Digital Solid-state Transistorized Apparatus for Analytical Measurements Athos Bellorno, Alessandro D e Robertis, Agatino Casale, and Domenico De Marco Institute of Analytical Chemistry, University of Messina, Messina, Italy
In a continuing program on theoretical and applied aspects of high frequency conductometry (1-4), the plan and the construction of an autonomous oscillometric system was of interest. The original apparatus presented in this paper is more versatile than the usual instruments constructed in a laboratory (5, 6) or available commercially (Sargent Model V, WTW model HFT 30, H.F.Titrator Touzart & Matignon, Oscillo-Titrator Type OH 302 Radelkis, Fischer Titrator). The utilization of FET in the oscillating circuit, in fact, gives high stability to the apparatus. In addition, the frequency discriminator detection system permits us to estimate the least values of frequency variations and, hence, of electrolytic conductivity or dielectric constant. Finally, the most original characteristic of the constructed apparatus is the direct reading on a digital millivoltmeter as a potentiometric variation of the frequency variation. It is possible to use a recorder, synchronized with an electric buret with our apparatus to obtain recorded automatic titrations of fast reactions. Apparatus. The apparatus consists of seven transistorized stages of which five are equipped with FET (Figure 1).The utilization of FET allows us to obtain the stability required for the high sensitivity of the apparatus, together with the high input impedance of the gate circuits. The first two stages form a Franklin oscillator (7) utilizing a FETBFW 11. The oscillating frequency of the LC circuit coupled with the gate of Tg may vary between 5.5 and 6.3 MHz; its value depends on the impedance of the cell containing the solution (1). The signal generated by LlCll is collected by the drain of Tlo and transferred to a separator and a large band amplifier (8) at constant amplitude between 5.5 and 6.3 MHz. LZC19 resonates at f = 6.3 MHz, L3Czz (central frequency) at f = 5.9 MHz, and L4Cz3 at f = 5.5 MHz. The output is therefore linear over a range of 800 kHz. The signal reaches L6 and L7 resonating at 5.9 MHz through Tlz. Inductance L7 together with diodes D7 and D8 form a frequency demodulator (9). Its output supplies a dc signal proportional to the frequency deviation of the signal applied to the primary L6. This direct current is applied, through the voltage divider R40, P6, P5 to the input of a digital millivoltmeter which is 199.9 mV full scale (f.s.). The signal for the writing recorder, for a f.s. of 10 mV, (1) A. De Robertis, A . Casale, D. De Marco, and A. Bellorno. Rass. Chim., 2, 121 (1973). (2) Atti SOC. Peloritana Sci. Fis. Mat. Nat., X V l l l , 65 (1972) (3) Reference 2, p 81. (4) A. De Robertis, D. De Marco, A. Casale, and A. Bellomo, Atti SOC. Peloritana Sci. Fis. Mat. Nat., X V I I I , 73 (1972). ( 5 ) E. Pungor, "Oscillometry and Conductornetry," Pergarnon Press, London, 1965. (6) A . Bellorno and G. D'Arnore, Atti SOC.Peloritana Sci. Fis. Mat. Nat., 5, 119 (1959). (7) D. Dobos, "Electronic Electrochemical Measuring Instruments," Terra Ed., Budapest, 1966. (8) S. Seeley. "Electronic Tube Circuits." Kogakusha Co.. Tokyo, 1967. (9) J. Hillrnan and H . Taub, "Circuiti ad irnpulsi digitalizzati," Bizzarri Ed., Rorna, 1969.
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is derived from the discriminator by means of the voltage divider R38,R43, R45. The apparatus is zeroed by means of C11 after having connected the cell. Every composition variation of the solution in the cell shows a numerical indication in absolute value. The variation towards higher frequency values (max f = 6.3 MHz) or lower (min f = 5.5 MHz) than the central value (f = 5.9 MHz) is furnished by the signal light L inserted in the circuit T5, T6, T7 and T8. The trigger T7-Ts (10) allows the commutation of the relays RL which pilots the signal light L to indicate negative values. The photo-resistance PH-R, set near L, has been inserted to reduce the trigger hysteresis. PH-R recompensates the polarization circuit of T7 in function with the commutation stage of the trigger. By means of L indication, it is possible to distribute within +1999, 0, and -1999 the indicated digital units of the frequency variation between 5.5 and 6.3 MHz. The power supply is stabilized within 0.01% for a variation of 20% of the electrical voltage by means of TI, Tz, T3, and T4. T3 and T4 connected to a differential amplifier compare the errors of the output voltage with the internal constant voltage generator made up of constant voltage diodes D1, Dz, D3,D4, and D5. Operational. The apparatus operates only a few minutes after its turn-on. The measuring cell containing the solution under examination is then connected to M and the digital display set in the zero position by means of C11. The titration may now be carried out. To carry out automatic titrations a recorder having a f.s. of 10 mV is connected to the output REC of the apparatus. Sensitivity of the Apparatus The versatility and sensitivity of the apparatus are shown by means of the comparison of concentration curves of 2 x 10-2NHC1 obtained by our apparatus (curve a in Figure 2) and Sargent Model V (curve b ) under the same experimental condition, the ordinates proportional to the maximum of the concentration curves. The same cell, the Sargent large model, was used to carry out the above-mentioned measurements. The acid was added into the cell containing 100 ml of water. It is to be emphasized that our apparatus allows us to carry out titrations in a wide concentration range without loss in sensitivity; in addition, the proposed apparatus still gives good results for concentrations higher than 4 x 1 e 3 N (curve a in Figure 2) when those of Sargent are almost nothing. To illustrate chemically the performance of the apparatus, two titrations at low and high concentration of the reagents were considered. In Figure 3, curve A is the titration of 20 ml of 0.4870N &Fe(CN)e with 0.5040N c u s o 4 in the presence of 0.5M KN03, while curve B shows the titration of 6 ml of 0.0100N &Fe(CN)6 with 0.0100N CuSO4, the latter under the conditions of formation of cu3[Fe(CN)& and the first one of KCulo[Fe(CN)6]7(11). (10) H. V . Malmstadt and C. G . Enke. "Digital Electronics for Scientists," Benjamin Inc.. New York, N.Y.. 1969. (11) A . Bellorno, A . Casale, and D.De Marco, Talanfa, 20, 335 (1973)
1 I
I I I I I
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Figure 1. Schematic of
r
the apparatus
Resistors (l/z W, tolerance 5%) R 1 = 1 Ohm, 2W R2 = 220 Ohm, 2W R 3 = 150 Ohm, 2W R4, R 6 = 150 Ohm, 1 w R 5 = 1.5 Kohm R,, Re, R22 = 470 Ohm Rg = 220 Ohm 1W Rlo = 5 Ohm RT1R , 1 2 . R 4 5 = 150 Ohm R13,R14 = 220 Kohm R15, R i a , R 3 4 = 820 Ohm R16 = 4.7 Kohm R T 7 = 68 Ohm R19, R3, = 1.2 Kohm R Z 0 = 56 Ohm R P 1 = 2 7 Kohm R Z j = 560 Ohm, 2W R241R Z 7 = 2.2 Kohm R25, R p 8 = 180 Ohm R26, R j 2 = 470 Kohm RZ9 = 4.7 Mohm R30 = 1 Kohm Rj3 = 82 Ohm RZ5, Rj6 = 270 Ohm R37,R 3 g = 10 Kohm Rj8. R43 = 22 Kohm R40, R 4 2 = 3.3 Kohm R4, = 15 Kohm R44 = 27 Kohm
Capacitors (25 WV) C i . C7 = 1OOOpF Cz = 2000 p F C3 = 100 pF c4, c6, c35, c 3 6 = 10 nF C5 = 1OpF Ca, C18 = 100 nF Cg = 5 p F Cio = 25 p F C11 = 10-100 pFvariable Ciz = 10 pF NPO c131 C l 5 ~ c161c20, C24. c 2 7 . Ci4 = 56 pF NPO C17 = 2 pF NPO C19, c 3 2 = 220 PF NPO c z i , c26 = 470 pF Czz. Cz3. Czg = 270 pF NPO c25 = 20 PF NPO c30 = 4.7 PF NPO c31 = 8.2 PF NPO C33, C34 = 3.3 nF Transistors T i = 2 N 3055 T2 = BC 140 T3, Tq = BC 107 T5, T6 = 2 N 3819 TI, T8 = 2 N 1984 Tg, Tio = B F N 11
= 4.7 nF
0
T i l , Ti2 = BFW 10 Diodes DT,Dz = BZX 61/C8V2 D3%D4, D5 = BZX 88/C6V2 Dg = BZY 88/C5V6 D7, Dg = 1 X 8055 Potentiometers P1, Pz = 1 Kohm-linear PJ. P4, P6 = 500 ohm-linear Ps = 4 7 Kohm-linear Inductors L> = 50 p H Lz, L3 = 27 p H L4, = 29 jLH L5 = 1 mH L7 = 32 p H Lg = 10 mH J = 100 mH Miscellaneous RS = Silicon bridge rectifier 35 V, 0.5 A S i . S p = switches RL = relays 12V, 420 Ohm PH-R = photo-resistance Philips ORP60 F1, F2 = fuses 250 mA TR = supply transformer 24V, 20W M = connection to cell DIGIT = digital voltmeter type VT 1290 Schlumberger
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1000
1000
~3000
-4000
~5000
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Figure 2.
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Concentration curves
The commercially available apparatus under the Same analytical conditions of curve A are not affected by the conductivity variations occurring in the cell.
4,' m' Figure 3.
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Titration curves carried out by means of our apparatus
( A ) 20 m l 0 . 4 8 7 0 ~K3Fe(CN)6 with 0 . 5 0 4 0 ~C U S O ~in the presence of 0.5M KNOj. ( B ) 6 ml 0.01N K3Fe(CN)6with 0.011'~' CuSOa
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CONCLUSIONS The versatility and sensitivity obtained by our apparatus are evident from the examination of the concentration curves and the titrations shown. Therefore, the apparatus is proposed as a valid instrumental complement in the resolution of inorganic and analytical problems such as the study of complexes in solution or precipitated, the determination of the dielectric constant, and all other applications where a resistive or dielectric constant variation may furnish helpful indications. In addition, the possibili-
ty of taking automatic measurements with great advantage regarding personnel and time without any loss in sensitivity and instrumental reproducibility is to be considered. ACKNOWLEDGMENT We thank C. D’Arrigo for his helpful collaboration. Received for review April 17, 1973. Accepted February 19, 1974. The present paper was supported by the Consiglio Nazionale delle Ricerche, Roma.
Use of a Microwave Oven in In-Process Control James A. Hesek and Robert C. Wilson Analytical Services, J. J. Baker Chemical Company, Phillipsburg, N.J. 08865
Microwave energy is receiving increasing use in the cooking of foods and in some industrial drying applications. Laboratory application for microwave energy has largely been in the research sector with little attention directed toward its use as an aid in routine analysis. The in-process control of inorganic chemicals manufactured by batch operations often requires partial analysis of the product during washing operations or before drying. Determinations required may include loss on drying, assay or metal content by titrimetric procedures, or the content of one or more impurities. Conventionally, a wet cake or paste is dried on a hot plate or in an oven (110 “C); for slurries, initial filtration is sometimes expedient. For some slurries, the elapsed time for drying may total 3-4 hours. Any measures that can be taken to reduce the laboratory time for such in-process assessment can often yield savings in plant costs and time, and improvement in utilization of equipment. EXPERIMENTAL Apparatus. A 550-watt Litton Minutemaster, Model 350 microwave oven was purchased in mid 1972 to assess its merit in the laboratory drying of in-process materials. This has recently been replaced with a Litton Minutemaster 70/20 with a 700-watt output. Our accumulated experience reveals that for most inorganic slurries and wet cakes, microwave drying can be accomplished successfully within 15 minutes. A portion of the slurry or cake (weighed if necessary) is placed in a porcelain or glass container and dried for a suitable period of time. Where experience has shown the material may spatter, a ribbed watch glass may be used as a cover. The establishment of an appropriate drying period is largely empirical. The water must be substantially removed (usually to less than 0.1%) and the compound must not decompose. Usually the appropriate drying time can be found with a few trials by drying a portion of the material to constant weight. The drying time may vary with sample size or the number of samples being dried. This time, however, is not always relatable to the number of samples. For example, it has been established that either one or six 20-gram samples of barium carbonate containing 68% water can be dried to constant weight in 15 minutes; 10 minutes is not adequate. (In a 105 “C drying oven, 3 hours was needed to reach a constant weight.) This indicates that small sample quantities do not approach a “full load” situation in the oven.
RESULTS AND DISCUSSION Materials dried successfully by microwave energy include carbonates of barium, cobalt, lead, magnesium, manganese, nickel, and zinc, and sulfates of barium, calcium, and lead. The savings in production time by the 1160
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earlier availability of dry basis analytical results has been significant, Our studies suggest that materials that can be dried in a conventional oven maintained somewhat below 150 “C can be dried successfully in a microwave oven. Copper(I1) sulfate pentahydrate, for example, loses all of its water of hydration at 150 “C; however, persistent microwave drying yielded only the monohydrate. Spent silica gel (20% water) can be regenerated to a useful desiccant in the microwave oven. A 50-gram sample reaches constant weight within 30 minutes; on subsequent furnacing of this material, the weight loss is typically only about 1.5% after 30 minutes at 400 “C. The drying of organic materials has not been of general interest. Some limited experience, however, may be noted: saccharin can be dried successfully, but the microwave heating of oxalic acid monohydrate resulted in the prompt sublimation of oxalic acid. Potassium biphthalate in the microwave oven showed a weight loss after 5 minutes, identical to that after drying for the conventional 2 hours a t 120 “C; however, on longer microwave treatment, continual small weight loss occurred. PRECAUTIONS Use of the microwave oven to dry small samples involves a situation where there is extremely small “load” of material capable of absorbing microwave energy. This situation normally exists for only short periods. Over 20 months, no effects to the magnitron tube were observed. However, a coincidental failure of the timer and the thermal safety switch resulted in a 20-minute cycle being extended to about 1 hour. At the end of the period, one of six samples of nickel carbonate was converted to the black oxide, and the Pyrex beaker was fused to the bottom of the oven. The magnitron tube was damaged during this cycle, probably by arcing, and heat damage to other parts made it uneconomical to repair the oven. Highly acidic materials should not be dried in a microwave oven, since the acid fumes may attack the unit. Small amounts of free ammonia, however, can be tolerated. Following good occupational safety practices, a microwave unit should be checked at intervals of 6 months to assure the absence of spurious radiation. Received for review December 13, 1973. Accepted April 8, 1974. This note is paper X in the Series, “The Practical Analysis of High-Purity Chemicals.”
