Measurement of Dissolved Gases

Electrochemical probes are available that measure the concentration of dissolved oxygen in both conducting and non-conducting media. Care has to be taken in the selection of probe type, as with some the elements are easily poisoned by certain species within the process fluid. 

The more reliable probes comprise a thin membrane that is porous to oxygen. The oxygen diffuses through the membrane and dissolves in the small body of electrolyte within the probe. The oxygen within the electrolyte is electrochemically reduced at an inert electrode and the corresponding current that flows between this and an auxiliary electrode gives a measure of the concentration of dissolved oxygen in the process fluid. 

Dissolved oxygen probes should not normally be inserted directly into the process stream, but fitted into a small flow chamber connected to a side stream or a process fluid off-take point. Proprietary kits may be used for the rapid on-site determination of oxygen, carbon dioxide, and hydrogen sulfide levels in aqueous process fluids. 

---

As shown in the right side of Fig. 9, a quadrupole mass spectrometer, MSG 300, with a gas-tight ion source, secondary electron multiplier, direction detector, and a turbo-molecular pump (TURBOVAC 150) is equipped with a membrane inlet (all from Nippon Shinku, Tokyo). The resolution scale is 300. Mass spectrometry can also be used for the measurement of dissolved gases in a liquid phase using a steam sterilizable membrane probe. Recently, the application of the mass spectrometer to fermentation processes has increased markedly. 

Considering the limitations of Eh measurement and the common occurrence of homogeneous disequilibrium Q), it would seem prudent to limit the use of Eh to either specific reactions or to qualitative descriptions when discussing natural systems. It has been suggested QS, I2 2Q) that the measurement of dissolved gases such as O2, H2S, NH4, CH4, and H2 may be a preferable method for characterizing the redox status of natural waters. 

Degn, H., Cox, R. P and Lloyd, D., Continuous Measurement of Dissolved Gases in Biochemical Systems with the Quadrupole... 

The work of Wallace and Wirick [60] shows the value of continuous measurements of dissolved gases using unattended gas sensors in situ. One promising... 

In situ NMR measurements can be made in conjunction with down-hole fluid sampling [5, 6]. The NMR relaxation time and diffusivity can be measured under high-temperature, high-pressure reservoir conditions without loss of dissolved gases due to pressure depletion. In cases when the fluids may be contaminated by invasion of the filtrate from oil-based drilling fluids, the NMR analysis can determine when the fluid composition is approaching that of the formation [5, 6]. 

For the estimation of dissolved gases in water the former are generally first removed by boiling the water or by generating carbon dioxide m it m the latter case the bubbles of this gas carry out any dissolved gas, from winch the carbon dioxide is easily removed subsequently. Carbon dioxide itself may be conveniently estimated by titrating a measured volume of the water with sodium carbonate solution until phenolphthalem becomes coloured, the method depending on the neutrality of sodium hydrogen carbonate towards phenolphthalein.2... 

Reaction of dissolved gases in clouds occurs by the sequence gas-phase diffusion, interfacial mass transport, and concurrent aqueous-phase diffusion and reaction. Information required for evaluation of rates of such reactions includes fundamental data such as equilibrium constants, gas solubilities, kinetic rate laws, including dependence on pH and catalysts or inhibitors, diffusion coefficients, and mass-accommodation coefficients, and situational data such as pH and concentrations of reagents and other species influencing reaction rates, liquid-water content, drop size distribution, insolation, temperature, etc. Rate evaluations indicate that aqueous-phase oxidation of S(IV) by H2O2 and O3 can be important for representative conditions. No important aqueous-phase reactions of nitrogen species have been identified. Examination of microscale mass-transport rates indicates that mass transport only rarely limits the rate of in-cloud reaction for representative conditions. Field measurements and studies of reaction kinetics in authentic precipitation samples are consistent with rate evaluations. 

Thus, ion-selective membrane electrodes can be defined as electrochemical sensors that allow potentiometric measurements of the activity of particular species in aqueous and mixed solvents or partial pressures of dissolved gases in water. However, these sensors may respond to certain other ions in the sample in addition to the selected i ion interferences by such j ions are usually expressed by the Nikolskii-Eisenman Eq. (17) ... 

Lpplications of continuous, underway measurement to marine science include measurements of chemical variability in the upper ocean, especially near coastal fronts and streamers (i, 2) determination of the in situ concentrations of trace metals (3) and of dissolved gases near the air-sea interface 4, 5) assessment of the types, concentrations, and variations of planktonic communities (6) investigation of the mixing and lateral dispersion of pollutants in the sea (7) and the acquisition bf spatially adequate ground truth for remote sensing of fisheries stocks and oceanic prop-... 

From expression (4.9) it follows that the quantity k] a can be determined via Ac and thereby via measurements of the concentration of dissolved gases in the liquid. Thus Ac is linked in a particular way to the saturation concentration c, see expres-... 

In many ground-water investigations, more data are collected than the minimum required to describe the carbonate system. Measurements of dissolved CO2 concentrations are often made as part of the suite of analyses for all dissolved gases or by base titrations in the field. Measurements of total dissolved carbonate concentrations made by precipitation of solid carbonate or evolution of CO2 following acidification may also be available. With such redundant analytical data, the internal consistency of all the carbonate data can be tested. 

If sampling for methane and sulfide has been performed as described above (see Sect. 3.3.2 Analyses of Dissolved Gases ), the analytical measurement of the already preserved samples does not have to take place immediately within the first few minutes. Yet, the analysis should not be deferred for more than a couple of hours. 

Refractive index detectors. There are three types of commercially available RI detectors, namely, deflection, Fresnal and interference. Each measures the change in the refractive index of the base eluant due to the presence of analytes and hence in principle should provide the basis for a universal detector. Difficulties in RI detection arise due to the sensitivity of the solvent RI to fluctuations in temperature, pressure, the presence of dissolved gases and eluant composition. The RI of a solvent changes by 4 X 10 " per °C and by 4 x 10 per atmosphere thus baseline noise is 1 X 10 with a mixed solvent giving corresponding sensitivities of 5 pg ml . ... 

The method of p-x curves was used for solubility measurements of noble gases in water at elevated temperatures (Potter and Clynne, 1978). The known amounts of water were injected in increments into the thermostated bomb with the known quantity of gas and pressure measured after thermal equilibration. As long as gas phase remains in the bomb along with liquid, those water injections alter pressure moderately. Only after the last bubble of gas dissolves in the water, further addition of water cause a relatively large increase in pressure. 

ESR has been useful in studying the influence of dissolved gases on polymer mobility [801]. Stable nitroxyl radicals, such as 2,2,6,6-tetramethylpipe-ridin-l-oxyl (TEMPO) are widely employed as spectroscopic probes for observing binding sites and molecular motion of macromolecules [802]. ESR spectra of the TEMPO free radical in PC film at various temperature and in solution were reported [795]. The TEMPO spin probe method was also used to study diisooctylphthalate (DIOP) plasticiser diffusion in suspension polymerised PVC particles [803]. Similarly, the compatibility Umit of PVAc and di-nonylphthalate (DNP) was studied by means of 2,2-di-n-nonyl-5,5-dimethyl-3-oxazolidinyloxy spin probe ESR measurements and DSC [804] DNP is an effective plasticiser for PVAc for concentrations not exceeding 17 wt.%. According to ESR evidence BBP in PVC forms radicals more easily than DOP [805]. 

---