of thiourea, and presoaked with the eluting solvent for zinc and cadmium after the resolution of a sample of zinc, cadmium, and mercury, before the introduction of another sample. This can be accomplished in one step by washing the resin free of the excess thiourea with the zinc and cadniiuni eluting solvent. If only zinc and cadmium are to be resolved. no washing step is required after the resolution and bcfore the introduction of another zinc and cadmium sample. because the rwin is in contact with the same
161 Kraus. K. A,. Xelson. F.. Proc. Intern. c‘onf. Peacefd Cses ’Atomic Energy, Geneca (1955) 7, 113-25 (1956). (7) hlartell, A . E., Calvin, ?VI., “Chemistry of the Metal Celate Compounds,” p. 487, Xew Tork, Prentice-Hall, 1953. ( 8 ) Riley. H. L.. Gallafent. V.. J . Cheni.
solvent a t the end of the separation as a t the beginning. LITERATURE CITED
(1) Burstall, F. H., Forrest, I-’. J., Kember, X . F., I n d . Eny. Chevz. 45, 1648 11953).
RECEIVEDfor review May 10, 1957. Accepted July 11, 1958. Presented in part at the Southwide Chemical Conference, Xemphis, Tenn., December 1956.
Interscienre. h e & York. 1947’. (5) Kraus, K.’ A., Moore: G . E., J . Ani. Chenz. Soc. 75, 1460 (1953).
Improved Saponification Number Determination by Use of Ion Exchange W. B. SWANN, R. J. ZAHNER, and 0.I. MILNER Research and Development laboratory, Socony Mobil Oil Co., Inc., Paulsboro, N. 1.
b In the determination of saponification number by the usual method, detection of the end point is often difficult because of the buffering effect of the weak acid salts. A method has been devised in which the saponified material, in nonaqueous medium, is passed through a cation exchanger. The excess caustic is converted to water and the salts of the organic acids are converted to the free acids, which are titrated potentiometrically. The procedure greatly simplifies the determination of the end point, eliminates the need for precautionary measures to prevent absorption of carbon dioxide, and gives additional information on the nature of the organic acids comprising the original esterified material.
S
number is widely used as a measure of the ester content of organic materials. Usually, the sample is saponified by refluxing with a known excess of standard caustic, which hydrolyzes the esters to the corresponding alkali salts. The excess caustic is titrated with standard acid to give the original ester concentration. K i t h mixtures of esters, it is often difficult to judge the end point of the titration because of the buffering effect of the weak acid salts formed during saponification. I n the petroleum industry, the saponification number is frequently determined on used engine oils. Because such oils are generally dark, a potentiometric titration is used. However, as used oils contain mixed esters, end point difficulties are still eneountercd. APOKIPICATIOS
1830
ANALYTICAL CHEMISTRY
13 E
+ 400
12 I
1 Figure 1. Titration curves from ASTM Method D 939-54 in nonaqueous medium
IO 4
‘ +
,oo,
8 7
2’
:I
1I:_
030.
.,
- 300
1 9 0
1
2
3 0.2N
To estimate the end point when a distinct inflection in the curve is not obtained, i t is customary to select an arbitrary point such as p H 10. I n nonaqueous media, where p H readings are not valid, the standard method recommended by A8TM ( I ) provides for the use of a nonaqueous basic buffer; the end point is taken a t the corresponding meter reading. This method still leaves some uncertainty as to the validity of the end point. Unless special precautions are taken, atmospheric carbon dioxide is absorbed during transfer and titration of the solution, introducing an additional error. The interference of carbon dioxide can be overcome by adding excess standard acid to the saponified solution, removing the carbon dioxide by purging with nitrogen and finally titrating in the reverse direction-i.e., with standard alkali. However, there is generally a mixture of organic acids having various dissociation constants, and it is difficult
4 HCI
5
5
7
8
MI
to determine the point a t which all the mineral acid has been neutralized. A new method is based on passing the saponified material through a cation exchanger in the hydrogen form. This converts the excess caustic to water and liberates the weak organic acids, which are titrated directly. This method gives well-defined titration curves and unmistakable end points. TITRATION
CURVES BY CONVENTIONAL METHOD
Figure 1 is a reproduction of A S T N titration curves ( I ) , typical of those expected when the nonaqueous titration solvent (1 to 1 benzene-isopropyl alcohol) is used. Curve d represents the titration of the standard alcoholic potassium hydroxide used in the saponification. The point a t +0.26 volt corresponds to the potential of the nonaqueous buffer and is used for estimating the end point from poorly defined
curves. Curve B represents the titrat,ioii of :i saponified castor oil. Becauscx castor oil is a rrfiiiecl ester, derived principally from ricinolcic acid, a welldefined inflection is obtained upon neutralization of the escess caustic.. C u r w C is the titration of a saponified samplc of osidizrd lubrirating oil. Rrcauso niatrriale of this type gmerally cwntain a series of estrrs derived from avids having difffvnt dissociation constants, thc, c u r ~is ~not n-ell defined. ~'onscquontly, the prrclt.tc.rniinet3 potontial of 0.26 WJlt is used to nirasurr tlw end point. The abnornial appearance of these curvcs-i.e., t h r sudden increase in potential prior to the inflectioii-is a c~haractc~ristic of the nonaqueous titration solvent anti is undesirable. Also. in nonaqueous titrations the e1ectrodt.s oftcm I w o m e fouled. For tliese reasons, u i i d to protect against atniosphrrir varhon rliositlc, the niethod has l x ~ nmodified a t the authors' laborator!-. Thv practiw is to add n-atc,r to thv flask :iftvr saponification, nliereby the s o a p anti ( w w s caustic arc extracted into the Ion-rr (wutrr) phase; the titration is c-arricd out in the aclueous part of the two-phasP system. By this procedure tlie curvw a p p ( w mor(' normal (Figure 2 ) . C u r w .1 again reprcsrnts the titration of thv stantlard alcoholic potassium Iq-droxitl(~u s c ~ l in the saponification. Points n, nnd a2 on t,he curve correspond to pH 10 and the point of inflection, respectivrjly. Curvr B represents thc. titration of ('X(WS caustic in n saponified sample of c.thyl-?2-butyrate. This r u r w is also w l l defincd. Curve C follows the titration of the escess caustic after saponification of an oxidized cwgine oil. This curvc' is poorly dcfined, and point el, corresponding t,o a p H of 10, nould normally be used t o (Istimate the end point. Hon-ever, the slight change in slope in the vicinity of +0.05 volt suggests that there may be t n o inflections. I n this ( m e the first inflection point, c p , could be considtvtl the end point. A third interpretation is that the slight change of slope in the vicinity of +0.05 rolt results
+
30OI
t 200
t
+
from either instrument instability or tardy electrode response. Thus, thc curve could be conqidered as having on(' inflection and the midpoint, c3, could be taken as the end point. Each of t h r three reasonable waj s of interpreting the titration curve results in a different saponification value. Unfortunately. with engine oils, conipl(Ju curves arc niuch niorc prevalcnt than well-defined ones. The effect of absorption of carbon dioside from the atmosphere is illustrated by the curves in Figure 3. Curve * I follows the titration of a carbon diouide-free solution of alcoholic potassimn hydrosidr. Curve B represents the titration of the caustic solution after deliberate exposure to air. Thc greatelst deviation between the curve< is a t p H 10, in the vicinity of most end points. T h ~ i sabsorption , of carbon diokidr can cauSe a serious wror. ION EXCHANGE
Van Etten anti \Lele (5) used the hydrogen form of a cation exchange resin to liberate frcc acids from aqueous solutions of organic salts. Jenkins ( 2 ) used the same form of resin to convert quantitativcJlJ alcoholic solutions of soaps to the corresponding acids. Teisenberger (6) and Samuelson (4) employed the ion euchange technique to remove the rscess caustic after saponification of simple esters, but confined their work to aquc'ous systems and used indicators to detect the end point. Mzukanii and Ieki ( 3 ) determined acetyl groups by an iodometrir method after removal of the raustic by a cation eschange resin. I n the proposed method, the saponified solution is passed through the hydrogen form of a ration resin. This converts the e s c w s caustic to water and the organic acid salts to free acids. Potassiuni caarbonate, formed by absorption of carbon d i o d e , is a t the same time converted to carbonic acid, which is e1iniinatc.d by purging with nitrogen. The organic acids are titrated directlv to give the original ester content. I n this mj., tlie potentio-
Efficiency of Conversion by Ion Exchange Column
Compound Ethyl n-butyrate n-But) 1 lactate Isopropvl benzoate Ethyl propionate n-Amyl acetate E t h l l acetate n-Butx 1 acetate
Ester, Meq. ConIon ventional exchange 1 55 1 15 1 05 1 45 1 29 I 53 1 86
1 54 1 16 1 07 1 47
1 26 1 53 1 35
metric titration does not h a w to distinguirh between excess strong basc and the salts of weak acids, and tho detection of the end point is greatlj simplified. To establish that the excess caustic and salts are converted quantitativelj. to water and the corresponding acids, srvwal commercially available esters nere tested. These esters were derived from single acids and ere thus suitable for analysis by both methods. After saponification, the saponified material was divided into two aliquots. Onr. aliquot JTas tested by the conventional method, and the other n-as passed through the resin and titrated avcording to the described procedure. A comparison of the results is shown in Table I. RECOMMENDED PROCEDURE
Apparatus. The saponification apparatus consists ol 50-ml. Erlenmeyer flask t o which a 200-mm. Liebig (may be purchased from A. H. Thomas, Philadelphia, Catalog KO. 3906) condenser is attached b y a 19/38 standard taper ground-glass joint. T h e ion exchange column consists of a standard 50-ml. buret with a plug of glass no01 a t t h e constricted end. A Precision-Don Recordomatic titrator. equipped x i t h a glass-calomel electrode system, was used to carry out the titrations. However, a manual titration assembly may be used. Reagents. Titration solvent. Add 500 ml. of benzene t o 500 ml. of Formula 30 alcohol. Alcoholic potassium hydroxide, 0 . 2 S . Dissolve 12 t o 13 grams of
12.1 B
t
300
t
200-
10.4
12 I
-
~-
~
10 4
e .7
100~
I
9000
7
-
100-
5.3
-
200
3 .6
0.
.o
0.2N
Figure 2.
Table I.
100 -.
- ~ _
-~
--
-
-
-I. B., AKAL CHEW25,1109 (1953). ( 6 ) Keisenberger, E., Makrochenzze Lei Mzkrochzm. -1cta 30, 241 (1942). RECEIVED for review February 26, 1958. Accepted June 24, 1958. Second Ilelaware Vallej- Regional Meeting, ACS, Philadelphia, Pa., February 5 , 1958.
Titration of Weak Acids in Nonaqueous Solvents Potentiometric Studies in Inert Solvents G. A. HARLOW and 0. B. BRUSS Shell Development Co., Emeryville, Calif.
b Phenolic and carboxylic acids have been potentiometrically titrated in inert solvents such as benzene, toluene, and gasoline. Nonaqueous solutions of quaternary ammonium hydroxides are used as the titrant and a glass-calomel electrode pair is used with a vibrating reed electrometer. Normal titration curves are obtained when dilute titrants are employed, but 1 N) an with concentrated titrants additional inflection frequently occurs near the mid-point. These mid-point inflections are believed to be due to acid-anion complexes.
(>
T
potentiometric titration of acids in inert solvents such as benzene, toluene, and gasoline is of interest from both the practical and theoretical points of view. This type of nonaqueous titration has received very little study because of experimental difficulties, one of LT hich is the shortage of suitable titrants. Conventional titrants such as alkali metal hydroxides and alcoholates combine with acids to form salts which are insoluble ininert solvents. These titrants also desensitize the glass electrode, making it almost useless for acidity measurements in solutions of 1-ery lo^ hydrogen ion content. Organic amines, on the other hand, are too weak for the titration of weak acids. The development of nonaqueous auaternari ammonium hydroxides has alleviated the titrant problem (3-6). S o t only are quaternary ammonium salts much more soluble in inert solvents than those of the alkali metals, but also the quaternary ammonium ions show no HE
tendency to inhibit the hydrogen ion response of the glass electrode. The second major difficulty is the accurate measurement of electrode potentials in inert solvents. Conventional titrometers and p H meters do not operate satisfactorily ith these solutions of extremely high resistance. This investigation had as one of its principal aims the development of apparatus which mould be satisfactory for this purpose. Published information on potentiometric titrations of acids and bases in inert solvents such as benzene and toluene is meager. The work of La Mer and D o m e s ( 7 ) , carried out 26 years ago, appears to be the major contribution. They titrated trichloroacetic acid n i t h diethylamine in benzene solution. d quinhydrone electrode was employed, and the potentials were measured with a ballistic galvanometer. The titration curves obtained n-ere normal in appearance. INSTRUMENTAL CONSIDERATIONS
The high resistance of inert solvents leads to two types of difficulties in potentiometric measurements. One, the electrostatic pickup of extraneous potentials, can be greatly reduced or eliminated by careful shielding and grounding of the titration apparatus. The other, the voltage drop across the electrodes and the cell solution, is a function of the current drawn by the voltmeter or p H meter. Titrometers and p H meters generally have input currents of to 10-12 ampere. The best of these instruments
are satisfactory for potentiometric titrations in nonaqueous solvents such a> acids, amines, and alcohols, where the total resistance is 10’0 ohms or less. They are inadequate, however, for inert solvents where resistances a thousandfold greater are encountered. Although the solvent resistance may be decreased by adding polar compounds, this n-ould defeat the purpose of many investigations. RIany of the instruments usually described as electrometers have input currents much smaller than conventional titrometers and p H meters (< ampere). An excellent review of the various types of electrometers has recently been published by Palevsky, Swank, and Grenchik (Q), n h o point out that the dynamic capacitor-type instrument is wperior in several respects. A number of excellent instruments of this type are being inanufactured, mainly for use in radiochemical work. I n this study, the Carey (Applied Physics Corp.) Model 31 vibrating reed electrometer 11-as employed. REAGENTS AND APPARATUS
Titrants. Tetra-n-butylammonium hydrovide in isopropyl alcohol solution was used throughout. (For t h e purpose of simplicity the titrant is referred t o as hydroxide, although it is actually a n equilibrium mixture of hydroxide and isopropylate.) T h e 0.2,V titrant was prepared by the ion exchange procedure (4). The stronger titrants were prepared by two different methods. One consisted of concentrating the 0.2N titrant in a rotating flask evaporator a t room temperature VOL. 30, NO. 1 1, NOVEMBER 1958
1833
