AIDS FOR THE ANALYST The instrument is very compact (8 X 8 X 8 inch sloping front cabinet, Figure l ) , and the cost of operation is negligible.
Simplifled Operating Dead-Stop Magic-Eye End-Point Indicator C. A. McCauley and W. J. Gresham, Monsanto Chemical Co., Monsanto, 111.
APPARATC S
T
HE use of an instrumental dead-stop end-point indicator for Karl Fischer determination of moisture is well known. Xumerous literature references describe the construction and application of this type of instrument (1-3). For application in an industrial control laboratory, where various products must be analyzed in a number of solvent systems, several objections are encountered with most instruments:
The circuit (Figure 2) was adapted from one described by Kieselbach ( 1 ) . Refinements include the substitution of a twinindicator “magic-eye” tube for a single indicator, a resistor step switch in place of a continuously variable control, and changes in resistor values to provide greater brightness of the magic-eye tube. These changes also appear to have minimized the occasional need for the critical matching of the 6SL7-GT duotriode (I).
Operation is too complex for routine control work, where the analysis of different types of materials frequently makes it necessary to change the adjustment of the instrument. Commercially available instruments are rather expensive for wide laboratory use. Many models are alternating-direct current-pols ered with the ever-present shock hazard in case of insulation or part failure.
0-
”i. A
L
...
7“
7-11 Figure 2. 1‘1. 1’2. T’3. v4.
T. L
Figure 1.
I
Optical view of instrument cabinet
With the increasing shortage of technically trained analysts, it is becoming more and more desirable to tailor analytical techniques to suit the available personnel, minimizing as many variables as possible. To answer this demand, an instrument described in the literature ( 1 ) has been adapted to combine the following desirable features. All variable controls are eliminated from the operating panel. A twin-indicator tuning-eye tube is employed to widen the effective viewing angle and sufficient brightness is obtained for accurate observation under almost all lighting conditions, including daylight or bright fluorescent light without shading the tube. No critical matching of tubes or other parts has been found necessary on 14 instruments so far constructed. Accommodations are provided for the selection of numerous solvent systems without readjustment, so that a variety of materials can easily be analyzed with the same instrument. Exceptional precision in detecting the same equivalence point repeatedly is possible with the circuit employed and adequate sensitivity is provided for all normal analytical requirements. The instrument is effectively stabilized against line voltage fluctuations up to at least &15%. Shock hazard is almost entirely removed, as this type of instrument features a transformer power supply. The complete instrument can easily be constructed from standard radio parts costing less than $25.
CH. C1. c2. c3. R1. R2. R3. R4.
6SL7 GF5-GT 6AF6-G 5Y3-6T Transformer Stancor 0010 12 H g Choke Triad C-BX 0.01-mfd. ceramir 8-mfd. 450-volt electrolvtic 30-mfd. 450-volt electrolytic 250,000-ohm 4703-ohm 1-meg.
2.2-meg.
Circuit diagram R.5. ~. ~
1-mro.
RO. R7. R8. R9. R10. R11.
3.3-0hiIl 1-watt 20.000-o~im10-watt 0000-ohm 10-matt 2 6 o h m mire-wound 0030-ohm 10-watt A . B. C . Adiust for solvent system and titration cell used Chas-iq, Bud Radio Co. No. C-B-38 Cahinet. Bud Radio Co. S o . C-1584 Switch. C R L No. 2503 Magic-eye bracket, Amphenol No. 58-hIE.4-8
All parts are mounted on a 7 X 6 X 2 inch chassis, except for the solvent selector switch with its resistor, R11, the magic-eye tube mounting clamp, and the binding post terminals for the electrode leads, all of which are mounted on the front panel. If interference from stray electrical fields makes it desirable to employ shielded leads, the binding posts may be replaced with a coaxial connector. Alternatively, adequate shielding can be secured more economically by the use of conventional microphone connectors, such as the Amphenol 75-PCIM chassis unit and 75-MCIF cable connector. The instrument is adjusted initially as follows: 1. After a 5-minute warm-up period, with the leads disconnected from the electrodes, turn the adjusting control, R9, to the stop nearest the junction of R2 and R8. Turn the solvent selector sir-itch to “Sone.” 2 . Adjust control R3 so that the eye just closes. 3. Connect the leads to the electrodes (mounted in the titration vessel) and make a temporary parallel connection from the binding posts to a potentiometer or vacuum tube voltmeter. 4. With methanol in the titration vessel adjust R 1 to read approximately 350 mv. on the potentiometer or voltmeter. 5 . Titrate with Karl Fischer reagent to a sudden large change in e.m.f. 6. Adjust R9 so the e) e just opens a t this point. The instrument is now ready for use. S o further adjustment should be required unless the 6SL7 tube is replaced.
Resistors R l l A , B , C, etc., are selected to compensate for differences in cell resistance, at the end point, for various solvent
1847
ANALYTICAL CHEMISTRY
1848
systems as compared to the cell resistance for methanol alone. As many solvent systems may be provided for as are desired, by the addition of suitable resistors to the selector switch. Preliminary values can be obtained easily by measuring endpoint cell resistance through the electrodes with au ohmmeter. Values for R11 are equal to the differencebetween thecellresistance for the solvent used and that for methanol. I n preparing the various solvent systems for measuring comparative resistance, the end points may be established by measuring the largest break with a potentiometer or voltmeter as was done x-ith methanol in steps 3,4, and 5. In most cases, standard radio-type 0.6watt resistors singly or in combination can be employed t o obtain the required values in ohms.
The length of the capillary inlet tubing, after bending, was extended to about 20 mm. and the inlet orifice was not constricted, but left the full 0.5-mm. diameter of the bore of the capillary tubing. The outlet orifice w m constricted, but to only about half the diameter of the bore (Figure 1). This permitted m atomization rate of about 2.5 ml. per minute with an air pressure of 10 pounds per square inch-several times that of the regular Beckman atomioer, which has very constricted inlet and outlet orifices. In the Perkin-Elmer flame photometer this atomizer is held in place with a rubber stopper, and the liquid samples are supplied to it from a 5-ml. beaker that is held under it by a spring steel attachment fastened to the photometer hy the right front screws (Figure 2).
RepresentativeVaIues of R11 for a Particular Cell Assembly Oil,”%
Methanol Methanol-benrene
0
BSO
Methanol-toluene Metiianol-soctic acid
10.50
200
Electrodes used were No. 16 gage platinum wire sealed in glass Interelectrodo spacing was not found to be critical. LITERATURE CITED
(1) Kieselbach, R.,ANAL.Cw~nr..21,1578 (1949). (2) McXinney, C. D., Jr., and Hall, R. L.,IND.END. CHEM.. ANAL ED.,15,460 (1943). (3) Serfass, E. J.,Ihid., 12,536 (1940).
Modifled Beckman-Type Atomizer in the Perkin-Elmer Flame Photometer
Figure 2
Rolond T. Mueller Deportment of Subtropical Horfkulhre, Univerrity of California, Lor Angele., Calif.
N RECENT years, flame photometry has become a n important
I technique in analytical chemistry, but the equipment has
not been perfected in every detail t o achieve the best results with the least inaccuracy snd inconvenience, particularly in regard to the atomizers. The Perkin-Elmer flame photometer has two types of atomizer: a metal atomizer containing a needle valve for regulating the rate of atomization, and B glass atomizer without a valve. Both have a “funnel” on top for introducing the sample and a tube on the bottom through which the compressed air enters. Neither atomiaer has been entirely satisfactory in this laboratory, because the glass one requires a large amount of sample to obtain an accurate reading; the metal one is subject t o frequent sir leakage and clogging; and with either one it is necessary t o wait until the SI\MPLE renainder of the sample in the Figure 1 funnel has been atomised before introducing a new sample, A Beckman-type atomizer, with slight modifications, was tried in the Perkin-Elmer flame photometer and found to be very convenient. This type of atomizer is similar t o that used with the early model of Beckman flame photometer. It was modified as folloms:
1
This type of arrangement has been in use in this laboratory for the past 3 years and has been found most satisfactory and timesaving far determination of sodium, potassium, and calcium in plant material, soil extracts, and water. No loss of instrument sensitivity or precision owurs with this modification, and there is no difference in instrument readings between the Perkin-Elmer atomizer and the modified Beckman atomizer. Small samples may be accurately read, very little clogging ocours, there is no air leakage of the atomizer so that a uniform rate of atomization is obtained, and it is possible to change samples immediately after a reading has been made, without waiting for the remainder of the sample to atomize. Readings can be made for about two samples per minute. This madifioation is less expensive and simpler than other modifications, such as that of Dubbs. [ANAL. CHEM:,24, 1654 (1952)l. Approximate dimensional mexrtsurements of modified atomizer (in millimeters) are:
O.D.
Bore Orifice at outlet O.D. at outlet Orifioe at inlet Len& of inlet extension alter bending
0
0.50 0.25 1.0 0.5
20
Air ohamber
O.D.
Length Orifice at outlet Air inlet tubing
12
30 2.0
0.n.
iength
40
