8.31 Mercury in Water: S. Nishi B. G. Lipták
8.31 Mercury in Water: S. Nishi B. G. Lipták
8.31 Mercury in Water: S. Nishi B. G. Lipták
Pretreatment of Sample: Wet oxidation is used for the detection of the total mercury level, and solvent extrac-
tion is used for the detection of organic mercury.
Ranges: The detection ranges for A and D are approximately 0.02 to 0.5 ppm of mercury in
water, and for B and C, approximately 0.1 to 10 ppb of mercury in water. For E, the
measuring ranges are from 01 g/l to 0100 ppm.
Partial List of Suppliers: (For a more complete list, for chromatograph suppliers, see Section 8.12; for colo-
rimeter suppliers, see Section 8.15; for atomic absorption analyzers, see Section 8.22;
for chromatographs, see Section 8.12; for ultraviolet analyzer, see Section 8.61; and
for wet chemistry analyzer, see Section 8.66.)
ABB (www.abb.com/analytical)
AMETEK, Process and Analytical Instruments Co. (E) (www.ametekpi.com)
Aurora Instruments (www.aurora-instr.com)
Bacharach Inc. (www.bacharach-inc.com)
Brooks Rand LLC (www.brooksrand.com)
Buck Scientific (www.bucksci.com)
Cetac (www.cetac.com)
Cole-Parmer (A) (www.coleparmer.com)
Davis Inotek Instr. (www.inotek.com)
EcoChem Analytics (www.ecochem.biz)
Fisher Scientific (www.fishersci.com)
LECO Corp. (www.leco.com)
Leeman Labs Inc. (www.leemanlabs.com)
Mercury Instr. GmbH (www.mercury-instruments.de)
Milestone (www.milestonesc.com)
Perkin-Elmer Corp. (B, E) (www.instruments.perkinelmer.com)
P.S. Analytical (www.psanalytical.com)
Rigaku MSC (www.rigakumsc.com)
Roentec GmbH (www.roentec.de)
Sick (www.sickmahaihak.com)
Shimadza Scientific Instruments
Tekran Inc. (www.tekran.com)
Thermo Automation Syst. (www.thermo.com)
1413
2003 by Bla Liptk
1414 Analytical Instrumentation
cobalt (Co), and iron (Fe) are not extracted from an acidic the concentration of dithizone do not affect the results. Hg
solution. Moreover, tin (Sn) and bismuth (Bi) are not extracted (HDz)2 gradually fades in the light. This tendency becomes
from a strongly acidic solution unless they are present in large pronounced when impure dithizone is used, whereas a solu-
quantities. tion of high-grade dithizone remains stable for several hours
in a lighted room.
Interference by Copper Interference of Cu may be elimi-
nated by one of the following methods: Atomic Absorption Spectrophotometry
1. When a small flow of a dithizone in carbon tetrachlo- In atomic absorption spectrophotometry, a solution contain-
ride solution is added to a weakly acidic solution con- ing mercury is introduced directly into a flame. This tech-
taining Hg and Cu, extraction of Hg is completed nique does not give high sensitivity in the analysis of mer-
before that of Cu begins. Separation of Hg and Cu is cury, and it is difficult to detect mercury at a concentration
thus possible. below 0.2 ppm. Mercury can be analyzed with high sensitiv-
2. Extraction of Hg and Cu can be carried out simulta- ity if the aqueous solution is first reduced to mercury vapor
neously with excess dithizone and then treated with a and then sent to the absorption cell by aeration, where it is
masking agent such as KBr, KI, and Na2S2O3 to analyzed by atomic absorption spectrophotometry without
2+ the use of flame. Stannous salts are best suited for reducing
decompose only Hg (HDz)2 and to transfer Hg into 2+
the aqueous layer. Thereafter, Hg is again extracted mercury in an aqueous solution. Mercuric (Hg ) ions are
0
with dithizone. reduced by stannous salts to metallic mercury (Hg ) accord-
3. Ethylenediaminetetraacetic acid (EDTA) in the form ing to Equation 8.31(1):
of the disodium salt can be added as a masking agent
2+ 2+ 0 4+
of Cu, and Hg alone is extracted with dithizone. The Hg + Sn Hg + Sn 8.31(1)
extraction is carried out at a pH of 2.5 or higher,
preferably 5.5, at which level CuEDTA is stable. The metallic mercury thus formed is vaporized by aeration
and is sent to the absorption cell (with quartz windows), where
2
A relatively large quantity of Cl also interferes with the its absorbance is measured at a wavelength of 253.7 nm.
2+
extraction of Hg under strongly acidic conditions. For Figure 8.31b illustrates both the closed and open systems
instance, the extraction of Hg can be done without difficulty, of measurement. Beers law holds for the mercury levels of
when up to 1 M of Cl is in 50 ml of 1 N sulfuric acid.
1
Extraction becomes incomplete as Cl exceeds 2.5 M. Under
neutral to alkaline conditions, some interference is also
observed when NH1+ 4
is present in large quantities. S M C D R
ments may be made at 485 to 490 nm, which is the maximum SAMPLE
6 ppb
ically switching from the sample stream to a calibration solu-
tion. Similarly, the zero can also be automatically reset, and
4 ppb rinsing steps can be automatically initiated for the purpose
3 ppb of providing automatic self-cleaning. This analyzer is illus-
2 ppb
trated in Figure 8.31e.
1 ppb
Hg
BLANK ORGANIC MERCURY DETECTION
Sample Treatment
5 MINUTES
In organomercury compounds of the RHgX type, R is an
FIG. 8.31c organic group, such as alkyl and phenyl, and X is an electrone-
UV absorption record of vapor sample produced by aeration. gative group, such as halogen and hydroxyl. Those carrying
FIG. 8.31e
Mercury process analyzer. (Courtesy of Mercury Instruments GmbH.)
methyl, ethyl, and propyl groups are known to be the causes where both R and R are organic groups and X is an elec-
of Minamata disease (a severe neurological disorder resulting tronegative group. Many of the organic compounds remain
from poisoning by organic mercury and leading to severe in the organic solvent phase, and the purpose of cleaning is
permanent neurological and mental disabilities or death). thereby accomplished.
When these compounds are present in water, it is first nec- The transfer of the organomercury compounds from the
essary to extract them with a suitable solvent. organic solvent phase to the aqueous phase is quantitative.
The solubility of a compound of the RHgX type in organic Therefore, when the backextraction is carried out by using a
solvents varies. If X is a halogen, the compound is soluble smaller volume of the aqueous solution against a known
in aromatic hydrocarbons such as benzene and toluene. If X volume of the organic solvent, concentration of the organo-
is an ion such as SO2 1
4 OH or CH 3CO 2 , the solubility in
mercury compounds may be determined simultaneously with
hydrocarbons is extremely low. Therefore, it is necessary first cleaning up.
to convert X to a halogen, and then the RHgX compound can The organomercury compounds extracted back into the
be extracted with an organic solvent such as benzene and aqueous layer are again liberated as RHgCl by addition
toluene. The aqueous solution is made acidic by addition of of hydrochloric acid to the aqueous solution according to
hydrochloric acid before extraction. The lower alkyl mercury Equation 8.31(3).
compounds are moderately soluble in water, and a relatively
large amount of solvent is necessary to effect quantitative RHgSR + HCI RHgCI + RSH 8.31(3)
extraction.
The organomercury compounds extracted in the organic The RHgCl liberated is extracted with a small amount of
solvent are generally contaminated and must be cleaned. The benzene and analyzed by gas chromatographic techniques.
cleaning of an RHgX-type compound can be performed
effectively by backextraction with an aqueous solution of a Gas Chromatography
sulfur-containing compound such as cysteine. Organomer-
Lower alkylmercury and phenylmercury compounds vapor-
cury compounds react with a sulfhydryl compound according
ize upon heating and can be analyzed by gas chromatography.
to Equation 8.30(2) and move from the organic solvent phase
The use of an electron capture detector gives high sensitivity
to the aqueous phase.
and is best suited for analysis of traces of organic mercury.
For example, methylmercury can be detected to a level of
11
RHgX + RSH RHgSR + HX 8.31(2) 1 10 g.
Organomercury compounds are highly reactive with met- three times, the aqueous layer is discarded, and the combined
als; consequently, it is not desirable to use metal tubing as benzene layer is washed with distilled water.
column materials. A glass column is preferable. Also, a polar The benzene layer is separated and is extracted back with
substance as a liquid phase in the column yields better results. 10 ml of a 0.1% aqueous solution of 1-cysteine. The aqueous
The liquid phases most frequently used are polyethylene layer is also separated, and 1 ml of concentrated hydrochloric
glycol, polydiethylene glycol succinate, and polybutanediol acid and 2 ml of benzene are added. After mixing, the ben-
succinate. zene layer is separated and dried over a small amount of
The amount of the liquid phase to be coated on the anhydrous sodium sulfate. A 10-l sample is analyzed by gas
support is preferably 5 to 10% for analysis of alkylmercury chromatography.
and about 2% for analysis of phenylmercury. An increase in The column recommended contains 5% polydiethylene
the coating amount causes an increase in bleeding gas and a glycol succinate on Chromosorb W, 60 to 80 mesh, packed
decrease in the standing current of the detector, with resultant in glass tubing, 1 m long, and with a 3-mm inside diameter.
lower sensitivity when a high column temperature is used. Column temperature is set at 266F (130C), and the flow
The size of the liquid phase is reduced for high-sensitivity rate of carrier gas is maintained at 60 ml/min. Methylmercury
analysis. chloride is eluted in 3 to 5 min under these conditions, and
The use of the substractive technique is recommended concentrations of 1 g/l or less can be detected by this
for simplified identification of organomercury compounds by method.
gas chromatography. When an organic solvent, such as ben-
zene or toluene, which contains organomercury compounds,
is mixed with the aqueous solution of a bivalent sulfur com- Thin-Layer Chromatography
pound, such as Na2S2O3 or cysteine, the organomercury com-
pounds disappear from the organic solvent. If one compares Thin-layer chromatography offers a simple and inexpensive
the gas chromatograms before and after this treatment, one method for analysis of organomercury compounds. Silica gel
will be noticed that the peaks corresponding to the organo- and alumina are mainly used as the adsorbent layer. The Rf
mercury compounds disappeared or diminished markedly values of organomercury compounds for a variety of devel-
after treatment. opers are shown in Tables 8.31f and 8.31g. The mercury
The analysis procedure is as follows: Ten milliliters of compounds are visualized by spraying the plate with a solu-
concentrated hydrochloric acid is added to 500 ml of the sam- tion of dithizone. Ordinarily, mercury of the order of 0.5 g
ple water containing methylmercury compounds. The result- can be identified visually in this manner. When developed as
ant solution is mixed with 100 ml of benzene, and the mixture organomercury dithizonate, visualization becomes unneces-
is allowed to settle. The aqueous layer is separated and is sary and mercury on the order of 0.1 g can be identified
extracted again with 100 ml of fresh benzene. This is repeated visually.
TABLE 8.31f
Rf Value of Organomercury Compounds
a
Developer
Compound A B C D E
Methylmercuric chloride CH3HgCl 0.35 0.59 0.29 0.42
Methylmercuric iodide CH3HgI 0.04 0.03
Ethylmercuric chloride C2H5HgCl 0.41 0.74 0.50 0.72 0.46
Ethylmercuric phosphate (C2H5Hg)2HPO4 0 0 0.49 0.67
Methoxyethylmercuric chloride CH3OC2H4HgCl 0.23 0.49 0.46 0.74
Phenylmercuric chloride C6H5HgCl 0.51 0.73 0.56 0.76
Phenylmercuric iodide C6H5HgI 0.74 0.84 0.92 0.86
Phenylmercuric acetate C6H5HgCH3CO2 0.39 0.64 0.86 0.83 0.48
a
Key:
Adsorbent layer; silica gel
A. n-Phexane:acetone (85:15)
B. n-Hexane:acetone (70:30)
C. Butyl alcohol saturated with water
D. Isopropyl alcohol:water (90:10)
E. Chloroform
TABLE 8.31g
Rf Value of Organomercury Dithiozonates
a
Developer
Compound 1 2 3 4 5 6
b
Methylmercury dithizonate CH3Hg(HDz) 0.64 0.48 0.57 0.77 0.89 0.86
Ethylmercury dithizonate C2H5Hg(HDz) 0.64 0.51 0.62 0.78 0.91 0.87
Methoxyethylmercury dithizonate CH3OC2H4Hg(HDz) 0.32 0.16 0.25 0.44 0.58 0.49
Ethoxyethylmercury dithizonate C2H5OC2H4Hg(HDz) 0.44 0.23 0.34 0.55 0.71 0.67
Phenylmercury dithizonate C6H5Hg(HDz) 0.48 0.34 0.46 0.62 0.72 0.69
Mercury dithizonate Hg(HDz)2 0.19 0.09 0.17 0.28 0.19 0.15
a
Key:
1. Hexane:acetone (9:1)
2. Hexane:acetone (19:1)
3. Hexane:acetone (93:7)
4. Petroleum ether:acetone (9:1)
5. Hexane:acetone (19:1)
6. Petroleum ether:acetone (19:1)
Adsorbent layer
14 silica gel
56 alumina
b
HDz = abbreviation for dithizonate ligand.