Indian Journal of Chemistry Vol. 41A, June 2002, pp. 1184-1187 Determination of osmotic coefficients of and can be used from 303 K to temperature below the aqueous solutions of polyhydroxylated freezing point of the solvent. compounds at various temperatures Determination of the osmotic coefficients of the aqueous solutions by VPO has been limited to a few 3 S 6 b papers . Continuing our previous work ,7, the present Zuo-Ning Gao* a. b, Jin-Fu Li a & Xiao-Lin Wen note deals with aqueous solutions of glycol, glycerol, a Department of Chemistry, Ningxia University, Yinchuan 750021, P. R. China sucrose and glucose at various temperatures. b Department of Chemistry, Lanzhou University, Lanzhou Experimental 730001, P. R. China All measurements were carried out on aqueous Received 3 May 2001; revised II December 2001 solutions of different solutes (glycol, glycerol, sucrose and glucose) at 313, 323 and 333 K over the Osmotic coefficients of aqueous solutions of polyhydroxylated compounds such as glycol, glycerol, sucrose and glucose at concentration range of 0.0 - 2.0 mol kg" for glycol various temperatures over the concentration range of 0.0 - 2. 1 and sucrose, 0.0 - 2.1 mol kg" for glycerol and mol kg ·1 and 0.0 - 2.0 mol kg·1 have been determined, glucose, respectively, from which the osmotic respectively. Using a linear least-squares fitting routine, the coefficients were derived, using a knauer vapour osmotic coefficients have been fitted by a simple polynomial pressure osmometer Model 11.0 in combination with equation. It is found th at the relation between the experimental 8 results of th e molal osmotic coefficients <t> and the molal a digital meter and a recorder . The instrument was concentration of the solution m are in agreement with that from calibrated with an aqueous standard sodium chloride the polynomial fitting at various temperatures. The experimental solution of known molality using the smoothed values results al so show th at over the studied concentration range and at of the osmotic coefficients reported by Herrington and various temperatures, the molar osmotic coefficients of aqueous 9 Taylor . Osmotic coefficients were measured to soluti ons increase with increase in concentration of solutes. The aqueous glycol solution system exhibits the properties of the ideal ±0.005. Glycol, analytical grade from Beijing solution. Chemical Works (China), was doubly distilled under reduced pressure of 666.5 Pa prior to use and the Vapour pressure osmometry (VPO), initiated by Hill' distillate between 347 to 349 K were collected. in the early 1930' s, are currently used to determine Glycerol, analytical grade was from Beijing Chemical 2 the osmotic coefficients of the solutions . The Works (China). Glucose and sucrose from Shanghai determination is based on the colligative properties of No.1 Reagent Works (China) were used as received the solutions in the dynamically steady-state rather without further purification. Sodium chloride, than in the thermodynamically equilibrated one. The analytical grade from Xi'an Chemical Works (China), method has several advantages, like simple measuring was recrystallized in doubly distilled and deionized process and simple operation, quick acqumng water, and desiccated for 24 hours in the vacuum experimental data and a wide range of experimental drying oven (78047 Pa) at 373 K. Standard aqueous conditions. The measurements are performed by solution with known molality was prepared using that conversion of the thermodynamic properties of the sample. Doubly distilled and deionized water was solution into electrical signals. Hence, both the used for its preparation of all solutions. steadiness and the reliability of the measurements can The general principle and operation of the be guaranteed. In addition to its higher sensitivity, 2 instrument have been described elsewhere . compared with other methods, VPO has further advantages. For example, cryoscope can only be used Results and discussion in the vicinity of the freezing point of the solvent Calibration of the vapour pressure osmometer for the while ebullioscopy in the vicinity of the boiling point osmotic coefficient determination of the aqueous of the solvent. However, VPO has just filled the "gap" solutions of the temperature range between these two methods Standard aqueous sodium chloride solutions with various concentrations were prepared first, their electrical signal intensities (G) were measured at E-mail: [email protected] various temperatures by vapour pressure osmometer, NOTES 1185 and finally (Glm)R'Z32, the values of the ratio of the where, the subscript NaCI means the reference electrical signal intensity to m, the molar substance; v, the number of ions generated in a concentration of the aqueous solutions of different complete dissociation; and the other symbols have the concentrations at various temperatures were usual significance. obtained. To fit the experimental data of (Glm) From Table 2 it is clear that the instrumental versus m as a simple polynomial Eq.(1) a linear calibration constant (KG) is only dependent upon the least-squares routine was used. temperature and not on the concentration (actually it s is almost a constant). According to Eq.(3) % = Ao + A,m + A2m2 + A3m3 + A4m4 + Asm .. (1) ... (3) According to the experimental results plot of (Glm) versus m over the concentration range of 0.1 - 2.3 where, the subscript x means the unknown mol kg" at 313, 323 and 333 K was drawn on Fig. 1. substance. The values of (Glm)x for the unknown From Fig.l it can be seen that the experimental results substances can be determined by the calibrated vapour (Glm) at various temperatures over the concentration pressure osmometer and from the instrumental range studied were in quite a good agreement with calibration constant (KG) it can give <l>x for the those calculated from the polynomial Eq.(l). Table 1 unknown substances. provides the fitting constants Ao, A" A2, A), A4 and As obtained from Eq.(l) and their correlation coefficients Determination of osmotic coefficients of aqueous R. solutions of different solutes at various temperatures According to Eq.(1) and the fitting constants in Molar osmotic coefficients (<I» of the aqueous Table 1, one can calculate the values of (Glm) for solutions of glycol, glycerol, sucrose and glucose at 0.10, 0.25, 0.50, 0.75, 1.00 and 2.00 mol kg" 313, 323 and 333 K were determined at various molar solutions at various temperatures. These calculated concentrations (m). The relation between the molar values of (Glm) can be substituted into Eq.(2) and the osmotic coefficients (<I» and their molar concentration instrumental calibration constants (Kd at various (m) are in quite a good agreement with that from the temperatures over the different concentration ranges polynomial Eq.(4) fitted using a linear least-squares can be obtained (see Table 2). routine. KG = (o/m) NnCl / (v <1» Nne l • . • (2) Table I - The constants Ai fitted to Eq.(1) and their correlation coefficients R for the aqueous sodium chloride solution systems (reference substance) at the various temperatures T/K Ao A, A2 A3 A4 As R 313 218.4663 -259.9438 519.0930 -449.4234 179.7026 -26.9221 0.9756 323 240.2965 -265.4753 494.0028 -411.1611 160.8310 -23.7451 0.9899 333 259.8077 -280.4486 550.8686 -472.2478 187.2147 -27.8687 0.9648 lO Table 2 - The relation between <1>Rer , the reference value of the osmotic coefficient of the aqueous sodium chloride solution (reference substance) and, <1>Cah the calculated one from Eq.(2), and the calibration constants of the instrument at various temperature over the different concentration range mlmol Kg" 0.10 0.25 0.50 0.75 1.00 2.00 <1>Rer 0.932 0.922 0.924 0.931 0.941 0.992 313K <1>Cal 0.931 0.925 0.923 0.930 0.942 0.992 KG 95.1 95.0 95.0 94.9 95.1 95 .9 <1>Rer 0.931 0.921 0.924 0.932 0.942 0.995 323K <1>CaI 0.929 0.924 0.923 0.930 0.943 0.995 KG 107.5 107.8 108.1 108.5 108.1 106.9 <1>Rer 0.929 0.920 0.923 0.931 0.942 0.996 333K <1>CaI 0.928 0.923 0.922 0.930 0.943 0.996 KG 115.7 116.1 116.2 115.7 115.9 116.0 1186 INDIAN J CHEM, SEC A, JUNE 2002 Relationship from Eq.(4) can also be given by the sucrose and glucose are in a nsmg trend, and their curves of Figs 2 - 4. It can be seen from the Figs. 2 - temperature dependences exhibit their own 4 that the experimental values of the molar osmotic regularities. In the low concentration range, the coefficients of the aqueous solutions with different changes in osmotic coefficients with temperature are solutes at various temperatures over the different not noticeable, when the concentration range is l concentration range selected are good agreement with greater than 0.7714 mol kg· , the dependence of the that from the polynomial fitted from a linear least­ osmotic coefficient on temperature is of the order of squares routine. Table 3 gives the fitting constants AI, <1>313 > <1>323 > <1>333, that is to say, the osmotic A2, A3 and A4 obtained from Eq.(4) by linear least­ coefficients decreased with increase in temperature squares routine and their correlation constants R. and the decreasing trend is not much. At 313 and 323 The experimental results over the studied K over the low concentration range, the osmotic concentration range and at 313, 323 and 333 K show coefficients of aqueous sucrose and glucose solutions that the concentration dependence of the molar approach each other and appear to crisscross.