Anal. Cham. 1962. 54. 2628-2629
2620
without additional water added to them were not wet enough to he sensitive within the limits of error to confirm or deny the presence of water. The samples shown in Table I1 were prepared hy huhhling HBr gas through a saturated H,O.HBr solution of 8 cm depth, a t room temperature. The gas was then passed through an empty trap and then was analyzed for water content. The results are shown for the analysis, performed three times. In looking a t these results, i t must he remembered that no attempt was made to achieve reproducible conditions between experiments in relation to attainment of isothermal equilibrium between the phases. However, the moisture content of this “wet” HBr gas can he considered as a rough approximation to the equilibrium value. It is clear that the Matheson HBr gas is not a “wet” gas as it leaves the cylinder as compared to the HBr stream analyzed after passing through the water (see results of samples numbered 1through 10 on Table I and compare with those in Table 11).
I
ACKNOWLEDGMENT
npure 1. Sample cell.
column =ppm of H,O found” in Table I have the meaning that less water was found in the sample than was determined to be present in the pyridine itself. These negative numbers are due to random experimental error. The samples analyzed
The authors wish to thank M. Dariel, former Director of the Research Division of Bromine Compounds Ltd.,for suggesting this topic and constant encouragement. We are indebted to Chaim Amit, the present Director of the Research Division, for permission to publish this work. The fine technical assistance of Z. Tannenhaum is greatly appreciated. The entire staff of the Research Division is noted for their willingness to share their experience and for technical assistance whenever needed. REcEnm, for renew March 2,1982. Accepted August 16,1982.
Glass and Teflon Closed-Loop Pumping System for Ultraviolet Absorbance Measurements Dennis R. MlgneauH and R. Ken Forci’ D8pariment of chemistry, University of Rhode wand, Kingston. Rhode Island 02881
Frequently the simultaneous acquisition of ultranolet absorhance spectra along with the measurement of some other parameter such as electrode potentials is desirable during the course of an experiment. Also, in order that solution experimental conditions be maintained the same for both measuring processes, a closed-loop system with pumping is frequently the optimum situation. In addition, since ultraviolet absorbance measurements in long path cells are very sensitive to trace levels of contaminants, the design of the system must be such that the construction materials are inert and, if adsorption of trace element contaminants be possihly suspected, the system must he available for the proper cleaning procedures such as acid leaching. Many commercially available pumps contain a variety of2ifferent types of plastic components, and many of these components are in contact with the working solution. Ultraviolet absorbing plasticizers and contaminants, under mildly acidic conditions, can frequently he leached from these plastics and into the solution. Of a simple flow-through system We report here the connecting a Cary Model 210 ultraviolet-nsihle spectrophotometer with a potentiometric experiment. The apparatus was designed with the following criteria in mind (1)materials of construction do not contaminate the solution of interest,
(2) the mixing time for the whole system he minimal, (3) the installation of the apparatus and the removal from the Cary he simple and quickly accomplished, and (4) constant temperature in the solution he maintained. EXPERIMENTAL SECTION The instrumentation used is the Cary 210 spectrophotometer with 10-cm fused silica cells. The electrodes employed were a Coming m p l e Purpose pH electrode with either a double junction SCE or double junction Ag/AgCI reference electrode. The double junction was filled with 4.0 M NaNO, in a viscous aqueous carbohydrate medium, The electrode potentials were monitored hy a Commodore PET Model 2001 mimmmputer with the necessary analog and digital interfacing as described elsewhere (1). The potentiometric cell was therkostated from a 50-L water bath controlled by a Haake water heater and pumped through coils surrounding the IeaCtion The design of the pump is simple and straightforward (Figure 1,, A 4,2-cm hole was through the bottom of a Pyrex beaker and a Pyrex petri dish of approximatelythe same size was attached to the bottom of the beaker by the chemistry department glassblower. An 8 mm 0.d. Pyrex glass tube was attached tangentidy to the side of the petri dish to provide egrw for the solution. A ret- tube f ” the spectrophotometer comea over the top edge of the beaker and hack into the solution. A
ooo3-27ool82io35c282s~Ol.2SlO 0 1982 American Chnmknl Socishl
Anal. Chem. 1982. 54. 2629-2631
2629
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\
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0.4
y 0,3
I
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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 construction two evaluation experiments were 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
0.2
OS
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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 glassblowing involved 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
