Anal. Chem. 1982. 54. 2629-2631
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0,6
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9 Flgure 1. Schematic diagram of the closed loop pumping system: (A) spectrophotometer cell compartment; (8) 1O c m cells In posltion; (C) 4-cm Teflon stir bar; (D) solution outflow; (E) return tube; (F) sample reservoir; (G) electrodes in position; and (H) titrant inlet tube.
'
4.0-cm Teflon-coated stir bar was inserted into the bottom chamber of the pump and the pump placed on a Corning magnetic stirrer. The vessel was first enclosed in a copper mesh Faraday cage connected to earth ground. This shielding helps to eliminate noise pickup by the high impedance electrodes. Coils of Tygon tubing from the water bath encircle the copper mesh, and finally the apparatus was enclolsed in Styrofoam insulation in order to maintain thermal stability. The glass tubing is Pyrex. The back panel of the Cary was duplicated in black opaque Plexiglas and ports were provided for the solution to flow into and out of the spectrophotometer sample compartment. The connections between lengths of glass tubing were made by stretching 8 mm 0.d. Teflon tubing over a no. 3, cork borer and were then slipped over both ends of the glass tubing to be connected. The Teflon was heated with a hot air gun until the shrinkage made a tight fairly rigid connection. These connections can be disassembled a number of times without stretching and leakage. If leakage at a connector joint occurs, additional heating easily eliminates the problem. Connection to the 10-cm cells was made with ground glass connectors. The male ends of the ground glass connectors were covered with a thin layer of Parafilm and heated with a hot air gun until translucent. The cell was then connected while the wax was still hot. The 110-cm cell is held in position by elastic bands to ensure proper and constant alignment. After constructiontwo evaluation experimentswere conducted, one to evaluate the mixing time of the solution and the second to determine the practical wavelength range of the system and the stability of the base lime. A wavelength range in the W region (220-320 nm) was chosen because our current reasearch is in this range. However, because of the many W absorbing contaminants that could leach into the Bystem from such materials as rubber, Plexiglas, and plastic components,this region is also an excellent test of system cleanliness. The wavelength range over which the pumping system can be used is limited only by the transparency range of the cuvettes employed. In the first experiment the system was base lined with distilled deionized water in both cells. With the pump operating, 1 mL of 3.16 M nitric acid was injected into the 500-mL sample volume. The spectrophotometer was set at 302 nm, the maximum absorbance for the nitrate anion, and the absorbance was monitored as a junction of time. In the second experiment the system was once again base lined with distilled deionized water in both cells. Hydrochloric acid was added to produce a solution of about pH 1. Spectra were recorded every hour for 4 h to monitor any
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Figure 2. Determination of the mixing tlme for the pumping system. The spectrophotometer was set to A = 302 nm and was scanned in time. At time zero, 3.16 mmol of HNO, was injected into the circulating system.
contamination which may have leached from the materials of the system.
RESULTS AND DISCUSSION The utility of this system and its effectiveness in providing an inert environment for combined potentiometric-spectrophotometric studies were evaluated. In the first experiment a constant absorbance was observed within 30 s indicating complete mixing with a very reasonable amount of time (Figure 2). The base line was constant (less than dz0.002 absorbance unit change per hour) for at least 5 h indicating the construction materials are suitable for even the most sensitive measurements. The lower wavelength limit in the ultraviolet is determined by any absorbing species present or by scattering of suspended particulate matter, not by any characteristic of the system. In addition the system can be installed in less than 15 min and dismantled in a similar amount of time. This allows for much flexibility in an analytical laboratory. The system has been in use for several months in our laboratory and has proven to be highly reliable, easy to maintain and to clean, and easily kept free of contaminants.
ACKNOWLEDGMENT The assistance of Andy Kocsi for the glassblowinginvolved in the construction of the pump is gratefully acknowledged.
LITERATURE CITED (1) For&, R. K.; Boyd, J.; Harrls, E. Interfaces Computing 1982, 1, 59.
RECEIVEDfor review June 23,1982. Accepted August 27,1982.
Modified Static Mercury Drop Electrode Peter E. Sturrock" and W. Kenneth Williams School of Chemistry, Georgia Institute of Technology, Atlanta, Georgia 30332
The PARC Model 303 static mercury drop electrode (1) is a welcome addition to the classical DME and HDME electrodes. The PARC 303 may be considered an automated
HMDE electrode, and in this laboratory it has been found to have excellent reproducibility in voltammetric and chronopotentiometric experiments. However, we have developed
0003-2700/82/0354-262g$01.25/00 1982 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 14, DECEMBER 1982
some modifications of this electrode t o make it even more suitable for our purposes, especially for pulse experiments with a moderately fast potentiostat.
EXPERIMENTAL SECTION The first change is only in the external wiring. The PARC 303 has a follower amplifier between the reference electrode and the outputs on connectors J1 and 52. This amplifier circuit has a long time constant to serve as a noise filter. Unfortunately, this is incompatible with a fast potentiostat. Rather than modifying the follower circuit, it is easier to connect the reference electrode lead to the back-panel test point instead of to J1 or 52 and thus simply bypass the follower amplifier circuit. We have had no problem with this configuration using an electrode cable over 10 ft long to the potentiostat. While evaluating the above change, we found high resistance in the working electrode circuit. A theoretical calculation of the capillary resistance, based on the capillary dimensions and the resistance of mercury, gives a result of about 8 a, significantly lower than for a typical DME. However, measured values varied from a little over 100 Q to several hundred ohms, depending on the capillary selected and the tightness of the capillary nut. This results in an RC time constant of almost 1ms, much too long for our needs. Examination of the electrode valving system revealed the problem. The tip of the solenoid plunger articulates directly on the blunt upper end of the capillary, and electrical conductance is provided between the capillary bore and the metal valve body via a thin film of tin oxide deposited on the end of the capillary. The tin oxide film has a significant resistance, and this resistance varies from capillary to capillary and even with one capillary as it is used. Our solution to this problem is to modify the valve seat and seal system to allow mercury, instead of tin oxide, to serve as the electrical conductor. With this modification, working electrode resistances are measured as 8-10 and drop areas are reproducible to better than 0.5% relative standard deviation (RSD) at each of the three drop sizes as evaluated by rapid-sweep square-wave voltammetery of 10 pM cadmium nitrate in 0.1 M potassium nitrate. In addition, any entrapped air bubbles can be pumped out by attaching a vacuum system to the capillary tip. We have experienced almost no problem with solution creep up the capillary orifice or capillary clogging. Modified Valve Seat. The valve seat modification entails machining a new seat from 304 stainless steel. All dimensions of the original seat are duplicated except as noted below. The height of the valve-seat body is 60 thousands of an inch taller than the original and this entire amount appears as a thickening of the seat which separates the mercury reservoir from the capillary. The hole drilled vertically through the center of the seat is I f l e or 1/32 in. in diameter (we have used both with no appreciable difference in performance), which is smaller than the outside diameter of the blunt end of the capillary. Thus the tip of the solenoid plunger seats across the upper end of the hole, and valving is achieved at this point rather than on the top of the capillary. The hole is countersunk from below as much as possible without enlarging the diameter of the hole at its top. Modified Seal. Successful operation requires a redesign of the seal around the top of the capillary. The original seal is a short length of rubber tubing slipped over the top of the capillary and compressed between the bottom side of the seat and the top of the metal ferrule sealed around the capillary. Tension is controlled by a capillary nut that presses upward on the bottom of the ferrule. This seal has only to withstand a positive pressure of mercury from the reservoir. With the modified seat, the seal has to withstand a positive pressure of mercury from the reservoir when the solenoid valve is open and also a negative pressure (from the mercury in the capillary)when the valve is closed. The original seal of rubber tubing is not adequate for this increased requirement. The new seal consists of a metal sleeve with an “0” ring at the top. The metal sleeve is made as follows: outside diameter matches that of the ferrule (0.36 to 0.37 in.); inside diameter to allow the sleeve to slip over the top of the capillary (-0.16 in., no. 20 drill bit); height, 0.325-0.33 in. (see below). The top of the sleeve is countersunk with an 82” countersink until the top lip is -0.02 in. wide. An “0” ring (Ifl6 in. cross section, 9/82 in.
Flgure 1. (A) Original configuratlon of valve seat and seal. (6) Modified valve seat and seal: (a) valve seat, (b) solenoid valve stem, (c) capillary, (d) ferrule, (e)capillary seal, (f) metal sleeve, (9) “0”ring seal, (h) cavity filled wlth mercury. Note: Capillary nut and ferrule support are not shown.
0.d.) rests against the countersunk surface of the sleeve and the beveled top of the capillary. As the capillary nut is tightened, the “ 0ring is compressed against these surfaces and the bottom of the seat, providing an effective seal. Figure 1A illustrates the original configuration and Figure 1B illustrates the modifications of the seat and seal.
DISCUSSION The only problem we have encountered with this seal comes from the variation from capillary to capillary in the distance from the top of the ferrule to the top of the blunt end of the capillary. We have found this to vary from 0.34 t o 0.37 in. If this dimension is too large, the top of the capillary can be driven against the countersunk bottom of the seat and the capillary broken as the capillary nut is tightened. If this ring is not compressed tightly dimension is too small, the “0” enough. For this reason we keep on hand several metal sleeves
Anal. Chem. 1982, 5 4 , 2631-2632
with slightly differing ‘heights and select an appropriate size for a given capillary. Modified Capillary. After the above modifications were developed and testedl, PARC sent the authors a modified capillary assembly for evaluation. Commercial availability of this capillary probably will depend on evaluations by PARC. In this laboratory the modified capillary performed significantly better than the original with measured resistance of about 25 Q and greatly reduced tendency toward clogging. The new capillary has an enlarged metal ferrule with an “0” ring seal. In addition, the top of the capillary bore is enlarged into a small cup or well which the solenoid-plunger tip seats across. This configuration allows mercury to contact a much larger area of tin oxide which has to provide conduction for a shorter distance, thus developing a much lower resistance. Furthermore, the center of the plunger tip, where foreign matter or trash tends to accumulate, makes contact with the mercury in the cup rather than the small bore of the capillary. Foreign matter can float on this small pool of mercury and not be
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tamped into the capillary bore. CONCLUSION
For routine applications, the modified capillary should be quite satisfactory. However, if fast rise times are required, the lower resistance is necessary and the modified seat is recommended. The modified capillary can be used directly with the modified seat, and in this case the metal sleeve is not required. If an original capillary is used, the metal sleeve is necessary. LITERATURE C I T E D (1) Peterson, W. M. Am. Lab. (Falrfield, Conn.) 1878, Dec, 69.
RECEIVED for review August 9,1982. Accepted September 17, 1982. This work was supported by a cooperative agreement between the School of Chemistry of the Georgia Institute of Technology and the Analytical Chemistry Branch of the Environmental Research Laboratory, EPA.
Field Technique! for Monitoring p-Cresol in Streams R. A. Coleman,” R. ID. Edstrom, and M. A. Unger Department of Chemistry, College of William and Mary, Wiliiamsburg, Virginia 23 185
I?.J. Huggett School of Marine Science, College of Wllllam and Mary, Gloucester Polnt, Virginia 23062
Several methods for determining phenols in aqueous environmental samples are available (1, 2). These methods proved to be unsatisfactory for our purposes for a number of reasons: they were slow, offered low recoveries, or were insensitive to 4-hydroxytoluene (p-cresol), the phenol we wished to determine. The method we chose to modify is based on the use of a Fisher Phenol Analyzer which utilizes the bathochromic shift from 270 to 291 nm, when a phenol solution is made alkaline (3). This paper describes the adaptation of that instrument for flow-through water analysis allowing reliable determination aC p-cresol in natural waters in the low milligrams per liter range with analysis times of 30-60 s. The need for the modification was brought about by an experiment at the EPA-Environment Research Laboratory in Monticello, MN, which was designed to determine the fate and effect of p-cresol in natural waters. The experiment involved continuous dosing of a 1-km oval-shaped, recirculating stream with the phenol to a desired level of 8 mg/L. The transit time around the stream was not sufficient for the p-cresol to completely dlegrade or be metabolized, so analyses of the water coming back to the dosing point were required in order to properly meter in the rquired make-up p-cresol. The time required for ciampling and analysis could not exceed 5-10 min or the dosing would be out of phase with the analyzed water. After the initial moldification it was a simple matter with pumps, valves, and pipes to draw and analyze water from anywhere in the 1-km utream with the only time constraint being the transit time of the sample in the pipe. EXPERIMENTAL S E C T I O N Reagents and Standards. The standard p-cresol solutions wed to calibrate the analyzer were prepared in a 70-L polyethylene container from reagent grade p-cresol and stream water. The acid used was 12 N reagent grade HC1 diluted 1:1with deionized water. The base was certified grade NaOH (50% w/w) also diluted 1:l with deionized water. 0003-2700/82/0354-263 1$01.25/0
Apparatus. A schematic diagram of the flow-through phenol analyzer is shown in Figure 1. Intake lines and valves were assembled from 3/4-in.polyethylene garden hose and PVC garden hose valves. The intake water pump (Little Giant Model U-110) was rated at 120 L min-l. Two glass wool filters (Fl-F2) were constructed from 50 cm lengths of 5 cm i.d. PVC water pipe with the glass wool plugs held in position via fine wires placed through holes drilled in the bottom of the pipe. Two filters were used in series to allow continuous filtration of suspended solids in the stream water when the filters had to be changed. The standpipes (Sl-S5) were also constructed from 30 cm lengths of 5 cm i.d. PVC water pipe. Outflow from the standpipe was through a 10 mm i.d. glass tube inserted in the side of the PVC pipe 5 cm from the bottom. Water flow through this tube was regulated by adjusting the height of a 25 mm i.d. glass tube standpipe inserted through a one-hole stopper in the bottom of the PVC pipe. Once the desired flow-standpipe height relationship was established, outflow was found to be quite stable throughout the analysis. Water overflow into the standpipe (considered wastewater) was carried back to the stream via Tygon tubing. Connections between the different parts of the apparatus were also made with Tygon tubing. Acid and base solutions were stored in 40-Lpolyethylene carboys and were metered into the apparatus via Cole-Palmer Masterflex peristaltic pumps. The pumps had delivery rates of 20 mL min-l. A Fisher Phenol Analyzer was modified to accept a 10-cm flow-through cell by drilling hose entry holes into the cell compartment cover. Black rubber tubing was used for the cell connections to prevent stray Iight entry into the cell compartment. To inhibit water condensation on the cell windows and the electrical circuitry under field conditions, the analyzer was placed in a custom built Plexiglas box which was continuously purged with a slow stream of dry N2. Output from the analyzer was recorded on a standard 10 mV chart recorder. A pH meter was placed in the water flow at the end of the apparatus to assure a proper pH shift had occurred during the analysis. Procedure. Prior to calibration all standpipe flow must be checked to assure constant head pressure and uniform flow through the system. The black tubing connecting S4 with the flow-throughcell is purged of air bubbles to avoid sporadic flow and/or spurious readings. The acid and base pumps are checked 0 1982 Amerlcan Chemical Society
