Thermochimica Actu, 73 (1984) 187-191 187
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
THE SOLUBILITY OF l-NAPHTHOL IN WATER AT DIFFERENT
TEMPERATURES
ANTHONY GUISEPPI-ELIE
Chemistty Department, University of M.I. T, Cambridge, MA (U.S.A.)
DOW M. MAHARAJH *
Chemistry Department, University of the West Indies, St. Augustine (Trinrdad)
(Received 30 August 1983)
ABSTRACT
The solubility of I-naphthol in water has been determined in the temperature range
lO.OO-45.OO”C. The heat of solution (- 2.23 X lo4 J mol-‘) and the entropy of solution (35.7
J mol-’ K-l) have been calculated. The free energy of solution decreases steadily with
increase of temperature. An equation is presented for the solubility of I-naphthol in water in
the range lo-45°C.
INTRODUCTION
There are no quantitative data in the literature on the, solubility of
I-naphthol in water. The present paper reports the solubility of l-naphthol
in water at different temperatures and the characteristics of this process.
This data was collected during research in this laboratory while studying the
solubility of 1-naphthol in 1,1,3,3-tetramethylurea-water mixtures which will
be reported in a later paper.
EXPERIMENTAL
Saturated solutions were prepared by stirring pure recrystallized l-naph-
thol in 25.00 cm3 volumetric flasks filled to the mark with distilled, deionised
water. Stirring was carried out through the bottom of a glass aquarium by
means of magnetic stirrers with stirring bars in the flasks. The aquarium was
thermostated to within f0.02”C by means of a Haake thermoregulator.
Saturation was attained overnight. At all temperatures investigated, an
excess of solid remained on the bottom of the vessel. The transparent
* Author for correspondence.
188
T
I.011
0.9.
r l-
a8-
/
Q7-
Q3-
i
0.2.
0.l
225 250 275 300 325 350
WAVELENGTH nm
Fig. 1. Absorption spectra of aqueous 1-naphthol.
saturated solution was removed at the given temperature by using a 5.00 cm3
pipette. The solution was transferred to a volumetric flask and diluted with
distilled water by a factor of 10. The concentration of 1-naphthol was
determined spectrophotometrically in the resulting solution in the UV region
at X = 322 nm. The absorption spectra was determined in a l-cm fused
quartz absorption cell at h = 322 nm. A typical absorption spectra is given in
Fig. 1. The UV was done using the Unicam SP 8000 and Zeiss. The content
of 1-naphthol in the sample was found with the aid of a calibration graph
and the concentration of the saturated solution was calculated taking into
account the dilution. The solutions follow the Lambert-Beer law plot as
shown by the linearity of the concentration vs. absorbance plot shown in
Fig. 2.
We did not use other methods for the analysis of the saturated 1-naphthol
solutions. The gravimetric method is inapplicable because of the appreciable
volatility of the test substance; the titrimetric determination by reaction with
alkali is impossible because of the low ionization constant of l-naphthol.
189
1.0
0.8
0.7
0.6
Y
z‘
2
8
$ 0.5
Q
0.4
0.3
0.2
0.1
I 1 I I I I
0.0
10 20 30 40 50 60
CONCENTRATION
mg / 1000 g
Fig. 2. Calibration plot for I-naphthol on SP 8000 at 322 nm.
The determination of l-naphthol concentration by a photoelectric colorime-
ter is tedious and subject to error.
RESULTS
At each temperature 4-5 concordant experiments were performed. The
equilibrium solubility data are tabulated in Table 1 and are plotted in Fig. 3
190
1.0
0.5
r --
__-
_____
oxn I
I I I I
30 31 32 3.3 3.4 3.5 :
JJQQ
K
Fig. 3. Solubility of 1-naphthol in water.
TABLE 1
Solubility of 1-naphthol in water
Temp. Cont. lo4 AG
(“C f 0.02) (g/l00 g) (J mol-‘) $?rnol-’ K-!)
10.00 0.088 1.31 36.1
15.00 0.102 1.30 36.0
20.00 0.113 1.29 35.4
25.00 0.135 1.28 35.2
30.00 0.153 1.25 35.0
35.00 0.187 1.24 35.5
40.00 0.220 1.21 35.9
45.00 0.275 1.19 35.9
191
over the temperature range lO.OO-45.OOOC. The results were used to plot log
L (g/100 g) against 103/T. The experimental points fit quite satisfactorily
on a straight line whose correlation coefficient is 1.03 and which has a slope
of - 1.16 X 103. Its equation is
where L, is the solubility at 10°C and L, is the solubility at higher
temperatures. The above equation gives the solubility of l-naphthol in water
at any temperature in the range lo-45°C.
Certain thermodynamic quantities for the solution process can be calcu-
lated from the experimental data. The heat of solution is -2.33 x lo4 J
mol-’ obtained from the slope of the above plot. The free energies ( AG) and
entropies (AS) were calculated by means of familiar equations (Table 1).
DISCUSSION
Thermodynamic theory of ideal solutions indicates that the slope of the
log solubility vs. reciprocal temperature plot should be constant and equal to
the heat of fusion of the solute. The solubility data, as shown in Fig. 3. give a
smooth curve when plotted, and over short temperature ranges the curve
approaches,a straight line, but nowhere does the slope approach the heat of
fusion of the solute. This is not unexpected, however, because both the
solvent and solute are polar molecules and their interaction is expected to be
nonideal.
As seen in Table 1, the free energy (AC) decreases steadily as the
temperature rises, and the entropy (AS) remains virtually constant. The
constancy of AS confirms the attainment of thermodynamic equilibrium in
the saturated solution. The decrease in free energy with increasing tempera-
ture indicates that the dissolution is more favourable at higher temperatures.
