D. W. COLVIN and
W. C.
PROPST
Savannah River Laboratory, E. 1. do Ponf de Nemoors and Co., Aiken, S.
+
A conductometric titrator was developed that automatically records the conductance of a sample solution as a function of the volume of titrant added. The instrument has a linear range up to 2.4 X l o 4 pmho in five overlapping steps, and conductance values as high as 3 X 1 Os pmho can b e recorded with a slight nonlinearity; however, the error is negligible and the distortion introduced into the titration curve does not limit the application of the instrument. Versatility of operation is assured since titration curves representing small changes in conductance (50 pmhos for the 350pmho range, 2500 prnhos for the 2.6 X 1 O4pmho range) can b e adjusted to occupy the full width of the recorder chart. The instrument has been utilized to titrate the free acid in a 25-pl. aliquot of a synthetic dissolver solution with a precision of 0.6%. The solution contained 1.8M AI(NO&, 0.4M HN03, 0.01M UOz(N03)2, and 0.01M Hg(N03)~. The precision for a manual titration of the same solution was 2.4%. TITRIMETRY is a little used but powerful analytical tool which should be considered in developing analytical procedures involving titrimetric end points. Frequently this procedure will permit the titration of solutions where colorimetric or potentiometric end points are inapplicable. The technique is capable of precision and accuracy comparable with other volumetric analytical procedures, and is sensitive to small changes in hydrogen ion concentration a t high dilution volumes. Automatic titration equipment would make the conductometric method more attractive particularly from the standpoint of a routine laboratory procedure. Such equipment would eliminate the labor involved in a manual titration and thus shorten the time required for an analysis. To be versatile, the instrument should be capable of recording a wide range of titrations with comparable sensitivity-i.e., the full width of the chart paper should be utilized for the titration regardless of the range of conductivity values encountered. Although the availability of commercial equipment for the automatic recording of conductivity has been reported (6),no seference to a suitable recording titrator was found in the literature. ONDUCTOMETRIC
C.
A literature search for a practical means of measuring conductivity on a continuous basis revealed numerous references to conductivity measurements and equipment. Although bridge circuits were described (1, 4, 9) which did not require balancing during titration, the majority of the instruments utilized bridge circuits that require both a capacitance and resistance balance for an accurate null. Fischer and Fisher (3) described an audiofrequency titrator which can be adapted for automatic recording; however, the circuit does not possess the versatility required for titrations involving small conductivity changes in solutions of high conductivity. DISCUSSION
The circuit developed is similar to the one described by McGuire (6). The alternating current voltage applied to the conductivity cell is derived from a low impedance source such that the cell voltage remains essentially constant over the conductivity range to be measured during the titration. It follows then that a t constant cell voltage, the current flowing through the cell is a measure of the reciprocal impedance or conductivity of the cell system. Linearity of response was ensured by designing the range (current measuring) circuit SO that its impedance was small as compared to the impedance of the conductivity cell. The prototype instrument utilized the measuring and recording circuits of an existiizg in-line conductivity monitor (a). The in-line instrument has been proved in plant service and exhibited excellent stability and reliability; therefore, the existing design was modified for use as a recording conductometric titrator. The titrator was intended for use in the control laboratory where repetitive analyses on samples from different sources were required. Since samples from different sources exhibit differences in the span of conductivity values experienced during a titration, whereas samples from the same source would all titrate within a predictable range, a means of adjusting the range of the instrument t o suit the sample was indicated. This feature would permit each titration to occupy the full width of the recorder and hence result in maximum sensitivity. Although the
above procedure would be satisfactory for the control laboratory where the range values could be specified in advance] to establish the range settings for maximum sensitivity samples of unknown origin would require a preliminary titration. This was not considered objectionable in view of the proposed use of the instrument. MEASURING CIRCUIT
The circuit that was designed to fulfill the above requirements is shown in Figure 1. The oscillator is a conventional stabilized type that is often used in audio generators and consists of the 6AU6 and 6CL6 tubes and associated circuitry. The sinusoidal voltage from the oscillator circuit is amplified by the 6AQ5 power amplifier and applied to the series combination of conductivity cell and primary winding of the monitor transformer through a low impedance output transformer. A 2.7-ohm resistor connected across the secondary of the oscillator output transformer ensures that a low impedance is presented by the oscillator output transformer t o the conductivity cell circuit so that the amplitude of the applied voltage remains essentially constant a t all times. The monitor transformer in serks with the conductivity cell functions as a low impedance input device for the current monitoring circuit. Current flow in the primary induces a voltage in the secondary, which is proportional t o the conductivity of the cell. The current flow in the primary winding, and hence the sensitivity, is controlled by shunting the primary winding with fixed resistors of the appropriate value. The voltage in the secondary winding of the current measuring transformer is rectified, filtered] and applied to one recorder input through a precision potentiometer, Helipot, which serves as a voltage attenuator. Frequently it is desirable to introduce a fixed bias or zero suppression voltage in series with one of the recorder inputs when the change in conductivity that occurs during a titration is small compared to the over-all conductivity of the solution. This feature is incorporated into the circuit by rectifying approximately lOyo of the voltage appearing a t the terminals of the oscillator output transformer and utilizing this voltage t o nullify a portion of the voltage from the current monitoring circuit. The rectified zero suppression voltage is applied to the second recorder input through a HeEpot that functions as a voltage attenuator. I n operation, the zero suppression voltage can be ad-
4SOV
450V
--
005pf
'OBTAIN 2 5 v P-P I K C BY ADJ. R I
RESISTORS 1/2 W UNLESS OTHERWISE NOTED H-POT
Figure 1 .
justed to eliminate that portion of the titration curve below a fixed value, and, by adjusting the sensitivity and zero controls, a titration curve can be made to utilize the full width of the recorder chart. Provision has also been incorporated to substitute either one of two variable resistors for the conductivity cell so that the impedance of the cell can be determined. The determination of the impedance of the conductivity cell will be described under Tests in Titrations. BURET DRIVE MECHANISM
Since the trend in the case of radioactive samples is toward microtitration techniquea utilizing small sample sizes, a I-ml. Gilmont buret driven by a synchronous motor was selected for the addition of titrant. This also simplified the problem of calibrating the recorder chart in terms of the volume of titrant added since both the buret and recorder chart would be driven by synchronous motors. A circuit diagram of the buret drive mechanism is shown in Figure 2. .Power for the buret drive mechamsm IS obtained from the instrument chassis as shown in Figure 1. TWOclutches were
- HELIPOT
O.lo/o LINEARITY
Measuring circuit diagram
necessary so that two drive speeds could be obtained. The buret can be emptied in either 1 or 10 minutes and refilled in 1 minute. The titration should require approximately 5 minutes to reach the end point. This condition can be met by selecting the appropriate strength titrant and sample aliquot. Limit switches are provided to prevent excessive travel of the buret plunger in either direction. RECORDER
The recorder selected for use with the instrument was a Varian Model G-11A with a A-2 input chassis. The range was adjusted to 0 to 25 mv. with a chart speed of 1 inch per minute. EVALUATION
Experiments were designed to determine the sensitivity, linearity, and range of the instrument, and to determine the precision of the results for volumetric titrations performed with the instrument. SENSITIVITY, LINEARITY, A N D RANG€
The five positions of the coarse range switch provide a linear response up to
2.6 X lo4pmhos in five overlapping steps as shown in Table I. Conductance values as high as 3 X 106 pmhos can be measured with a slight nonlinearity. Thus the instrument will record accurate titration curves for solutions covering a wide range of conductance values. Sensitivity, linearity, and range studies. were made by substituting a precision resistance decade for the conductivity cell. The instrument was calibrated for each position of the coarse range switch by initially setting the zero suppression potentiometer to read zero. The range potentiometer was then adjusted to its maximum value and was not changed throughout the experiment. The resistance decade was then adjusted to give a full-scale reading on the recorder. The rePistance decade reading was noted, and the zero suppression potentiometer was adjusted to return the recorder pen to zero. The resistance decade was again adjusted until the recorder again gave a full-scale reading. This process was repeated until the maximum conductivity reading for a particular range was obtained. Therefore the distance between successive points on the calibration curves represented a full-scale deflection on the recorder. A larger VOL 32, NO. 13, DECEMBER 1960
1859
4 Figure 2. Circuit diagram of buret drive mechanism
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f7
*
WOrnlr Cluloh NO. 5F-100
Figure 3.
conductance change for a full-scale deflection could have been obtained for any position of the coarse range switch by adjusting the range potentiometer to a lower value. The results are given in Table I. Although the instrument exhibits a slight nonlinearity a t the extreme ends of all ranges, the distortion introduced is negligible and does not seriously limit the application of the instrument. TESTS IN TITRATIONS
The precision and reproducibility of the results for titrations performed by the instrument were demonstrated in a series of titrations involving strong acid and strong base. To test the ability of the instrument t o perform titrations where the over-all change in conductivity during the titration was small compared to the average conductance, titrations involving nitric acid in the presence of hydrolyzable ions were made. The results for the analysis of 10 synthetic samples indicate a coefficient of variation of 0.6% for 0.01 mmole of nitric acid when titrated with 0.lM sodium hydroxide in the presence of 0.05 mmole of hydrolyzable ions. Additional titrations involving neutralization and precipitation were made te demonstrate the versatility of the instrument. All titrations were made in the titration vessel shown in Figure 3. Magnetic stirring was employed and the jacketed titration vessel was positioned so that an air gap of 2 to 3 mm. existect between the top of the stirrer case and the bottom of the titration vessel. This arrangement kept heat from the stirrer niotor from raising the temperature of the solution during the titration. Water a t constant temperature (25" C.) was circuIated through the jacket by a Labline Model 3052, constant temperature bath. Standardized solutions of acid and base were employed for the precision studies. Solutions for the exploratory titrations were prepared from reagent grade chemicals and were not standardized. An exploratory titration was normally required with each new sample to adjust the instrument for maximum sensitivity. This adjustment was accomplished by setting the zero suppression potentiometer to zero and the range e
ANALYTICAL CHEMISTRY
potentiometer to full scale before the start of the titration. The coarse range switch was then adjusted to bring the recorder pen on scale, and the titration was initiated. Upon completion of the titration, the standard switch was adjusted to either the high or low standard as required, and the appropriate standard potentiometer adjusted until the recorder pen position coincided first with the high and then the low conductivity points on the recorder chart. The readings corresponding to these positions then represented the low and high impedance values encountered during the titration. Once the over-all impedance change to be encountered during a titration had been determined, the instrument was adjusted so that the titration curve would occupy the full width of the re-
Table 1.
Switch Position
a
Diagram of titration vessel
corder chart. This maximum sensitivity was achieved by adjusting the standard potentiometer to correspond to the highest impedance value and then advancing the zero suppression potentiometer until the recorder pen read approximately 10% of full scale. The lowest impedance was then set on the standard potentiometer, and the range potentiometer was adjusted until the recorder read approximately 90% of full scale. In some cases it was necessary to increase the sensitivity by changing the coarse range witch t o a higher sensitivity position. Since there was some interaction of controls, the adjustments were repeated until the high and low impedance values corresponded to the 10 and 90% readings, respectively.
Instrument Range, Sensitivity, and Linearity
Linear Range, fimhos
Maximum Sensitivitya Mv./finiho X 'lo2
5000-26 000 1000-7000 500-3500 200-1300 0-380
0.097 0.462 0.867 2.27 5.06
biaximum Conductance, Mmho X 30.3 11.4 2,314 0,909 0.481
Range Helipot set at maximum
90 80
70
I-
60
4 u"
50
E
40
a:
30
20 10
V.lmc
Figure 4.
d
TNtrmt
Titration curves of strong acid-strong base
Marker “blips” were produced on the recorder chart t o indicate the start of the titration and the total volume of titrant added. The blips were produced by the standard switch. At the start of a titration the standard switch was turned from standard to titrate, and simultaneously the buret drive switch was thrown from stop t o start. Upon completion of the titration, the procedure was reversed to produce a blip at a definite volume. The volume of titrant consumed was calculated from the ratios of the distances from the starting blip to the end point, and from the starting blip to the total volume marker blip. The results are given in Table 11, and some typical titration curves are reproduced in Figure 4. A synthetic dissolver solution [ simulates the composition of a fuel element of uranium-aluminum alloy dissolved in HSOs with Hg(NO3)z added as a catalyst] containing 1.8M Al(K03)3, 0.4J-f Hr\’os, o.o11%f UO2(N03)2, and 0.01M Hg(P\TO& was analyzed according to the method of PepkomTitz ( 7 ) . The coefficient of variation for this titration was 0.6%, which was four times better than the 2.6y0qbtained previously where manual equipment rTas employed (8). The positive bias of 4% was attributed to the presence of mercury in the sample. The coefficient of variation of 0.3% obtained for the titration of strong acid with strong base was approximately equal to that obtained previously in this laboratory for a potentiometric procedure in which !Vas used comniercially available automatic equipment. Exploratory titrations of nitric acid with ammonium hydroxide, acetic acid with sodium hydroxide, acetic acid with ammonium hydroxide, uranyl nitrate with potassium ferrocyanide, uranyl nitrate with sodium hydroxide, and barium chloride with sodium sulfate all gave excellent results with titration
Table II.
Series
No. of
Samples
Results of Tests in Titrations
NaOH Titrant, N 0.9964 0,1016 0.0099 0,1016
Acid, bImole Coefficient Found, of Added av. Variation, % 0,4946 0.4950 0.3 0.04946 0.04951 0.3 0.4 0.00514 0.00512 0,0107 0.0111 0.6
Sample, N 0.9892 HNOa 2 9 0.9892 HNOa 10 0,1029 HGl 3 4 10 synthetic dissolver so1n.a a Composition: 1.8X iil(NO3)3, 0.4144 “03, O,OliJf UOs(N08)n, and 0.01M Hg(N0a)z.
No. 1
10
curves, which exhibited the same general shape as those obtained with a manually operated bridge equipment. CONCLUSION
The results of the performance studies indicate that the instrument can be utilized to titrate a wide variety of samples quickly and accurately and with speed comparable to other automatic titrators. Although the instrument has proved adequate for all problems encountered to date. the linearity in the case of solutions of high conductance may be improved by decreasing the values of the resistors that shunt the current monitoring transformer, and adding a linear amplifier between the transformer and rectifier circuit. Hovever, in view of the simplicity and reliability of the present design, this improvement does not appear worthwhile. The zero suppression circuit provides adequate sensitivity by permitting the titration of solutions where the total change in conductivity was small as compared to the over-all conductivity of the solution. It is felt that this feature will encourage the utilization of conductometric titration techniques for micro analytical work. The precision studies indicated that the “blip” method for determining the
total volume of titrant added was reproducible to within 0.1%. Thus, on a practical basis, essentially the total error lies in the determination of the end point, which must be determined by conventional graphical methods of construction. LITERATURE CITED
(1) Anderson, L. J., Revelle, R. R., ANAL.
CHEX.19,264 (1947). (2 Colvin, D. IT., E. I. du Pont de i‘emours & Co., AEC Research and Development Rept. DP-361, ,4pril 1 953.8. .. _
(3) Fischer, R. B. Fisher, D. J., ANAL. CHEM.24, 1458 1952). (4)Garman, R. IKD.ENG. CHEW, AKAL.ED.8, 146’[1936). ( 5 ) Lingane, J. J., “Electroanalytical Chemistry,” 2nd ed., p. 193, Interscience, New Tork, 1958. (6) AIcGuire, ]IT,S.,J . Chent. Educ. 17, 381-2 (1940). ( 7 ) Pepkonritz, L. P., Sabol,, R. W., Dutina, D., Knolls Atomic Power Laboratory, KAPL-757, July 1952, classified rept. (8) Propst, R. C., E. I. du Pont de iiemours & Co., AEC Research and Developnient Rept. DP-76,July 1955. (9) Treadwell, W. D., Helv. Chim. Acta 8, 89 { 1925). RECEIVEDfor review April 7, 1960. Accepted August 17, 1960. 11th Conference on Bnalytical Chemistry and Applied Spectroscopy, February 1960. Work supported by the U. s. Atomic Energy Comniission under Contract AT(07-2)-1.
Improved Alpha-Cowntin NAOMI A. HALLDEN and JOHN H. HARLEY Health and Safety laboratory, United Stafes Atomic Energy Commission, 376 Hudson Sf., New York 14,
b A scintillation technique for alpha counting with a zinc sulfide phosphor is described, which exhibits higher efficiency and lower background than She standard techniques. The zinc sulfide i s coated on a Mylar base and disks of the phosphor are applied directly to the sample. Counter efficiencies of about 47% are attained and the background is iess than 1 count per hour.
T
proportional flon- counter and the scintillation counter are the instruments commonly used for measuring alpha activity. Both are stable and can be operated with high efficiency and low background, so that the choice between them is largely a matter of personal preference. The proportional counter, however, is less suited to counting nonconducting samples unless a window is used, and thus the scintillaHE
N. Y.
tion counter has been the choice in the Health and Safety Laboratory. The counters, designed and built by our Instrumentation Division, include both manual and automatic units. Both have used a zinc sulfide phosphor coating on a Vycor light pipe which is cemented in turn t o the face of a multiplier phototube. Air dust samples and chemical precipitates on filter papers tend to dust VOb. 32, NO. 13, DECEMBER 1960
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