Effect of Diverse Ions. X t r a t e , sulfate, and chloride ions did not interfere, b u t other anions caused the absorbance of the cerium-2-methyl8-quinolinol complex in carbon tetrachloride to decrease. The limiting amounts of these interfering anions are shown in Table 111. The ammonium ion and the alkali and alkaline earth ions did not interfere. However, a trace of iron(II1) or iron (IV) ions did provide interference; therefore, it was necessary to remove these species completely if they were not masked before determining cerium. Thorium(1V) and the rare earth(II1’i ions interfere slightly, but when present a t a concentration less than 3 pg. per ml., they provide no interference. Many cations have a pale yellow or pale yellowgreen color in carbon tetrachloride, but when present a t a concentration less than five times that of cerium, they do not interfere. The cations in this category include: Pb+2, Ag+, Hg+2, Cd+2, Zn+2, C U + ~ Bi+3, , Al+3, Sn+4, Ga+3, Zr+4, Cr+3, lIn+z, CO+~, and U02+*. As described above, a trace of iron
Table IV. Determination of Cerium in the Presence of Iron When KCN Is Used as the Masking Agent
Ce(II1) Fe(II1) Ce(II1) taken, added, found pg./ml. fiug./ml. Mg./mi. 9.2 1.0 9.1 9.2
18.4 18.4 13.8 4.6 4.6
0.6 1.0 0.5 1.0
0.5 0.5
8.9 18.2 18.2 13.6 4.7 4.4
Error, pg./ml. - 0.~ .1
-0.3 -0.2 -0.2 -0.2 $0.1 -0.2
masked by KCN, addition of more than 2.5 mg. of KCN per ml. interfered with the absorbance of the cerium complex in carbon tetrachloride. Results are shown in Table IV. ACKNOWLEDGMENT
The authors thank the Minister of Education for the financial support given to this research. LITERATURE CITED
(1) Antoniates, H. N., Chemist-Analyst. 44, 34 (1955).
(111) or iron(1V) ions interfered, but when iron was present at less than 1 pg. per ml. in the initial aqueous solution, it could be masked by adding a suitable amount of KCN. After addition of 1.5 ml. of 10% malic acid solution to a sample solution of Ce(II1) containing iron a t this concentration, 1 ml. of 10% KCN solution was added a t pH 10. Then, 5 ml. of 0.1% 2-methyl-8-quinolinol solution was added. The extraction of the cerium complex was carried out in the normal manner. Although 1 fig. or less of iron per ml. could be
(2) Misumi, S., Taketatsu, T., Memoirs of the Faculty of Science, Kyushu Uni-
versity, Series C. Chem., Vol. 3, No. 2, 55 (1958). (3) Reinhardt, R. W., ANAL.CHEM., 26, 1820 (1954). ( 4 ) Sarma, B., J. Sci. Ind. Research ( I n d i a ) 1413, 538 (1953). (5) Wendlandt, W. W., J. Inorg. Nucl. Chem. 2, 133 (1957). (6) Wendlandt, W. W., Science 124, 682 (1956). (7) Westwood, W., Mayer, A., Analyst 73, 275 (1948).
RECEIVED for review December 26, 1961. Accepted July 23, 1962.
Automatic Digital Recording Thermob a la nce WESLEY W. WENDLANDT AFOSR Center for Molecular Research, Department of Chemistry, Texas Technological College, Lubbock, Tex. An automatic recording thermobalance is described which records the voltage variations that are proportional to the weight change of a sample, as it is heated, in an analog (X-V recorder) as well as digital (digital recorder) form. The system consists of a conventional recording thermobalance in which the voltage outputs from the balance accessory and the furnace thermocouple, after amplification, are converied to a series of pulses. These pulses are counted on an electronic counter and then printed on a digital recorder. Provision is made for printing the furnace temperature voltage, then 5 seconds later the sample weight voltage, a t intervals of 1 minute. The instrument is useful in providing input data for digital computer calculations of thermogravimetric and kinetic quantities of interesi.
A
a number of automatic recording thermobalances have been described ( I , 3, 4, 6), none of these instruments is capable of recording the weight-loss and temperature in digital as well as analog form (strip chart or X-Yrecorders). Such an instrument is LTHOCGH
1726
ANALYTICAL CHEMISTRY
desirable for use in thermogravimetric analysis and for reaction kinetics studies of liquid and solid inorganic and organic compounds. The data for the kinetics studies may be obtained by a thermogravimetric (2) or an isothermal method. For both methods, data obtained in a digital form are more convenient than those in a n analog form, and they lend themselves more accurately and readily to various computer programming methods (5). Under certain experimental conditions, the data recorded by digital techniques are more accurate than their analog counterparts. The instrument described consists of a conventional automatic recording balance and furnace attachment, with provision for recording the weight change us. furnace temperature on an X-Y recorder. The voltage outputs from the recording balance accessory and the furnace thermocouple are amplified and connected to a synchronous motor-driven, cam-actuated switch. The resulting voltage output from the switch is converted to a series of pulses; these pulses are counted on an electronic counter and then printed on a digital recorder. Thus, the weight and temperature of the sample are recorded in
an analog form on the X-Yrecorder and in a digital form on the digital recorder. EXPERIMENTAL
Recording Balance. The general arrangement of the components is given in Figure 1.
The automatic recording thermobalance has been described (8). The furnace and power supply (7) were altered slightly, in that the two-holed thermocouple insulator tube was replaced by a four-holed tube of similar dimensions. D.C. Amplifiers. The thermocouple (Chromel-Alumel) amplifier circuit is illustrated schematically in Figure 2 . The amplifier employed was the Acrostat Model 164 low-level d.c. magnetic preamplifier (Acromag, Inc., Southfield, Mich.). As the amplifier is linear only over a 0- to 20mv. range, the voltage output from the thermocouple was fed into the amplifier through a 4000-ohm voltage divider. If only lower temperatures are to be recorded (20 mv. equals 485’ (3.)) the voltage divider may be eliminated. However, the upper limit of the thermobalance was about 800’ C. (33.30 mv.), so that the voltage divider was necessary. The voltage
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Figure 1. Schematic representation of digital recording thermobalance
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Figure 2.
output from the amplifier was adjusted to 0 to 1 volt by the 5000-ohm and 50ohm 10-turn precision Micropots and then filtered by a simple LC filter.
It was found more convenient to use two furnace thermocouples, one for the
X-Y recorder X axis and the other for the d.c. amplifier. This particular amplifier produces an output signal containing a 120-c.p.s. square wave frequency which blocks the chopperstabilized recorder servo-amplifier. Perhaps another filter could be designed for the input of the amplifier which would reduce the square wave component and thus enable the use of' only a single thermocouple in the furnace. The thermobalance accessory d.c. amplifier was a Keithley Model 151 null detector (Keithley Instruments, Inc., Cleveland, Ohio). The circuit arrangement is given in Figure 2. Output from the null detector, which wy9s operated in the 0- to 30-mv. range, was connected to a 30,000-ohm Micropot. The potentiometer served as a voltage divider, so t h a t the output voltage varied from 0 to 0.1 volt. Motor-Driven Switch. T h e switching circuit, illustrated in Figure 3, consisted of three niicroswitches actuated by circular cams connected to a common motor-driven shaft. A
Schematic illustration of amplifier circuits
All resistances in ohms and all capacitors in pf. ohm potentiometers are Barg 1 0-turn Micropots
1-r.p.m. synchronous motor (R. W. Cramer Co., Inc., Centerbrook, Conn.) was used to rotate the cams, although other motor speeds could be employed, depending upon the printing rate desired. 3Iicroswitch MI connected the thermobalance amplifier, while Jf2 connected the thermocouple amplifier to the voltage-to-frequency converter. Microswitch 1113 was connected to a 6.3-volt a x . relay which reset the electronic counter. The switching sequence is illustrated in Figure 4. Thus, sample weight and temperature were printed within 5 seconds of each other a t 1-minute intervals. Other Components. The voltage output from the motor-driven switch was converted to a series of pulses by a Dymec Model 2210 voltage-to-frequency converter (Dymec, Inc Palo Alto, Calif.). The converter was operated on the 1-volt range, which produces 10,000 C.P.S. for a full scale input. Output from the converter was counted on a Hewlett-Packard Model 521 A/D counter and then printed on a Hewlett-Packard Model 560AR digital recorder. Both instruments were obtained from the HewlettPackard Co., Palo Alto, Calif. The
50-, 5000-, and 30,000-
Model 2210 voltage-to-frequency converter can also be obtained with a 0to 0.1-volt range. This would simplify construction of the instrument, in that the d.c. amplifiers could be eliminated. Operation. After a suitable "warniup" time, usually 30 minutes, from 50 to 70 mg. of sample are accurately ( 1 0 . 0 5 mg.) weighed out into the platinum sample holder. T h e sample holder is attached to the balance and the furnace raised into position around it. Adjustments on the X and Y axes are made on the X-Y recorder and the entire system is allowed to come to equilibrium for about 10 minutes. With SW1 in the off position, bypass switch SW3 is closed and the weight of the sample observed on the counter, using a 1-second gate time. If the weight reading on the counter is not the same as that of the sample, the null detector zero-adjust is rotated until they are identical. Then SW3 is opened and bypass s\yitch SW2 is closed, so that the initial temperature of the furnace can be observed. Switch S W 2 is then opened and the run begun by closing SWr, putting the counter display time on infinite, closing the print switch on the printer. and starting the furnace heating cycle. Normally, a furnace heating rate of 5' C. per minute was employed.
x x 1 1
RESULTS AND DISCUSSION
The accuracy of the instrument is illustrated by the thermogram of
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Schematic illustration of switching circuit
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Switching sequence for motor-driven switch VOL. 34, NO. 13, DECEMBER 1962
1727
CuS04,5H20, as given in Figure 5 . The digital recorder weight losses n-ere in good agreement with those obtained from the X - Y recorder curve. For a 25-mg. weight change, the accuracy arid precision of the digital recorder were about *0.57,. The temperature was recorded to an accuracy of about =t1% and a precision of about +0.25%. Because of the 5-second difference in printing times, the weight value will not coincide nith the exact value a t the printed temperature. For a furnace heating rate of 5" C. per minute, the temperature difference bet\\-een the weight values is about 0.4' C. For most cases, this can be neglected or, if desired. a correction factor can be applied. For the size of the samples employed and the weight range of the X axis on the X-Y recorder, the digital recorder is probably more accurate than the former. I n the former type of recorder, the weight axis can be read only t o h 0 . 2 mg., which for a 25-mg. weight change would involve an accuracy to about +170.
ACKNOWLEDGMENT
The helpful assistance of Karner Iiendall is gratefully acknodedged.
56.5 MG.
LITERATURE CITED 16.0CURVE 16.1 DIGITAL
100
200 TEMP.
Figure 5.
t
20.6CURVE 20.5 DIGITAL
300
400
'C.
Thermogram of CuSO4.-
5H20 5' C. per minute furnace heating rate and air furnace atmosphere X.Y. Recorder values Digital. Digital recorder values
This instrument has been in daily use in this laboratory for the past six months.
(1) DuvaI, C., "Inorganic Thermogravi-
metric Analmis." Elsevier, Houston,
Tex., 1953.
( 2 ) Freeman, E. S.,Carroll, B J Phys Chem. 62, 394 (1958). (3) Gordon, S., Campbell, C., L Z ~ ~ ~ . CHEM.32, 271R (1960). ( 4 ) Fen-in, S. Z., J . Chem. E d x . 39, -4975 (1962). ( 5 ) Soulen, J. R.,ANAL.CHERf 34, 136
(1962). (6) Wendlandt, W. W.,Zbzd., 30, 56 (1958). ( 7 ) Wendlandt, W. W.,J . Chem. Educ. 38,571 (1961). (8) Wendlandt, W. W., George, T. D., Horton, G. R., J . Znorg. Nucl. Chem. 17,273 (1961).
RECEIVEDfor review June 25, 1962. Accepted September 24, 1962. Work supported by the Directorate of Chemical Sciences, Sir Force Office of Scientific Research, through Contract AF-AFOSR 23-63.
Precise Automatic Spectrophotometric Analysis in the Low Parts per Billion Range R. D. BRITT, Jr. Savannah River laboratory, E. 1. du Pant de Nemours &
b The sensitivity and precision of spectrophotometric methods were improved by means of automatic instrumentation. Quantitative analyses were automated for the determination in water of chloride, nitrate, nitrite, ferrous, ferric, and ammonium ions with a sensitivity of a few parts per billion. The precision of these methods was 2 to 3% at a concentration of 10 p.p.b. The automated methods are rapid and applicable to continuous in-line analysis as well as batch sample analysis.
P
and accurate ionic analysis of the heavy water (DzO) that is used as a moderator in the Savannah River Plant reactors is required to determine the rate of corrosion of reactor components, the fate of nitric acid used in p H control, and the mechanism of formation of radioactive impurities. The ions of interest are present in the concentration range of 1 to 100 p.p.b. Routine manual methods of analysis are time-consuming, lack the required sensitivity and precision, and are susceptible to trace contamination durHECISE
1728
ANALYTICAL CHEMISTRY
Co., Aiken, S. C.
ing sampling or analysis. The concentration of samples by ion exchange or evaporation into the range where reliable results could be obtained by conventional methods was unsuited for this work. Consideration was given to extending the range of spectrographic, coulometric, potentiometric, isotope dilution, and spectrophotometric techniques. Of these, only the spectrophotometric method appeared to be applicable for the determination of both cations and anions at these low levels. Another reason for this choice mas the availability of a commercial automatic colorimeter, the AutoAnalyzer, manufactured by the Technicon Instruments Corp., Chauncey, N.Y. The AutoAnalyzer is widely used for clinical and biochemical analyses (1, 3, 9 ) as well as for the determination of silica in boiler water ( I ) , nitrate in fertilizer ( I ) , and phosphate in detergents (6). KO information has appeared in the literature on the use of this instrument for analysis in the range of parts per billion, but the design and method of operation are such that a high degree of reproducibility and sensitivity appeared possible.
INSTRUMENT DESCRiPTlON
The basic Autohnalyzer unit has been discussed (1, 9) and only a description of the components pertinent to parts per billion analysis and the principles of operation is given here. The instrument used consists of a proportioning pump, mixing coils, a filtration unit, a heating bath, and a colorimeter equipped with a 50-mm. cell. The colorimeter output is connected to a range expander and a stripchart recorder. The filtration unit is an AutoAnalyzer dialyzer with a 0.4micron pore size membrane filter for aqueous solutions (Schleicher and Schuell Type A, coarse), substituted for the dialysis membrane. Filtration is accomplished by pumping the sample stream a t a rate of 7.8 cc. per minute. along with air segmentation a t a rate of 0.6 cc. per minute, into the upper plate of the filtration unit. The outlet from the upper plate is connected t o a tube in the pump which has a flow rate of 0.8 cc. per minute. The difference in flow rates forces the sample into the bottom plate a t a rate of 7.6 cc. per minute. The air segmentations serve to wipe the membrane and prevent plugging. These filters mere used for several months mithout plugging.
