Gaseous standards

A secondary standard is prepared by mixing the substances in a dilution chamber with a voliune of more than 10 m. The larger the better, since the final concentrations of individual components have to be very low. Most of the halocarbons are liquids at room temperature, and are mixed at about the same relative ratios as found in seawater. The gaseous methyl chloride, methyl bromide and CFC-12 are released into the dilution chamber from glass ampoules. The toxicity of methyl halides deserves special attention. An evacuated stainless-steel cylinder, electropolished inside, with a volume of 1-5 L, is opened in the chamber. The gas mixture let into the cylinder is then pressurised with nitrogen gas to reach the required concentration level (mixing ratio). 

A new secondary standard is not stable the first weeks after preparation. In particular carbon tetrachloride adsorbs to the cylinder walls and decreases in concentration. Water vapour in the standard mixture seems to prevent the decrease of carbon tetrachloride concentrations and should therefore be added to the mixture. After calibration, the secondary standard is left to stabilize for at least one month, after which time it is re-calibrated. 

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Comparing Equations 26 and 27 to Equation 8 implies a new standard state (to be denoted as the Henry s Law standard state or HL s.s.) whose chemical potential is related to that for the gaseous standard state by... 

Fig. 2-43. Energy balance in the reaction of normal hydrogen electrode H2(sid.p>j = hydrogen molecule in the gaseous standard state (at 1 atm) H( gro. i) = hydrated proton of unit activity = real potential of the hydrated proton of unit activity a.ajHE) = real potential of the equilibrium electron of NHE (= Fermi level cpcnhe) of NHE).

L OX , + eligand coordinated with redox particles and ejsTD, is the electron in the gaseous standard state at the outer potential of the aqueous solution (Refer to Chap. 4.). The following reaction cycle may be used to obtain the energy relationship between the two redox reactions ... 

First, the PTR-MS accurately measures the test-gas concentration across almost the entire range of concentrations, with most values lying close to or within 10% of the predicted concentrations. This is comparable to observations made by Hansel et al. (1998) who carried out a similar experiment with benzene and toluene gaseous standards. Furthermore, the value of 10% is close to the lower range of the accuracy of the procedure used to standardize the test gas and is well within the 30% uncertainty range given by Lindinger, Hansel and Jordan (1998). 

We can now compute the exergy of the gaseous Standard Chemical Exergy ... 

For the ideal gaseous standard state, is evidently the molar enthalpy of an ideal gas. For standard states based on Henry s law, where y 1 as X ot m 0,lTi is the partial molar enthalpy of the solute in the hypothetical pure substance having yg = 1 or the hypothetical ideal one molal solution respectively. Substances in these strange states have partial molar enthalpies (and volumes) equal to that at infinite dilution, hence providing a method of measurement. This can be seen by considering Equations (8.38) and (8.39), which show that 71° becomes equal to // when y is 1.0. Therefore for Henryan standard states where y, -> 1 as X or m 0, must be the partial molar enthalpy of i at infinite dilution, and for Raoultian standard states where y, 1 as Xj -> 1, //° must be the partial molar enthalpy (the molar enthalpy) of pure i (confirming what we stated by simple inspection, above). 

A third fit parameter (in addition to a and b) is a scaling factor (1 — ) which is multiplied by Equation (13.47) written for the pure solvent (water) and then subtracted from the binary (13.47). The final form for the difference in chemical potential for the nonelectrolyte in the aqueous and gaseous standard states is... 

The exact volumes of the loojis for water and gaseous injections are determined following a procedure described by WMe et al. (1993). One loop at a time is filled with distilled and degassed water, and the mass difference between the filled and empty loop (while connected to the valve body) is measured to an accuracy and precision of +0.05 %. The internal loops in valve 2 are calibrated against gaseous injections into valve 3 of easily quantifiable halocarbons available as both liquid and gaseous standards, e.g., carbon tetrachloride and CFC-11. The volume of the loop can hardly be determined with an accuracy better than 5 %. On the other hand, these loops are used for applications where this level of data quality suffices. 

Calibration A precise comparison to a known quantity. Adjusting an instrument to reflect the value of a known gaseous standard. 

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