Photometric Titrations in Nonaqueous Solvents CHARLES N. REILLEY and BARBARA SCHWEIZER University o f N o r t h Carolina, Chapel H i l l , N. C. ,A8 photometric titrations have proved valuable in circumventing difficulties experienced with other techniques of end-point detection, a study of the appliration of the photometric method to the field of acid-base titrations in nonaqueous media seemed desirable. Glacial acetic acid was employed as the solvent and eeveral weak bases were titrated with a glarial acetic acid solution of perchloric acid. The titration cell was placed directly in the light path of a Becltman DU spectrophotometer, and the absorbancy at an experimentally determined wave length was plotted against the corrected volume of the titrant. Bases, such as ochloroaniline and quinoline, gave distinct breaks at the equivalence point which agreed well with potentiometric end points. By the addition of an absorbing species of w-ealter basicity i t was also possible to titrate a substance where neither acid nor base forms of the substance show absorbancy in the ultraviolet region.
As more substances absorb in the ultraviolet region of the spectrum, the number of reagents that can be used is greatly increased. Other advantage. not previously discussed can also be included. I n titrations in which the reaction is slom in the vicinity of the end point or in which the equilibrium is poor, the possibility of determining the end point by extrapolation is of importance. Also, the end point is not affected by stray electrical currents, which can cause serious difficulty in potentiometric titrations rrith solvents of low dielectric constant. As a titration procedure is followed in determining the concentration of base present, no extinction coefficient or Beer's law plot is needed. In addition, the presence of other inert, but absorbing, substances is not particularly harmful, as their effect may be canceled to a large extent by increasing the light intensity. Because of the sensitivity of the photometric method and the small amount of solution needed, relativelv small quantities of material are required in these titrations.
T
HE photometric end-point technique appears to offer several unique advantages for acid-base titrations in nonaqueous solvents. The use of visual indicators in these titrations is limited, as only a few indicators are suitable. Many substances, however, absorb in the ultraviolet region of the spectrum, and the necessity for adding chemical indicators is thereby eliminated, The dependence of the extinction coefficient on wave length also permits a degree of selectivity vihich is not present in other end-point detection techniques. The sensitivity can be varied simply by a variation in the wave-length setting. Much interest has been shown in recent years in the field of acid-base titrations in nonaqueous medium. Many organic acids and bases which are too weak to make aqueous titrimetry feasible may be accurately titrated in nonaqueous media. Riddick ( 7 ) and Fritz (2) have reviewed this literature. Visual indicator and potentiometric methods for the determination of the end point were the first two methods to receive attention in nonaqueous acid-base titrations. Hall and Spengeman ( 4 ) titrated conductometrically eleven organic bases using glacial acetic acid as the solvent with a glacial acetic acid solution of perchloric acid as the titrant. From these titrations it as concluded that, although conductometric measurements are applicable, potentiometric titrations have certain advantages over conductometric titrations in glacial acetic acid solution. Another method of end-point detection, high-frequency titrations, was investigated by Wagner and Kauffman ( 8 ) , and the results were found to agree satisfactorily with the theoretical values. However, some of the disadvantages of conductometric titratmns would also be present in the high-frequency method, because highfrequency titrimetry is closely related to conductance titrations (6). The subject of photometric titrations has been reviewed by Qsborn, Elliot, and Martin ( 5 ) and Goddu and Hume (3). Bricker and Sweetser ( 1 ) made the first use of the ultraviolet region of the spectrum for photometric titrations. Bricker and Sweetser mentioned the advantages of using the ultraviolet region of the spectrum instead of the visual region for photometric titrations. The molar extinction coefficients of many volumetric reagents and base samples have their maximum values in these wave lengths. The sensitivity is therefore greater in this region, making possible the titratium uf more &lute solutions. A better agreement with Beer's law IS obtained in the ultraviolet.
s BAslc v
0.
x
/
250
300
275
325
350
W A V E LENGTH ( m p )
Figure 1. Spectral Curves for Acidic and Basic Forms of o-Chloroaniline
There are a few disadvantages to this method. The fact that not all compounds absorb in the ultraviolet region of the spectrum is a limiting factor in its applicability. The cost of the spectrophotometer may be a hindrance in the broad use of ultraviolet photometric titrations. However, for the determi-
Table I. Titration of Organic Bases
Base m-Cliloroaniline
Quinoline
a-Chloroaniline hlixture : Sodium acetate
a-Chloroaniline
(HC104 = 0.1010N) ~ ~ 1Used, 0 , Normality Base PhotoAv. Taken, metric potentiox 111. M 1. metric 314 10 6.87 0.0694 10 5.88 Av. 9 34 0 0471 350 20 20 9.33 20 9.26 Av. 312 20 11.68 0.0588 20 11.71 Av.
312
...
10
10
6,96 5.96
4.67 4.68
of Base Photometric 0.0893 0.0594 0,0594 0.0472 0 0472 0.0468 0.0471 0.0590 0.0591 0 . 0.590
(O.OG01)" 0,0602 Av. (0.0471)" Av.
0.0602 0.0602
0,0472 0.0473 0,0472
a Calculated on separate titrations, as successive potentiometric end points nere unobtainable for mixture.
1124
VOLUME
26,
NO. 7, 1 U L Y 1 9 5 4
1125
nation of certain bases that can be determined only poorly, if at all, by other methods, photometric titrations have a definite application. As dilution affects the absorbancy readings, they must be corrected for the volume change accompanying the titration. At elevated temperatures the procedure for these titrations would be necrssarily more complicated. APPARATUS
All absorbancies were measured on the Beckman Model DU quartz spectrophotometer, using only the ultraviolet region of the spectrum. The titrations were carried out in a titration cell similar t o that described by Bricker and Sweetser (I). The regular cuvette holder cover was replaced by a felt-padded board by which the titration cell wa3 held in place. To prevent any light from entering, the sides of the beaker and the cell extending above the board were painted black, and the top was covered with a Bakelite cover drilled with holes in which the tip of the microburet and the stirrer were placed. All potentiometric titrations were carried out as described by Fritz ( 2 ) . A Beckman Model M pH meter was used for the "pH" readings in conjunction with a glass electrode and a silver-silver chloride reference electrode. All titrations were carried out in a controlled temperature room, in an effort to keep the solution a t approximately the same titer. The high coefficient of thermal expansion of glacial acetic acid (approximately five times greater than that of water) made this necessary.
A standard solution oi approximately O.1N perchloric acid was prepared, according t o the directions given by Fritz ( B ) , by dissolving 8.5 ml. of 72% perchloric acid in 200 to 300 ml. of glacial acetic acid. Enough acetic anhydride was added t o remove the water present in the perchloric acid, and the solution was then diluted to 1 liter with glacial acetic acid. The 0.1N perchloric acid was standardized using potassium acid phthalate as the standard. The potassium acid phthalate was dissolved in 75 ml. of glacial acetic acid and the titration carried out potentiometricany . Solutions of approximately 0.0j.\7 quinoline, o-chloroaniline, nt-chloroaniline. and sodium acetate were made UD bv dissolving appropriate amounts of the compounds in glacial -acetic acid: These solutions vere all standardized potentiometrically using the previously standardized perchloric acid. Twenty-milliliter portions of the bases were taken and diluted to 100 ml. for the titration. For the photometric titrations of these compounds curves of transmittancg us. wave length were determined for both the acidic and basic forms of the compounds in glacial acetic acid, and the wave lengths for the titrations mere selected in the usual manner from this information. Previous to the titration the wave length was set at the predetermined value. The titration cell contain-. ing the base was set in place and the instrument adjusted to readx the desired absorbancy. The value of this absorbancy was determined from the acidic and basic curves of the base. The titration was then carried out with no further readjustment of the instrument. ;120-ml. portion of the base diluted to 100 ml. with glacial acetic acid was used in these photometric titrations. A volume correction was applied to all absorbancy readings, and the data were plotted according to the method of Bricker and Sweetser (I). RESULTS
400Q U I N O L I N E 1 5 I IO"N
1
I-
s
0
300
325
h
350
375
400
WAVELENGTH ( m p ) Figure 2.
Spectral Curves for Acidic and Basic Forms of Quinoline
Quinoline, na-chloroaniline, c-chloroaniline, and a mixturlt of o-chloroaniline and sodium acetate were titrated photometrically. The equivalence points found in this way were compared with the corresponding results obtained by potentiometric titrations as shown in Table I. Figures 3, 4, and 5 show graphs of the photometric titrations as compared with the corresponding p tentiometric titrations. Figure 1 shows the spectral curves for the acidic and bash form of o-chloroaniline. This is typical of the m-choloroaniline also. From this it is seen that, during a titration of the base with perchloric acid, there is a steady decrease in absorbancy until the equivalence point is reached, when the curve levele off at almost zero absorbancy. This has a distinct advantage
\
D
'.
m i . HCIO,
( 0.1010 N )
Figure 3. Photometric and Potentiometric Titration Curves for Quinoline
ln
'P =
mi. HC104 (0.1010 N 1 Figure 4. Photometric and Potentiometric Titration Curves for o-Chloroaniline
ANALYTICAL CHEMISTRY
1126
I in photometric titrations, as the wave length may be easily adjusted and greater sensitivity obtained. For quinoline (Figure 2) the spectral responses of the acid and base 107 forms are more alike, but reversed compared to Figure 1. The absorbancy increases as the titration is carried out, and I the wave length and sensitivity are not as I easily adjusted. 5’ M I X T U R E ( 3 1 2m y ) In the titration of quinoline (Figure 3), SODIUM A C E T A T E the break in the potentiometic curve is o - C H LOROANlLl N E 4sharp and in close agreement with the end point obtained photometrically. As quinoline is a moderately strong base in glacial acetic acid, this is to be expected. There is considerably less rounding off of the photometric curve a t the end point than in the case of the weaker bases. The end point of the m-chloroaniline is also sharp potentiometrically and in close agreement with the end point obtained photometrically. Although m-chloroaniline m l . H C I O4 ( 0 . l O I O N ) is a slightly weaker base than quinoline, the Figure 5. Photometric and Potentiometric Titration Curves for Mixture absorbancy readings still form two straight of Sodium Acetate and o-Chloroaniline lines with no tendency toward rounding near the equivalence point. The end point on the potentiometric curve of o-chloroaniline lence point of the o-chloroaniline. In practice it was necessary (Figure 4) is less sharp than that of the other two. To obto extrapolate to the end point, as shown in Figure 5 . Thus a tain the photometric end point for o-chloroaniline it was necessubstance rhose acid and base forms do not absorb-e.g., acesary to make all absorbancy readings within about 2 ml. prior tate ion-can nevertheless be titrated by addition of an absorbing substance of weaker basicity--e.g., o-chloroaniline. to the end point. Absorbancy readings measured more than a few milliliters before the end point gave poor results. Quick titration and estimation of the end point were possible, howACKNOWLEDGMENT ever. Khen done in this way, the photometric and potenThe authors gratefully acknowledge the suppol t of the Retiometric equivalence points were in good agreement. To insearch CorP. for Part of this stud?.. crease the sensitivity of these measurements near the end point, the wave length was adjusted to give a fairly high degree of LITERATURE CITED absorbancy. The actual change in absorbancy was made greater in this way. . 24, 409 (1952). Hricker, C. E ., and Sweetser, P. B., A N ~ LCHEY., A mixture of o-chloroaniline and sodium acetate could not Fritz, J. S., “Acid-Base Titrations in Sonaqueous Solvents.” Columbus, Ohio, G. Frederick Smith Chemical Co., 1952. be analyzed a-ith any degree 01 accuracy (Figure 5 ) by the Goddu, R. F., and Hume, D. N., A N ~ LCHEM., . 22, 1314 (1950). potentiometric end-point procedure. Separate standardizations (4) Hall, N. F., and Spengeman, W. F., Trans. W i s c o n s i n A c a d . Sca., of the two indicate that the end points of the photometric 30. 51 (19371. titrations are, however, very accurate. Neither the acidic nor (5) OYborn, R. H., Elliot, J. H., and Martin, A. F., ISD.EKG.CHEM.. ANAL.ED.,15, 642 (1943). the basic forms of sodium acetate absorb in the ultraviolet (6) Reilley, C. N., and hlcCurdy, W.H., Jr., A N ~ LC.H E J I . , 25, 86 region of the spectrum. As sodium acetate is a strong base (1953). in acetic acid, it is neutralized before the o-chloroaniline, (7) Itiddick, J. A , , Ihid., 24, 41 (1952). which is a comparatively weak base. The first break in the (8) Wagner, IT‘. F., and Kauffman, W. R.,Ibid., 25, 538 (1953) curve will, therefore, indicate the equivalence point of the RECEIVED for review March 1, 1954. Accepted May 6, 1954. sodium acetate; the second break will be reached a t the equiva-
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