Extrapolation Plot for Photometric Titration of Weak Bases in Aqueous and Nonaqueous Systems CARL REHM' and TAKERU HlGUCHl School of Pharmacy, University of Wisconsin, Madison, Wis.
b A method is described for plotting photometric titration data in a way that permits linear extrapolation of information obtained beyond the end point to give the stoichiometric value. Data are given illustrating the application of the procedure to titration of extremely weak bases in water and to titration of practically nonbasic compounds such as urea and amides in acetic acid. The procedure permits acid-base titration with much greater accuracy than do visual systems. Many systems which do not yield suitable results b y potentiometric means are amenable to this technique because of its inherently greater sensitivity.
For systems containing a n e a k indicator, one can expect t h e acid-base ratio of the indicator to change linearly beyond the end point TTith the amount of strong acid or base added. For example, if a neak base, B, is being titrated in n-ater with a strong acid, the system \vi11 contain essentially t v o acidic species, BH' and H30+. If, for a weakly basic indicator, the equilibrium
is far to the left. the concentration of BH+ will have very little effect on eliciting a color change. Beyond the end point, hoviever, when the concentration
mo
THEORY
Titrations in Aqueous Systems. Present address, Analytical Research Department, Ciba Pharmaceutical Products, Inc., Summit, K . J.
H30+
+I
HsO
+ IH+
comes into play. The constant for the above equilibrium can be m i t t e n as
or
BH++IeB+IH+
T
TYPES of linear plots have been discussed previously (4) for the determination of indicator titration end points b y photometric methods. These plots, based on equilibria involving both the indicator and the weak base or acid undergoing titration, permitted accurate extrapolation to the stoichiometric end point from regions prior to the end point. This work is concerned with a third type of linear plot obtained by use of relatively weakly basic indicators, such that the changes in the indicator color occur primarily after a basic sample has been titrated with an acidic titrant. The extrapolations to the stoichiometric end points in these cases are from regions well past the end points. The Type I11 plots are particularly suited for titration in water of weak bases such as pyridine, aniline, acetates, benzoates, and the like, which are very difficult to titrate visually or potentiometrically. The method is also applicable to titration, in the presence of indicators, of extremely weak bases (urea, certain amides) in nonaqueous systems such as acetic acid.
of H30' becomes appreciable, the equilibrium
If the amount of excess acid present in the system is made to equal X - S, n-herexis the total amount of the standard acid added and S is the volume of the acid required to convert stoichionietrically the amount of base B origiiially present to its conjugate form (BH+),the resulting equation is IH X-S=K-V I
+
(3)
where V is the total volume of solution contained in the titration vessel. Thus, a plot of ratio
IH +
vs.
X will yield a
o'2: 6.60
700
7.40
7.80
8.20
8.60
ML. 1.304 N HCI
Figure 1. Type 111 plot for titration of aniline in water with 1.304N hvdrochloric acid using Metanil fellow indica tor
~
I
straight line with a n intercept corresponding to S. T o obtain analytically useful forms of the plot from the titration of weak bases, the indicator should besufficiently xeak so that the observed color change is that defined by Equation 2. For titrations of iTeak bases with pKb
Figure 2. Titration of sodium citrate in water with 1.304N hydrochloric acid using Metanil Yellow indicator
i, M L 1304 N HCI
VOL. 29, NO. 3, MARCH 1957
367
Table I.
Determination of W e a k Bases from Photometric Data by Titration in Water and Acetic Acid
Base
Indicator
a b c
Base, Mg.
Precision, mg.c
Foundb
Weak Bases with 1.3045 Hydrochloric Acid in Kater Metanil Yellow 522 4 523 1 573 1 572 3 Metanil Yellox Rletanil Yellow 639.5 637 9 Metanil Tellow 694.0 691.4
0 5 0 5 5.8
Very Weak Bases with 0 10345 Perchloric -4cid in Acetic Acid 65.6 65.3 Sudan 111 Sudan I11 200.1 199.7 Sudan I11 88,9 87.8
0.1
Sodium acetate Sodium benzoatea Aniline" Pyridine Crea Antipyrine N-Met hylpyrrolidone
Taken
2 0
0.2
2.0
Ethyl alcohol-Lvater, 1 to I, used as solvent Average of three. Standard deviation.
values of 9 or less, indicators with gK, values of 1.5 to 2.0 and strong mineral acids of moderate concentration (0.5 to 2.0,V) haye been found useful. For the titration of weaker bases, correspondingly weaker indicators and considerably stronger acid should be used. The Type I11 plots are closely related to the absorbance-volume plots of Goddu and Hunie ( 2 , s ) . As Equation 3 shows, a plot of the volume of the titrant added against IH+, as determined by the absorbance of the solution (it is assumed that the base form of the indicator does not absorb), would be approximately linear if the extent of the conversion of the indicator to its acid color n-ere kept very small. Direct absorbance plots, however, would have been of little value in the systems investigated here. as they would have led to highly curved lines. Furthermore, the direct plot of absorbance is of little use for indicators whose base
form absorbs significantly a t the wave length employed for the photometric measurements.
and the corresponding equilibrium constant is
Titrations in Acetic Acid Systems.
(I.HCl0a) K = (I)(HC104)
Several indicators which have been recently evaluated for t h e acetic acid system are so weakly basic that they exhibit a color change only in the presence of a strong acid such as perchloric acid (4). The equilibrium corresponding to this color change can be written as I
By rearranging the above equation into the form HC10, )There I.HClO4
+ HC104 e I.HC104
(I.HC10a) I
K =
the acid form of the indicator resulting from interaction with HC10,
4 Figure 4. Type 111 plot for titration of urea in acetic acid with 0.1 034N perchloric acid using Sudan 111 indicator
ML.
01034N H C l 0 4
J
Y*[ O
IO
o
2.0
L
methylpyrrolidone Figure 5. Titrationin of acetic Nacid with 0.1 034N perchloric acid using Sudan 111 indicator
ML. 0.106 N PERCHLORIC ACID
Figure 3. Blank titration of 25 ml. of glacial acetic acid with 0.106N perchloric acid using Sudan 111 indicator
368
ANALYTICAL CHEMISTRY
(5)
~
0
0
I /I 86
/
/
o
, , , , 90
9.4
98 ML. 0.1034 N
10.2 HClO,
106
11.0
11.4
I
=
the base form of the indicator
a plot of ratio (1'HC104) against the
I
volume of added perchloric acid will yield a straight line with a n intercept corresponding to the stoichiometric end point of the titration. I n order to obtain linearity of such plots, the following equilibrium must be far to the left:
+ BH+C104- e IH'ClO4- + B
I n here
B BHClOl
= =
the base being titrated the perchlorate salt of the base
In cases where equilibrium b e t w e n the indicator and the base perchlorate is significant, curvature of such plots ran be expected, especially near the cnd point. EXPERIMENTAL
Reagents and Chemicals. Hydrochloric acid solution, 1 . 3 s in water, standardized against sodium carbonate. Perchloric acid solution in acetic acid, O . l S , prepared according to Fritz ( I ) and standardized against triphenylguanidine. Acetic acid, ACS reagent grade, glacial. Sodium acetate, ACS reagent grade, anhydrous. Sodium benzoate, USP. Aniline, 4CS reagent grade, redistilled. Pyridine, ACS reagent grade, redistilled. Urea, recrystallized from ethyl alcohol-n ater mixture and dried under vacuum. Antipyrine. recrystallized from ethyl alcohol-n ater mixture and dried under vacuum. N-~Iethylpyrrolidone, General Aniline and Film Corp., fractionally redistilled (50-plate Oldershaw) under reduced pressure. Xetanil Yellow, Eastman Kodak. Sudan 111, Kational Aniline Division, lllied Chemical and Dye Corp. Procedure. T h e experimental procmlures used 11ere essentially t h e same as previously described (j),except t h a t t h e absorbance measurements n e r e made using a Bausch & Lomb Spectronic 20 colorimeter. Indicator acid-base ratios were calculated from the indicator absorbance data by means of the equation IH' - A - A b I -4a - A \\.here A , = absorbance of pure acid form of indicator
-4
=
Qb
=
absorbance during titration absorbance of pure base form of indicator
The absorbance of the pure acid form of the indicator was obtained a t the end of the titration after a large excess of acid had been added to the system. The absorbance of the pure base form of the indicator, if significant a t the wave length used, was obtained a t the beginning of the titration before the addition of standard acid. The nave lengths used in these titrations corresponded to the absorption peaks of the acid forms of the indicators. TYPICAL RESULTS
Titrations in Aqueous Systems. I n Figure 1 is s h o n n a typical plot obtained during photometric titration of aniline in water with 1 . 3 s hydrochloric acid in the presence of Metanil Yellow. T'alues of
IH -, I
as computed from the
absorbance data obtained a t 530 mp, were plotted against the volume of standard acid added, giving a straight line, the intercept of which corresponded to the stoichiometric end point. Similar linear plots were obtained for the titrations of sodium acetate, sodium benzoate and pyridine. The results of titrations of these weak bases (Table I) show that precisions of the order of a few tenths of a per cent ivere obtained in most instances. It is questionable whether any other titrimetric procedure is inherently capable of yielding superior results for these same systems. Titiations of sodium citrate (pKb for dihydrogen citrate = 10.9) and glycine (pKb = 11.6) in the presence of lletanil Yellow gave plots which approached linearity only a t relatively high values
IH + Accurate eatiapolations to I '
of --
the end points in these cases are, as may be expected, somen hat difficult. Figure 2 is a typical plot obtained for sodium citrate. Extrapolation of the linear portion of the curve permitted estimation of the amount of citrate present to about 2%. The curlre obtained in the case of glycine did not permit simple extrapolation. Better plots for these compounds may be expected by using a 11-eakei indicator and a somewhat more concentrated standard acid solution. Titrations in Acetic Acid. Sudan 111, which behaves as a n evtremely gave weak indicator in acetic acid (j),
Type I11 plots for hases even as \Teak as urea. It was thought that the small amounts of m t e r commonly present in acetic acid might interfere a i t h the titration of weak bases by this method. However, a blank titration in acetic a d containing about 0.2% water gave a Type I11 plot (Figure 3 ) which n a s linear and passed through the origin upon exti apolation, indicating that no detectable titiatable impurities 1% ere present in the solvent. ,4 standardized solution of perchloric acid in acetic acid containing Sudan I11 indicator was used to titrate several very Tveak bases in acetic acid by the method described. Linear plots of
IH I
+
__ z's. the volume of perchlorir acid
added were obtained for urea and antipyrine. both evtrcmely weak bases. A typical plot obtained for the titration of urea is shown in Figure 4, the inteicept representing the end point. Ahsorbance nicasurements were made a t 615 mp. The data in Table I sho\v that these compounds nerc titratrd with a piecision of about 0.1%. .1 similar plot for the titration of .I-methylpyi rolidone, lion ever. did not approach linearity until relatirrly high indicator acid-base ratios n ere ieacalied, making accurate extiapolation to the end point somen-hat difficult. Figui e 5 shom. however, that it was possible to extrapolate the linear portion of the cuire. permitting estimation of the amount of Y-meth? lpyiiolidoiie pi cwnt to about 1 2 7 , . ACKNOWLEDGMENT
This study n-as supported in part b y the rrsrarch committee of the Graduate School from funds supplied by the TTisroPsin Alunini Research Foundation. LITERATURE CITED
(1) Fritz, ,J. S., "Acid Base Titrations in Son:iqueous Solvent," G Frederick
Smith Chemical Po., Columbus, Ohio. 19.52. Goddu, R. F., Hume, D. S.,As i~,, C H E \ f . 26, 1679 (1954). Zbzd., 2 6 , 1740 (1954). Higiichi, T., Feldman, J. A , Rehm, C.. Zbid., 28. 1120 11956). Hiauchi T.. Rehm. C.. Barnpteiri. C.. hid.,28,'1506 (1956).
RECEITED for review July 27, 1956. Accepted Sovpmber 29, 1956. Division of Analytical Chemistry, 130th meeting, ACS, Atlantic City, N. J., September 1956.
VOL. 2 9 . NO. 3, M A R C H 1957
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369
