The Spectrophotometric Determination Of An Equilibrium Constant

The Spectrophotometric Determination of an Equilibrium Constant

A two wavelength ratio method as applied to polyelectrolyte solutions is compared to the standard method which makes use of the previous determination of the dye-metal ion formation constant. The ratio method is found to be a convenient and reliable method which is not influenced by decomposition of the dye or by statistical errors in the extrapolation procedure. The activity coefficients as determined by the two wzwelen h dye spcctrophotometric method are compared to results of Donnan exclusion measurements, and of EhlF measurements using a calcium read more selective electrode.

The results of the spectrophotometric method are equal to those of the two other methods within the limits of error in the latter. The spectrophotometric measurements can The Spectrophotometric Determination of an Equilibrium Constant to much lower ion activaties than the other two methods, and can be done in the presence of a large excess of added electrolyte, yielding results of considerably improved precision: when com- pared to Donnan and EMF methods. Thus it is of The measurement of the divalent metal ion activity considerable interest to see how the degree of binding of or the free divalent metal ion concentration in poly- earth alkaline and transition metal ions to a variety of electrolyte solutions containing both uni-univalent and polyions corresponds to this theoretical treatment. The binding of multivalent metal ions by poly- a comparison is very limited.

This ionic strength released univalent ions. However, it is far better to study dependence has recently been examined in detail by the binding of the divalent ion directly. For this pur- Manning [I 3 and Iwasawho considered the speci- pose, measurements in mixed counterion polyelectro- fic polyelectrolyte The Spectrophotometric Determination of an Equilibrium Constant binding process.

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Donnan equilibrium measurements often suffer equilibrium in eq. In practice this means that for activity ranges determinations, especially at low metal ion concentra- of physiological interest the apparent equilibrium con- tions or when the amount of metal ion bound is small. The dye should advantages of relative simplicity, applicability in the have large extinction The Spectrophotometric Determination of an Equilibrium Constant in both complexed range of metal ion concentrations of physiological in- and uncomplexed forms, and the extinction maxima of terest, and absence of interference by excess 1 : 1 elec- the two forms should be well separated.

Finally, the trolyte. It is the purpose of this paper to is pH independent K, should not vary with pH at examine the applicability of the dye spectrophoto- least in the pH range studied by these authors. Method gions where continue reading techniques provide reliable data.

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In a subsequent paper the Kohn method will be ap- Consider the dye-metal ion equilibrium: plied to the study of Mg? Moreover, the formation constant of the ted. At a given wavelength the total absorbance of HD and The two wavelength ratio method first suggested by MD is given by: Buddecke and Drzeniek [18] was later well investigated by Kohn et al.

HD and MD at the given wavelength. The M, and AE the absorbance in the presence of excess M. For un- trapolation is needed to obtain K from the absorbance known solutions ahi, the activity of the free divalcnt measurements in the reference solutions, resulting in metal ion, can be obtained by again determining Q for large uncertainties in K which in turn can seriously these solutions, and then using the calibration curve to influence the activity determinations in the unknown.

The long extrapolation involved in this mum. Combining eqs. We will call this procedure the modi- The Spectrophotometric Determination of an Equilibrium Constant.

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This is zero. When we have a large excess of metal ion Q ap- 58 J. The buffer concentration in all linear in anI. Thus we can predict the general shape of solutions was less than 2. There are two rhe calibration curve.

All solutions were made in deionized water of uncertainties associated with it.]

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Furthermore, after the completion of this experiment, the formation of the equilibrium constant of the iron thiocyanate complex could be determined. In order to accomplish these tasks, five different solutions were made. Upon completion of the experiment, the results of the aforementioned experiments were calculated to an average of the final formation constant of Introduction: The experiment for the Spectrophotometric Determination of Equilibrium had the main objectives of determining the position of equilibrium and the formation of the equilibrium constant of the iron thiocyanate complex. In order to do so, the use of a spectrophotometer was incorporated into the experiment. In order to obtain an accurate end result, the absorbance of five various samples was used. In order to study the position of equilibrium and the formation of the equilibrium constant, within the realm of the experiment, the absorbance was used for the purpose of monitoring the changes in concentration.