Anal. Chem. 1981, 53. 135-138
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I1 illustrates the dependence of the LEI signal for 100 ng/mL indium in sodium matrices on electrode separation. One millimeter diameter rods are used as electrodes with -800 V applied. At each electrode separation, the recoverable signal decreases to zero as the sodium matrix concentration is increased. ks the electrode separation is increased, 100% signal suppression due to the matrix occurs a t lower sodium concentrations. The sodium concentration range diminishes from approximately 40 pg/mL sodium a t a 9-mm electrode separation to approximately 20 pg/mL sodium at 10 mm. Thus, the sodium matrix concentration range was decreased by a factor of 2 by a 1-mm increase in cathode separation. Documentation of the need for precise sampling height adjustment is given in ref 2. The severity of these position effects are mitigated somewhat by operating a t higher applied voltages, but they still remain important for achieving maximum signal and precision. The position of the electrodes plays an important role in the LEI signal collection process. The use of this electrode positioner permits recovery of the maximum LEI signal with sufficient spatial resolution. The electrode position has been shown to be particularly important for low ionization potential matrices. The flexibility afforded by the electrode positioner is also necessary when different flames are used because of their different chemical and physical characteristics (2). Detailed plans for the electrode pwitioner may he obtained from the authors upon request.
I
i
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Figure 1. Schematic diagram of helectrode positionw: (A) vertical translation assembly. (A-I) 'I, in. X 20 in. threaded steel rod, (A-2) ' I sin. brass support rods, (A-3) vertical kandabbn wlumn. (A-4) k n M wheel, (A-5) nylon washer, (A-6) ball bearing pivot, (A-7) spacer, (A-8) vertical translation column base, (A-9) positioner base: (6) electrode holder assembly. (El) right- and lefthand thread 3/8 in. X 2 4 in. brass rod, (6-2) right- and left-hand thread electrode holder mounts, (6-3) nonconducting spacers. (8.4) brass electrode holders, ( 8 5 ) knurled wheel. ( 8 6 ) track for electrode holder mounts, ( 8 7 ) end blocks. ( 8 8 ) base plate for electrode holder assembly. (6.9)tension and alignment boltlnut for 8 1 : stipple, brass: crosshatch, nonwnductw: slanted lines, threads: the rest is aluminum stock
ACKNOWLEDGMENT The authors acknowledge Carl Wise (West Virginia University) for machining the prototype model, Terry Trask (UA) for suggestions for improvement of the prototype, and George Kirsch (UA) for helpful suggestions and machining of the fnal product.
indexing system for both horizontal and vertical translations would he useful for reproducing settings. If the burner head is used as the anode, the high-voltage leads which are terminated with banana plugs are plugged into holes in the brass electrode holders (B-4). Figure 1depicts flat electrode (plate) holders but other types of electrodes may be used with minimal modification. The voltage applied to the electrodes is isolated from the rest of the assembly with nonconducting spacers (B-3).The electrode holders are also isolated from each other so that one electrode mav be used as the cathode and the other the anode. The importance of the electrode separation is illustrated by the data in the tables. In these experiments, the electrodes in the holder are held at a negative voltage and the burner head is used as the anode. Table I gives the LEI signals for 100 ng/mL indium (303.9 nm) for 30-mm plates a t various plate separations. An acetylene/air flame is used with -500 V applied. The LEI signal is significantly diminished when the separation is increased 1mm (0.5 revolution of B-5). Table
LITERATURE CITED (1) Green. R. 8.: HavriL, 0.J.: Trask. T. 0. Awl. Spechosc. 1980, 34,
561-569. (2) Havrilla. G. J.; Green.
R. B. Awl. Chem. 1980. 52. 2376.
RECEIVED for review J d y 28,1980. Accepted October 14,1980. This research was supported by the National Science Foundation under Grant No. CHE79-18626. This paper was taken in part from the d h r t a t i o n written by G. J. Havrilla in partial fulfillment of degree requirements for a Doctor of Philosophy in Chemistry from West Virginia University, Morgantown,
wv.
Differential Thermometric Titration Apparatus
Basil H. Vassos' and Re& A. Roddguez Chemistry Deparfment. Universily of Puerto Rico, Rio Riedras, puerlo Reo 0093 1
Thermometric titration started with the pioneering work of Bell and Cowell (I) in 1914 and through the years has been 0003-2700/81/0353-0135$0i.00/0
developed into a competent analytical technique (2,3). In general, calorimetry is done with instruments of simple design 0 1980 American Chemical Society
7 I
!I II
PUMP
DIFFERENTIAL THERMONETER
SUPPLY TIMER
i n u u
I
MAGNETIC STIRRERS
Figure 1. General schematic of the system. The thermistors are marked by T, and A is the titration tip, which must be bent upward like a letter J to avoid loss of reagent during Mle periods. E designates two glass electrodes and C is a Joule effect calibrator (a resistor enclosed in an oil-filled glass tube). Note that pH measurements can be done simultaneously with the thermometric titration if desired.
which require difficult and slow methods of operation (4). Less common are the differential calorimeters that are characerized by their more complex construction but easy and rapid operation (5, 6 ) . In a simple calorimetric experiment one must correct for any heat effect additional to the reaction under study by separate experiments and subsequent calculations. The major factors involved are the heats of dilution, evaporation, and stirring, the heat loss, and the Joule effect. The application of differential calorimetry to thermometric titrations (7, 8) provides the possibility of eliminating these factors. However, the degree of succes in elimination of these factors varies with different experimental systems. The majority of the calorimetric systems are differential only as to their thermometry. The thermochemical part is usually nondifferential, so all the heats are measured together. The system we report is differential in both its chemical and its thermometric aspects, including the case where both calorimeters suffer strong heat effects produced by secondary reactions. Thus the system was useful in determining the heat of complex formation (a few kilocalories per mole) in the presence of heats of dilution and neutralization (about 50 kcal/mol) which result on the addition of NaOH solution of metal ions and protonated amino acids (9).
EXPERIMENTAL SECTION The operation consists of simultaneous addition of equal amounts of the same reagent to two matched Dewars filled with the same volume, of which one contains the sample of interest and the other contains a blank. The two temperature changes are subtracted by an electronic circuit of very high common mode rejection ratio (CMRR), in terms of electrical drifts and other types of noise. The system is shown in Figure 1. The pumps (piston burets, Metrohm Dosimat E-412) are actuated by a variable-repetition-rate timer of Phipps and Bird. The reagent is added in small increments every few seconds at a rate depending on the reaction involved. The stirring motors are adjusted to equal speed of about 2-3 rps. The temperature sensors are thermistors made by Victory Engineering, Model T-41A28 of 10 kQ at 25 “C. A large number of thermistors were purchased and the closest matching pair was selected for use in the instrument. This matching is critical for obtaining high CMRR. Ideally the thermistors should be matched, both in resistance at one temperature and in temperature coefficients. The resistance matching is essential for the electrical CMRR while the temperature coefficient matching is required
Figure 2. The electronic circuit. The two halves (a, upper part and b, lower part) are interconnected at point I. All the resistor values are in kQ, while the capacitor values are in microfarads. Resistors are of 1 % precision except those around amplifiers A and D, which must be of the closest matching and the smallest temperature coefficient available. The 8038 oscillator is not entirely satisfactory and a modular oscillator such as a Burr Brown 4023125 is preferable. The diodes are 1N4448 type. The variable capacitor of the bridge Is a small air dielectric unit.
by the thermal CMRR. Both were performed, and the best pair from some 40 thermistors was selected. The thermistors, T, are connected in a lo00 Hz active bridge circuit, Figure 2, around operational amplifier A with very high CMRR. The amplifier model used was MONO-OP-O5C from Precision Monolithics Inc., which has a CMRR of about 120 dB and a noise density of only about 10 nV/Hz1/2. The thermistors change resistance by approximately 4% / “ C , which corresponds to 4 nV/OC if the bridge is excited with 100 mV. The design goal of loT5OC requires therefore readings of 40 nV. The value of 100 mV was chosen after some experimentation to balance the effect of increasing in S/N upon increasing the excitation voltage and the concurrent increase in the self-heatingof the thermistors with higher voltages. Our value of 100 mV generates a self-heating effect of (0.1)2/10000 = 0.1 pW. The bridge itself is of a proven good CMRR (IO). The 500-!I balancing helipot is driven by a 401 drive microvernier (Raytheon). This gives, for a 10 turn potentiometer, a total of 400 turns and permits zeroing over about f1.25 “C with a resolution of a few microdegrees. The capacitive component is nulled by a small variable air capacitor, in parallel with TI.The procedure consists in connecting an oscilloscope at the points marked “test points” in the figure and alternately balancing for R and C. Since balancing can be done only for a single frequency at a time, noise of higher frequencies cannot be balanced out. This is, however, of no consequence in the actual measurement, We have tested that there is no measurable difference in amplitude between titration curves done starting on-balance and starting moderately off-balance. The output from amplifier A, even with careful shielding, contains a few millivolts of noise mostly at 60Hz but also some Johnson noise generated by the 10-kQthermistors. The bridge output is amplified by a variable factor in B and
ANALYTICAL CHEMISTRY, VOL. 53, NO. 1, JANUARY 1981
137
73
8" I'
---_
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-
C
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t
AT= 0
t
START
END
t
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N t O H ADDITION
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Figure 3. Differential titration of HCI with NaOH demonstrating the common mode rejection: (a)2 X lo3 M HCI vs. water in the reference M HCI in both Dewars. Actual traces of recorded Dewar; (b) 2 X runs.
P
START
EHO
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0 NiOWAWlTlOH
Flgure 4. Examples of differential titrations of HCI with NaOH: (a) medium sensitivity, (HCI] = 2 X IO3 M, 250 ml; (b) highest sensitivity, [HCI] = 2 X M, 250 mL. The uitimate sensitivity can be seen to be in the micromole region. Actual traces of recorded runs.
C (along with the much larger noise) and fed into the twin-T filter, D, which is the major noise-reducing element, since it attenuates strongly all frequencies outside a small 5-Hz band around 1000 Hz. Note that the twin-?' has to be carefully tuned to the oscillator frequency. In our case the actual frequency was 998 Hz. The 200-kQresistor serves to control the filter gain and band-pass. The filtered signal is then rectified (11) in a perfect rectifier E followed by a low-pass filter F and output stage G. The variable gain of B is chosen to give between 5 X lo4 O C and 2 O C full scale (100 mV out). RESULTS AND DISCUSSION In Figure 3a one can see the differential titration of HC1 (2 X M) with NaOH vs. HzO with NaOH. The heat of dilution of NaOH, the heat of stirring, and the heat loss to the surroundings are all eliminated. The only measured effect is the heat of neutralization (about 13 kcal/mol), which produces a differential rise of 2.5 X "C.
1
START
o
t
END
t
HEAT
Illustration of common mode rejection under hlgh sensitivity. Both thermistors were placed in the same Dewar. The curves show (a) the base line without heating, (b) heating at 0.10 cal/min-', and (c) heating at 1.00 callrnin-'. In this case the common mode rejection ratio is over 1500:l. Flgure 5.
The power of differential operation is apparent when one switches to two identical solutions, as shown in Figure 3b. In this case, both Dewars have the same acid at the same initial temperature and rise by the same 2.5 X "C. The common-mode rejection ratio is so good, that the output is very close to zero. Please note that all graphs are actual recorder traces. A study of the sensitivity limits is shown in Figure 4, where titrations at millimolar and micromolar levels are compared. Note that curve 4a is a different run from that of Figure 3a, but the two curves are identical, except for the speed of reagent addition. The curve in Figure 4b, made a t 2X M, represents the neutralization of 5 pmol of HC1. At such low concentration, one must keep in mind that the curve amplitude is not completely linear with the concentration and a calibration curve must be used. Inspection of Figure 4b shows that the ultimate sensitivity should approach 10 p o c . At moderate concentration the data are reproducible to better than 1%. For absolute measurements, however, the instrument must be thermally calibrated. A pair of matched resistances of about 2000 R was used to produce Joule heating. A good test of the common mode rejection of the thermometer can be done by placing the two thermistors in the same Dewar and introducing a given amount of heat into the Dewar by Joule heating. Figure 5 indicates the resulting absence of any detectable reading even at a scale of 1000 g"C/in. The lower curve is a background reading, curve b corresponds to a heating of about 0.1 cal/min while curve c is done while heating with 1 cal/min. Note that there is less than 20 p"C common mode reading when the actual temperature rise is about 0.03 "C, a common mode rejection ratio of a t least 1500:l. The cost of the electronics is below $200.
LITERATURE CITED Bell, J. M.;Cowell, C. T. J. Am. Chem. SOC. 1963, 35,49. Tinell, H. J. V.; Beever, A. E. I n "Thermometric Titrimetry". Chapman and Hall: London, 1988. Hansen, L. D.; Izatt, R. M.; Christensen, J. J. Anal. Cabrim. 1974, 3, 237. Wiihoit, R. C. I n "Topics In Chemical Instrumentation"; Ewing, G. W., Ed.; Chemical Education Publishing Co.: Easton, PA, 1971; pp 200-224. ReM, D. S. J . phys. E 1976, 9, 601. k i t e s . T.; Mites. L. J . Phys. Chem. 1868, 73,3801.
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 1, JANUARY 1981
(7) Tyson, B. C.; McCurdy, W. H.; Bricker, C. E. Anal. Ghem. 1961, 33, 1640. (8) Holmes, F.; Williaps, D. R. J . Chem. SOC. A 1961, 1256. (9) Mulr, M. M.; Rodriguez, R. A., unpubllshed results. ( I O ) Vassos, B. H.; Bonilla, A. Ghem. Insfrum. ( N . V . ) 1977, 8, 31.
(11) Vassos, B. H.; Ewing, Q. W. "Analog and Digital Electronics for Scientists", 2nd ed.;Wiiey: New York, 1980.
RECEIVED for review July 26,1979. Accepted August 4,1980.
