Nonaqueous Quaternary Ammonium Titrants. G. A. Harlow and G.
E. A.
Preparation by the Potassium Hydroxide Method
Wyld, Shell Development Co., Emeryville, Calif.
solutions of quaternary N ammonium bases are excellent titrants for the determination of acids ONAQUEOCS
in organic solvents. They are superior to the alkali metal hydroxides and alcoholates because they can be used with the glass electrode without introducing the “alkali error” and because they form salts which are more soluble in nonaqueous solvents than are those of the alkali metals. In the past these titrants have been prepared by either the ion exchange (3) or the silver oxide (1) method. Although both of these methods yield satisfactory titrants, the procedures involved are time-consuming. It has long been known that quaternary ammonium hydroxides can be prepared from ethyl alcoholic potassium hydroxide and a quaternary ammonium chloride (5). Apparently, however, this method has not been previously tested for the preparation of nonaqueous titrants. Solutions of potassium hydroxide in isopropyl alcohol are extensively used in petroleum laboratories for the determination of saponification numbers (ASTM D 939-54) and of acidity (ASTM D 664-54). It occurred to the authors that this solution should make a n excellent starting material for the preparation of quaternary ammonium titrants. This paper describes the results obtained in developing and testing such a method of preparation.
The general procedure used in testing the method consisted of taking a measured volume of potassium hydroxide titrant solution, adding a slight excess of the quaternary ammonium chloride, shaking the mivture vigorously, and allowing the precipitated potassium chloride to settle. Samples of the clear supernatant liquid were withdrawn by pipet, with proper precautions being taken to avoid contamination by atmospheric carbon dioxide. Standardization was carried out in isopropyl alcohol withO. 1N isopropyl alcoholic HCl. The same general procedure was used successfully for the preparation of hexadecyltrimethylammonium hydroxide and tetramethylammonium hydroxide titrants. For the sake of brevity, however, details are given only for the preparation and testing of the hexadecyltrimethylammonium hydroxide. The folloB-ing procedure is specifically designed for the preparation of 100 ml. of 0.2N hexadecyltrimethylammonium hydroxide.
To 100 ml. of 0.2N potassium hydroxide in isopropyl alcohol add 7.0 grams of dry hexadecyltrimethylammonium chloride (K and K Laboratories, Inc.). Shake vigorously a t intervals over a period of 15 minutes and then allow the precipitate to settle. Decant the supernatant solution into another container, preferably one which permits delivery of titrant and yet protects it from carbon dioxide.
PREPARATION OF TITRANT
The potassium hydroxide method of preparing quaternary ammonium hydroxides is based on the reaction between potassium hydroxide and quaternary ammonium chlorides in isopropyl alcohol solution.
+ KOH
NR,Cl
(2) XR,OH 4- KCl $
The reaction is forced to completion by the precipitation of potassium chloride, which is only slightly soluble in isopropyl alcohol. Quaternary ammonium bromides and iodides are less suitable starting materials, because the corresponding potassium salts are more soluble. 172
0
ANALYTICAL CHEMISTRY
Table 1. Reproducibility of Hexadecyltrimethylammonium Hydroxide Preparations Trial
Normality
Conversion Factor
7
0.1686
0,923
AV.
0.924
The above procedure provides for an excess of the quaternary ammonium salt for cases where the potassium hydroxide normality is 0.2X or less. For stronger hydroxide solutions more of the salt is required. If the normality of the resulting solution is to be determined by titration, the volume of potassium hydroxide solution and the weight of the quaternary ammonium salt need only be approximate. If, on the other hand, the normality is to be calculated, the volume of potassium hydroxide solution should be known to 0.1 nil. and the quaternary ammonium salt should be weighed to the nearest 0.01 gram. CALCULATION
OF
NORMALITY
The conversion of a potassium hydroxide titrant to a quaternary ammonium titrant results in a decrease in normality, because quaternary ammonium ion occupies a greater volume than the potassium ion which it replaces. It was of interest to determine if a correction factor for this change in volume could be applied, so that the normality of the quaternary ammonium titrant could be calculated from the normality of the potassium hydroxide. To test the reproducibility of the method seven preparations of hexadecyltrimethylammonium hydroxide were made from 20-ml. portions of the same stock solution of potassium hydroxide (0.1826N) by adding equal amounts (1.6 grams) of hexadecyltrimethylammonium chloride. The resulting solutions were standardized by potentiometric titration with 0.liV hydrochloric acid. The results are shown in Table I. The standard deviation of the conversion factor is about 0.003. For many routine determinations of acidity the calculated normality would be sufficiently accurate. One difficulty with the use of a conversion factor is that an identical amount of quaternary ammonium chloride must be added each time. Since it is more convenient to weigh out a less specific amount of the salt, an attempt has been made to apply a more general formula. It was determined by experiment that the volume of the titrant solution was increased by 1.04 ml. for
"I".. OrQJm nf e9-h ". .hexadecyltrimethylammonium chloride added. The use of this factor perm its the calculation of the normality of the resulting hexahydroxide decyltrimethylm monium sointion from the normality of the starting potassium hydroxide titrant and the weight of the hexadecyltrimethylammoniinn chloride taken. ~
...
I
COMPARiSOhI OF METHODS OF PRElPARATION
The major advantages of the potassium hydroxide method of preparation are its convenience and speed. No special apparatus is required, and operator time is kept a t a m i n i u m . A batch .. of titrant can he. prepared . . . .. in less tnan I nour. m contrast, the suver oxide method ( 1 ) requires 2 to 3 hours,
.
..
~
the ion exchange method (3) 4 to 8 hours. An additional advantage is elimination of the standardization whenever high precision is not required. As with both other methods of preparation, care must be taken to avoid contamination by carbon dioxide. The titrants prepared by thiz;method are also subject to slow decor(position in the same manner as similsr solutions prepared hy other method s. This decomposition, which results iin the formation of trialkylamines, can be minimized by storing the titrant in a refrigerator. A disadvantage of the potassium hydroxide method is that it yields titrants with a relative17i high potassium
potentiometric titration of very weak acids (2). The potassium ion can he reduced to a negligible concentration by treating the titrant with a strong acid ion exchange resin in its quaternary ammonium form. However, this adds step to the preparation and makes it necessary to standardize the solution. LITERATURE CITED
(1) Cundiff, R. H., Msrkunas, P. C., ANAL.CAEM.28, 792 (1956). (2) Harlow, G. A,, Ibid., 34, 148 (1962).
(3) Harlow, G. A,, Noble, C. M., Wyld, G. E. A,, Ibid., 28, 787 (1956). ( 4 ) Hummelstedt, L. E. I., Hume, D. N., Ibid.,32, 1792J19~0). ,E\
color-indicator (4) or most . . . . . ^titrations . .. . . potentiometric titrations, it may, unaer certain conditions, interfere in the
Txrhi+mnl^
in Dart conference on- ~
PREsENTEo
at Gordon Research ~ chemistry, ~
August 1961.
A pH-STAT Lars Josefsson, C. E. Ryberg, and Runa Svensson, Department of Physiological Chemistry, University of Lund, Lund, Sweden
INa widerangeyearsof applications the pH-stat has found andisnow RECENT
becoming a standard instrument in many laboratories. Today several commercial instruments are available and the theoretical and practical use of the pH-stat in biochemistry has been reviewed (1). Nevertheless, there s e e m to be some justification for describing the instrument developed several years ago in our laboratory, because it still
has major advantages over others in use, especially in studying reaction systems of low concentrations and for long periods of time. For our purpose it was necessary to keep the pH constant within jzO.01 pH unit for one or more days. This puts very high demands on the pH-meter to be used. Our requirements were met by Titrator TTT 1 (Radiometer, Copenhagen). The principle by which this
instrument operates is described elsewhere ( 1 ) . However, to obtain a suitable control of the buret as well as of the recorder, a special buret control circuit has been devised (Figure 1). This controller is connected via a relay to the socket marked "valve" of the titrator.
Our Of the buret is On the same principle as that of Lingane (6)i,e,, a motor-driven buret syringe, It is shown in ~i~~~~ 2 and consists of a motor (Bodine, Type W C 23) with transmitters, an AGLA - micrometer syringe (Burroughs Wellcome and Co., London), with a total capacity of 0.5 ml. and a ten-turn potentiometer of
"ELlPD,
Figure 1.
Wiring diagram of buret control
Heiipot, 1 0-turn potentiometer, 1000 ohms, 0.5% linearity C1. 4 pf., 450 volts C2. 3 2 mi., 500 volts, d.c. R1. 1 0 0 0 ohms Rs. 22,000 ohms Ra. 470,000 ohms Rn. 220,000 ohms Rs. 100.000 ohms Rs-s. 100 ohmr wirewound Rp. 5000 ohms, linear REG. 2 0 0 volts, 3 0 ma. RECv 2 5 0 volts, 75 ma. REL Telephone relay T. Primary 2 2 0 volts, 5 0 periods per second, recondory 1 5 0 volts, 50 ma. VI. OA 2 V% B 5 A 2
Figure 2.
Buret
S. Gear train U,V. Friction drive W. Potentiometer X. Motor Y. Worm gear 2. Micrometer screw Bodine Type KYC is now available, with reduction gear tiam built in, IO that Worm gear Y could be eliminated VOL. 34,
NO. 1,
JANUARY 1 9 6 2
173
