Zero gas generators

To set analyzers to zero, that is, to adjust the signal value to zero, nitrogen is usually used. In environmental analysis, in which contaminants in the air are to be determined, synthetic air is employed. When larger amounts of zero air are needed, zero air can be generated with a zero gas generator. The task of this apparatus is to condition the ambient air so that the contaminants that are to be measured later (such as CO, CH4, NO, NO2, SO2, and NH,) are removed. 

In the case of zero gas generators, the above-named contaminants are usually removed by adsorption because this separation process is the easiest to handle (for instance in comparison to washing processes). Gas is sucked in through a dust niter and is compressed to between 7 and 8 bar. If possible each adsorption step should be settled in this region, because the grades of separation rise in conjunction with the partial pressure. Water vapor is removed from the air initially, because water vapor is the most prevalent accompanying component (yH20 approximately 3<7o absolute). 

In the case of zero gas generators for low flow rates and mobile employment, an adsorber which can be utilized only once is used and then it is exchanged upon saturation and disposed of, or it is regenerated externally. The other contaminants are of such minimal concentrations that theses adsorbers provide sufHcient exposure time. 

The generating of zero air will usually not remove all components from the air (nitrogen, oxygen, and argon will remain) but merely those components that are to be analyzed later on. Thus most zero gas generators usually include only a subset of the named separation procedures. 

The zero-air purge gas is prepared with an AADCO 737A zero-air generator and is stored in 35-L stainless steel containers at 400 psi. Zero air was chosen because helium will not cryocondense in the sample cans at liquid N2 temperatures, and commercial prepurified N2 contains too much con-... 

The cathode formulations further employ metallic catalysts to promote the recombination of hydrogen gas generated within the zinc anode at very low (0.025 %) or zero mercury content [41,42,43]. The hydrogen scavenger can form a discrete auxiliary electrode in electronic contact with the cathode or can be introduced simply as a cathode additive in a ratio of 30/70 in the cathode preparation process. A very reliable catalytic material even at elevated temperatures (65°C) is Ag20. The auxiliary electrode may be an annulus or a disk of similar... 

Air Generator An air generator is, in fact, a sophisticated air purifier. Typically, the source is house air from a compressor or low-grade compressed-air cylinders. When properly installed (Figure 10.4), a zero-air generator will provide air at a purity exceeding the quality demands of your gas chromatograph. 

From Table 6.2, the potential for the first half-reaction is 2.866 V. The second half-reaction is the reference (relative to which the first is measured), and so its potential is zero. Thus, an electrochemical cell that generates HF from hydrogen and fluorine gas generates a potential of 2.866 V under standard state conditions. The Gibbs energy change under standard state conditions is obtained directly via Equation 6.63. 

Figure 7. Adsorption of an electronegative species from the gas phase onto a metal surface generates a dipolar layer due to electron transfer from the metal to the species. Adsorption of anions onto an electrode simulates the situation when the positive charge on the metal compensates for the adsorbed negative charge (zero diffuse-layer charge), and not when the charge on the metal is zero.

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