Cell potential seawater

Another fresh-water method which holds some promise for seawater analysis is twin cell potential sweep voltammetry, as proposed Afghan et al. [138]. In this method, semicarbazones are formed by reaction with semicarbazide... 

Tschaikowski obtained a British patent for developing a method to prevent rusting and fouling of ship s bottoms by continuous formation of molecular hydrogen and caustic soda formed by the electrolysis of seawater. The author mentions a current density of 6.5/iAcm in the electrochemical cell formed by graphite anode and the underwater part of the vessel, cathode, and at a cell potential of 4.0 V. ... 

The seawater will be involved in the oxidation half-cell reduction (i.e., it acts as the reductant) with Eox -(0.600 V)= -0.600 V. When the seawater is in equilibrium with the iron system Eceii = rcd + ox = 0 or red = 0.600 V. For nonstandard concentrations at 298K, the total cell potential developed by the iron system when paired with the hydrogen half-cell is given by Eq. (6.26). However, the hydrogen half-cell generates zero electrode potential. Therefore, the electrode potential developed by the iron system is, from Eq. (6.26)... 

Spacer would reduce the corrosion rate in the pipe. The rate of corrosion is partially determined by the difference in the standard cell potentials of the two metals in contact (see Table 9.4). The relative potential of metals in seawater is given in Table 10.1 and represents the driving force of the corrosion which includes the current, or more precisely, the current density, that is, A/cm. ... 

Half-Cell Potential Criterion For steel stmctures, cathodic protection is achieved when polarized at the iron (Fe) equilibrium half-cell potential [3]. In neutral environments (soil and seawater), the half-cell potential is based on the following reactions and it is determined by the Nemst equation... 

The protective potentials for steel in seawater at 25°C and soils with respect to commonly used reference electrodes are shown in Table 5.15. Several reference electrodes can be used for measurement of half cell potential as shown in Chapter 2. 

It is experimentally difficult to obtain numerical estimates of the total number of bacteria present in seawater, and the contribution of ultramicroorganisms that have a small cell volume and low concentrations of DNA may be seriously underestimated. Although it is possible to evaluate their contribution to the uptake and mineralization of readily degraded compounds such as amino acids and carbohydrates, it is more difficult to estimate then-potential for degrading xenobiotics at realistic concentrations. 

Chronopotentiometry has also been used to determine chloride ions in seawater [31]. The chloride in the solution containing an inert electrolyte was deposited on a silver electrode (1.1 cm2) by the passage of an anodic current. The cell comprised a silver disc as working electrode, a symmetrical platinum-disc counter-electrode and a Ag-AgCl reference electrode to monitor the potential of the working electrode. This potential was displayed on one channel of a two-channel recorder, and its derivative was displayed on the other channel. The chronopotentiometric constant was determined over the chloride concentration range 0.5 to 10 mM, and the concentration of the unknown solution was determined by altering the value of the impressed current until the observed transition time was about equal to that used for the standard solution. 

Stolzberg [143] has reviewed the potential inaccuracies of anodic stripping voltammetry and differential pulse polarography in determining trace metal speciation, and thereby bio-availability and transport properties of trace metals in natural waters. In particular it is stressed that nonuniform distribution of metal-ligand species within the polarographic cell represents another limitation inherent in electrochemical measurement of speciation. Examples relate to the differential pulse polarographic behaviour of cadmium complexes of NTA and EDTA in seawater. 

Fig. 22.6. Redox potentials (mV) of various half-cell reactions during mixing of fluid from a subsea hydrothermal vent with seawater, as a function of the temperature of the mixture. Since the model is calculated assuming 02(aq) and H2(aq) remain in equilibrium, the potential for electron acceptance by dioxygen is the same as that for donation by dihydrogen. Dotted line shows currently recognized upper temperature limit (121 °C) for microbial life in hydrothermal systems. A redox reaction is favored thermodynamically when the redox potential for the electron-donating half-cell reaction falls below that of the accepting half-reaction.

Electro-active labile metal contents have also been measured by using a combination of electro-deposition and analysis by graphite furnace AAS (Batley and Matousek, 1977). Metals (e.g. Pb, Co, Ni, Cr from seawater) are plated on to a short graphite tube by application of a suitable potential. At the end of the electrolysis period, the graphite cell (plus pre-concentrated metal) is placed in an electro-thermal atomiser attached to an AAS spectrometer, and the element content determined. 

As DMS is ultimately derived from intracellular DMSP, it is necessary to quantify this source strength to characterise the potential for DMS production, which may be modified by alternative sinks for DMSP. Figure 5 shows the relationship of DMSP to cell carbon (mmol DMSP (S) /mmol C, H seawater) for blooms of two common algal species, P. pouchetii and G. aureoleum. From this data it appears that P. pouchetii produces about seven times more DMSP than G. aureolum. This might be taken to suggest that P. pouchetii is seven times more prolific a DMS-producer than G. aureolum. However, the ratio of DMS concentration (per unit carbon) for the two species is only three, which implies that there must be quite different rates of DMS production and/or DMSP comsumption for the two cases. 

Thus, if dissimilar pipes are butt-welded with the electrolyte flowing through them, the most severe corrosion will occur adjacent to the weld on the active metal. The current of the galvanic cell takes the path of least resistance and this affects corrosion in that current does not readily flow around corners. In soft water, the critical distance between copper and iron may be 5 mm in seawater it may be several decimeters. The critical distance is greater the larger the potential difference between anode and cathode. Then, the geometry of the circuit affects galvanic corrosion and this is observed in the case of stray current corrosion.7 (Baboian)5... 

These potentials are standardised, that is, they only apply to systems at unit activity, 25°C, and latm pressure. Deviation from these conditions will result in quite different values being obtained. For example, if two pieces of zinc are dipped into zinc sulphate solutions of different concentration (the solutions being connected via a salt bridge), a difference in potential will be measurable between the two electrodes. The electrode in the more concentrated solution will be the cathode and that in the dilute solution the anode. This is a differential concentration cell, many examples of which are found in corroding systems. Thus, any electrochemical series may be produced for a given environment, that is metals and alloys may be arranged in order of corrosion resistance to that environment. Table 1 illustrates this point for alloys immersed in flowing seawater [3],... 

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