Frank I. Miller and Henrik M. Fog'
Univers~tyof Aberdeen Old Aberdeen. Scotland
I
I I
Reversible hrma#ion of Aluminum XyIenoI Orange by Temperature Variation An experiment demonstrating the entropy effect
Aluminum ions have been reported to react with xylenol orange forming 2:1,1:1 (2:2), and 1:2 complexes depending on pH and concentration ratios (1-4). The rate of formation of the 1:l complex a t pH 6 3 with excess of aluminum is increased by heating to boiling ( 2 , 5 , 6 ) .The present work is based on the observation that the red complex formed by heating certain solutions of colorless A13+ and yellow xylenol orange, dissociated again when the solutions were allowed to cool. A similar effect was noticed with other metal indicator systems. Positive values of entropy changes, AS, by chelate reactions have been reported by Calvin (7). The multidentate xylenol orange ligand favors formation of multi-ring structure chelates, thus giving rise to more degrees of freedom in the system. One would therefore expect a high positive A S value for reactions of xylenol orange with metal ions in aqueous solution. The equilibrium constant for the reaction of aluminum with xylenol orange can he evaluated-by photometric deAI(H,0),3+
+ H.Xm ==+ AIX'~-"'"" + n
+
~ + 6H,O
I
I
I
I
1.5
2.0
2.5
3.0
pH termination of the aluminum xylenol orange complex a t 550 nm (c -25,000 1molkl cm-1). The changes in standard free energy AG (kcal mol-'1, the standard enthalpy AH (kcal mol-I), and the standard entropy A S (cal mol-'K-') of the reaction are related to the stability constant by the equation ( ~ ) -RT I n K = AG = AH-TAS = A H - TJAG' \
and may thus he determined. Experimental
~
Figure 1. Of pH.
Formation of aluminum xylenol orange complex as function
.,
Apparatus. EEL Titrator with filter no. 605, Tmara t 550 nm. EEL Unigalvo 20. Reagents. Aluminum nitrate (analytical grade). Xylenol orange, standardized by. .photometric titration with lead nitrate (analytical grade). Stoiehiometry. A series of solutions was prepared in 25-ml M in xylenol orange, 4.00 X 1 0 - W in alubeakers, 1.42 X minum and varying in p H from 1.36 to 3.00. It was necessary to adjust the p H before mixing the aluminum and aylenol orange, since in very dilute acid a t room temperature an inert complex is formed which does not break down a t pH 1.0. The solutions were heated to boiling and the absorbance (arbitrary unit) measured a t fixed sensitivity (Fig. 1). The chemical equilibrium was checked by variation of the concentration ratios. Temperature Effect. The measurements were made with a solution at p H 1.7, since all the complex formed at that p H dis~iociated completely during coaling. The 25-ml beaker was equipped with a mercury thermometer through a rubber bung and placed on the EEL Titrator. The formation and dissociation of the complex were followed by absorbance measurements. The solution was either heated to the appropriate temperature or heated just to boiling and then allowed to cool. Investigation a t different cooling rates showed that the system was only in equilibrium on slow cooling.
Temperature I T Figure 2. Concentration of aluminum xylenol orange complex as function of temperature.
Results Figure 1, referring to boiling temperature, shows that almost no complex was formed at pH 1.36 and that the reaction was complete a t pH above 2.4. The number n of protons released by complexation appears to he 4, supposing i t to he a whole number. The results from Figure 2, obtained on slow cooling, are used in Figure 3 for plotting Iog~oKversus 1/T. Since
'Present address: Chemistry Department A, the Technical University of Denmark, Lyngby, Denmark.
Volume 50, Number 2,February
1973 / 147
Equilibrium Constant 4.01
-LOgt~K
/
3.5
Conclusion The reaction of aluminum ions with xylenol orange in aqueous solution as described above yields qualitatively a simple and instructive lecture demonstration of the entropy effect connected with chelate formation. The determination of the thermodynamic properties should be well suited for a university and secondary school exercise. Demonstration In a 25-ml beaker are mixed 2 ml of 2 X 10-4 M xylenol orange with 4 ml of 0.1 M hydrochloric acid. Then 1 ml of 10-3 M aluminum nitrate is added and the volume made up to about 25 ml. On heat in^ the yellow solution, the color slowly changes to red with increasing temperature, but returns to yellow on cooling. l h i s process may be repeated as many times as desired with the same solution. Literature Cited
2.7
Figure 3:
2.8
2.9
3.0
3.1
Reciprocal Temperature 10'1 K+ Variation of equilibrium constant with temparature.
the graph yields AH = mol-'K .'-
kcal mol -1 and AS = 38 cal
148 /Journal of Chemical Education
(I1 Otomo. M..Rull. Chsm Soc. Jopan.36,8W (19631. (2) BuddinskY, B , Z h Anolit. Khim.. 18. 1011 (19631. (31 Akhmedli. M. K.. andGiushchenko. E. L..Zh. Neorgon. Khim.. 10,98(19651; CA 62
...
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(41 Nazarenko, V. A , and Nevskaya. E. M., Zh. A w l . Khim., 24. 536 (19691; CA 71, 70378. I51 Chorkeson. A. I., Kazskov. B. I., andTonikoshumv. V. S.,Zh. A w l . Khim.. 22.14M (6) Cherkemn, A. I.. and Kazskov. B. I., Fia-Khim. Issled. Prir SorbenlovRvodoAnol Slsl., (19671 106. CA 71.25082. (71 Csivin.M.. sndBailes. R. H . . J A m s r Chrm. Soc.. 68.949(1946).
