V O L U M E 20, NO. 9, S E P T E M B E R 1 9 4 8
795 authors are deeply grateful to the Food and Drug Administration and the Antibiotics Section of the National Institute of Health for penicillin samples. LlTERATURE CITED
E 5
( I ) Iniencan Society for Testing Materials, Philadelphia, Pa..
X-Ray Diffraction Patterns. I., and Schrenk, H. H.. Bur. Mine>. Rept. Invest., 3520 (June 1940). 1.3) Ballard, J. W., Oshry, H. I., and Schrenk, H. H., J . Optical Soc. 1 2 ) Ballard, J. W., Oshry. H.
:a
Am., 33, 667 (1943). 14)
(5)
(6) (7)
3 3
(8)
0 2
I 0
1 20
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I 40
9 . SODIUM
I
I 60 PENICILLIN
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80
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I00
G
Figure 12. Working Curve for Determination of Crystalline Sodium Penicillin G
essentially the same as that of the standards used in making up the working curve. It is advisable, therefore, that all unknowns be observed microscopically prior to x-ray analysis. ACKNOWLEDGMENTS
I t is a pleasure to record the authors’ indebtedness t o Dan NcLachlan, Jr., E. F. Champaygne, and other members of the staff of these laboratories for much help in this work. The sample of crystalline sodium penicillin dihydro F was furnished through the courtesy of the Chas. Pfizer Company, Inc. The
Ballard, J. W., and Schrenk, H. H., Bur. hiines, R c p i . I n w s t . . 3888 (June 1946). Barnes, R. B., Gore, R . C., Williams, E. F., Linsley, S. G . , and Petersen, E. hf., A 3 . 4 ~ CHEM., . 19, 620 (1947). Buhler, J. S., Elec. Mfg., 35, No. 6 (June 1945). Carl, H. F., Am. Aif21ineral.,32, 508 (1947). Clark, G. L., and Reynolds, D. H., IND.ENG.CHEM.,.LXAL. ED.,
8, 36 (1936). (9) Committee on Medical Research, OSRD, Washington, and Medical Research Council, London, Science, 102, 627 (1945). (10) Food and Drug Administration communication: method of Chas. Pfizer & Co., Inc. (11) Friedman. H.. Electronics. 18. 132 (1945). (12j
Gross, S. T., and Martin, ‘D. E., IND. E&. CHEM.,ANAL.ED.,
16, 95 (1944). (13) Hanawalt, J. D., Rinn, H. W., and Frevel, L. K., Ibid., 10, 457 (1938). (14) Lonsdale, K., Bm. Mineral., 33, 90 (1948). (15) Redmond, J. C., BNAL.CHEM.,19, 773 (1947). (16) Smith, C. S.,and Barrett, R. L., J. Applied Phys., 18, 177 (1947). (17) Strong, F. C., AN.AL.CHEM.,19, 968 (1947). (18) Strong, John, “Procedures in Experimental Physics,” pp. 296300, New York, Prentice-Hall, 1938. (19) Wilsey, R. B., Am. J . Roentgenol. Radium Therapy, 32, 789 (1935). RECEIVEDFebruary 7, 1948. Presented, in part, a t the Fifth Annual Pittsburgh Conference on X-Ray and Electron Diffraction, Rlellon Institute, Pittsburgh, Pa., Sovember 7 and 8, 1947.
Electronic Trigger Circuit for Automatic Potentiometric and Photometric Titrations RALPH H. MULLER, New York University, New York,N . Y.,AND JAMES J. LINGANE, Harvard Unieersity, Cambridge 38, Mass. A Schmitt trigger in combination with a single-stage pentode preamplifier provides a circuit that can be made to switch abruptly and definitely at a preset level of input potential. The trigger action is reversible, with a small dead zone. A stabilized line-operated instrument is described which is suitable for automatic potentiometric and photometric titrations, with a sensitivity of * 5 mv. and 1 2 X 10-6 lumen, respectively.
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recent paper (1) it was shown that precise automatic titrations can be made with a motor-driven syringe type buret, the delivery of which is stopped exactly a t the equivalence point by a mercury switch attached to the slide wire of a recording potentiometer. Intermittent action in the vicinity of the end point is obtained by appropriate adjustment of the distance between the indicator electrode and buret tip. A Brown Electronik recording potentiometer was used in the previous investigation. The present paper describes an amplifier trigger combination which will switch abruptly at a definite predetermined potential and switch back again for a slightly smaller potential and which can thus be used for automatic titrations in place of an expensive recording potentiometer. The new circuit is also suited, without modification, for phototube control, and is therefore very useful in automatic titrations using indicators or other color changes.
The behavior of the circuit is best understdod by reference to the simplified schematic of Figure 1. The twin triode represented by TI and Tz is the trigger circuit essentially as described by Schmitt (3). Section Tn has a relay connected in the anode circuit and the grid of this section is connected to the anode of the first section through a resistor and through another resistor to ground. A common cathode resistor, Re,is returned to a point 150 volts above ground and the common plate supply is 300 volts above ground. With no signal applied to the input of T I this section is nonconducting and section T Zis fully conducting. This is a steady state and will prevail indefinitely. If the grid of T1 is made somewhat more positive, the plate potential will decrease and therefore drive the grid of T I negative. The decrease of current in section T1 decreases the voltage drop across R, and therefore makes the grid of TI still more positive. This action is cumulative and during a period of the order of microseconds, T I becomes fully conductive and 2’2 is cut off.
ANALYTICAL CHEMISTRY
f196 The full range of current in T 2 between conduction and cutoff can be made much larger than the difference between pull-up and drop-out values of the relay; consequently the switching action is very positive. Both states of equilibrium are stable; at one value of potential on the grid of T1 the first section is conducting, at a somewhat more negative value it will be cut off and the second section will conduct.
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0+300
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+I50
o +
the trigger circuit. A change in E , of only a few millivolts in either direction is sufficient to cause positive reversible trigger action. The complete circuit is shown in Figure 2. Its similarity with the schematic of Figure 1 requires no further explanation of fundamental behavior. The 300- and 150-volt direct current supplies are regulated by the two VR-150 tubes, TPand T B . The desiled signal potential (0 t o 1 volt) a t which switching 1s to occur is set by the coarse and fine potentiometers, R I and ~ R13, and indicated on the voltmeter, V . The agxiliary bias is then adjusted by the coarse, medium, and fine potentiometers, Rlr, R15,and Rls, until the circuit triggers. Triggering is indicated by red and green pilot lamps actuated by one side of the double-pole double-throiy relay. If the system is now switched to the eleetrodes of the titration cell by reversing switch SI the circuit will trigger as soon as the cell potential reaches the preset valu+i.e., the equivalence point potential. The second side of the double-pole double-throw relay acts as an on-off switch in series in the circuit of the motor which drives the syringe buret (1). Inasmuch as in various titrations the cell potential may either decrease or increase toward the equivalence point potential, switch SOis provided so that the syringe motor circuit can be set to open for either direction of potential change. The input resistance of the circuit is approximately 5 megohms, which is suitable for all potentiometric titrations in which the cell resistance is not larger than a few thousand ohms. The present
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Figure 1.
Schematic Circuit
The difference in these two triggering levels becomes qrogressively smaller as the value of the cathode resistor, R,, is reduced, and may be made as small as 0.1 volt. Schmitt has shown that if R, is reduced still more the circuit fails to trigger, but becomes a very sensitive amplifier. In the present application a very conservative dead zone of 0.5 volt was chosen, first of all t o preserve the certainty of triggering, and secondly because preliminary amplification of the signal was necessary anyway to apply the circuit to potentiometric titrations. The function of the pentode amplifier, Ta, is also shown schematically in Figure 1. A slight amount of degeneration in this stage was provided by a cathode resistor.
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Figure 2. R2,R;. 100000 ohms (0.5-watt) RI. Rj.
Ri. RE.
5 meeohms (1-watt) lOO,O?lO ohms ( l - n a t t )
Ro. 200 ohms (1-watt)
1 m'egohm (O.5-watt) RlO. 500,000 ohms (1-watt) 4000 ohms (1-watt) 100 000 ohms (1-watt) Rll. 2 megohms (0.5-watt) 100-ohm potentiomLters (wire-wound, linear taper) R12. 1000-ohm potentiometers (wire-wound, linear taper) RlS. Ria. 10,000-ohm potentiometer R I S . 100,000-ohm potentiometer Ris. 1-megohm potentiometer 5 megohms (0.5-u.att) Rl7. V. Weston voltmeter Model 269 (0 t o 1 volt in O.Ol-voit divisions a n d 1000 ohms resistance) BI. 22.5-volt dry battery (Burgess 4156) 1.5-volt dry battery (Burgess 2F) 8 2 . TI. 524 rectifier tube TP,Ta. VRl5O voltage regulator tubes T4. 6SJ7 pentode amplifier (6.3-volt heater connection not shown) Ts. 6 S S 7 twin triode (6.3-volt heater connection not shown) RY. Double-pole double-throw relay (C. P. Clare and Co., Chicago, S o . 40 E. C., 11.,300 cihms: L. Choke (UTC 30 henrys 100 ma. 375 ohms d.c.1 TR. Transformer '(UTC, 35OlO-350, 9b ma.) CI, C2. 8 p f . Aerovox electrolytic capacitors (475 volts d.c.) CY. 1 pf. Pyranol capacitor (500 volts d.c.) P. 2-amp. fuse si,8 2 . Telephone type single-pole single-throw switches (General Control Co., Boston, Mass.) P.L. Pilot lights P.T. High vacuum phototube (R.C.A. 929)
Re.
The plate of the pentode is connected to the grid of 7'1 through a resistor and the input potential of the pentode, E,, which is the sum of a variable bias plus the desired signal voltage, issoadjusted that the plate potential of the pentode is of the order of 1.50 volts above ground. This is the order of magnitude of the grid potential of 7'1 in its untriggered condition. If, now, the grid of the pentode is made slightly more negative, its plate potential riyes and initiates mitching in
Complete Circuit
V O L U M E 20, N O . 9. S E P T E M B E R 1 9 4 8
Figure 3.
Assembled Instrument
cirouit is not applicahle with high resistance glms eleotrodes, and
if it is desired to make titrations with the glass electrode a preliminary resistance matching stage may be added. The instrument was assembled as a portable unit in a radiotype steel cabinet, as shown in Figure 3. The only extern1 power supply required is a 110-volt, 6D-cycle, alternating current line. All wiring, except the leads to the bias batteries and panel controls, was securely anchored to the under side of the chassis. and shielded cable, mounded at several points
197
Tho Jensitivity of the instrument-i.c., changc in input signal required to actuate the relay-is ahout 1-3 millivolts. This is more than ample for the great majority of potentiometric titrations. When the instrument was powered from a 110-volt, 60eyole, Sola constant voltage transformer, rather than the ordinary house line, the dead zone was decreased to less than 2 millivolts. Theresponsetimeisveryshort, of.theorderof afewmilliseconds. The stability of the instrument is very satisfactory. After an initial warni-up period of about 15 minutes, the rote of drift from the preset triggeringpotential is only ahout 5 mv. per hour or less. This is entirely negligible for a11 automatic titrations. For photometric titrations a high vacuum phototube, P.T., is connected as shown by the dashed lines in Figure 2. An R.C.A. 929 tube was used with an unregulated incandescent lsmp source. The instrument is set for photometric titrations with a solution that has the same spectral characteristics as the titrated solution will have a t the equivalence point, and potentiometers Ru, Rls, and RI. are adjusted until triggering occurs. No refinements were made to ascertain the limiting precision attainable, hut in gross photometric terms the differential sensitivity is extremely lumen. With a properly d e high;of the order of *2 X signed titration cell, and o phototube and filter of optimum characteristics for a particular case, color changes too slight to he perceptible to the eye are sufficient to cause triggering. The paper of Muller and Partridge (S) may be consulted for technical detail8 of automatic photometric titrations. The autotitrator previously described (.1.) is easily admtahle t o such titrations, and some typical applications will be described in a later paper.
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LITtiRAlUKti ti1IEP.u
Syringe buret. These prekutions rindered the cirouit completely immune to bodv cspaoity effectsand stray fields. The voltmeter, V , i s a high quality instrument (Weston Model 269) with an internal resistance of 1000 ohms, a range from 0 t o 1 volt. and 0.01-volt divisions. which oermitted readinesto *0.001 volt. For many purposes'a cheape: probably suffice.
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Lingme, ._ .. J. J., ANAL. . Cni~x..20,285(1948). . "~ (21 IMuller, K. H., and Yarl,ridge. H. M., I d . Eng. Cltm.,Z", 4'J (1)
(1928).
....... .E (3) Sohmitt, 0.E.,J . Sci. Inarrummca, si), "" L* ~
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Automatic rorenriometric Titr; Automatio potentiometric titrations of Fe(II1) and T i W ) with chromous ion are described. The automatic titration of ferric ion in sulfuric acid medium with a platinum indicator electrode is precise and The optimum conditions for the aocurate to *O.l$&. titration of Ti(IV) with chromous i o n are the use of a mercury indicator electrode rather than platinum, and of sulfuric acid rather than hydrochloric acid media. Under these conditions the titration is precise and accurate to ahout 1-0.2970. Titration curves
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(71 an autotitrator was described which performs potentiometric titrations automaticrtlly, with p r e cision and accuracy fully equal to that obtainable in the same titrations by conventional manual techniques. It was shown (7) that the instrument is applicable t o oxidation-reduction, precipitation, and protolytie titrations, and to slow as well a rapid reactions, so that it is capable of general application. The primary purpose of the present investigation was to estahlish optimum conditions for the titrimetric reduction of f 4 N A previous paper
demonstrate the erroneous behavior of a platinum indicator electrode in solutions whose oxidation potential is below that of the hydrogen-hydrogen ion aouple. Very satisfactory determinations of iron and titanium io mixtures of the two can be obtained by titration with chromous ion in solutions containing about 4 N sulfuric acid, using a platinum indicator electrode for the iron titratinn and a meroury indicator electrode for the subsequent titration of the titanium.
titanium to the +3 state with chromous ion, and the use of the autotitrator was incidental. However, the autotitrator proved t o be ideally suited to titrations involving air-sensitive suhstances, and it is muoh more convenient than the usual manual method for establishing the characteristics of potentiometric titration curves BS well as for routine titrations. The only previous study of the titration of +4 titanium with chromous ion is that of Brintzinger and Schieferdecker (0,who recommended titration in a very conoentrated chloride solution
