Automatic Coulometric Titrator for the Karl Fischer Determination of Water Application of the Leeds & Northrup Alternating Current-Operated pH Indicator to the Control of Titrations M. T. KELLEY, R. W. STELZNER, W. R. LAING, and D. J. FISHER Analytical Chemistry Division, Oak Ridge National laboratory, Oak Ridge, Tenn.
bA constant-current coulometric method was developed b y Meyer and Boyd for the determination of water. Coulometric instrumentation was built to perform these titrations autornatically. An automatic cutoff attachment has been designed for the Leeds & Northrup pH indicator so that it may b e used to control this titration automatically. The modified Leeds & Northrup instrument can b e used to sense and control other pH, millivolt, or dead-stop titrations
M
PERFORMANCE
(3) has recently described several novel applications of p H meters in analytical chemistry. Another useful application is described in this paper. A constant-current coulometric method developed by AIeyer and Boyd for the determination of water (8) depends upon the coulometric regeneration of Karl Fischer reagent ivhich occurs when iodine is electrolytically generated in a solution containing depleted reagent. The end point is reached when an excess of iodine is generated and is detected by the depolarization of platinum indicator electrodes. These electrodes are polarized by a 10-pa. current obtained by plugging the long probe into the Leeds & Northrup p H indicator. In coulometric titrations, the number of gram equivalent weights of generated titrant required to reach the end point is calculated from the number of coulombs used to generate that titrant. With this titrator, a known constant current is used, and the length of time required for this current to generate that amount of the reagent is shown on a timing clock. The calculation of the amount of water titrated coulometrically is based upon Faraday’s law and upon the assumption of 100% current efficiency. The validity of the latter assumption is regularly checked by standard solutions of water in methanol. To prepare the standards, known amounts of water 220
are added to methanol, and correction is made for a blank on methanol alone. The relationship that 10.71 coulombs of generating current is equivalent to 1 mg. of water is used for water titrations (8). This titrator has been used for the determination of water in organic liquids such as lJ2-diarninopropane (lJ2-propanediamine), Dowanol-8, glycols, long-chain secondary amines, and benzene dried over sodium potassium alloy, and also to determine the water of hydration in solid salts.
ULLER
ANALYTICAL CHEMISTRY
The useful range of the coulometric titrator for the determination of water is from 5 seconds a t 50 ma. to 100 seconds a t 300 ma., which corresponds to 0.02 and 3 mg. of water, respectively. The relative standard deviation of this method for the determination of water in IJ2-diaminopropane is about 5%. Typical results are shown in Table I for the determination of water in methanol standards. Under ideal conditions, with little variation in the amount of compensating current needed, the relatire standard deviation is about 1% for titrations of several milligrams of water. DISCUSSION
Automatic Cutoff Circuit. A Leeds & Northrup No. 7664 alternating current-operated p H indicator assembly provides, by use of the long probe, the polarizing current to the end point-detecting electrodes. By means of a simple cutoff circuit (shown in Figure 1) located within the indicator case, the assembly is used to on-off con-
Table 1.
Precision Data, Titration of W e t Methanol Standards
Generating Current, Ala. 50 100 300 KO.of standards titrated.
8,Seconds (Corrected for Blank)
17 18 18 (1
b
trol the generating current automatically. The assembly is switched to the 700- to 0-mv. range and is connected to read the potential developed across the polarized platinum wire electrodes. The added circuit consists of a 12Ax7 triode having a relay as its plate load. When the signal to its grid from pin 3 of the 5963 drops to -2 volts, this relay is actuated. The setting of the “cutoff adjust” potentiometer added to the front panel of the pH indicator determines the reading in millivolts of the pH indicator a t which the relay is actuated. The zero set adjustment is repeated after the cutoff adjust setting is made so that the pH indicator remains calibrated in millivolts. The actuated relay simultaneously turns off a timing clock and the generating current, whose magnitude has been preset. By use of the cutoff adjust control, the relay can be set to actuate a t any selected potential between 320 and 170 mv. By manipulation of the resistances in the network attached to the cutoff adjust potentiometer, this range may be shifted as desired. The combined mixing and reaction times are sufficient to demand end point anticipation, as seen from the fact that the potential developed across the indicator electrodes will drift donaward after cutoff to a degree dependent principally upon the magnitude of the generating current. To obtain electronic end point anticipation, the automatic cutoff is set to occur a t a potential numerically greater than the true end point potential. The required amount of anticipation is determined experimentally so that the titrator automatically cuts off and drifts to the end point. From complete titration curves, the end point potential was selected to be between 180 and 100 mv. I n the
Relative standard deviation.
6.8 52.6 88.7
L S )5 ~ 9 2 3 5
1.1
- Water, Mg.
Taken
Found
0 0309
0.0317 0 492 2 48
0.499 2.49
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Figure 1.
5 ~ 1 - 3~ o ~ 3 , z ’~t s Ij , 5 1 ~ - , . A ~ U N I “ ~ J ~rSh i i i A - J R E CASE A Y 3 a 3 L T TO TOP O F L A V 2 *: IPIhER C - A 5 S 8 S
Automatic coulometric titrator for Karl Fischer determination of water
presence of water, several hundred millivolts are developed across the polarized electrodes. Generating Current Supply. The generating current is obtained from six. 6-volt batteries connected in series: Ray-0-Vac No. A6, or the equivalent. A six-position “coarse adjust” switch and a “fine adjust” rheostat are used to control manually the magnitude of the generating current t o a value constant t o ilyo between the limits of 50 and 300 ma. The approximate value of this current is indicated on the “generating current” panel meter. The exact value can be measured by connecting a Pointerlite (Rubicon Co., Catalog No. 2731) millivolt potentiometer, or the equivalent, across the I-ohm precision resistor. $Then the “standby-titrate” switch is in the “titrate” position, the generating current is turned off or on by the relay controlled by the automatic cutoff circuit. Khenever the generating current is on, the yellow panel light is illuminated. The use of batteries to supply high generating currents is more practical than the use of a line-powered, vacuum tube, regulated constant-current supply and has proved entirely satisfactory, although a constant-current supply using transistors might be desirable. The instrument has operated for several years with very infrequent maintenance, other than replacing batteries. Compensating Current Supply. A compensating current is needed in the coulometric titration of water because esterification and other side reactions occur, electrolyte diffuses from the
cathode compartment into the sample compartment of the cell, and \yater vapor leaks into the cell from the atmosphere ($). These iodine-consuming side reactions are minimized if reagents aged for a meek or more are used.
Instrumental Details. A detailed operating procedure has been written for the routine use of this titrator (1). The circuit diagram is shown in Figure 1.
From 4 to 34 ma. of compensating current is supplied to the generating electrodes. This current bypasses the generating current panel meter and the 1-ohm resistor and is shown on the “compensating current” panel meter. A Type 6AG7 constant-current pentode supplies the compensating current. Khenever this current is on, a red panel light is illuminated. After the “compensating current on-off” switch is thrown on, its magnitude is manually adjusted by the “compensating current adjust’, potentiometer to a value such that the cell electrolyte remains a t end point conditions while the “standbytitrate” switch is in the “standby” position. If, during standby, the p H indicator meter remains a t the end point potential, the compensating current is adjusted properly. In this manner, the effects that necessitate the compensating current are balanced out. Cell. A suitable cell of the internally generating type for use with this titrator has been described by Meyer and Boyd (8). Platinum gauze generating electrodes are used and the cathode compartment is isolated by a fritted-glass barrier. One terminal of the cell is grounded t o minimize the danger of electrical shock. The direct current resistance of the electrolysis cells ranges up to about 1000 ohms. The anode compartment contains a magnetic stirring bar.
CONCLUSION
By the addition of the automatic cutoff circuit shown in Figure 1, the Leeds & Northrup pH indicator may be generally used as the sensing and control component of various automatic titrators, an example of which is described. The variable used as the indicator of titration progress could be either p H or millivolts, including dead-stop measurements. ACKNOWLEDGMENT
The data shown in Table I are the work of Bryce Philpot and precision statistics were calculated by Bobby Ginocchio. LITERATURE CITED
(1) Fisher, D. J., “Automatic Coulometric Titrator (300 ma.),” Oak Ridge National
Laboratory hIaster Analytical Manual, Method Nos. 1 003023 and 9 003023 (10-24-56), TID-7015(Section l),Office of Technical Services, U.S. Department of Commerce, Washington 25, D. C. (2) hleyer, A. S., Jr., Boyd, C. hf., ANAL. CHEX 31, 215 (1959). (3) hluller, R. H., Zbid., 29, No. 9, 778 (1957).
RECEIVED for review October 9, 1957. Accepted October 14, 1958. VOL. 31, NO. 2, FEBRUARY 1959
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