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High frequency titrations of some organic functional groups
VOL. 15
HIGH
r\NAL\“I-ICA
(1956)
L~IIEQUENCY
CHlhSlCA
TITI
ACTA
SOME
193
ORGANIC
FUNCTIONAL
The clctcrrninations of functional groups in organic compounds is an important and common l~roblcm. Various chemical and physico-chemical methods have been proposed for determining amine, oxime, iminc, carboxyl and other groups. The present study was undertaken in order to show the applicability of high frequency titrations for the dctcrmination of various functional groups both in aqueous and in non-aqueous solutions. High frcclucncy titrations holcl consiclcrablc promise for USC in studying nonaqueous or dilute aclucous systems. ‘l’hc systems being studied arc completely isolated from clcctrodcs and other active parts of the instrument ant1 yet the instrument is itself responsive to slight changes in the composition of the system. High frequency m&hods have been cstablishcd for use in dctcrmining various organic bases through studies made by WAGNER AND KAUPFMAN~ and by MASUI’L I_ANE3 has described high-freclucncy titrations of organic bases, phenols and cnols. The present study was initiated to investigate some of the broader aspects of high frcclucncy titration for functional group analysis.
Chewiculs
urrd rcngruls.
All ChcmlCillS 11w2d wcrc of rcagcnt graclc. Purities wcrc chcckctl by ancl whcrc ncccusary, rcpurificntions wcrc carriccl out by appropriate methods. Apfmrcrtrts. A Sargent i’vlodcl V Oscillomctcr was usctl for high frequency mcasurcmcnts. Hyclrogcn ion conccntmtmne wcrc tlctcrmincd by means of a Ucckman Motlcl C, 1x1 mctcr.
stirndartl
mcanh,
The carboxyl group may be titratccl in cithcr aclucous or non-aqueous media. In most cases, the carboxyl group tends to make compounds water soluble, but such solubility does not necessarily infer that water will be the best reaction medium. As a general rule. acids with very small dissociation constants should bc titrated in a non-aqueous medium. The recommended procedure, however, is to first try the acid in aqueous solution to dctcrmine whether or not the end-point is sufficiently well -__.---_* I’crnlancnt adtlrcss: 1,t30 I-loriuclli, Ilaynma-Slachi, lianagawn-Ken, Japan. Refcrcrrces p.
200
I<. lIARA, I’. w. WEST
194
VOL.
15
(1956)
dcfincd to permit satisfactory titrations. If good end-points are not obtained, then the titrations should bc carried out in pyridine, which has been found to be especially suitable for use in high frequency work.
moo-
E2000 -
Q _
# cf 3000. $ c “n = 4000 -
5000 -
6000 -
I Fig.
1
2
I. Titrations
3
4
5
of acids tn
6
7 ml NaOH
i1CIllC(JllS
solution.
I'cJr the purpoxc of tlctcrmining the applicabihty of high frcl’ifrdio,r itr uqucf~ffs solitllo~. quency titrations of carboxylic acltls in qucous solution, cxnct amounts of sclectcd acids Wcrc of the acicl solution being taken so as to obtain I /~oooM solutions. One huntlrctl milliliters stuclictl wcrc then transferred lo the large titration ccl1 of tho Osclllomctcr ant1 titratcci with 0.0278 M sodium hytlroxitlc. Titration clIrvcs arc shown in Fig. 1 for mnlonic acid ( Iin = 1.G x 10~~ ;Ind 2. I x lo-O), tartnric acicl (I<;, = I.1 x IO+ ant1 6.9 x 10’~). phtholic acid ( ICrl = I .26 x 10.~ nntl 6.9 x IO-O) anti picric acid (I<, = 1.0 x 10’1). ‘L’hc curves for malonlc, tnrtaric and phthalic acids show only one inflection point at 3.5 ml of sodium hyclroxidc. This vohimc of titrant rcprcscnte one mol cquivalcnt of hasc, indicating that only the first hytlrogun can be titrated in aqueous solution and that tlw scconcl hyclrogcn is too wcalc to bc titrated Iinclcr the conditions tlcscribccl. ‘The titration curves for mnlonic and phthalic acids inclicatc a linear rcsponsc for the instrument once the cncl-point has been rcachecl. In the cast of tartaric acid, the ascending portion of the titrntiun curve exhibits a chanfiing slope. This tliffcrcncc is probably due to the fact that the scconcl dissociation constant for tnrtaric acid is grcatcr th;rn the sccontl dissociation constant is, thcrcforc, pnrtlally neutralized for malonic and phthalic acids. l’hc sccontl hytlrogcn probably in the cast of tartaric acid. Picric acid shows il sharp inflection point at onr niol cquivalcnt of base.
Refercrtccs p.
200
VOL.
ORGANIC
15 (xc)g(,)
FU. CTIONAL
GROW15
195
Tifrnf~ns in ~yridrmc. FRITZ AND LISICKI* have proposed the titration of weak organic acids using sodium methoxidc in bcnzcne-methanol mixture. The course of the titration can be followed either by potcntiometric methods or by the USC of color indicators. These investigators have also suggested the use of basic solvents, such as butylarnine and pyricline. ISHI~ATB AND MASUX~ used srmilar tcchmques for dctcrmining organic acids, hut used high frcqucncy oscillators for dctcrmining the end-pomts. The present study has indicated the suitability of pyridinc as the solvent with alcoholic pot.assium hytlroxide as titrant. Standardization of filvnrrt. The alcoholic potassium t~yclroxldc used as the tttrant for organic ;ic~rls was prcparcd by dissolving potassium hydroxiclc in ~~b!dUtC cthvl alcohol. *rhc solution wits kept in g bottle closctl in s~tcli a way as to cxcluclc carbon tlioxidc am1 water. Ucnzoic ilCid was found to bc the most satisfactory primary stnntlard fur USCincstnblishing the strength of the potassium bydroxulc 9olutlon. The titration end-point. as tlctcrmlnctl by means of the Oscillomctcr, was well dcfincd and reprocluciblc. For the standardization, one huntlrctl milliliters of pyritlinc were placed in the titration ccl1 of the inntriiriicnt. 1\ wcighctl amount of
I
primary
2
1
3
4
5
6
8 ml
7
Fig. 2. Stnntlnrtliaation of (a) alcoholic potassium hydroxide in pyriclinc against bcnsoic acid and (1~) pcrchloric acid in acetic acid against potassium acid phtlialntc.
12
3
4
5
Fig. 3. Titrations of acids in pyridinc potassium hydroxide. Rcfcrcnces
fi. aoo
6
7ml with
alcoholic
standard
bcnzoic
ilCid
was
then cllssolvccl in the pyridinc and titrated with tlrc alcoholic potassium 1rydroxltlc. The solution was stirrctl hy means of an electric stirrer having a KlilSS lmpcllcr. The presence of water In the solution must In: avoided as much as posaiblc bccausc cvcn trace .aniounts tcnrl to introduce errors into tlic titration. Vrcshly prcpnrccl nlcoliolic potassium hyclroxitlc should bc ll!wl, ancl tlic pyridinc ~l~ould IJC rcpurifictl if it shows cvcn the slightest discoloration. It should bc notccl that with the progress of the titration a prccipitatc of potassium bcnzoatc scparntcs from the nolvcnt. This precipitation dots not intcrfcre with the titration and in fact, is ncccssary to the success of the mctliod. Small amounts of water which might cause dissolution of the potassium bcnzoatc convert tlic salt into the ionic form which results in serious loading of the Oscillometcr. Titration curves exhibit a slight rise during the initial stages of tlic titrationsThis probably is tluc to the prcscncc of trace amounts of water which dissolve some of the potassium bcnxoatc that is first formed. The curves ultimately flatten until the cndpoint is rcachcd, after which the curves riscstecplyand approximatelylinearly. Curve (a) in Fig. z shows a titration
19G
R.
ffAKA,
i’.
w.
VOL.
WEST
I5
(rggG)
using o.2+t.4 8 of kma0ic acid rlissolvcrl in 100 ml of pyriclrne i~~cl tilratcrl with 4.40 ml of 0.0.~5~~~&2 zllcoholic potassium hytlroxicle. r‘ilrfffions of ucids, l’itrations of acids in pyricline mcctiilm can bo carried out using the same genera1 procctlurc DY that tiescribcd for the standarckation of the alcoholic potsssium hydroxide ayninst pure bcnaoic acid. The cxatnplcrof such titrations arc shown in L:EK. 3. Using 0.09070iW potasstum hy~lroxrdc, 0.03950 g (If lactic acict and 0.03320 g uf tartaric acid were tttratecl (curves a zlntl b respcctivcly). 111 both CLLSC’Y the inflection point is tn;lrkecl ~11rrpproxlmately 5 ml of titrnnt. Tnrtnric acid is titrutcd iis a di-basic acid in pyritlinc mcclium. In the bamc way the tlctcrmin;d,ion of succinic acltl (ICir = 0.6 % lomGand 2.8 X toSo), citric acid (Ii:, = 1.7fi x 10~~ and acid (I& = 1.4 r: I.8 x 10-6 and ,~.a x lo+), formic acid (Ii:, = I .76 X IO+) and propionic roSG) wcrc succc?~ful~y cikrricd out. ‘J’hc wcond hydrogen (of sllccinic ncrtl and the thircl hydrogen tltr;~tutl as lzclng monoof citric acid wcrc only piWtldly titrntcd. Thcrc ncitls wcrq thercfolc, basic unci cli-basic acids respcctivdy. Curve ubte1nct1
Many organic bases arc too weak to Ix titrated in wpcotrs solution, ancl it is recommcndecl, thcreforc, that some suitable non-aqueous solvent IJC tisecl for such titrations. CONANT, HALL, AND WERNER~ have shown that the basicity of organic coml>ounds is increased in acetic acid medium. Votcntiometric or indicator methocls for determining bases have been commonly made in acetic acid7s*[email protected] frcclucncy titrations of some bases have been carried out in acetic &cl or in benzcnc-methanol mixtur@. In the prcscnt study, some analytically important bases were titrated using standard perchioric acid as the titrant and glacial acetic acid as the solvent. The method seems quite suitri?>lc for gcncrztl q@cation.
(CJ Phenonthr&a -(d~Ethyknedkamfne
.Sfutlfif~~di~ufrorc of Itfvunl. Lkrclilor~c acid of z\ppi-tJ@atC strcngttz was prcparccl by simpie ciilutron of the 70$5&acid using glacial iwzctic acid as the cltlttent. Stanrlnrdizaticm of the pcrchloric acid was made using potassium acid phthaltrtc as lhc pritnnry stanrinrcl*~D and employing the Osciilomctcr for cstnblishing titration end points. One lrunclrcci milliliters of glacial ncctic acid were placed in the titration cell of the O~~i~l~)n~~tcr, and an appropriate Etmount of potassium acid phthdatc was then crcIclccl. ‘Chc solution was stirred continumtsly during tlra titration with the pcrchloric acid. A typid titrution curve is shown 111Fig. 2, curve (b). ‘f’hc titration curves obtainctl in st;lnctardization amploying tlic high frcqucncy C?scillomctcr wcrc wcfl &2fincd. Uclvrc
Kefcivttccs
p. 200
VOL.
15
(r956)
ORGANIC
FUNCTIONAL
GROUPS
=97
the equivalence point, very little change in instrument response could be noted. After the equivalence point, the cnrvcs rose steeply givmg well defined, reproducible end-points. I3derminution. Titrations of bases were carried out in the same manner as that described under standardization. The titratlon of some typical organic bases is illustrated by the cnrvcs shown in Fig. 4. In each titration, well defined inflection points were obtained. The cnrves for brucinc (Kb = 7.2 X IO+ and a.5 X 10~~~) and phcnanthrolinc (l
The acidity of imine groups is dependent upon the nature and position of other groups attached to the molecuic. The irninc group may act as a weak acid, and can be titrated as such using pyridine as the solvent, The procedure used in the high frequency studies was the same as that used in the titration of weak acids described above. Fig. 5 shows the results of one such titration. The end-points obtained arc not as satisfactory as might be dcsircd. Very distinct breaks in the titration curves arc usually obtained, but the exact location of the end-point is somewhat uncertain because of the non-linearity of response exhibited, especially in the vicinity of the end-point.
Titvtitio9ts of oxime
gro~t#w
chlfiti~trg
mtd othr
Salinogenic groups may be so weakly acidic that they cannot be titrated directly in either aqueous or pyridine solutions. Where a donor group is located near the salinogcnic group, it is often possihk to curry ottt titrations by using a Chelation
5000 7 Fig.
5.
Refe.rences j2.
200
l?
3
‘Citrntion of ctipicrylamine
4
5
6
7
B
ml
with l~O~I~~icol~ol in pyritline
mcdinm.
R.
vJ8
IKARA,
P.
W,
VOL.
WEST
15
(X956)
reaction to enhance the acidity. The oxime group, for example, can be titrated when a suitable metal is added, provided some second Functional group is present and so located that &elate salts are formed. Upon chelation, one or more protons are liberated and these can then be titrated in aqueous solution using a standard base. The end-poi.nt can be determined accurately by means of a high frequency oscillator, As examples of such titrations, cr-benzoinoximc and salicylaldoxime were determined. l’hcsc compounds react with copper in the following manner: /\__c \ .___/ / -... -L-c I
\.._._/
FI
“-‘-\.___c -_ \__..f 1.. Y-z
/ &-l-2 - --.-
1
-2,
+zii+
I
f‘--\
+
H+
The liberated protons are completely dissociated so that in effect the chelation changes the organic reactants from very weak acids to strong acids. Fig. G shows titration curves obtained by titrating o.oor molar solutions of u-henzoinoximc, nitroso-Ii-salt and salicylaldoximc. 0.002 M bsnzolnaximt M COSO
l0.0005
G, Titrations of functional groups in the prcscncc of mctaf salts,
Fig.
VOL.
15
ORGANIC
(1956)
FUNCTIONAL
GROUPS
199
‘The organic compounds were first dissolvccl in 25 ml of ethyl alcohol and subsequently diluted to 500 ml with distilled water. One hundred milliliters portions of these solutions were then placed in the titration cell of the Oscillometer, and copper sulfate was then adclcd to liberate the hydrogen ions. The role of the metal is illustrated in the case of a-benzoinoxime. If the metal present is in c~~ccss, or in an amount equivalent to the chelating agent, the end-point obtained upon titration with standard sodium hydroxide falls at z mol equivalents (curve “a”), In curve “b”, the mol equivalents of metal ion used was only half of the amount of chelating agent. The titration curve obtained in this case shows an end-point at I mol equivalent of sodium hydroxide, intlicnting that only half of the hydrogen had been rclcascd and that the other half could not bc titrated under the conditions used. Curve “c” shows tha result of 8 typical titration of salicylaldoximc containing one mol cquivatcnt or more of copper ion. Only one hydrogen ion can be relcascd by one mol cquivalcnt of metal antl additional metal ctocs not, thcrcforc, shift the position of the end-point.
Salinogenic groups other than the oxime group can be titrated by raising the reactivity through chelation. For example, nitroso-R-salt reacts with cobalt, 0 = N-O
N=O
(“l‘;““. HO,S’\ / \ fiSO,H
co-t-2 _--__->
-j-I-I-I-
with the release of one proton for each molecule of chelating agent. The released hydrogen can be titrated in the manner described previously, and the titration curves have the general appearance of the one shown in curve “d” of Fig. 6. DISCUSSION
Titrations of organic acids can be made using the high frequency oscillator as a means of detecting end-points. Such titrations can be performed in aqueous solution, provided the dissociation constant of the acid is in the order of 10’~. For weaker acids, some basic solvent such as pyridinc must be used. With pyridine as the solvent, it is possible to titrate acids having dissociation constants in the order of 10‘~. The usual precautions against carbon dioxide absorption should be observed. Other basic solvents, such as diethylamine and ethylcnediamine, were investigated, but were found unsatisfactory because of their hygroscopic character, volatility, or loading effect on the Oscillometer. Organic bases can be titrated using acetic acid as the solvent. The lowest limit of basicity that can bc successfully titrated is represented by bases having dissociation constant of the order of x0-10. For example, urea (Kb = 1.5 x 10’~~) and thiourea (Kt, = r.r x TO-15) could not be titrated. Brucine (Ku = 7.2 x xom4 and 2.5 x 10’11) gave a single inflection point, which is in agreement with the findings of FRITZ’ who reported that only a single break was obtained when brucine was titrated in acetic acid medium. Similarly, phenanthroline gave only one inflection point. KOLTHOFF, LEE AND LEUSSING had reported previously that phenanthroline behaves as a monoacidic base in aqueous solutionlO. High frequency titrations of functional groups hold considerable promise. In the present study’the application was limited to chelation. Liberated hydrogen ions were then titrated directly by means of standard base. With such reactions it is important to note that the course of chelation may be altered depending on the reaction environment. a-Benzoinoxime behaves as a d&basic acid in its rc ctions ,Refemrces p. zoo
R.
200
ILARA,
P.
W.
WrSST
VOL.
15
(rggbj
on the other hand, is known to react in neutral or with copper 13. SaIicylalcloximc, slightly acidic solution with the hydroxyl group serving as the salt forming grouping and the oximc group serving as a donor group. Inner complex salts are thereby formed and one hydrogen ion is released. In an ammoniacal solution, salicylaldoximc reacts to form a normal salt in which the hydroxyl group lends itself to salt formation, but the oximtl group also reacts to form a normal salt with the replacement of a hydrogen ion. Under such conditions, a total of two protons are released. The above reactions arc cited merely to show that it may bc necessary to know the reaction behavior of compounds hcing titrated. In most cases, the titrations are straight forward and a wide variety of compounds can be determined in the manner described, It is also possible that other reactions can be used in which standard solutions of metal ions are used as titrants and normal salt forming reactions, as well as inner-complex salt formation, can be followed by means of high frequency measurements. ACKNOWt.IGDGl
nuttrors wish to cxprcss tldr nssistnncc during thcsc strtrlics. 3%~
appreciation
to IS, 7-I. SnXcirWr
AND Cob~x*hNv fr8r financial
S~~~~ARY ‘Cho higii frequency titration of various urgnnic cation of high freqircnq tecttniqucs in titrations
aci& nncl basca has l3ccn ~titdfc& X%c at@of iminc, oximc and other functional groups has been fmtnti trst?ftrl. In yencral. these mcthocls scum widcty applic:&tc in the dctcrmination of organic compuuncis and funclicmsl groups rrncl can be iisccl for the nnnlysis of cliluto systems.
l~l%XJMnr”t Le titragc h IltLute fr&:qucncc de nombreux rtciclcs ct bases orgitnicjtlcs (L610 &tudib. L’application tics tcchniqucs h hautc frdtjacncc cut ovnntngcusc pour Ie titrayc clcs gronpemcnta iminc, oximc,
etc. En gdn&d, ccs mbthocles somblcnt largcmcnt npplicnblcs nu dosage da compost% ct de fonctions 0rgnniquc.q ct pouvcnt citrc utilindcs pour i’annlyst? WI solutions aqr~cuuo ct ~O~-~~IICIIYC.
ZUSAMMENl’ASSUNG IDiu l-locllfrcrlucn~-Titr,7tioll
vcruchicclencr
organischcr
Stiurcn
unct 13itsen wurclc unteruucht.
Die Anwcndung von I-lochfrecpcnz-Vcrfnhrcn auf die Titration van Iminon, Oximcn uncl andcren Funktionen wurdc niitzlid~ bcft~nclcn. Im Allgcmcinon kdnncn clicsc Mcthoclen zur Bcstimmung von or~~n~~~IlcnVcrbindungen tincl f~~llktioncl~c;~ Gruppcn and zur An;dySc vcrdtinntcr I.tlsnnpcn ~~cit~~}~e~~~lnn~ewcnctct wcrckm.
HIGH
r\NAL\“I-ICA
(1956)
L~IIEQUENCY
CHlhSlCA
TITI
ACTA
SOME
193
ORGANIC
FUNCTIONAL
The clctcrrninations of functional groups in organic compounds is an important and common l~roblcm. Various chemical and physico-chemical methods have been proposed for determining amine, oxime, iminc, carboxyl and other groups. The present study was undertaken in order to show the applicability of high frequency titrations for the dctcrmination of various functional groups both in aqueous and in non-aqueous solutions. High frcclucncy titrations holcl consiclcrablc promise for USC in studying nonaqueous or dilute aclucous systems. ‘l’hc systems being studied arc completely isolated from clcctrodcs and other active parts of the instrument ant1 yet the instrument is itself responsive to slight changes in the composition of the system. High frequency m&hods have been cstablishcd for use in dctcrmining various organic bases through studies made by WAGNER AND KAUPFMAN~ and by MASUI’L I_ANE3 has described high-freclucncy titrations of organic bases, phenols and cnols. The present study was initiated to investigate some of the broader aspects of high frcclucncy titration for functional group analysis.
Chewiculs
urrd rcngruls.
All ChcmlCillS 11w2d wcrc of rcagcnt graclc. Purities wcrc chcckctl by ancl whcrc ncccusary, rcpurificntions wcrc carriccl out by appropriate methods. Apfmrcrtrts. A Sargent i’vlodcl V Oscillomctcr was usctl for high frequency mcasurcmcnts. Hyclrogcn ion conccntmtmne wcrc tlctcrmincd by means of a Ucckman Motlcl C, 1x1 mctcr.
stirndartl
mcanh,
The carboxyl group may be titratccl in cithcr aclucous or non-aqueous media. In most cases, the carboxyl group tends to make compounds water soluble, but such solubility does not necessarily infer that water will be the best reaction medium. As a general rule. acids with very small dissociation constants should bc titrated in a non-aqueous medium. The recommended procedure, however, is to first try the acid in aqueous solution to dctcrmine whether or not the end-point is sufficiently well -__.---_* I’crnlancnt adtlrcss: 1,t30 I-loriuclli, Ilaynma-Slachi, lianagawn-Ken, Japan. Refcrcrrces p.
200
I<. lIARA, I’. w. WEST
194
VOL.
15
(1956)
dcfincd to permit satisfactory titrations. If good end-points are not obtained, then the titrations should bc carried out in pyridine, which has been found to be especially suitable for use in high frequency work.
moo-
E2000 -
Q _
# cf 3000. $ c “n = 4000 -
5000 -
6000 -
I Fig.
1
2
I. Titrations
3
4
5
of acids tn
6
7 ml NaOH
i1CIllC(JllS
solution.
I'cJr the purpoxc of tlctcrmining the applicabihty of high frcl’ifrdio,r itr uqucf~ffs solitllo~. quency titrations of carboxylic acltls in qucous solution, cxnct amounts of sclectcd acids Wcrc of the acicl solution being taken so as to obtain I /~oooM solutions. One huntlrctl milliliters stuclictl wcrc then transferred lo the large titration ccl1 of tho Osclllomctcr ant1 titratcci with 0.0278 M sodium hytlroxitlc. Titration clIrvcs arc shown in Fig. 1 for mnlonic acid ( Iin = 1.G x 10~~ ;Ind 2. I x lo-O), tartnric acicl (I<;, = I.1 x IO+ ant1 6.9 x 10’~). phtholic acid ( ICrl = I .26 x 10.~ nntl 6.9 x IO-O) anti picric acid (I<, = 1.0 x 10’1). ‘L’hc curves for malonlc, tnrtaric and phthalic acids show only one inflection point at 3.5 ml of sodium hyclroxidc. This vohimc of titrant rcprcscnte one mol cquivalcnt of hasc, indicating that only the first hytlrogun can be titrated in aqueous solution and that tlw scconcl hyclrogcn is too wcalc to bc titrated Iinclcr the conditions tlcscribccl. ‘The titration curves for mnlonic and phthalic acids inclicatc a linear rcsponsc for the instrument once the cncl-point has been rcachecl. In the cast of tartaric acid, the ascending portion of the titrntiun curve exhibits a chanfiing slope. This tliffcrcncc is probably due to the fact that the scconcl dissociation constant for tnrtaric acid is grcatcr th;rn the sccontl dissociation constant is, thcrcforc, pnrtlally neutralized for malonic and phthalic acids. l’hc sccontl hytlrogcn probably in the cast of tartaric acid. Picric acid shows il sharp inflection point at onr niol cquivalcnt of base.
Refercrtccs p.
200
VOL.
ORGANIC
15 (xc)g(,)
FU. CTIONAL
GROW15
195
Tifrnf~ns in ~yridrmc. FRITZ AND LISICKI* have proposed the titration of weak organic acids using sodium methoxidc in bcnzcne-methanol mixture. The course of the titration can be followed either by potcntiometric methods or by the USC of color indicators. These investigators have also suggested the use of basic solvents, such as butylarnine and pyricline. ISHI~ATB AND MASUX~ used srmilar tcchmques for dctcrmining organic acids, hut used high frcqucncy oscillators for dctcrmining the end-pomts. The present study has indicated the suitability of pyridinc as the solvent with alcoholic pot.assium hytlroxide as titrant. Standardization of filvnrrt. The alcoholic potassium t~yclroxldc used as the tttrant for organic ;ic~rls was prcparcd by dissolving potassium hydroxiclc in ~~b!dUtC cthvl alcohol. *rhc solution wits kept in g bottle closctl in s~tcli a way as to cxcluclc carbon tlioxidc am1 water. Ucnzoic ilCid was found to bc the most satisfactory primary stnntlard fur USCincstnblishing the strength of the potassium bydroxulc 9olutlon. The titration end-point. as tlctcrmlnctl by means of the Oscillomctcr, was well dcfincd and reprocluciblc. For the standardization, one huntlrctl milliliters of pyritlinc were placed in the titration ccl1 of the inntriiriicnt. 1\ wcighctl amount of
I
primary
2
1
3
4
5
6
8 ml
7
Fig. 2. Stnntlnrtliaation of (a) alcoholic potassium hydroxide in pyriclinc against bcnsoic acid and (1~) pcrchloric acid in acetic acid against potassium acid phtlialntc.
12
3
4
5
Fig. 3. Titrations of acids in pyridinc potassium hydroxide. Rcfcrcnces
fi. aoo
6
7ml with
alcoholic
standard
bcnzoic
ilCid
was
then cllssolvccl in the pyridinc and titrated with tlrc alcoholic potassium 1rydroxltlc. The solution was stirrctl hy means of an electric stirrer having a KlilSS lmpcllcr. The presence of water In the solution must In: avoided as much as posaiblc bccausc cvcn trace .aniounts tcnrl to introduce errors into tlic titration. Vrcshly prcpnrccl nlcoliolic potassium hyclroxitlc should bc ll!wl, ancl tlic pyridinc ~l~ould IJC rcpurifictl if it shows cvcn the slightest discoloration. It should bc notccl that with the progress of the titration a prccipitatc of potassium bcnzoatc scparntcs from the nolvcnt. This precipitation dots not intcrfcre with the titration and in fact, is ncccssary to the success of the mctliod. Small amounts of water which might cause dissolution of the potassium bcnzoatc convert tlic salt into the ionic form which results in serious loading of the Oscillometcr. Titration curves exhibit a slight rise during the initial stages of tlic titrationsThis probably is tluc to the prcscncc of trace amounts of water which dissolve some of the potassium bcnxoatc that is first formed. The curves ultimately flatten until the cndpoint is rcachcd, after which the curves riscstecplyand approximatelylinearly. Curve (a) in Fig. z shows a titration
19G
R.
ffAKA,
i’.
w.
VOL.
WEST
I5
(rggG)
using o.2+t.4 8 of kma0ic acid rlissolvcrl in 100 ml of pyriclrne i~~cl tilratcrl with 4.40 ml of 0.0.~5~~~&2 zllcoholic potassium hytlroxicle. r‘ilrfffions of ucids, l’itrations of acids in pyricline mcctiilm can bo carried out using the same genera1 procctlurc DY that tiescribcd for the standarckation of the alcoholic potsssium hydroxide ayninst pure bcnaoic acid. The cxatnplcrof such titrations arc shown in L:EK. 3. Using 0.09070iW potasstum hy~lroxrdc, 0.03950 g (If lactic acict and 0.03320 g uf tartaric acid were tttratecl (curves a zlntl b respcctivcly). 111 both CLLSC’Y the inflection point is tn;lrkecl ~11rrpproxlmately 5 ml of titrnnt. Tnrtnric acid is titrutcd iis a di-basic acid in pyritlinc mcclium. In the bamc way the tlctcrmin;d,ion of succinic acltl (ICir = 0.6 % lomGand 2.8 X toSo), citric acid (Ii:, = 1.7fi x 10~~ and acid (I& = 1.4 r: I.8 x 10-6 and ,~.a x lo+), formic acid (Ii:, = I .76 X IO+) and propionic roSG) wcrc succc?~ful~y cikrricd out. ‘J’hc wcond hydrogen (of sllccinic ncrtl and the thircl hydrogen tltr;~tutl as lzclng monoof citric acid wcrc only piWtldly titrntcd. Thcrc ncitls wcrq thercfolc, basic unci cli-basic acids respcctivdy. Curve ubte1nct1
Many organic bases arc too weak to Ix titrated in wpcotrs solution, ancl it is recommcndecl, thcreforc, that some suitable non-aqueous solvent IJC tisecl for such titrations. CONANT, HALL, AND WERNER~ have shown that the basicity of organic coml>ounds is increased in acetic acid medium. Votcntiometric or indicator methocls for determining bases have been commonly made in acetic acid7s*[email protected] frcclucncy titrations of some bases have been carried out in acetic &cl or in benzcnc-methanol mixtur@. In the prcscnt study, some analytically important bases were titrated using standard perchioric acid as the titrant and glacial acetic acid as the solvent. The method seems quite suitri?>lc for gcncrztl q@cation.
(CJ Phenonthr&a -(d~Ethyknedkamfne
.Sfutlfif~~di~ufrorc of Itfvunl. Lkrclilor~c acid of z\ppi-tJ@atC strcngttz was prcparccl by simpie ciilutron of the 70$5&acid using glacial iwzctic acid as the cltlttent. Stanrlnrdizaticm of the pcrchloric acid was made using potassium acid phthaltrtc as lhc pritnnry stanrinrcl*~D and employing the Osciilomctcr for cstnblishing titration end points. One lrunclrcci milliliters of glacial ncctic acid were placed in the titration cell of the O~~i~l~)n~~tcr, and an appropriate Etmount of potassium acid phthdatc was then crcIclccl. ‘Chc solution was stirred continumtsly during tlra titration with the pcrchloric acid. A typid titrution curve is shown 111Fig. 2, curve (b). ‘f’hc titration curves obtainctl in st;lnctardization amploying tlic high frcqucncy C?scillomctcr wcrc wcfl &2fincd. Uclvrc
Kefcivttccs
p. 200
VOL.
15
(r956)
ORGANIC
FUNCTIONAL
GROUPS
=97
the equivalence point, very little change in instrument response could be noted. After the equivalence point, the cnrvcs rose steeply givmg well defined, reproducible end-points. I3derminution. Titrations of bases were carried out in the same manner as that described under standardization. The titratlon of some typical organic bases is illustrated by the cnrvcs shown in Fig. 4. In each titration, well defined inflection points were obtained. The cnrves for brucinc (Kb = 7.2 X IO+ and a.5 X 10~~~) and phcnanthrolinc (l
The acidity of imine groups is dependent upon the nature and position of other groups attached to the molecuic. The irninc group may act as a weak acid, and can be titrated as such using pyridine as the solvent, The procedure used in the high frequency studies was the same as that used in the titration of weak acids described above. Fig. 5 shows the results of one such titration. The end-points obtained arc not as satisfactory as might be dcsircd. Very distinct breaks in the titration curves arc usually obtained, but the exact location of the end-point is somewhat uncertain because of the non-linearity of response exhibited, especially in the vicinity of the end-point.
Titvtitio9ts of oxime
gro~t#w
chlfiti~trg
mtd othr
Salinogenic groups may be so weakly acidic that they cannot be titrated directly in either aqueous or pyridine solutions. Where a donor group is located near the salinogcnic group, it is often possihk to curry ottt titrations by using a Chelation
5000 7 Fig.
5.
Refe.rences j2.
200
l?
3
‘Citrntion of ctipicrylamine
4
5
6
7
B
ml
with l~O~I~~icol~ol in pyritline
mcdinm.
R.
vJ8
IKARA,
P.
W,
VOL.
WEST
15
(X956)
reaction to enhance the acidity. The oxime group, for example, can be titrated when a suitable metal is added, provided some second Functional group is present and so located that &elate salts are formed. Upon chelation, one or more protons are liberated and these can then be titrated in aqueous solution using a standard base. The end-poi.nt can be determined accurately by means of a high frequency oscillator, As examples of such titrations, cr-benzoinoximc and salicylaldoxime were determined. l’hcsc compounds react with copper in the following manner: /\__c \ .___/ / -... -L-c I
\.._._/
FI
“-‘-\.___c -_ \__..f 1.. Y-z
/ &-l-2 - --.-
1
-2,
+zii+
I
f‘--\
+
H+
The liberated protons are completely dissociated so that in effect the chelation changes the organic reactants from very weak acids to strong acids. Fig. G shows titration curves obtained by titrating o.oor molar solutions of u-henzoinoximc, nitroso-Ii-salt and salicylaldoximc. 0.002 M bsnzolnaximt M COSO
l0.0005
G, Titrations of functional groups in the prcscncc of mctaf salts,
Fig.
VOL.
15
ORGANIC
(1956)
FUNCTIONAL
GROUPS
199
‘The organic compounds were first dissolvccl in 25 ml of ethyl alcohol and subsequently diluted to 500 ml with distilled water. One hundred milliliters portions of these solutions were then placed in the titration cell of the Oscillometer, and copper sulfate was then adclcd to liberate the hydrogen ions. The role of the metal is illustrated in the case of a-benzoinoxime. If the metal present is in c~~ccss, or in an amount equivalent to the chelating agent, the end-point obtained upon titration with standard sodium hydroxide falls at z mol equivalents (curve “a”), In curve “b”, the mol equivalents of metal ion used was only half of the amount of chelating agent. The titration curve obtained in this case shows an end-point at I mol equivalent of sodium hydroxide, intlicnting that only half of the hydrogen had been rclcascd and that the other half could not bc titrated under the conditions used. Curve “c” shows tha result of 8 typical titration of salicylaldoximc containing one mol cquivatcnt or more of copper ion. Only one hydrogen ion can be relcascd by one mol cquivalcnt of metal antl additional metal ctocs not, thcrcforc, shift the position of the end-point.
Salinogenic groups other than the oxime group can be titrated by raising the reactivity through chelation. For example, nitroso-R-salt reacts with cobalt, 0 = N-O
N=O
(“l‘;““. HO,S’\ / \ fiSO,H
co-t-2 _--__->
-j-I-I-I-
with the release of one proton for each molecule of chelating agent. The released hydrogen can be titrated in the manner described previously, and the titration curves have the general appearance of the one shown in curve “d” of Fig. 6. DISCUSSION
Titrations of organic acids can be made using the high frequency oscillator as a means of detecting end-points. Such titrations can be performed in aqueous solution, provided the dissociation constant of the acid is in the order of 10’~. For weaker acids, some basic solvent such as pyridinc must be used. With pyridine as the solvent, it is possible to titrate acids having dissociation constants in the order of 10‘~. The usual precautions against carbon dioxide absorption should be observed. Other basic solvents, such as diethylamine and ethylcnediamine, were investigated, but were found unsatisfactory because of their hygroscopic character, volatility, or loading effect on the Oscillometer. Organic bases can be titrated using acetic acid as the solvent. The lowest limit of basicity that can bc successfully titrated is represented by bases having dissociation constant of the order of x0-10. For example, urea (Kb = 1.5 x 10’~~) and thiourea (Kt, = r.r x TO-15) could not be titrated. Brucine (Ku = 7.2 x xom4 and 2.5 x 10’11) gave a single inflection point, which is in agreement with the findings of FRITZ’ who reported that only a single break was obtained when brucine was titrated in acetic acid medium. Similarly, phenanthroline gave only one inflection point. KOLTHOFF, LEE AND LEUSSING had reported previously that phenanthroline behaves as a monoacidic base in aqueous solutionlO. High frequency titrations of functional groups hold considerable promise. In the present study’the application was limited to chelation. Liberated hydrogen ions were then titrated directly by means of standard base. With such reactions it is important to note that the course of chelation may be altered depending on the reaction environment. a-Benzoinoxime behaves as a d&basic acid in its rc ctions ,Refemrces p. zoo
R.
200
ILARA,
P.
W.
WrSST
VOL.
15
(rggbj
on the other hand, is known to react in neutral or with copper 13. SaIicylalcloximc, slightly acidic solution with the hydroxyl group serving as the salt forming grouping and the oximc group serving as a donor group. Inner complex salts are thereby formed and one hydrogen ion is released. In an ammoniacal solution, salicylaldoximc reacts to form a normal salt in which the hydroxyl group lends itself to salt formation, but the oximtl group also reacts to form a normal salt with the replacement of a hydrogen ion. Under such conditions, a total of two protons are released. The above reactions arc cited merely to show that it may bc necessary to know the reaction behavior of compounds hcing titrated. In most cases, the titrations are straight forward and a wide variety of compounds can be determined in the manner described, It is also possible that other reactions can be used in which standard solutions of metal ions are used as titrants and normal salt forming reactions, as well as inner-complex salt formation, can be followed by means of high frequency measurements. ACKNOWt.IGDGl
nuttrors wish to cxprcss tldr nssistnncc during thcsc strtrlics. 3%~
appreciation
to IS, 7-I. SnXcirWr
AND Cob~x*hNv fr8r financial
S~~~~ARY ‘Cho higii frequency titration of various urgnnic cation of high freqircnq tecttniqucs in titrations
aci& nncl basca has l3ccn ~titdfc& X%c at@of iminc, oximc and other functional groups has been fmtnti trst?ftrl. In yencral. these mcthocls scum widcty applic:&tc in the dctcrmination of organic compuuncis and funclicmsl groups rrncl can be iisccl for the nnnlysis of cliluto systems.
l~l%XJMnr”t Le titragc h IltLute fr&:qucncc de nombreux rtciclcs ct bases orgitnicjtlcs (L610 &tudib. L’application tics tcchniqucs h hautc frdtjacncc cut ovnntngcusc pour Ie titrayc clcs gronpemcnta iminc, oximc,
etc. En gdn&d, ccs mbthocles somblcnt largcmcnt npplicnblcs nu dosage da compost% ct de fonctions 0rgnniquc.q ct pouvcnt citrc utilindcs pour i’annlyst? WI solutions aqr~cuuo ct ~O~-~~IICIIYC.
ZUSAMMENl’ASSUNG IDiu l-locllfrcrlucn~-Titr,7tioll
vcruchicclencr
organischcr
Stiurcn
unct 13itsen wurclc unteruucht.
Die Anwcndung von I-lochfrecpcnz-Vcrfnhrcn auf die Titration van Iminon, Oximcn uncl andcren Funktionen wurdc niitzlid~ bcft~nclcn. Im Allgcmcinon kdnncn clicsc Mcthoclen zur Bcstimmung von or~~n~~~IlcnVcrbindungen tincl f~~llktioncl~c;~ Gruppcn and zur An;dySc vcrdtinntcr I.tlsnnpcn ~~cit~~}~e~~~lnn~ewcnctct wcrckm.