the solution would be isolated around one of the electrodes, could present advantageous electrometric comparison of the isolated solution with the main solution after an increment of titrant is added to the latter, thus eliminating the need for preparation of a filling solution for a reference electrode tube. Further experimentation in this direction might result in even more sensitivity at the end point and greater precision of the determination.
hydroxide crystallizes out (as observed by development of turbidity), the potential rises. As more titrant is added, the alcohol content rises, contributing more solubility to the hydroxide, whereupon the potential may decrease again. Since these events occur considerably beyond the end point, they do not interfere with the analysis. The drift of the palladium-hydrogen electrode during a titration is essentially negligible during the period of a titration. For example, in an aqueous buffer solution unprotected from the atmosphere, the electrode was observed to drift LO millivolts over a period of 18 hours. If the maximum duration of a titration is arbitrarily considered one hour, the drift should be less than one millivolt during the titration. In most cases of routine titrations, the duration should be considerably less than one hour, since only the data in the region of the end point need be collected carefully. With rapid titrations. it is believed that the titration cell may be an open beaker, unprotected from the atmosphere. Observation of recorded data which were collected carefully for single titrations over periods of approximately one hour showed potential-volume data equivalent to rapid titrations. The palladium-hydrogen electrode responds rapidly to changes of acidity in the titration cell as titrant is metered in. If the solution is stirred continuously, equilibrium of the system is attained within a few seconds following the addition of an increment of the titrant. The negligible resistance of the electrode pair contributes materially to this rapid response. After sustained use of the electrode, the palladium may become poisoned, but it is easily restored to its originally useful condition by immersing it briefly in hot nitric acid, rinsing, and gently heating to redness in a hydrogen flame. No attempt has been made to use two palladium-hydrogen electrodes in a direct differential titration system. It is considered that such a system, wherein a small portion of
ACKNOWLEDGMENT The suggestion of using the palladium-hydrogen electrode for the application described here came from Sigmund Schuldiner of the Naval Research Laboratory. The author is indebted to him for his suggestion, subsequent advice, and criticism of this manuscript. LITERATURE CITED J. 1.Stock and W. C. Purdy, Chem. Rev., 37, 1159 (1957).
G. A. Harlow, C. M. Noble, and G. E. A. Wyld, Anal. Chem., 28, 784 (1956). V. 2. Deal and G. E.A. Wyld, Anal. Chem., 27, 47 (1955). J. S. Fritz, "Acid-Base Titrations in Nonaqueous Solvents," G. Frederick Smith Chemical Co., Columbus, Ohio, 1952. C. W. Pifer. E. G. Wollish, and M. Schmall, Anal. Chem., 25, 310 (1953). J. A. Riddick, Anal. Chem., 28, 679 (1956). J. S. Fritz and L. W. Marple, Anal. Chem., 34, 921 (1962). R. H. Cundiff and P. C. Markunas, Anal. Chem., 28, 792 (1956). G. A. Harlow, C. M. Noble, and G. E. A. Wyld, Anal. Chem., 28, 787 (1956). J. S. Fritz and N. M. Lisicki, Anal. Chem., 23, 589 (1951). H. Ravner and S. Kaufman, ASLE Trans., in press. P. Nylen, 2.Nectrochem.. 43, (12), 915 (1937). P. Nylen, Z.Electrochem., 43, (12), 921 (1937). S. Schuldiner. G. W. Castellan, and J. P. Hoare, J. Chem. Phys., 28, 16 ( 1958). D. P. Smith, "Hydrogen in Metals," University of Chicago Press, Chicago, Iil., 1946. R. G. Bates, "Electrometric pH Determinations," John Wiley and Sons, New York, N.Y.. 1954.
RECEIVEDfor. review September 11, 1974. Accepted November 14,1974.
Evaluation of Unique Directly Digital Computer-Controlled and Hardware-Controlled Automatic Titrators T. W. Hunter, J. T. Sinnamon, and G. M. Hieftje Department of Chemistry, Indiana University, /3Ioomington, Ind. 4740 1
Several automatic titrator configurations, all of which incorporate a unique digital titrant delivery system, are compared and the performance of each is evaluated. The configurations differ mainly in the method used to control the delivery of titrant (either computer, hardware, or manual) and the end-point detection technique (either fixed-level or derivative). The best performance, 0.16% relative standard deviation, was obtained with a configuration that was computer-controlled and utilized a first or second derivative end-point method. The computer-controlled titrator was also superior in its versatility, in the choice of end-point detection, and its adaptability to non-routine analysis using operator interaction. The hardware-controlled titrator, utilizing a fixed-level end-point technique, produced relative standard deviations of only 0.4%; however, it was less expensive than the computerized system and was well suited to rou-
tine applications. In all titrator configurations, the precisionlimiting factor appeared to be the end-point detection process rather'than the titrant delivery system.
Titration has been a standard analytical procedure for many years. In applications where large numbers of routine samples are analyzed, however, manual titration becomes tedious and time consuming. To solve this problem, automatic titration procedures have been developed, and many systems are in wide use today. These systems range from simple operator-controlled titrant delivery devices to complex assemblies that handle titrant delivery, titration curve measurement, end-point detection, sample changing, and readout of final concentration values. Most of these automated systems control titrant delivery by means of a mechanical displacement as in the case of a motor driven buret. ANALYTICAL CHEMISTRY, VOL. 47, NO. 3, M A R C H 1975
497
/-
COUNTER! FREQUENCY
u
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For maximum versatility and ease of operation, it is desirable to operate an automatic titrator with a digital computer or controller. Of course, the interfacing of an automatic titrator to a computer or hardware system is greatly facilitated if the titrator is directly digital, and if it operates on a compatible time scale. One such device, which has been previously described ( I ) , enables the introduction of titrant solution in the form of small uniform droplets whose introduction can be controlled by a digital pulse train. In this way, the frequency of the pulse train is proportional to the titrant delivery rate, while the cumulative pulse count is a directly digital measure of titrant volume. The present paper describes and compares several titrator configurations using this digital titrant delivery system operated under digital hardware or computer control. In this study, both the hardware-controlled titrator and the computer-controlled titrator showed improved performance over the previously reported system ( I , 2 ) . However, computer control produced greater precision and enhanced versatility over hardware control.
TITRATOR DESIGN Figure 1 is a functional block diagram showing the general organization of the titrator systems used in this study. In various configurations, the functions illustrated in Figure 1 are performed by the operator, by the computer, or by a hardware device. The functions and their implementation are described below. The titrant delivery system used in this study was operationally identical to the device reported in an earlier paper ( I ) and thus will not be described in great detail here. During a titration, the titrant delivery system sends a submicroliter droplet of titrant into the titration vessel each time a pulse is sent to it from the control section. These droplets have been determined to be uniform in volume to within 0.1% relative standard deviation (1-3). Thus, the rate of titrant delivery is proportional to the pulse frequency while the total volume of titrant is proportional to the cumulative number of pulses supplied to the system. These digital values, respresenting titrant delivery rate or volume, are available on the counter/frequency meter shown in Figure 1. As pointed out earlier ( I ) , this titrant dispenser is particularly well suited to digital automation because of its digital output, totally electronic input, and lack of mechanical moving parts. The titration monitor in Figure 1 is used to measure the progress of the titration. This monitor can be any device which responds to the concentration of a chemical species involved in the titration. In this study, the monitor generally incorporated an electrometer, which was used to meaANALYTICAL CHEMISTRY, VOL. 47, N O .
-
Figure 2. Schematic titration curve to illustrate fixed level end point detection
Figure 1. Functional organization of the titrator
498
--
3F TITRHNT
3, MARCH 1975
At level 1 the rate of titrant addition is reduced: at level 2 the titration ISterrninated
sure the potential difference between a reference electrode and the appropriate indicator electrode; the electrodes in turn indicated the changing concentration of one of the titration-active species. Of course, the system is not limited to potentiometric titrations and could employ other kinds of monitoring systems as well (e.g., photometric). The control section of the instrument consists of two parts: a decision unit and a timing unit. The decision unit uses the information about the progress of the titration provided by the titration monitor to determine whether to add additional titrant. The rate of titrant delivery is also selected by the decision unit. In the timing unit, the information provided by the decision unit is translated into control pulses of the voltage and timing required by the titrant delivery system. Logic level pulses are simultaneously sent to the counter, which accumulates a number equal to the total number of droplets delivered to the titration vessel or. alternatively, indicates the titrant delivery rate. The control section of the titrator not only determines the titration rate but also detects the end point and the approach of equivalence. It is primarily in this section where the titrators discussed in this paper and in the previous paper ( I ) differ. Details concerning these differences and the end-point detection methods and control operations which were used will be discussed below. Three basic titrator configurations were employed in this work. In the manual system, the role of titration monitor and decision unit were performed by a human operator. In this mode, a visual end-point indicator was used, and the operator made decisions about the rate of titrant delivery. Thus, the end-point detection method was identical to that used in standard manual procedures, so that errors caused by the titrant delivery system were isolated and could be evaluated. In the hardware-controlled titrator, an electrometer with appropriate reference and indicator electrodes was used to monitor the titration. The output of this monitor, which is a voltage related to the concentration of the species of interest, was sent to the decision unit, which consisted of a digital logic circuit. This logic circuit then selected the rate of titrant addition and detected the end point by comparing the voltage signal from the titration monitor to appropriate reference levels. This “fixed-level” end-point detection technique is illustrated in Figure 2, which shows the variation in the monitor output as the titration progresses. As illustrated in Figure 2, two decision levels are established which represent, respectively, an output reflecting the approach of the equivalence point and, second, the equivalence point itself. When
OSCILLATOR or DI 0 I TAL CLOCK
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SCALER
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I
COUNTER
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INPUT -1 FROM TITRATION MONITOR
I
REFERENCE VOLTAGE (Level I)
I 142 I
-
I I
PULSE DRIVER
I I I
I I I I
REFERENCE VOLTAGE (Level 2)
I
-
I I
I I
DECISION UNIT
TIMING UNIT
Figure 3. Schematic logic circuit diagram of the hardware-controller C1 and C2 are comparators: G1-G6 are NAND gates; 11 and 12 are inverters; MI, M2 are monostables adjustable from 100 psec to 1 rnsec; T I is a 2N3439 transistor
the output from the titration monitor reaches level 1, the rate of titrant addition is reduced by the control system. This reduction in rate compensates for electrode response time and serves to minimize end-point overshoot due to slow titration reactions. Next, when the output of the titration monitor reaches level 2, the titration is terminated. The total volume of titrant required to reach this end point is contained in the counter. A schematic diagram of the hardware digital logic circuit that performs the function of timing unit and decision unit is shown in Figure 3; the circuit operates in the following manner. The input signal from the titration monitor is examined by analog comparators ( 4 ) C1 and C2 both of which are initially in a logical zero state. As long as the input voltage is less than the reference level of C1, the output of C1 stays at logical 0, so that gates G2 and G3 are open, and all the pulses from the oscillator reach the timing unit. When the voltage from the titration monitor reaches the reference level of C1, the comparator changes states, closing G2 and opening G1, so that every lOnth pulse reaches the timing unit. At this point, the rate of titration is reduced by a factor of lon. In the present system, a rate reduction by a factor of 10 or 100 is switch-selectable. When the output of the titration monitor reaches the reference level of comparator C2, the comparator output goes to a 1 state, and resets to zero the output of the R-S flipflop formedby gates G4 and G5. This closes G6 and halts the titration. To initiate a new titration, push button PB1 is pressed to set to a logical one the flip-flop formed by G4 and G5, and open gate G6. The timing unit consists of monostables M 1 and M2, and the pulse driver. M1 determines the delay time between the generation of a pulse by the decision unit and its appearance a t the titrator, while M2 determines the width of the pulse sent to the titrator. These parameters are adjusted by potentiometers for proper operation of the titrant delivery system, and need be set only once for each titrant during
OPERATOR
DISPLAY
INITIALIZATION
A
OkTA TREATMENT
TITRATION MONITOR
t
4
DECISION UNIT
TITRATION VESSEL
TIMING UNIT
'4
TITRANT DELIVERY SYSTEM
Figure 4. Organization of the software for the computer-controlled
titrator the initial setup. The output of the monostable network is sent to the pulse driver and to a digital counter. The pulse driver is used to convert the pulses from M2 to an amplitude (about 300 volts) suitable for controlling the titrant delivery system. In the computer-controlled titrator, the titration monitor was the same electrode-electrometer combination as described above. However, the decision unit, timing unit, and the counter were all embodied in computer software, a block diagram of which is shown in Figure 4. To start the program, the operator supplies the computer with a set of initialization parameters, including the initial and reduced rate of titrant addition, the type of data pretreatment to be performed, the location on mass storage where the data are to be stored, and the end-point detection method to be used. Once the program has been initialized, the titration is A N A L Y T I C A L CHEMISTRY, VOL. 47, NO. 3, M A R C H 1975
499
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i
,
I
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V O L U ~ E OF T I T I I H h T
OF T L T Y R C T
VOLURE OC T I T R R Y ?
-
~-
I
VOLL’lE OF T I T K P h T
YOLURE OF T I T R H N T
Figure 5. Effect of digital smoothing on a titration curve and its derivatives
Unsmoothed exDerimental tltration curve; (Bj First derivative of A: ( 0 Second derivative of A, (0 A after digital smoothing; (4 First derivative of D; (0Second derivative of D (A)
started upon operator command. After this command and at the time of each synchronization pulse from the titrant delivery system, the decision unit determines whether to add a droplet of titrant, based on the information from the titration monitor. If a droplet is to be added, this decision is passed on to the timing unit, which produces a pulse of the delay and length required by the titrant delivery system. Each time a droplet of titrant is added to the titration vessel, the voltage from the titration monitor is digitized and stored as a data point. After treatment, the data are sent to the decision unit and displayed on a cathode ray tube. The titration curve is thus displayed in real time during the titration, so that a t any time the operator can override the decision unit. This enables manual intervention in the case of non-routine titrations. Decisions concerning the rate of titration and the location of the end point may be based on the input data or on its first or second derivative. Thus, three software endpoint detection techniques are available for use with the computerized titrator. A fixed level technique, similar to the one used in the hardware-controlled titrator, employs reference levels which are numbers stored in the computer, thereby overcoming the need for highly stable voltage sources. The computerized titrator affords several advantages over the hardware system. Some of these advantages are especially important when derivative end-point detection is utilized. Derivative end-point detection eliminates the problem of drift in the reference levels and the titration monitor, and also obviates the need to know the absolute titration monitor voltage a t the equivalence point. These points were amplified in an earlier paper ( 1 ) which reported the use of a second derivative end-point detection system with a hardware controller. Although this earlier system exhibited relative freedom from drift, there were severe problems with noise. In the present study, the computer-controlled titrator was used with first and second derivative end-point detection, and was relatively immune to noise because of the computer’s ability to digitally smooth incoming data. The importance of this smoothing process is illustrated in Figure 5 . 500
Figures 5,A-C show an unfiltered experimental titration curve and its first and second derivatives, respectively. The large noise component in the second derivative ordinarily makes it difficult to detect the zero crossing with the desired precision. In comparison, the smoothed titration curve and its derivatives are shown in Figures 5,D-F. Smoothing and differentiation was performed by a least squares fit to a 5th order polynomial using the algorithm of Savitzky and Golay (5, 6). The original titration curve, it is seen, appears relatively unchanged, but considerable improvement is apparent in the derivatives. This digital smoothing and differentiation was performed on data already collected and stored by the computer, since data collection rates of up to 1200 pointshec did not allow sufficient time to perform the computations in real time. The smoothing and differentiation algorithm requires that data be collected beyond the end point. For this reason, data collection was continued 1-2% of the total titration volume beyond the end point of the titration. Collection of these additional data did not significantly alter the 1-4 minutes required to perform a typical titration. In these results, the derivative operations provide freedom from drift while digital smoothing increases the noise immunity of the system. Another advantage of the computer is its ability to store the titration data on magnetic tape, thereby allowing more sophisticated data processing techniques to be performed after the titration is complete. For example, Gran plots (7) and interactive data analysis (8) can be performed in addition to smoothing.
EXPERIMENTAL The titration monitor consisted of an electrometer (Model 6lOA Keithley Instruments, Cleveland, Ohio) with indicator and reference electrodes. pH measurements were performed with a combination glass microelectrode (S-30070-10 Sargent-Welch), and precipitation and redox titrations were performed with silver or platinum wire indicating electrodes and a calomel reference electrode (Model S-30080-27 Sargent-Welch). The hardware controller (see Figure 3) was breadboarded on a commercial unit (model EU-gOlA, Heath Co., Benton Harbor, Mich.). The computer used in these studies was a PDP-12/40 (Digital Equipment Corp., Maynard, Mass.).
ANALYTICAL CHEMISTRY, VOL. 47, NO. 3, MARCH 1975
Tabla I. Summary of Results of Titrations Performed with Several Chemical Systems a n d Several Titrator Configurations Mon it071control system
Titrant-titrate a
Concentration of timate,N
0.01 0.01 0.01 0.01 0.01 0.02 0.002 0.01 0.01 0.01 0.01 0.01
Computer HC1-NaOH Computer HC1-NaOH NaOH-HC1 Com pu t e r Computer NaOH-HC1 C o mput e r NaOH-HC1 Computer NaOH-HAC C oniput e r NaOH-KHP Computer HC1-AgNO, Computer K,Cr,0,-Fe2+ Computer Fe2+-K,Cr,0, Computer F e *+-Ce4+ Hardwar e NaOH-HC1 Hardware H C 1-N aOH Hardware NaOH-HAC M a m a1 K2Cr207'--FeZ* Manual NaOH-HC1 Manual NaOH-HAC a KHP = Potassium Hydrogen Phthalate. HAC= Acetic Acid.
0.01
0.01 0.01 0.01 0.01
All software was written in assembly language and assembled with a modified version of FPP Assembler (Digital Equipment Corporation).Analog datn were input to the computer via a 10-bit successive approximation .Analog to Digital Converter. A real time clock (KWl2-A, Digital Equipment Corporation) was used for timing and synchronization. All reagents were prepared by dilution of stock solutions made from reagent grade chemicals.
RESULTS A N D DISCUSSION The factor limiting the accuracy and precision of titrations performed with the titrator configurations in this study was the uncertainty in end-point detection. As mentioned before, the titrant delivery device is capable of 0.1% precision; however, even in the manual mode, only 0.3%relative standard deviation could be experimentally realized. Under manual operation, the end point was detected in the same manner as in conventional manual titrations (Le., a visual indicator) so that comparable precision should be expected. Titrations performed with this manual system include errors due to the titrant delivery system, manual pipetting, slow titrant reactions (not a factor in the present study, however), and errors in judgment on the part of the operator. Fixed-level end-point detection was less accurate than visual or derivative end-point detection. When the fixedlevel method was used, the hardware controlled titrator produced typical standard deviations of 0.4% while the computer-controlled system generated relative standard deviations of 0.3%. The better precisioin of the computercontrolled system is a result of the computer's ability to digitally integrate incoming data and its freedom from the drift associated with the analog reference voltages used by the hardware controller. Derivative end-point detection was used in the computer-controlled titrator, and produced much better precision than the hardware analog described earlier (I). With the computer, end-point detection based on either the first or second derivative produced precisions typically around 0.16%. This result, which is better than the values of 0.40.9% obtained with the hardware system ( 1 , 21, is in part due to the smoothing and data analysis possible with the computerized system. Although both first and second derivative techniques gave equivalent performance in our studies, the second derivative technique is preferred be-
End-point detection system
R e l s t d d e v , c/o
Fixed level 2nd Derivative Fixed level 1st Derivative 2nd Derivative 2nd Derivative 2nd Derivative Fixed level 2nd Derivative 2nd Derivative 2nd Derivative Fixed level Fixed level Fixed level Visual Indicator Visu a1 Indicator Visual Indicator
0.32 0.16 0.28 0.13 0.16 0.18 0.17
0.18 0.14 0.13 0.20 0.39 0.33 0.32 0.28 0.28 0.33
cause it is informationally and computationally easier to detect a zero crossing than a peak maximum. The hardware- and computer-controlled titrators were used with several titrant-titrate combinations. The results of these investigations, summarized in Table I, indicate that the titrators perform equally well with all titrant-titrate combinations studied. In particular, interferences caused by absorption of atmospheric gases by the titrant droplets were absent. Because the small droplets (0.15 microliter) produced by the titrant delivery system have a relatively large surface area-to-volume ratio, it was initially feared that problems might arise from absorption of atmospheric gases (COz or 0 2 ) when bases or reducing agents were employed as titrants. Fortunately, no absorption of gases was detectable during the passage of the droplets through the air, as determined by titration of samples of titrant taken from the titrant reservoir and from a vessel into which titrant droplets had been sent. The computerized titrator also performed well in nonroutine titrations. In this mode, a human operator observes the developing titration curve as data arrive, and interactively selects a suitable rate of titrant addition. The computer meanwhile collects and stores the data for later analysis. Upon completion of the titration, the computer can be used to differentiate the titration curve and determine the end point. In this way, titrations can be performed on unknown or uncharacterized samples with no preliminary setup procedure, and with maximum freedom from interference effects. The hardware-controlled titrator, on the other hand, requires initial setup and adjustment for each type of sampie being run. CONCLUSION The computerized titrator was superior to the hardware device in that it is much more versatile and possesses the capability to store data and perform numerical enhancement on incoming data. Interactive titrations and interactive data analysis can also be utilized to improve results on nonroutine samples. Furthermore, because the computerized system is largely embodied in software, it can be easily modified; for example, the system could readily be used as a p H stat (9) or modified to perform Gran plots (7) merely by minor alteration of several software routines. Work in this area is now under way in this laboratory.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 3, M A R C H 1975
501
On the other hand, the hardware controlled titrator, while less versatile and less accurate than the computerized titrator, is much simpler and less expensive. It is particularly well suited to routine applications where relative standard deviations of the order of 0.4% are adequate. The titrant delivery device could also be modified to produce smaller droplets of titrant ( 3 ) .This would enable titration of very small samples, such as single cells, provided that suitable micromonitors are available.
LITERATURE CITED (1) (2) (3) (4)
(5) (6) (7) (8) (9)
Hieftje and B. M. Mandarano, Anal. Chem., 44, 1616 (1972). D. G. Mitchell and K. M. Aldous, Analyst, 98, 580 (1973). G. M. Hieftje and H. V. Malrnstadt, Anal. Chem., 40, 1860 (1968). H. V. Malrnstadt, C. G. Enke and S. R. Crouch, "Electronic Measurements for Scientists,"W. A. Benjamin, New York, N.Y., 1974. A. Savitzky and M. J. E. Golay. Anal. Chem., 36, 1627 (1964). J. Steiner, Y. Terrnonia, and J. Deltour. Anal. Chem., 44, 1906 (1972). G. Gran, Analyst, 7 7 , 661 (1952). J. Frazer, personal communication, 1974. H. V. Malrnstadt and E. H. Piepmeier, Anal. Chem., 37, 34 (1 '365). G. M.
ACKNOWLEDGMENT
RECEIVEDfor review August 8, 1974. Accepted. November
The authors express their appreciation to Maurice Williams and Larry Sexton for their help in construction of some of the apparatus used in this study. Thanks are also due to B. M. Mandarano, who performed some of the first fixed-level titrations with the reported titrator.
27, 1974. Support of this work by grant GM17904-03 from the National Institutes of Health is gratefully acknowledged. Presented in part at the 25th Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 1974.
Indirect Polarographic Microdetermination of Fluorine in FluoroOrganic Compounds after Oxygen-Flask Combustion Y. A.
Gawargious, Amir Besada, and B. N. Faltaoos
Microanalytical Research Laboratory, National Research Centre, Dokki, Cairo, Egypt
Two indirect polarographic methods are described for the microdetermination of fluorine In fluoro-organlc compounds after oxygen-flask combustion. In one, the liberated F- is precipitated as PbCIF, In the presence of CI- and 70% ethanol, wlth excess Pb(N03)2 and is measured indirectly by recording the polarographic cathodic wave of the unconsumed Pb2+. In the other, the F-, in about 62% acetone medium, is Precipitated as CaF2 with excess calcium iodate and is measured indirectly by polarographically recording the cathodic reduclion wave of the free iodate released. Nine fluoro- and perfluoro-organic compounds were analyzed by the two methods. For each method, the error is about f0.4%.
Several methods exist for the determination of fluorine in organic compounds; the subject has already been reviewed by Macdonald ( I ) . After the sample is decomposed by one of various techniques (2, 3 ) , the F- can be determined gravimetrically (4-6), titrimetrically (7, 8 ) , potentiometrically (9-1 1) using fluoride ion-specific electrode, or spectrophotometrically (12-14). Titration of the F- with thorium nitrate (7), the procedure most commonly used, has limitations; the reaction is reported ( 1 5 ) to be nonstoichiometric over the F- range 1to 50 mg. As already known (15), many difficulties are associated with the methods available for the determination of fluorine in organic samples, particularly for the microdetermination of perfluoro-organic compounds. Moreover, no polarographic methods, neither direct nor indirect, exist for such a determination, primarily because F- does not exhibit polarographic characteristics (16). Although previous work in this laboratory has already established polarographic methods for the microdetermination of other halogens in organic compounds (17, 181, such methods for fluorine determination are still lacking. 502
*
In the present work, two new indirect polarographic methods were developed for the microdetermination of fluorine in fluoro-organic compou.nds after oxygen-flask combustion. One depends on precipitation of the released F- as lead chlorofluoride (PbClF) with Pb(N0& followed by polarographic recording of the cathodic wave of the excess Pb2+. The equations involved in the precipitation and redox reactions may be represented as follows: F-
+
Pb(NO,),
+
Cl-
-+
PbClF1
Pb2+ 1- 2e ---
+
2NO,-
Pb
(1) (2)
In the other method, F- is reacted with calcium iodate, Ca(IO&, and the cathodic reduction wave of the free 103liberated is measured polarographically. The precipitation and redox reactions involved may be expressed as follows: 2F-
+
IO,-
Ca',IO,),
+
6H'
+
-
6e
CaF,
-
I-
1 + 2103-
(3)
+
(4)
3H,O
EXPERIMENTAL Apparatus. An Orion-KTS 510 polarograph (Hungarian make) with accessories was used. The combustion was carried out in a 500-ml oxygen flae k. The electrolytic vessel was an ordinary Kalousek cell with a i:athode compartment that allowed sample solutions as small as 4 ml to be polarographed. The dropping mercury electrode (DME) had a drop time of 3 to 4 sec under an open head of 75 cm of mercxry. A saturated calomel electrode (SCE) was the anode. Reagents. A l l reagents were AR or MAR grade, and doubly distilled water wiis always used. Lead nitrate, about 0.01M solution, prepared by c'iissolving about 3.3123 grams of Pb(N03)z in doubly distilled water and made up to one liter with water. Potassium iodate, about 0.01M solution, prepared by dissolving about 2.1401 grams of K'J.03 in doubly distilled water and made up to one liter with water Calcium iodate was measured as a 10% suspension in water. Proced ures. The sampling, weighing, and combusting of the or-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 3, M A R C H 1975
