Company, and C. R. Knowles. We gratefully acknowledge permission to use data obtained for P. M. Cook of the National Water Quality Laboratory, Duluth, Minn. The work could not have been completed without the expert and tireless assistance of the following from The Dow Chemical Company who helped with the fiber counting: R. E. Cook, J. W. Edmonds, C. W. Kocher, D. L. Miller, C. K. Niemi, L. A. Settlemeyer, and H. J. Walker.
LITERATURE CITED (1) (2) (3) (4)
A. M. Langer and I. J. Selikoff, Arch. Environ. Health, 22, 348 (1971). H. M. Cunningham and R. Pontefract, Neture (London), 232, 232 (1971). Can. Res. Develop., 7 (6), 19-36 (1974). E. C. Hammond and H. Seidman, lnsul. Hyg., Progr. Rep., 8, No. 3, 1974. (5)P. Gross, R. A. Harley, L. M. Swinburne, J. M. G. Davis, and W. B. Greene, Arch. Environ. Health, 20 341 (1974). (6)M. F. Stanton, J. Natl. Cancerlnst., 52, 633 (1974). (7) W. C. McCrone and i. M. Stewart, Am. Lab., 1074, (4), 13. (8) G. H. Kay, Water Pollut. Control, 3 (5),33 (1973). (9) F. D. Pooiey, Brit. J. lnd. Med., 20, 146 (1972). (10) E. J. Chatfield and H. Pullan, Can. Res. Develop., 7 (6),23 (1974).
(11) B. Blles and T. R. Emerson, Nature (London), 210, 93 (1966). (12) W. J. Nicholson, C. J. Maggiore, and i. J. Selikoff, Science, 177, 171 (1972). (13) P. Cook, G. Glass, and J. Tucker, Science, 185, 853 (1974). (14) J. R. Kramer and 0. Mudroch, Can. Res. Develop., 7 (6), 3 1 (1974). (15) E. H. Kalmus, J. Appl. Phys., 25, 87 (1954). (16) M. A. Jaffe. in Proceedinas. Electron MiCrOsCODe Societv of America. Toronto, Sept. 1948. (17) P. Gross, R. T. P. de Trevllie, and M. N. Halier, Arch. Environ. Health, 20, 571 11970). - -, (18) L. Sturkey, The Dow Chemical Co.. Walnut Creek, Calif., private communication. (19) D. R. Beaman and J. A. isasi, "Electron Beam Microanalysis", ASTM STP 506, American Society for Testing and Materials, Philadelphia, Pa. 1972. (20) S. L. Bender and R. H. Duff, in "Energy Dispersion X-ray Analysis: X-ray and Electron Probe Analysis", ASTM STP 485, American Society for Tasting and Materials, Philadelphia, Pa., 1971, p 180. (21) A. M. Langer, I. Rubin, and i. J. Seiikoff, J. Hlstochern. Cytochem., 20 (Q),735 (1972). (22) J. Am. Water Works Assoc., "A Study of the Problem of Asbestos in Water", Sept. 1974, part 2.
.
RECEIVEDfor review April
22, 1975. Accepted September
26, 1975.
Automated Method for the Determination of Total and Inorganic Mercury in Water and Wastewater Samples Abbas A. El-Awady,"
Robert B. Mlller, and Mark J. Carter
U S . Environmental Protection Agency, Central Regional Laboratory, 18 19 West Pershing Road, Chicago, 111, 60609
An automated method for the determination of total as well as inorganlc and organic mercury by the cold vapor method is given. The method is suitable for the analysis of samples in a varlety of environmental water matrices. A detection limit of 0.05 pg/i. is obtained by the use of a highly sensitive spectrometer. The method Is suitable for the analysis of samples with mercury concentrations in the range 0.05-6 pgA. and a COD of less than 700 mg/i. The use of potassium persulfate, potasslum permanganate, potasslum dlchromate, and mixtures of these salts as oxldizing agents for the digestion step Is discussed, and a study of sample preservation is given. Twenty samples and/or standards per hour can be analyzed uslng this method.
These methods are highly suitable for the analysis of clean water samples and other samples with a very low content of oxidizable materials. However, for samples with a high content of particulate matter as well as for those with high concentrations of oxidizable impurities, the suitability of these methods is questionable. An automated method that addresses itself to these questions is presented in this paper. The comparability of the described method has been checked against the manual method presently accepted by EPA (14) and has been found suitable for the analysis of mercury in all types of water samples including those samples with a high content of particulate matter and oxidizable impurities.
EXPERIMENTAL In recent years, a number of methods have been introduced for the determination of mercury in a variety of matrices (1-10). The most widely used method utilizes a flameless atomic absorption technique first introduced by Hatch and Ott (10). Most of these are time consuming, however, and do not allow for the analysis of a large number of samples, such as is generally encountered by environmental laboratories. The solution to this problem has been to move in the direction of establishing automated procedures, which will allow either continuous monitoring or the analysis of reasonable numbers of samples per day. This increase in the sample analysis rate should be done without affecting either the sensitivity or the accuracy of the procedure. Recently a number of automated methods (11-13) for the determination of mercury have appeared in print. Present address, Department of Chemistry, Western Illinois University, Macomb, Ill. 61455. 110
ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
Apparatus. All glassware used in this work was borosilicate glass. Standard mercury solutions were prepared in volumetric flasks with glass stoppers, All glassware was first washed with water, soaked for 2 hr in a 1% potassium permanganate solution, soaked for an additional 2 hr in a 1:l mixture of concentrated nitric and sulfuric acids, and then washed with doubly deionized water. The glassware was then baked for 3-4 hr a t 400 "C. It was found that for subsequent use of the same glassware, a rinse with concd "03 followed by several rinses with doubly deionized water was sufficient. No traces of mercury were observed in these flasks. All domestic and industrial waste samples were stored in high density polyethylene, 1-liter screwcap bottles with polyethylene lined caps and preserved to give a final concentration of 0.5% "03. Liquid transfers for dilution purposes were made with Eppendorf pipets of 0.1,0.25,0.5, and 1-ml capacity. Instrumentation. The instruments used consisted o f 1) Spectro Products Mercury Analyzer Model HG-2; 2) Perkin-Elmer Model 56 multi-range chart recorder; 3) Harmonically smoothed voltage stabilizer; 4) Technicon AutoAnalyzer Unit consisting of a) Sampler IV, b) Proportioning Pump 111, and c) Heating bath with heating coil (20 f t long and 2.4-mm i.d.; 5) Gas-Liquid Separator; 6) A rotameter to measure the rate of air flow in the gas-liquid separator; and 7 ) High speed blender for sample homogenization.
SAMPLLR W A S H SOLUTION 6 0
C O N C H2504
I
I
Figure 1. Flow chart of the mercury manifold using K2S208as the sole oxidizing agent S M C , LMC, and J M C are small, large, and jacketed mixing coils. DO, GO, and KO are 3, 4, and 5-way glass joints
The operating principle of the mercury analyzer is based upon balancing the intensity of the mercury line a t 2537 A from a hollow cathode lamp “A” against the intensity of one or more lines from a reference hollow cathode lamp “B” in the same wavelength region as observed by a single detector. Lamp A is a pulsed mercury hollow cathode lamp, and lamp B is a pulsed iron hollow cathode lamp. The two lamp sources are pulsed 180° out of phase above a sustaining base current in a square wave mode. The instrument is balanced with the absorption cell swept free of any residual mercury. In the absence of any interfering substance, any observed absorption is solely due to mercury. The net intensity due to mercury absorption is directly obtained from the output of a phase sensitive, lock-in amplifier. The instrument is equipped with an automatic gain control to the lock-in amplifier, so that compensation for the background or extraneous absorption is accomplished automatically. In addition, a 1OX scale expansion is provided for the analysis of samples containing low concentration levels of mercury. It was necessary to modify the absorption cell supplied with the instrument. The main modifications were: 1) to decrease the internal diameter by a factor of two; 2) To position the inlet and exit tubes as close as possible to the quartz windows so as to decrease the dead air space in the vicinity of the window. To compensate for the factor of four decrease in signal strength caused by these modifications, it was necessary to increase the gain (voltage) on the photomultiplier tube. The present cell dimensions are: 22 cm long, 7-mm i.d., and 11-mm 0.d. The cell is constructed completely from quartz. The chart recorder is equipped with a variable input voltage, allowing several scale expansions. The recorder was operated a t its lowest chart speed of 5 mm/min. Reagents. All chemicals used are analytical reagent grade or better. The water used was doubly deionized. The reagents and their concentrations are as follows: 1) Concentrated sulfuric acid; obtained from Baker and designated as “suitable for Hg determination”. 2) 10% stannous chloride solution; prepared in a solution 10% in HC1. 3) 1%potassium permanganate solution; a fresh stock solution was prepared every three weeks. 4) 2% potassium dichromate solution. 5) 3% hydroxylamine hydrochloride; prepared in a solution 3% in sodium chloride. 6) 4% potassium persulfate; this solution was prepared fresh weekly. 7) Concd nitric acid. 8) A tank of purified nitrogen. 9) Mercury stock solutions; 1 mgiml (= 1000 mgil.) of either mercuric chloride (Fisher No. SO-M-114) or methyl mercuric chloride (Alpha Inorganic No. 88036). 10) Mercury
standard solutions. Mercury standard solutions in the range 0.056.0 pgil. were prepared by proper dilutions of a 100 pgil. stock solution prepared from solution 9 above. All standards were prepared in a solution 0.5% in “ 0 3 and 0.05% in KzCr20: as a preservative. 11) Activated charcoal, as an absorber for elemental Hg. Procedure. The flow diagrams for the mercury manifold are given in Figures 1 and 2. Figure 1 represents all experiments with KzS208 as the only oxidizing reagent used for sample digestion. Figure 2 represents all experiments in which KMn04 or K2CrZ07 is used as an oxidizing agent in addition to KgSz08. In the second system, a 3% hydroxylamine-hydrochloride solution is used as a reducing agent for the excess KMn04 or K2Cr20; which was not reduced in sample digestion. It should be noted here that all connections, coils, etc. in the sample train past the pump tubes are made of 2.4-mm i.d. borosilicate glass. The heating bath is set at 100 f 2 “C, and the mercury analyzer is allowed to stabilize for a 1-hr warm-up period. The system is then flushed with a 1%“ 0 3 solution and the absorption cell is flushed with purified nitrogen gas. The rotameter is set to give a constant gaseous flow rate of 15-25 cm3/min. The inlet to the segmenting air tube as well as the outlet for the absorption cell are connected to a tube filled with activated charcoal which acts as a mercury absorber. The reagents are then passed through the system in the order: A) H2S04, B) SnC12, C) NHZOH-HC1-NaC1, D) KMn04 or K2Cr207, E) K2S2O8; while the automatic sampler is kept in the wash cycle. The wash is made of a 1% ” 0 3 solution. The flow of all reagents is maintained for a period of 15-30 min or until a stable base line is obtained. In all experiments done using K2S2O8 as the sole oxidizing agent, both the hydroxylamine and permanganate or dichromate reagent lines are disconnected, and the reagents introduced in the order HzS04, SnC12, and K2S208. After a stable base line is obtained, standards in the range 0.05-6.0 figil. Hg are placed in a sample tube (prerinsed with the same solutions), and transferred to the sampler. For all samples judged to be high in mercury, i.e., 0.5-6 fig/l., only standards in that range are used. For low level mercury determination, i.e., less than 0.5 fig/l., standards in the range 0.05-0.5 fig/l. are used, and the mercury analyzer is set a t its 1OX scale expansion. In addition, a recorder scale expansion (a factor of 2) is used for runs with mercury concentrations in the range 0.2-3 pg/L Samples to be analyzed are then placed in the sampler while the standards are running. Standards were prepared fresh daily from the stock solution and analyzed by the system before and after each run to provide a calibration curve and to check for ANALYTICAL CHEMISTRY, VOL. 48,
NO. 1,
JANUARY 1976
111
-00
LYC 2 2 x 0 1 cm CELL
C H A R T RECORDER
U V MONITOR 2 5 3 7 n m
U
Figure 2. Flow chart of the mercury manifold using K2S208-KMn04 or K2S208-K2Cr207 as oxidizing agents All abbreviatlons are the same as in
Figure 1
the stability of the standards during the analysis period. In some experiments, the same standards were run over a period of 2 to 4 weeks to check for the effectiveness of the preservatives used (0.05% HN03 and 0.05% K2Cr20,). Both mercuric chloride and methyl mercuric chloride standards were used to check for the recovery of organomercury compounds. All sampling was done using a 2 0 h r sample cam with 2:l washsample ratio. For recovery studies, arbitrarily selected samples were spiked with the equivalent of 0.3 pg/l. Hg for samples with less than 1 pgh. Hg, and with 2-3 pg/l. Hg for samples containing 1-4 pg/l. Hg. All samples containing high concentrations of particulate matter were first homogenized using a Techmar Co. high speed homogenizer Model SDT. In addition, samples were stirred before and during sampling. After the analyses are completed, all lines with the exception of the sulfuric acid line are placed in a 1%H N 0 3 solution until all reagents are completely flushed out. This is followed by placing the sulfuric acid line in the wash. All the lines are then flushed for 10 min with 3% NH20H.HC1 to remove any build-up of manganese oxides. This is then followed by flushing the system with 1% "03 for a period of 20-30 min. The above flushing includes all coils in and outside the high temperature bath. For all experiments, using K2S208 or K2Cr207 as oxidizing agents, the system was flushed with a 1%HN03 solution for 20-30 min.
SYSTEM DEVELOPMENT During the early stages in the development of the system, several studies were undertaken to optimize the analysis conditions. Nitrogen Flow Rate. Table I gives the relationship between the scale reading on the recorder and the flow rate of nitrogen gas used for aspiration. The results obtained, depend on the flow rates of the reagents used. The total area under the peak of a given mercury concentration remained virtually constant up to 30 cm3/min. Peak separation, however, decreases with the decrease in nitrogen flow. Preservation of Samples and Standards. It has been recognized for a number of years (15, 16) that aqueous solutions of mercury compounds normally lose their strength on storage. This result was observed for samples stored in glass as well as polyethylene vessels. Although the mechanism for the loss of mercury is currently unknown, several interpretations have been given. The absorption of mercury on the surface of the container is perhaps the most common of these interpretations. The amount of mercury lost from aqueous solutions decreases, however, when the sample is stored in acid solution (17, 18). Feldam (18) showed that mercury standards preserved in a solution 5% " 0 3 0.05% K2Cr207 will maintain their mercury concentrations over a period of 10 days. In an attempt to establish the stability of the standard mercury solutions, as well as that of mercury in environmental samples, several solutions were prepared and analyzed over a l-month period. Table I1 gives data collected on samples prepared in deionized water, surface water containing industrial waste, sewage treatment plant (STP) effluent, and S T P influent. Both polyethylene and glass bottles were used for the unpreserved samples. All other samples were prepared in polyethylene bottles. The mercury content of all samples was first analyzed. Each sample was then spiked with a known amount of HgC12, and the total mercury present was determined within 10 minutes of sample preparation. The volume of the samples was measured using a graduated cylinder, and hence a difference of 2-3%
+
Table I. Variation of Scale Reading with Rate of N, Flow Flow rate,a cm3/rnin
Scale reading,b 0.5
1.0
2.0
3.0
4.0 Hg/l.
25.0 59.0 69.0 72.5 13.0 26.0 42.0 58.5 76 13.0 25.0 44.0 63 77 77 12.0 23.0 40.0 60 75 10.0 20.0 35.0 55 70 60 9.0 19.0 32.0 50 a Flow rates are those of the N, used f o r aspiration only. b F u l l scale 100 division a t 50% absorption. 0 10 15 17 19 22 24 25 31 37
112
10.8 12.5
22.0 24.0
32.5 41.0
48.0 58.0
ANALYTICAL CHEMISTRY, VOL. 48, NO. 1. JANUARY 1976
between the mercury found at zero time and the sum (Hg present and Hg added) is not significant. Table I1 shows that unpreserved standards (samples prepared in deionized water) lost 20% of their strength within 10 minutes of preparation and 60% over a 10-day period. Standards prepared in 0.5% H N 0 3 lost 6% and 30% of their strength over a 10-minute and a 10-day period, respectively, The preservation of standards in a solution containing 0.05% K2Cr207, however, resulted in no sig0.5% "03 nificant mercury loss over a four-week period. Of particular interest is the comparison of the data for unpreserved samples over a 24-hour period. Here we observe that deionized water samples lost 30% of their strength, while environmental samples lost up to 10% of their strength over the same period. This result bears very heavily on the method of sampling. It is suggested that grab samples are collected followed by immediate preservation with 0.5% HN03-0.05% K2Cr207 solution. This is preferred over, for example, a 24-hour nonacidified composite sampling method. Any loss of mercury due to surface adsorption in a grab sampling method should be recovered upon the addition of the preservative. Thus, if a mixture of 0.5% HN03-0.05% K2Cr207 was added to a neutral aqueous solution of HgClz (2 pg/l.) which had been left standing for 24 hours, all the Hg was recovered within 20 minutes. In addition, a comparison (Table 11) of the Hg concentration in the samples prior to the addition of the spike (HgC12), shows that the nitric-dichromate values are significantly higher than those of nitric alone, which in turn are higher than the unpreserved samples. The difference is attributable to the redissolution of Hg adsorbed on the surface of the flask, since the reagents did not show any measurable Hg contamination. In a recent study, Bothner and Robertson (19) observed an increase in mercury concentration in acidified sea water samples stored in polyethylene bottles. This was attributed to either "leaching of mercury from the container surfaces or from passage of mercury vapor from the ambient air through the container wall into the solution or from both sources." Similar mercury increases were not observed in this study. Organic and Inorganic Standards. Methyl mercuric chloride and mercuric chloride produced the same standard graphs over the range 0.05-6.0 bg/l. Hg. Plots of the percent absorption due to mercury as well as the absorbance (optical density) vs. the mercury concentration, gave very good straight line fits.
+
-
o m w
000 tiwm
RESULTS AND DISCUSSIONS Figures 3 and 4 give typical chart recordings of measurements made on standard solutions. The detection limit of the method (defined as the concentration that gives a signal that is twice the level of the base-line noise) on the range 0-0.6 pg/l. is 0.05 pg/L Inspection of Figures 3 and 4 shows that the wash sample ratio of 2:l is sufficient for our system with a complete return to the base line between samples. The shape of the peaks is quite characteristic a t all levels of mercury, and with a 1-minute sampling time, a steady state concentration of mercury is reached in the absorption cell. This was established by sampling for an extended period of time and observing that the percent absorption attained is the same. The reproducibility of the results (precision of the method) was established by replicate analyses of the same sample on the same day as well as over an extended time period. Table I11 gives the standard deviations and the coefficients of variations (percent relative standard deviation) at various levels of mercury, Data for the manual method are ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
113
70-
c
I
'~
60-
0 4OyPil
50-
0 3041II
l
40-
I I 1
1
1
1
b
'
'
-
30-
, I,
0 1041/1
I
1
! I l l L I
40
0 20yglI
1
I
y&$
'
I
0 O~YK/I
k
20
-
41 \! ibi h/ h& h/mt
I '
I!\
10-
f
Figure 3. Recorder plot with standard solutions of mercury at the
0.5, 1.0, 2.0, 3.0, and 4.0 ygll. level Data obtained on the 1X scale of the mercury monitor and a scale expansion of 2X on the chart recorder at a damping control of 10 sec
also given as a reference. Reproducibility data for real samples were generally within the standard deviation of the standards for samples with small amounts of particulate matter when run on the same day, and within 2-3 standard deviations of the standards for samples run on different days. This could be attributed to sample deterioration, preferential adsorption of mercury on particulate matter, or day to day changes in the elasticity of pump tubing. Samples with considerable amounts of particulate matter were sampled representatively by homogenization using a high speed blender. The reproducibility of these samples was within 2-3 standard deviations of the standards. Since mercury in water can exist as organic mercury, it was necessary to establish the completeness of the digestion procedure as well as the effectiveness of the various chemicals involved in the break-down of organic mercury compounds to ionic mercury. This is necessary since SnC12 will not reduce organic mercury under the experimental condition given. Table IV gives recovery data for methyl mercuric chloride using various reagent combinations. Potassium permanganate alone gave less than 30% recovery of the mercury present. Potassium persulfate alone was sufficient to recover all the mercury present. Variation of the concentration of K2S208 between 0.5-5% did not alter the results obtained for methyl mercuric chloride. Since the Table 111. Reproducibility Automated Hg level, g/l.
Std d e v c
0.05a 0.10a
0.005 0.007
0.20a
0.01
0.400
0.02 0.04 0.04
0.60a 0.25b 0.50b 1.Ob
2. O b 3.0b 6.0b
0.04 0.04
0.09 0.08
Manual
Re1 std dev, %
Std de$
Re1 std dev, 70
10
7 5
CH,HgCI added,
5 4 17 7
0.063
28.0
4
0.083
8.0
5
3 0.1 3 4.0 8 standards run using l o x expansion of the Hg monitor scale of the (automated system). b Standards run using IX Hg monitor and 2 x expansion of the recorder (automated system only). c Based on 1 0 or more replicates.
114
Table IV. Recovery of Known Amounts of Methyl Mercuric Chloride by KMnO,, K,Cr,O,, and K,S,O, Oxidation Procedures0
0.48
ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
Pg/1.
Thermal decomposition,
1%
2%
4%
KMnO,,
K,Cr20,,
K,S,03,
%b
%
%
%C
45.5 100.1 27.3 0.5 40.0 100.5 1.0 25.0 35.9 98.5 2.0 18.0 23.4 39.2 103 3.0 23.8 29.7 40.6 98.0 4.0 24.2 31.5 a Based on HgCl, standards. b No oxidizing agent added; all other reagents are used, however. c Similar results are obtained using 1%KMn0,-4% K,S,O, and 2% K,Cr,O,-4% 17.4 20.0
K2S203'
Table V. Comuarison of Automated and Manual Methods,
!all. Hg
Automated 4% KzS,O,,
Sample t y p e
Reagent blank Well
Stream Industrial intake effluent
Raw sewage
COD
... ...
