Photometric Titration of Weak Bases in Nonaqueous Media L. E. 1. HUMMELSTEDT and DAVID N. HUME Department of Chemistry and laboratory for Nuclear Science, Massachusetts Institute of Technology, Cambridge 39, Mass.
A photometric titration technique for the determination of weak bases uses perchloric acid in acetic acid as titrant and acetic acid or acetonitrile as solvent. Differentiating titration of as many as four components in a single mixture is possible. Comparison with potentiometric titrations in the same media shows the photometric technique to b e preferable in many instances where the bases to b e differentiated are very similar in strength. The versatility of the photometric procedure in differentiating bases is greatly increased by changing the wave length during titration of mixtures.
S
few applications of the 'photometric titration method to acid-base titrations in nonaqueous solutions have been reported (1, 8, 12-14), although it was long ago pointed out that the method probably has its potential use in the nonaqueous field ( 7 ) . The present investigation was undertaken to explore the usefulness of the photometric technique for titration of bases,, particularly differentiating titrations involving mixtures of weak bases, in nonaqueous solutions. URPRISIXGLY
O-. 0
576
I
The theoretical fundamentals of the photometric titration method have been discussed by Goddu and Hume (6, 7 ) . These authors used the method for determination of weak acids in aqueous solutions (6) and pointed out that the same general principles apply to the titration of weak bases. I n aqueous solutions, the basicity of the solvent limits the titratability to relatively strongly basic compounds, while an inert or acidic solvent permits titration of much weaker bases. Solvents such as acetonitrile, dioxane, and glacial acetic acid have long been used as titration media for potentiometric or visual titration of bases, but these are unsatisfactory in titrations of very weak bases, or in differentiating titrations of mixtures where the components are similar in basic strength. I n such cases it may be difficult or impossible to locate the inflection in a potentiometric curve or the color change in a visual titration. If the titration reaction involves a change in absorption characteristics, it is then advantageous to follow its progress by measuring the abporbance of the solution which is directly proportional to the concentration of the absorbing species. A particularly attractive
feature of a photometric titration is that the location of the end point does not depend on measurements taken in the immediate vicinity of the stoichiometric equivalence point. The linear portions of the curve may be extrapolated to intersect, thereby giving an estimate of the end point which can be determined with good accuracy even where the titration curve shows a rounded break due to incomplete or slow reaction. EXPERIMENTAL
Apparatus. Many of the titrations in t h e visible wave length region were performed using a Beckman Model B spectrophotometer with the simple titration assembly developed by Goddu and Hume ( 7 ) . For titrations in the ultraviolet region, a Beckman 3Iodel DU spectrophotometer was adapted by replacing the metal cover of a standard 10-cm. cell compartment with one of wood, through which a hole for the buret tip had been drilled. The cell carriage was removed from the cell compartment so that the titration beaker could be placed directly on the floor and the spectrophotometer was raised on wooden blocks to provide space for a conventional magnetic stirring motor directly underneath the cell compartment. For titrations in the visible wave length region, 150-ml. borosilicate glass beakers served as titration cells. I n the ultraviolet, 150-ml. beakers of Vycor grade 7910 (Corning Glass Works, Corning, X. Y,), transparent down: to about 241 mp, were used. Because run-to-run reproducibility of the light
-I
1.00 2 00 ml. 0.4391M HClO4
3 00
0
IO0
2 00 m 1 . 0 4 3 9 l ~HC104
3 00
Figure 1. Photometric titration of 1.109 meq. of m-nitroaniline
Figure 2. Photometric titration of 0.794 meq. of 8chloroquinoline
In glacial acetic ocid with 0.4391 M perchloric acid
Conditions or in Figure 1
ANALYTICAL CHEMISTRY
path is not a requirement, a simple arrangement of sponge rubber ads keeps the titration beaker in a [xed position during a run. Effective stirring was provided by Teflon-covered magnetic stirring bars, the speed being adjusted so that the vortex generated in the center of the beaker did not descend into the light path. Titrant was delivered from a 5-ml. Exax microburet, painted black below the graduated portion. The buret tip extended through the rubber-lined hole in the titration cell cover into the solution in the beaker. For nearly all purposes this simple beaker and magnetic stirrer titration setup are adequate, and recourse to elaborate titration systems involving circulation pumps and shortpath cells is unnecessary. For comparison purposes, a number of potentiometric titrations were per-
01 0
I
I
formed using a Leeds & Korthrup p H meter equipped with a Leeds & Northrup glass electrode (Std. 1199-30) and a sleeve-type calomel electrode. Solvents, Chemicals, and Reagents. Most of the titrations were performed in glacial acetic acid or acetonitrile because these two solvents are sufficiently transparent for most purposes and are well established solvents for the titration of basic compounds. Ordinary reagent grade glacial acetic acid was an adequate solvent for the titration of all b u t extremely weak bases ( ~ K in B water greater than about 12.5). For these, anhydrous acetic acid was prepared by addition of the calculated amount of acetic anhydride (as determined by Karl Fischer titration) and sulfuric acid catalyst, and subsequent distillation of the anhydrous solvent through a 60-cm. Vigreux
!
1
4 00
500
I
2 00 3 00 mi 0 4 3 9 1 ~ H C 1 0 4
IO0
Figure 3. Photometric titration of mixture of 0.502 meq. of 2-methyl-5-nitroaniline and 1.007 meq. of 4-methyl-2-nitroaniline Conditions as in Figure 1
I
900 1
0
I
IO0
2 00 3 00 ml. 0.4391&jHClO4
Figure 4. Potentiometric titration of mixture of 0.502 meq. 5-nitroaniline and 1.007 meq. of 4-methyl-2-nitroaniline Conditions as In Figure 1
400
of 2-methyl-
column (10). Glacial acetic acid prepared by this method contained less than 0.002% water, as determined by Karl Fischer titration. Technical grade acetonitrile (Union Carbide Chemicals) was sometimes useful as such, but was of varying quality. Occasionally the addition of perchloric acid resulted in formation of a red color which interfered with the absorbance measurements. The colorforming impurity could be removed satisfactorily by distillation from phosphorus pentoxide and most of the acetonitrile used was purified in this manner. Reagent grade acetonitrile can be used without purification. Most compounds titrated were of Eastman White Label grade, used without purification when possible. All nitroanilines and N,N-dimethylsubstituted nitroanilines were purified by recrystallization from water or waterethyl alcohol mixtures. Perchloric acid (0.5JJ) prepared by dissolving 70 to 72% acid in glacial acetic acid was used as titrant throughout the work, even when the sample solvent was acetonitrile. This watercontaining titrant was adequate for titration of all but the weakest bases. When an anhydrous titrant was required, the water was removed by adding the calculated amount of acetic anhydride, as determined by Karl Fischer titration. The anhydrous titrant mas slightly yellowish, but no interference with the titration was observed a t any wave length. The report that solutions of perchloric acid in acetonitrile are unstable 112) was verified in the present study. Water, introduced with the 70 to 72% acid normally used in preparing the titrant, hydrolyzes acetonitrile in the strongly acidic solution. The primary product of this hydrolysis, acetamide, is a weakly basic compound which decreases the sharpness of the end points. If the titrant stands for several months, the hydrolysis procceds further to form ammonium acetate, with eventual precipitation of ammonium perchlorate. For this reason perchloric acid in acetonitrile is satisfactory only when freshly prepared, while a titrant made up in glacial acetic acid is stable indefinitely. The perchloric acid titrants were standardized by visual titration of potassium acid phthalate in glacial acetic acid, using crystal violet as indicator. In general, 0.4 to 0.5M acids were used, less concentrated solutions, if needed, being prepared by dilution. Titration Technique. Most titrations were performed a t the 10-2M level using a sample volume of 100 to 110 ml. Thus, 1 to 3 ml. of 0.5M perchloric acid were normally consumed in a titration. The effect of dilution on the absorbance readings was, therefore, small and no corrections were applied in plotting the titration curves. If the absorbance a t the selected wave length was known to increase during the titration, the spectrophotometer was set to read zero absorbance before adding the VOL. 32, NO. 6, M A Y 1960
577
53
,
Photometric Titrations of Single Bare in Acetonitrile and Acetic Acid 0.4 t o 0.5.U Perchloric Acid Titrants in Acetic Acid Titrant Consumption % Solvent Calcd. Obsd. Deviation Compound ?.oo -1.0 1.98 Acctorii trile A n i 1i lie 1.99 -2.0 ?.03 .Y,,Y- tliinctii~lu:!ilinc Acotoiiitrile 1 .oo 0 0 1 .oo Acetonitrile .Y,.Y- Die t hvlani!ine 1 .!I5 -1.0 1.W Acetonitrile p-C tilorondine 1 .05 0.0 1.05 Acetonitrile ut-Chloroaniiine 1.X -1.0 1.99 Acetoiii trile o-Chloronniline ? . 19 -0 D 2.li o-Chloronniline Acetic acid 2.41 0.0 2.41 .4cetic acid p-Chloroiinilirie ?.34 A 1.i 2.30 .4cctic acid 2,1-l~ichloronni!itie 2.37 -3.0 ?.30 Acetic acid "5-l)ichioronniline 2.32 0.0 2.32 In-Sitroaiiiiine hcctic acid 2.34 71.3 2 31 Acetic w i d p-Sitronniline .Y,.YDi me thy- tn-n i t roAcctic ncici 2.29 2.30 "0.4 :iiiiline
fable I.
!Y,!Y- Iliniethyl-p-ni tro:ini linp
'?-.\Iethyl-j-nitronniline 4-~Icthyl-?-nitronniline
Table II,
Acetic acid Acetic acid Acetic acid
3.36 2 29 3.29
2.42
2.29 2.31
-0.9
Differentiating Titrations of Bases in Acetonitrile
Aiiilinc
71-Chioronniliiie o-C hloroanil ine Aniline 5-Yit-ro-I-naphthylamine
Sodium acetnte j-Sitro-I-naphthylaminc Di-n-butylamine Di-n-butylamine h:,h'-Diethylaniline d nil ine Aniline o-Chloronpiline h n il i n e m-Chloroaniline
p&( H20) 7,B 9.4 10.0
11.4
1.9i 1.99
9.4
1,05
II.?
0.5i 2.20 0.5i 0.55
9.2
11.2
2.G I .3
99.4 .4
11.4 9.4 10.5
titrant. In titrations involving an absorbnnce decrease, the instrument was adjusted to zero absorbance against the pure solvent before addition of the sample. The titration wave length v a s selected to give absorbance readings below 1.0 to minimize any interference from stray light. Titration of Single Bases. In discussing the different possible shapes of photometric titration curves, Goddu and Hume (7) pointed out that it is advantageous to have a decrease rather than an increase in absorbance during the titration because the former type of curve is less susceptible to errors resulting from dilution and light leakege than the latter. A lnrge and important group of organic bases, the aromatic amincs, gives this favorable curve shape when tho titration is performed u t R wave length somewhere on the long wave length absorption peak of the basic form. Protonation ties up the tree electron pair on the nitrogen atom, thereby eliminating this absorption band. Aromatic nitrogen heterocyclics, such as pyridine and quinoline and their derivatives, can also be titrated photometri-
*
AF(ALYllCA1 CHEMlSTRY
Titrant Consumption Ca!cd. Obsd. 1 .QO 1 .oo LO8 LOO 1,95
1 .ns
1.96 1.96 0.56 2.23 0.56 0.53 0.78 l.9i
0.09
0.99
1.98 LO5
1.91 1.11
n 0 , 5sn0
,&
+2.5 0.0
0.4 to 0 . 5 M Perchloric .4cid Titrants in Acetic Acid
Coniponcnts .\',.Y-I~ictt;~.laniline
I
7c Deviation 0,o $1.0
-1.0 -l,5 -0.5
-1.8 -1.4 -1.8 -3,G - 2 . 55 -0,s
0.0 -3.5 +5.7
cally. For these compounds, however, protonation causes a red shift of the absorption peak and, therefore, titration a t the long wave length absorption edge results in a curve showing increasing absorption. Figures 1 and 2 show the titration of representative compounds which illustrate these two main types of titration curve. Table I presents some results of photometric titrations of aromatic amines in acetic acid and acetonitrile, The agreement between calculated and observed titrant consumption is satisfactory in view of the fact that most of tho compounds titrated were not of highest purity. If the aqueous pKB value for a base is lower than about 10, the base cannot be determined by direct photometric titration in ncetic acid becausc it is completely protonated by the solvent and no change in absorbance would be observed during the titration. Such a compound can be titrated in acetic acid if a weaker indicator base is added, but it is often simpler to use a nonacidic solvent such as acetonitrile, For titration of extremely veak bases acetic acid is the
wove
,iP-
do
icpFtb,rnp
Figure 5. Absorption spectra of 4,06X 1 O-'M aniline and 4.09 X lO-'M N,N.diethylaniline in acetonitrile 1 ,OO-cm. cells Aniline (bosic forml N,N.Dielhylanlline (botic form)
0 3
preferred solvent. p-Sitroaniiine ( p I h in water = 13.02) and .I-:nethyldnitroanilinc (cstimated pK8 in water about 13.5) are still titr:itnblc, though they give somewlint rouiitleci breaks. For such compounds, nholutclg dry solvent and titrant :LIT rcconiixnded. o-Sitroaniline (p& in water = 14.28) is too weak to give a useful photometric end point evcn i n anhydrous acetic ncid. Differentiating Titrations, Wliile the photometric tcclinique is useful for titration of single bases, its greatest potential is probably it1 differciitiating titrations in miutuws contnining components of similar basic strength. Figure 3 s h o w the photoiiietric titration curve for n misture of 2-methyl-5nitroaniline and 4-methyl-2-nitroaniline and comparison with Figure 4 shows the advantage over potentionictric titration. Figure 3 illiistratca a photometric titration in which onl!. the weaker (tornponent absorbs ? $ thc selected \rave length. This sitil:ltion is encoantered when the wenker component absorbs nt longer wave lengths than the etroiiger one. There are qlro nunibcr of cases in which the reverse is true, as illustriitcd by Figure 5 , which shorvs the b:isic absorption spectra of aniline [pK~(H20) = 9.421 Snd X',.~-diethylniiiliiie [ pIL(&0) = 7,451 i n ncetonitrilc. There the nbsorption of the weaker component is obscured by that of the stronger one. Fortunately, the absorption of N,Ndiethylnnilino is sliifted to sliortcr wnve lengths upon protonntion, tluis mnking the aniline absorption accessible to measurement, Hence, brcnks can be obtained for both components by c h a w ing to a shorter wave length after ob. taining the first brc:ik, as sho~viiin Fig-
I
1 l
Rcl$\
In acetonitrile with 0.51 1 5 M perchloric acid 0 Titrated at 3 3 7 mp A At 3 2 0 mp
$1
kl 0 0 0 .
0
I
0
1
,
l
I
\
'
IO0 ml 0 5115
"
1
-1
.1
-
'
P
2 00 Hc10.1
Figure 6. Photometric titration of mixture of 0.512 meq. of N,N-diethylaniline and 1 . 1 04 meq. of aniline
I I
1
1
3 00
I
I
I
I
I 00
200
300
4 00
I 500
m1.0.4391 f4-HC10.1
Figure 7. Photometric titration of mixture of 0.407 meq. of p-chloro-N,Ndiethylaniline, 0.951 meq. of o-chloroaniline, and 0.506 meq. of 2,5-dichloroaniline Conditions as In Agure 1 Titrated a t 3 4 1 rnp A At 3 2 7 rnp
0
Figure 8. Potentiometric titration of mixture of 0.407 meq. of p-chloro-N,Ndiethylaniline, 0.95 1 meq. of o-chloroaniline, and 0.506 meq. of 2,5-dichloroaniline Conditions as in Flgure 1
ure 6. The only drawback of this wave length change technique is that the untitrated residue of the first component absorbs strongly at the shorter wave length and causes nonlinearity of the titration curve, because it is titrated faster than the weaker component. I n the present example, where the final break is rather sharp, this nonlinearity introduces little uncertainty because the readings close to the end point can hc used for extrapolation. If the second break were rounded, the nonlinearity would have a detrimental effect on the accuracy. I n spite of such occasional difficulties, wave length changes were extremely useful in differentiating titrations of aromatic amines. Figure 7 illustrates the photometric titration of a three-component mixture consisting of p-chloro-X,S-diethylaniline, 2,5-dichloroaniline, and o-chloroaniline. The first break is obtained a t 341 mp where only p-chloro-S,X-diethylaniline absorbs. The wave length setting is then changed to 327 mp where the absorption of 2,5-dichloroaniline can be measured, while the o-chloroaniline absorbs a t slightly shorter wave lengths. Although the first two breaks are rounded, this titration compares very favorably with the corresponding potentiometric curve (Figure 8) which shows only two poor inflections. Differentiating titrations of bases in glacial acetic acid must probably be restricted to mixtures containing no more than three components because the available basicity range in this solvent is limited by the leveling effect. If the nonleveling solvent acetonitrile is used, even four-component mixtures may be titrated. Figure 9 shows the titration of a mixture of di-n-butylamine, h',N-diethylaniline, aniline, and o-chloroaniline. The first two breaks are obtained a t 337 mp, where only the second component, N,N-diethylaniline, absorbs. The setting is then changed to 323 mp where both aniline and o-chloroaniline absorb, the latter compound more strongly. The marked curvature before the third break is due to the titration of the last traces of N,N-diethylaniline. I n spite of this complication the photometric curve compares favorably with the corresponding potentiometric curve shown in Figure 10. Tables I1 and 111present some results of differentiating photometric titrations of commercial materials dissolved in acetonitrile and acetic acid, respectively. I n titrations of bases of very similar strength, such as aniline and m-chloroaniline (Table 11), o-chloroaniline and 2,5-dichloroaniline, or o-chloroaniline and 2,4-dichloroaniline (Table 111), relatively large deviations are observed. With such systems, however, the potentiometric method fails to give usable results. VOL. 32, NO. 6, MAY 1 9 6 0
e
579
fable 111.
Differentiating Titrations
of Bases in Acetic Acid
0.4 to 0.5M Pcrchloric Acid Titrants in Acetic Acid
Components
Titrant Consumption Calcd. Obed.
pKn( HtO)
I'yriilirie
o-Ctilorociiiiline p-Chlnro-,\',h;-diethylaniline 0-Ch1oronni:ine o-Chloronniiine 2,5-Dichloroaniline o-Chloronriiline 2,4-Dichloronniline
8,s
1.41
1.43
9.5' 11.4
1.10
-5.2
12.5
1.16 2.17 1.08 1.15
2.23 1.04 1.19
+3.8 -3.7
11.4
1.08
1.08
11.4
11.4
12.2
2-~lethvl-5-;iitronniline
1.24
12.04
4-Methyl-24 troaniline p-Cliloro-h:,N-dieth~laniline o-Chloroarii!ine 2.5-nichloronniline Estimated.
% Deviation
1.14
13.5O 9.5' 11 4
2,29 0.93 2.17 1.16
12.8
1.08
1.12 1.20 1.14
2.33 0.88 2.18 1.13
$1.4 0.0
4-3.5 +3 7 -3 2 0.0 +I.?
-5.4
+0.5 -1.7
0
Nonlinear Titration C W e S . While most compounds gave photometric titration curves in close agreement with the expected linesr shape, excoptions were found, as illustrated by the titrations of p-chloroaniline and p-nitmmiline, shown in Figure 11. close inspection of the curves Of numerous other bases in acetic acid revealed curvature of the Bame type to a much smaller degree. One of the bases exhibiting a moderate curvature \\as o-chloroaniline, a compound which also mas titrated by Reilley and
Iv* Recommended Wave Lengths for 10-IM Bases in 5-
Table
Cm. Cells Titrntion
~~~~~h Aceto- Acetic Coll>pollnd ( ~ ~ nitrile 0 ) acid Aiiiline 9.4 319mu .. m-Chloroaniline 10 5 325 o-Chloroaniline 11 , 4 324 320 m/r pChloronniline 10.0 333 327
pKs
p-Chloro-.V,iy diethylaniline 9 . 5 " 2-Chlorop yrjdine 13 3 ,. SChloroquinoline 11.2' .. * 2,4-Dichloroaniline 12.2 IE,?i-Dichloroani1i n e 12.8 .., N,N-Die thylaniline 7 . 5 337 e . .
Schvveizer It significant that these authors state that the end point for o-chloroaniline had to be extrapolated from readings close to the break because earlier readings gave poor resuits. The shape of the beginning of the titration curve was not shown but it seems reasonable t o assume that the Curvature Rns of the type encountered in the present study. Some nonlinearity Of the observed type would be expected to result from light leakage or dilution effects. However, light leakage .R;ould have caused similar deviations in all titrations with the same initial absorbance a t these Wave lengths, regardless of which base wB8 titrakd or which solvent r v used, ~ Actually, curvature *'as observed only for certain compounds and it was almost entirely rmtricted to the low-dielectric solvent acetic acid. (p-Chloroaniline exhibited some curvature also when titrated in acetonitrile,) The dilution
corrections were much too small to atcount for the observed curvature, unrl impurities in the samples could also Le excluded as a possible explanation. Sonlinear photometric titration curves have also beeen obsencd when phenolic compounds in lowdielectric media are titrntcd with bases. Thcsc result from forination of phcnol-phenolate complexes during the titration (9). This suggests that the anomalies observed for some aromatic amines in acetic acid may be due to hydrogcnbonded complexes bctwcen the amines and their conjugate acids. Such hydrogen bonds mny cause wave length shifts in the ultraviolet, usually toward shorter wave lengths (re), and recent work by Forbes et al. (5-6) indicates that the absorption intensity may be affected. Because the concentration of the complex between the base and its conjugate acid must go through a nlaximum during the titration, a nonlinear curve is to be espected, acid, an additional fartormay be operative if the base is sufficiently strong to be partly protonated, I n such a cnse, the degree of protonation by the solvent may decrease during the titration as the free (or more weakly hydrogen-bonded) bas0 is transferred into a strongly hydrogen-bonded complex with its con\upst,o This additional effect -,ycuid explain lj+v the stronger base, p-chloroaniline, gives a more pronounced curvature than the weaker base, p-nitroaniline p i g u r e 11). formationon The effect of the absorption spectrum probably depends strongly on the structure of the compound, horrever, and such comparison may, therefore, notbe justified, DISCUSSION
Table I V lists recommended titration
312
294L
am* 335 328
W 0 C
. . I
N,h'-Dirnetliyl-
aniline 8 9 334 N,N-Dimethylnt-nitrotiniline 11.00 . . . 518 N,N-Dimethylpnitroaniline 12.51 . . 483 2-h!~thyI:5; nitroaniliiie 12.00 . . 470 4-hf ithyl-2nitroaniline 13 5 O 520 n-Nitroaniline 11.5 . . . 461 p-Xitroaniline 13 0 . . 448 &Nitro-1-naphthylnmine 11.2 555 655 Estimated values. Absorbance i n c r e w during titration. I , .
e 5: 4
I
I
I
0
APIALWCAL CHEMISTRY
mi 0 5115
M HCIO4
Figure 9. Photometric titration of mixture of 0.283 meq. of di-n-butylamine, 0.409 meq. of N,N.diethylaniline, 1.014 meq, of aniline, and 0,509 meq, of o-chloroaniline CondlHoru (11 In Rgurr 6 0 TtkotrdolJ3tinjl A M323mjl
wave lengths for a number of bases in acetonitrile and/or acetic acid. The wave lengths refer to 10-*M bases in a cell with a path length of 5 cm. For lower concentrations or shorter cells, correspondingly shorter titration wave lengths should be chosen. While most of the titrations in this study were performed a t the 10-*M level, occasional checks showed t h a t many bases may be successfully titrated in the 10-4M range. For example, 5-nitro-1-naphthylamine ( ~ K B = 11.2) gave a n excellent titration curve in acetic acid when titrated a t 3.6 X lO-'M concentration with O.05M perchloric acid at 490 mp. At 3.6 X 10+M a satisfactory end point was obtained but the presence of traces of basic materials in the solvent led to a n unsatisfactory result. I n fact, for relatively strong
organic bases, such as N,N-dialkylanilines, the lower concentration limit for titratability may be set by the molar absorptivity which often is about 2500 liters per mole cm. The results indicate that bases with as high as 13.5 an aqueous ~ K value B give useful breaks in photometric titrations at the 10-*M level when glacial acetic acid is used as solvent. For very weak nonabsorbing bases, such as urea, the photometric indicator titration techniques of Higuchi et al. (8, 18) are necessary. Taken as a group, the photometric titration techniques are distinctly superior to potentiometry for determination of extremely weak bases. I n media having very low dielectric constants, reliable potential measurements are extremely difficult to obtain due t o the high resistances involved.
1200
This limitation does not apply to photometric measurements. However, the photometric titration technique has its own set of limitations. I n solvents such as benzene, for example, titration with perchloric acid generally leads to precipitation and consequent disruption of the photometric measurements. The photometric technique is particularly advantageous for differentiating titrations. Photometric differentiation of two bases appears possible if the difference between their aqueous pKB values is 1.5 or greater. I n the case of o-chloroaniline and 2,4-dichloroaniline (Table 111) A pKB is only about 0.8, showing that analytically useful, though less accurate, breaks may be obtained even in such situations. There is, of course, a limit set on how close the strengths of tFvo bases can be in such a titration by the fact that the weaker begins to protonate before the stronger is completely protonated. ii'ot only does this lead to curvature at the end point but if the tn.0 components are not present in nearly equal amounts, the cancellation of errors no longer takes place and the two lines do not intersect exactly at the end point. The aqueous pKB values, although not strictly applicable in other solvents, give a fairly reliable estimate of the relative strengths of bases of the same type in a given solvent when pK values determined in that solvent are not available. LITERATURE CITED
(1) Aronoff, S., J . Phys. Chem. 62, 428 (1958).
I
0
IO0
I
I
2.00
3 00 m1.05115M HC104
1
400
500
Figure 10. Photometric titration of mixture of 0.283 meq. of di-n-butylamine, 0.409 meq. of N,N-diethylaniline, 1.014 meq. of aniline, and 0.509 meq. of 0-c hloroa niline Conditions as In Figure
6
(2) Brealey, G. J., Kasha, M., J . Am. Chem. SOC.77,4462 (1955). (3) Forbes, R. F., Knight, A . R., Can. J . Chem. 37,334 (1959) (4) Forbes, W. F., Templeton, J. F., I b i d . , 36, 180 (1958). (5) Forbes, K. F., Templeton, J. F., Chem. & I n d . ( L o n d o n ) 1957, 77, 600. (6) Goddu, R. F., Hume, D. N., ANAL. CHEM.26,1679 (1954). ( 7 ) Ibid., p. 1740. ( 8 ) Higuchi, T., Rehm, C., Barnstein, C , Ibid., 28, 1506 (1956). (9) Hummelstedt, L. E. I., Hume, D. N , ,
in preparation.
Figure 1 1 . Photometric titrations of 1.053 meq. of p-chloroaniline and 1 .O1 6 meq. of p-nitroaniline
(10) Kelly, H. J., Ph.D. thesis, Massachusetts Institute of Technology, 1956. (11) Lavine, T. F., Tocnnies, G., J . B i d . Cham. 101, 727 (1933). (12) McKinney, R. TT'., Reynolds, C. A,, Talanta 1,46 (1958). (13) Rehm, C., Higuchi, T., AI,. CHEY. 29, 367 (1957). (14)Reilley, C. S . , Schweizer, B., Ibid., 26, 1124 (1954).
Conditions as in Figure 1
0 p-Chloroaniline at 327 m p A p-Nitroaniline at 448 m p
RECEIVEDfor review October 21, 1959. Accepted February 8, 1960. Initiation of this investigation was made possible by an ASLA-Fulbright grant obtained through the cooperation of the U. S. Department of State, the Institute of International Education, and the U. S. Educational Foundation in Finland to L. E. I. H., as well as a grant from Svenska Vetenskapliga Centralrddet (Finland). Work supported in part b y the U . S. Atomic Energy Commission under Contract AT(30- 1)905.
VOL. 32, N O . 6, MAY 1960
581
