfur was quantitatively retained in the apparatus. This is true for a11 substances so far analyzccl. An other advantage is that it is not necessary to operate the apparatus in a hood. The semimicro type can be operated anywhere LLS it is, and if the macro npl>aratus is provided with a funnel, mounted upside down over the top part and conncctecl to a water suction pump, all escaping gases can be rcmovcd safely. ZLS shown
320
P.
0.
VOL.
BETHGE
10
(1954)
SUMMARY An apparatus for the wet ashing of organic material is dcscribccl. It makes possible the ashmg of lar e quantities of organic matter with pure! ash-free reagents and the determination oB various constituents in the samples. This procedure is most useful for the determination of trace elements. It has mamly been used so far for the detcrmination of small amounts of sulfur. RtiSUMX? de substances organiqucs par voie humidc cst Un a parcil de min&ralisation 1 crmet la min&ralisation dc grandes quantitbs dc mat&es organiques d&rite pas de r6sidu par 6vaporation et avcc dcs% r, actifs purs, sans cendres, ne la&ant Ce proc&li: est dcs plus utilcs pour le dosage de divers 6ldments dc l’dchautillon. utllis6 pour ddtermincr dc falbles le dosage dcs tracts. 11 a 6th principalemcnt quantitds dc soufrc. ZUSAMMENFRSSUNG von organischcm Material wird bcschricben. Ein A parat fiir Nassverbrennung ltijnncn lnlt aschcfreien Reagcnzicn verbrannt werden Grosse Z ubstanzmengcn und es ist m6iglich, verschicdcnc l3cstsndtcile zu bcstimmen. Der Apparat ist besonderes ftir die Bestimmung von Spurenelemcnten gccignet. Bis jctzt ist cr haupts%chlich ftir die Bestimmung von lcleinen Mengen Schwcfcl bcnlitzt worden. ICcccivcd
October
zdth,
1953