Table I. Results of Isotope Dilution Analyses NBS 673, S B S 835, Run p.p.m. Cu p.p.m. Sb 1 21.0 113 2 21 8 121
Av. NBS value
21.0 i 1 . 3 20
121 f 6 100
politan Vickers MS7 mass spectrograph. This method involves a polyisotopic element of known isotopic composition. Tin was used in all experiments because it has ten isotopes covering a wide range of abundances. After the tin spectrum has been recorded on the plate, a curve is constructed which relates the absorbance of the tin lines and their known isotopic abundances. From this curve the ahundances of the isotopes in question may be found. Sample Dissolution. The materials investigated were YBS 673 (analyzed for copper) and NBS 835 (analyzed for antimony). Weighed samples were degreased and cleaned and the appropriate spike was added before dissolution. Then the samples were dissolved and alternately taken to dryness and redissolved several times to ensure mixing of the isotopes.
Spiking before and after dissolution produced no change in the result, indicating no loss of spike during dissolution. The copper in M3S 673 was concentrated by electrolysis and subsequent redissolution. Because the antimony content of NBS 835 was five times greater than the copper in NBS 673, it was not subjected to a concentration step. The solution was then coated onto pure electrodes of gold or aluminum by pipetting under a heat lamp. The metals had been previously analyzed to ensure that no interferences were present. Next, the coated electrodes were placed in the mass spectrogiaph and a series of ten exposures were taken. The gold electrodes were replaced with tin and three exposures of tin were placed on the same plate for calibration of the emulsion. Blank Determination. The blank for a n analysis is carried out in the normal manner. A small addition of spike is made to the acids and this is carried through the entire analytical procedure. Measurement of the spike ratio after this leads to a n evaluation of the blank. In practical analyses some difficulty is encountered in measuring ratios greater than 20. In these cases we have measured the stronger line on a shorter exposure and used the difference in exposures to calculate the ratio. In the two analyses cited above, blanks were of the order of 1%.
RESULTS A N D DISCUSSION
Once a calibration curve has been established, the isotopic composition of the mixture of spike and sample may be determined. Then, from the known amount of spike and the known isotopic ratios of spike and sample, the concentration of the element is found from the following relation: Wt. of c u = (Wt. of spike) K,(R,,
- RJ
Ki(Rm - E,) where R, = isotopic ratio of mixture; R, = isotopic ratio of sample; R,, = isotopic ratio of spike. The results of these analyses are shown in Table I. The standard deviation of 5y0 is caused by errors in weighing, micropipetting, standardization of the spike, and densitometry. III all cases, the amount of spike added was adjusted so the ratio was close to one, the most favorable for the measurement of isotope ratios from a photographic plate. LITERATURE CITED
(1) Mattauch, J., Ewald, 31, 487 (1943).
H., N U ~ U T .
(2) Webster, R. K., “Isotope Dilution Analysis, Advances in Mass Spectrometry,” p. 103, Pergamon Press, Sew kork, 1959.
Arbitrary-Set, Proportional Temperature Controller T. R. Mueller, Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tenn.
the development of a highD sensitivity conductometric titrator URING
( 2 ) the need arose for control of the cell solution to within 0.01” C. Although the temperature can be so regulated with a good contact thermometer and “on-off” control of the heater, noise which makes difficult the location of end points is produced on the recorded conductivity curve. With proportional control, excursions about the control tenqierature are less abrupt and the recording is smooth. Commercially available proportional temperature controllers having the required sensitivity and using a d.c. bridge supply are costly. Cheaper units described in the literature have neither the sensitivity nor the long-term drift charactpristics required. An arbitrary-set, proportional controller was designed and built as an accessory to the conductometric titrator. The controller may be used with other types of instruments having plus and minus 300volts d.c. regulated power supplies. A transformer (2 amperes at 6.3 volts, 172
ANALYTICAL CHEMISTRY
ax., center-tapped) is then required to supply power to the filaments and chopper of the USA-3 amplifier (pins 7 and 8). The circuit diagram of the controller is given in Figure 1. Components required for construction are described in the parts list. The controller consists of a bridge, amplifier, and control circuit. A lowresistance thermistor (chosen to minimize noise pickup) in one arm of the d.c.-powered bridge serves as the temperature-sensing element. Imbalance of the bridge is sensed by the high-gain, chopper-stabilized, VSA-3 amplifier (G. A. Philbrick Researches, Inc.) The output of the amplifier drives a silicon-controlled rectifier (SCR) circuit. The SCR circuit is identical to one described in the General Electric Co. SCR Manual ( I ) , except that the manually operated variable resistor is replaced by a transistor. The firing angle of the SCR is controlled by changes in the resistance of the collector-eniitter path of the
transistor, and power is delivered to the heater during a larger fraction of each cycle as more heat is required. The controller, less isolation transformer and power supply, can be mounted in an 8-inch high X 10-inch wide X 7-inch deep cabinet. Bridge resistors are mounted in a 51/4-inch long X 3-inch wide X 21/2-inch high aluminum box inside the main cabinet. The box is lined with a ‘,14-inch thickness of polyurethane foam for heat insulation. The SCR is attached to a small heat sink isolated from the chassis. Front panel controls are “amplifer bias,” “heater,” “bridge (on-off),” and “coarse” and “fine” potentiometers for setting the control temperature. Heatconducting tube shields are placed around all vacuum tubes. The cabinet should be vented at top and bottom. Ports of about 1-sq. inch total area are required for proper convective cooling. The mercury cells may be stabilized by placing them in series and connecting a 100-ohm, 1-watt resistor across them for two days. The battery voltage i q
,
I
,
t
I
Y
:
2
i I
002.C
Y
5
I
I _ -
Figure 1.
Arbitrary-set proportional temperature controller
Pin numbers refer to contacts on connector socket (supplied with amplifier, Amphenol Blue Ribbon No: 2 6 - 4 2 0 0 - 1 6s) Do, De, Cr, and Rlo mounted on amplifier connector socket Filament !eodr ore 2-conductor No. 22 shielded cable; shield common with chassis
not critical when the controller is operated at 25' C., but the requirements for stability increase as the control temperature deviates from 25' C. The regulating capabilities of the controller are well within the design goal of 0.01' C. Electrical instability corresponds to less than =k0.002' C. per day and is independent of line variations between 100 and 125 volts. Cell temperature regulation depends on cell design, stirring efficiency, and proper adjustment of heating and cooling rates. I n the conductometric work, a 400-ml. beaker served as the cell, and about 300 ml. of solution were tempered. Stirring was accomplished magnetically. The cell was heated with a 100-watt projection lamp and cooled with a 13-mm. 0.d. cold finger operated at a temperature about 2' C. below the control temperature. Short term regulation and stability were checked by using the unit to regulate the temperature of 300 ml. of 0.3JI KC1-1 x 10-4.11 HC1 solution during the conductometric titration of the HC1 with a solution of NaOH. Noise associated with "on-off" control was absent from the recorded curve. Cell temperature measurements made with a second thermistor indicate regulation within ~t0.002' C. during titration. Temperature regulation deduced from changes in conductance of KC1 solutions is within +=0.002" C. also. Ambient temperatures during these tests ranged from 21' to 27' C. The cell temperature was 25' C. Although
the cell was shielded from direct drafts, it was not totally enclosed. Temperature regulation obtained in the cell described is shown in Figure 2. Solution temperature changes were monitored with a second 32A1 thermistor and recorded with the conductometric titrator used in the high-sensitivity mode. For maximum stability, a break-in period of about 100 hours should be allowed. Warm-up normally requires about 2 to 3 hours. The USA-3 amplifier is biased with the thermistor immersed in the solution to be regulated and with SI in the "bridge off" position. The amplifier output should be set with the bias control to about 0 volt d.c. when the solution is at the desired temperature. I n practice, it is convenient to observe the heating lamp filament (Sz "on") and to bias the amplifier so that the filament just 0'-1 ows. PROPORTIONAL TEMPERATURE CONTROLLER
,
0 I
Q l : '25338
13,: 251671B
4;:
SCR, 2N1774A T : l5OW isolation transformer, 117 VAC, 60 c.p.5. Chicago Stancor P 6371 3: mercurv batterv holders ("D" cell size) 1: female, chassis mounting, NEJIA socket SI: SPST toggle switch, mounted front panel SO: DPST toggle switch, mounted front _panel 1: heat sink, type UP2-T03B, inter. electronics research corp., or equivalent 2: fuse holders 2: 2AMP, slow-blow fuses 1: amphenol connector, Type ILIS-310214s-5s 1 : amphenol connector, Type lIS-310814s-5P 1: amphenol connector, Type MS-310814s-5s 1: amphenol connector, Type JlS-310214s-5P 2: test jacks, mounted front panel 1: amphenol connector, Type AIS-3102R14s-9s 1: amphenol connector, Type lIS-310814s-9P 1: pilot light assembly, for operation at 117 T'AC &conductor: $0. 20 cable, length as required 2-conductor: No. 22 shielded cable, length as required Chassis base Cabinet (see text) Miscellaneous hardware and wire 3: lIercury batteries, 3Iallory Type RRI-42R Tube shields: 3, IE & RC Type 6020H; 1, Type 6025H
PARTS LIST R,, R2: 2000 R, wire wound, low temp. coeff. (Daven, Type 1252) R,: 5000 R, 10-turn helipot, mounted front panel Rd: 10 fl, 3-turn helipot, mounted front panel R,. Rp: 300K n, 5'7,, carbon resistors, 2W R6: 10K R, wire wound potentiometer, mounted front panel R7, R,: 10 KR, dual potentiometers, wire wound R,o: 4-meg. a, deposited rarbon film, 1W RI1: 9100 R, carbon 1/2W, 10%
LITERATURE CITED
(1) Gutzwiller,
F. W., ed., "SiliconControlled Rectifier Manual," 2nd ed., 116, General Elertric Co., Auburn, Y., 1961. (2) Mueller, T. R., Stelzner, R . W., Fisher, D. J., Jones, H C , ANAL.CHEW37, 13 (1965).
Ri.
Research sponsored by the U. S. Atomic Energy Commission under contract with the Union Carbide Corp. VOL. 37, N O . 1 , JANUARY 1965
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