Coulometric generation of hydrogen ions by anodic oxidation of some organic compounds in nitromethane, sulpholane, acetonitrile and acetic acid—acetic anhydride

Analytzca Chumca Acta, 265 (1992) 35-42

35

Elsevler Science Pubhshers B V , Amsterdam

Coulometric generation of hydrogen ions by anodic oxidation of some organic compounds in nitromethane, sulpholane, acetonitrile and acetic acid-acetic anhydride R MihaJloviC Department of Chemutry, Faculty of Sconces, Utuverscty of k?agu]evac, R Domanovl~a 12, Kragujevac (Yugoslavud

V VaJgand Department of Chemlstty, Faculty of Sconces, Unwerscty of Belgrade, Studensk~ trg 16, Belgrade (Yugoslavia)

Z SlmiC Department of Chemistry, Faculty of Scumces, Unrversrty of Kragyevac,

R Domanovda

12, KraguJevac (Yugoslavza)

(Recewed 16th September 1991)

Abstract

The coulometnc generation of hydrogen ions by anodlc oxldatlon of cyclohexa-l,Cdlene, cyclohexa-1,3-dlene, 9,10-dlhydroanthracene, cyclohexene and 1,2,3,4_tetrahydronaphthalene m acetomtnle and acetlc acid-acetlc anhydrlde (1 + 6, v/v> IS described The coulometnc generatlon of protons by anodlc oxldatlon of 2,3,4-trihydroxybenzoolc acid, some dlhydnc and tnhydnc phenols and esters of galhc acid, m mtromethane and sulfolane as solvents IS also reported The current-potential curves recorded for these depolanzers, the titrated bases, Indicators and solvents showed that the mvestlgated depolanzers are oxldlzed at more negatwe potentials than the oxldatlon potentials of the titrated bases and other components present m the solution The generated hydrogen ions were used for the tltratlon of some organic bases (p-tolmdme, tnethanolamme, sodium acetate, potassmm hydrogenphthalate, pyndme, plpendme, tnbutylamme, colhdme and 2,2’-blpyndme) \\rlth Msual and potentlometnc end-pomt detectlon The current efficiency was 100% for dlenes, 2,3,4- and 3,4,5-tnhydroxybenzolc acid, phenols and esters of galhc acid Keywords Coulometry, Potentlometry,

TltrlmetIy, Hydrogen Ions, Orgamc bases

Nltromethane IS a protogemc solvent which exhibits weak acid-base properties (pK, = 10 6 m water) As an acid, it can be titrated wrth sodmm methylate m butylamme [l] and with tetrabutylamrnomum hydroxide m pyrldme as solvent [2] In sulphurlc acid it behaves as a base [31 Although It has a relatively high dlelectrlc constant (E = 371, the dlssoclatlon of salts in this

Correspondence to R MlhaJlovk, Department of Chemlstly, Faculty of Sciences, Umverslty of Kragujevac, R Domanovka 12, Kragujevac (Yugoslavia) 0003-2670/92/$05

solvent 1s not pronounced Not many morgamc compounds are soluble m mtromethane Like acetomtnle, mtromethane 1s an excellent solvent for the tltratlon of weak organic bases Fritz and Fulda [4] used this solvent or its nuxture with acetic anhydride (4 + 1) for the titration of some orgamc bases (caffeine, 1-methyl-Zpyrrohdone, mcotmamlde, theobromme), Streuly [5] applied It m a nuxture with fonmc acid for the tltratlon of some ammes, amme salts and quaternary ammomum salts with perchlorlc acid m dloxane, whereas Clacclo et al [6] used this solvent for the titration of ptperazme with perchlorlc acid

00 0 1992 - Elsevler Science Pubhshers B V All rights resewed

36

m acetic aad with potentlometrlc end-point detection using a glass electrode with a saturated calomel reference electrode as the electrode couple Casassas et al [7] applied this solvent for the determination of ultramlcro amounts of alkaloids with potentlometrlc end-point detection, using Ag/AgSCN as the reference electrode The low solublhty of salts and the effect of water on the potential Jump are great disadvantages of this solvent Traces of water m this solvent make it impossible to determine bases, which have pK, values less than 14 m water Sulpholane (tetrahydrothlophen-l,l-dlo=de) 1s a more recent aprotic solvent in investigations of protolytlc reactlons [S-11] and m voltammetry of morgamc compounds [12] It has a high freezing pomt (28 45°C) [131, high bodmg pomt (28Y’C) [14], high density (12526 g cmp3> [15] and high dielectric constant (relative permlttmlty) (43 3) [15,16] This solvent 1s a very weak Bronsted acid, pK,(S) > 31 [17], and 1s very reslstant to oxldlzmg and reducing agents [15] The range of accessible working potentials m this solvent 1s large m both the cathodic and anodlc regions With sodium perchlorate solutions, the hmltmg anodlc potential at a platinum electrode 1s 2 3 V, whereas the hmltmg cathodic potential is - 4 V relative to the Ag/Ag+ electrode [18] In weakly acidic solutions, the potentials of hydrogen and glass electrodes are a linear function of the pH m buffered solutions but m strongly acidic solutions (HClO,, HSbCl,) this dependence 1snot linear Sulpholane 1s a good solvent for morgamc (NaClO,, LlClO,, NH,PF,) and organic salts Owing to these good properties, it 1s widely applied as a solvent for anodlc reactions instead of acetomtrde or mtromethane Among the sulphones, dlmethylsulpholane 1s preferred for electrochemical mvestlgatlons [ 19,201 Although mtromethane and sulpholane are solvents with good properties, few data have been reported on their analytical apphcatlon, partlcularly m coulometrlc determmatlons All the determmatlons described so far are tltratlons with perchlorlc acid solutrons m an organic solvent which, under the most suitable condltlons, contams some water, the latter having an adverse effect on the final results

R Mhallouti et al /Anal

Chun Acta 265 (1992) 35-42

It is clearly desirable to ehmmate the use of perchlorlc acid solutions for titrations m nonaqueous solutions and also to automate the tltratlon process In previous work it was shown that bases can be titrated with CH,CO+ ions, generated by the oxldatlon of mercury m acetlc anhydrlde or m acetic acid-acetic anhydride 1211,and with hydrogen ions generated by the oxldatlon of mercury m acetone, or by the oxldatlon of thlol compounds, or of some phenols, ascorbic acid and esters of galhc acid m acetomtrle, propylene carbonate and acetic acid-acetic anhydride [22241 This paper describes mvestlgatlons of the posslblhtles of coulometrlc generation of hydrogen ions by anodlc oxldatlon of dlenes, dlhydroanthracene, tetrahydronaphthalene and cyclohexene m acetomtrde or acetlc acid-acetic anhydrlde (1 + 6, v/v) and also by anodlc oxldatlon of 2,3,4-tnhydroxybenzolc acid, 3,4,5-tnhydroxybenzolc acid, some phenols and esters of galhc acid m sulpholane and mtromethane as solvents

EXPERIMENTAL

Reagents All the investigated depolarizers and titrated bases were of analytical-reagent grade from Merck or Fluka Before use, the liquid depolarlzers were redistilled Liquid organic bases were dried over fused potassium hydroxide and then redistilled under reduced pressure The concentrations of bases m solutions were checked by titrating them with hydrogen ions generated by the oxldatlon of hydrogen dissolved m palladium [24] Solutions of sodium acetate and potassium hydrogenphthalate were prepared as described earlier [25] Before use, acetomtrlle and mtromethane were purified according to the procedure of Kreshkov et al [261 Acetic acid and acetic anhydride were redistilled and fractions bodmg at 116°C and 138”C, respectively, were collected For visual end-point detection m mtromethane and acetomtrde, 0 1% crystal violet m methanol was used, whereas m acetic acid-acetic anhydnde, 0 1% malachite green m the latter solvent mixture was applied

R Mha&wrE et al /Anal

Chm Acta 265 (1992) 35-42

37

form of a palladium wire spiral sealed mto a glass tube by means of a platinum Hnre The reference electrode was a modified saturated calomel electrode or a H,/Pd electrode The reference H,/Pd electrode was also made of palladium saturated with hydrogen, and it was immersed m 0 2 M sodium perchlorate m the correspondmg solvent, placed m a glass tube with a smtered-glass bottom The potentials m the course of the titrations were measured with a Radiometer pH meter or a Digital 870 pH meter

Fig 1 Schematlc diagram of the apparatus for coulometnc tltratlons m sulpholane, 1= current stablhzer, 2 = pH meter, 3 = reference electrode, 4 = indicator electrode, 5 = Pt cathode, 6 = thermostat, 7 = Pt anode, 8 = magnet, 9 = stirrer

The supportmg electrolyte m sulpholane, acetomtrlle and acetic acid-acetic anhydnde was 0 2 M sodium perchlorate solution, m mtromethane, a saturated solution of the same salt was used Apparatus and electrodes The apparatus for the coulometrlc titration of bases wrth visual end-point detection [251 and that with potentlometrlc end-point detection [271 were described previously The current source was a current-voltage stablhzer (type STNS, VmEa, Belgrade) The anode and cathode compartments were separated by a smtered G4 glass disc The volume of the anolyte was 20 cm3 (for sulpholane 30 cm3) and that of the catholyte 5 cm3 The anode and cathode were platinum wire spirals each with a surface area of 25 nun’ The apparatus for the coulometrlc titration of bases m sulpholane IS shown m Fig 1 The solution was thermostated at 30°C (thermostat from NBE, Dresden) Solutions of bases for the titration m sulpholane were prepared m mtromethane The indicator electrode was a glass electrode (Radiometer 62OOB)or a laboratory-made H,/Pd electrode The HJPd electrode was made m the

Procedure for vwal end-point detectron The supportmg electrolyte was poured mto the anode (about 3 cm3) and cathode (about 5 cm31 compartment up to the same level The platinum electrodes were immersed m the catholyte and anolyte, and about 100 mg of a solid or 2-3 drops of a liquid depolarizer and 2-3 drops of the mdlcator, were added to the anolyte The current was switched on and hydrogen ions were generated until the indicator colour changed (titration of the supportmg electrolyte) A weighed amount of the investigated base (0 6-l 00 cm31 was then added to the anolyte and the current was again switched on The generation of hydrogen ions was contmued until the same colour change of the indicator was obtained A new ahquot of the base was then added and hydrogen ions were generated until the same indicator colour change Procedure for potentlometnc end-poznt detection The supportmg electrolyte was added to a certain level into the cathode compartment of the vessel and a platinum spiral was mnnersed m it, a titrated supporting electrolyte solution was poured mto the anode compartment up to the same level, and to it 100 mg (2-3 drops) of the depolarizer were added The platinum anode and an electrode couple (glass electrode-SCE or HJPd-SCE or H,/Pd-H,/Pd) were placed m the anolyte After the addition of a weighed volume of the investigated base, the current was switched on and hydrogen ions were generated dlscontmuously, the potential being measured after each generation The end-point was determined from the second derivative or by means of a Granov diagram Several samples can be deter-

38

R MI~~J~OVZC et al /Am!

mmed successwely m the same supportmg trolyte

Chm. Acta 265 (1992) 35-42

elec-

4 RESULTS AN DISCUSSION

Oxuhztwn of dmes Shono et al [28] have estabhshed that cyclohexa-1,4-dlene IS oxrdlzed to benzene m a mixed solution of acetic acid and methanol with a current efficiency of 55-71% The oxldatlon potenteal of cyclohexa-1,4-dlene m this solvent mixture IS 174 V Much earlier, Geske [291 had reported that m acetomtrlle cyclohexa-1,4-dlene IS oxldlzed to benzene but at a lower potential (16 V) The oxldatlon of cyclohexa-1,Cdlene gives rise to two hydrogen ions per dlene molecule As cyclohexa1,4-dlene IS oxldlzed to benzene at a much more negative potential than the oxldatlon potentials of the solvents (acetonltnle, acetic acid-acetic anhydride), mdlcators (malachite green, crystal violet) and bases (tnethanolamme, p-tolmdme) [23-251, it was considered that this compound could be used as a depolaruer for the coulometnc generation of hydrogen ions m acetorutnle and acetic acid-acetic anhydride

r

mA/cd 4

1

Fig 2 Change m the anodlc potenual with current density m 0 25 M sodmm perchlorate m acetomtrde 1 = Solvent, 2 = crystal vlolet, 3 = pyndme, 4 = cyclohexa-1,3-dlene, 5= cyclohexa-1,Cdlene

3

2

1

Fig 3 Change m anodlc potential with current density m 0 25 M sodmm perchlorate m acetic acid-acetic anhydnde (1 + 6, v/v) 1 = Solvent, 2 = malachite green, 3 = sodmm acetate, 4 = cyclohexa-1,4-dlene, 5 = cyclohexa-1,3-dlene

No data have been reported so far on the oxldatlon of cyclohexa-1,3-dlene However, on the basis of its structure, it would be expected that, like cyclohexa-1,4-diene, it would be oxldlzed to benzene, liberating an equivalent amount of hydrogen ions Current-potential curves recorded for cyclohexa-1,3- and -1,4-dlene m anhydrous acetomtrlle and acetic aad-acetic anhydride (1 + 6, v/v) (Figs 2 and 3) sh owed that the oxldatlon potenteals of these compounds m the investigated solvents are lower than those of the mdlcators, solvents used and titrated bases, mdlcatmg that the first condltlon for the apphcatlon of an anodlc depolarizer 1s satisfied The results of the titration of bases with hydrogen ions generated by the oxldatlon of the investigated dlenes (Table 1) m acetlc acid-acetic anhydride show that the current efficiency IS lOO%, hence the second condition for the apphcatlon of an anodlc depolarizer IS also satisfied The oxldatlon of cyclohexa-1,4-dlene m acetomtnle also proceeds with 100% current efficlency (Table 1) However, when cyclohexa-1,3dlene was oxldlzed m this solvent, a black preclpitate separated at the anode, so this depolarizer

R

Mha~lovrc'

et

al /Anal

Chun Acta 265 (1992) 35-42

39

TABLE 1 Results of coulometric tltratlons of bases \mth hydrogen Ions obtamed by the omdatlon of some orgamc compounds (supportmg electrolyte 0 2 mol dmm3 sodmm perchlorate) Depolanzer

Solvent

Titrated base

Endpomt a

nb

Taken (mg)

Recovery (%I

Cyclohexa-1,Cdlene

CH3CN

p-Tolmdme Tnethanolamme Sodmm acetate Potassmm hydrogenphthalate

V

6 6 6 9

2 16 5 61 203 161

1001*03= 1001*03~ 1000~06 = 1000*09 c

Sodmm acetate Potassium hydrogenphthalate

V

8 6

203 761

1001*O1e 1001*02=

V

CH,CN

Sodmm acetate Plpendme Plpendme

6 6 I

3 19 3 61 431

1022kO4 d 1014*05d 1014kO6 d

CH,COOH-(CH,CO),O CH,CN

Sodmm acetate Plpendme

V V

3 19 3 61

1024f05d 1033f04d

1,2,3&Tetrahydronaphthalene

CH ,CN

Pyndme

V

425

1018*06

d

2,3,4-Tnhydroxybenzolc acid

Nltromethane

Tnbutylamme Colhdme 2,2’-Blpyndme

P P P

6 6 6

3 80 254 344

999kO2 999f02 1000*05

e = e

3,4,_5-Tnhydroxybenzolc acid

Nltromethane

Trlbutylamme

P

6

3 80

1001*02e

3,4,STnhydroxybenzolc acid

Nltromethane

Colhdme 2,2’-Bqyndme

P P

6 6

254 344

1000*02 e 1000*02”

Hydroqumone

Nitromethane

Tnbutylamme Colhdme 2,2’-Blpyndme

PYv P, v P, v

5 6 6

3 17 254 3 05

1002*01” 1000*01~ 1001*03’

Pyrocatechol

Nltromethane

Tnbutylamme Colhdme 2,2’-Blpyndme

P P P

6 6 5

3 17 254 3 05

1001*02’ 1000*02 = 1001*03”

Pyrogallol

Nltromethane

Tnbutylamme Colhdme 2,2’-Blpyndme

P P P

6 6 6

3 17 254 305

1001*02= 1000*02 e 1001*02e

Methyl gallate

Nltromethane

Tnbutylamme Colhdme 2,2’-Blpyndme

P, v P, v P, v

6 5 4

331 254 3 05

1001*02e 1002kO3 999+02

CH,CN CH,COOH-(CH,CO),O

Cyclohexa-1,3-dlene

9,10-Dlhydroanthracene

CH,COOH-(CH,CO),O

CH,COOH-KH,CO),O f=3a

Cyclohexene

V V V

V

V V

Ethyl gallate

Nltromethane

Tnbutylamme 2,2’-Blpyndme

P P

5 6

3 31 3 05

1001*03e 1001*02”

Propyl gallate

Nltromethane

Butyl gallate

Nltromethane

Tnbutylamme Colhdme 2,2’-Blpyndme Tnhutylamme Colhdme 2,2’-Bipyndme

P, v P?v P, v P. v P. v P, v

6 5 6 6 6 5

3 31 270 3 05 3 80 270 344

1001*03e 999k02e 1000*04e 1000*ole 1001*02e 999*03”

= e

40 TABLE

R Mdza~lovrc’ et al /Anal

Chm

Acta 265 (1992) 35-42

1 (Contmued)

Depolarizer

Solvent

Titrated base

Endpoint a

nb

Taken

Recovery

(mg)

(%I

Octyl gallate

Nitromethane

Tributylamme Colhdme 2,2’-Bipyridme

PYv P? v P, v

6 6 6

3 80 254 344

999+02 999*04e 1000*02

Dodecyl

Nltromethane

Trlbutylamme Colhdme 2,2’-Blpyrldme

P? v P, v P? v

6 6 6

3 80 254 344

1001f03e 999*03e 999f03e

Hydroqumone

Sulpholane

Trlbutylamme Colhdme Cyclohexylamme

P P P

6 6 6

648 3 80 5 92

1001*03c 999*04’ 999+02’

Methyl gallate

Sulpholane

Trlbutylamme Colhdme Cyclohexylamme

P P P

6 6 6

641 3 80 5 92

1000*02 1000*03c 998*02’

Pyrogallol

Sulpholane

Trlbutylamme Colhdme Cyclohexylamme

P P P

6 6 6

641 3 80 5 92

998*02= 999*03c 999*04=

gallate

a p = Potentlometrlc end-pomt, v = wsual end-point I = 12 mA e Current density Z = 5 mA

cannot be applied for the coulometrlc of hydrogen ions m acetomtrlle

b Number of determmatlons

generation

Oxzdatzon of dzhydroanthracene, tetrahydronaphthalene and cyclohexene It 1s known that the oxldatlon of 9,10-dlhydroanthracene and 1,2,3,4_tetrahydronaphthalene with aqueous potassium permanganate gives rise to anthracene and naphthalene, respectwely The electrochemical oxldatlon of these compounds also gives anthracene and napthalene, but with the liberation of an equivalent amount of hydrogen ions Cyclohexene 1s electrochemlcally 0x1dlzed to benzene, four hydrogen ions being hberated The oxldatlon potentials of 9,10-dlhydroanthracene, 1,2,3,4_tetrahydronaphthalene and cyclohexene m acetomtrlle are 1 53, 157 and 193 V, respectively [28,30,31] The oxldatlon potentials of cyclohexene m mtromethane (183 V), propylene carbonate (1 86 V) and sulpholane (182 V) [31] do not differ much from the potential of this compound m acetomtrlle As the oxldatlon potentials of the above-mentioned compounds are lower than those of the titrated bases, mdlcators and solvents used, It was

’ Current density Z = 9 mA

e =

=

d Current dens@

expected that the oxldatlon of these compounds m acetomtrlle and acetic acid-acetic anhydride at the anode would proceed with 100% current efficiency However, the results showed that the current efficiency was 1014-103 3% which mdlcates that, m addition to the oxtdatron reaction of these compounds to anthracene, naphthalene and benzene, some secondary reactions also occur Oxzdatwn of phenols, esters of galizc aczd, 2,3,4trzhydroxybenzozc and and 3,4,5-tnhydroxybenzozc aczd zn nztromethane and sulpholane It has been shown previously that some dlhydrlc and trlhydrlc phenols and some esters of galhc acid can be applied as anodlc depolarizers for the couIometnc generatlon of hydrogen ions m acetomtnle, acetic aad-acetlc anhydride and propylene carbonate [25,27] As mtromethane and sulpholane are very good solvents for acid-base determmatlons, and as the literature contains no data on their application to the coulometric titration of bases, it was consldered of interest to investigate the posslblhty of applymg these depolarizers to the coulometrlc generation of hydrogen ions m these solvents The current-potential curves for phenols and

R Mha~lovti et al /Anal

41

Chrm Acta 265 (1992) 35-42

Concluswn The coulometrlc

generation of hydrogen Ions 1s successful when cyclohexa-1,C and -1,3-dlenes m acetic acid-acetic anhydride (1 + 6, v/v), cyclohexa-1,4-dlene m acetomtnle, 2,3,4_tnhydroxybenzolc acid, 3,4,5-tnhydroxybenzolc aad, pyrocatechol, pyrogallol and gallates (methyl, ethyl, propyl, butyl, octyl, dodecyl) m mtromethane and hydroqumone, methyl gallate and pyrogallol m sulpholane are oxldlzed anodlcally Fug 4 Change m anodtc potenttal wtth current dens@ m 0 25 M sodmm perchlorate m mtromethane 1= Solvent, 2 = mdtcator, 3 = colhdme, 4 = hydroqumone, 5 = pyrocatechol, 6 = methyl gallate, 7 = ethyl gallate, 8 = propyl gallate, 9 = pyrogallol

esters of galhc acid m mtromethane (Fig 4) showed that the oxldatlon potentials of these compounds are about 1 V lower than those of other components m the solution The oxldatlon potentials of the investigated depolarizers m sulpholane are more positive, but m this solvent the difference between the oxldatlon potentials of the investigated depolarizers and those of other components (m the solution) 1s large Hydrogen ions formed by the oxldatlon of the investigated depolarizers were used for the tltratlon of tnbutylamme, colhdme and 2,2’-blpyndme with visual and potentlometrlc end-point detection, usmg HJPd-H ,/Pd and glassHJPd as electrode couples With pyrocatehol, ethyl gallate, pyrogallol, 2,3,4-tnhydroxybenzolc acid and 3,4$tnhydroxybenzolc acid the titration end-point could not be determined visually, as the indicator colour change was slow and not sufficiently sharp In potentlometrlc end-point detection, the potentials of all the investigated depolarizers were very stable m the course of the titrations, and were promptly established The potential Jumps at the end-point, m electrolysmg for 15 s at a current of 6 mA, were higher than 200 mV The results obtained m the determmatlon of investigated bases (Table 1) show that the current efficiency is 100%

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Chum Acta 265 (1992) 35-42

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