ANALYTICAL CHEMISTRY, VOL. 51, NO. 2, FEBRUARY 1979
increased linearly with the amount of HMDS or MSA added. Furthermore, the concentrations of HMDS and MSA obtained by this standard addition technique were in good agreement with the initial analysis. T h e BSA solutions used in analyses were found to be unchanged after two days a t probe temperature. Other samples were prepared and analyzed after being heated at 60 and 90 "C, respectively, for 30 min in the variable temperature probe. N o significant decomposition was detected. As expected t h e addition of water to solutions under test produced rapid hydrolysis of BSA t o H M D S and MSA. 0 - S I (CH,), --+ (CH3I3Si-0-Si(CH,),
2 CH,' C XN-Si
- 2CH,-C:/
(cH,),
0
NH Si(CH
,I3
Subsequently, measured amounts of water (0.02-mL aliquots of a 4% w / v solution of water in pyridine) were added and the production of H M D S and MSA was monitored. Results after regression analysis confirmed t h a t each mole of BSA yielded 0.5 mol of HMDS a n d 1.0 mol of MSA as expected. This confirms the accuracy of our method. No significant decomposition occurred when solutions were treated with dry
313
oxygen, and sparging with nitrogen produced no notable improvement in the spectra. Therefore it seems that degassing is not necessary.
LITERATURE CITED R. H. Horrocks, E. J. Hindle. A. P. Lawson, D. H. Orrell, and A. J. Poole. Clin. Chim. Acta, 69, 93 (1976). A. J. Poole, D. I. Siater. and D. H. Orrell, C h . Chim. Acta, 73, 527 (1976). M. Lauwerys and A. Veroruyssa, Chromatographia, 9, 520 (1976). G. Munro, J. H. Hunt, L. R. Rowe, and M. B. Evans, J. pharm. Ff?armaco/., 28. Suppl. 27P (1976). P. R . King, Glaxo-Allenburys Research (Greenford) Ltd., Greenford, Middlesex, England, Private Communication. E. D. Smith, J . Chromatogr. Sci., 10, 34 (1972). R . K. Stevens and J. P. Mold, J . Chromatogr., 10, 398 (1963). F. Kasler, "Quantitative Analysis by N.M.R. Spectroscopy", Academic Press, New York, 1973. 0 . LevensDiel. "Chemical Reaction Enaineerina", 2nd ed. John Willev. New York; 1972, p 107. M. B. Evans, Chromatographia, 3, 337 (1970). L. Birkofer. A. Ritter, and W. Giessler, Angew, Chem.,75, 93 (1963). J. F. Klebe, H. Finkbeiner, and D. M. White, J . A m . Chem. Soc., 88. 3390 (1966).
RECEIVED for review J u n e 21, 1978. Accepted October 11, 1978.
Pressure-Volume Technique for the Calibration of Ozone Analyzers Ikuo Watanabe' and Edgar R. Stephens* statewide Air Pollution Research Center, University of California, Riverside, California 9252 I
T h e calibration of instruments for monitoring ozone or oxidant in polluted air has always presented a special problem because ozone is an unstable gas not readily handled in pure form. Historically, analysts have made use of the oxidation of t h e iodide ion in buffer solution with the assumption t h a t iodine is formed in stoichiometric yield. Attempts to verify this assumption have sometimes given positive ( I , 2) and sometimes negative results (3, 4 ) . T h e situation was complicated by the fact t h a t many variations of the iodide method were in use. I t is unlikely t h a t any two laboratories ever used iodide methods which were identical in every detail. The most recent version uses a boric acid buffer ( 5 , 6). Direct comparisons finally produced such discordant results t h a t regulatory agencies began to explore alternatives not dependent on the stoichiometry of iodide oxidation. T h e two most popular alternatives are gas phase titration ( G P T ) ( I ) and ultraviolet absorption spectrophotometry (UV) (7). The G P T method depends on the stoichiometric fast reaction of ozone with nitric oxide and requires accurate measurement of flow rates. I n addition, t h e user must either prepare a known mixture of nitric oxide in a n inert gas (which has its own pitfalls) or trust such a primary standard provided by some third party. This may not be readily available in some parts of the world. T h e ultraviolet method is more direct but does require a special long p a t h cell (1-5 m) since 1 ppm of ozone absorbs only about 3% of an ultraviolet beam per meter. T h e error in measurement of either light intensity (with and without 0,) is magnified about thirty-fold by t h e necessary Beer-Lambert law calculations of ozone for these parameters. T h e method described in this paper depends on the measurement of ozone by t h e pressure change which accompanies the conversion of a small portion of oxygen to ozone in a closed system:
302
-
203
(1)
Present address: Institute of Public Health 4-6-1 Shirokanedai. Minato-Ku, Tokyo 108, Japan. 0003-2700/79/0351-0313501 O O / O
A very brief discharge of 12 000 V through a trapped volume of pure oxygen a t atmospheric pressure produces about 1% ozone. The decrease in total moles causes a decrease in either volume or pressure or both depending on the apparatus. This change can be used to calculate the amount of ozone formed. In this study, the ozone formed had a volume of only a few hundredths of a cubic centimeter so pressure change a t constant volume was used. After equilibration and measurement. this ozone is swept into a dilution vessel of large, known volume to achieve a known concentration in the sub-ppm range. This method had previously been used to determine the infrared absorptivity of ozone ( 8 , 9) and to calibrate an older model ozone photometer as well as t o validate a potassium iodide procedure ( I O ) . In all four studies the P / V "absolute" ozonizer gave consistent, reliable results. In this study, a coulometric (Mast Development Co. Model 724.2) potassium iodide analyzer and two ultraviolet photometers (Dasibi Model 1003AH) were tested. Each of these could, in principle, be an "absolute" analyzer (i.e., not requiring calibration). The ultraviolet absorptivity of ozone is accurately known and this instrument measures fractional light absorption by digital techniques which can be used directly to compute concentration. T h e coulometric analyzer is "internally calibrated" if a stoichiometric yield and measurement of electrons is assumed. B u t no instrument is foolproof, so independent calibrations are required a t least for verification. Ozone analysis is especially vulnerable because decomposition to oxygen is thermodynamically favored and, if it occurs in air, it would likely go undetected because of the excess of oxygen, and traces of catalyst could easily cause such decomposition. T h e present method permits the monitoring of such ozone loss for all steps except the transfer from ozonizer to dilution bottle.
EXPERIMENTAL A p p a r a t u s . The major elements of the calibration (shown in Figure 1) are the €';\. ozonizer, the dilution bottle (46.6 L). and the instrument tu be calibrated. The dilution flask should c 1979 American Chemical Society
314
ANALYTICAL CHEMISTRY, VOL. 51, NO. 2 , FEBRUARY 1979
c 600
>
3
500
5
n
a a
v
k-
a (L
c
z Figure 1. Ozone generated in the 1.89-mL volume by electrical discharye is measured by pressure change on the oil manometer and aiiuted into the large bottle
be large so that the instrument being calibrated will reach a steady reading without excessive dilution of the calibration mixture and so that the amount of ozone generated in the ozonizer will be as ppm of ozone in 46.6 L, 23.3 KLof pure large as possible. For ozone is required. If the discharge converts 1% of the trapped oxygen to ozone, the volume of the ozonizer should be 2.33 mL. This is i~iuchsmaller than the ozonizers used in previous studies which ranged from 11.0 to 205 mL. The ozonizer volume was measured by weighing with and without water before sealing the 5-legged manometer to the ozonizer body. The volume of the Coliiicctirlg capillary was estimated to be 0.27 mL from its diiiieiisioiis. This was added to give a total volume of 1.89 mL. This is probably the minimum practical since the connecting volume is an appreciable fraction of the total, and the accuracy of the method depends directly on the accuracy of measurement of this -.~ ~ l i l m eThe . volume of the dilution bottle was also measured by water filling, one of the simplest and most reliable of all physical iiicasiirements. It is important that the temperature of the azoiiizer be the same after ozonization as before. This was kept to iU.02 O C for 1h, in spite of a 2 O C change in room temperature, by immersing the ozonizer in a water bath in a vacuum jar to provide a large heat sink and by covering the water with silicone oii. Minimizing the time of ozonization (3-15 s ) is also advantageous. Provided uniform, constant temperature is maintained, the volume of ozone can be calculated by applying conservation of mass and the perfect gas law: V O Q = 2VAp/P (2) where cos and V are the volumes of ozone and of the ozonizer (1.89 iiLj, respectively, and I p / P is the fractional change in pressure which accumpanies ozone formation. Silicone oil of 0.963 g/mL d-c;nsity was used in the 3-legged manometer so formation of 1% ozone causes a pressure decrease of 5.37 cm a t constant volume and I atin pressure. The third leg of the manometer was capped a i t h a small syringe and a screw to manipulate the manometric fluid so that the trapped volume would be constant. The viscous silicone oil reyuired some time to drain from the walls of the 2-mm i.d. maiiometer. Complete drainage is not necessary, however, so long as pressure equilibration is attained. In addition, a few ixiiiules were required to establish thermal equilibrium. Keeping the discharge time short also minimized the time necessary for Chis. It should be noted that ozone decomposition which occurred in the ozonizer during the equilibration period would not cause any error since the pressure a t any instant truly reflects the amount of ozone present. Nor is there any error caused by lack of ozone generation in the connecting capillaries. Again the pressure reflects the total number of moles without any assumption of uniformity of concentration. Any ozone which dissolved in the manometric fluid would of course be lost but the coiiiact here is minimal. Procedure. After measurement of the pressure difference, the ozone was swept immediately into the dilution bottle with a small volume of oxygen and allowed to mix. Since the P / V ozonizer has a volume of only 1.89 mL, 20 mL of oxygen is more than adequate to transfer the ozone with little loss in the bottle due to dilution. Contact time in this transfer is minimized. Then che analyzer was connected to the bottle and the concentration w-as recorded vs. time. Sampling from a limited volume of course causes dilution of the ozone a t a measurable rate as shown in
E
300
Z 0
0
m 0 I
200
0
IO
20
30
40
TIME (MINUTE), t Figure 2. Sampling at 1.51 L/min dilutes t h e standard at t h e rate calculated for perfect mixing (UV photometer)
Figure 2. Charcoal filtered room air was allowed to enter the dilution bottle through the tube shown. For perfect mixing the concentration should decrease a t an exponential rate
with F = 1.51 L/min as sampled by one of the UV photometers (Dasibi 15) and V = 46.6 L; the halftime for dilution is (46.6/1.51) X In 2 = 21.4 min. The flow rate through another photometer (Dasibi 06) was 0.6 L/min giving a halftime of 53.8 min. (These photometers have built-in flowmeters.) The solid line in Figure 2 shows this theoretical dilution rate superimposed on the concentrations from the recorder trace. The agreement shows two things. (1)The assumption of good mixing is valid (otherwise the plot would not be linear). (2) No significant decomposition of ozone occurred during the sampling period (otherwise the rate of decrease would exceed that caused by dilution). Since ozone decomposition in the ozonizer is also monitored by the pressure measurement, only the transfer step itself (a few seconds) is unvalidated. A loss here would not be the same from test to test. The calibration point was taken to be the concentration extrapolated to zero time. In routine use, it would not be necessary to measure the sample flow rate since it does not enter into any calculation. This is an important advantage of the P / V method. To verify the stability of the ozone in the bottle, sampling by the UV photometer was interrupted for various time intervals and the concentration (back extrapolated to time of reconnection) was compared with the final value of the preceding sample period. Figure 3 demonstrates the good agreement which was obtained. In this figure, the number of minutes of interruption is shown adjacent to each point.
RESULTS AND DISCUSSION Having demonstrated that the bottle did not destroy ozone appreciably, comparisons were made between t h e ozone calculated from the pressure change and that indicated by the two photometers. Each of these instruments had been calibrated by comparison with t h e standard photometer of t h e California Air Resources Board prior to these tests. Combining data from t h e two photometers yielded a least squares equation of [O,(UV)] = 0.97[O3(IP)] - 1 (in ppb) with a correlation coefficient R of 0.998. In these tests, t h e slopes of the logarithmic concentration vs. time plots (of which Figure 2 is an example) gave calculated dilution flow rates within 10% of the measured flow rates.
ANALYTICAL CHEMISTRY, VOL. 51, NO. 2, FEBRUARY 1979
+
0 DASlBl 0
/-
DASlBl 15
by the analyzer was compared with that calculated from the ozone concentration according t o the formula
i = 0.0392CFP/T
06 X I
DASIBI 06 #'2
-
-
1
PI6
-
315
(4)
where i is current (PA), C = concn of 0, in ppb, F = sample flow rate mL/min, P is barometric pressure (atm), and T i s absolute temperature (K). Linear regression yielded the best - 0.03 with R = 0.999. fit equation imeasd= 1.04iCalcd Again, it would not be necessary to measure flow rate in a routine calibration of this analyzer. These results indicate t h a t the P / V ozonizer can be depended on t o produce accurately known quantities of ozone for the preparation of primary standards for the calibration of ozone analyzers in the range below 1 ppm. I t offers a viable alternative t o the UV and G P T methods.
+2/I; 1
500
LITERATURE CITED
1
I000
0 3 CONCENTRATiON ( p p b v ) BEFORE INTERRUPTION
Figure 3. No significant loss of ozone occurred during time intervals comparable to those needed to calibrate the U V analyzer
T h e P / V ozonizer was also used to evaluate a coulometric KI analyzer (Mast Development Corp.). Although the contact time between the air sample and the reagent in this analyzer is of t h e order of seconds, the time for nearly full response is greater than 20 min. This long lag increases the potential error in the back extrapolation so the equilibration time was reduced by pre-equilibrating the analyzer with a second ozone stream of similar concentration generated by an ultraviolet lamp. This procedure reduced the equilibration time by one half. Trials with charcoal-filtered room air as a diluent gas gave very low results, so most of the experiments were done with cylinder nitrogen. T h e current (in microamperes) produced
(1) J. A. Hcdgeson, R. E. Eaumgardner, R. E. Martin, and K. E. Rehme, Anal. Chem., 43, 1123 (1971). (2) S. L. Kopcynski and J. J. Bufalini, Anal. Chem., 43, 1126 (1971). (3) A . W. Boyd, C. Willis, and R. Cyr, Anal. Chem., 42, 670 (1970). (4) J. N. Pitts, Jr.. J. M. McAfee, W. D. Long and A. M. Winer, Environ. Sci. Technol., 10, 787-793 (1976). (5) D. L. Flamm, Environ. Sci. Technol., 11, 978 (1977). (6) R. J. Paur, R. K. Stevens, and D.L. Flamm, EPA-600/3-771a, Jan. 1977, InternaMnal Conference m PhotochemicalOxidant Pollution and Its Control, Proceedings, Vol. I , "Status of Calibration Methods for Ozone Monitors". (7) W. E. DeMore and M. Patapoff, Environ. Sci. Technol., 10, 897 (1976). (8) P. L. Hanst, E. R. Stephens, W. E. Scott, and R. C. Doerr, Anal. Chem., 33, 1113-1115 (1961). (9) J. M. McAfee, E. R. Stephens, D. R. Fitz, and J. N. Pitts, Jr., J . Quant. Spectrosc. Radiat. Transfer, 16, 829 (1976). (10) F. R. Burleson and E. R . Stephens, "ChemicalMethods in Air Pollution", California Department of Health, "Volumetric Calibrations of the Kruger Ozone Photometer", 1960. (11) F. R. Burleson and E. R. Stephens, California Department of Health, Conference Workshop 1959, "The Formation and Recovery of Absolute Quantities of Ozone".
RECEIVED for review June 22, 1978. Accepted October 20, 1978.
Wet Digestion Method for the Determination of Mercury in Biological and Environmental
J. Ross Knechtel" and J. L. Fraser Waste water Technology Centre, Environmental Protection Service, Environment Canada, Burlington, Ontario L 7R 4A6, Canada
Cold vapor atomic absorption spectrophotometry has proved very effective for the determination of mercury in a wide range of sample types. The original procedure described by Hatch and O t t ( 1 ) a n d modifications such as those suggested by Uthe e t al. (2),Bishop e t al. ( 3 ) ,and Hendzel et al. ( 4 ) depend on wet oxidation by a combination of concentrated nitric and sulfuric acids, some heat, and usually an oxidizing agent such as potassium permanganate. When measuring the mercury content in botanical samples and sewage sludges, the main consideration is the high carbon content (vegetation samples: 40-50% ; dried sewage sludges, 15-30% C). Carbon acts as a strong reducing agent on mercury during sample digestions. This commonly leads to volatilization losses of mercury. Because of this difficulty, usually only small amounts of sample (about 0.1 g) are taken for analysis when evaluating plant tissue or sewage sludges for mercury content by cold vapor atomic absorption (2-4). As the mercury content in vegetation samples is usually very low (1G50 ng/g), the 0.1-g sample weight would be insufficient for a reliable and accurate mercury determination. T h u s a 0003-2700/79/0351-0315$01.00/0
need for a method capable of handling larger sample weights exists. Malaiyandi and Barrette ( 5 ) published a wet oxidation procedure for total mercury analysis capable of the digestion of up to 5 g of sample. The method, however, required special glassware, a t least a 1-h digestion, and constant attention. These aspects made it impractical for the routine determination of mercury. Deitz et al. (6) described a variation in the Malaiyandi e t al. ( 5 ) method. T h e Deitz method appeared t o be more practical than the Malaiyandi method as the special glassware was eliminated and constant attention was no longer necessary. However difficulty was experienced in obtaining accurate results using the Deitz e t al. (6) method and t h e use of long neck volumetric flasks presented some practical difficulties. T h e proposed method is a modification of t h e Deitz (6) procedure. An aluminum hot block with digestion tubes is utilized for sample preparation. The new method has proved to be sensitive, fast, accurate, and reproducible. I t has been used successfully on grass tissue, fish tissue, sewage sludges, c' 1979 American Chemical Society
