(15) R. B. Dean, R. T. Williams, and R. H. Wise, Environ. Sci. Technoi, 5, 1044 (1971). (16) A. F. Westerhold. The Digester, 22, 4 (1965). (17) A. F. Westerhold, The Digester, 22, 18 (1965). (18) J. S. Jeris, Water Wastes Eng., 4, 89 (1967). (19) T. K. Wu. Michigan Department of Natural Resources Laboratory, Lansing, MI, private communication, 1974. (20) J. M. Foulds and J. V. Lunsford, Water Sewage Works, 115, 112 (1968). (21) W. N. Wells, Water Sewage Works, 117, 123 (1970). (22) L. E. Shriver and J. C. Young, J. Water Poiiut. Contr. fed., 44, 2140 (1972). (23) W. R. Bloor, J. Bioi. Chem., 77, 53 (1928). (24) M. J. Johnson, J. Bo/.Chem., 181, 707 (1949). (25) R. R. McNary, M. H. Dougherty, and R. W. Wolford, Sewage ind. Wastes, 20, 894 (1957). (26) N. Chaudhuri, S.Niyogi, A. De, and A. Basu, J. Wafer Pollut. Contr. fed., 45, 537 (1973). (27) A. F. Gaudy and M. Ramanathan, J. Water Pollut. Conk Fed., 38, 1479 (1964). (28) A. H. Molof and N. S.Zaleiko, 19th Purdue Industrial Waste Conference, Lafayette, IN, May 1964. (29) J. H. Ickes, E. A. Gray, N. S.Zaleiko. and M. H. Adelman in "Automation in Analytical Chemistry, Technicon Symposia 1967". Mediad Inc., Tarrytown, NY, 1968.
(30) M. H. Adelman. 18th Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, PA, March 1967. (31) M. H. Adelman in "Automation in Analytical Chemistry, Technicon Symposia 1965", Mediad Inc., Tarrytown, NY. 1966. (32) "industrial Method No. 137-71W," Technicon Instruments Corp., Tarrytown, NY, 1973. (33) E. C. Tifft and B. E. Cain in "Automation in Analytical Chemistry, Technicon Symposia 1972", Mediad Inc., Tarrytown, NY, 1973. (34) "Technicon Operation Manual," Technicon Instruments Corp., Tarrytown, NY, 1973. (35) Fed. Reg., 38, 28759 (1973). (36) W. A. Moore and W. W. Walker, Anal. Chem., 28, 167 (1956). (37) J. A. Winter, "Method Research Study 3, Demand Analyses", Environmental Protection Agency, Cincinnati, OH, 1971.
RECEIVEDfor review November 13, 1974. Accepted March 13, 1975. The mention of trade names or commercial products does not constitute endorsement or recommendation for use by the Central Regional Laboratory or the Environmental Protection Agency.
Investigation of lsothiocyanatopentaaquochromium(lll) as a Reagent for the Separation and Identification of Nanogram Quantities of Mercury(l), Mercury(ll), and Methylmercury(l1) Richard J. Baltisberger' and Curtis L. Knudson Department of Chemistry, University of North Dakota, Grand Forks, N.D. 58202
The reagent, isothiocyanatopentaaquochromium(ll1) (CrNCS"), forms polynuclear species with CH3Hgf, Hg2+, and HgZ2+ with the stoichiometry (CrNCS),MeXf where n equals 1 or 2. An ion exchange procedure was Investigated for the separation of nanogram quantities of Me equal to CH3Hg+, HgZ2+, and Hg2+ in water based on the formation of the (CrNCS),Me complex Ions. The three mercury species can be isolated by variation of the eluent acidity. Possible analytical applications for samples containing down to 20 nanograms of mercury are discussed.
Polynuclear species involving a sharing of the thiocyanate ion between complex ions of the type, L5MNCS2+ (where M equals Co3+ or Cr3+ and L equals H20 or NH3), and Hg2+ have been shown to exist in aqueous solution ( I , 2 ) . For example, when a solution containing Cr(OH2)bNCS2+ is added to a solution of Hg2+, the following equilibria occur: Hg2'
+
Cr(OH,)5NCS2+ = (H20)5CrNCSHg4+ (1)
Cr(OH,),NCS2+
+
(H,0)SCrNCSHg4' =
[(HzO) ,CrNCS12Hg6+ (2) The values of the stability constants for the above equilibria have been measured by Armor and Haim (2) to be K1 equal to 1.66 X lo4 and K2 equal to lo2. The binuclear species, (H20)&rNCSHg4+, is kinetically stable for several hours in solution a t 2 5 O . The rate coefficient for the dissociation of the dimer to form Cr(OH2)e3+ and HgSCN+ is 8.5 X sec-' at 25' and 1.0 ionic strength (2). This rate coefficient corresponds to a half-life of the order of one day. Schwarzenbach ( 3 ) found that the cation, CH3Hg+, Author to whom all correspondence should be addressed. 1402
ANALYTICAL CHEMISTRY, VOL. 47, NO. 8, JULY 1975
forms a dimer with Co(NH3)5NCS2+having a value of K1 equal to lo3. These complex polymers have been found to exist long enough to carry chemical separations of mercury cationic species at 0'. In particular, isothiocyanatopentaaquochromium(II1) can be used in conjunction with cation exchange techniques to separate and identify Hg22+,Hg2+, and CH3Hg+ in a concentration range from 1 to 100 ppb mercury in aqueous solution. Trace analysis methods for the differentiation of inorganic and organomercury ions in aqueous solution have been proposed by several authors. One of the earliest procedures was investigated by Polley and Miller ( 4 ) who reported that dithizone extraction could be used to differentiate between Hg2+ and CH3Hg+ at the ppm level. Three methods have recently been reported which are based on the selective reduction of Hg2+ in the presence of CH3Hg+ (5-7). In the latter three procedures, the mercury vapor produced is measured in a gas stream using the well known Hatch and Ott flameless atomic absorption technique (8). Magos found that only Hg2+ is reduced by acidic stannous chloride whereas in basic stannous solution (with CdCl2 added) methylmercury(I1) is reduced ( 5 ) . Umezaki and Iwamoto (6) found that cupric chloride when added to a basic solution of stannous ion could be used to reduce methylmercury(I1). Baltisberger and Knudson (7) found that hydrogen peroxide added to acidic solution completes the reduction of methylmercury(I1). Only Hg2+ is reduced if no peroxide is present in acidic solution. The three methods are applicable to a concentration range of 1 to 10 ppb. A few liquid chromatographic methods for the differentiation of the cationic forms of mercury have been reported. In the analysis of hair, CH3Hg+ and Hg2+ were separated as dithizonate complex ions on alumina columns for samples containing a few micrograms of mercury (9). Several hundred micrograms of Hg22+ and Hg2+ were sepa-
rated by TLC techniques using chromatographic solvent of benzene and trichloroacetic acid (10). From 0.05 to 250 pg of Hg2+ were measured in water by neutron activation analysis after preconcentration of the chloro-mercury(I1) complex on anion exchange resin (11).No liquid chromatographic separation of mercury species has been reported a t the nanogram level. One difficulty with isolating Hg2+ a t this level is the tendency for the irreversible absorption of Hg2+ on polystyrene matrices as first noted by Walton (12). Initial work in our laboratory indicated this to be a difficulty in both anion and cation exchange procedures involving less than 1 pg quantities of Hg2+. This paper describes the use of CrNCS2+ as a reagent for the cation exchange separation of Hg2+, Hg2*+, and CH3Hg+. The cations once separated can be measured quantitatively by flameless atomic absorption methods. The advantage of this technique is that three forms of mercury can be isolated and identified. A relatively simple method for the differentiation of Hgz2+ and Hg2+ a t 1 to 100 ppb level is proposed.
EXPERIMENTA L Reagents. [Cr(OH2)5NCS](C104)2solution was prepared by cation exchange chromatographic means. A solution containing ca. 50% Cr(OH2)5NCS2+, 30% Cr(OH2)c3+, and 20% cis- and transCr(OH2)4(NCS)2+ was prepared by combining 0.1 mol [Cr(OH2)6](C104)3,0.1 mol KSCN, and 0.1 mol HC104 in 1 liter of aqueous solution. (Chromic perchlorate can be obtained commercially, although in this work the salt was prepared by a previously described method (13).) The mixture was heated to 60' for 24 hr, cooled to 0' and the resulting KC104 removed by filtration. A 25-ml aliquot of the solution was then placed on a cation column containing 100 ml of Bio-Rad Ag 50W X 8, 50-100 mesh, strong acid resin in 0.1M HC104. Perchloric acid was used in this work because it was found to be more free of mercury contamination than other acids. Any Cr(NCS)Z+ species was eluted immediately upon washing with 0.1M perchloric acid. The reagent, Cr(OH2)5NCS2+, was collected from the column by elution with 1.OM perchloric acid. The collected reagent was stored a t 0" C in the dark. The remaining Cr3+ can be eluted with 1-2M sulfuric acid. Small quantities of Cr3+ polymer were held tightly to the resin. The polymer can be removed by cleaning process in which the resin is heated in NaOH and H202 media. The resulting sodium chromate can be washed from the resin with water. The CrNCS2+ solution is stable for several months. The aquation of the cation to produce Cr3+ and NCS- is slow a t 0'. At 25O, the rate coefficient is sec-' (14). No problems were experienced with the reagent if remade every three months. Mercuric and mercurous perchlorate stock solutions were prepared from salts obtained from the G. F. Smith Chemical Company. Methylmercury(I1) acetate, obtained from Alfa Chemical Company, was used to prepare 2-10 ppm solutions in 0.1M perchloric acid. The solution was stored a t Oo in the dark to prevent photodecomposition of CH3Hg+ ( 1 5 ) .Dilution of this solution to the desired ppb level was used to calibrate the UV readout of the FAA instrument. Perchloric and hydrochloric acid solutions used in this work were prepared from reagent grade chemicals. Doubly distilled water was used in the preparation of all stock solutions. Stannous sulfate used in the reduction of mercury solution was obtained from Alfa Chemicals. Analyses. Mercury content of solutions was determined by use of a flameless atomic absorption setup consisting of an aeration vessel (impinger type percolator), an LDC model 90073, 30-cm path, 13.7-ml volume gas cell, an LDC model 1235 UV monitor, and an LDC model 3300, 1-100 mV recorder. A calcium sulfate drying tube was initially placed in the gas line between the percolator and the gas cell. Ultimately, the system was used without the drying tube a t 1-10 ppb level. A constant 920 ml/minute air flow was maintained in the setup by use of a Master flex model 8540 constant speed pump fitted with a model 7015 head. Sample injections were made with Plasticpak, 5602, 1-cc T B disposable syringes to which were attached N722 Hamilton stainless steel needles. Repeated rinsing with 1M HCl solution and distilled water cleaned the syringe. If high UV absorption blanks were obtained, the syringe was discarded. Mercury(I1) was analyzed by direct injection of a sample con-
taining about 1 to 10 ng of mercury into the percolator containing ca. 5 ml of a solution of 2.5M sulfuric acid and 0.05M stannous sulfate. The volumes of the injections varied from 0.2 to 1.0 ml. Methylmercury(II), was quantitatively measured only when the cation was oxidized by addition of hydrogen peroxide prior to injection of the sample into the percolator. Various hydrogen peroxide concentrations and conditions were tested. A reasonably rapid (15 minute) decomposition and analysis of methylmercury(I1) is obtained if 0.2 ml of 30% hydrogen peroxide is added to 10 ml of sample in 0.1M perchloric acid and allowed to stand for 15 minutes. After standing, an aliquot of the treated sample is removed by syringe, a volume of stannous solution equal to the aliquot is drawn into the syringe, the two solutions are mixed by shaking for 10-15 seconds, and the mixture is injected into the percolator containing the stannous reducing solution. The premixing of the stannous solution in the syringe is necessary, and attempts to leave out this step result in incomplete recovery of methylmercury in the analyzer. Hydrogen peroxide in too large a concentration prevents complete reduction of Hg(I1) to Hg in the percolator. Details of the testing of this procedure are described in another paper (7). The peak shape of the UV detector response to injections was found to be dependent upon the volume of the injection, volume of solution in the percolator, and the speed of the injection. The quantities of mercury were proportional to peak areas. The standard deviation for ten injections of 0.2 ml of the same 20-ppb Hg2+ solution was 5%. The standard deviation for ten injections of 1.0 ml for a 1-ppb solution was 11%. Ion Exchange Separation. The highly charged complexes of the type, CH3HgSCNCr3+, CrNCSHgz4+, and (CrNCS)2Hg6+,are very strongly retained on cation exchange resin. Methylmercury(I1) and mercury(I1) are removed from the column under quite different situations which results in a rapid, simple and complete separation of the two species. Evidence was also found that Hgz2+ undergoes complex reactions with Cr(OH2)5NCS2+without complete dismutation of the mercurous species. A mercury species was eluted between CH3Hg+ and Hg2+ in the cation exchange separation. A column 0.5-cm diameter by 5.0-cm length of Bio-Rad AG 50W X 8, 100-200 mesh, is washed with 0.1M perchloric acid. The column is jacketed and maintained near 0" with cooling water to prevent dissociation of CrNCS2+. One ml of 0.15.44 CrNCS2+ solution in 0.6M perchloric acid is added to the column and washed with 0.1M perchloric acid until loaded. The CrNCS2+ does not elute a t this acid strength. One ml of the CrNCS2+ solution is mixed with an aliquot of water sample containing mercury species. Normally the sample volumes were in the range from 5 to 10 ml which resulted in a final perchloric acid content of 0.05 to 0.1M. The acidity should be maintained near 0.1M. The sample is then added to the ion exchange column and rinsed with 5 ml of 0.1M perchloric acid after loading. The methylmercury(I1) cation is eluted by the addition of 1.OM perchloric acid to the column. This addition results in the elution of the excess CrNCS2+ from the column. CrNCS2+ is highly colored blue which can be seen to elute from the column. Methylmercury(I1) elutes from the column immediately after the CrNCS2+ breakthrough. The CH3Hg+ is collected in a 10-ml volume of 1.OM perchloric acid that follows the CrNCS2+ from the column. In this work, CH3Hg+ was collected as soon as the faint blue of CrNCS2+ disappeared in the eluent. Collecting in this manner eliminates diluting the sample with wash containing no CH3Hg+. Mercury(I1) and 2 equivalents of coordinated CrNCS2+ are retained on the column during this rinse. Analyses with large concentrations of Hg2+ and CrNCS2+ show that nearly a 2:l ratio of CrNCS2+/Hg2+is maintained even when the column is washed with 4M perchloric acid. The complex ion of Hg2+ and CrNCS2+ is eluted from the column by washing with ca. 20 ml of a 1.2M HCl-1.8M HzSO4 mixture. Approximately 95% of the mercury(I1) eluted appears in the first 10 ml of the wash. We recommend collecting the first 20 ml of the wash for Hgz+ analyzed as opposed to further dilution of the sample. The 1.2M HCI-1.8M wash analyzed to contain about 0.2 ppb mercury which made difficult the analyses a t less than 1.0 ppb Hg(I1). The presence of CrNCS2+ ion does not interfere in the mercury flameless atomic absorption technique. One molar perchloric acid contains about 0.05 ppb mercury and is the most useful acid for elution and washing columns. When Hg22+ is present in a sample, the ion was found to elute in the same manner as CH3Hg+. The mercurous ion elutes immediately after the disappearance of CrNCS' from an ion exchange column when washing with 1.OM perchloric acid. The elution of CH3Hg+ and Hgz2+ together causes no problem in their identificaANALYTICALCHEMISTRY, VOL. 47, NO. 8, JULY 1975
1403
Table I. Magnitude of Blank in the Analysis of Hg(I1)a X d n o g r o m s Hq(I1) a n a l ) zed (52-nanoqram loads) Elucnt
Loading 1Al HClOd
Collectcd, nil
prc -Blank, ng Hq
"9 H d W
post-Blank, ng Hrj
10 10
0.5
10
0.6
5 5 5
5.0'
54.4b'C
4.8'"
1.9 1.9
2 .o
1.3
1.4 -
.8 -
1.2,11 HC1-1.8J1 H2SO.j
0.8
1.o -
5
9.8
0.9 0.4 0.5
0.3
...
0.5
...
...
6.6
57.8
-8.2
49.6 (95%) a
All mercury analyses were based on the average of five 0.2-ml injections of a stock 10.4 ppb Hg(I1) solution. These values were averaged
from 2, 4, and 5 injections. respectively. Ten ml collected instead of 5,
Table 11. Recovery of Mercury(I1) from Cation Exchange Resin Using Cr(NCS)2- a
Table 111. Elution of Methylmercury(I1) from Cation Exchange Resina
H d W , ng
A110 U O t , b
~~
Added
Foundb
Yo. of analyaes
R e c o v e r , , "6
21 15 i 1 3 72 52 46 i 3 5 88 1000 1040 i 20 2 104 "5.0- x 0.5-cm column of Bio-Rad 50W - X8, 100-200 mesh cation exchange resin, 1 ml of 0.15M CrNCS2- used as the complexing agent. b Value reported is the average of the analyses and the uncertainty is the range of the mean. A blank correction of 8 ng was subtracted from the total mercury collected.
m l t i a l , [CHJHg 1, ppb
m1
\o. of 3 * , . I l > E L ,
RLCO, q ,
9.9 10 2 100 i 5 99 5 1 104 472 5 2 99 i 2 596 5 2 95 7 944 5 4 99 i 6 0 5.0- x 0.5-cm column of Bio-Rad 50W - X8, 100-200 mesh, cation exchange resin, 1 ml of 0.15M CrNCS2' used as the reagent Value reported is the average of the analyses and the uncertainty is the range from the mean
*
tion. Inorganic and organomercury species can be analyzed independently by the methods previously described (5-7).
RESULTS AND DISCUSSION Mercury(I1) Recovery. A standardized 0.05M mercuric perchlorate solution was diluted with 0.1M perchloric acid to obtain a series of Hg2+ solutions ranging in concentration from 2 to 500 ppb. An aliquot of a known solution was placed in a 0.5- X 5.0-cm column of cation exchange resin and analyzed by the use of CrNCS2+ as described in the experimental section. The results presented in Table I illustrate the magnitude of the Hg blank readings one observes for the various eluents used in the separation procedure. In particular, apparently some Hg2+ was concentrated from the eluent reagents which was eluted with 1.2M HCl-1.8M HzS04 solution. This blank reading was observed before and after the collection of 54 nanograms of Hg2+ as shown in Table I. For most accurate analysis, a blank reading should be made for the column before and after analysis. The average of the two blank readings can then be subtracted from the total mercury observed upon elution with 1.2M HC1-1.8M H2SO4. The magnitude of the blank is about 5 nanograms of mercury, and therefore no samples containing less than 20 nanograms of Hg2+were analyzed. Table I1 illustrates the percentage recovery of Hg2+ at various loadings. The analysis was within 15% of the correct value if at least 50 nanograms of mercury(I1) were loaded onto the column. The results were low when 21 nanograms of mercury was loaded. The loss of this mercury could be attributed to irreversible absorption of the Hg2+ by the polymer matrix of the resin. We have measured the recovery of Hg2+ from columns containing various ion exchange materials. Polystyrene matrices such as Bio-Rad AG 2 X4, Amberlite, IRA-400, Bio-Rad AG 50 X8, and Amberlyst all absorb some portion of a Hg2+ solution. The problem is decreased if the contact time on the column is 1404
ANALYTICAL CHEMISTRY, VOL. 47, NO. 8 , JULY 1975
shortened. Fritz and Seymour (16) obtained quantitative recovery of Hg2+ on anion exchange resin using a 1M HCl-5M HC104 eluent using high speed techniques and loading milligram quantities of mercury. The other possible explanation is that the remaining Hg2+ is lost under the tail of the peak. With greater than 50-nanogram loadings, 99% of the mercury is eluted within the first 20 ml. This was the volume collected for analysis in all cases and may not be sufficient for 20 nanograms of mercury. Methylmercury(I1) Separation a n d Recovery. A standardized 0.01M methylmercury acetate solution was prepared and diluted to 10 to 1000 ppb in 0.1M HC104 for analysis using the CrNCS2+ elution separation. In the case of methylmercury(I1) analysis, the elution solvent used was 0.3 to 1.OM HC104. The blank UV reading with this eluent is unimportant as evident from Table I. Table I11 presents the recovery percentages of various loadings of methylmercury(I1) on a 5-cm cation exchange column. The uncertainty of the analysis is f 5 % for 5 analysis which wasabout the same as the uncertainty of the FAA technique used. Mercury( I) Separation a n d Recovery. Preliminary work in this laboratory has shown that the dismutation of Hg,"
-
Hg2* + Hgo
can be followed by measurement of the Hgo produced. Dissolved mercury(0) can be measured quantitatively by purging from a percolator system containing no reducing agents. The forward rate constant for Reaction 3 was calcuto lated to be 5 X sec-l at 25' in 0.001M HC104. The reaction must be carried out at concentrations of HgZ2+ of the order of 10-6M so that the Hgo produced does not exceed the known solubility of Hgo of 3 X 10-7M at 25' (ca. 60 ppb) (17, 18). The reaction is very sensitive to im-
60
c.' r W 7
40
1c 'c W
Hq2+
W c Y
1
r; .r
L rn
20
,
C H ~ H ~ +
n
a
n
0 Load
Figure 1. Chromatograph of a 5-ml
I
20 30 Volume o f 0.6 M HC104 (ml)
R e c o ~rii d IHc, species
$1 x 10'
5 10 1.5 M HC1
sample of 150 ppb Hg(l),20 ppb Hg(ll), and 30 ppb CH3Hg(II)
Table IV. Recovery of Mercury Species from Hg(1) Stock Solutions ~d,i,.d O I + ) I ,
40
C H ~ J,~ "
[I hiZ* 1 ,
I! X l O Y
M x 105
6.75" 2.4 2.8 3.75 0.83 1.1 0 335 0.05 0.044 Hg(1) added reported as molarity of strenth 0 01M perchloric acid
[i k,2 1,
v
x lo4
Kb
4.3 6.6 x 3 .O 9 x 10-8 0.65 9.5 x 10-8 HgZ2+. 25 " C ; ionic
purities present in the acid and water used. Therefore, the addition of CrNCS2+ to a solution of Hg(1) may induce rapid dismutation during the column separation or it may retard dismutation. Three Hg(1) stock solutions in 0.01M HC104 were prepared to be 270, 150, and 13.5 ppb Hg(1) from 0.01M mercurous perchlorate standard solution. The diluted solutions were equilibrated at 25 OC for several weeks before use. At equilibrium, the Hg(1) solution contains a mixture of Hgo, Hg22+, and Hg2+. The equilibrium constant for Reaction 3 was measured to be 5.3-5.5 X a t 25 "C by Moser and Voigt (17). The constant can be used until the solubility of Hgo in water of 3 X lO-'M a t 25" is exceeded. At equilibrium, all of the three stock solutions would contain less than 3 X 10-7M HgO.If no oxidation of the Hgz2+ occurs during preparation, equal quantities of Hg2+ and HgO would be present in the stock solutions. Aliquots of the stock solution were chromatographed by the CrNCS2+ separation technique previously described. Figure 1 presents a typical chromatograph for the separation of an equilibrated mixture of Hg(1). Table IV reports the concentrations of mercury species measured. The first mercury species eluted in 0.01M HC104 was identified as HgO. A solution was saturated in H$ by equilibration of mercury metal with 0.01M HC104. An aliquot of this solution when chromatographed resulted in a species eluting with the same retention time. However, less than 75% of the HgO was collected from this solution. The loss of mercury(0) to the exchange resin would be expected due to its greater solubility in the organic matrix than water. For this reason, only 75% of the total Hg(1) added to the ion exchange column was recovered. The second species eluted from the ion exchange column immediately following the CrNCS2+ in 1.OM HC104 was identified to be Hg22+. The third species eluted in 1.2M HC1-1.8M HzSO4 and was identified to be Hg2+. If no air oxidation of the Hgz2+ occurs during sample
preparation and separation, then equal amounts of Hgo and Hg2+ should be formed during equilibration of a Hg(1) stock solution. The validity of the separation and amount of separation induced dismutation can be checked by calculation of K. Since H$ was lost due to absorption, the measured Hgo concentration was not useful in calculating a value of K for Reaction 3. The value of K was calculated in Table IV from the equation: K = [Hg2+12 ( 4) [Hgz2+1 derived from Equation 3 assuming that the concentrations of H e and Hg2+ are equal. The assumption appears to be good in the case of the two larger concentrations of Hg(1). The summation of the concentration of and Hg2+ should equal the initial Hg(1) concentration, if only dismutation occurs. The recovery of mercury is 105 and 109% on this basis for the two larger initial Hg(1) concentrations. For the 3.35 X 10-9M Hg(I), the Hg2+ concentration is nearly twice the Hg(1) value. This would indicate a sizeable amount of oxidation of Hgz2+ as well as some dismutation. The data thus are not conclusive as to whether the CrNCS2+ induces any dismutation during separation. Further experiments are to be carried out to measure the dismutation rate directly. Mixtures w i t h Cation Exchange Resin. Mixtures containing 30 ppb methylmercury(II), 150 ppb Hg(I), and 20 ppb Hg(I1) in distilled water were prepared from stock solutions. These mixtures were separated using columns and conditions as described previously. The elution profiles for elution with 0.6M and 1.OM perchloric acid are presented in Figure 1. Table V presents the data for three mixture elutions and one elution of a Hg(1) solution. The contribution of Hg(1) to the H$ and Hg(I1) peaks was observed whenever a Hg(1) stock solution was analyzed. However, the Hg(1) peak is consistent in that for 150 ppb solutions of Hg(I), 39, 47, 44, and 45 ppb was observed to elute as the Hg(1) species. The methylmercury(I1) analysis exhibits reasonable results considering that errors due to analysis by differences have been introduced by the presence of Hg(1). The Hg(1I) was found to be 79 f 5 ppb in the mixtures. After accounting for the contribution of Hg(1) to the Hg(I1) concentration, the value of 19 ppb for a corrected Hg(I1) concentration compares favorably with the mixed value of 20 ppb. Interferences. The interference of chloride and of hydrogen sulfide were investigated in order to determine a t what level these anions would interfere in the analysis. Chloride concentrations of greater than 1 X 10-3M in the original sample result in a loss of 20-30% of a 20-ppb ANALYTICALCHEMISTRY. VOL. 47, NO. 8 , JULY 1975
1405
Table V. Mixture Separations with Cation Exchange Resina M!122 1, I ,,,er,,ne1,1
1 2
3 4c Added valuesd
[H~1~1,pph [ C H j l i g I,&
ppb
[Hg2 I , P P ~
NA* NA
NA 25
39 47
73 83
16 17
31
44
82
45
60
0
30
150
20
Loading and column conditions were as described on Table I. Concentrations for the species are corrected to the original 5-ml sample loaded. Data were inadvertently not taken. In this experiment, neither methylmercury(I1) or Hg(I1) was added. Solutions for experiments 1, 2, and 3 were added from stock solutions to yield these mixed concentrations.
methylmercury(I1) solution during the column loading step. A chloride concentration of lOd3M results in a similar loss of a 20-ppb Hg2+ solution. A chloride concentration of 10-'M can be tolerated if less than 20-ml volume is loaded into the ion exchange column. Hydrogen sulfide concentration of 10-5M results in a loss of 25% Hg(I1) during loading of a 20-ppb sample. Hydrogen sulfide and chloride were chosen as likely contaminants in an environmental water sample. No other possible interfering anions were checked at this time. The large stability constants for the formation of the isothiocyanatochromium(II1)-mercury complex ions would require that ligands must form complex ions with stabilities similar to those of HgCl+ and HgC12. For example, chloride does not interfere at 10-4M which is about a factor of ten lower in concentration than the CrNCS2+ added to the ion exchange column. Other cations form complex ions with CrNCS2+ and would compete with Hg2+ for the CrNCS2+. The cations T1+, Ag+, Cd2+,and Pb2+ were tested for retention on cation ion exchange resin. Thallium(1) elutes similarly to CH3Hg+ immediately after elution of CrNCS2+ with 1.OM HC104. Lead(I1) and cadmium(I1) can be eluted with 2.OM HC104 after removal of the excess CrNCS2+ with 1.OM HC104. Mercury(I1) is retained on the cation column when washed with 3.OM HC104. Silver(1) elutes in a similar manner as Hg(I1). The interference of these cations in the presence of Hg(I1) was not studied. SUMMARY The ion exchange procedure offers an ion exchange method of quantitatively separating CH3Hg+ and Hg2+. The species once separated are then measured by FAA means. The differentiation of submicrogram quantities of inorganic and organic mercury compounds has been carried out by measurement of one species and the second species by other techniques. For example, methylmercury(I1) can be measured by means of gas chromatography using an electron capture detector. Mercury(I1) can be measured by FAA means in the presence of methylmercury(I1); stannous ion reduces Hg2+ but not CH3Hg+ in acidic media (7, 8). The advantage of the ion exchange procedure described in this paper is that the same instrumental technique can be used and that sample concentration should be possible. For
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ANALYTICAL CHEMISTRY, VOL. 47, NO. 8, JULY 1975
the ion exchange to be useful for environmental water samples, several additional points would have to be checked. Environmental water samples generally contain less than 0.1 ppb mercury. This lower range has not been tested in this work. A problem may occur in the concentration of mercury from the reagents. At 2 ppb Hg2+, we found that 20% of the mercury peak isolated was due to impurities in the reagents. This background could lead to blanks as large as the sample if 100 ml of a 0.05-ppb environmental sample were loaded onto the column. In summary, the concentration level and the effects of suspended organic matter on separation need to be investigated before use with environmental water samples. The ability of the method to isolate the species, Hg22+,is interesting; although it appears that the separation procedure induces further dismutation of the mercurous ion. The observation that the dismutation of Hgz2+ is slow appears to be contrary to previous studies. Several investigators (19, 20) have found the exchange of Hg2+ a n d Hg22+ to be immeasurably fast at 25'; this observation has been interpreted to mean that the dismutation reaction is rapid. Several investigators have assumed a rapid dismutation in interpreting Hgz2+ reaction data (21, 22). The exchange of Hg22+-Hg2+ can be explained without a rapid dismutation step. It is therefore our contention that the rate of dismutation of Hgz2+ is slow in acid media and some separation scheme should be possible for Hgp2+ and Hg2+ based on this slow reaction. Unfortunately, in this study, the data are inconsistent with the CrNCS2+ inducing an increase in the amount of Hg2+ during separation. The dismutation reaction of Hg22+is to be studied further to determine if its quantitative separation is possible. The reagent, CrCNS2+, can therefore be used only for the quantitative separation of Hg2+ and CH3Hg+. LITERATURE CITED (1) L. C. Falk and R. G. Linck. lnorg. Chem., 10, 215 (1971). (2) J. N. Armor and A. Haim, J. Am. Chem. Soc., 9 3 , 8 6 7 (1971). (3) G. Schwarzenbach and M. Schellenberg, Helv. Chim. Acta, 48, 28 (1965). (4) V. L. Miller, D. Polley, and G. J. Gould, Anal. Chem., 23, 1286 (1951). (5) L. Magos, Analyst, (London),96, 847 (1971). (6) Y. Umezaki and K. Iwamoto, BunsekiKagaku, 20, 173 (1971). (7) R. J. Baltisberger and C. L. Knudson, Anal. Chim. Acta, 73, 265 (1974). (8) W. R. Hatch and W. L. Ott, Anal. Chem., 40, 2085 (1968). (9) A. Nishikata, K. Tanzawa, Y. Takeda, T. Osawa, and T. Ukita, Eisei Kagaku, 14, 211 (1968): Chem. Abstr., 70, 36116b. (10) P. B. Janardhan and A. Paul, lndian J. Chem., 7, 66 (1969). (11) D . E. Becknell, R. H. Marsh, and W. Allie. Jr., Anal. Chem., 43, 1230 (197 1). (12) H. F. Watton and I. M. Martinez, J. Phys. Chem., 63, 1318 (1959). (13) T. J. Weeks, Jr. and E. L. King, J. Am. Chem. Soc., 90, 2545 (1968). (14) C. Postmus and E. L. King, J. Phys. Chem., 59, 1216 (1955). (15) P. L. Goggen and L. A. Woodward, Trans. Faraday Soc., 58, 1495 (1962). (16) M. D. Seymour and J. S. Fritz, Anal. Chem., 45, 1394 (1973). (17) H. C. Moser and A. F. Voigt. J. Am. Chem. Soc.,79, 1837 (1957). (18) S.S.Choi and D. G. Tuck, J. Chem. Soc.. 1962,4080. (19) E. L. King, J. Am. Chem. Soc., 71, 3553 (1949). (20) R. L. Wolfgang and R. W. Dodson. J. Phys. Chem., 56, 872 (1952) (21) A. M. Armstrong and J. Halpern, Can. J. Chem., 35, 1020 (1957). (22) J. Doyle and A. G. Sykes, J. Chem. Soc. ( A ) ,1968, 215.
RECEIVEDfor review February 6, 1975. Accepted April 14, 1975. Based in part on the Ph.D. thesis of C.L.K., Dec. 1974, University of North Dakota, Grand Forks, N.D. Grateful acknowledgment is made to the Department of Interior, Office of Water Research and Technology, which supported all this work through Grant B-020 NDak.
