The measurement of organically complexed FeII in natural waters using competitive ligand reverse titration.

Whilst there is increasing evidence for the presence of stabilized Fe(II) associated with organic matter in aquatic environments, the absence of a reliable method for determining Fe(II) speciation in solution has inhibited the study of this aspect of Fe biogeochemistry. A technique is described here for the determination of Fe(II) organic complexation in natural waters that is based on competitive ligand reverse titration and a model fit to experimental results, from which ligand concentration and a conditional stability constant can be obtained. Spectrophotometry was used to detect the Ferrozine (FZ) complex with reactive Fe(II), which in combination with a liquid waveguide capillary cell (LWCC) enabled high sensitivity and precision measurements of Fe(II) to be made. A series of samples was collected in the Itchen River in Southampton, UK to test the method at a wide range of salinities including river water. Levels of Fe(II) and total dissolved Fe were within previously reported values for this system. Fe(II) was found to occur organically complexed with values for K'(Fe(II)L) (conditional stability constant for Fe(II)-natural ligand complexes) of ≈8 at salinities between 0 and 21, whilst no measurable complexation was detected at a salinity of 31. This work demonstrates that spectrophotometry can be used in combination with ligand competition to investigate metal speciation in natural waters.

Thiols in coastal waters of the western North Sea and English Channel.

Thiols were determined in coastal waters of the western North Sea and English Channel. Detection was carried out on-board ship on-line by flow-analysis with detection by cathodic stripping voltammetry and calibration with thiourea. The thiol concentrations ranged from 0.70 to 3.60 nM (thiourea equivalents) and were found to vary over a relatively short distance perpendicular to the coast. Low concentrations in the area of greatest estuarine input (the Humber-Wash area) indicated that the thiols did not originate from low-salinity waters. Instead, variations in the thiol concentration were found to parallel those of chlorophyll. This correlation was confirmed by depth profiles which showed a trend for the thiols similar to that of chlorophyll. The data demonstrates that thiols are more widespread than anticipated, and that marine phytoplankton is an important source of the thiols. In view of their known ability to bind with metals, these data indicate that the thiols could be an important candidate to act as such ligands in the marine system. The measurements gave no evidence for the presence of sulfide in these waters which means that it must be present at less than 20% of the detected thiol levels.

Determination of picomolar levels of iron in seawater using catalytic cathodic stripping voltammetry.

A new procedure for the direct determination of picomolar levels of iron in seawater is presented. Cathodic stripping voltammetry (CSV) is preceded by adsorptive accumulation of the iron(III)-2,3-dihydroxynaphthalene (DHN) complex from seawater, containing 20 microM DHN at pH 8.0, onto a static mercury drop electrode, followed by reduction of the adsorbed species. The reduction current is catalytically enhanced by the presence of 20 mM bromate. Optimized conditions include a 60-s adsorption period at -0.1 V and a voltammetric scan using sampled dc modulation at 10 Hz. In these conditions, a detection limit of 13 pM iron in seawater was achieved which can be lowered further by extending the adsorption time to 300 s. The new catalytic CSV method is approximately 5 times more sensitive than existing CSV methods and was tested on samples from the Atlantic Ocean.

Determination of the complex stability of zinc with carbonic anhydrase in sea-water.

Carbonic anhydrase (CA) is inactive unless associated with zinc, with possible substitution by cobalt. In this work, the complexation of zinc by CA was determined in sea-water using cathodic stripping voltammetry (CSV) with ligand competition. The zinc was found to be released from the CA over a period of 3 h when equilibrated with a competing complexing ligand and the complex was re-formed with the CA when zinc was added. A value of 8.90+/-0.27 was found for logK'ZnCA where K'ZnCA is the conditional stability constant for the complex of Zn2+ with CA in pH 8 sea-water. A value for the molecular weight of CA was calculated from its equivalent ligand concentration (in nM) obtained by titrations with zinc at various CA concentrations (1-4 mg l(-1)). The value found (34740 g mol(-1)) for the molecular weight is consistent with values found previously by other methods (29000-31000 g mol(-1)) confirming that the stoichiometry of the complex between zinc and CA is 1:1. This work confirms that the zinc-CA complex is reversible and that the interaction between zinc and CA can be determined using CSV with ligand competition.

Determination of cobalt in seawater by catalytic adsorptive cathodic stripping voltammetry.

A new procedure for the direct determination of picomolar levels of cobalt in seawater is presented. Cathodic stripping voltammetry is preceded by adsorptive accumulation of the cobalt-nioxime (cyclohexane-1,2-dione dioxime) complex from seawater containing 6 µM nioxime and 80 mM ammonia at pH 9.1, onto a hanging mercury drop electrode, followed by reduction of the adsorbed species. The reduction current is catalytically enhanced by the presence of 0.5 M nitrite. Optimized conditions for cobalt include a 30 s adsorption period at -0.7 V and a voltammetric scan using differential pulse modulation. According to the proposed reaction mechanism, dissolved Co(II) is oxidized to Co(III) upon addition of nioxime and high concentrations of ammonia and nitrite; a mixed Co(III)-ammonia-nitrite complex is adsorbed on the electrode surface; the Co(III) is reduced to Co(II) (complexed by nioxime) during the voltammetric scan, followed by its chemical reoxidation by the nitrite, initiating a catalytically enhanced current. A detection limit of 3 pM cobalt (at an adsorption period of 60 s) enables the detection of this metal in uncontaminated seawater using a very short adsorption time. UV digestion of seawater is essential, as part of the cobalt may occur strongly complexed by organic matter and rendered nonlabile. The method was applied successfully to the determination of the distribution of cobalt in the water column of the Mediterranean.

Determination of cobalt in blood using cathodic stripping voltammetry.

A method was developed to determine low levels of cobalt in whole blood, serum and plasma. Samples of blood (0.2 ml) were mineralized at 160 degrees C in the presence of concentrated nitric acid. Residual organic matter was destroyed by digestion using ultraviolet irradiation after dilution with water. The cobalt concentration was determined by catalytic cathodic stripping voltammetry (CSV) preceded by adsorptive collection of cobalt complexed with diphenylglyoxime (DPG). The optimized analytical conditions for the CSV analysis included a DPG concentration of 0.5 mumol l-1, 0.05 mol l-1 ammonium chloride buffer (pH 9.3), 0.15 mol l-1 nitrite, a deposition potential of -0.75 V, an adsorption time of 30-120 s and a negative potential scan using the differential-pulse modulation. The limit of detection was 40 pg of cobalt in 0.2 ml of blood, which was limited by the blank level of cobalt in the reagents after purification.

Effect of the deposition potential on the voltammetric determination of complexing ligand concentrations in sea-water.

The effect of the deposition potential on the determination of complexing ligand concentrations in natural waters was investigated by titration with Cu from sea-water samples originating from the North Sea and monitoring the labile Cu concentration after deposition at two potentials, viz., -0.05 and -0.7 V. The amount of deposited metal was measured by cathodic stripping voltammetry with ligand competition, using tropolone as the added ligand. It was found that the complexing ligand concentration detected decreased by more than 60% when the more negative deposition potential was applied. The data indicate that a significant fraction of the organic complexes of Cu in natural waters is electrochemically labile at negative deposition potentials and is not determined by anodic stripping voltammetry. The concentrations and conditional stability constants of the complexing ligands in the North Sea are similar to those detected previously in the Irish Sea.

Determination of borate ion-pair stability constants by potentiometry and non-approximative linearization of titration data.

Borate anions, B(OH)(-)(4), are known to associate with alkali and alkaline-earth metal cations in sea-water. The borate cation ion-pairs are of the general form MB(OH)((n-1)+)(4), where M(n+) is the cation. In this work, the cation borate stability constants (K*(MB)) have been evaluated for Na(+), Li(+), Mg(2+), Ca(2+) and Sr(2+) where K*(MB) = [MB(OH(4))((n-1)+)]/[M(n+)][B(OH)(-)(4)]. The K*(MB) values were obtained from values found for the stability constant of boric acid (K*(B)) in various electrolyte media at 25 degrees and an ionic strength of 0.7. Acid-base potentiometric titrations were performed in the electrolyte media with a standard Pt/H(2) electrode and a junctionless Ag/AgCl reference electrode to monitor the emf. A non-approximative equation was used to linearize the titration data. The values obtained were: K*(Lib) = 0.89 +/- 0.02, K*(NaB) = 0.44 +/- 0.01, K*(MgB) = 13.6 +/- 0.7, K*(CaB) = 11.4 +/- 0.15, K*(SrB) = 3.47 +/- 0.06. The values for K*(MB) correlate with the charge-density parameter z(2)/(r + 0.85), where r is the radius of the cation. The speciation of boron in sea-water was predicted from the K*(MB), data for the major cations present.

Direct determination of sub-nanomolar levels of zinc in sea-water by cathodic stripping voltammetry.

A novel technique for the determination of nanomolar levels of zinc in aqueous solution is presented. The zinc complex with ammonium pyrrolidine dithiocarbamate is adsorbed on a hanging mercury-drop electrode and the reduction current of zinc is measured by voltammetry. The detection limit for zinc is 3 x 10(-11)M, with 10-min collection time. A procedure is suggested for the simultaneous determination of Ni and Zn in a single sample.

Nembutal treatment of the VMH (rat): effects on feeding and sexual behaviour.

In male rats two brain cannulae were implanted bilaterally and directed to an area just dorsal of the ventromedial hypothalamic nucleus. The behaviour of these animals was observed before and after the injection through these cannulae of either 1 micronl saline or 1 micronl Nembutal. Injections were performed under three behavioural conditions: (1) animal alone in cage with food, (2) animal in the presence of an oestrous female and (3) animal in cage with food and oestrous female. Following the Nembutal injection, sniffing and feeding are disinhibited temporarily in condition 1, whereas in condition 3 only sniffing, but not feeding, is disinhibited. In conditions 2 and 3 male sexual behaviour is not influenced by the Nembutal except for an increase of sniffing at the female. It is concluded that a temporary elimination of the VMH leads to a disinhibition of feeding only if the external conditions are favourable for feeding. The same elimination of the VMH does not influence the occurrence of male sexual behaviour under the conditions used in these experiments.