814
ANALYTICAL CHEMISTRY
3.622. Mold appeared in five other saturated solutions at ages of 15, 8, 30, 30,and 8 days. After 17 days the p H of these solutions had increased by 0.000, 0.040, 0.008, 0.001, and 0.028 unit, respectively, All'pH value5 were computed from the e.m.f. at 25" of a doulile hydrogen-electrode cell in which a saturated solution of potassium chloride was interposed between the trvo electrode compartments. A phosphate buffer mixture 0.025 X with respect to both potassium dihydrogen phosphate and disodium hydrogen phosphatr, pH 6.860 a t 25" ( 2 , S), served as a reference standard. The author's experience fails t o confirm Lingane's statement ( 4 ) that. solutions of potassium hydrogen tartrate appear more stable than those of potassium hydrogen phthalate. The pII of a 0.05 Jf phthalate buffer solution, prepared and preserved in the same manner as the tartrate solutions, was found to have increased 0.005 unit in 130 daj-s. The stability of Lingane's tartrate solution over an extended period illustrates the diffwences that may be encountered as the result of chance contamination by molds and other microorganisms. Inasmuch as sterilr conditions are not conveniently maintained in the preparation and use of tartrate p H standards, it is important that these wlutions l x ~made as needed.
vides compensation for line voltage fluctuations. The grid of 1-2 reaching a potential PI will 6re the tube, thus operating the relay. Cpwill now discharge through I, and Ks. When the voltage of C2 has dropped sufficiently to reduce t'he potential at the grid of 1.2 to P2 the tube will suddenl!- stop conducting, allowing the relsy to drop out and thus initiating another cycle. The other set of contacts on L supply power to the outlet 1 ' ~ c*c,ptacles. The pilot lights indicate the duration of, the timed intervals, permitting easy calibration. Switch So will turn off both outlets without shutting off the timer. Sawill turn on outlet .VO without affecting the timer. This swit,ch could IF :i double-pole switch and turn on both outlets.
54
LlTER.4TURE CITED
Figure 1. Diagram of Timer-Controlier
(1) Bates, 11. G.,Chevi. Rccs., 42, 1 (1948). C,. ( 2 ) Bates, R. G., Rnd Aciee. H. F.,J . Rcseirrcii S o t i . Bur, . S t r n ~ ~ r l u ~ d s , Cz. 34, 373 (1915). (3) Hitchcock, D. I., and Taylor, .4. C., J . A F J ZC.l i o ~ , . SI^,, 59, 1812 (4)
(1937). Iingane, ,J. J., h . 4 L .
&Eli.,
19, 810 (1917).
Electronic Timer-Controller.
Donald 11. Peppard, I\-yandotte Chemicals Corp., Wyandotte, Mich.
the past few years magnetically operated still heads D for distilling columns (Oldershaw, Piros-Glover, D u Pont, and others) have become increasingly popular. A necessary auxiliURISG
ary instrument with such a still hea,d is a timer-controller to operate the valve and control the reflux ratio. The timer-controller described is simple, inexpensive, accurate, and extremely easy to use, and its reliability has been demonstrated by steady use for 3 years in this and other laboratories. I t has all the flexibility associated with the electronic timers previously described (3)plus certain added features. Thus, the present instrument has a circuit compensated for line voltage variations, and the time intervals are independent of the relay adjustment and remain constant for the useful life of the tube. Furthermore, it is usually possible to replace the tubes without having t o make compensating adjustments. The circuit uses a condenser-resistor combination as the basic timing element (3). It consists of a condenser whose charge and discharge times determine the off and on periods of the controlled circuit. The desired periods may be varied over a wide range by the choice of resistance and capacitance values. Those given are for a particular model designed for distillation purposes and will serve as guides to the constructor building a unit for other purposes. Figure 1 gives a diagram of the timer. The transformer, 2',supplies the filament voltages for tubes VI and V2, and also the high voltage for VI. VI rectifies the high voltage supplying about 250 volts across RIRp. At the start of a vycle condenser C p is charged through the resistors of switch Sa and relay L. C? is connected t o the grid of V pthrough 1 1 7 arid Ra. The grid potential of V2 therefore rises as C? is charged. Superimposed on this direct current voltage is an alternating current voltage, from CS,Rs through C,, which is 90 electrical degrees ahead of the voltage applied to the plate of T,i ( 1 ) . This leading voltage does several things-it ensures that V 2will start conducting and stop conducting instantaneously, thus making the timing interval independent of the relay t y p e or adjustments; it minimizes the effects of individual tube characteristics; and it pro-
1 mid., 600 volts .i t o 10 mfd. Hermetically
sealed with high internal resistance. Ceresin wax or silicone used t o stop surface leakage a t all high impedance points C3. 4 mid., 450 volts electrolytir, Cd. 0.02 nifd., 600 volts. Hrrmetically sealed n.ith liieh internal resistance Cs. 0.05 mfd.. 600 volts I,. D.P.D.T. relay, 1-10 kiloohms d.c. Cereiiri was or silicone used to stop snrface leakage a t all high impedance points PI,1'2. NE51 neon XI. 10,,000 ohms, 10 watt-, adlustable
Rs.
10,000 ohms, 10 u-atts
Ha,R I . 200,000 ohms, 0.3 x a t t
0.5 megohm 500 ohms, 5 watts 127. 5 megohms. 1 watt Rs. 1 mewohm 1 watt Ro. 4500 "om,' 1 watt Rla. 60,000 ohms, 0.5 watt ,Si,S 2 . SI.S.P.S.T. toggle svitcti .S,, Ceramic selector siritch (Mallory 17%). Resistors chosen t o suit time intervals desired (0.22. 0 . 2 2 , 0 . 2 2 , 1.2 megohm-, etc.) 7, 240-0-240, 6.3. 5 volts 11. RY3 1-2, 2050 1'. Fuse, 1 amp. .YC. Outlet .YO, Outlet Rs. Ra.
Four timers have been built and used to operate magnetic still heads, reciprocating stirrers, and magnetic pumps ( 4 ) . Like a11 devices using relays, the contacts may require occasional cleaning. The frequency of servicing is determined to a large extent hy the amount of dust and fumes t o which a given relay design is SUI)jected. Sealed relay contacts, if available, will, in most instance., eliminabe any trouble due to this cause. Relay coil resistances are not critical within the maximum current rating of the t.ube, 1000 ohms being a safe lower limit. Above 3000 ohms it is suggested that the value of R6 be increased or the value of C3 decreased, so that the time constant of the relay circuit will not be made too long, particularly when time intervals of less than I second are desired. With reference to the voltage divider, 83,heating the resistors while soldering them in place may alter their values. If this happens, t'his resistor is discarded and replaced with one with longer leads. I t is bett,er and easier to use two resistors in series or parallel to get the exact value desired, rather than to buy expensive precision resistors or try t o select a particular value from an assortment. If a suitable transformer is not available, a high voltage supply may be made by substituting a selenium rectifier volt'age-douhler for the transformer-rectifier tube combination shown. The heater of the 2050 could be operated by a small transformer 0 1 i n series with a resistor across the line. LITERATURE CITED
(1) Cockrell, W. D.. "Industrial Electronic Control," p. 186, Sew York, M c G r a w - H i l l Rook Co., 1944. (2) Electronics, 14, Reference charts. 33-64 (June 1941). (3) Harvey, R. B., IND.EXG.CHEM.,Ax.4~.ED., 18, 331 (1946). (4) Oliver, G. D., Bickford, JV. G., Todd. 9. S., and Fynn. P. J., Ibid., 17, 158 (1945).
