ANALYTICAL CHEMISTRY, VOL. 51, NO. 12, OCTOBER 1979
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be regarded as a “limiting case”, Le., in some special cases, e.g., a t very slow thermal perturbations (5), a quite good linearity may be observed. But even in these cases the physical significance of the parameters derived, remains rather obscure. This is why we proposed to derive E and 2 from a single signal and not from a set of signals obtained in different working conditions. The dependence of the peak height upon the distance z between the solid state surface and the measured absorption point is not surprising. This distance is one of the working conditions and its modifications lead to the modification of E and 2,and consequently to the modification of the shape and position of the signal. This is why we emphasized the importance of studying the influence of working conditions upon the kinetic parameters E and 2. The only point in which Tessari and Torsi agree with us is a case of misunderstanding. This concerns the relationship between the peak height and the heating rate. Torsi and Tessari presume on the basis of their first model and of a supplementary simplifying hypothesis a linear relationship, but they find experimentally a nonlinear one (2). They explain this disagreement by the imperfection of their model. We have shown ( I ) , that actually the experimentally found curve does agree with the model and only the supplementary hypothesis is wrong. In their comments Tessari and Torsi state once again the presumed linearity to be a consequence of their model and they believe that we are of the same opinion. But we maintain exactly the contrary. As the initial conditions T = To and 29 = 29 instead of T
= 0 and 29 = 1 are concerned, at the integration of the differential Equation 8 in ( I ) , we believe these not to be important. If we take the theoretical curve discussed in Tessari and Torsi’s comments, i.e., by presuming E = 320 kJ mol-’, 2 = 6.0 X l O I 7 s-l, and a heating rate of 50 K s-l, at the beginning of the atomization (29 = 0.99 29 O) the exponential integral value is p ( x ) = 2.16 X lo-=. By taking T = 298 instead of T = 0, a correction ought to be made, but its value is only of A p ( x ) = 1.07 X 1040. At higher temperatures and larger heating rates the importance of this correction is even less. On the other hand, 2 9 O can just be a scaling factor, and by taking 29’ # 1, the absolute value of n, cannot be calculated either, even if one of the more sophisticated models is used.
Sir: After Zsak6’s reply we feel that the main subject of our discussion has been shifted from arguing on some specific use of a kinetic model to whether it is at all feasible to use a kinetic model for describing the release of metal at the solid/gas interphase. In fact, in his extreme position ( E and Z depending on heating rate a and observation height z ) , Zsakd is getting down to a mathematical equation of a curve that might be useful for information storage and retrieval of data, but which contains parameters that no longer carry a physical meaning. We thus maintain that our model approach to metal
release at the solid/gas interphase is instead physically legitimate, supported by the experimental results and provides a better physical insight of the process.
LITERATURE C I T E D (1) J. Zsak6, Anal. Chem., 50, 1105 (1978). (2) G. Torsi and G. Tessari, Anal. Chem., 45, 1812 (1973). (3) J. ZsakB, J . Thermal. Anal., 5, 239 (1973). (4) S. L. PaverCFontana, G. Tessari, and G. Torsi, Anal. Chem., 46, 1032, (1974). (5) G.Torsi and G. Tessari, Anal. Chem., 47, 839 (1975); 48, 1318 (1976). (6) G. Tessari and G. Torsi, Anal. Chem., 47, 842 (1975).
JBnos Zsak6 Babes-Bolyai University Department of Chemical Technology str. Arany Janos 11 3400- Cluj-Napoca Rumania
RECEIVEDfor review January 25,1979. Accepted June 6,1979.
Gin0 Tessari* Giancarlo Torsi Istituto di Chimica Analitica Via G. Amendola, 173 70126 Bari, Italy
Separation of Humic Substances by pH Gradient Desorption from a Hydrophobic Resin Sir: Humic substances consist of a complex mixture of organic acids which are extracted from soils, sediments, and natural waters (1-3). The chemistry of humic substances is of particular interest owing to the significant role which these materials play in the natural environment. For example, humic substances are effective complexing agents for metal ions ( I , 4 , 5 ) and are active in various pedologic and geochemical processes (1-3, 6). No satisfactory fractionation method for humic substances has yet been developed. Consequently, humic fractions are operationally classified, on the basis of their aqueous solubility, into the common fractions of humic acid (soluble only a t pH 2 6.0), fulvic acid (soluble a t all pHs) and humin (insoluble at all pHs). When dealing with a soil or sediment source, the humic and fulvic acids are generally separated from the humin and other insoluble matter by means of an alkaline extraction. The humic acid is then precipitated by adjusting the pH to a low value (typically l.O), thus separating it from the fulvic acid. 0003-2700/79/035 1-2041$01 .OO/O
However, there are a number of drawbacks with this separation methodology: (i) The choice of precipitation pH is rather arbitrary and the humic acid precipitation may not be complete; and (ii) some fulvic acid is generally coprecipitated with the humic acid. Repeated washings of the precipitate may be necessary to signficantly reduce the amount of fulvic acid associated with the humic acid. Other techniques are employed to achieve the humic/fulvic extraction and separation when working with natural water samples. These generally involve preconcentration of the organic matter by adsorption on some solid matrix followed by its removal using a suitable solvent. Riley and Taylor (7) used Amberlite XAD-1 resin for this purpose, and Mantoura and Riley (8)reported a detailed study on the adsorption and desorption of humic substances with respect to XAD-2 resin. Mantoura and Riley (8)showed that different fractions of the organic matter could be successively eluted by washing the resin with buffer solutions in increasing pH values. Subse0 1979 American Chemical Society
