Anal. Chem. 1989, 61 I 285-288 285 excitation source. In addition to a gain in sensitivity, the instrument also acquires the complete molecular spectrum thereby increasing the qualitative information obtained with every measurement. The ability to normalize the fluorescence signal to the scattered light signal pro6des a simpler and more direct method of correcting for source and readout detector drift than methods employing beam splitters and auxiliary detectors. The results of this work point to the use of the CCD fluorescence spectrograph as a practical detector for use in high-performance liquid chromatography and flow-injection analysis. ACKNOWLEDGMENTThe authors thank Gary Sims and Richard Aikens of Photometrics, Ltd., and Robert W. Fitts of RCA for their assistance and support in the design and use of scientific grade CCD detector systems. LITERATURE CITED(1) Bradley, A. B.; Zare, R. N. J. Am. Chem. Soc. 1976, 98, 620. (2) Rlchardson, J. H.; Ando. M. E. Anal. Chem. 1977. 4 9 , 955-959. (3) Roach, M. C.; Harmony, M. D.
CONCLUSIONSThe isoextraction method has been shown to be useful for the determination of the dissociation constants of organic acids. The primary advantage of the method is that it needs only pH measurements and concentration determinations by direct analysis. In this study, for example, no information was needed regarding the interactions of the extracting species in chloroform such as dimerization or hydration. Since concentrations can be determined a t very low levels, this method should be particularly useful for highly hydrophobic acids and bases which have low solubilities in water or which show effects of aggregation or other competing equilibria in dilute solutions. In parallel work, we have determined the pK, values of some bile acids, which exhibit such characteristics, a t concentrations of the order of M (13). The isoextraction cell should be readily adaptable for studying other types of equilibria such as self-association (3-5), mutual association, or complex formation and for other combinations of solvents.(11) R . A. Robinson and R. H. Stokes, "Electrolyte Solutions", 2nd ed., is expected to be negligible. Solvent transport can often be controlled to some extent. For example, it is possible to reduce or eliminate the difference in the initial water activity, a,, and, therefore, the transport of water, by the addition of nontransportable solute species such as inorganic salts. The duration of an experiment can be reduced by bringing the initial solute activities close to equilibrium a t the beginning of an experiment on the basis of preliminary runs.In the experiments reported here, a, was above 0.998 in all the limbs with a maximum difference of less than 0.0004. The transport of water was thus estimated to be of negligible consequence on theoretical grounds. Since an order of magnitude for rates of water transport in the isoextraction cell may be of some interest for the general use of the method, an experiment was conducted in which 10 mL each of four concentrated NaCl solutions, 5.79 m, with the initial water activity of 0.7698 (111, were placed in alternate limbs of the isoextraction cell along with four 10-mL samples of water. Other experimental conditions were identical to the ones described earlier. On shaking for 68 h, it was found that the salt solutions were diluted by an average factor of 1.08. The initial difference in a, between the water limbs (a, equaling unity) and the salt solutions in this experiment was 0.230 compared to values less than 0.0004 in the experiments reported earlier and the value of about 0.002 corresponding to an osmolality difference of 0.1. The time of shaking was roughly three times the 24 h found satisfactory for solute equilibration earlier. These data indicate that concentration changes due to water transport can be kept well below 0.1%) in the water-chloroform system for solutes with moderate partition coefficients. On the other hand, if isopiestic measurements are difficult for a solvent because of low vapor pressures, equivalent measurements may be possibl...
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