THE YELLOWNESS INDEX USE FOR THE ACID-BASE EQUILIBRIUM STUDY IN XYLENOL ORANGE AQUEOUS SOLUTIONS

The possibility of the yellowness index use for the acid-base equilibrium study and ionization constants determination in dyes solutions is studied. It is shown that the yellowness index is suitable for the set task. Its use enables a more reasonable mathematical apparatus algorithmization, automation, simplification and in comparison with the existing approaches of the chemical chromaticity method and classical physico-chemical methods.


Introduction
The aim of study is to establish the dyes physicochemical and acid-base characteristics of the reagents that are realized in the solutions with a broad values change of acidity or solvents.Also there is the determination of ionization constants (pK) and the spectral characteristics of individual reagents states or forms.The knowledge of pK values provides a basis for understanding of the chemical reactions between the compound of interest.Additionally, they play a ma�or role in acid-base titrations, complex formation and various analytical procedures.Also, the pK values of a compound influence many characteristics such as its reactivity, spectral properties (color).Conse�uently, several publications were devoted to the determination of pK experimentally and theoretically [1,2].
Nowadays interest in the chemical chromaticity (CC) method (tristimulus colorimetry method) has grown very much.The CC method approaches have been applied to create test methods [3], to assess the food �uality [3,4], as well as to study acid-base e�uilibrium in organic dyes solutions and analytical reagents [3,5].It is recommended to use a saturation (S) function and its derivative -the specific color discrimination for ionization constants determination in the dyes solutions [3,6].Yellowness Index is the indicator that describes the color change of the sample from white to yellow, and is calculated from the array of spectrophotometric data.Its calculation is methodologically simpler and easy to automatize.The yellowness index is used to assess the textile and food industries �uality [5][6][7][8].In this study we investigated the possibility of a more informative yellowness index (Y I ) use for acid-base e�uilibrium study and ionization constants determination.

Apparatus
Acid-base e�uilibrium in dyes solutions was studied by the �V/VIS-spectroscopy and chemical chromaticity methods using a spectrophotometer SF-56 (OKB «LOMO-Spektor», St.-Petersburg, Russia) in the wavelength range 380-780 nm in �uartz cuvettes with the thickness of the absorbing layer 1 cm.The medium's acidity was controlled by pH meter I-130 («ZIP», Gomel, Belarus).

Reagents
Xylenol orange (Merck) (XO) was chosen as a dye, whose functional -analytical groups are characterized by the reliably determined pK (table 1).The initial XO solution with 10 -3 M concentration was made by sample dissolution (m = 0,1682 g) in 250 ml of distilled water.Solutions with lower concentrations were prepared by dilution of initial solution.The medium acidity was created by sulfuric acid and sodium hydroxide.

Procedures
The necessary medium acidity was created for рК determination.It was provided by putting the dye's ali�uot of solution into the 50 ml flask.The obtained solutions absorption spectrum were registered and coordinates of color XYZ were calculated from the array of spectrophotometry data.Parameter Y I was calculated by formulae: where Y I -yellowness index; X, Y, Z -coordinates of color in the CIEXYZ color space.

Effect of the medium acidity
The absorption spectra of XO in a�ueous solution were studied by various the medium's acidity.Figure 1 (parts a and b) shows the changes in the absorption spectra of XO.
Figure 1 shows the changes in the absorption spectra of XO.As can be seen from fig. 1 (a) in the acidic medium XO has absorption bands at 430 -460 nm (curves 2, 3), 505-515 nm (curve 1) and absorption band with maximum at 670±10 nm (curve 4).Increasing the concentration of OH-groups in the solution contributes to formation of new acid-base forms of XO.They are characterized by isosbestic point at 490 nm and absorption maxima at 575 nm (fig. 1 b, curves 1-3).The observed changes in the absorption spectra are explained the acid-base e�uilibria in XO solution.Each balance describes appropriate pK values.

Determination of pK values
We studied the possibility of the yellowness index use for determining the dye рK values.Figure 2 shows the Y I dependence of the medium acidity.It is known that for an absolutely white body -Y I = 0, for yellow and red ones -Y I >0; for blue and green ones -Y I <0.The fracture's points on the curve of Y I values dependence from pH corresponds to the areas of transition from one form to another.The corresponding values -lgCH + are received by finding the coordinates of each peak, which are numerically e�ual to the values of the XO рК in solution.The formation of 9 peaks confirms the presence of 9 values of рK (table 1) for different molecule forms of the XO in the solution, each of which is characterized by specific Y I values, whose sign indicates the solution reagent color in a wide range of medium's acidity.The graphic dependence analysis (fig.2) points to the existence of ten differently charged forms of XO in solution, which depend on the medium acidity and are in dynamic e�uilibrium; each of which is characterized by pK value (table 2).The results (Table 2) correlate with those presented in Table 1 literature data, which indicates that the results are reliable and the yellowness index use for the acid-base properties study and the establishment of the corresponding values of the dyes' molecules and organic analytical reagents рK are suitable.
The yellowness index use allows to simplify the calculations in comparation with the specific color differences parameter.The algorithm of рK definition with the specific color discrimination has the following form:

absorption spectrum -the color coordinates XYZ -the color coordinates LAB -the color saturation S -parameter specific color discrimination -the dependence identification of the SCD = f (-lg CH + ) -definition of the рK.
The algorithm is simplified by using the yellowness index:

Conclusion
Thus, the use of color characteristics of chemical systems in chemistry of dyes enhances the study of their acid-base e�uilibria.Also it should be noted that the chemical chromaticity method compared to other physico-chemical methods (spectrophotometry, potentiometry, etc.) provides more complete information about available and appropriate protolytic e�uilibria constants.As a result of our work we established the principal possibility of the yellowness index use for the acid-base e�uilibrium study and pK determination in the dyes solutions.The yelowness index allows to simplify and reduce the mathematical apparatus instead of the existing approaches of chemical chromaticity method and classical physico-chemical methods.

Fig. 2 .
Fig. 2. The Y I dependence of the xylenol orange solution on the medium's acidity