Anal. Chem. 1982, 5 4 , 1216-1217
Determination of Active Sulfur Trioxide in Sulfur Trioxide-Pyridine and Sulfur Trioxide-Trimethylamine Complexes David H. Sieh" and John M. Dunham Squibb Institute for Medical Research, Analytical Research and Development, Georges Road, New Brunswick, New Jersey 08903
Sulfur trioxide-pyridine complex is a versatile reagent widely used in synthetic procedures (1). In our laboratories, this reagent is used in various sulfation processes. Large variations in the yields from the sulfation processes have occurred with different batches of sulfur trioxide-pyridine complex. Since the complex is extremely hygroscopic and degrades in the presence of moisture, some batches of the complex are contaminated with the pyridine sulfate hydrolysis product. Consequently, an assay for the determination of the active sulfating moiety in the sulfur trioxidepyridine complex was needed. Two procedures for assaying sulfur trioxide-pyridine complex have come to our attention. Eastman Kodak (2)uses an infrared method that measures the ratio of absorbance for pyridine sulfate and pyridine-sulfur trioxide as a function of time. Extrapolation of the graph of percent pyridine sulfate vs. time back to t = 0 provided the amount of pyridine sulfate in the sample. Guiseley (3) reported a titrimetric method to determine the active sulfating reagent in sulfur trioxide-dimethylformamide complex. The complex was allowed to hydrolyze in water and react overnight in anhydrous methanol, and the resulting solutions were titrated with sodiym hydroxide. The amount of active sulfating reagent was calculated from the difference between the two titrations. Our procedure also involves measuring the difference between two titrations but the assay can be completed in 2 h. When sulfur trioxide-pyridine complex is dissolved in water, 1h is required for complete hydrolysis. However, when the complex is dissolved in a 0.1% water in pyridine solution, complete hydrolysis takes place in a maximum of 5 min. This enhanced rate of hydrolysis allows the quantitation of the reaction by a Karl Fischer titration. Determination of sulfur trioxide by Karl Fischer titration is not without precedent. For example, a procedure has been published (4)for the determination of sulfur trioxide in fuming sulfuric acid based on the hydrolysis of the material followed by titration of the excess water with Karl Fischer reagent.
EXPERIMENTAL SECTION Apparatus. A Photovolt Aquatest IV coulometric Karl Fischer titrator was used. All glassware was oven dried and allowed to stand for 1 h at room temperature and humidity before use. Reagents. All water was distilled before use. Sulfur trioxide-pyridine complex was prepared in-house or obtained from Aldrich Chemical Co. Sulfur trioxide-trimethylamine complex was obtained from Aldrich Chemical Co. Pyridine was obtained from Fisher Scientific. Preparation of the Karl Fischer Apparatus. The Karl Fischer apparatus was readied for operation following the guidelines found in the instruction manual. Enough titration vessel solution was withdrawn to account for the volume of liquid to be added (usually 25 mL was sufficient). Because of the dilution of the vessel solution by the added pyridine, the water standard was periodically rechecked. The water standard was prepared in a concentration of about 100 pg/pL of anhydrous methanol. If the amount of water titrated for the standard differed by more than &2.5%, the vessel solution was changed an! the standardization repeated. A 0.5-min stir time was programmed into the apparatus to allow complete delivery and mixing of the sample. Determination of Water in Pyridine. In a well-ventilated hood, 50.0 mL of pyridine from a freshly opened bottle was transferred into a glass-stoppered 125-mL conical flask. With a volumetric pipet, a 3.0-mL portion of this pyridine was with-
Table I. Determination of Active Sulfur Trioxide in Various Sulfur Trioxide-Pyridine Preparations batch
% active SO,a
6 months old, prepared in-house fresh sample, prepared in-house freshly opened, Aldrich Chemical Co.
35.7, 35.3, 35.6 45.6,45.7,45.a 46.6, 46.0, 46.4
a The theoretical SO, content is 50.3%. The values listed are triplicate titrations from a single weighing.
drawn and delivered below the surface of the solution in the titration vessel. After the titration was completed, the reading (micrograms of water titrated) was recorded. The determination was repeated and the average calculated. If the titrations did not agree within &2.5%,replicate titrations were performed until the desired precision was reached. Distilled water was then added to the remaining dry pyridine in the freshly opened bottle to increase the water content to approximately 1mg/mL (0.1% v/v). Determination of Active Sulfur Trioxide. By use of a volumetric pipet, 50.0-mL portions of wet pyridine (1mg/mL) were transferred into each of two glass-stoppered 125-mL conical flasks. Since sample handling was found to be crucial to this assay, the following weighing procedure was followed. The top 5 mm of sample was discarded and the remaining material mixed well with a spatula for about 15 s. The sample vial was then accurately weighed. Approximately 150 mg of material was transferred to one of the conical flasks, taking care to exclude lumps. The flask and the sample vial were resealed and the sample vial was reweighed. Both flasks were placed in an ultrasonic cleaner for 30 min to ensure complete dissolution of sample. The water content of 3.0-mL portions of the resulting solutions was determined then as described previously. Duplicate titrations from both flasks were carried out. The active sulfur trioxide was calculated according to 50 80.06 ( B - S)- 3 ia.01 ,n2 % active Pn (1) simplifying % active SOB= - X 7.41
where B is the amount of water titrated for the wet pyridine in micrograms, S is the amount of water titrated for wet pyridine plus sulfur trioxide-pyridine complex in micrograms, and W is the weight of the sample in milligrams.
RESULTS AND DISCUSSION The precision of the titration and assay was good. For example, multiple assays using 3.0-mL portions of a sample of wet pyridine and wet pyridine plus sulfur trioxide-pyridine complex yielded relative standard deviations of 0.8% (n = 6) and 1.2% (n = 6), respectively. When several samples from the same batch of sulfur trioxide-pyridine complex were assayed, the relative standard deviation was found to be 1.9% (n = 6). Three batches of sulfur trioxide-pyridine complex were assayed with the results given in Table I. The 6 month old sample that was prepared in-house had been exposed to moisture several times. A 6 month old batch of sulfur trioxide-pyridine complex that had been prepared in-house was found to contain 35.5% active sulfur trioxide (Table I). In a freshly prepared batch,
0003-2700/82/0354-1216$01.25/00 1982 American Chemical Society
Anal. Chem. 1982, 5 4 , 1217-1219
the active sulfur trioxide was found to be 45.7%. Therefore, storage and handling of the material are critical in maintaining potency. Other solvent [systemswere used in an attempt to improve the method. Sulfur trioxide-pyridine complex was found to be very soluble iin dimethylformamide (DMF) and reacted rapidly with the water in the DMF. However, DMF was found to be a poor solvent for Karl Fischer determinations. Wide variations (f20S6) were noted when a 0.1% water in DMF solution was assayed repeatedly for water content. Acetonitrile was found to be an excellent solvent for Karl Fischer determinations and the complex was very soluble in it. However, the reaction of sulfur trioxide-pyridine complex with water in acetonitrile was too slow to be used advantageously. This method a m also be used to determine the active sulfur trioxide in sulfur trioxide-trimethylamine complex. A sample from a freshly alpened bottle assayed 56.0% active sulfur trioxide (theoretical value 57.5%) with a relative standard
deviation of 2.0% (n = 6). This indicates general utility for the method. This assay can be performed easily with a minimum of effort and, since the Karl Fischer apparatus is ubiquitous in analytical laboratories, a minimum of cost. Another advantage is that it allows the chemist to determine the activity of a sample of sulfur trioxide complex before using it in a reaction.
LITERATURE CITED (1) Fieser, L. F., Fieser, M. "Reagents for Organic Synthesis"; Wiley: New York, 1969; Vol. I,pp 1127-1129 and references therein. (2) Hawryiuk, R. M., Laboratory & Specialty Chemicals Division, Eastman Kodak Co., Rochester, NY, personal communication. (3) Guiseley, K. 8."Carbohydrate Sulfates"; Schwelger, R. G., Ed.; American Chemlcai Society, Washington, DC, 1978; Chapter 9, pp 152153; ACS Symp. Ser., No. 77. (4) Mltchell, J.; Smith, D. H. "Aquametry", 2nd ed.; Wlley: New York, 1980; Part 111, pp 823-824.
RECEIVED for review December 7,1981. Accepted February 16, 1982.
Flow Cell folr the Determination of Mercury in Water by Electrodeposition Followed by Atomic Absorptlon Spectrometry Darryl A. Frlck' and Dennis E. Tallman" Department of Chsmlstry, North Dakota State University, Fargo, North Dakota 58705
The electrodeposition (ED) of trace metals followed by atomic absorption (AA) determination of the deposited metal is a combination technique (EDAA) having several inherent advantages (1-5). The electrodeposition step effectively preconcentrates the analyte and removes it from the sample matrix, thereby extending downward the working concentration range of the AA measurement step and eliminating many matrix interactions which might otherwise interfere in the AA determination. A further potential advantage of EDAA is the ability to speciate certain elements by selective electrodeposition under controlled conditions (5). A convenient approach to EDAA is to carry out the electrodeposition at a graphite tubular electrode which can subsequently by placed directly into an electrothermal atomizer (4, 5). In this report we describe an electrodeposition flow cell which uses a commercially available graphite furnace tube as the working electrode in a thin-layer, flow-through configuration. The thin-layer design enhances deposition efficiency and, hence, sensitivity compared to deposition carried out with an open tubular electrode. Application of the flow cell for the determination of mercury in water is demonstrated.
EXPERIMENTAL SECTION Instrumentation. Electrodepositions were performed with a PAR 174A polargraphic analyzer (EG&G Princeton Applied Research, Princeton, NJ), the tubular graphite working electrode flow cell described below, a counterelectrodeof 52 mesh Pt gauze 52 mm square contained in an isolation bridge, and a AglAgC113.5 M KC1 reference electrode. Solution was pumped through the flow cell using a :Masterflex variable speed peristaltic pump equipped with a 701.6 head (Cole-Parmer Instrument Co., Chicago, IL). Atomic absorption measurements utilized a Perkin-Elmer Model 603 spectrometer equipped with the HGA-2100 graphite tube furnace (Perkin-Elmer Corp., Norwalk, CT). Absorbance peak heights were recorded digitally by the 603 microprocessor Present address: PPG Industries, Research and Development,
Coating and Resin ]Division, Springdale, PA 1.5144.
and absorbance-time profiles were recorded on a Heath/ Schlumberger Model EU-205-11 strip chart recorder. Noncoated graphite furnace tubes (0.2222 A 0.0005 in. i.d., Perkin-Elmer Corp.) were found to give higher sensitivities than pyrolytically coated tubes, probably due to a higher surface area for deposition, and were used throughout this work. Reagents. Solutions were prepared in distilled, demineralized water (four cartridge Milli-Q system, Millipore Corp., Bedford, MA). Nitric acid was Ultrex brand (J.T. Baker Chemical Co., Phillipsburg, NJ). A11 other chemicals were reagent grade. Flow Cell Design. The flow cell incorporates a nylon insert which is centered coaxially inside a graphite furnace tube so as to force solution flowing through the tube into a thin layer adjacent to the tube wall. The nylon insert, 4.50 in. long, varies in diameter along ita length as follows: for 0.30 in. from each end the diameter is 0.17 in. (for easy coupling to flexible tubing) and for 0.60 in. beyond that the insert is sized for and threaded with 12-24 threads. The central 2.70 in. of the insert is precision machined to 0.213 in. in diameter. A 0.104 in. diameter channel is drilled coaxially down the center of the insert to a depth of 1.70 in. from each end, each channel intersecting and terminating at a set of two orthogonal bisecting holes 0.052 in. in diameter, forming entrance and exit ports for solution flowing through the graphite tube (Figure 1). The graphite tube is centered around the insert and held in place by means of cylindrical Teflon centering pieces 0.75 in. in diameter with a 45O bevel at one end tapering to a 0.213 in. hole drilled coaxially through the piece. The graphite tube is positioned on the insert followed by the Teflon centering pieces which are forced against the graphite tube by means of cylindrical Teflon nuts (12-24 threads) 0.5 in. in diameter and 0.5 in. long. The sample introduction hole in the graphite furnace tube was sealed prior to electrodeposition with a small piece of adhesive tape which was removed before installing the tube in the HGA2100 atomizer. An alligator clip was used t o establish electrical connection with the tubular electrode. Components of the flow cell and an assembled flow cell are shown in Figure 1. The solution flow path through the cell is depicted in Figure 2. The thickness of the thin layer flow channel is 0.0046 in. and length of the thin layer region is 1.10 in., yielding a volume of 57 pL for the thin-layer region. A Reynolds number calculation for flow rates used in this study indicated that the flow is laminar (6).
0003-2700/82/0354-1217$01.25/00 1982 American Chemical Societv