紫外-可见吸收光谱结合高斯多峰拟合技术测定甲基红酸离解常数

张建华 刘 琼 陈玉苗 刘兆清 徐常威

(广州大学化学化工学院,广州510006)

1 Introduction

The acid-base ionization equilibrium exists in aqueous solution of many organic dyes.Acid dissociation constant is a very important parameter to indicate degree of ionization at the different pH values in acidic organic dye solutions.1-3The acidbase ionization equilibrium of organic dyes and the determination of acid dissociation constant in aqueous solution are of great significance for many practical applications and scientific research areas such as acid-base titration,complex formation, solvent extractions,capillary electrophoresis,4chromatographic retention characteristics,5,6potentiometric titration,fabric dyeing and finishing,7environmental monitoring and protection,8-12drug research and development.In particular,drug synthesis, production,purification,formulation,dissolution,absorption, distribution and metabolism processes are closely related with the pKa.13,14Many determination methods for acid dissociation constants of organic dyes have been developed.3,15-17The most commonly used methods are potentiometric titration,5,18-21conductivity,22,23capillary electrophoresis,15capillary electrophoresis mass spectrometry,24nuclear magnetic resonance spectroscopy,25-28liquid chromatography,6,16,29infrared spectroscopy,30,31Raman scattering,32fluorescence spectroscopy,33,34UV-visible spectrophotometry,35-37theoretical calculation38and so on.Among them,spectrophotometric method has high precision and accuracy.The linear relationship between absorbance and concentration of color solution with a certain thickness is determined by Lambert-Beerʹs law,which gives a theoretical foundation of the spectrophotometric determination of the pKaof an acidbase indicator.

In 1958,Tobey39determined the pKaof methyl red using singlewavelength spectrophotometry and the pKais 5.02 at 300.35 K.Since then,a lot of work to determine the pKaof organic dyes using single-wavelength spectrophotometry has been reported.3,35-37Single-wavelength spectrophotometry is very suitable to determine the position of absorption peaks of acid and base species which are separated.However,in most cases,the maximum absorption peaks of acid and base species are association with each other in organic dye aqueous solutions.This situation leads to difficulties in data processing and determination of the pKa.3The multi-wavelengths spectrophotometric method has been adopted to determine the value of pKa.4,40,41Target factor analysis,4rank annihilation factor analysis42,43and other methods6,44have been developed to deduce the pKavalues from the multi-wavelengths spectrophotometric data obtained at different pH values.

UV-visible absorption spectra of methyl red were measured at different pH values regulated by a series of acetic acid (HAc)-sodium acetate(NaAc)buffer solutions with different concentrations and characterized by an overlap of a principal peak of acidic specie of methyl red(HMR)at 520-550 nm and a shoulder peak of basic specie of methyl red(MR-)at 425-460 nm.A multi-peaks Gaussian fitting method based on Origin from Microcal Company was used to interpret the spectra in this study.The multi-peaks Gaussian fitting calculation45-47on the overlap peaks gave the integrated absorbance ratioA1/A2,then the pKaof methyl red was obtained.

The studies on organic dye-surfactant interactions in aqueous buffered systems are of great importance in analytical chemistry,pesticide efficiency,pharmaceutical development, fabric dyeing and so on.Therefore,a new procedure based on multi-peaks Gaussian fitting method was firstly performed in dye-surfactant interactions of methyl red with sodium dodecyl sulfate(SDS),cetylammonium bromide(CTAB),which will enrich research methods in this research area.

2 Experimental

2.1 Reagents

Methyl red,anhydrous NaAc,CTAB,SDS were of analytical grade and purchased from Sigma-Aldrich.Ethanol,HAc, HCl were of analytical grade and purchased from Guangzhou chemical reagent factory.All solutions were prepared with distilled water.

(1)All the solutions of methyl red were prepared according to the literature37and the pH values were achieved using a certain concentration of NaAc-HAc buffer solution.

(2)A series of methyl red solutions with SDS were prepared by a concentration range of SDS from 0.001 to 0.018 mol·L-1and the concentration of methyl red was fixed.

(3)Instead of SDS,a series of methyl red solutions with CTAB were prepared by the concentration of CTAB at 0.0001 mol·L-1(below the critical micelle concentration(CMC)of CTAB)and 0.001 mol·L-1(above the CMC of CTAB)and the concentration of methyl red was fixed.

2.2 Apparatus

UV-visible absorption spectra were recorded on Shimadzu UV2550-UV-visible spectrophotometer(Japan)equipped with 10 mm path length quartz cell.Distilled water was used as reference solution.All the spectra were obtained between 320 and 750 nm and the sampling interval was 0.5 nm.The pH values of the solutions were measured by PHSJ-4A-type PH meter furnished with a combined glass electrode(Shanghai Precision Division-Shanghai Lei magnetic)which was pre-calibrated with at least two buffer solutions at pH 4.00 and 10.00.Each pH value was obtained from the average of three measurements.All pH values and spectra were measured at a constant temperature which was controlled by a super-heated water circulating thermostat bath.The measurement data were imported into PC with Microcal Origin 7.0 for data processing.

2.3 Multi-peaks Gaussian fitting method

The UV-visible spectrum data were imported and plotted with Origin 7.5 software.When menu command of Analysis/ Fit Multi-Peaks/Gaussian was selected,a dialog box with the number and the initial half-width estimated default values of the peaks appeared in the current graphics window.After doubleclicking the mouse at 425 and 520 nm of the spectra respectively,Origin 7.5 automatically completed a multi-peaks Gaussian fitting procedure on certain spectra and gave the line-shape parameters of the UV-visible spectra in the result window.

3 Results and discussion

3.1 Principle of multi-peaks Gaussian fitting method

Ionization equilibrium of methyl red in aqueous solution is given as the following equation

The pH value range of color change of methyl red in aqueous solution is well known as 4.4-6.2.When pH values are 4.63, 4.93,5.39,5.68,the UV-visible absorption spectra and their multi-peaks Gaussian fitting results are shown in Fig.1.It is shown that integrated absorbanceA1of base MR-peaks increases and integrated absorbance A2of acid HMR peaks decreases with the increase of pH value.The increase and decrease of the integrated absorbance of the MR-and HMR are objectively due to the change of the relative concentrations of MR-and HMR.

Existence simultaneously of HMR and MR-in methyl red solution results in spectra with two peaks at a certain pH value. Multi-peaks Gaussian fitting on the spectra with two peaks satisfies following equation

where,y0is baseline,λmax1and λmax2are the maximum absorption wavelengths,w1and w2are half peak widths,A1and A2are the integrated absorbances of the two peaks for MR-and HMR.

Multi-peaks Gaussian fitting method based on the spectragives the λmax1,λmax2,w1,w2,A1,A2and these data are listed in Table 1.

Table 1 Results of the multi-peaks Gaussian fitting method on the spectra of methyl red solution at different pH values and 298.15 K

3.2 Relationship between pKaand the relative integrated absorbance

Acid dissociation constant of methyl red is given as following equation

where,ε1and ε2are the molar absorption coefficients of MR-and HMR,respectively.Then,the methyl red absorption spectra(Fig.2)were measured in base(pH=9)and acid(pH=2)conditions respectively.

When the value of pH is 9,there is only MR-in methyl red solution and the[MR-]=C,then

When the value of pH is 2,there is only HMR in methyl red so-lution and[HMR]=C,then

Fig.2 UV-visible absorption spectra of methyl red solution measured in basic(pH=9)and acid(pH=2)solutions

Setting ε=ε1/ε2and A1or A2are obtained with the same concentration of methyl red,then

The value of ε is obtained as 0.56.

From Eqs.(4)-(8),pKacan be calculated from Eq.(10)

The value of pKais obtained from Eq.(10)at different pH values.The Multi-peaks Gaussian Fitting results are shown in Table 2.

3.3 Error analysis for determination of pKa

It is very clear that the values of pKalisted in Table 2 are slightly lower than the values(4.90±0.20)in literature.39,42,48However,the results are within the error range for the values of pKaand prove the reliability of multi-peaks Gaussian fittingmethod for determination of pKa.The concentrations of MR-and HMR in solutions are determined by the curves for absorbance of maximum absorption wavelength at 425 nm for MR-and that at 520 nm for HMR.However the concentrations of MR-and HMR are determined by the methyl red color range which are unreliable because of blue shift of MR-and red shift of HMR absorption peak.18The absorption peak of MR-bluely shifts from 456 to 436 nm and the absorption peak of HMR redly shifts from 531 to 546 nm when the pH value increases from 4.63 to 5.68 in Table 1.The shifts of absorption peaks make the relationship of the concentration and absorbance diverge obviously from the standard curve and cause about ±(3%-5%)systematic error for pKameasurement which is higher than true value.

Table 2 Relationship between Kaand temperature

A multi-peaks Gaussian fitting based on the absorption spectra of methyl red within the color change interval has been made and gives the relative integrated absorbance of MR-and HMR absorption peaks which is used to determine the relative concentration of MR-and HMR instead of using the unreliable standard curves obtained from the acid and base conditions in this study.So the pKameasurement results are more reliable and repeatable.

3.4 Thermodynamics on ionization equilibrium of methyl red

The pKavalues of methyl red were determined by UV-visible absorption spectroscopy with multi-peaks Gaussian fitting method at different temperatures and shown in Table 2.The values of pKadecrease considerably with increase of temperature,which is consistent with the literature.49Generally it is believed that the increase of temperature promotes ionization of organic dyes and leads to decrease of pKa.

A few of thermodynamic models have been developed for acid dissociation equilibrium of oganic dyes.49Here,the‘density’model has been selected.49The standard Gibbs free energy of reaction(ΔrG⊖)for the ionization equilibrium of methyl red satisfies the following Eq.(12)

where pKa=-lgKa,ρwis water density(kg·m-3),T is the thermodynamic temperature(K),and a-g are model parameters.The relationship between the lgKaand 1/T is shown in Fig.3.The ΔrG⊖obtained from Eq.(12)with parameters which were obtained from non-linear curve fitting on data of Fig.3 with change of temperature is shown in Fig.4.The ΔrG⊖increases with increase of temperature and is consistent with the literature.49

3.5 Effect of surfactants on pKa

3.5.1 SDS-methyl red system

The variety of CMC of surfactants can be measured accurately by UV-visible absorption spectrum combined with multipeaks Gaussian fitting method in order to study their aggregation behavior.45To study the effects of aggregation behavior of surfactant on pKa,SDS-methyl red and CTAB-methyl red aque-ous solution systems were selected.Fig.5 shows the relationships between pKaof methyl red and the SDS concentration at different temperatures.The values of pKadecrease considerably with the increase of temperature,indicating that the ionization equilibrium of methyl red moves to the right in the existence of anionic surfactant SDS.When the SDS concentration is lower than the CMC,no significant change of pKavalues is observed.When the SDS concentration is higher than the CMC,pKavalues increase with the increase of SDS concentration.It is undoubted that the pKais sensitive to the CMC and used for determination of the CMC of SDS,for example the CMC of SDS are 8.54×10-3,8.68×10-3,8.82×10-3,9.66×10-3mol·L-1at 303.15,308.15,313.15,318.15 K,respectively, which are consistent with the literature.50

Fig.3 Relationship between lgKaand 1/T

Fig.4 Relationship between ΔrG⊖and temperature

Fig.5 Relationships between pKaof methyl red and the SDS concentration at different temperatures

Table 3 Spectrum line-shape parameters of methyl red with different concentrations of SDS at 298.15 K and pH=5.40

In order to investigate further effect of SDS on ionization equilibrium of methyl red in solution,a multi-peaks Gaussian fitting method on UV-visible spectra of the series of solutions (pH=5.40)containing different concentrations of SDS from 0.001 to 0.018 mol·L-1gives spectral line-shape parameters such as A1and A2,λmax1and λmax2,w1and w2of MR-and HMR at 298.15 K in Table 3.

The spectral line-shape parameters listed in Table 3 are sensitive to the SDS concentration and changes suddenly at CMC of SDS which are shown in Fig.6 and Fig.7.The red shift of λmax1and blue shift of λmax2with the increase of the SDS concentration are observed and more obvious after formation of the SDS micelle.There is a sudden change at 0.0083 mol·L-1in Fig.6 and Fig.7.At first,the λmax1of MR-absorption peak increases slowly and then increases quickly with the increase of SDS concentration.Sudden change occurs at the CMC of SDS.The change of λmax2is exactly the opposite with λmax1.The w1of MR-absorption peak increases then decreases quickly with the increase of SDS concentration.Sudden change occurs at the CMC of SDS.The change of w2is exactly the opposite with w1.Generally,relationship between spectral line-shape parameters such as A1,A2,λmax1,λmax2,w1,w2and concentration of surfac-tant has been used by us for CMC determination.45

Fig.6 Dependence of the maximum absorption wavelengths of MR-and HMR on the concentration of SDS at 298.15 K

Table 4 Spectrum line-shape parameters and effect of aggregation behavior of CTAB on pKaof methyl red with different concentra

3.5.2 CTAB-methyl red system

CTAB with methyl red system is much more complicated than SDS with methyl red system.The UV-visible absorption spectrum of CTAB with methyl red solution is shown in Fig.8 when CTAB concentration is bellow and above the CMC.Table 4 gives the spectrum line-shape parameters and the effect of aggregation behavior of CTAB on the pKaof methyl red solutions at 303.15 K.

When the CTAB concentration is 0.0001 mol·L-1which is bellow the CMC of CTAB,the UV-visible absorption spectrum of CTAB with methyl red solution is similar with that of methyl red solution.However,the pKavalue of methyl red with 0.0001 mol·L-1CTAB obtained by multi-peaks Gaussian fitting method is 4.67 which is 0.09 lower than that of the methyl red(4.76)at the same temperature.Furthermore,pKavalue decreases slowly with the increase of CTAB concentration.When the CTAB concentration is 0.001 mol·L-1,the pKavalue decreases rapidly to below 3.32 and the color of solution system changes completely from red to yellow at the same time,which indicates that there is mainly MR-species in the solution. These results illustrate that formation of the CTAB micelle can changes the range of changed color of methyl red from 4.4-6.2 to 3.0-5.0 and makes the pKavalue of methyl red decrease remarkably.Generally the significant decrease of pKavalue is attributed to electrical double layer of CTAB micelle which absorbs selectively MR-and makes the ionization equilibrium of methyl red move obviously to the right.

Fig.7 Dependence of the half-widths of MR-and HMR on the concentration of SDS at 298.15 K

Fig.8 UV-visible absorption spectra of methyl red solution with different concentrations of CTAB at 303.15 K

4 Conclusions

In this study,a multi-peaks Gaussian fitting method based on the UV-visible absorption spectra is firstly used to determine the pKavalue of organic dyes such as methyl red.The reliability of the method is adequately proved by an excellent agreement of the measurement results with literature.There are several advantages,such as the easy operational procedure,explicit physical meaning,and the accurate measurement results for the method.

The relative concentration of the MR-and HMR in solution has been determined by the relative integrated absorbance calculated from multi-peaks Gaussian fitting method based on the UV-visible spectra.This method avoids successfully the systematic error of 3%-5%of pKameasurement from the standard curves which is established from the absorbance of MR-maximum absorption wavelength at 425 nm and HMR maximum absorption wavelength at 520 nm versus concentration of MR-and HMR at the methyl red color range,respectively. Moreover,the effect of SDS and CTAB on ionization equilibrium of methyl red also has been studied and some reliable results with the method have been obtained.The spectral lineshape parameters such as A1,A2,λmax1,λmax2,w1and w2of MR-and HMR that obtained from multi-peaks Gaussian fitting method on the UV-visible spectra of SDS-methyl red and CTAB-methyl red solutions have been firstly discovered to be sensitive to aggregation behavior of surfactants SDS and CTAB.The CMC values can be determined by the dependence of the three sets of parameters on the surfactant concentration which should support each other.

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