Differentiating Titration of Acid Mixtures in Acetone JAMES S. FRITZ and STANLEY S. YAMAMURA Institute for Atomic Research and Department o f Chemistry, lowa Stafe College, Ames, lowa
b A
wide variety of organic acids can b e titrated in acetone using a tetraalklylammonium hydroxide in benzenemethanol as the titrant. Weakly acidic substances can b e titrated and the products of titration are soluble. The titrations can b e followed potentiometrically using a glass-modified calomel electrode system. The titration of phenols, carboxylic acids, enols, imides, and sulfonamides was studied. In general a differentiating titration is feasible if the potential at the start of the inflection of the stronger acid and the potential at the beginning of the curve of the weaker acid differ by more than 100 mv.
T
I I R ~ T I O N of acidic orginic compounds in nonaqueous solvents has received considerable attention in recent year< In 1948, Moss, Elliott. and Hall (IO) ihowed that many weakly acidic phciiolr could be titrated in a nonacidic solvrmt such as ethylenediamine. Fritz anti others used alkali metal alcoholates for the determination of phenols (3, 7 , 8 ) , enolq anti imides ( 5 ) , salts (d), sulfa druqs and sulfonamides (6), and carboxylic acids (8). More recently tetrabutylammonium hydroxide in benzenemetliitnol ( 2 ) and in absolute isopropyl alcohol (9) has been used in the titration of a variety of organic acids. Cundiff ant1 Narkunas (2) investigated many solvents but performed most of their titrations in pyridine. Harlow, Noble, ant1 K y l d (9) concentrated their efforts or1 titrations in ethylenediamine, pyridine, and alcohols, but also recomnicnded methyl ethyl ketone for differeritiating titrations. Rruss and Wyld re( ommended methyl isobutyl ketone f(J1 the resolution of certain acid mixtuws and nitrogen bases (1). I'ntil recently (1, 2) relatively little attention has been paid to the problem of tletermining individual acids in mixturm using differences in acidity of the acidic constituents of the mixture as the basis for analysis. The purpose of this paper is to study in a systematic manner the titration of acidic organic compounds in a suitable nonaqueous medium, and from this t o be able to predict reasonably well which compounds can be differentiated quantitatively in mixtures. For this investigation a tetraalkylammonium hydroxide (triethyl-n-butylaninionium hydroxide) n nc: chosen a$
the titrating base. This titrant has two major advantages over the alkali metal bases previously used. I n virtually every case the tetraalkylammonium salt of the organic acid titrated is soluble; precipitate formation is a more frequent occurrence using sodium or potassium alkoxide as the titrant. The other point in favor of tetraalkylammonium hydroxide is that excellent potentiometric curves are obtained in a variety of solvents using a glass-calomel electrode system. T h e n this electrode system is used for titration of weak acids with a solution of an alkali metal base, the curves are less reproducible and the end point breaks are greatly reduced. Acetone has been chosen as the sol\-ent medium for most of the titrations reported here. It is sufficiently lacking in acidic properties to permit titration of weak acids, but is also nonbasic and therefore is not a leveling solvent for acids (as are ethylenediamine. butvl-
amine, and the like). Acetone has ci sufficiently high dielectric constant to permit potentiometric titration with steady readings using conventional electrodes. It is inexpensive and can be used without purification. Its only major limitation is that it apparently enters into reactions with rather strong acids, as evidenced by loss of stoichionietry when titrating such compounds. REAGENTS
Triethyl-n-butylammonium hydrouide, 0.1N. The method of preparation used was essentially that described by Cundiff and Markunas ( 2 ) . Triethyln-butylammonium iodide was prepared by refluxing equivalent quantities of triethylamine and n-butyl iodide, The air-dried quaternary ammonium salt was converted to the hydroxide by agitating 30 grams of the salt for about 90 minutes with 90 ml. of absolute methanol and 27 grams of powdered silver
I. 2,4-0INITROPHENOL
2p-NITROFWNOL
+
3.2,6.MBROMOPHENOC 4. p-NiTROPHENOL
loo
5. SUCYLALDEHYDE 6. m NITROPHENOL
-
I
--t
I0,FHYDRIXYACETOPHENONE
1
11.2-NAPHTHOL
12. PHENOL 13. METHYLSAUCYLATE
I 20
I 40
I M)
I
en
I m
I
IP
I
PER CENT N E U W Z A T K N
Figure 1 .
Titration of phenols VOL. 29, NO.
7,JULY 1957
1079
oxide. The mixture was then filtered under nitrogen and the filtrate was diluted to 1 liter with anhydrous benzene. Acetone, reagent grade. Acetonitrile, Matheson or Eastman. Benzoic acid, reagent grade. Diniethylforniamide, technical grade. Sitrogen, prepurifid Samples of acids, analyzed a? received; mostly 98 to 1007,purc'.
+
zoc
+ 'OC
0
Figure 2.
- 10:
1
1
2,4
APPARATUS
Beckniaii general-purpose glass electrode KO. 1190-80. Beckmaii fiber-tjpe calomel electrode, Yo. 11i0, or sleeve-type calomel electrode, S o . 1170-71. Both electrodes wcre modified by replacing the aqueous potassium chloride solution by a saturated solution of potassium chloride in methanol. Buret, 10 ml., Normax. Precision-Shell Dual AC Titroiiieter, or other direct-reading titration apparatus Magnetic stirrer. PROCEDURE
Titrate 0.4-to 0.8-meq. samples of the acid or acid mixture with 0.1S triethyl - n - butylaninioniuni hydroxide, using 40 ml. of acetone as the solvent. Follow the titration potent,iometrically and determine the end point by plott'ing potential us. volume of titrant. Subtract the solvent blank from the total volume. K h e n two or more inflections are observed. the difference between successive end points should be used to calculate the, volume of titrant equivalent to thc aci I ~ p r c sented. The solvent blank i; .utitracted from the last interval. St ind: rdize the titrant by potentiometric titration of a weighed portion of brnzoir acid. The usual method of performing potent'iometric titrations is to add a set increment of titrant before the end point and a smaller series of increments near the end point. This procedure is satisfactory but relatively tinieconsuming. It is more convenient and faster to add increments of such size that a constant change of approximately 30 mv. is obtained. By this method, the size of the increments near the end point' is automatically decreased. EXPERIMENTAL
I n this study five classes of acidic organic compounds were consideredphenols, carboxylic acids, enols, imides. and sulfonamides. I n each class individual compounds were titrated potentiometrically. The individual compounds selected for titration contained a variety of electron-withdrawing functional groups. The number of these groups and the position of the functional groups relative to that of the acidic group were also varied. The potentiometric curves of individual acids can be used to predict the
1080
ANALYTICAL CHEMISTRY
Titration
of phenol mixtures - DINITROPHENOL
vi
5 -200 > 0
-
J
-
= -300
LL
0 4 _I
z
-400
w
c a 3
- 500
-EO0 - 700 -S
E
-w
1 I
I
I
3
2
I
4
V O L U M E OF
Table I.
5
Phenol o-Bromophenol p-Bromophenol ni-Bromophenol 2,4-Dibromophenol 2,6-Dihromophenol o-Hydroxybenzaldehyde (anlic.ylaldchyde) p-Hydrosybenzaldeh!-de m-Hydroxybenzaldehyde o-Sitrophenol p-Xitrophcnol vi-Xitrophenol 2,4-1)initrophenol o-Hydrosyacetophenone p-H ydrosyacetophenone Ethyl vanillin llethyl salicylate o-Phenylphenol 2-Saphthol l-Sitroso-2-nn~~htliol Table II.
Compoiiiid
5,5-Dimethyl-1,3-c.1.clohe\alledione
2,4-Pentanedione (acetylacetone) Acetoacetanilide Ethyl malonate Malononitrile Cyanoacetamide Ethyl cyanoacetate
I 7
I
,
Results of Titration of Phenols
Conipound
Dibenzo! lmethane
6
TITRANT
lf illieqnivalent Taken Found 0 265 0 259 0 514 0 514 0 531 0 521 0 391 0 372 0 418 0 410 0 473 0 466 0 377 0 375 0 425 0 406 0 540 0 521 0 511 0 522 0 519 0 350 0 405 0 413
0 503 0 528 0 487
c c Recovery
97.8 100.0 98.2 95.2 98.0 98.4 99.4
0 422 0 401 0 534 0 518 0 503 0 516 0 512 0 345 0 401 0 406 0 497 0 527 0 481
99.3 98.7 98.9 99.6 98.5 98.5
98.7 98.7 98.9
98.4
98.9 99.8 98.7
Results of Titration of Enols
\Iilliequivaleiit Taken Found 0 489 0 479 0 543 0 538 0 353 0 353 0 542
, Recovery
0 534
Decompoeition or ieaction solvent 0 300 0 297 0 398 0 404 0 363 0 3T6
98 0 99 0 98 2 98 5
nith 99 0 98 4
96 6
feasibility of a differentiating titrat,ion of two or more acids. By moving the curve of t'he weaker acid horizontally so that i t begins a t the stoichiometric point of the stronger acid, an estimation of the curve for titration of a mixture of two acids is obtained. The curve actually obtained is usually a t least as good as predicted by this method and is often somewhat better. A useful gcneralization is t h a t the differentiating titration is feasible if the potential a t the start of thc inflcction of t'he stronger acid and the potential a t tlic beginning of the cwrvc for the n-eakpr acid differ by approxiniat,cly 100 niv. Phenols. Eightwn individual phenols w ( ~ r vtitratcd 1,oteatiomctri~ally. T h t,it r:i t i o lis a 1 c Ft o i hi om c t ri c ( Talilc I) Iwcai!i:i~ the IcroYcries are n-itliin thv 9; t o 100yGraiipc. which is t h c c~stimatctlpurity of t h e samples tnkc,ii. Titration curves of rPprcwiit:itirc, i)lic~noli::ii'c plottcd in Figurc 1. Ihaiiiination of the curves obt ai n c d sh o \v t h a t i . 1 c~t i,on-\\-it h tl r a wing groul s incw:ise the acidity of plic~iiolsin (3
(1
0
//
0
// > ---C-R
thv ordcir -SO, > -C-H > C'l. Rr > -C6Hs. d group in the ortho or para position affects t,he acidity more than in the nieta position. Hytlrogeii h i d i n g :ipparently reduces the :idit!- \vlicn thc proper group is in the ortho position. Thus. para - S O 2 , 0
/i
0
/
-C-H. anti --C-C'H, phenols are each niore acidic than the corrcsponding ortho compounds. Thc stwp slope of many of the curves reduces the possibilities for differentiating titrations, but several are still feasible. 2.4-Dinitrophenol is quit,e acidic and can be titrated in the presence of any of thf, other phenols listed. 0- and p-nitrophenol. 2,B-dibromophe1101, and salicyaldehyde are moderately acidic and (>anhe titrated in the presence of phenol. alkyl-substituted phenols. methyl salicylate, and o-phenylphraol . I n addition, 2,6-dibroniophenol can bc titrated in the presence of any cif the nionohalogenated phenols. Titration curves of some mixtures are shown in Figure 2. Carboxylic Acids. I n Figure 3 , titration curves for several carboxylic acids are given. Simple carboxylic acids all tend t o he of about t,he same acid strength. The presence of halogens on t h e rarbon atom does increase tht. acid strengbh significantly, as dori: one or more nitro groups in t h e bcnzc.ne ring of a n aryl carboxylic acid. Ilichloroacetic acid can be titrated in the presence of acetic acid, but not in the presence of chloroacetic acid. Enols. The curves for enolic comuounds of t h e type A-CH2-A'
are given in Figure 4. -4 a n d A' represent various electron-withdraw0 0
// mg groups such as -C-R,
s
// -C-OR,
anedione. arr flat and ideally suited to differentiating titration$. Data for the quantitative titration of individual enols are given in Table 11. Imides and Sulfonamides. Imides 0
H0
//
B
-C--SH2, -C--SHCeHj, and -C=S. (-c/TH-C-), thioimides, sulfonThese curves. with the exception amides. and acidic thioureas are inof that for ~.~-dimethyl-l,3-~yclohelr- cluded in this group. Pome titration
Table Ill.
Results of Titration of Imides and Sulfonamides
Slilliequivalent Found
Compound Hydantoin Phthalimide Dithiobiuret Succinimide
Takrn 0 475 0.418
l-;lcetyl-2-thiohydantoin
0 502 0.57ti 0 532 0 445 0 571
sU,,1-DiphenSlthiourea Siilfanilamide Snlfathinzole Sulfapyridine
0.451 0 523
YGRecoyery 99.7 100 3 100.2 99.4 100.2 99.9 99.6 98.6 99.4
0.4T-I
0.419 0,452 0,520 0.503
0.575 0,530 0,439 0 . 567
Table 1V.FResults of Differentiating Titrations
Slilliequivalent Conipoiinds 2,6-~ibronnophenol, p-bromophenol 2,4-Dinitrophenol, o-nitrophenol p-Sitrophenol, phenol p-Hydro.c\-benzaltiel~~ de. phenol Ethyl vanillin, methyl salicylate 2,&Dinitrophenol, 2,6-dibromophenoi 2,4-Dinitrophenol, p-nitrophenol 2,4-Dinitrophenol, o-nitrophenol, methyl salicylate 2.6-Dibromophenol, o-bromophenol 5,5-Dimethyl-1,3-cyclohexanedione, dibenzoylmethane 1 - 4cetyl-2-thioh\-dantoin,
succinimide I -Acetyl-2-thiohydant oin, hydantoin Sulfathiazole, hydantoin Sulfathiazole, sulfapyridine Salicylic acid, methyl salicylate Sulfathiazole, sulfanilamide 2,4-Dinitrophenol, sulfapyridine 2,4-Dinitrophenol, acetoacetanilide 2,4-Dinitrophenol, 2,4-pentanedione 2,6-Dibromophenol, dibenzoylmethane S,S'-Diphenylthioiirea, phenylthiourea
Taken
0,292 0,256 0,342
0.223 0.148 0 402 0,226 0.41s 0 227
0,287 0.30: 0.356 0.222 0,444
0.228 0,303 0 151 0.277 0 31
