INDUSTRIAL A N D ENGINEERING CHEMISTRY
926
TRAY
Figure 2.
Flow Controller
ground to a sharp edge, would be at least as satisfactory, and probably better than the shape shown in Figure 1. Although the direction of the V as shown does not make the slot self-cleaning, it is necessary to have it as shown in order to feed all the liquid to the guide plate. (A disk-type metal filter with low pressure drop was used on the feed line to remove any scale or sediment from the liquid.) With the exception of the slot, which must be fixed accurately, none of the dimensions are critical, and they may be varied to suit the individual requirements. The plates are adjustable, so that the liquid head necessary to maintain the proper flow does not become unreasonable. The
Vol. 18, No. 11
plates are adjusted a t both ends, two feeler gages being used to ensure parallelism. The slot width is determined by the allowable liquid head and the density, viscosity, and surface tension of the liquid. A spacing of 0.0045 inch in a 4-inch long slot is sufficient to carry a flow of 5 to 25 gallons of gasoline per hour at a maximum head of 15 inches, and to provide a continuous sheet of liquid. This slot \vas tested with gasoline a t reflux ratios in the range of 1 to 30. The ratios predicted from the position of the divider were obtained with an accuracy varying from * l % a t a ratio of 1 to * 10% a t a ratlo of 30, throughout the entire range of 5 to 25 gallons per hour. This degree of precision is entirely adequate for distillation work, but can be further increased by lengthening the slot. While the present device was designed for reflux control, it is evidently applicable to many installations where it is desired to split a liquid stream into two parts of given, constant ratio. Its advantages include: no moving parts in operation; wide and continuous range of ratios, independent of throughput, pressure, viscosity, etc.; continuous, rather than intermittent, operation with relatively low holdup. LITERATURE CITED
(1) Berg, L., ISD. ENQ.CHEM.,ANAL.ED., 18, 54 (1946). (2) Brunn, J. H., Ibid., 7, 359 (1935). (3) Carter, A. 9.. and Johnson, F. W., U.5.Patent 2,251,185 (July 29, 1941). (4) Podbielniak Centrifugal Contactor Co., Circ. 22 (Nov. 1942).
Improved Apparatus for Karl Fischer W a t e r Determination RICHARD KIESELBACH Bakelite Corporation, Bound Brook,
TFHE
need for excluding atmospheric moisture from a Karl ischer titration is well known; it is equally necessary, for high precision pork, to protect the apparatus used from moisture before the titration. Several types of apparatus to accomplish this end have been proposed. Almy, Griffin, and Wilcox (1) suggested a stopper drilled to take the buret tip, stirrer, and electrodes, and machined to fit into an ordinary Erlenmeyer flask. Wernimont and Hopkinson (4)modified this apparatus by machining the stopper to a standard taper, which with a standardtaper flask gives a more nearly air-tight joint, and replaced the glass stirrer with stainless steel for the sake of durability. More recently, an all-glass apparatus n a s described by Zerban and Sattler (6), in which magnetic stirring is used with a semi-ballneck flask having a side neck for the electrodes. The apparatus shown in Figure 1 is an all-glass modification of Wernimont and Hopkinson’s apparatus. In use, it is held by a clamp on the upper portion and connected by the standard-taper 10/30joint t o an automatic buret of the overhead reservoir type. The use of a seating-type joint prevents trapping of an air bubble at this point. A radio tube base and a cable connector afford a convenient and positive means of electrical connection to the titrimeter. The one-piece construction of the apparatus eliminates possibility of leakage and adds t o convenience of assembly. A glass stirrer is used, since stainless steel is subject to corrosion by the reagent. Less than 5 seconds are required by an experienced operator to remove the flask from the apparatus and unstopper and connect the next flask to be titrated; this feature is also found in the first two types cited. All else being equal, the speed of this operation determines the degree of precision obtainable in the titration. The dimensions of the apparatus permit use of a 250-ml. Erlenmeyer flask with a 24/40 standard-taper joint, but it could be modified to accommodate other sizes. For small-scale titrations, a vessel in the form of a test tube with a 24/40 joint will permit titration of volumes down to about 5 ml. A solvent-resistant grease should be used on the buret stopcock and the 10/30 joint, to prevent serious leakage. The 24/40 oint should not be greased, since a slight amount of outward air eakage is necessary to permit addition of the reagent; it is some-
i
N. J.
times necessary to grind a slight groove in the male joint to t,his end. An efficient moisture trap should be used on the air inlet of the buret. (The drying tube usually supplied with the buret is too small to be effective.) The stirrer should be connected to the motor with a short length of soft-rubber tubing to prevent damage due t o misalignment. Care should be taken to ensure that the stirrer paddles clear the electrodes. A drop or two of light machine oil in the glass bearing will reduce wear. The electrodes should be kept covered with a spare flask between titrations, that flask being kept stoppered when not in use. TITRIMETER
Serfass (3) has described an electronic titrimeter suitable for detecting end points in conductometric and potentiometric titrations, featuring the use of a “magic eye” electron ray tube for the indicating instrument. MeKinney and Hall ( 2 ) modified the circuit by introducing a voltage doubler, giving a brighter image in the eye. This circuit, although it has the advantage of simplicity and low cost, suffers from two faults which diminish its usefulness. Because it is not isblated from the power line, and because of an inefficient filter circuit, the magic eye image is fuzzy and very susceptible t o interferences such as that caused by a stirrer motor. In addition, flickering of the eye is sometimes caused by a ground to the electrodes through the operator’s body and a conducting film on the surface of the buret, when the buret is connected to the titration vessel, as in a Karl Fischer titration. These disadvantages may be eliminated by adding a power transformer, choke, and condenser, in accordance with the diagram shown in Figure 2. A clear, sharp image in the magic eye results from elimination of alternating current component through the improved filter circuit, the by-pass condenser between the grid and cathode of the magic eye tube, and the connection maimaining the heater of the triode a t the same direct current potential as the cathode. Although the low potential side of the circuit is grounded, no deflection of the eye is caused by grounding the buret through the operator’s body. This effect is eliminated by reduction of the above-ground potential of the
ANALYTICAL EDITION
November, 1946
II i
4
I
t
inherent in the apparatus and the technique involved in its use, a s opposed t o sampling errors and chemical or physical considerations based on the constitution of the sample. The standardization of Karl Fischer reagent against water was chosen as meeting these requirements.
TRUBORE TUBING
l
+!
'
A solution containing 11.66% by weight of water in methanol was made up in a dropping bottle having a ground-in medicine dropper. This solution, rather than pure water, was used as a standard, to reduce the effect of weighing errors upon precision. Approximately 30 ml. of nearly dry methanol were placed in a 250-ml. standard-taper Erlenmeyer flask, and connected to the titration assembly. The traceof water present was titrated with Karl Fischer reagent until one drop caused the magic eye to remain open for nbout 30 seconds. (Although this point may be determined with a stop Tvatch, the difference in time required for the eye to close after the next-to-last drop and the last drop is so great as t o be readily apparent.) Approximately 1 ml. of the standard water solution was introduced into the flask as rapidly as possible, and titrated as before. The weight of standard solution added was determined by difference.
z " c
t 3 Y z
-APPROX
I ID
m
3
-P
c
24/40
Y
a
. m
m
-
t
2 ID, I
WALL
The results of t,en such titrations are given in Table I. The precision is of the same order of magnitude as that obtainable wit,h moderate rare in any simple acid-base titration.
-
COPPER WIRE S I L V E R /1SOLDERED T O PLATINUM 2 ELECTRODES SPACED 120* FROM B U R E T T I P , AROUND C E N T R A L STIRRER
4
-24
727
G A . PLATINUM WIRE
LITERATURE CITED
(1) Almy, E. G., Griffin, W. C., and Wilcox, C.
S.,IND. ENG.CHEM.,h s a ~ED., . 12,392 (1940). (2) McKinney, C. D., Jr., and Hall, R. T., I h i d . , 15, 460 (1943). (3) Serfass, E. J., Ibid., 12,536 (1940). (4)
Wernimont, G., and Hopkinson, F. J., Ibid., 15, 272 (1943).
Figure 1.
( 5 ) Zerban, F. W., and Sattler, L., Ibid., 18, 138 (1946).
Karl Fischer Electrode-Stirrer-Buret Assembly
amplifier grid to approximately 30 volts, from about 140 in the case of the voltage doubler circuit. Possibility of greatly increased sensitivity can be introduced by means of the potentiometer in the triode cathode circuit. This is sometimes very desirable in conductometric titrations, where impurities cause a high initial condurtivity of the solution being titrated. 4 similar result could be obtained by replacing the potentiometer nith a fixed resistor, and varying the grid bias, as in Serfass' original circuit. However, the desirability of keeping the grid-to-ground potential as low as possible makes the arrangement shown more satisfactory. Operation of the titrimeter is identical 1%-iththe original model. Like the voltage doubler model, hon ever, it can be used only on an alternating current pon er source. PRECISION
In order to measure the precision obtainable with the apparatus, it was desirable t o confine possible deviations to those
Table
115 V 4c
I. Standardization of Karl Fischer Reagent
Weight of 11.66% Water Solutlon Grams 0,8613 0,9231 1.0123 0.8252 0.9138 0.9330 0.9561 0.7196 0.8518 0.8181
Volume of K.F. Reagent
Water J i g /ml.reagent
Deviation from Mean
% 0.0 0.2 0.2 0.1 0.1
0.0 0.2 0.1 0.1 0.3
Av.
3.265
0.1
GROUND
0
+ Figure 2.
Electrometric Titrimeter
