Dissolution oxygen/nitrogen

In addition, Grabke (95) has determined the rate of dissolution of nitrogen in iron and the rate of the reverse reaction at temperatures between 700 and 1000°C. A thin iron foil was exposed to N2-H2 mixtures at various compositions and pressures, and the resistance of the iron foil was measured as a function of time. In essence, the resistivity of iron is proportional to the nitrogen content and, therefore, can he used as a measure of the nitrogen concentration. Thoroughly dried N2-H2 mixtures rather than pure N2 were used in order to ensure an iron surface practically free of oxygen, which easily blocks the surface (81, 92, 93, 96, 97). At 700 to 900°C and low H2 partial pressures, the rate of the forward reaction... 

After dissolution of the oxide, Ti may be directly determined by atomic absorption using either the nitrous oxide-acetylene [31] or oxygen-nitrogen-acetylene [32] flame systems. The method is straight-forward, and no interference has been noted from Cr, Co, Mn, Mo, Nb, W, Ta, or Cu [33]. An improvement in the determination of Ti using the nitrous oxide-acetylene flame system was noted when the analysis was performed in a buffered HF-boric acid mixture [34]. 

Solution We will calculate the amount of oxygen by neglecting the dissolution of nitrogen or any other gases present in air. At 25 °C the saturation pressure of water is 0.03175 bar. Appl)dng eq. (12.1. ) we have... 

Similar relationships can be written for the dissolution of hydrogen and oxygen. These relationships are expressions of Sievert s law which can be stated thus the solubility of a diatomic gas in a liquid metal is proportional to the square root of its partial pressure in the gas in equilibrium with the metal. The Sievert s law behaviour of nitrogen in niobium is illustrated in Figure 3.8. The law predicts that the amount of a gas dissolved in a metal can be reduced merely by reducing the partial pressure of that gas, as for example, by evacuation. In practice, however, degassing is not as simple as this. Usually, Sievert s law is obeyed in pure liquid metals only when the solute gas is present in very low concentrations. At higher concentrations deviations from the law occur. 

Dissolution requires 1.5-2 hr. If the solution is not properly degassed, it will turn green prematurely. The green color indicates that oxygen absorption by the cuprous chloride-pyridine complex has occurred, but it also means that any undissolved cuprous chloride has been oxidized. Therefore, nitrogen should be bubbled into the flask at a brisk rate and stirring should not commence until addition of the cuprous chloride is complete. 

Use high quality pure nitrogen, do not use liquid air or liquid nitrogen that has been in contact nrith air for a long period (due to the dissolution of oxygen in it) which could EXPLODE in contact with organic matter. If the quality of the liquid nitrogen is not known, or is uncertain then it should NOT be used. 

Catalyst deterioration due to gas poisoning is only avoided by careful gas cleaning. Anodic oxidation followed by dissolution of Pt and transfer to the cathode is a serious cause for Pt loss. It is potential dependent and accelerates as the cathode potential increases, for instance under partial load or in off-time, when the cathode potential drifts toward the oxygen equilibrium potential. Therefore it is of utmost importance that whenever the fuel cell is switched off, the oxygen in the cathode lumen is rapidly exchanged by inert nitrogen and that the cell voltage under operation does not surmount 0.8 V. 

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