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V O L U M E 22, NO. 6, J U N E 1 9 5 0 parison tube (downward arrow) until the micrometer reading reached the base line value. At that time the flow through the comparison tube was stopped (upward arrow), the comparison tube was again closed, and the effluent was collected in the graduated vessel as usual. This procedure of crossing over and crossing back was repeated when each succeeding component emerged and reached its equilibrium concentration. The effect was to produce a curve, each step of which arose from the base line. Presumably, in a large chromatographic experiment, this process could be re eated indefinitely, if no single step up in concentration exceedeithe ran e of the micrometer screws. If at the beginning of a $isplacement analysis experiment the two channels are connected so that the effluent traverses both of them, the observations made then represent concentration gradients across a volume interval of effluent equal to the volume of the capillary connecting the two channels. The curve obtained then represents the rate of change of concentration, a rough differentiation of the curve obtained with the ordinary technique (curve A ) . To make the differences in concentration in the two channels conveniently measurable, a spacer can be introduced between the two channels. The spacer used in experiment B is a flexible capillary tube of approximately 2-ml. volume. Curve B shows that as each component emerged, the refractive index increment across the spacer rose sharply until the equilibrium concentration of that component had emerged. At that time the increment decreased roughly to zero. The refractive index increment decreased to a newtive value on the first component, could be predicted from the shape of curve A .
=
The latter type of manipulation, crossing over, is particularly
useful in preparative experiments where the localization of the steps or components is more important than the measurement of the equilibrium concentrations, and where the use of high concentrations is desirable. I n Figure 2 is shown a displacement of lauric and myristic acids by a 3.0% solution of palmitic acid. This displacer concentration is three times as high as that normally heretofore used in fatty acid separations (8,3).The three components of the displacer diagram are clearly apparent in the differential diagram, and the range of measurement has remained within the scope of the instrument. Curves obtained with the crossing over technique (Figure 1,B, and Figure 2) representing concentration gradients are basically similar to those obtained with the Tiselius electrophoresis apparatus. It is likely that quantitative memurements based upon the integration of these curves can also be made, thus extending the analytical use of existing instruments to higher concentration ranges. LITERATURE ClTED (1) (2) (3) (4)
Claesson, S., Arkiv Kemi Mineral. Geol., 23A, 1 (1946). Hagdahl, L., and Holman, R. T., J . Am. Chem. Soc., 72,701 ( 1950). Holman, R. T., and Hagdahl, L., J . Biol. Chem., 182,421 (1950). Tiselius, A., and Claesson, S., Arkiu Kemi Mineral. Geol., 15B,18 (1942).
RECEIVED December 27,
1949. Supported in part by a grant from the William-Waterman Fund of the Reaearch Corporation.
Antperometric Titration Cell for Use with Dropping Mercury Electrode H. A. LAITINEN
AND
L. W. BURDE'IT', University of Illinois, Urbana, Ill.
MPEROMETRIC titrations with the dropping mercury A electrode suffer from the disadvantage that they must usually be performed in solution that is free of oxygen. This ri
condition is usually attained by passing an inert gas through the solution for a few minutes after each addition of reagent. The necessity of repeated bubbling to remove oxygen from the solution may be obviated by maintaining oxygen-free reagents under an inert atmosphere of hydrogen or nitrogen. The solution to be titrated is freed of oxygen before the titration and is then titrated with the oxygen-free reagent; the solution is stirred between additions of reagent. The apparatus of Kolthoff and Langer ( 8 )is a good example of this approach to the problem. For many reagents which are stable in air, however, this method involves an unnecessarily cumbersome type of apparatus. Because a means of stirring between increments must be provided, the gas stream used for this purpose might as well also serve to remove the oxygen introduced with the reagent. Another approach to the problem is the designing of a cell in which the titration may be performed while a continuous flow of inert gas is passed through the cell. This continuous gas flow would provide for both oxygen removal and thorough mixing of the solution. A primary requirement of such a cell would be an effective means of shielding the dropping mercury electrode from the flow of gas. Such shields have been described by Wise (6) and by Laitinen, Higuchi, and Czuha ( 4 ) in other analytical applications of the dropping electrode. APPARATUS
The amperometric titration cell designed to utilize the principle of continuous gas flow for oxygen removal and solution mixing is shown in Figure 1. The flow of nitrogen (or hydrogen) gas enters through the side arm and is controlled by the 2-mm. stopcock, by means of which it may be shut off or allowed to pass through the coarse fritted-glass disk in the bottom of the cell. 1
Present address, Standard Oil Company, Inc., Whiting. Ind.
The main body of the cell is about 16 cm. high (from the fritted-glass disk) and 3 om. in diameter, and has a total capacity of about 100 ml. The dropping electrode shield consists of two parts-an outer shield and an inner shield. The outer shield, which is suspended from the top of the cell by means of a stiff platinum wire, was made by cutting off a weighing tube, 22 mm. in diameter, about 3.75cm. from the bottom by means of a Carborundum lass saw. By the same method, four slits, just above and paralfel to the glass bottom of the tube, were cut at e ual distances from each other. These slits were about 0.5 cm.long and 1.5 mm. wide. Four similar slits were made 2 cm. from the bottom of the tube but not dhectl above those made at the bottom. Finally, this outer shieldlwas notched, by means of the saw, so that the platinum wire, by means of which it was suspended from the top of the cell, could be firmly attached. The inner shield was made from a piece of glass tubing with an inside diameter about 3 mm. greater than the diameter of the dro ping electrode used. The tube was cut about 2.5 cm. long a d s i x slits, 0.5 cm. in length, were made at one end parallel to the length of the tube. Two other slits, parallel to the ends of the tube, were cut half-way between the perpendicular slits and the top of the tube. These slits, which are about 0.5 cm. long, were cut on opposite sides of the tube. This inner shield was placed inside the outer shield and when in use the dropping electrode was inserted into it to a distance about half-way between the parallel upper slits and the perpendicular slits. Three holes were bored in a rubber stopper of suitable size to fit the top of the cell and cut off, if necessary, so that it extended only a short distance into the cell. The hole made for the insertion of the dropping electrode was bored so that the electrode fitted tightly. The stopper was left permanently attached to the electrode, which wm inserted far enough into the cell so that it extended into the shield the proper distance when the stopper was in place. The other two holes were provided for the insertion of a salt bridge from a saturated calomel electrode and the tip of the buret to be used in the titration. The end of the salt bridge waa made from a glass tube, one end of which consisted of a porous fritted-glass disk and which was partially a l e d with an agar plug made from a 3% agar solution saturated with otassium chloride. This tube was then attached by means o?a piece of rubber tubing, as described by Hume and Harris (I), to a saturated calomel cell made from a bottle large enough ( 5 to 7.5 cm., 2 to 3 inches, in diameter) to give a reference electrode which would not become polarized by the passage of the current.
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ANALYTICAL CHEMISTRY
A simple circuit for current meaaurementa is shown in Figure 2. The applied e.m.f. was measured by a high resistance vole meter, V . A microammeter, A with a scale reading of 0 to 50 p a , was provided with a 2000-okm Ayrton shunt, S, to control its sensitivity. The damping circuit of Philbrook and Grubb (6),utilizing two 1000-mfd. electrolytic condensers, was found effective for practical work. 0PERATION
The solution to be titrated should be placed in the cell only while the gas is passing through the cell; otherwise some solution will pass below the fritted-glass disk, spoiling the titration, If solution does get below the disk it may be effectively removed by putting suction on the side arm and rinsing the cell several times with water and finally with alcohol or acetone, after which it is sucked dry. The gas should be shut off during insertion of the electrode to prevent air bubbles from attaching themselves to the end of the capillary.
W Figure 2. Diagram of Circuit A . Microammeter
V. Voltmeter
S. Ayrton shunt
plate to a point where some solution will be allowed to pass through the plate. A liquid preasure valve consisting of a column of water was found advantageous in controlling the gas flow at the cell rather than a t the gas cylinder.
Table I. Time Required for Removal of Oxygen from 50 MI. of 0.1 N Air-Saturated Potassium Chloride (As indicated by oxygen diffusion current measurements) Current, Time, Solution pa. Seo. 3.0 0 50 ml. of air-saturated 0.1 N potassium chloride 0.6 30 0.4 60 0.4 90 10 ml. of above solution added after 0.4
