fective stability constants for the various metal-chelates not only decrease with decreasing p H but also a t different rates. The dotted lines in Figures 8 to 11 demonstrate the formation of hydrogen or hydroxy chelates which increase the effective stability constant. This effect is especially prominent with Me-DTPA"-b and HgHEDTA-l chelates which readily form hydrogen or hydroxy chelates, respectively. Inspection of Figures 8 to 11 allows many interesting comparisons, but only a few of which will be pointed out here. iit p H > 7, tetraethylenepentamine (tetren) is superior to any chelon studied for the titration of mercury or copper; yet at pH 3 tetren would be one of the poorer titrants. At p H > 8, DTPA forms a more stable chelate with zinc than any chelon investigated; but in acidic solutions CyDTA is superior. Triethylenetetramine (trien) is, by far, the poorest titrant for zinc. Except from p H 8 to 10 (where EGTA is superior), CyDTA is the best titrant for Ca+2. Because of formation of Hg(OH)HEDTA-2, the effective stabil-
ity of Hg-HEDTA-1 has been increased by 4 log K units a t p H 12. I n summary, evaluation of the various chelons based solely on the comparison of log K values is deceptive, as only after the effects of p H are considered can really justified conclusions be made concerning their relative merits. ACKNOWLEDGMENT
One of the authors (J. H. H.'r expresses his deep gratitude to the American Enka Corp. and to the R. J. Reynolds Tobacco Co. for their financial support of his research program. The authors are also indebted to B. D. Armes for invaluable aid in obtaining certain experimental data. LITERATURE CITED
(1) Anderegg, G., Nageli, P., RIuller, F., Schwarzenbach, G., Helv. Chim. Acta 42,827 (1959). (2) Bjerrum, J., Schwarzenbach, G., Sillen, L. G., Chem. SOC. (London), Spec. Publ. 6, Part I (1957). (3) Chaberek, S., Martell, A. E., J . Am. Chem. Soc. 77, 1477 (1955).
(4) Durham, E. J., Ryskiewich, D. P., Zbzd., 80,4812 (1958). (5) Kroll, H., Powers, J., Pinchin Butler, F., Division of PhyaicaK) a% Inorganic Chemistry, 124th Meeting, ACS, Chicago, Ill., September 1953. (6). La$,mer, W. M., "Oxidation Potentials, 2nd ed., p. 179, Prentice-Hall, New York, 1952. (7) Matyska, B., Dolezol, J., Reubolova, D., Collection Czechoslov. Cliem. Commun. 21,107 (1956).
(8) Reilley, C. N.,Hollo\Tay, J. H., J . Am. Chem. SOC.80.2917 (1958). (9) Reilley, C. N., Pbrterfield, w. w., ANAL.CHEM.28,443 (1956). (10) Reilley, C. N., Schmid, R. W., J . Elisha Ji'itchell Sci. SOC. 73, 279 (1957). ( 1 1 ) Schmid, R. TV., Reilley, C. N., ANAL.CHEW29, 264 (1957). N.. (12) Schmid, R. W., Reilley, J . Am. Chem. SOC.78,5513 (1956). (13) Schwarzenbach, G., Helv. Chirn. Acta. 32,1682 (1949). (14) Schwarzenbach, G., Senn, H., Anderegg, G., Ibid., 40,1886 (1957). (15) Wanninen, E., private communica-
c.
tion.
RECEIVED for review November 10, 1959. Accepted November 27, 1959. Research supported by the United States Air Force through the Air Force Office of Scientific Research, Air Research and Development Command, Contract No. A F 49 (638)-333.
A Digitizing Print-Out Mechanism Capable of Automatically Carrying Out Simple Mathematical Conversions D. H. SIMMONDS and R. J. ROWLANDS Department of Agricultural Chemistry, Waite Agricultural Research Institute, University o f Adelaide, South Australia, and Division of Protein Chemistry, Wool Research Laboratories, Commonwealth Scientific and Industrial Research Organization, Parkville N. 2, Victorio, Australia
b A simple and relatively inexpensive printing mechanism automatically prints and records in tabular form colorimetric data presented to it. It consists of a typewriter modified so that the tabulating mechanism is solenoid-operated. Data are printed by an engraved counter wheel driven either by mechanical rotation or by a servomotor balanced by a Wheatstone bridge and an alternating current amplifier circuit. Simple mathematical transformations can b e automatically effected by making the resistance in the two arms of the Wheatstone bridge obey different laws. The speed and accuracy of the printing mechanism are limited only by the speed and accuracy with which the servo-system can b e balanced. In practical terms this means about 10 to 20 printing operations a minute. 256
ANALYTICAL CHEMISTRY
T
conversion of angular rotational movement into a printed read-out is now a well established process, being accomplished by a digitizer coupled to a decoder and a n automatic typewriter. Such equipment is available commercially or is in the process of development in many countries ( 1 ) . To achieve high standards of accuracy and resolution, however, the digitizer or coding section of the unit must be a highly precise and therefore expensive piece of equipment. Because of the necessity of encoding and decoding the digital information and presenting it in sequential form to an automatic typewriter, reasonably complex electronic circuitry is also required. Furthermore, the automatic typewriter is itself expensive. I n developing equipment for the automatic colorimetric estimation of amino HE
acids ( 3 , 4 ) ,it became desirable to print the absorbance readings given by the photometer in some form of sequential arrangement. Furthermore, because the one photometer monitored the effluent from eight ion exchange chromatographic columns, it was necessary to tabulate the information to simplify subsequent calculations. An approach was developed, in which the digitizing and printing mechanisms were combined in one unit mounted on a converted typewriter. GENERAL CONSIDERATIONS
By suitable reduction gearing, a counter of the type used to record automobile mileage transforms angular rotational movement of the road wheels into a digitized read-out. Similarly, the position of the dial in any null-balance
instrument can he recorded in suitable units by appropriate gearing to such a counter. For this reason, a SigristPhotometer (Type UPZLD, manufactured by Sigrist & Weiss, Ltd., Falkenstrasse 23, Zurich I/S, Switzerland) was selected for the colorimetric estimations required (S,4). I n this instrument there is a direct relationship between the angular rotation of the dial drum and the linear movement of the wedge which reduces the light intensity through the standard solution to a point of balance. The drum itself is linearly graduated in percentage transmittance units. I n addition, a logarithmic absorbance scale has been engraved on the drum to facilitate use of the instrument. As the counter
mechanism is of necessity a linear arrangement, at some point between the rotating photometer drum and the counter wheels, a logarithmic conversion must be introduced if the final reeord is required in absorbance units. The use of a carefully wound logarithmic potentiometer was considered for this purpose. However, because of the extreme difficulty associated with accurately winding a potentiometer following a logarithmic law over the required range (about 1.5 decades) in the angular space available (about 240°), a linear potentiometer was used instead. This, attached to the scale drum spindle, could be electrically balanced against an esponentially wound potentiornetcr connected to the counter.
This paper describes the construction of a digitizing print-out mechanism which automatically converts from a linear to a logarithmic scale the readings given by the self-balancing photometer. These readings are tabulated in columns corresnondine to the color source beine monitored. Althoueh a snecific aoolication is de.. scribed, the print-out mechanism is gcneral and versatile in its application, and could in many instances replace the type of potentiometer recorder currently in use. It can he simply coustructed in a laboratory workshop for about one sixth the cost of a multipoint recorder. ~
~
-
:ONSTRUCTION OF PRINTING MECHANISM
_...
Tl.l"h a mmn1nt.o nrintins nnit. . ahnwvn ""..-r.l"~ F..l"...i? " i ._ l . . l
in Figure 1. Its construction may he divided into three parts: modification of a standard typewriter, the print wheel and servo-balancing mechanism, and the servo-balancing amplifier.
ure 1.
Comulett printing unit
-.%&lad
E
G ^^.
-.
X J L t l 401-v
tigure 2. A. B.
Lounter wheel digit king and printin(1 tlead
C.
Counter wheel with raised figurer 0 Gear wheel Counter wheel rhoft
D.
idler g e m drive from main shaft
to
9
I!. F.
G.
Main shaft Yoke for print whee Shah for connection to p&t solenoid
Modification of a Standard Typewriter. A new or used standard typewriter is modified as follows: The keyboard and typing mechanism are completely removed. On some makes this part can be d r a m out as a unit hy pressing two buttons at the side of the machine. The tabulator key is connected by a lever to a small solenoid-e.g., the type used in conventional magnetic contactor assemblies. T h e n the solenoid is energized, the tabulator mechanism is operated and the typewriter carriage moves down to the next position. The necessary number of tabulator check stops are set so that the carriage stops at a different position for each column of figures required. The small ratchet wheel operated by the back spacer key is driven through a series of reduction gears and a slipping clutch by a small brush electric motor. This mechanism resets the carriage at the end of one complete cycle of readings. Typewriters having an electric carriage return motor built in are probably preferable. The ribbon - winding mechanism is connected by a lever to the tabulator mechanism, so that the ribbon advances each time the tabulator key is operated. A spool holder is constructed to take the paper rolls used for tabulating the results. The width of these rolls depends on the number of columns in operation, and they are mounted so that the paper feeds freely under the carriage roller. The rolls are held in place hy a narrow roller mounted above the carriage roller. Alternatively a single wide roll of paper may be used, with less chance of the outside strips not feeding through properly. It ma,v also he desirable to construct a cover to prevent the emerging paper from becoming tangled in the typewriter. The roller turning lever is bent or replaced by a suitably shaped lever which points tou-ards the rear of the machine. When the carriage is reset, VOL. 32, NO. 2. FEBRUARY i960
a
257
this lever contacts a stop nhich automatically turns the roller around one line. This operation is made easier and more positive if the spring controlling the indexing of the roller is removed. In most models this involves only moving a small lever. The roller then turns freely when the paper advance lever contacts the stop. Thcse modifications convert the typenriter into an autoniatic tabulating mechanism, which presents the results from each ion exchange column as a column of figures. Print Wheel and Servo-Balancing Mechanism. The print wheel and servo-balancing mechanism are made up of three units mounted in the body of the typewriter in place of the standard keyboard: the printing head, the exponential potentiometer, and the balancing servomotor geared t o the first two units.
PRINTING HEAD. Thr printing head (Figure 2) consists of a small unit counter of the type used in a car trip mileage recorder-e.g., general-purpose counter made by Veeder-Root, Ltd., Kilspindie Road, Dundee, Scotland. The engraved figures are replaced by raised type, four cylinders carrying the numbers 0 to 9 being required. The unit cylinder is connected to the main shaft by a 1 to 1 gear and idler wheel. The print wheel mounting frame is pivoted a t its mid-point by two ball races through ivhich the main shaft rotates. The lower end of the mounting frame is connected to a small solenoid of the type used to oaerate the tabulator key mechanism When this solenoid is energized, the print n-heel is thrown forward onto the ribbon and paper on the roller. BALAKCING SERVOMOTOR. Also connected to the main shaft are the balancing servomotor and the exponential receiving potentiometer. The servomotor drives the main shaft through an approximately 3 to 1 reduction gear. The exact ratio is unimportant, but a stepdown is desirable to reduce the rotational speed of the print wheel, and to increase the power available from the servomotor. The latter is a Leeds &. Korthrup pen-balancing motor as used in the Nodel G Speedomax recordrr. EXPOXENTIAL POTCSTIOMETER. If readings were to be recorded as percentage transmittance values, no scale conversion would be necessary, and only a linear potentiometer matching the 10,000-ohm linear potentiometer attached to the photometer drive shaft would be required. To effect a conversion continuously from transmittance t o absorbance units, however, the last few turns of wire in the 10,000-ohm transmitting potentiometer, corresponding to absorbance values of 1.25 and above, are shorted out. The position of this potentiometer is then balanced against an exponentially wound potentiometer driven by the print wheel servo258
0
ANALYTICAL CHEMISTRY
SERVOMOTOR DRIV:
, 3 INCH
u
Figure 3. A-G.
Constructional details of switched exponential resistance bank
Same as in Figure 2
G.
10 to 1 reduction, snap action, stepping gear RED.
Figure 4 .
Circuit for servo-balancing amplifier
motor. OnlJ-recently has it been possible to wind a sufficiently accurate esponential potentiometer and in the first model this unit was replaced by a si-itched resistance bank and rotating linear rheostat ah suggested by Hibbard and Piddington ( 2 ) . The construetional details of this nrrangenient are shown in Figure 3.
It consists of a lo-watt, 3000-ohm 360" linear potentiometer geared to the main shaft by a 1 t o 10 reduction. Also driven by the main shaft is a multiposition switch so arranged that as the continuously rotating potentiometer crosses its zero position, the multiposition witch rapidly moves from one position to the nest. The switch has 14 contacts spaced at 22l/,O intervals around its circumference. The gearing required is such that the 3000-ohm rheostat rotates once for each ' / d h rotation of the snitch. The rotational movement of the switch wiper arm, JT-hen it occurs, is required to be almost instantaneous. This is achieved by gearing it to the second pinion wheel of a counter mechanism similar to that
y:,;
i -, ( L ~ NBALANCINO
SERVOMETER
employed in the print wheel, but more sturdy in its construction. Wired to the nlultiposition switch is a blank of fixed and adjustable resistors arranged so that as the switch moves from one position to the next, successive increases of resistance are switched in. The incrrase a t any one step is determined by the exact value of the rotating potentiometer. The smoothness of the transition a t each switching point is achieved by careful adjustment of thr wire-ii-ound rheostats in the resistance bank. Subsequent printing units have incorporated :in exponentially PT ound potentiometer of conventional dcsign. .1 Bakelite card, cut to obey an exponential curvc, is close wound by hand with 37 s.n-.g. Eureka enamel-covered I\ ire. After the finished card has been coated with Perspei cement to hold the \iire in place, it is bent around the outside of a standard potentiometer case and strapped into position so that the
rotating arm engages the flat edge of the card. The potentiometer arni is geared to the final digit of the print wheel by a worm gear whose ratio corresponds to the range covered by the potentiometer --e.g., if the resistance of the potentiometer covers 1.2 decades in 3603 of rotation, the reduction between the final digit of the printwheel and the niper arm is 120 to 1. Suitable potentiometers wound to these specifications are nom available through E.3I.I. Electronics, Ltd. (Salisbury), South Australia. To avoid ambiguity in the position of the final digit an indexing mechanism has also been incorporated on the main shaft. This consists of a 10-toothed star wheel, about 7 / ~ inch in diameter, pinned to the main shaft. 4 pawl attached to a fairly heavy solenoid (1- to 2-pound pull), engages the star ahpel when the solenoid is energized, pulling it around into a position corresponding t o an unambiguous, in-line digital read-out. This solenoid “locks” the whole main shaft while the printing movement takes place. The Servobalancing Amplifier. The circuit for this unit is shown in Figure
4. The transmitting and receiving POtentiometers attached to the photometer scale drum and the print wheel servomotor, respectively, form two arms of a Wheatstone bridge, across which is plnccd a small alternating current voltage (6.3 volts from transformer 2‘). The out-of-balance voltage IS amplified, and after a phase inversion,
is fed to the print wheel servomotor t’o drive it to a position of balance. OPERATION AND CALIBRATION OF PRINT-OUT MECHANISM
The sequence of events involved in making a reading is as follows: The photometer servomotor drives the optical Yedge, scale driini, and 10,000-ohm linear potentionieter to a position of balance. The print R heel servomotor drives the print wheel and exponential POtentiometer to a position of balance with the 10,000-ohm linear transmitting potentiometer. The indexing solenoid is energized, locking the main shaft. The printing solenoid is energized and released, pressing the print wheel onto the ribbon and paper. The indexing solenoid is released, freeing the main shaft for the next reading. The tabulator solenoid is energized, operating the tabulator mechanism and moving the carriage t o the next column. Once in every cycle the carriage return motor is switched on to reset the carriage. This sequence of events is controlled by a sequence timer of the type described by Simmonds (3) and Sininionds and Ro\>-lands (4). Such a sequence timer is normally required for the operation of the equipment associated with the print-out mechanism, and so the extra contacts for the above switching operations can readily be incorporated.
Three print-out mechanisms of the type described have been constructcd and tm-o have been used successfully for over a year. The first utilized the switched exponential resistance bank, while the second two used exponentially wound potentiometers. Each was calibrated by rotating the scale drum of the photometer manually and noting the absorbance readings on thc drum and on the print wheel. respectively. Excellent agreement vias achieved in both caqeq. ACKNOWLEDGMENT
It is a pleasure to acknoidedgc tlic assistance of K. I. TTood, who designed and constructed the print wheel servoamplifier, and W.J. Sutherland and E. h’. Paton, Paton Industries Pty., Ltd., Adelaide, South Australia, who constructed the print wheel assembly and carried out the modifications required on the typewriter. The assistance of J. C. Cockram, E.RI.1. Electronics (Sslisbury), Ltd., in the development of the exponential potentiometer is also gratefully acknon-ledged. LITERATURE CITED
Evans, D. S., Brit. Communications and Electronics 4, 334 (1957). (2) Hibbard, L. U., Piddington, J. H., J. Sci. Instr. 24, 92 (1947). (3) Simmonds, D. H., ANAL. CHEW30, 1043 (1958). (4) Simmonds, D. H., Rowlands, R. J., IbLd., 32, 259 (1960). RECEIVED for review April 2, 1959. Accepted October 14, 1959. (1)
Automatic Equipment for Simultaneous Determination of Amino Acids Separated on Several Ion Exchange Resin Columns D. H. SIMMONDS and R. J. ROWLANDS Department o f Agricultural Chemistry, Waite Agricultural Research Institute, University o f Adelaide, South Australia, and Division o f Protein Chemistry, Wool Research laboratories, Commonwealth Scientific and Industrial Research Organization, Parkville N. 2, Victoria, Australia
b Construction of automatic equipment to monitor the effluent of eight ion exchange chromatography columns simultaneously was undertaken to speed amino acid determinations b y the methods of Moore, Spackman, and Stein. A unit fractionates the effluent from each column, treats each fraction with ninhydrin reagent, dilutes, and estimates the resulting color. The measured absorbances are recorded as a printed column of figures on paper tape. This record requires only integration to complete the analysis.
Six analyses for the common amino acids can b e completed in 48 hours. Different samples can b e analyzed simultaneously on different columns, allowing closer and more accurate intercomparison of results. Use of the equipment to analyze four insulin hydrolyzates is described.
A
capable of automatically determining amino acids in mixtures or in protein hydrolyzates has been described (IO). Subsequent MACHINE
publications ( 2 , 8) describe its performance and compare the results with analyses carried out manually by the techniques of Rioore and Stein (5, 6) and Moore, Spackman, and Stein (4). This paper describes a machine which automatically estimates the amino acid composition of up to eight different samples simultaneously. The equipment controls the operation of eight ion exchange chromatographic columns, and presents the results from each as a printed record. This is easier to VOL. 32, NO. 2, FEBRUARY 1960
259
