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Thermogravimetric characteristics of barbituric acid and some of its derivatives
ANALYTICA
THERMOGRAVIMETRIC AND ALEXANDRE Deparlwzed
CHIMICA
ACTA
427
OF CHARACTERISTICS SOME OF ITS DERIVATIVES NURRAY
l3ERLIN*,
of Chetuislvy.
Univevsily
(Rcceivcd
E.
TAYLOR
AND
ACID
BARBITURIC RES
J.
ROI3INSON
of lVaslrir@ou, Senflle, Wrislr. (U.S.A.)
November
I st,
x gGo)
,
INTRODUCTION
g-Nitrobarbituric acid has been used for the determination of metal cations’-5 and organic bases 590.The thcrmogravimetric characteristics of some inorganic and organic salts of 5-nitrobarbituric acid have been given by the authors in previous papers490. The thermogravimetric characteristics of barbituric acid, dilituric acid (5-nitrobarbituric acid), violuric acid (5-isonitrosobarbituric acid), I ,3-climcthylvioluric acicl and barbital (5,5-cliethylbarbituric acid) are prcscntecl in this paper. reported that dibarbituric acid is formed by the loss HOTCHKISS AND JOHNSON’ of one mole of water of composition per two moles of barbituric acid when the latter is heated in glycerol at 170~. The authors have been interested in observing if such a reaction also took place in the solid state Dilituric acid, as stated previously, has been used for gravimetric determinations. Thermal stability studies of dilituric acid ancl the cliliturates should prove of value in investigating the proper temperature of heating the diliturates. Such a temperature should be high enough to decompose or sublime any coprecipitated dilituric acid but not high enough to decompose the cliliturates. Violuric and 1,3-dimethylvioluric acid have been usccl by TAYLOR* for the spectrophotometric determination of the alkali and alkaline earth metals. A thermogravimetric study of these acids and salts is reported in this paper to indicate the degrees of hydration as well as the range of stabilities of the anhydrous compounds. The decomposition of barbital has also been made to study the effect of alkyl substituents in the 5 position on the decomposition. EXPERIMENTAL
Prepavntion
and p&f
icatiovt of the acids
Ravbitwic acid. Technical grade barbituric acid from Eastman Organic Chemicals was recrystallized from 50% ethanol solution, washed with ether and dried in air overnight. Dilitzrric aci+. Technical grade dilituric acid from Eastman Organic Chemicals was recrystallized from water ancl dried in air overnight. Violuric acid. Technical grade violuric acid from Eastman Organic Chemicals was recrystallized from a 50% ethanol solution and dried in air overnight. x,g-DimetlzyZvioZzcric acid. The method used for the preparation of I,g-dimcthyl* Present address: Chemistry York 3, N. Y. (U.S.A.).
Department,
New
York
University,
Washington
Anal.
Ada,
Ciha.
Square,
New
24 (1961) 427-431
428
A. BERLIN, iM.E. TAYLOR, R. J. ROBINSON
violuric acid was essentially modifications. &W%zl. 13arbita1, from further recrystallization.
that
of BILTZ” and BILTZ AND HAMBURGER~~ with minor
Eastman
Organic
Chemicals
was used clirectly
without
A.~fiural~lrs An A.D.A.M.E.L. rccorcling thcrmobalance (Chcvenarcl System) was usecl to obtain the thcrmolysis curves. The samples were heated in No. 000 porcelain crucibles, having a volume of about G ml. PROCEDURE The samples wcrc in the form of powclcrs passing through CLQ-mesh sicvc. All the compounds were hcatcd in air with the exception of violuric acid, which was run in clry nitrogen, flowing at the rate of 2 l/h. The weights of the samples and the corrcsponcling rates of lleating xc given in the figures. In the graphical rcprcscntations of the tliermo~ravimetric clcconipositions we have normalizecl the curves to a percentage change in weight WS.tcmpcraturc. In this way differences in wei&t of the samples are eliminated and comparison is easier. Another way of presenting tlicsc thermogravimctric decompositions is hy a plot of the chan~c in the apparent molecular wci&t zts. tcmpcrature. Such plots give a clircct measurement of the molecular wci&t of the fraction clccomposing and permit an easy correlation between reactions of similar compounds. An cxnmplc of such a rcprcsentation will be shown. RESULTS
AND DISCUSSION
The thermal decomposition curves of barbituric, clilituric and violuric acids are shown in Fig. I, with the data normalizccl for percentage decomposition. The thermal dccomposition curves of x,3-climethylvioluric acid and of barbital arc shown in Fig. 2. In Fig. 3 we have the clccomposition curves of the same acids as in Fig. I but the data in this cast have hccn normalized with respect to changes in the apparent molecular weight. A cliffcrential thermal analysis curve for the dchydrntion and decomposition of clilituric acid is shown in Fig. 4.
I 200
I 400
I 600
Tenpffature Oc Fig. I. Thcrmtil decomposition of dcrivativcs of bnrbituric acid. A. Violuric acid, 45.3 mg llentcd at 65”/11;13. 13arlJituricacid, 88.9 mg hcatcd at G5”/11; c. Dilituric ncid, G9.3 mg hcatcd at G5O/h. AwI~. Cili?#l.A&a, 24
(1961)
427-431
THERMOGRAVIhIETRY
OF BARBITURIC
I
ACID
I*
429
130
w 2. Thermal
280 0 270
I
I
200 Tcmpemture
Fig.
A
300
OC
ticcomposition
GgJ/li;
of clcrivativcs of barbituric acid . A. HarlGtal, 3 I .8 mg hcntccl 13. I ,3-l.Jirncthylvioluric acid, 40.0 rng 1leiltW.l at 65”/11.
nt
200 &
____^^.. t‘i
Temperature
OC
Fig. 3. Thermal decomposition of clcrivativcs of barbituric acid. Curves normalized with respect to changes in the apparent molecular weight. A. 13nrbituric acicl, 88.9 mg hcatcd at GsO/h; B. Violuric acid, 45.3 mg hcatctl at 65”/l1; C. Dilituric acid, G9.3 my hcatcd at GsO/h.
I 200
I 400 Temperature
Fig. 4. Differential
thermat
decomposition
I
I
800
600 *C of dilituric
acid, hcatcd
Anal.
Claim.
at 3o0°/t1.
Ada,
24 (rgGr)
427-431
A.
430 Burlritzwic
acid (Fig.
BERLIN,
k1. E. TAYLOR,
R. J. ROBINSON
I)
The dehydration of barbituric acid dihydrate began at 20~ and terminated at 70”. In spite of the easy dehydration, this water must be water of crystallization since the precipitate had been wasl~ccl with ether, and normally water of absorption would be removed by this treatment. Anhydrous barbituric acid was stable to 180”. An accelerated loss in weight followed to 320~. From 320~ to 400” the residue, rcprescnting 22% of the initial weight, was stable. If clibnrbituric acid had. been formed by a solid state reaction a plateau or a break in the slope of the thermolysis curve would have been observed corresponding to a loss in weight of 27.5’7;. No such break occurred; we can therefore concluclc that clibarbituric acid is not formed by tllermal decomposition. The stable residue formed at 320” clecomposecl slowly to 600” at which temperature no residue was left. I)iLitwic acid (Fig. 2) A trihyclrntc was formccl upon recrystallization from water which dehydrated bediscussion of the dehydration of clilituric acid tween 50” ancl 90”. A more complete ancl of the kinetics of this clehyclration will be given in a later paper. The anhydrous form of clilituric acid was stable to 140”. A decomposition then was initiated and at 185” clilituric acid exploclccl, even thou@ there was a-low rate of heating. It appears that this csplosion will occur under any heating conditions since an explosion also occurred after a certain time, when clilituric acid was heated at a constant temperature of ISOO. This is reasonable in view of the high esothcrmicity of the autocatalytic reaction as shown by the differential thermal curve in Fig. 4. The residue, left after the explosion, dccomposcd very slowly to 700~ where no residue remained. Dilituric acid is an almost white crystalline compound which dissolves in water to yield a yellow solution with high absorbancy at 380 rnp. The ultraviolet spectrum of this solution was more comples having peaks at 218 and 318 rnp and a shoulder at 237 m,u. It seems that the 2r8-rnp peak is due to the primary ionization of dilituric acid, while the 318-rnp peak is clue to a secondary ionization and is strongly dependent upon the pH of the solution. The thermal decomposition product of clilituric acid which appeared just before the esplosion had a deep red color; it was more easily solul~lc in water than dilituric acicl itself giving a clecp red solution with high absorbancy at 520 rnp. This absorption peak disappeared completely within 3 h. The PH of such a solution was about 3 and clid not change with the change in color. If this same clecomposition product was dissolved in ethanol then the absorption peak was shifted to 450 mp and it had a higher stability. The U.V. absorption spectrum of, the residue of clilituric acid was only slightly different from that of dilituric acid itstilf, the peaks having been shifted to 212, 311 and 249 rnp respectively. Let us note finally that upon evaporation of the faded solution the original red residue was obtained again. This substance dissolved in water forming a red solution which faded on standing in the same manner as the original red solution. This is an indication that the red color is clue to an anhydrous form and that the decay is simply a reversible hydrolytic process. The infrared spectrum of the residue showed the complete disappearance of the G-75-, G.cJ~- ancl 7.00~,u peaks of clilituric acid due to the nitro group. A,laC.
Cjriwr. n&u,
24
(1961)
427-431
THERMOCRAVIfilETRY
Violzrric acid
(I;@.
OF BARBITURIC
ACID
431
r)
The rnonohyclrate of violuric 1x4~ and 130°.The anhydrous curve followed to 500~ where curve. The decomposition was
acid was quite stable. Dehydration took place between form was stable up to 195”. A S-shaped decomposition an explosive like break occurred in the thermolysis terminated at 715".
This acid did not form a hydrate. Sublimation started at 130~ and proceeded where zS’$& of the initial weight remained. With the without side reactions to 230°, inflection point at 230~ the differential change in weight became less accentuated. This can be interpreted as clue to coating of the undecomposed dimethylvioluric acid with decomposition product rendering further sublimation difficult. Another break in the curve occurred at 270~ indicating a f@ler reduction in the differential change in weight. The residue disappeared completely at 370’. The decomposition curve of r,3-dimethylvioluric acid obtained at a rate of heating of 300”/11had an inflection point at 275”. There was a 50’;1; loss in wci@t, indicating decomposition as the major process whereas sublimation predominated at the slower rate of heating. Bnvbital (I;+. 2) Barbital was also present as the anhyclrous compound. It started to sublime at 145” accelerating to zSo” whcrc no residue remained. Comparing the various thcrmolysis curves it is seen that substitution of an alkyl group for a hydrogen either on the nitrogen or on the 5 carbon makes the resulting compound less stable and with a greater tendency to sublime. The stability of the unsubstituted compound can be attributed to hydrogen bonding within the crystal lattice. SUMMARY The tliermolysis curves of barbituric acid and some of its clcrivativcs liavc been detcrminctl. Barbituric acid, violuric acid and dilituric acid form hydrates while r,3-dimcthylvioluric acid and barbital arc anhydrous. Barbital and 1,3-dimcthylvioluric acid sublime before decomposition. The diffcrcntial thermal analysis for dilituric acid sl~owecl a sharp cxothcrm at rgo” indicating a violent explosion. RI%UMti Lcs autcurs ont examine les courbcs dc pyrolyse dc l’aciclc barbituriquc ct de quelqucs-uns scs d&iv&: acide violuriquc, acide dilituriquc, acicle dimbthyl-r,3-violuriquc et barbital.
Bcschreibung cincr Untcrsuchung einigen BarbitursPurcdcrivatcn.
ZUSAMMENPASSUNG iibcr das thcrmolytische
Vcrhalten
von
REFERENCES 1 H. FREDHOLM,%. unal. Citenr.,104 (1936) 400. 2 I. 1)ICK AND 1%. MIIKAI, Acad. vep. popuhve IZotnErle, Baza cevcefdvi @in;. cevcet&i qfiint. Ser. I (Sliinle claimice), 4 (1957) 73. 3 C. L. DE GHAAFF AND E. C'.NOYONS, Clrcm. Weelrblad, 43 (1947) 300. 4 A. BERLIN ANU R. J. ROBINSON, Anal. Cltim. Ach, 24 (rgGr) 224. 6 c. 1;. REDEMANN AND C. NIEMANN,~. Am. Chem. Sot., G2 (1940) 590. 0 A. BERLIN AND R. J, ROBINSON, Anal. Chim. Acla, 24 (rgGx) 3rg. 7 R. D. HOTCHKISS AND T. D. JOHNSON, J. Am. Chem. SOC., 58 (x936) 525. 8 RI. E. TAYLOR, Thesis, University of Washington, Seattle, rgGo. 9 H. BILTZ, Bev. deut. clrem. Ges., 45 (xgrz) 3547. 10 H. BILTZ AND T. HAMBURGER, Uer. de&. &em. Ges., 49 (x916) 649.
Anal.
Ckim.
A&a,
Barbiturslknx
de
und
Tirr&i$oaru, Sludii
24 (1961)
427-431
THERMOGRAVIMETRIC AND ALEXANDRE Deparlwzed
CHIMICA
ACTA
427
OF CHARACTERISTICS SOME OF ITS DERIVATIVES NURRAY
l3ERLIN*,
of Chetuislvy.
Univevsily
(Rcceivcd
E.
TAYLOR
AND
ACID
BARBITURIC RES
J.
ROI3INSON
of lVaslrir@ou, Senflle, Wrislr. (U.S.A.)
November
I st,
x gGo)
,
INTRODUCTION
g-Nitrobarbituric acid has been used for the determination of metal cations’-5 and organic bases 590.The thcrmogravimetric characteristics of some inorganic and organic salts of 5-nitrobarbituric acid have been given by the authors in previous papers490. The thermogravimetric characteristics of barbituric acid, dilituric acid (5-nitrobarbituric acid), violuric acid (5-isonitrosobarbituric acid), I ,3-climcthylvioluric acicl and barbital (5,5-cliethylbarbituric acid) are prcscntecl in this paper. reported that dibarbituric acid is formed by the loss HOTCHKISS AND JOHNSON’ of one mole of water of composition per two moles of barbituric acid when the latter is heated in glycerol at 170~. The authors have been interested in observing if such a reaction also took place in the solid state Dilituric acid, as stated previously, has been used for gravimetric determinations. Thermal stability studies of dilituric acid ancl the cliliturates should prove of value in investigating the proper temperature of heating the diliturates. Such a temperature should be high enough to decompose or sublime any coprecipitated dilituric acid but not high enough to decompose the cliliturates. Violuric and 1,3-dimethylvioluric acid have been usccl by TAYLOR* for the spectrophotometric determination of the alkali and alkaline earth metals. A thermogravimetric study of these acids and salts is reported in this paper to indicate the degrees of hydration as well as the range of stabilities of the anhydrous compounds. The decomposition of barbital has also been made to study the effect of alkyl substituents in the 5 position on the decomposition. EXPERIMENTAL
Prepavntion
and p&f
icatiovt of the acids
Ravbitwic acid. Technical grade barbituric acid from Eastman Organic Chemicals was recrystallized from 50% ethanol solution, washed with ether and dried in air overnight. Dilitzrric aci+. Technical grade dilituric acid from Eastman Organic Chemicals was recrystallized from water ancl dried in air overnight. Violuric acid. Technical grade violuric acid from Eastman Organic Chemicals was recrystallized from a 50% ethanol solution and dried in air overnight. x,g-DimetlzyZvioZzcric acid. The method used for the preparation of I,g-dimcthyl* Present address: Chemistry York 3, N. Y. (U.S.A.).
Department,
New
York
University,
Washington
Anal.
Ada,
Ciha.
Square,
New
24 (1961) 427-431
428
A. BERLIN, iM.E. TAYLOR, R. J. ROBINSON
violuric acid was essentially modifications. &W%zl. 13arbita1, from further recrystallization.
that
of BILTZ” and BILTZ AND HAMBURGER~~ with minor
Eastman
Organic
Chemicals
was used clirectly
without
A.~fiural~lrs An A.D.A.M.E.L. rccorcling thcrmobalance (Chcvenarcl System) was usecl to obtain the thcrmolysis curves. The samples were heated in No. 000 porcelain crucibles, having a volume of about G ml. PROCEDURE The samples wcrc in the form of powclcrs passing through CLQ-mesh sicvc. All the compounds were hcatcd in air with the exception of violuric acid, which was run in clry nitrogen, flowing at the rate of 2 l/h. The weights of the samples and the corrcsponcling rates of lleating xc given in the figures. In the graphical rcprcscntations of the tliermo~ravimetric clcconipositions we have normalizecl the curves to a percentage change in weight WS.tcmpcraturc. In this way differences in wei&t of the samples are eliminated and comparison is easier. Another way of presenting tlicsc thermogravimctric decompositions is hy a plot of the chan~c in the apparent molecular wci&t zts. tcmpcrature. Such plots give a clircct measurement of the molecular wci&t of the fraction clccomposing and permit an easy correlation between reactions of similar compounds. An cxnmplc of such a rcprcsentation will be shown. RESULTS
AND DISCUSSION
The thermal decomposition curves of barbituric, clilituric and violuric acids are shown in Fig. I, with the data normalizccl for percentage decomposition. The thermal dccomposition curves of x,3-climethylvioluric acid and of barbital arc shown in Fig. 2. In Fig. 3 we have the clccomposition curves of the same acids as in Fig. I but the data in this cast have hccn normalized with respect to changes in the apparent molecular weight. A cliffcrential thermal analysis curve for the dchydrntion and decomposition of clilituric acid is shown in Fig. 4.
I 200
I 400
I 600
Tenpffature Oc Fig. I. Thcrmtil decomposition of dcrivativcs of bnrbituric acid. A. Violuric acid, 45.3 mg llentcd at 65”/11;13. 13arlJituricacid, 88.9 mg hcatcd at G5”/11; c. Dilituric ncid, G9.3 mg hcatcd at G5O/h. AwI~. Cili?#l.A&a, 24
(1961)
427-431
THERMOGRAVIhIETRY
OF BARBITURIC
I
ACID
I*
429
130
w 2. Thermal
280 0 270
I
I
200 Tcmpemture
Fig.
A
300
OC
ticcomposition
GgJ/li;
of clcrivativcs of barbituric acid . A. HarlGtal, 3 I .8 mg hcntccl 13. I ,3-l.Jirncthylvioluric acid, 40.0 rng 1leiltW.l at 65”/11.
nt
200 &
____^^.. t‘i
Temperature
OC
Fig. 3. Thermal decomposition of clcrivativcs of barbituric acid. Curves normalized with respect to changes in the apparent molecular weight. A. 13nrbituric acicl, 88.9 mg hcatcd at GsO/h; B. Violuric acid, 45.3 mg hcatctl at 65”/l1; C. Dilituric acid, G9.3 my hcatcd at GsO/h.
I 200
I 400 Temperature
Fig. 4. Differential
thermat
decomposition
I
I
800
600 *C of dilituric
acid, hcatcd
Anal.
Claim.
at 3o0°/t1.
Ada,
24 (rgGr)
427-431
A.
430 Burlritzwic
acid (Fig.
BERLIN,
k1. E. TAYLOR,
R. J. ROBINSON
I)
The dehydration of barbituric acid dihydrate began at 20~ and terminated at 70”. In spite of the easy dehydration, this water must be water of crystallization since the precipitate had been wasl~ccl with ether, and normally water of absorption would be removed by this treatment. Anhydrous barbituric acid was stable to 180”. An accelerated loss in weight followed to 320~. From 320~ to 400” the residue, rcprescnting 22% of the initial weight, was stable. If clibnrbituric acid had. been formed by a solid state reaction a plateau or a break in the slope of the thermolysis curve would have been observed corresponding to a loss in weight of 27.5’7;. No such break occurred; we can therefore concluclc that clibarbituric acid is not formed by tllermal decomposition. The stable residue formed at 320” clecomposecl slowly to 600” at which temperature no residue was left. I)iLitwic acid (Fig. 2) A trihyclrntc was formccl upon recrystallization from water which dehydrated bediscussion of the dehydration of clilituric acid tween 50” ancl 90”. A more complete ancl of the kinetics of this clehyclration will be given in a later paper. The anhydrous form of clilituric acid was stable to 140”. A decomposition then was initiated and at 185” clilituric acid exploclccl, even thou@ there was a-low rate of heating. It appears that this csplosion will occur under any heating conditions since an explosion also occurred after a certain time, when clilituric acid was heated at a constant temperature of ISOO. This is reasonable in view of the high esothcrmicity of the autocatalytic reaction as shown by the differential thermal curve in Fig. 4. The residue, left after the explosion, dccomposcd very slowly to 700~ where no residue remained. Dilituric acid is an almost white crystalline compound which dissolves in water to yield a yellow solution with high absorbancy at 380 rnp. The ultraviolet spectrum of this solution was more comples having peaks at 218 and 318 rnp and a shoulder at 237 m,u. It seems that the 2r8-rnp peak is due to the primary ionization of dilituric acid, while the 318-rnp peak is clue to a secondary ionization and is strongly dependent upon the pH of the solution. The thermal decomposition product of clilituric acid which appeared just before the esplosion had a deep red color; it was more easily solul~lc in water than dilituric acicl itself giving a clecp red solution with high absorbancy at 520 rnp. This absorption peak disappeared completely within 3 h. The PH of such a solution was about 3 and clid not change with the change in color. If this same clecomposition product was dissolved in ethanol then the absorption peak was shifted to 450 mp and it had a higher stability. The U.V. absorption spectrum of, the residue of clilituric acid was only slightly different from that of dilituric acid itstilf, the peaks having been shifted to 212, 311 and 249 rnp respectively. Let us note finally that upon evaporation of the faded solution the original red residue was obtained again. This substance dissolved in water forming a red solution which faded on standing in the same manner as the original red solution. This is an indication that the red color is clue to an anhydrous form and that the decay is simply a reversible hydrolytic process. The infrared spectrum of the residue showed the complete disappearance of the G-75-, G.cJ~- ancl 7.00~,u peaks of clilituric acid due to the nitro group. A,laC.
Cjriwr. n&u,
24
(1961)
427-431
THERMOCRAVIfilETRY
Violzrric acid
(I;@.
OF BARBITURIC
ACID
431
r)
The rnonohyclrate of violuric 1x4~ and 130°.The anhydrous curve followed to 500~ where curve. The decomposition was
acid was quite stable. Dehydration took place between form was stable up to 195”. A S-shaped decomposition an explosive like break occurred in the thermolysis terminated at 715".
This acid did not form a hydrate. Sublimation started at 130~ and proceeded where zS’$& of the initial weight remained. With the without side reactions to 230°, inflection point at 230~ the differential change in weight became less accentuated. This can be interpreted as clue to coating of the undecomposed dimethylvioluric acid with decomposition product rendering further sublimation difficult. Another break in the curve occurred at 270~ indicating a f@ler reduction in the differential change in weight. The residue disappeared completely at 370’. The decomposition curve of r,3-dimethylvioluric acid obtained at a rate of heating of 300”/11had an inflection point at 275”. There was a 50’;1; loss in wci@t, indicating decomposition as the major process whereas sublimation predominated at the slower rate of heating. Bnvbital (I;+. 2) Barbital was also present as the anhyclrous compound. It started to sublime at 145” accelerating to zSo” whcrc no residue remained. Comparing the various thcrmolysis curves it is seen that substitution of an alkyl group for a hydrogen either on the nitrogen or on the 5 carbon makes the resulting compound less stable and with a greater tendency to sublime. The stability of the unsubstituted compound can be attributed to hydrogen bonding within the crystal lattice. SUMMARY The tliermolysis curves of barbituric acid and some of its clcrivativcs liavc been detcrminctl. Barbituric acid, violuric acid and dilituric acid form hydrates while r,3-dimcthylvioluric acid and barbital arc anhydrous. Barbital and 1,3-dimcthylvioluric acid sublime before decomposition. The diffcrcntial thermal analysis for dilituric acid sl~owecl a sharp cxothcrm at rgo” indicating a violent explosion. RI%UMti Lcs autcurs ont examine les courbcs dc pyrolyse dc l’aciclc barbituriquc ct de quelqucs-uns scs d&iv&: acide violuriquc, acide dilituriquc, acicle dimbthyl-r,3-violuriquc et barbital.
Bcschreibung cincr Untcrsuchung einigen BarbitursPurcdcrivatcn.
ZUSAMMENPASSUNG iibcr das thcrmolytische
Vcrhalten
von
REFERENCES 1 H. FREDHOLM,%. unal. Citenr.,104 (1936) 400. 2 I. 1)ICK AND 1%. MIIKAI, Acad. vep. popuhve IZotnErle, Baza cevcefdvi @in;. cevcet&i qfiint. Ser. I (Sliinle claimice), 4 (1957) 73. 3 C. L. DE GHAAFF AND E. C'.NOYONS, Clrcm. Weelrblad, 43 (1947) 300. 4 A. BERLIN ANU R. J. ROBINSON, Anal. Cltim. Ach, 24 (rgGr) 224. 6 c. 1;. REDEMANN AND C. NIEMANN,~. Am. Chem. Sot., G2 (1940) 590. 0 A. BERLIN AND R. J, ROBINSON, Anal. Chim. Acla, 24 (rgGx) 3rg. 7 R. D. HOTCHKISS AND T. D. JOHNSON, J. Am. Chem. SOC., 58 (x936) 525. 8 RI. E. TAYLOR, Thesis, University of Washington, Seattle, rgGo. 9 H. BILTZ, Bev. deut. clrem. Ges., 45 (xgrz) 3547. 10 H. BILTZ AND T. HAMBURGER, Uer. de&. &em. Ges., 49 (x916) 649.
Anal.
Ckim.
A&a,
Barbiturslknx
de
und
Tirr&i$oaru, Sludii
24 (1961)
427-431