Polarographic Cell for Continuous Analysis of Flowing Solutions W. J. Blaedel and J. H. Strohl, Chemistry Department, University of Wisconsin, Madison, Wis.
Ab ALYSIS with the dropping mercury electrode (DME) in flowing solutions is well established, and measurements have been made successfully for many years. Literature references are given in a recent article (2). Various cell designs continue t o appear in the literaturs (1, 3 ) . X polarographic ce.1 for the continuous analysis of flowing solutions should be designed to have a low holdup volume with respect to the flow rates used, so that the response to changes in composition will be rapid. This paper describes the construction and characteristics of a simple Teflon-glass cdl with a smaller holdup volume than previously described cells. OLAROGRAPHIC
Construction. A ,schematic of the cell is shown in Figure 1. The axial 1/18-inch hole is drilled first on a lathe, and the ends are en:.arged ‘by drilling 0.010 inch undersize to receive the inlet and outlet tubes by press fit. The D M E and reference electrode holes are drilled (also 0.010 inch undersize) with a drill press to a de.3th a t which the drill point just enters the axial hole. The entry to the axitil hole is finished with a l/le-inch drill, care being taken not to go past the center of the axial hole. After drilling, the holes are examined and all burrs are removed with tweezers, burring tools, or sinal1 drills. The axial hole should be smooth; it should contain no rough spohs, burrs, or traps that might hold gas bubbles or mercury drops. Before pressing the glass fittings into the Teflon block, they are all ground to a taper slightly greater than that of the hole bottoms. The grinding is not critical; it may be done by rotating the fitting manually while holding it against the side of a glass cutting wheel. The D M E cs,pillary is ground with mercury flowing through it, so that it will not plug. A circle of about
DME CAPILLARY
1/16-inch diameter around the orifice should be left unground. In a n alternate version of the cell, a Tygon inlet tube is used instead of glass. The inlet hole is drilled t o the same size as the outside diameter of the Tygon tubing, which can be easily inserted and removed by hand. A Tygon outlet tube is not recommended because the mercury drops have a tendency to stick to the Tygon making operation of the cell more troublesome. Characteristics and Use of the Flow Cell. Figure 2 shows the cell response when abrupt changes in composition are made in the flowing stream. Data similar t o those i n Figure 2 showed t h a t on changing sharply from 0.2M NaCl solution to 0.01M CdClz in 0.2M NaC1, the current rose to 90% and t o 99% of the steady state value after passage of less than 0.5 and 0.75 ml., respectively, when t h e flow rate was less than 5 ml. per minute. Flushout volumes were measured by changing sharply from 0.01M CdClz in 0.2M KaC1 to 0.2M NaCl; here, the current fell to 0.1% of its steady state value above background after passage of less than 0.3 ml., when the flow rate was below 10 ml. per minute. The response volumes appear to increase for flow rates above 10 ml. per minute, but the cell can still be used satisfactorily up to flow rates of at least 90 ml. per minute. At high flow rates, fluctuations become large; at 90 ml. per minute, the fluctuations in the steady state current amount to about 4%. At a constant flow rate, and in the limiting-current region, i,/C values were shown to be constant within 10% by rough measurements on 0.2M NaCl containing lop2to 10-4LTfCdC12. However, the limiting current observed for a particular solution does depend upon the flow rate. Figure 3 shows the dependence of limiting current upon flow rate for two cells with axial holes of inch and 3/32 inch. In principle, the limiting current should
increase with flow rate, because electroactive material is brought more rapidly to the mercury surface. However, the drop size and drop time both decrease as flow rate increases, causing a decrease in limiting current. These two opposing effects can be used to explain qualitatively the shapes of the curves in Figure 3. Different DME’s were used in the two cells of Figure 3, which accounts for the different currents observed at zero flow rate in the two cells. For precise work, the flow rate should be controlled. I n the range below 20 ml. per minute, control to about 0.75 ml. per minute is necessary to control the limiting current to 3%. (Less precise control would be necessary a t higher flow rates, but such high flow rates are not generally of analytical interest .) Gravity flow at constant head is not generally recommended; any collection of mercury or air bubbles in the system causes fluctuations in the flow rate and also in the limiting current. The effects of such fluctuations are greatest a t low flow rates. Control of flow rate with a pump, such as a peristaltic type, is generally preferred, especially at low flow rates. The apparent resistance of the cell of Figure 1, measured with a Serfass conductance bridge (RCM 15), was 2200 ohms in a static system and 2600
1
1
fi
Figure 2.
Cell response
A. Solution changed from 0.2M NaCl to 0.01M CdCIz-0.2M NaCI. Flow rate, 1 ml. per
Figure! 1 .
Polarographic cell for flowing solution
minute. 8. Solution switched back to 0 . 2 M N a C l Sorgent XV polarograph. Sensitivity, 1 .O pa. per mm.; no damping; 1.40 volts applied cathodic potential (against an Ag-AgCI reference electrode in 6 M NaCl); opporent cell resistance, 2200 ohms; DME characteristics, f = 1.46 second, h = 6 8 . 0 cm., m = 1.64 mg. per second
VOL. 36, NO. 2. FEBRUARY 1 9 6 4
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a S/Tinch i.d. hole, required passage of 0.5 ml. and 0.9 ml. to reach 90% and 99% of steady state after a change from 0.2M NaCl t o 0.0liM CdC12 in 0.2M NaC1. A flushout volume of about 1.0 ml. was required to bring the current to 0.1% of its steady state value above hackground after a change from 0.01M CdCla in 0.2M NaCl to 0.2M NaCI. Axial holes larger than inch have a tendency to hold gas bubbles and are not recommended. ACKNOWLEDGMENT
200
0 120 160 IATF (MI /MIN )
Thanks are extended to M. Evenson for his help in machining of the cells.
Figure 3. Variation of limi LITERATURE CITED
Figurer on ~ u r v e sdenote intern01 d 0.01M. CdCldI.ZM NaCl
ohms at a flow rate of about 200 ml. per minute, when 6 M NaCl was used as the electrolyte in the Ag-AgCI reference electrode, and when 0.2111 NaCl was the flowing sample solution. Gas bubbles that enter the system are not trapped, but because of the small diameter of the axial hole, are carried
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ruption of the polarographic current. This is a very convenient characteristic, for it permits the cell to function unattended for long periods of time. Cells with larger axial holes were also constructed and studied. A cell identical to that of Figure 1, hut with
(1) Blaedel, W. J., Strahl, J . H., ANAL. CHEM.33, 1331 (1961). (2) Parker, W. J., Metal Ind. loo, 82 (1962). (3) Tamamushi, R., Momiyama, S., Tauaka, N., Anal. Chirn. Acta 23, 585 (1960).
Work supported in part by grant No. AT(ll-1)-1082 from the Atomic Energy Commission.
Useful Thin layer Chromatogra P'i y Techniqul T. L. Brown and J. Benjamin, Research Di
I.
N THE PHARMACEUTICAL INDUSTRY it 1s mandatory that the most powerful separatory techniques known he utilized to establish the purity of organic compounds. This is particularly true of the finished products sold to the public.. I t is also a necessary condition prior to the preliminary pharmacological evaluation ' of new synthetic organic substances. Thin layer chromatography has great utility in modern research chemistry because of its separatory efficiency, rapidity, simplicity, and relatively low cost. When one is analyzing large numbers of new synthetic organics with markedly different- chemical properties, it 'is convenient to use a nearly universal detect-
ing agent. It is the practice in this laboratory to spread a thin layer of silica-gel G onto borosilicate plates and then locate the spots on the developed chromatogram by charring the organic substances over a Bunsen burner flame subsequent to spraying with a sulfuric acid plus dichromate solution. This solution is prepared by saturating concentrated sulfuric acid with sodium dichromate and then diluting 1 part of this solution with 5 parts of concentrated sulfuric acid. It has been reported elsewhere (4),that aluminum plates coated with silica-gel G can be used satifactorily with sulfuric acid spray. It has been found in this laboratory that the commercially available silicagel contains smsll amounts of organic
on the acid-sprayed chromatogram (Figure 1). A number of techniques have been evaluated to eliminate this interference. One effective method of eliminating this interference has been described in the literature (3, 6 ) . This method consisted of prewashing the chromatoplate with a suitable solvent prior to use. This was accomplished by placing a chromatoplate in a chromatographic development chamber containing a small quantity of solvent. The solvent was allowed to flow, by capillary action, up the entiie length of the chromatoplate. The purified plate was then dried, activated, and used. This procedure, altbough effective, is unnecessarily wasteful of time and equip-
Figure 1. Chromatogram that was not methanol washed prior to development
Figure 2. Chromatogram that was methanol washed prior to development
Figure 3. Chromatogram illuminated b y reflected light only
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ANALYTICAL CHEMISTRY
