A. Ford and C. E. Meloan Kansas State University Manhattan, 66502
I
I
An hexpenrive Conductivity Monitor for Column Operadon
This detector was built because the authors tired of avoiding ion exchange, gel permeation, and ordinary column chromatography experiments because they didn't have the funds available to buy a uv or ir detector and because something sturdy and portable was needed for undergraduate laboratories. The components for this detector cost less than $25. It fits into a 4 x 4 x 6 cabinet and weighs about 2 lb. It requires about one day to put together if classical wiring and soldering techniques are used and about two days if a circuit hoard is made. A Xerox copy of the circuit board can he obtained from the authors if desired. The detector was tested by measuring the response of various gums going through a Sephadex column. 50 id of a 0.1% aaueous solution of cum acacia (2 x 10-lo moles) produce a signal that easily Gives a recorder (1V) off scale after it has been diluted by HzO in going through a 3-ft column of Sephadex 6-10, All of the electrical components are standard and can be purchased through normal channels. Circuil Descriptions The circuit diagrams are shown in Figures 1-4. Figure 1 shows the total system with Figure 2 giving the details of the oscillator, Figure 3 the details of the power supply, and Figure 4 the operational amplifier bridge circuit. The circuit (Fig. 1) incorporates the basic operational amplifier bridge of Knudson, Ramaly, and Holcomhe' and is shown in schematic form in F i p r e 4. Using E for voltage, I for current, R for resistance, and G for conductance we can derive the transfer function for this The finanical assistance of the National Science Foundation is gratefully appreciated. Knudson. Ramaley, and Haleomhe, Chem. Instrumentation, I (4), 1969. %Tobey,G., Graeme, J., and Huelsman, L., "Operational Amplifiers-Design and Applications," MeGraw-Hill Book Co., New York, 1970.
circuit using the usual ideal assumptions associated with operational amplifiers2; that of infinite gain, infinite input impedance, and zero output impedance. Assuming no current enters the (+) or (-) inputs, E, is the RMS value of the bridge input voltage (a constant) and E, is the output voltage feed to the amplifier
Where E, is the common mode voltage at the (+) and (-) inputs. Solving for E,
The bottom leg is a voltage divider giving the common mode voltage
OSCILLATOR
BRIDGE
RECIlFlEl
bHPLIFIEII
Figure 1.
General wiring diagram.
Figure 2.
Oscillator circuit.
List of Components
7
R16 = 20,000 R17 = 6,800 R18 = R18 = 10 Rr2 = 25K pot pf = microfarads pf = picofarads Dl= D. = IN 3125 Cn = 0.01pf = Cz= Cs = 0.25pf50V mylar Cesl = 1-15 pf C( = 0.33 pf 15V electrolytic = Cs = C g = 0.01 fif disk ceramic C7 = 1000 pf 25 V electrolytic Clo = 10 pf 25 V electrolytic
The integrated voltage regulator, Silicon General SG 3501, may he obtained from Baddel Sales, Chicago, Ill. All other semiconductors may be purchased from Semiconductor Specialists, Chicago, 111. All other components may be purchased from Allied Radio in Chicago, Ill.
Figure 3.
Power supply circuit.
Es R~
+'I
Figure 4.
R3
'
3
_
Operational amplifier bridge circuit.
Volume 50, Number I , January 1973 / 85
were silver dipped mica and the resistors were 1%thin film. The diode may be any signal diode of 15 V or greater t a oabilitv. The one megohm potentiometer is a 1 turn trimmer and need be adiusted onlv initiallv. In this instrument i t was adjusted to-a 1 V P.P. output &d the buffer amplifier was used to vary the signal by varying R12. The output impedance of the buffer amplifier is a function of the 25 K potentiometer, Rlz, hut, for the circuit values shown. is calculated to be less than an ohm. The supply (Fig. 3) was an integrated model made from a Silicon General integrated circuit and is of the dual tracking variety. A small heat sink should be employed on the can to assure adequate heat dissipation near its680-mw rating
Combining equations and assuming Ro = RI
Since only E, and G , are variable, differentiation of E, with respect to G, should give us the slope of this equation a t any value of G,.
Thus the relation between cell conductance and output voltage is linear. If the bridge is balanced so that G , and Gb are equal and there is no output signal then any change in the conductance G , will give an output signal a t point A of
Conductivity Cell
An attempt was made to use a cell made of platinum wire imbedded in glass tubing but the cell impedance was t w high and the cell tended to drift with temperature. A cell (Fig. 51 was constructed to fit into a large crystalizing aish fitted with inlet and outlet hoses running to and from a constant temperature bath. Circular platinum electrodes of 1-cm diameter were welded to 2-cm pieces of platinum wire with an arc welder. The platinum wire was run through a small cork which was pushed into the end of a bent 10130 ground glass joint. The corks were covered with epoxy cement and a copper wire was soldered to the end of the platinum wire for electrical connection. These two electrodes were placed in ground glass fittings in apposite sides of the small volume cell. The uutput siennl a a s ],near in X u p t o n I - \ ' ot.tpur of thr entire C I T P UAI ~ plot . 0 1 ~ ~ n t l u c ~ a nversus c e M Y output ttmtchcn the rheorrticnlls expected i\mrnerr nhnut AG for nrautwe a n d pus)tive AG values
where G, is the cell conductance and Gb is the halancing conductance. In practice the conductance cell is balanced, using both potentiometer Rb and a srpall variable capacitor Cb until the voltage at point A is zero. Any change in the conductance of the ceil is then reflected in the voltage a t point A. Both positive and negative changes in conductance give an increase in the outnut voltaee. Cell connections should he short and of low capacitance. The voltage obtained a t point A is amplified by amplifier 4 and further amplified and rectified by amplifier 5 giving a total gain of about 30. Since tuned filters require precise matching of components and the primary noise source is 60 Hz line noise or 120 Hz ripple from full wave rectification, it was decided to use the high pass capability of the dc blocking capacitor as a high pass filter with the cutoff frequency % KRC somewhere between 120 and 550 Hz, Ra and Rs were thus selected as 3600 ohms and Cz and Ca as 0.25 pf. Rectification is done bv amnlifier 5. a half wave rectifier with a gain of 3, and thk o u t b t coubled directlv to a recorder. Dc offsets may be corrected by adjustment of Rlo. The oscillator circuit (Fig. 2) is a standard Wein Bridge oscillator with a buffer amplifier a t the output. The buffer amplifier is used to give a constant loading at the output of the oscillator. The output freauencv is controlled bv the values of R . and Cs which muit be well matched. ~ h output k frequent; is % pRC. Values of 25,000 ohms and 0.01 pf wereused bere giving a frequency of about 550 Hz. The capacitors
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-iiz Figure 5
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86
/ Journal of Chemical Education
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Cell consfruct~on
