Seawater mercury analyses

Garcia-Monco Carra et al. [296] have described a hybrid mercury film electrode for the voltammetric analysis of copper (and lead) in acidified seawater. Mercury plating conditions for preparing a consistently reproducible mercury film electrode on a glassy carbon substrate in acid media are evaluated. It is found that a hybrid electrode , i.e., one preplated with mercury and then replated with mercury in situ with the sample, gives very reproducible results in the analysis of copper in seawater. Consistently reproducible electrode performance allows for the calculation of a cell constant and prediction of the slopes of standard addition plots, useful parameters in the study of copper speciation in seawater. 

Fitzgerald, W.F., 1975. Mercury analysis in seawater using cold-trap preconcentration and gas phase detection. Adv. Chem. Ser., 147 99—109. 

To date, a few methods have been proposed for direct determination of trace iodide in seawater. The first involved the use of neutron activation analysis (NAA) [86], where iodide in seawater was concentrated by strongly basic anion-exchange column, eluted by sodium nitrate, and precipitated as palladium iodide. The second involved the use of automated electrochemical procedures [90] iodide was electrochemically oxidised to iodine and was concentrated on a carbon wool electrode. After removal of interference ions, the iodine was eluted with ascorbic acid and was determined by a polished Ag3SI electrode. The third method involved the use of cathodic stripping square wave voltammetry [92] (See Sect. 2.16.3). Iodine reacts with mercury in a one-electron process, and the sensitivity is increased remarkably by the addition of Triton X. The three methods have detection limits of 0.7 (250 ml seawater), 0.1 (50 ml), and 0.02 pg/l (10 ml), respectively, and could be applied to almost all the samples. However, NAA is not generally employed. The second electrochemical method uses an automated system but is a special apparatus just for determination of iodide. The first and third methods are time-consuming. 

Garcia-Monco Carra et al. [405] have discussed the use of a hybrid mercury film electrode for the voltammetric analysis of lead (and copper) in acidified seawater. 

In many applications, such as the analysis of mercury in open ocean seawater, where the mercury concentrations can be as small as 10 ng/1 [468,472-476], a preconcentration stage is generally necessary. A preliminary concentration step may separate mercury from interfering substances, and the lowered detection limits attained are most desirable when sample quantity is limited. Concentration of mercury prior to measurement has been commonly achieved either by amalgamation on a noble-metal metal [460,467, 469,472], or by dithizone extraction [462,472,475] or extraction with sodium diethyldithiocarbamate [475]. Preconcentration and separation of mercury has also been accomplished using a cold trap at the temperature of liquid nitrogen. 

Olafsson [478] has reported on the results obtained in an international intercalibration for mercury in seawater. Sixteen countries participated in this exercise, which involved analysis of a seawater and seawater spiked with 15.4 and 143 ng/1 mercury. The results show good accuracy and precision in the recovery of mercury for the majority of calibrations, but serious errors in the low-level determinations on the seawater. 

Stroh and Voellkopf [746] utilised flow injection analysis coupled to ICP-MS to determine down to 0.6 ppt of antimony, arsenic, and mercury in seawater. 

Advantages High analysis rate 3-4 elements per hour Applicable to many more metals than voltammetric methods Superior to voltammetry for mercury and arsenic particularly in ultratrace range Disadvantages Nonspecific absorption Spectral interferences Element losses by molecular distillation before atomisation Limited dynamic range Contamination sensitivity Element specific (or one element per run) Not suitable for speciation studies in seawater Prior separation of sea salts from metals required Suspended particulates need prior digestion About three times as expensive as voltammetric equipment Inferior to voltammetry for cobalt and nickel... 

Scarponi et al. [781] studied the influence of an unwashed membrane filter (Millpore type HA, 47 mm diameter) on the cadmium, lead, and copper concentrations of filtered seawater. Direct simultaneous determination of the metals was achieved at natural pH by linear-sweep anodic stripping voltammetry at a mercury film electrode. These workers recommended that at least 1 litre of seawater be passed through uncleaned filters before aliquots for analysis are taken the same filter can be reused several times, and only the first 50-100 ml of filtrate need be discarded. Samples could be stored in polyethylene containers at 4 °C for three months without contamination, but losses of lead and copper occurred after five months of storage. 

High concentrations of sulfide may interfere with the CVAA mercury measurement. Free chlorine that is generated in the oxidation step of seawater samples or in other samples with high chloride contents absorbs radiation at 253 nm. Sulfide and chloride can be chemically removed from the sample matrix prior to analysis. 

Solvent extraction, coprecipitation and ion-exchange techniques are the main concentration methods used for seawater analysis. Other interesting concentration techniques, such as electrodeposition, amalgam trap (for mercury), a cold trap-vaporization system for hydride generation, and recrystallization, are often used by marine and analytical chemists. The first three methods are briefly reviewed here. 

Copper, nickel, and cobalt were found by Seitz (5) to diminish the height of the zinc current peak by broadening it. Although the concentration of cobalt in seawater was deemed too low to cause serious problems, the eflFect of copper and nickel required further study. The interference by copper in the stripping determination of zinc was extensively investigated by Bradford (8). He concluded that in the mercury film, copper and zinc formed a 1 1 intermetallic compound that dissociated to release zinc during the oxidation. Thus zinc peak areas remained proportional to the zinc concentration even in the presence of copper, and the analysis of zinc by standard addition was not affected. The interference from nickel was found to be similar to that from copper although the stoichiometry of the intermetallic compoimd could not be determined. 

A cold-trap pre-concentration procedure, which is incorporated into a standard jlameless atomic absorption analysis of mercury in environmental samples, has been used for both shipboard and laboratory analyses of mercury in seawater, The coefficient of variation for seawater containing 25 ng Hg/l, is 15%, and a detection limit of approximately 0,2 ng Hg is attainable. In surface seawaters of coastal and open regions of the northwest Atlantic Ocean mercury concentrations appear to decrease with increasing distance from terrestrial sources. In the open ocean samples they are less than 10 ng/l. and rather uniformly distributed. The amounts of mercury in inshore samples can approach 50 ng/l, A significant mercury fraction characterized by a stable association with organic material may be present in coastal waters. 

In Figure 2, a composite calibration curve for seawater is reproduced. This curve is based on the response obtained for spike additions of 1.0, 2.5, 5.0, and 10 ng Hg to different 100-ml seawater samples analyzed over three weeks. The number of spikes making up this composite graph are 3, 19, 20, and 9, for the 1.0-, 2.5-, 5.0-, and 10-ng mercury additions, respectively. The average value for each addition was plotted, and the brackets indicate the standard deviation. The precision of analysis reported as a coefficient of variation for these spike additions is 30% at 5 ng Hg/L, 20% at 10 ng Hg/L, 15% at 25 ng Hg/L, and 10% at 50 ng Hg/L... 

The results of this sampler study are summarized in Table I, where the value for the mercury concentration in each case is the mean mercury concentration found by triplicate analysis. The mercury concentrations found in the surface seawater collected in Teflon agree within experimental error with those for this same water after 1 hr in the PVC sampler subjected to the hydrocast procedure and sample transfer usually used. The mercury concentrations found at other depths (Table II) show little variation from the surface values. This suggests that the open samplers are not contaminated or affected signiflcantly by the hydrocast procedure. Additional details regarding this sampling study can be found in Fitzgerald and Lyons (27). 

The analysis for Cd, Pb, and Zn in seawater was performed by preconcentration by electrodeposition from the salt solution onto a hanging mercury drop (729). The mercury was then transferred to a graphite furnace and removed by evaporation. The metals were then determined by AAS. 

Robertson, D. E. Mercury distribution in the Western Atlantic Ocean based on analysis of Geosecs seawater samples. Pacific N.W. Lab. Ann. Rep. Ecol. Sci. 2, 116 (1973). 

Mercury electrode was utilized for the quantification of pyrithione in seawater. The cathodic stripping analysis was performed in the presence of Triton X-100, with the aim of separating the desired peak from the signal from the interfering thiol compounds. A detection limit of 1.5 x 10 mol was attained using 60-s pre-concentration. 

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