The Stoichiometric Technique

The simplest method to measure gas solubilities is by what we will call the stoichiometric technique. It can be done either at constant pressure or with a constant volume of gas. For the constant pressure technique, a given mass of IL is contacted with the gas at a fixed pressure. The liquid is stirred vigorously to enhance mass transfer and allow approach to equilibrium. The total volume of gas delivered to the system (minus the vapor space) is used to determine the solubility. If the experiments are performed at suSiciently high pressure that the ideal gas law does not apply, then accurate equations of state can be employed to convert the volume of gas to moles. For the constant volume technique, a known volume of gas is contacted with the stirred ionic liquid sample. Once equilibrium is reached, the pressure is noted, and the solubility is determined as before. The effect of temperature (and, thus, enthalpies and entropies) can be determined by repeating the experiment at multiple temperatures. 

An alternative technique to the stoichiometric method for measuring gas solubilities has evolved as a result of the development of extremely accurate microbalances. The gravimetric technique involves the measurement of the weight gain of an IL sample 

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The advantage of the stoichiometric technique is that it is extremely simple. Care has to be taken to remove all gases dissolved in the IL sample initially, but this is easily accomplished because one does not have to worry about volatilization of the IL sample when the sample chamber is evacuated. The disadvantage of this technique is that it requires relatively large amounts of ILs to obtain accurate measurements for gases that are only sparingly soluble. At ambient temperature and pressure, for instance, 10 cm of l-n-butyl-3-methylimida2olium hexafluorophosphate ([BMIM][PFg]) would take up only 0.2 cm of a gas with a Henry s law constant of... 

The main advantage of the gravimetric technique is that it requires a much smaller sample than the stoichiometric technique. In many cases, samples as small as 70 mg are sufficient. Accurate temperature and pressure control and measurement are still required, but gas adsorption on the metal walls of the equipment is no longer a concern because it is only the weight gain of the sample that is measured. 

Although the solubilities of gases in ILs are extremely important, at the time of this writing the number of published studies are limited. Some measurements were presented in oral and poster presentations at a five day symposium dedicated to ionic liquid research at the American Chemical Society national meeting in San Diego in April, 2001. Scovazzo et al. [14], for instance, presented preliminary results for CO2 and N2 solubility in [BMIM][PF6], and Rooney et aL [15] presented the solubihty of several gases in several different ILs as determined by the stoichiometric technique. A recent manuscript [16] presented Henry s law constants for H2 in two ILs. Given the lack of availability of other data, we concentrate below on the data collected in our laboratories. 

Various spectroscopic techniques can also be used to measure gas solubilities in ILs. For instance, Welton and coworkers have used proton NMR spectroscopy [16] to determine the solubility of hydrogen in a series of ILs. Since hydrogen exhibits low solubility in ILs and has such a low molecular weight, it is difficult to measure gravimetrically or by any of the stoichiometric techniques. As a result, it is particularly well suited to determination by spectroscopy. In addition, Kazarian and coworkers have measured CO2 solubility by infrared spectroscopy [17]. In general, spectroscopic techniques are quite attractive, as long as extinction coefficients do... 

As we saw in Section L, titration involves the addition of a solution, called the titrant, from a buret to a flask containing the sample, called the analyte. For example, if an environmental chemist is monitoring acid mine drainage and needs to know the concentration of acid in the water, a sample of the effluent from the mine would be the analyte and a solution of base of known concentration would be the titrant. At the stoichiometric point, the amount of OH " (or 11,0 ) added as titrant is equal to the amount of H30+ (or OH-) initially present in the analyte. The success of the technique depends on our ability to detect this point. We use the techniques in this chapter to identify the roles of different species in determining the pH and to select the appropriate indicator for a titration. 

We have already seen how to estimate the pH of the initial analyte when only weak acid or weak base is present (point A in Fig. 11.6, for instance), as well as the pH at the stoichiometric point (point S). Between these two points lie points corresponding to a mixed solution of some weak acid (or base) and some salt. We can therefore use the techniques described in Toolbox 11.2 and Example 11.6 to account for the shape of the curve. 

A simple, reliable, and fast method of determining the pH of a solution and of monitoring a titration is with a pH meter, which uses a special electrode to measure H 0+ concentration. An automatic titrator monitors the pH of the analyte solution continuously. It detects the stoichiometric point by responding to the characteristic rapid change in pH (Fig. 11.9). Another common technique is to use an indicator to detect the stoichiometric point. An acid-base indicator is a water-soluble organic dye with a color that depends on the pH. The sudden change in pH... 

The average rate of a reaction is the change in concentration of a species divided by the time over which the change takes place the unique average rate is the average rate divided by the stoichiometric coefficient of the species monitored. Spectroscopic techniques are widely used to study reaction rates, particularly for fast reactions. 

The technique of titration is equally useful for the titration of an unknown base by a solution of strong acid. The calculations proceed exactly as described previously. For the titration of a base, the stoichiometric point is reached when the number of moles of added acid in the titrant equals the number of moles of base in the unknown solution. Stoichiometric point Moles H3 O added = Moles base present... 

XPS investigations of the composition of the anodically grown passive layer on Ti electrodes were performed by Armstrong and Quinn [123, 124], The formation of a suboxide layer between the underlying Ti metal substrate and the stoichiometric Ti02 on top was demonstrated using XPS, AES and electrochemical techniques. 

For mechanisms that are more complex than the above, the task of showing that the net effect of the elementary reactions is the stoichiometric equation may be a difficult problem in algebra whose solution will not contribute to an understanding of the reaction mechanism. Even though it may be a fruitless task to find the exact linear combination of elementary reactions that gives quantitative agreement with the observed product distribution, it is nonetheless imperative that the mechanism qualitatively imply the reaction stoichiometry. Let us now consider a number of examples that illustrate the techniques used in deriving an overall rate expression from a set of mechanistic equations. 

Many different analytical techniques were developed for the estimation of hydroperoxides. Among them, the iodometric technique has been used for a long period of time [60]. According to this method, peroxide is reduced by HI, and diiodine is formed in stoichiometric quantity. The amount of the formed I2 is measured by reduction with thiosulfate using any titrometric technique or photometrically. 

During the latter part of the nineteenth century and the early years of the twentieth century, there was considerable controversy over the composition of chemical compounds—were compounds strictly stoichiometric, with an immutable composition, or could the composition vary. Indeed, at the turn of the twentieth century, even the existence of atoms was a subject of debate. The principal techniques involved at this epoch were accurate quantitative chemical analysis and metallo-graphic studies of phase equilibria. The advent of X-ray diffraction studies effectively resolved the problem, and the experimental evidence for composition ranges of many solids became incontestable. 

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