Indian Journal of Chemical Technology Vol. 8, July 2001, pp. 273-277
Beta-correction spectrophotometric investigation of aluminium complex solution with new ligand, dibromo-o-nitrophenylfluorone
Hong-Wen GAO•*, Shu-Ren SHib & Yu-Cheng u•
a School of Chemistry and Chemical Engineering, Anhui University, Hefei 230039, P.R. China bSchool of Life Science, Anhui University, Hefei 230039, P.R. China
Received 13 March 2000; accepted 23 February 2001
The new ligand, dibromo-o-nitrophenylfluorone (DBNPF) was found to sensitively complex aluminium (Al3+) at pH 8.5. The ordinary spectrophotometry was limited for use because of the serious interference of excess of DBNPF. The fj- correction theory was applied to the analysis of this reaction instead of the single wavelength spectrophotometry. It gave the simple determination of the complex characteristic factors. Results showed that the formed complex can be expressed as Al(DBNPF)5 at pH 8.5, the cumulative stability constant of complex equaled to 1.48x1020. For analysis of water samples, the detection limit of aluminium was 0.005 mg/L and the recoveries vary between 92.0 and 108% with the relative standard deviations less than 4.4%.
Aluminium (Al) exists widely in nature. The chromo- genic agents CAS1·2, aluminon3, were studied and ap- plied to the determination of trace aluminum by spec- trophotometry. In laboratory, the synthesis of new chromogenic reagent, dibromo-o-nitrophenylflurone (DBNPF) was reported and its structure is as follows:
HO,Jy~o
HOv;::OH
u
It was earlier applied to the determination of trace amounts of vanadium4
. In the reaction between Al(III) and DBNPF at pH 8.5 the solution changed into red from blue because of the sensitive absorption of the complex product at 560 nm and that of DBNPF at 480 nm. Because the difference in the two wavelengths is only 80 nm, the excess of DBNPF will affect notably the measurement of real absorption of the complex formed. As a result, the single wavelength spectro- photometry is unfair to the analysis of Al(III) com- plex solution with DBNPF. The updated dual- wavelength spectrophotometric method, beta- correction method5 have been applied6'8
for the inves-
*For correspondence (E-mail: gaohongw@ mail.hf.ah.ca;
Fax: 0086-551-5106110)
tigation of many complexes and the determination of trace amounts of metals because it may eliminate the effect of excess of DBNPF from the complex solution to give out the real absorption of Al-DBNPF complex.
This updated method was different from other dual- wavelength spectrophotometric methods9-11
. In this paper by means of the new principle, trace amounts of aluminium was determined accurately as well as some properties of Al-DBNPF complex solution can be worked out easily like the complex ratio, real non ap- parent absorptivity and stepwise stability constant. In this aspect, the recommended method was different from the classical methods, for example the molar ratio12, continuous variation13, equilibrium move- ment14, etc. For the determination of trace amounts of aluminium, this work produced satisfactory results.
The analysis of some samples showed that the recov- ery was between 92.0 and 108% and the relative stan- dard deviation was less than 4.4%.
Principle
Following expression is developed for the determi- nation of the real absorbance (Ac) of metal (M) com- plex (MLy) produced with a ligand (L):
M-{3M' A=----'--
c 1-af3
The symbols M and M' are the absorbances of the mixed solution of MLr and excess L measured at
274 INDIAN J. CHEM. TECHNOL., JULY 2001
wavelengths A.2 and A.1 against the blank reagent, re- spectively. The coefficients, a and ~ are named cor- rection factors and can be measured from only MLr solution and L solution and then computed as follows
E AI
a = - - -ML, E A2
ML,
and
E A2
~=-L
E AI L
Th e terms EMLy , EMLy , EL AI A2 AI an d EL A2 are t e mo ar h J absorptivities of MLy and L at wavelengths A.1 and A.2,
respectively.
The amount ratio (y') of L to complex M in their reaction may be expressed as follows
y'=l]X-L
c eM
where
aM-M' l] = (l-a~)A'o
The symbol YJ indicates the reacted percentage of L and the terms CM and CL are the concentrations (moi/L) of M and L in the beginning. A' 0 is the ab- sorbance of the blank reagent measured at wavelength A.1• If y' reaches maximum and remains constant, it was thought that y = y' where y is a natural number and it is named as the stoichiometric ratio ofthe com~
plex produced. In addition, the following expression was established for the stepwise stability constant (Kn)
of complex MLy from the reaction: MLn.1 + L ===
MLn. For this' purpose, such an M-L solution must be prepared to form the complex ratio y' between n-1 and nand studied successjvely.
K = y'+l-n
n (n-y')(CL -y'CM)
From each Kn the cumulative constant (K) of complex MLy can be calculated from the following expression:
K= K1xK2x. .. xKn ... XKy. In addition, from such a M-L reaction the stepwise absorptivity (real EML/2 not ap- parent EaA2
, n=1, 2, ... , y) of complex MLy may be ex- pressed as follows:
n-y' A ---=--E '
y'+ J-n ML •. ,
Here, the symbol, 8 the cell thickness (em) and the others have the same meaning as in the equations above.
Experimental Procedure Apparatus and Reagents
Absorption spectra were recorded with a Lambda 19 (Perkin-Elmer) double-beam recording spectro- photometer with 1.0-cm cells and pH was measured with a Model PHS-2C acidimeter (made in Xiaoshan, China).
Standard Al(III) solution, 1000 mg/L was prepared by dissolving some of high-purity AI in hydrochloric acid solution and diluted with non-ion water. Standard Al(III) working standard, 5.00 mg/L must be prepared daily with the above standard AI solution. DBNPF solution, 1.00 mrnoi!L DBNPF was prepared by dis- solving dibromo-o-nitrophenylflurone (DBNPF) (pro- vided by Changke Reagents of Shanghai) in acetone (A. R., Shanghai Reagent). It should be stored in a dark bottle and at less than 5°C (pH 8.5). Buffer solu- tion was prepared with borate so as to adjust the acid- ity of the complex solution. Masking reagent solution was prepared by mixing 2% potassium-sodium tar- trate (A.R., Shanghai Chemicals), 5% thiourea (Bei- jing Chemicals) and 2% ascorbic acid (A.R., Shang- hai Reagents).
···Method
A known volume of a sample solution containing less than 10 )lg of AI was taken in a 25-mL volumet- ric flask. Added deionized water to about 10 mL.
Added 1 mL of masking reagent solution and 3 mL of 1.00 mrnoi!L DBNPF. Diluted to volume and mixed well. After 15 min, measured absorbances at 560 and 480nm against a reagent blank, respectively.
Results and Discussion Absorption Spectra
Fig. 1 showed the absorption spectra of DBNPF and AI complex solution at pH 8.5, two wavelengths should be selected such that the difference in absorb- ance was a maximum: 480 (valley absorption) and 560 nm (peak absorption) from curve 3. The term, J3 of DBNPF solution was equal to 0.360 from curve 1.
At the same time, a of Al-DBNPF complex was equal to 0.560 from curve 2. The Ac expression was taken as follows: Ac=l.25(M-0.360M').
GAO et al.: BETA-CORRECTION SPECTROPHOTOMETRIC INVESTIGATION OF ALUMINIUM COMPLEX 275
0.6 0.5
II) 0.4 u 0.3
§
1
.D .... 0.2
0 til
.D 0.1
<
0 2
. _I . • - •• - • l -.. ·.'. - . -l... •• - -• - 't • - •• - .
-0.1 3 -0.2
400 450 500 550 600 650
Wavelength (run)
Fig. !-Absorption spectra of DBNPF and Al-DBNPF complex solutions at pH 8.5: 1- 1.00 J.lmoU25 mL DBNPF against water; 2- AI (1 mg) -DBNPF (0.50 J.lmol) complex soiution against water;
3-AI (10 J.lg)-DBNPF (1.00 J.lmol) reacted solution against a rea- gent blank
0.8 0.6
8 0.4
t:: c<S ..0 ... 0.2
0 til
~ 0
-0.2
··· ·········-.... .... 2
~0.4
0 0.5 1.5 2 2.5 3 3.5 4
Addiionofl.OO 11mJVLDBNPF (mL)
Fig. 2-Effect of 1.00 mmoi/L DBNPF addition on absorbance of AI (10 J.lg) complex solution: 1 - 560 nm and 2 at 480 nm, against reagent blank
Effect of DBNPF Concentration
Fig. 2 gave the effect of the addition of DBNPF solution. From curve 1, it is difficult for the complex ratio of AI to DBNPF to be calculated accurately with the conventional molar ratio method because of the uncleamess of the inflexion point. From equations in principle, Ac and y' of each solution were calculated and their curves are shown in Fig. 3. From curve 3, the maximum of y' remained to be 5 under the addi- tion of DBNPF solution over 2.4 mL. Therefore, Al(DBNPF)5 was formed here. From curve 2 in Fig. 3 the effective percentage of DBNPF was only about 62% on addition of 3.0 mL. The excess of DBNPF approached 40% and it was inevitable for the free DBNPF in its AI complex solution to affect the accu- rate measurement of the absorption of the formed· complex.
0 0.5 1.5 2 2.5 3 3.5 4 Addition of 1.00 nunoVL DBNPF (ml) Fig. 3--Effect of 1.00 mmoi/L DBNPF addition on Ac, T) andy' of AI (10 J.lg) complex solution: I -Ac at 560 nm; 2-T) and 3- y'
1 0.8
~ 0.6
~ ~
0.4"'
~
0.20 -0.2
6 7 8 9 10 11 12
pH
Fig. 4-Effect of pH on absorbance of AI (10 J.lg)-DBNPF com- plex solution : 1-Ac at 560 nm, 2- M at 560 nm and 3-M' at 480nm
EffectofpH
Effect of pH on absorbance is shown in Fig. 4. It was found that the absorbance reached maximum when pH was between 8 and 9. In this study, pH 8.5 buffer was selected for use, which gave the maximal sensitivity.
Effect of Time
The effect curves of the reaction time for Al- DBNPF complex are shown in Fig. 5. From curve 1 the reaction was complete because of the reach of maximal Ac after 15 min. The colour was found to remain almost constant for at least 2 h.
Determination of Stability Constant and Absorptivity The following solutions were prepared for the de- termination of stepwise stability constant and step-
276 INDIAN J. CHEM. TECHNOL., JULY 2001
Q) u
-e
§ 0{/)
.0 ~
0.6 0.5 0.4 0.3 0.2 - 0.1
0 -0.1 -0.2 -0.3
0
~~·~--~·--_,·~--··--~·
~· . . . 2
___ l. ___ I _ ___ I _ . _ . _I_. _ __j __ ....:... _ J __ •
·--·- - · --·-- - - · - - -- ·
---- · - - - - ·
10 20 30 40 50 60 Time (min)
3 70
Fig. 5--Effect of colour-developed time on absorbance of AI ( 10 )..lg)-DBNPF complex solution: 1-Ac at 560 nm, 2-M at 560 nm and 3-M' at 480 nm
Table 1-The determination of stepwise stability constant and stepwise real absorptivity of Al-DBNPF complex at pH 8.5
n-th Al(DBNPF)5
Kn. ionic strength 0.025 mol/L and temperature I 0 °C I" 6.48x104
2nd 5.64x103
3'd 3.84x103
4'h 4.25x104
5'h 2.48x104
Cumulative K=l.48x1020
E,, Lmor'cm-1 at 560 nm
1.30x104 2.60x104 4.10x104 4.92x104 6.21x104
wise absorptivity of the complex according to the equations described in principle: 10.0 J-!moV25 mL Al(III) with 0.42, 1.08, 1.50, 2.10 and 2.50 J-!moV25 mL DBNPF at pH 8.5 and temperature 10°C and in ionic strength 0.025 mol/L. All results were listed in Table 1. The cumulative stability constant of Al(DBNPF)5 was calculated to be 1.48xl020 and the final real molar absorptivity to be 6.21xl04 Lmol-1cm-1 at 560 nm which was more than the ap- parent value, that is 4.45xl04 Lmor1cm·1•
1
0.8
Q)
§
0.6-e
0{/)
.0
<
0.40.2 0
0 1 2 3 4 5 6 7 8 9 10 11 12 Al(lll) (microgram)
Fig. 6----Calibration graphs for the determination of aluminium at 560 nm: 1-Ac; 2-M
Table 2-Determination of aluminium in water samples
Sample AI concentration, mg!L RSD,% Recovery,%
Added Found with CAS
Water 1# 0 0.081
0.082 0.078
0.076
0.083 0.079 4.4
0.084
0.076 0.084-
aver 0.080 aver 0.080
0.100 0.188 108
0.183 103
Water2# 0 0.153
0.144 0.152
0.141
0.151 0.167 4.0
0.157
0.150 0.155
aver 0.149 aver 0.158
0.100 0.241 92.0
0.256 107
GAO eta/.: BET A-CORRECTION SPECTROPHOTOMETRIC INVESTIGATION OF ALUMINIUM COMPLEX 277
Calibration Graph
A series of standard AI (0-10 ~g/25 mL) solutions were prepared and the absorbance of each was meas- ured. Ac of each solution was obtained. Calibration curves are shown in Fig. 6. All points around curve 1 (the linear correction coefficient, R=0.9995) were much more linear than that around curve 2 (R=
0.995). In addition, from the slope of curves 1 and 2 the analytical sensitivity of beta-correction method was one and half times that of the ordinary spectro- photometry. As a result, the recommended method gave out the better precision and higher sensitivity than the single wavelength spectrophotometry. The following equation was established for the determina- tion of aluminium: Ac=0.0930X + 0.006.
Precision, Accuracy and Detection Limit
Six replicated determinations of standard solution containing 5.00 ~g AI were carried out. The relative standard deviation (RSD) was 1.9%, whereas, the RSD was equal to 5.8% by the ordinary spectropho- tometry.
Lmin=kSJ.IS was used to calculate the detection limit of AI, where k=3, Sb named as standard deviation and S named as sensitivity. Replicate determination of twenty reagent blanks gave Sb of Ac equal to 0.004.
The analytical sensitivity S was equal to 0.0930.
Therefore, the detection limit of AI was Lmin=0.13
~g/25-mL (0.005 mg/L).
Effect of Foreign Ions
Once the masking reagent was added and the rec- ommended procedure was carried out, none of the following ions will affect the direct determination of
10 Jl.g of AI (<20% error): 5 mg of Cr, SOl, N03-, F, PO/, NH/; 0.5 mg of Ca(II), Mg(II), Mn(II), Ti(IV), Sb(III), Ag(l); 0.1 mg of Sn(II), Zn(ll), Pb(II), Fe(II), Cr(III), Ni(II), Mo(V) and Cu(II).
Samples Analyzed
As a test of the method AI was determined in water.
The results are listed in Table 2. It was found that the results obtained by the recommended method ap- proached to that obtained by the conventional method with CAS as chromogenic reagent. The RSDs were less than 4.4% and the recovery rate of AI between 92.0 and 108% for the recommended method.
Acknowledgements
This work was supported by both of the Natural Science Foundation of Anhui Province (no.99045332) and '99 Climbing Program of China (special).
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