Photoelectric Colorimeter for IUU
111
111
Ghe use of small volumes and a long absorption path, the determnation of copper by the carbamate method may be considered. Assume that the blank is set at 100 on a 0-100 scale. In the usual micracolorimeter a volume of 2 ml. is required for a 1-cm. absorption path. Approximately 0.25 microgram of copper as the carbamate per ml. of cmhon tetrachloride or s, total of 0.5 microgram of copper will he required to give a reading of 90. With the smallest cuvette daacrihed below, 1ml. of solution is adequate for an absorption path of 10 cm. or a total of 0.025 microgram of copper to give the same reading. Thus the sample required is reduced by B factor of 0.05. In the c a e of copper in plant and animal tissues where ashi w is necessary, 0.025 microgram of copper is about the minim,im t,hat can be dealt with hecause unavoidable contamination and lasses hecome limiting factors. I n other methods, the point a t which these and other factors become limiting is reached with larger volumes and s h o r t e r a b s o r p t i o n paths, and the authors feel that in the determination of the trace elements, the oolorimeter described will be found adequate in a l a r g e m a j o r i t y of t h e methods now available. T h e e a r l y w o r k of Muller (6) demonstrating the value of a photocellamplifier combination led to it,s use in the present rolorimeter.
shows the arrangi >in the light shields. All components, except light shields, are mounted on an optical bench consisting of two lengths of 1 X 1 inch (2.5 X 2.5 om.) angle iron positioned as shown in Figure 1. The henoh rests on mounting blocks and is secured to the base of the cabinet. The bases of the component,R a m grooved to fit the lips of the bench and are secured hy bolts through the slot in the bench. The lamp, 8. 50-cp. prefocus auto headlight lamp (Mazdh No. 2531),, is mounted on a support which permits adjustment of the lamp in a plane normal t o the optical axis of the inst,rument and n i t h the axis of the filament coil about 6O'from the o p t i d axis. The lamp is operated from either a 6-volt storage battcry or a suitable transformer. The lens is mounted so that adjustments in the plane normal to the optical axis and on a line parallel to the axis m e possible. A lens board mounted in a frame aa shown in Figure 1 permits the adjustments normal to the axis. The lens is mounted in a barrel which screws into the lens board, thus permitting adjustment along the optical axis for accurate focusing. The barrel also accommodates fixed diaphragms. The American Optical Company 116-mm. doublet No. 11161-701 has provcd satisfactory in this service. The filter holder consists of a simple frame with spring clips to hold 2 X 2 inch glass filtws. The holder fits into an opaning in the first light shield, uhich is an integral part, of the cabinet.
DESCRIPTIOW O F COLORlMErER
The xrrangenent of the components of the colorimeter and the g e n ~ r s lcons t r u c t i o n a l d e t a i l s are ahown in Figure 1, Figure 2
ure 1. i46
Exposed View of Colorimeter
V O L U M E 21, NO. 12, D E C E M B E R 1 9 4 9
1547
. .
.
. .... .
rBllS-
idled LY he :omly be 'yrea wnThe glw ,any, aster press Car-
Colorimeter Arrangement in Cabinet
Figure 2.
The cuvette carriage assembly consists of three parts: a plaG form, the carriage, and tho cuvette support. The top plate of the olatform (the bottom d a t e of which rests on the optical bench) &rLrries the guides, c&iiage stop bosses, and adjustable diaphragm. The carriage slides on two parsllel guides made of ,'/, inoh steel rod. .~~~~ ~. > t,he -~~~ contact beine t,wo brass strim in one of whnh is cut a 90"V to hold the carrizge in &gnment'. Screws running through the carriage Stop bosses pcrmit adjustment, of the lateral travel of the carriage. The adjustable diaphragm consists of a brass disk pivoted a t the ccntm to another brass plate, which in turn is fastened to the platform. S ~ v e r dsets of holes of different diameters located a t various rndii ~~
~~~~~~
~~~~~~~~~
~
~~~~~~
~
~
have The : solm to 3 tiltI. sisr led it ettes (5) or and Bessey (4), both of which are particularly adapted for use with small volumes of aqueous solutions, could be used in place of the cuvettes dsswihed here. The amount of light energy to be measured io this colorimeter is rather small, but the high sensitivity of either of the amplifiers described below makes pissible the use of light - filters transmitting a relatively narrow naption of the spectrum.
---__.__ . . N nPE,-I2rmln
Two amplifier circuits have hL-".._. ~"-. scribed sboye. One, a~hattery-operatedVanee agplifier of high sonsiitivity, is shown schematically in Figure 3. The other, line__.
~
~
used to raise cuvettes of sm& diameter into"t.he fight beam, Wire drills No. 30, 40, 50, and 60 give a sat,isfactory selection of size, and steps of '/la inch are setisfactory for h8ight adjustment. The carriage is moved either by a small rod threaded into either side of the carriage and extending through the
three p&'of carefully michinod V-ways mounted on a plate, pivoted on the carriage so that it may be swung out of the way when the holder for square cuvet.tes and test tubes is slid onto the two tongued bosses shown. The plate is positioned by means of two bosses fitted with adjusting screws which press against the ends of the twn V-ways. A spring clip is used to hold the plate firmly in position. A simple lever-type light shutter is provided to cover the hole in the light shield located between the carriage
the sh&r a ~ m .1 s the lid is raised, the shutter arm drops by gravity, closing the opening into the phototube housing. The photocell is mounted on a support similar to the light support, permitting adjustments in the plane normal to t.he optical axis of the instrument. A brass shidd fitted with a thin glass window (microscope cover slips me satisfactory) fits over the phototube. The shidd 8wves to protect the tube from dust and stray light. The cuvettes are ovlindriral. with dane. oarallel endsof C o r a or Pyrex gla& 1 mm. 'thiok.' Thef are filled and emptied through a tubular opening approximntely 1.5 cm. from the front end. On the smdlllcr cuvettes this opening is approximately 4 mm. in diameter and 1 em. in length. A small tab of Lucite or similar material !/* inch thick is cementedon the cuvettes for convenience in handling and to avoid heating during the filling operation. If the cuvettes are held in the fingers, the heat causes the formation of tin," bubbles in many organic liquids, and & wait of
Figtire 3.
Modified Vunee Amplier
0.01 pf. psper 2 megohms, 2 watt! 25 K / W W potentiometer 30 K/2 watts
9 K/Z watts 100 K/2 Watt* 100/w w potentiometer 3O/W w pot 1 K/2 watts 3 K/2 watts 10 K/2 watt 10 K / W W i 1 megohm 10 megohms 100 megohm 300 megohm 1000 rnegohn io00 rnegohii
3-poLe single-throw switob 7-position DBT&rniD low-loss switoh 88. 4-position oeramit low-lor@ switoh B I ,Ba. BI. 22% volt B battery B4 1% volt A battery SI. 81.
ANALYTICAL CHEMISTRY
1548 operated, is shown in Figure 4, with a built-in power supply for a photomultiplier tube. Neither of these circuits possesses complete stability over a period of several hours, but such stability was not considered essential because a rapid interchange of reference blank and sample is provided for in the colorimeter. The battery-operated amplifier is essentially that described by Vance ('7). Because portability is not of prime importance, the 1B4 tubes have been replaced with 6C6 tubes, and a 6-volt storage battery is used to supply the heater current of approximately 1 ampere. A Weston h i d e 1 622 0-100 microammeter is used instead of the 0.1-volt meter of the original design. Except for the filament battery which must be recharged regularly, the dry battery life is about one year when used 2 to 4 hours daily. The amplifier consists of a simple three-stage cascade of sharp cut-off pentodes with a high degree of negative feed-back. Resistors Rs, Rlo, R11, and Rlz form a meter-sensitivity network for adjusting the meter to 100. Resistors RI and Rs will zero the instrument and are sufficient to handle dark currents normally found with the RCA 929 phototube used. The Potentiometer, Ra, permits adjustment of the first stage screen potential. The input resistor bank, Rls through Ria, serves as the primary range contiol. The meter sensitivity network overlaps the range between steps in the input resistor bank. Full-scale meter deflection will be obtained with approximately a 100-millivolt signal. This is equivalent to an approximate full-scale current sensitivity of 10-'0 to 10-11 ampere, using the 3000-megohm input resistor. Two inputs are provided. The input terminals marked zero and grid are used for potentiometric titrations or for a phototube with external driving potential. The terminals marked 221/2 volt and grid provide a 22.5-volt driving potential sufficient for direct connection of the RCA 929 phototube as shown. In the line-operated amplifier (Figure 4), some stability and amplifier-sensitivity have been sacrificed for the convenience of line operation. However, if the 931-A photomultiplier tube is used the over-all sensitivity for photometric purposes is greater than the battery-operated system described above. The amplifier proper is a simple vacuum tube bridge circuit, similar in most respects to the mu-bridge circuits described by Turner (6) and Garman and Droz ( 2 ) . The mu-bridge conditions have not been strictly satisfied, and the meter is used for direct reading of in-balance rather than as a null point indicator. The two halves of the 6SN7 form two arms of a resistance bridge, while the resistors RI,, RIC, and R ~ form I the other two arms. One half of the tube acts as a fixed-reference resistor, the other half as a variable resistor controlled by the signal voltage developed across the input resistor, Rs plus El. Potentiometer R11 serves to balanre the meter to zero for zero signal. Full-scale adjustment is made with the input resistor. A full-wave rectifier 5W4 and a simple RC filter circuit supplies the plate potential to the bridge. This potential is stepped down by the bleeder-voltage divider, RI, and regulated by the VR-75 voltage regulator tube. The tap on R1 is adjusted so that the VR tube will fire under all line conditions and draw about 15 ma. on open circuit. A highvoltage power supply using a half-wave 2 x 2 rectifier is built into the unit to supply the 931-h photomultiplier tube. The amplifier requires a signal of approximately 200 millivolts for full scale. The input resistor is limited to about 2 megohms, ampere which is equivalent to a current sensitivity of about for full scale of meter. With an input resistance of up to 0.05 to 0.1 megohm, the grid return resistance of the reference tube is not too critical. With higher input resistances, the resistance in the two grid circuits should be matched. Although photocurrents of 100 to 150 microamperes are easily obtained with the 931-4, operation of the tube at this level is not satisfactory. Best operation, from the standpoint of fatigue and stability, is obtained with current drains of less than 50 microamperes. Resistor R7 is so chosen that the amplifier cannot be adjusted for photocurrents in excess of 40 pa,, except by rrducing the light intensity. Resistor Re is then chosen to cover the desired operating range. The stability of the two measuring devices as indicated by reproducibility of readings taken on standard solutions or neutral wedges in the colorimeter is less than 0.5% full scale for the battery-operated circuit. The stability of the bridge-type lineoperated circuit is about 1% full scale when erratic values due to line transients are ignored and drift corrections made. The response of both circuits with their photocells is strictly linear when tested with varying concentrations of colored solutions known to follow the Lambert-Beer law, The authors gratefully acknonledge the help given by F. h l . RZcNall in adapting the Vance amplifier to the present application.
Figure 4. Schematic Diagram of Line-Operated Amplifier; T?. Power transformer, 650 CI, cz. 2/1000 volt electrolytic VCT/40MA CI, c4. 26/400 volt electrolytic cs, Ca. 0.05 rf. paper (Stancor P-6010 or R I , Rr. 400/10 watts Rr, Ra, Ro. 220 K/2 watts 50 K/50 w'atts R4. RK.
Rs. Ri.
equivalent) Power t r a n s f o r m e r , 700 VI100 .I.I (Stancor P-6013 or equivalent) Td, Filament t r a n s f o r m e r 6.3 V / l O a (Stancor P-3061 or equivalent) SI.S?. Sq. Sinnle-Dole sinele-throw Coggle switcLes Pi, P2. Standard power plug and receptarle Pa. Low loss shielded plug and receptacle Meter. 0-100 pa. meter (Weston 622 or eouivalent)
TJ.
300/2 watts
a0 K / W W potentiometer
5 K/2 watts
270 K/2 watts RIO. 250 K/potentiometer R I ~Ria , , Rn. 10 K / 2 watts Rl*. 300/W W Dotentiometer RM. 200/2 watts Rli. lOK/W W potentiometer RIB. 1 megohm(10-100 K/2 watts) Ti. Constant voltage transformer or supply. Approx. 100 1' A required
RII.
A11 resistors in ohms;
all capacitors in microfarads
Table I. Dimensions of Cuvettes Diameter Inside Outside
~~
Length
Type
Capacity
Ml. 3.0 10 2.2 6.5 0.4 5.0 6.5 2.0 lo 10 8.5 10.5 6.0 Square, test tubeo 13.0 16.0 3 . 0 min. a I n one colorimeter, V-ways can be rotated 90°, leaving space near front of carriage where a detachable holder for cuvettes of this type is plaoed. Light beam passes through these cuvettes near bottom, so t h a t only 3 ml. of liquid are required. Cylindrical
Cm.
Mm.
Mm.
5
8.5
10.5
LITERATURE C I T E D
(1) Ellis, G. H., and Thompson, J. F., IND.EXG.CHEM.,ANAL.ED., 17, 254 (1945). (2) Garman, R. L., and Droz, M . E., Ibid.,7, 341 (1935).
(3) Kirk, P. L., Rosenfels, R. S., and Hanahan, D. J., AXAL.CHEM. 19, 355 (1947). (4) Lowry, 0. H., and Bessey, 0. A., J. Bio2. Chem., 163, 633 (1946). (5) MUlier, R. H., IND.ENG.CHEM.,AX.AL.ED., 7 , 223 (1935). (6) Turner, L. .1.,Rea. Sci. Instruments, 4, 665 (1933). (7) Vance, A. W., Ibid.,7 , 489 (1936). RECEIVED April 12, 1949.
