High Frequency Titrations of Organic Bases In Glacial Acetic Acid with Perchloric Acid WILLIAM F. WAGNER AND WILLIAM B. KAUFFMAN’ Department of Chemistry, University of Kentucky, Lexington, K y .
The purpose of the investigation was to determine the applicability of a high frequency oscillometer to titrations in nonaqueous solutions. High frequency titrations of organic bases in glacial acetic acid with perchloric acid agree well with potentiometric and visual end-point methods. For nonaqueous titrations, the high frequency method, which eliminates electrodes in contact w-ith the solution, overcomes the limitations of the lack of suitable visual indicators and adequate electrode systems for potentiometric end points. The authors believe that the high frequency method will find wide application in the field of nonaqueous ti trimetry.
T
The titrations were carried out in a modified dielectric constant cell shown in Figure 1. The inside of the cell was provided with spring wire clamps attached t o the walls of the cell. A 25ml. weighing bottle, held securely in place by the clamps, was used for the titration vessel. Only one half of the bottle was in the cell, the top half making no contact with the condenser. A 10-ml. sample in the cell brought the liquid just even with the top of the condenser plate, making it possible to perform the titration without changing the volume of solution between the plates of the condenser, This cell was plugged into the top of a General Radio Co. Type 722-D precision condenser connected by shielded cable to the circuit of the variable oscillator. This condenser, having a capacity of 110 ppf, on the low drum, could be read to the nearest 0.01 pyf. A stirring motor was in position to be raised or lowered into the titration cell, which was fitted with a rubber stopper through which the stirrer and buret tip were inserted. A 5-ml. microburet which could be read accurately t o the nearest 0.005 ml. v-as used for all titrations. The beat freauencv was brought to a null after each addition of titrant by var$ng ihe precisiGn condenser in parallel with the titration cell. The null was detected by a 6E5 “tuning eye” tube. At high beat frequencies the eye nas blurred, but when the beat fell below the persistence period of vision, the blinking of the eye could be seen. .4t zero beat the eye remained open. A cathode ray oscilloscope was also used to detect the null point by the appearance of the circular Lissajous pattern, but it offered no advantage over the use of the 6E5 tube.
HE extension of high frequency titrimetry to nonaqueous solutions appears to be a promising field for investigation. The lack of suitable visual indicators and adequate electrode systems for potentiometric titrations is a major limitation. It is usually necessary to use the potentiometric end point as a test or control on the visual end point. I t was the purpose of the present investigation to determine the applicability of a high frequency oscillometer to one type of nonaqueous titration. Xumerous high frequency titrators have been developed in recent years for use in analytical chemistry. The literature on the rapid development of this field has been adequately reviewed by Jensen and Parrack (1O), West, Burkhalter, and Broussard (19),Blaedel and Malmstadt (S), and Hall and Gibson (9). The advantages resulting from the use of a capacitor cell in these titrations has been presented by Blaedel and %Jmstadt (S), Sance, Burkhalter, and Monaghan (15), and Blaedel, Burkhalter, Flom, Hare, and Jensen ( 2 ) . Although most of the work has been done on the development of suitable types of apparatus, there have been several practical applications reported. The use of high frequency titrations in aqueous solutions has been reported for the determination of thorium (6) and the chloride ion by Blaedel and Malmstadt (C), of beryllium by Anderson and Revinson ( I ) , and of calcium and magnesium ions hp Jensen, Watson, and Vela (12). A study of the saponification reaction rate of ethyl acetate R as made by Jensen, Watson, and Beckham (11). An analysis of the system: water-benzene-methyl ethyl ketone was performed by West, Robichaux, and Burkhalter (20). The detection of chromatographic zones by means of a high frequency oscillator was studied by Nonaghan, Moseley, Burkhalter, and Nance (14). Recently, there has been much interest in the field of nonaqueous titrations. The literature has been reviewed for the first time by Riddick (16) and the present status of the field was summarized a t a round-table discussion ( 1 7 ) . The authors investigated the high frequency titration of organic bases in glacial acetic acid because methods using visual and potentiometric end points discussed by Fritz (8) and Markunas and Riddick ( I S ) were available for comparison.
CONDENSER Puns
Figure 1. Titration Cell
APPARATUS
The high frequency titrator was a modification of the heterodyne beat type described by Chein (6) for use in dielectric constant measurements. The simplicity of construction of this instrument as well as its high stability when operated in the vicinity of 1megacycle made it desirable for this investigation. However, a t this frequency it was necessary to work a t lower concentrations for maximum sensitivity, as pointed out by Forman and Crisp ( 7 ) .
Figure 2.
* Present address, American Cyanamid Co., Stamford, Conn. 538
High Frequency and Potentiometric Standardization Curves
539
V O L U M E 25, NO. 4, A P R I L 1 9 5 3
B
41
Ik
0340 18
1.9
Figure 3.
20
el---- 51 32 M L . HCL04 ADDED.
3.3--- 4.2
4.3
4.4
4.5
High Frequency and Potentiometric Titration Curves for Aniline
The potentiometric titrations were carried out with a Beckman Model G pH meter using a shielded glass indicator electrode and a silver-silver chloride reference electrode as described by Fritz (8). The titrations were carried out in a 50-ml. beaker fitted with a rubber stopper in which the electrodes were mounted and two holes were provided for the buret tip and stirrer. An airdriven stirrer was used because erratic readings were obtained when an electric motor stirrer n-as used in the vicinity of the p H meter .
The instrument was allowed to warm up for 2 hours. A 10-ml. sample was pipetted into the titration cell; an indicator was added if desired, and the stirrer and buret were lowered into the cell. The stirring motor was started and the beat frequency was brought to a null by adjusting the precision condenser. The titration was carried out by adding portions of the standard titrant and recording the condenser reading when the null was established after each addition. Any sudden change in the capacitance increments signified an end point, after which five or six more readings were obtained. The condenser readings were plotted against milliliters of titrant added; the intersection of the extrapolation of the straieht line Dortions of the Dlot was taken as the end point of the titration. Standardization of Perchloric Acid Solution. Standard 0.1 N perchloric acid was prepared by dissolving 4.24 ml. of 70 to 72% perchloric acid in glacial acetic acid and diluting to 500 ml. I t was found that the addition of acetic anhydride to remove the small amount of water had no noticeable effect on the titrations, as reported by Fritz (8). Perchloric acid solutions were standardized using acid potassium phthalate according to the procedure of Seaman and Allen (18). Figure 2 shows the curves obtained for standardization by high frequency and potentiometric titrations. Standardization, using the visual change of crystal violet indicator to blue, gave values in agreement with the potentiometric and high frequency methods.
REAGENTS
Alceticacid, glacial, ACS specifications. ilcetic anhydride, ACS specifications. Perchloric acid, 70 to 72%, .4CS specifications. Acid potassium phthalate, primary standard, Xational Bureau of Standards. Pyridine, Eastman yhite label. piToluidine, Eastman white label. The aniline, freshly distilled, and ,V,',S'-bis (2-cyanoethyl)-2,5dimethylpiperazine were obtained from Union Carbide and Carbon Corp. .\I1 inorganic salts met ACS specifications for purity. EXPERIMENTAL
The instrument n-as tested for titrimetry using aqueous titrations of hydrochloric acid xith sodium hydroxide and sodium chloride ith silver nitrate. The acid-base titration gave a very sharp end point in the concentration range calcul:tted from the relation of Forman and Crisp ( 7 ) . The titration of 2 X 10-4 to 3 x 10-4 jf sodium chloride \\ith silver nitrate gave good results. The follon ing procedure ~ 3 used ' for all high frequency titrations:
Table I. Base .hiline
Method High frequency
p-Toluidine
Average Potentiometric High frequency
Average Potentiometric High frequency Pyridine Visual N,N'-Bis (2-cyanoethy1)- High frequency 2,5-dimethylpiperazine Average Potentiometric Average
Figure 4. High Frequency and Potentiometric Titration Curves for p-Toluidine
The problem of determining the exact end point in high frequency titrations is shown by the tQo curves, 3 A and 3B in Figure 2. In 3 A the titrant was added in increments of 0.05 to 0.1 ml. while in 3B, in increments of 0.02 ml. I n the latter case, the end point agrees with the potentiometric end point. It is generally advantageous first to determine the approximate end point using large increments of titrant and then to carry out the titrations using small increments in the vicinity of the end point. The precipitate which was formed during the titration of acid potassium phthalate with perchloric acid had no deleterious effect on any of the titraTitration of Organic Bases tions. Normality of HC104 0.1074
0.1009
0.1210
, 0.1074
....
HClOi Base Used, M1. Taken, Gram 1.99 0.01982 1.99 0.01982 3.24 0.03234 4.35 0.04347 1.99 3.46 3.62 3.90 3.90
0.01982 0.02655 0.03910 0.04206 0.04206
3.90 1.96 1.95 3.42 4.08 4.44
0.04206 0.01896 0.01896 0.04024 0.04800 0.05240
4.10 4.45
0.04800 0.05245
Base Purity Found, Gram of Base, % 0.01990 100.4 100.4 0.01990 0.03241 100.2 100.1 0.04351 100.3 0.01990 100.4 0,02659 100.2 0.03913 100.1 0.04216 100.2 0.04216 100.2 100.2 0.04216 100.2 98.94 0.01876 98.48 0.01867 0.04046 100.5 0.04827 100.6 0.05253 100.2 100.4 0.04850 100.6 0.05264 100.4 100.4
TITRATION OF ORGANIC BASES
High frequency titrations of several organic bases were attempted and compared to the corresponding potentiometric titrations. The results obtained for aniline, p-toluidine, pyridine, and N,N'-bis (2-cyanoethyI)-2,5dimethylpiperazine are summarized in Table I and in Figures 3, 4, 5 , and 6. Aniline ( K b = 5X and p-toluidine ( K b = 2 x 10-10) gave similar high frequency curves. The potentiometric end points agreed well
540
ANALYTICAL CHEMISTRY
with those obtained from the oscillometer but they were not quite as sharp as those obtained in the standardization. The visual end point using crystal violet was not sharp and was difficult to detect. .4s may be seen from curves 1A and 1B in Figure 3, and 3 A and 3B in Figure 4, the high frequency end point was the same when 0.1-ml. or 0.02-ml. increments of perchloric acid were added near the end point. The break in the high frequency curve for aniline became less abrupt a t higher concentrations.
I
J
. 30
32
34
36
36 K
40 HCL04ADDEO
42
44
46
48
Figure 6. High Frequency and Potentiometric Titration Curves for N,N’-Bis (2-cyanoethyl)-2,5-dimethylpiperazine
o-nitroaniline (Ka = 1.5 X with the oscillometer.
and urea (Ka = 1.5 X lo-“)
ACKNOWLEDGMENT
The authors wish to thank John P. Fletcher for supplying aome of the samples of organic bases. LITERATURE CITED
I
0
I ML. HUO, ADDED
2
Figure 5 . High Frequency and Potentiometric Titration Curves for Pyridine
An unusual curve was obtained for the titration of pyridine (Kb = 1.4 X as shoun in Figure 5. The time interval from A to B was very short, the solution coming to equilibrium within 5 seconds after each addition of perchloric acid. -4t point B a white flocculent precipitate began to form and the time required for titration from B to C was 50 minutes. Equilibrium was established very slowly after each addition of titrant. A titration using methyl violet indicator as described by Fritz (8) gave results slightly lower than the oscillometer, hut the process proceeded much faster. The curves for the high frequency and potentiometric titrations of N,iV’-bis (2-cyanoethyl)-2,5-dimethylpiperazineshoa n in Figure 6 resemble more closely the titration of acid potassium phthalate with perchloric acid. The titration of both basic constituents of the compound gave only one break in the curve. The titration of p-nitroaniline (Kb = 1 X was attempted but no satisfactory high frequency end point n-as achieved, although a slight break did occur. This was apparently the lower limit of bases that could be determined by the procedure used in the above investigation. No potentiometric curve could be obtained with p-nitroaniline. It was impossible to titrate
(1) Anderson, K., and Revinson, D., A s . 4 ~ CHEM., . 22, 1272 (1950). (2) Blaedel, W. J., Burkhalter, T. S., Flom, D . G., Hare, G., a n d Jensen, F. W., Ibid., 24, 198 (1952). ( 3 ) Blaedel. W.J.. and hlalmstadt. H. V.. Ibid.. 22.734 (1950). (4j I b i d . , p.’1410. ( 5 ) I b i d . , 23,471 (1951). (6) Chein, J. Y., J . Chem. Educ., 24,494 (1947). (7) Forman, J., and Crisp, D. J., Trans. Faraday SOC.,42A, 186 (1946). ( 8 ) Fritz, J . S., A s a ~CHEX, . 22, 1028 (1950). (9) Hall, J . L., and Gibson, J. A , , J r . , Ibicl., 23, 966 (1951). 110) Jensen. F. IT.. and Palrack. A. L.. IND. EXG.CHEY..ANIL. ED.. 18,595 (1946). (11) Jensen, F. W.,Watson, G. ll,,and Beckham, J. B.. ANAL. CHEM.,23,1770 (1951). (12) Jensen, F. R., Watson, G. hl., and Vela, L. G., I h i d . , 23, 1327 (1951). (13) Markunas, P. C., and Riddick, tJ. .i.,I t i d . , 23, 337 (1951). (14) Monaghan, P. H., XIoseley, P. B.. Burkhalter, T. S., and S a n c e , 0.A, Ibid., 24, 193 (1952). (15) Nance, 0. A , , Burkhalter, T. S . , and Nonaghan, P. H., I b i d . , 24,214 (1952). (16) Riddick, J . A , , Ibzd., 24, 41 (1952). (17) Riddick. J. A , . Fritz, .J. S.. Davis. M . 11..Hillenbrand. E. F.. Jr., and hlarkunas, P. C . , Ibid , 24,310 (1952). (18) Seaman, IT., and.Illen, E., Ibid., 23, 592 (1951). Burkhalter, T. S.,and Broussard, L., Ibid., 22, (19) West, P. W., 469 (1950). (20) West, P. W.,Robichaux, T., and Burkhalter, T. S., Ibid., 23, 1625 (1951). RECEIVED for review August 20,1952. Accepted November 3, 1952. From a thesis submitted by W. B. Kauffman to the Graduate School of the University of Kentucky in partial fulfillment of the requirements for the degree of master of science.
