ANALYTICAL CHEMISTRY, VOL. 51, NO. 4, APRIL 1979
591
Automatic Device to Monitor and Terminate a Distillation Robert
R. Lowry
Department of Agricultrual Chemistry, Oregon State University, Corvallis, Oregon 9733 1
T o obtain t h e level of purity presently required for many chromatographic techniques, it is often necessary t o redistill solvents. This is true even with reagent grade material, as well as lesser grades and solvents t h a t have been stored for a period of time. Distillation is time-consuming and can be hazardous. If left unattended, broken hoses result in floods while overheated mantles result in damaged mantles and/or broken flasks, any of which may cause fires. These hazards are minimized using the device described below. Any failure of either the power or the water flow right up to the exit into the drain will result in the shutting off of both the water and the power. Further, an adjustable sensor permits the distillation to be turned off, both power and water, with a previously chosen level of solvent remaining in the still flask. This level can be chosen a t any time before or during t h e distillation.
-s v
HEATER
LINE
F1
11
52
C I
DESIGN All electrical connections and circuits are outside the still itself with only a float made of glass or glass and metal in the solvent chamber. Standard flasks and mantles are used; the only requirement is that the flask have two necks, one of which should be a 5 45/50. If necessary, all the circuitry can be solid state. T h e drawings shown, however, have an electromechanical relay and two manual switches. A low voltage system is used to minimize arcing and other electrical hazards. Figure 1 shows t h e equipment added to a regular still, in this case a 5-L flask and mantle. A glass guideway is made using a through-type 5 24/40 joint with a 12-mm upper diameter. This in turn sits upon a stock 5 24/40 t o 45/50 adapter. Inside. a float consisting of two glass spheres on the ends of a wire, is free t o rise and fall with the solvent level. T h e upper sphere is made of a n opaque glass. If necessary, t h e wire could be replaced with a fine glass rod or tube. Also shown is the sensor head containing a light source, PL; a light activated silicon controlled rectifier, PC; a switch, S1;
Figure 2. Schematic of device
Table I. Parts List for the Device
c1 c2 PL PC MR F1 T1
sv
CR s1 s2
Figure 1. View of flask, solvent, float, float guideway, and sensor head
0.1 p f , 6 0 V dc
100 pf, ,io V d c 18-24 V
General Electric L 9 1 1 F Potter & Bruinfield KRPlDG for mantle plus T1 1 4 V, 0.25 A secondary Sporlan, 115 V, NC SEKBA1 push button, NO Cherry = E 2 2 , NO
-
capacitor capacitor pilot light photo SCR relay fuse
transformer solenoid valve
bridge rectifier switch microswitch
and the resistors, R1 and R2, shown i n Figure 2 and listed in Table I. T h e sensor head height is adjusted by a rubber grommet stop that is on the glass tube. When the opaque sphere interrupts the light beam from PL to PC. the circuit path to the relay coil is broken. This can only be reset by manually closing switches S1 and S2 simultaneously. Switch S1 is a momentary push button and S2 is a flow switch consisting of a microswitch with a small plate of aluminum attached. This is located immediately above t h e drain and is so positioned that the exit stream of t h e condenser cooling water holds it in the closed position. T h e relay controls the 110-V power t o the heater and solenoid valve t h a t controls the water supply. T h e latter is "hard" plumbed into the line and contains a constrictor for t h e water flow. With this arrangement, any loss of a hose connection or breakage of the condenser immediately stops t h e water flow and turns off the power. In normal operation, once the distillation is proceeding and the solvent cut desired is being collected, it is necessary only
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 4, APRIL 1979
t o have the sensor head located a t the correct height for unattended operation. The device described has been in use for eight years without a failure of any sort. It has permitted the distillation of hundreds of liters of a variety of organic solvents safely and economically in a system that is essentially of all glass
construction with a minimum of technical attention.
RECEIVED for review December 18, 1978. Accepted .January 15,1979. This work was supported by the Oregon Agricultural Experiment Station, Technical Paper No. 4917.
CORRECTION Anodic Stripping Peak Currents: Electrolysis Potential Relationships for Reversible Systems In this article (Zirino, A.; Kounaves, S. P. Anal. Chem. 1977, 49, 5 6 ) , an inconsistency for the derivation of an equation for the half-wave ( E , L ) potential of a "polarogram" generated from peak currents or peak areas (charges) obtained by anodic stripping voltammetry has been found. T h e corrected equation should be
This expression differs from the previously derived equation by the "2" in
T h e difference occurs from our failure to integrate the equation for the surface concentration of the reduced component under conditions of constant flux (Shain, I.: Lewinson, J. ilnai. Chern. 1961. 33. 187.). We equated C R ( 0 ) with CR(0) which in practice sets the mean value of C R ( 0 ) over the interval to the final value of C K ( 0 )at the end of the electrolysis. T h a t this is incorrect can also be seen intuitively from the following. Since diffusion within the drop can be neglected, C R ( 0 )can be shown to increase linearly with t even a t very low overvoltages. T h u s C R ( 0 i lies between zero and CR(0) a t t , and C R ( 0 ) = C',iO);Z. The resolution of our data is not sufficient to clearly distinguish the factor of 2 experimentally. Differences in junction potentials and E" between our experimental conditions and those which produced reference values (Harned, H. S.; Owen, B. R. "The Physical Chemistry of Electrolyte as well as the lack Solutions": Keinhold: New York. 1958.), of comparative values for -,o and *,K make this impossible at present (Ben Yaakov, S.; private communication.). In accordance with the above. the corresponding equation for the half-wave potential of a stripping polarogram generated on a thin Hg film of thickness " i " should he
T h e expanded equation f o r t h e potential current relationship should be similarly corrected.
