INTRODUCTION:

Dyes, paper, pesticides, polymeric material, pharmaceutical and petrochemical product etc are produced by using hydroquinone as the major benzene metabolite. It is used as a developing agent in photography, dye intermediate, stabilizer in paints, varnishes oils and motor fuels. In addition, hydroquinone has been used as an antioxidant in the rubber and food industry. From 1950 to 2001 hydroquinone is applied in the commercially available cosmetic skin lightening formulations in European Union countries and since 1960 hydroquinone is commercially available as a medical product, cosmetic formulations of products for coating finger nails and hair dyes^1,2.

The antimicrobial properties of arbutin as the main compound and hydroquinone as the active metabolite was determined and compared with the antimicrobial properties of A. unedo leaf extracts so as to test the extent to which arbutin is responsible for antimicrobial activity³. Wide used of such substances increase phenolic compounds in industrial wastewater, these are toxic to aquatic life and human bring⁴.

Solubility data is required for selection of proper solvent and design an optimized crystallization process, in this paper the systematic study of solubility and densities of hydroquinone in water, ethanol and water + ethanol binary solvents over the entire composition range from zero to one mole fraction at temperatures (293.15 to 313.15) K is reported. The thermodynamic functions for saturated hydroquinone solution are calculated using modified van’t Hoff equation.

MATERIALS AND METHOD:

Material:-

Triple distilled water is used in all experiments. Other chemicals is supplied by

Chemical Name	Supplier Name	Percentage purity	Standard
Hydroquinone	Sigma-Aldrich co.	99%	Reagent Grade
Ethanol	Merck, Darmstadt, Germany.	≥99.8%	G.R.

Selected solvent ethanol is very common and mainly used for many industrial processes. Temperature selected is closer to room temperature.

Apparatus and Procedure:-

Many methods are available to determined the solubility^{5, 6}. In this work the solubility of hydroquinone is measured using an apparatus similar to that described in the literature^7,
8. In this work an excess amount of hydroquinone is added to the binary solvents mixtures prepared by weight (Shimadzu, Auxzzo Ltd.) with an uncertainty of ±0.1 mg, in a specially designed 100 ml double jacketed glass flask. Water is circulated at constant temperature in jacket between the outer and inner walls of the flask. The temperature of the circulating water is controlled by auto temperature control thermostat within (±0.1)K. The solution is continuously stirred using a magnetic stirrer to assured equilibrium and no further solute dissolved. The temperature of solution is same as that of circulating water. The stirring is stop and the solution is allowed to stand to get the supernatant liquid, which was withdrawn from the flask in a weighing bottle with the help of pipette which is hotter than the solution. The weight of this sample was taken and kept in an oven at 343 K until the whole solvent was evaporated and the residue was completely dry. This is confirmed by weighing two or three times until a constant weight was obtained after keeping the sample in an oven for another 30 min every time. The solubility has been calculated using weight of solute and weight of solution. Each experimental value of solubility is an average of at least three different measurements and the standard uncertainty of the experimental mole fraction solubility, value is ± 0.003. The mole fraction solubility, initial the mole fraction of ethanol (), were calculated using usual equations⁹. The standard uncertainty for is 0.0002. Densities are determined using a 15 cm³bicapillary pycnometer as described earlier. For calibration of pycnometer triply distilled and degassed water with a density of 0.99705 g·cm⁻³ at 298.15 K was used. The pycnometer filled with air bubble free experimental liquids was kept in a transparent wall thermostat (maintained at constant temperature ± 0.1 K) for (10 to 15) min to attain thermal equilibrium. The heights of the liquid levels in the two arms were measured with the help of a travelling microscope, which could read to 0.01 mm. The estimated standard uncertainty of the density measurements of the solvent and binary mixtures was 10 kg·m⁻³. ^10-13

RESULTS AND DISCUSSION:

Solubility: Table 1 show the experimental and calculated (using Apelblat equation) values of solubility of hydroquinone at 293.15 to 313.15 K in water, ethanol and water + ethanol respectively. The density of each saturated solution is also reported. Variation of solubility with is visually shown in Figure 1.

Table 1: Experimental _(exp_.)and Calculated Values of Mole Fraction Solubility and Density (ρ) of Hydroquinone for Various Initial Mole Fractions, (), of Ethanol at Temperatures (293.15 to 313.15) K and Pressure 101.32 kPa^a.

			RD	*ρ·10⁻³/* *kg·m⁻³*				RD	*ρ·10⁻³/* *kg·m⁻³*
293.15 K					305.15 K
0.0000	0.0102	0.0103	0.0079	1.0090	0.0000	0.0161	0.0162	0.0090	1.0119
0.1008	0.0354	0.0354	0.0012	0.9992	0.1008	0.0536	0.0537	0.0013	1.0081
0.2000	0.0703	0.0702	0.0015	0.9980	0.2000	0.0933	0.0932	0.0010	1.0063
0.3001	0.1003	0.1001	0.0022	0.9941	0.3001	0.1239	0.1236	0.0022	1.0004
0.3999	0.1250	0.1244	0.0044	0.9875	0.3999	0.1473	0.1468	0.0036	0.9921
0.5000	0.1434	0.1433	0.0011	0.9795	0.5000	0.1664	0.1658	0.0037	0.9827
0.5999	0.1575	0.1575	0.0002	0.9698	0.5999	0.1830	0.1817	0.0073	0.9728
0.7002	0.1697	0.1695	0.0008	0.9598	0.7002	0.1928	0.1908	0.0108	0.9619
0.8000	0.1788	0.1790	0.0011	0.9489	0.8000	0.1993	0.1989	0.0022	0.9508
0.8999	0.1834	0.1834	0.0000	0.9379	0.8999	0.2030	0.2027	0.0014	0.9396
1.0000	0.1877	0.1883	0.0030	0.9259	1.0000	0.2063	0.2072	0.0042	0.9280
295.15 K					308.15 K
0.0000	0.0112	0.0112	0.0000	1.0097	0.0000	0.0180	0.0180	0.0009	1.0127
0.1008	0.0387	0.0386	0.0024	1.0009	0.1008	0.0580	0.0581	0.0019	1.0099
0.2000	0.0745	0.0744	0.0012	1.0001	0.2000	0.0991	0.0984	0.0072	1.0083
0.3001	0.1045	0.1045	0.0000	0.9950	0.3001	0.1292	0.1288	0.0031	1.0016
0.3999	0.1281	0.1285	0.0035	0.9882	0.3999	0.1520	0.1519	0.0006	0.9934
0.5000	0.1475	0.1476	0.0008	0.9795	0.5000	0.1698	0.1705	0.0042	0.9838
0.5999	0.1625	0.1621	0.0028	0.9702	0.5999	0.1885	0.1870	0.0082	0.9732
0.7002	0.1737	0.1737	0.0000	0.9602	0.7002	0.1938	0.1950	0.0066	0.9624
0.8000	0.1829	0.1824	0.0023	0.9491	0.8000	0.2036	0.2036	0.0003	0.9514
0.8999	0.1871	0.1869	0.0007	0.9381	0.8999	0.2070	0.2071	0.0002	0.9398
1.0000	0.1920	0.1918	0.0010	0.9268	1.0000	0.2109	0.2113	0.0019	0.9286
298.15 K					310.15 K
0.0000	0.0122	0.0122	0.0037	1.0102	0.0000	0.0199	0.0201	0.0071	1.0135
0.1008	0.0419	0.0420	0.0034	1.0024	0.1008	0.0627	0.0628	0.0015	1.0123
0.2000	0.0786	0.0788	0.0027	1.0014	0.2000	0.1032	0.1038	0.0061	1.0098
0.3001	0.1085	0.1090	0.0046	0.9961	0.3001	0.1339	0.1341	0.0012	1.0029
0.3999	0.1322	0.1328	0.0045	0.9892	0.3999	0.1569	0.1572	0.0021	0.9943
0.5000	0.1518	0.1521	0.0021	0.9805	0.5000	0.1751	0.1752	0.0009	0.9845
0.5999	0.1671	0.1668	0.0019	0.9707	0.5999	0.1918	0.1924	0.0031	0.9743
0.7002	0.1775	0.1780	0.0025	0.9606	0.7002	0.1984	0.1993	0.0047	0.9632
0.8000	0.1854	0.1862	0.0039	0.9494	0.8000	0.2072	0.2087	0.0072	0.9525
0.8999	0.1903	0.1906	0.0016	0.9384	0.8999	0.2109	0.2116	0.0034	0.9408
1.0000	0.1964	0.1955	0.0046	0.9271	1.0000	0.2156	0.2156	0.0003	0.9295
300.15 K					313.15 K
0.0000	0.0134	0.0134	0.0036	1.0108	0.0000	0.0226	0.0224	0.0080	1.0146
0.1008	0.0458	0.0457	0.0029	1.0041	0.1008	0.0679	0.0678	0.0020	1.0148
0.2000	0.0829	0.0834	0.0057	1.0024	0.2000	0.1095	0.1094	0.0003	1.0119
0.3001	0.1135	0.1137	0.0022	0.9974	0.3001	0.1394	0.1396	0.0015	1.0045
0.3999	0.1370	0.1373	0.0020	0.9900	0.3999	0.1627	0.1628	0.0006	0.9957
0.5000	0.1565	0.1566	0.0003	0.9815	0.5000	0.1803	0.1801	0.0012	0.9856
0.5999	0.1701	0.1716	0.0087	0.9713	0.5999	0.1974	0.1979	0.0026	0.9754
0.7002	0.1821	0.1822	0.0009	0.9608	0.7002	0.2044	0.2037	0.0035	0.9641
0.8000	0.1909	0.1901	0.0038	0.9501	0.8000	0.2150	0.2140	0.0046	0.9532
0.8999	0.1944	0.1945	0.0007	0.9384	0.8999	0.2167	0.2163	0.0019	0.9419
1.0000	0.1993	0.1992	0.0003	0.9271	1.0000	0.2206	0.2200	0.0023	0.9302
303.15 K					^aStandard uncertainties in u are u(T) = 0.1 K, u(x_C⁰ ) = 0.0002, u(x_B) = 0.003, and u(ρ) = 10 kg·m⁻³. The relative uncertainty in pressure ur(p) = 0.05.
0.0000	0.0147	0.0147	0.0025	1.0113
0.1008	0.0496	0.0495	0.0019	1.0065
0.2000	0.0885	0.0882	0.0033	1.0046
0.3001	0.1188	0.1186	0.0016	0.9990
0.3999	0.1425	0.1419	0.0040	0.9910
0.5000	0.1615	0.1611	0.0022	0.9819
0.5999	0.1755	0.1766	0.0061	0.9720
0.7002	0.1864	0.1865	0.0004	0.9615
0.8000	0.1943	0.1944	0.0004	0.9502
0.8999	0.1989	0.1985	0.0019	0.9391
1.0000	0.2034	0.2031	0.0011	0.9279

^aStandard uncertainties in u are u(T) = 0.1 K, u(x_C⁰) = 0.0002, u(x_B) = 0.003, and u(ρ) = 10 kg·m⁻³. The relative uncertainty in pressure ur(p) = 0.05.

Fig. 1-Mole Fraction Solubility of Hydroquinone () Variation with Initial Mole Fraction () of Ethanol at Temperatures (◆T=293.15 K, ■T=295.65 K; ▲T=298.15 K; ×T=300.65 K; ○T=303.15 K; ●T=305.65 K; +T=308.15 K; -T=310.65 K & ▬ T=313.15 K).

The solubility of hydroquinone in all solvents increases with temperature. At the same temperature, the solubility trend in solvent is ethanol > water + ethanol >water. This trend implies that solubility of hydroquinone increases with increasing with mole fraction of ethanol, it is prefer to dissolve more in ethanol than water. The solubility of hydroquinone in water-ethanol mixture with increases with increases in x_B up =1. This implies that there is strong dipole-dipole interaction between solute and solvent molecules. As temperature increases density goes on decreasing. But here increase of temperature and mole fraction of ethanol density goes on increases; this is because of increase of solubility with temperature and mole fraction of ethanol.

Apelblat Model:

Among the different methods, the modified semi-empirical Apelblat model (eq 1) is a suitable way to correlate solubility data against temperature^{14, 15}.The equation is based on solid-liquid equilibrium theory provide excellent agreement between experimental and calculated values of solubility¹⁶.

A, B, and C are the model parameters and T is temperature in Kelvin. A and B represent the non-idealities of the solutions in terms of the variation of activity coefficients, C reflects to the effect of temperature on the enthalpy of fusion¹⁷. A, B, and C parameters are determined using non-linear least square fitting¹⁸.Solubility values of hydroquinone in water, ethanol and their mixtures are calculated by eq 1. Relative deviation (RD)¹⁹ is calculated using eq 2.

The data of experimental mole fraction solubility, calculated solubility and RD in monosolvent (water, ethanol) and water-ethanol mixtures are listed in Table 1. The values of parameters A, B, C along with co-relation coefficient (R²) are listed in Table 2.

Table 2. Model Parameters and Correlation Coefficient of the Apelblat Equation.

Solvents	Mole fraction	Parameters			R²
Solvents	Mole fraction	A	B	C	R²
Ethanol	0.0000	-528.615	20611.26	79.8717	0.999
	0.1008	8.448876	-3051.92	-0.2426	1.000
	0.2000	-4.96045	-1622.33	1.3797	0.999
	0.3001	-26.3022	-208.589	4.3502	1.000
	0.3999	-87.4798	2809.789	13.3454	0.999
	0.5000	-5.71715	-717.335	1.0951	0.999
	0.5999	-41.0135	879.6565	6.3662	0.995
	0.7002	8.305781	-1167.21	-1.0736	0.994
	0.8000	-121.689	4717.937	18.2855	0.996
	0.8999	-71.2879	2498.368	10.7503	0.999
	0.0000	-528.615	20611.26	79.8717	0.999

NIBS-Redlich-Kister Model:

The solubility data at constant temperature is fitted into combined NIBS-Redlich-Kister model^20-23.

Where is initial mole fraction of water and , are solubilities of hydroquinone in pure ethanol and water respectively. M_i is curve fit parameters (four parameter equation). All values of M_ialong with R² value are listed in Table 3. The values of R² are close to unity shows that NIBS-Redlich-Kister model is very well applicable for this solubility data.

Thermodynamics Functions of Dissolution:

According to the van’t Hoff equation, the standard molar enthalpy change of solution ΔH⁰_soln is generally obtained from the slope of the ln vs 1/T plot. Average temperature T_mean is introduced to obtain a single value of ΔG⁰_soln and ΔS⁰_soln in the temperature range studied.

Where n is the number of experimental points. In the present work, T_mean = 302.98 K and the temperature range is (293.15 to 313.15) K in both pure solvents and binary solvent mixtures. Heat capacity of the solution can be assumed as constant. Hence values of ΔH⁰_soln are derived using eq 5.

The lnx_B Vs 10000 (1/T − 1/Tmean) plot of different solutions including pure solvents and binary solvent mixtures are displayed in Figures 2. From these figures, it can be seen that a trend of increasing solubility with temperature is observed. The slope and the intercept for each solvent are listed in Table 4. Thus the modified van’t Hoff equation can be thought to be fit to calculate the enthalpy change of solution. The standard molar Gibbs energy change for the solution process ΔG⁰_soln, can be calculated in the way similar to Krug et al²⁴ as

Table 3. NIBS-Redlich-Kister model parameters.

T/K	Range of	M₀	M₁	M₂	M₃	R²
Water + Ethanol + Hydroquinone
293.15	0.101-0.90	4.730	-3.569	3.161	-1.801	1.000
295.65	0.101-0.90	4.584	-3.397	3.271	-2.166	1.000
298.15	0.101-0.90	4.470	-3.305	3.327	-2.439	1.000
300.65	0.101-0.90	4.352	-3.159	3.518	-2.712	1.000
303.15	0.101-0.90	4.244	-3.129	3.575	-2.748	0.999
305.65	0.101-0.90	4.170	-2.939	3.630	-3.023	0.999
308.15	0.101-0.90	4.219	-2.998	4.342	-3.550	0.998
310.65	0.101-0.90	3.845	-2.750	3.579	-3.123	0.999
313.15	0.101-0.90	3.676	-2.572	3.510	-3.082	0.999

In which the intercept is used to obtained from plots of lnx_B as a function of (1/T − 1/Tmean). The standard molar entropy change ΔS⁰_soln is obtained from

Both ΔG⁰_solnand ΔS⁰_soln pertain to the mean temperature T_me_an = 302.92 K.

The results are shown in Table 5, together with %ζH and %ζTS. It is worthy to note that relative contribution of enthalpy %ζH and %ζTS which are defined as

can be simply used to calculate the main contributors of enthalpy or entropy to ΔG⁰_soln ²⁵.

The values of ΔH⁰and ΔS⁰ for all solutions are positive indicating the solution process as endothermic. The contribution of enthalpy to positive molar Gibbs energy is more as compared to entropy for all solutions.

Table 4. Slope(m) and Intercept (c) of the lnx_B vs. 10000(1/T − 1/T_mean) Plot along with R²

Water + Ethanol+Hydroquinone
*x_c⁰*	m	c	R²
0.0000	-3584	-4.201	0.995
0.1008	-2978	-3.005	0.999
0.2000	-2040	-2.427	0.999
0.3001	-1526	-2.131	0.999
0.3999	-1233	-1.949	0.997
0.5000	-1049	-1.825	0.999
0.5999	-1048	-1.732	0.994
0.7002	-841.9	-1.679	0.993
0.8000	-821.3	-1.633	0.992
0.8999	-758.2	-1.614	0.997
1.0000	-715.1	-1.591	0.996

CONCLUSIONS:

Solubility data and Thermodynamic functions including ΔH⁰_soln, ΔG⁰_soln, and ΔS⁰_soln of hydroquinone in water, ethanol and their mixture are more useful in field of physical chemistry and chemical engineering calculations involving fluid flow, heat and mass transfer, pharmaceutical industry, agriculture, biology, medicine. Solubility data is required for selection of proper solvent and design an optimized crystallization process.

The solubility of hydroquinone is more in ethanol than in water and increases with increase in mole fraction of ethanol. ΔH⁰_soln values are higher in water than water+ethanol mixture and lowest in ethanol indicates the solubility trend in various solvents. Density of solution is depends on solubility and solvent system both. Increase of density with temperature and mole fraction of ethanol indicates higher solubility. Also lower values of υ-OH shows increase of molecular interaction and hence the increases solubility of hydroquinone.

ACKNOWLEDGMENTS:

The authors express their sincere thanks to Dr. Apoorva Hiray Coordinator M. G. Vidyamandir and Principal M.S.G. College, Malegaon Camp, Malegaon for providing laboratory facility and encouragement.

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Received on 22.03.2020 Modified on 11.01.2021

Research J. Pharm. and Tech 2022; 15(1):40-46.

DOI: 10.52711/0974-360X.2022.00008

x_C⁰	ΔH⁰sol/kJ·K⁻¹·mol⁻¹	ΔG⁰soln/kJ·K⁻¹·mol⁻¹	ΔS⁰soln/KJ·K⁻¹·mol⁻¹	TΔS⁰soln/KJ·K⁻¹·mol⁻¹	%ζH	%ζTS
Water + Ethanol
0.0000	29.7974	10.5882	0.0634	19.2092	0.6080	0.3920
0.1008	24.7591	7.5738	0.0567	17.1853	0.5903	0.4097
0.2000	16.9606	6.1170	0.0358	10.8436	0.6100	0.3900
0.3001	12.6872	5.3709	0.0241	7.3162	0.6343	0.3657
0.3999	10.2512	4.9122	0.0176	5.3389	0.6575	0.3425
0.5000	8.7214	4.5997	0.0136	4.1217	0.6791	0.3209
0.5999	8.7131	4.3653	0.0143	4.3478	0.6671	0.3329
0.7002	6.9996	4.2317	0.0091	2.7678	0.7166	0.2834
0.8000	6.8283	4.1158	0.0089	2.7125	0.7157	0.2843
0.8999	6.3037	4.0679	0.0074	2.2358	0.7382	0.2618
1.0000	5.9453	4.0099	0.0064	1.9354	0.7544	0.2456

Ethanol ()	Experimental υ-OH Hydroquinone+ Water+ Ethanol cm^-1	Solid Hydroquinone υ-OH cm^-1
0(Water)	3387	3224.98
0.2	3387
0.5	3379.29
0.7	3363.86
1(Ethanol)	3332.99