AIDS FOR THE ANALYST Evaluation of Paper Chromatograms
For the nonpolarizable electrode, a saturated calomel half cell is used. The container, K , is made of plastic. Glass wool is immersed in the potassium chloride solution and exposed through a slot in the cover t o a porous porcelain tube, T , about 1.5 inches in diameter and several inches wide. This tube is supported on its shaft by rubber stoppers so that it can be rotated a t low speed (about r.p.m.) by motor .If?. The filter paper is supported by the drum and passed sl0.sly under the amalgamated gold wire. The whole unit is enclosed in a tight, transparent box with a removable front to facilitate the placing of the filter paper. Air in the box can be replaced by purified nitrogen through nozzle S t o eliminate the interference of dissolved oxvgen _ - ivith the oolarographic process. 1he nitrogen is first moistened bv bubbling through bottles containing Rat&; some wet glass \vl.Obl is also :lacedit the bottom of the box t o keep the atmosphere saturated with water vapor. In this way, drying out of the filter paper and evaporation of the potassium chloride solution are minimized.
by Direct Polarographic Scanning Alois Langer, Westinghouse Research laboratories, East Pittsburgh, Pa.
polarographic study of solid microelectrodes of the D goid, aplatinum, or gold-mercury amalgam type useful URI\G
(i?),
curves were obtained by diminishing the amount of supporting electrolyte and having the solid electrode just touch a filter paper moistened with the electrolyte being investigated. Relativelq slow scanning was used, so that the resulting current-potential curves resembled the shape of a normal polarographic wave ( 4 ) . Thus it was found that b y placing the filter paper on a porous porcelain block partly immersed in the saturated potasqium chloride solution of the calomel electrode, and using an amalgamated gold bead as the cathode, polarographic spot test evaluations could be made. The encouraging results of these tests led to the construction of a simple apparatus for the continuous evaluation of paper chromatograms viith inorganic or organic spots (Figure 1).
Figure 1.
EXPERIMEhTAL
Polarographic scanner for paper chromatograms
The polarizable electrode is a mercury amalgamated 40-mil gold wire, A , wound tightly in a shallow groove of a plastic wheel about 3 inches in diameter and held tightly by the tension of a small spring, B. The annealed gold wire was amalgamated b y immersing it in a mercury pool and brushing slightly. The wheel can be rotated s l o ~ d pby a 1-revolution-per-hour motor, Mi, attached through a flexible coupling. It can be detached from the motor drive by loosening screw C, thus allowing it t o rotate freely on its shaft, driven only by contact with the filter paper on roll T. Sidewise adjustments of the wheel can be made by sliding it along the shaft. Vertical adjustment was allowed for by the use of slotted bearings. This allowed the wire t o be pressed against the filter paper by adjusting the tension of spring D attached t o wiper E, which also served as the electrical connection for the wire electrode. 426
Many experiments Tvere made under varying conditions, mainly to establish the feasibility of the procedure rather than to find the optimum conditions for a given problem. First, individual droplets of solution ranging in cation concentration from 0.1M to 0.OlM were placed on the filter paper strip, xvhich had been previously moistened with the supporting electrolyte and blotted t o remove the excess. Potassium chloride, potassium nitrate, ammonium nitrate, and others in 0.131 or 0.01JI concentrations were used as supporting solutions. I n a limited number of actual chromatograms, the buffer solution used as a migration medium served also as the supporting electrolyte. To determine the presence of a reducible substance on the moistened paper, the polarizing potential of the wire electrode 11as made as negative as the amalgamated electrode n ould allou , Rithout having too high a background current. Depending 011 the electrolyte, this was about - 1.6 to - 1.8 volts. I n all cases, the useful range of potentials for the amalgamated electrode was lower than for the mercury dropping electrode with the same electrolyte. When the paper mas scanned b? rolling it between the wire electrode and the porcelain tube, a current 11as recorded as soon a s the mire made contact with a spot containing a reducible substance. The Sargent Model XXI recording polarograph a a s used for recording. Because the recorder paper moved continuously M ith time while a constant polarizing potential Fl-as maintained, individual spots n ere recorded as peaks. After a more or less sudden rise, the tops of the peaks corresponded to the size of the spot; they nere usuallj not flat but showed some fluctuations as indicated in Figure 2 4 . The fluctuations may be due to unevenness of the filter paper (therefore, a variable contact area) more than to concentration variations across the drop, because they can be minimized by the adjustment of the tension of spring D. For quantitative estimation of the ion concentration, it Tvas found that the average current is roughly proportional to the concentration over a wide range. This was shonn nhen spots of a confined area mere made x i t h different concentrations of the solutions and care was taken to prevent an evaporation of the supporting solution. Hovever, only a rough estimate of the amount could be made from average peak height and peak top length, because the width of the spot area was not considered. When the movement of the filter paper was stopped a t these peaks and a current-voltage curve taken with a stationarl electrode or b y rotating the wire vr ith motor to expose new surface of the wire to the paper, current-potential curves nere obtained. These waves closely resembled the ones obtained mith the mercury dropping electrode but did not have the characteristic oscillations, rllthough this is a procedure to which objections can be raised ( I ) , some useful information can be obtained. Here,
V O L U M E 28, NO. 3, M A R C H 1 9 5 6
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too, the half-wave potentials were a qualitative indication of the reducible ion present in the spot. Shifts in the half-wave potentials of the order of 0.2 volt or more were observed, but these may be partly due to the varying I R drop depending on the moisture content of the filter paper. No detailed experimental evaluation of these phenomena was made. Normal waves were usually obtained but sometimes maxims. were indicated (Figure 2,B). These maxima usually were not .so pronounced as those with solid microelectrodes in a. liquid medium, but showed some
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ic d
Modified Thermocouple for Peak Exotherm Measurement John F. Palmer, Jr., and Robert S. Bomer, Momonto Chemical Co.. St. Louis, Mo.
thermocouple has been successfully adapted A to the measurement of the peak exotherm temperature MINIATURE-SIZE
exhihited by unsaturated polyester resins undergoing catalytic cure. The need for a miniature thermocouple centers around the necessity of avoiding underestimation of the peak exotherm temperature in the preparation of castings of polyester resins. For example, in the preparation of a 100-pound casting containing 25% glass and having a specific heat of 0.32 (Sonnehorn, R. H., “Fiberglas Reinforced Plastics,” 1st ed., p. 116, Reinhold, New York, 1954) an error of 40” F. underestimation of peak extherm temperature would result in 1280 B.t.u. of unexpected heat being transmitted t o the mold. This error might have occured in the small sample determination beomuse of the larger heat capacity and slow response of the temperature sensing element. This could have caused a very serious situation to exist if proper means had not been previously provided for the removal of this extra heat load. Work with the gel time and peak exotherm procedure, as outlined in Military Specification MIL-R-?575A, April 27, 1953 [Smith, A. L., Proceedings of Sixth Annual Technical Session, Reinforced Plastics Division, Society of Plastics Iudustry, Inc., Sec. 1, p. 3 (1951) ]led to an investigation of thermocouples permitted by the speoification. Most satisfactory results have been obtained with a miniature iron-constantan thermocouple fabricated into a No. 18, 2-inch-long, stainless steel hypodermic needle. The couples themselves were prepared from 30-gage enameled and cotton-covered iron and constmtm thermocouple wire with the junction silver soldered into the tip of the hypodermic needle. The ferrule of the hypodermic needle was, in turn, soldered to the end of a piece of X 4 inch brass tubing which served as a handle.
Figure 1. Comparison of hypodermio needle t h e r m o c o u p l e a n d mdlike cornrneroial couple
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1.
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T o facilitate uniform placing of the thermocouple in the test tube, 8 centering ring and plug cap were fabricated from Teflon and attached to the upper shank of the thermocouple (Figure 1). Temperatures were recorded by a single-pen Leeds & Northrup Speedomax Type G strip recorder modified to have a l i n e a chart speed of 1 inch per minute. These small thermocouples, &s a result of the radically reduced mass of metal in contact with the test sample, conduct less heat away than conventional couples and thus indicate a higher temperature, this. being closer t o the true peak exotherm temperature that would be reached in s mold. T h e lower ma68 of the thermocouple a180 results in more rapid response to temperature changes, thus more accurately indicating gel and cure time.
