AIDS FOR ANALYTICAL CHEMISTS
Determination of Mercury in Water Marvin J. Fishman U.S.Geological Survey, Denver, Colo. 80225
RECENT PUBLICATIONS by Brandenberger and Bader (1,2) and Hinkle and Learned (3) describe methods for the determination of mercury in various media. The following describes a technique for the determination of mercury in fresh waters based on the above methods. Advantages of this technique are the ease in cleaning the silver wires and the availability of atomic absorption instrumentation. Mercury is collected from an acidified water sample by amalgamation on a silver wire. The silver wire is then electrically heated in an absorption cell placed in the light beam of a n atomic absorption spectrophotometer. The mercury vapors are drawn through the cell with a water aspirator and the absorption is plotted on a recorder. Samples containing at least 0.1 pg of mercury per liter can be analyzed directly; samples containing more than 1.5 pg/l. must first be diluted. None of the substances commonly occurring in natural water interferes, although iron (> 5 mg/l) and sulfide, if present, cause low recovery of mercury.
Table I. Atomic Absorption Conditions Grating Ultraviolet Wavelength counter 253.7 (2537A) Slit
Scale expansion Lamp current Source Noise suppression Chart speed
3 1x 350 mA
Mercury vapor-discharge lamp 1 20 mm/min
Table 11. Observed Absorbance Mercury concn, Absorbance /e/l. 0.0 0.10 0.25 0.50 1.0 1.5
0.013 0.041 0.099 0.187 0.362 0.530
EXPERIMENTAL
Apparatus. A Perkin-Elmer Model 303 atomic absorption spectrophotometer and a Perkin-Elmer Model 165 recorder are used. Operating conditions are given in Table I. The absorption cell is identical to that described by Brandenberger and Bader (1, 2). Reagents. All chemicals are reagent grade. Solutions are prepared using demineralized water. The silver wire is B & S gauge No. 30 and is cut into 250-mm lengths and formed into approximately 10-mm coils on a 4-mm diameter glass rod with leads about 10 mm long coming off each end in the same direction. To expel all traces of mercury, the wires are heated for a t least 2 hours at 750 "C in individual crucibles in a muffle furnace, just before use. Procedure. Samples to be analyzed for mercury should be filtered through a 0.45-pn membrane filter at the time of collection and the filtrate immediately acidified with HNOa. Clean all glassware with warm, dilute nitric acid (approximately 3 M ) and rinse with demineralized water before beginning the analysis. Pipet a volume of sample containing less than 0.15 pg of Hg (100 ml max.) into a 200-ml volumetric flask and adjust the volume to approximately 100 ml. Prepare a blank of demineralized water and sufficient standards, and adjust the volume of each to 100 ml with demineralized water. Add 10 ml concd HCI to each flask. Remove the silver coils from the furnace, let cool 5 minutes, and add one coil to each flask. Place the flasks in a shaker and mix overnight. Decant the sample solutions and rinse each coil four times with demineralized water and once with acetone. Transfer the coils to individual plastic vials, allow them to (1) H. Brandenberger and H. Bader, Helc. Chim. Acta, 50, 1409, 1414-15 (1967). (2) H. Brandenberger - and H. Bader, At. Absorptiojz Newslett., 6 , 101-103 (1967). (3) M. E. Hinkle and R. E. Learned, U.S. Geol. Sum. Prof: Pap., 650-D, D251-4 (1969). _
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air dry (approx. 2 min), and cap the vials until ready for analysis (avoid unnecessary delay). If the coils are gray in appearance, it is an indication of iron interference or a n excessively high concentration of mercury and the determination should be repeated with a smaller volume of sample. Connect the leads of the coil across the alligator clips mounted in the rubber stopper of the absorption cell (handle only with clean forceps), and replace the stopper. Vaporize the mercury by closing the circuit to the battery and record the resulting absorption. The aspirator should be adjusted so that maximum absorbance is reached approximately 5 seconds after absorbance begins. A wide area under the peak (compare with standards) may indicate a n iron interference or a n excessively high concentration of mercury; in such cases, the determination should be repeated using a smaller sample aliquot. Convert absorption to absorbance and determine the Hg concentration (Fgil.) in the sample from a plot of absorbances of standards. The absorbances in Table I1 are typical of the data obtained. Exact reproducibility is not obtained, and a working curve must be prepared with each set of samples. RESULTS AND DISCUSSION
The amalgamation of mercury onto the silver wire is complete in 5 to 6 hours and remains unchanged even after overnight mixing. Since the better part of a day is required for complete amalgamation, it was convenient to shake the solutions overnight before proceeding with the analysis. N o attempt was made to determine mercury concentrations below 0.1 pg per liter using the scale expansion provided on the atomic absorption spectrophotometer because the hydrochloric acid contains traces of mercury. The amount of mercury in the reagent acid will vary from one bottle to the next and high blank readings may be found. In our work, the
ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970
absorbance of the blank (demineralized water plus the acid) varied from 0.007 to 0.056. This corresponds to 0.02 to 0.17 pg of mercury per liter. The precision of the method was tested by replicate determinations on three natural-water samples over a period of 2 weeks. The standard deviations, based on five determinations at the 0.13-, 1.88-, and 87.8-pg/l. levels, were 10.04, j~O.21, and i ~ 7 . 1respectively. , Several natural-water samples were analyzed for mercury by t h e proposed method and found to contain less than 1 pg per liter. These samples were reanalyzed after adding 0.5 pg per liter of mercury to each. The recovery ranged from 80 t o 128%.
Three additional samples containing 80, 184, and 228 pg of mercury per liter, as determined by a dithizone extractioncolorimetric method, were analyzed for mercury by the silver wire-atomic absorption technique. The concentration of mercury found was 88, 188, and 208 pg per liter, respectively. Extreme dilutions were used because the upper limit of the procedure is only 1.5 pg per liter. The method has proved useful for the determination of trace amounts of mercury in samples of fresh water. RECEIVED for review June 4, 1970. Accepted July 13, 1970. Publication authorized by the Director, U.S. Geological Survey.
A Method for Calibration of Nuclear Magnetic Resonance Standard Samples for Measuring Temperature Osamu Yamamoto and Masaru Yanagisawa Government Chemical Industrial Research Institute, Shibuya-ku, Honmachi, Tokyo, Japan
IN THE PRESENT DAY high resolution nuclear magnetic resonance (NMR) measurement, variation of temperature is accomplished by passing heated or cooled nitrogen gas to a cylindrical Dewar vessel in which an NMR sample tube is inserted. The cylindrical vessel also serves as a detection coil support. By controlling the gas temperature by means of a suitable device, the sample temperature may be maintained within jZ1 “C. Unfortunately, however, there seem to be only a few reliable methods for determining the sample temperature, especially the temperature of the part where the NMR signal is detected. A conventional method which has been widely employed by Varian users is that of using a supplied “standard” sample. The temperature is obtained by measuring the chemical shift of a temperature-dependent signal of a standard sample which will provide corresponding temperature in terms of the supplied calibration chart. As standard samples, acidified methanol or ethylene glycol are usually employed. But actually the supplied chart does not seem to be sufficiently accurate, and it has been indicated that the standard sample method for measuring temperature with the aid of the supplied calibration chart is not suitable for studying the system involving spin exchange, where it is frequently desirable to extract kinetic data from the spectral pattern ( I ) . In this case, temperature is one of the major spectral parameters, and it should be determined as precisely as possible. Another method for determining temperature in the NMR sample comprises the direct measurement of gas temperature by a thermocouple or a thermistor suitably placed in the NMR probe. But this method also cannot provide actual temperature, because the heat-sensor cannot be placed near the detection coil. When the heat gradient, which is always present along the elongated cylindrical Dewar, is not maintained sufficiently small, or the flow rate of the nitrogen gas is not kept constant, the determined temperature would be different from that of the sample part where the signal is detected. Thus it was hoped to determine the actual temperature of (1) A. L. Van Geet, ANAL.CHEM., 40, 2227 (1968).
the sample readily and with reasonable accuracy, particularly for kinetic studies. Recently, some attempts have been made to solve the problem. Van Geet ( I ) proposed a method of measuring temperature of the sample in the NMR probe with a thermistor directly inserted in a spinning sample tube. This elaborate device comprises a thermistor probe covered by a tubing of polytetrafluoroethylene in the lower portion and a support which permits the thermistor to stay at rest around the spinning sample tube without touching its wall. The direct temperature reading is provided by a resistance bridge and meter placed as far from the probe as desired. He also built a spinning thermistor probe which spins along with the sample tube remaining closed in a pressure cap (2). He observed by this technique that the chemical shift between the CH2 and O H groups of ethylene glycol fits a straight line, while that of methanol does not, and should be approximated by a quadratic equation ( I ) . Forstn and his coworker (3) employed a method of calibrating temperature, which consists of making a capillary containing methanol-ethylene glycol mixture, and inserting it in a NMR sample tube together with a liquid sample of which NMR spectrum will be measured. Temperature is obtained from the shift of the OH proton of the mixture before and after an actual NMR measurement. The relation between the OH proton shift and temperature is previously determined by inserting the capillary into a xylene solution in the NMR sample tube, and measuring its shift values at various temperatures, which in turn are calibrated by a thermocouple inserted in the sample tube. Neuman and Jonas also reported another calibration method with a thermocouple in a spinning sample tube (4). Although the above methods succeeded in giving accurate measurements of the sample temperature in the NMR probe, they seem to put the operator to some bother in measuring temperature because of delicacy of the device or the trouble of (2) A. L. Van Geet, Rea. Sei. Instrum., 40, 177 (1969). (3) K.-I. Dahlqvist and S. ForsCn, J . Phys. Chem., 73, 4124 (1969). (4) R. C. Neuman and V. Jonas, J. Amer. Chem. SOC.,90, 1970 (1968).
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