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Journal of the Chilean Chemical Society

versión On-line ISSN 0717-9707

J. Chil. Chem. Soc. v.48 n.3 Concepción sep. 2003

http://dx.doi.org/10.4067/S0717-97072003000300002 

J. Chil. Chem. Soc., 48, N 3 (2003) ISSN 0717-9324

COCRYSTALLIZATION OF LOW AMMOUNTS OF M2+ IONS DURING
CoSO47H2O CRYSTALLIZATION

Marek Smolik

Institute of Chemistry, Inorganic Technology and Electrochemistry
Silesian University of Technology
Gliwice, Poland

(Received: October 24, 2002 ­ Accepted: March 29, 2003)

ABSTRACT

The equilibrium cocrystallization coefficients DM/CoSO4.7H2O of low amounts (10-3- 10-1%w/w) of M2+ ions (M2+ = {Ni2+, Mg2+, Cu2+, Zn2+, Fe2+, Mn2+, Cd2+, Ca2+}) with CoSO47H2O have been determined with the method of long-time stirring of crushed CoSO47H2O crystals in their saturated solution at 20oC and compared with coefficients determined by means of the method of isothermal decreasing of supersaturation during 3 ­ 360 hours of stirring. This enabled the time needed to reach equilibrium to be found. It is different for different microcomponents. The determined cocrystallization coefficients are diverse: from <0.008 for Ca2+ to 1.20 for Fe2+. Their dependence on some physicochemical and crystal-chemical properties of both sulfate hydrates (CoSO47H2O and MSO4nH2O) and metal M2+ ions has been discussed. They depend mainly on solubility in water and structure of corresponding sulfate hydrates as well as on radii of M2+ ions. It is possible to calculate cocrystallization coefficients with empirical formula based on determined relationships between some of the considered properties of macrocomponent and microcomponents and DM/CoSO4.7H2O coefficients at the average relative error not exceeding 10%.

INTRODUCTION

Cocrystallization is one of the major factors which influence the effectiveness of crystallization as a purification process. One of the indicators of this effectiveness is the cocrystallization coefficient of the microcomponent, D2/1 (Henderson ­ Kra_ek [1], Khlopin [2]). Knowledge of such coefficients allows to predict the applicability of crystallization for the removal of definite microcomponents from a given substance. However the literature data concerning cocrystallization of microamounts (less than 0.1%w/w) of salts M(2)bXa with macroamounts of salt M(1)bXa are not so ample and their cocrystallization coefficients D2/1 hardly can be calculated (particularly if these salts are not isomorphous) by means of general thermodynamic formula:

where: C01(C02) ­ molar concentration of the saturated binary solution of the salt M(1)bXa (M(2)bXa) , n = b + a; (gc01) = [(gc0M(1))b(gc0X) a]1/n ­mean molar activity coefficient of the salt M(1)bXa in its saturated solution; (gc02)=[(gc0M(2))b (gc0X)a]1/n - mean molar activity coefficient of the salt M(2)bXa in its saturated solution; C1 (C2) ­ molar concentrations of the salt M(1)bXa (M(2)bXa) in the ternary solution being in equilibrium with solid solution (M(1),M(2))bX a ; (gc1) = [(gcM(1))b(gcX) a]1/n- mean molar activity coefficient of the salt M(1)bXa in this solution; (gc2)=[(gcM(2))b(g cX)a]1/n - mean molar activity coefficient of the salt M(2)bXa in this solution; x1(x2) ­ mole fraction of M(1)(M(2)) ion in the solid solution (M(1),M(2))bX a ; f1(f2) ­ activity coefficient of ion M(1)(M(2)) in the solid solution (M(1),M(2))bX a;

the Gibbs free energy of the phase transition DGII-I of the salt M(2)bXa from its structure (II) into the structure (I) of the salt M(1)bXa

This is because of lack of the appropriate data (gc01, gc02, gc1, gc2and particularly f1, f2 ). Therefore general dependence of coefficients D2/1 on numerous physicochemical and crystal-chemical factors (such as: ionic radii, similarity of crystal structures, ability to form solid solutions as well as solubilities of corresponding salts) is searched for, which would allow to evaluate the values of coefficients D2/1 for various crystallization systems which have not been investigated so far.

Investigations on cocrystallization of microcomponents in several sulfate systems (triclinic: CuSO45H2O [3] and MnSO45H2O at 20oC [4], monoclinic: FeSO47H2O at 20oC [5], MnSO47H2O at 2oC [4] MnSO4H2O at 50oC [6], orthorhombic: ZnSO47H2O [7], NiSO47H2O [8] and MgSO47H2O [9]) have shown that in different crystallization systems, different factors mentioned above have predominant influence on the coefficients D2/1 and very often these influences overlap. It was interesting how these factors influence the coefficients D2/1 in the case of crystallization of monoclinic CoSO47H2O.

Cocrystallization of hydrates of M(II) sulfates with cobalt(II) sulfate heptahydrate has been recognized to some extent [10-24]. However, those investigations concerned mainly macroamounts (more than 1% w/w) of the mentioned sulfates. Cocrystallization of microamounts (10-3-10-1% w/w) of M2+ ions was investigated only in few cases: Ni2+ (92-4600 ppm - DNi = 0,53) [15], (510 ppm - DNi = 0,9®1,3) [16], and Cu2+ (traces) , DCu = 0,91 at 20oC[18], DCu = 0,87 at 25oC[19], DCu = 1,33 at 5oC[20].

The purpose of the present investigations was to determine cocrystallization coefficients DM/CoSO4.7H2O of low amounts (10-3- 10-1%) of M2+ ions (M2+ = {Ni2+, Mg2+, Cu2+, Zn2+, Fe2+, Mn2+, Cd2+, Ca2+}) with CoSO47H2O as well as to find out the influence of physicochemical and crystal-chemical properties of both sulfate hydrates (CoSO47H2O and MSO47H2O) as well as M2+ ions on DM/CoSO4.7H2O coefficients.

EXPERIMENTAL

Reagents and solutions: Cobalt(II) sulfate heptahydrate, p.a. (POCh ­ Gliwice) was additionally purified by crystallization. Standard solutions of Fe(II), Zn(II), Cu(II), Cd(II), Mg(II), Ni(II), Mn(II) sulfates and Ca(II) nitrate were used. Ammonia solution (14 M) was obtained by saturating distilled water with ammonia gas. Sodium versenate p.a. (POCh ­ Gliwice) ­ 0.1 M water solution, hydrazinum sulfate p.a. (POCh ­ Gliwice) ­ saturated water solution and murexide ind. (POCh ­ Gliwice) were used

Apparatus: Atomic absorption spectrometer model 3300 manufactured by Perkin Elmer was applied.

Analytical methods: The macrocomponent: cobalt(II) was determined by complexometric titration with sodium versenate in ammonia acetate buffer solution in the presence of murexide[25]. The microcomponents (Fe, Ni, Cu, Zn, Cd, Mn, Mg and Ca) were determined by means of direct atomic absorption spectrometry (Perkin Elmer 3300 Atomic Absorption Spectrometer) from 0.1423 mol/L (or less) solutions of CoSO4 in 0.02 mol/L sulfuric acid. Absorbances of the samples and of the sets of standards having the same concentrations of the matrix [cobalt(II) sulfate], were measured under the same conditions.

The method of determination of distribution coefficients DM/CoSO4.7H20

After crystallization the crystals were separated from mother solutions by means of filtration through a Büchner funnel with a sintered glass disk, weighed, washed with saturated purified CoSO4 solution and dissolved in water. Mother solutions were diluted with water in volumetric flask. From the cobalt(II) contents in both solutions (mother solution and solution containing dissolved crystal) the degree of CoSO4·7H2O crystallization b was found, as well as the volumes of these solutions were calculated, necessary to determine the microcomponents.

The relative concentrations of microcomponents ([ppm] in relation to CoSO47H2O) in the mother solution - a'r and in the washed crystal - a'k determined by direct atomic absorption made it possible to calculate the homogeneous distribution coefficient ­ DM/CoSO4.7H2O (Henderson-Kraek, Khlopin):

Conditions of determining cocrystallization coefficients

The determination of cocrystallization coefficients, DM/CoSO4.7H2O was carried out by means of the following two methods, which enable homogeneous distribution of microcomponents during the crystallization to be achieved [26]:

a) the method of isothermal decreasing of supersaturation during 3 ­ 360 hours of stirring;
b) the method of long-time stirring of crushed CoSO47H2O crystals in their saturated solution.

a) Establishing the dependence of coefficients DM/CoSO4.7H2O
on time of crystallization of CoSO47H2O at 20oC

The supersaturated cobalt(II) sulfate solutions containing different initial amounts of Fe2+, Co2+, Mn2+, Cu2+, Cd2+, Mg2+, Zn2+ and Ca2+ were poured into beakers with water jacket. After having the beakers covered with watch glass and their contents cooled to 20oC, the solutions were stirred with magnetic stirrer (~300 rpm) at mean temperature 20±1oC over 3 -360 h. The results are presented in Fig. 1 and Table 2.


Fig.1: The dependence of coefficients DM/CoSO4.7H2O on time of crystallization of CoSO47H2O at 20oC (a'o)M - initial concentration of microcomponent (M2+) ([ppm] in relation to CoSO47H2O)

Crystallization of CoSO47H2O at 20oC

*- Average DM/CoSO4.7H2O for given range of time of crystallization; D - mean value of DM/CoSO4.7H20; ta - value of Student t-test for (n-1) degrees of freedom and for the confidence level of (1-a) = 0.95; n ­ number of determinations; s ­ standard deviation; Domin, DoMAX ­ initial minimum and maximum values of DM/CoSO4.7H2O respectively.

b) Determination of the equilibrium distribution
coefficients DM/CoSO4.7H2O at 20oC.

The equilibrium was reached starting either from initial concentration ratio of a microcomponent in crystal and in solution exceeding the

 
áko("conta min ated" crydstal)
=
 
áro("purified" solution)

expected value of its equilibrium coefficient Domax

  áro("purified" crystal)    
o
from this ratio lower than the expected value  
  áko("conta min ated" solution    

Domax

When selecting values Domax and Domin the highest and the lowest values of D obtained during crystallization of CoSO4·7H2O by the first method were taken into consideration. The experiments were carried out in the following way:

Crushed contaminated" CoSO4·7H2O crystals (f<0.1mm) were introduced into several beakers together with saturated purified" cobalt(II) sulfate solution. Crushed crystals of purified" CoSO4·7H2O (f<0.1mm) and contaminated" saturated solution of cobalt(II) sulfate were introduced to some other beakers. Contents of the beakers were stirred for ~360 h with a magnetic stirrer in a closed room. Fluctuations of temperature facilitated recrystallization of CoSO4·7H2O and helped attain equilibrium and homogeneous partition of microcomponents in crystal [27]. Its mean value was equal to 20±1oC.

While stirring was continued N2H4.H2SO4 was added in small portions (100 ml of saturated water solution) in order to avoid oxidation of iron(II). The results are given in Table 2.

* The upper limit of Dca/CoS04.7H2O. has been taken into consideration.

RESULTS AND DISCUSSION

The cocrystallization coefficients DM/CoSO4.7H2O determined with the first method of isothermal decreasing of supersaturation during 3­360 hours of stirring (Fig. 1) vary distinctly in the initial period of crystallization (they increase for Fe2+ and Cu2+ and decrease slightly for the other microcomponents) to achieve for each microcomponent, after different period of time of crystallization, constant values (plateaus) close to equilibrium ones, determined by means of the second method of long-time stirring of crushed CoSO47H2O crystals in their saturated solution. (Table 2).

The distribution coefficients of ions M2+{Ni2+, Fe2+, Zn2+, Cu2+, Mg2+ , Mn2+ , Cd2+} for two series Domax and Domin do not differ from each other essentially, which means that the equilibrium condition was reached for them, and average values for both series are equilibrium values (Table 2).

The run of the curves DM/CoSO4.7H2O = f (time) in Fig. 1 and comparison of plateaus of these curves with corresponding values of the equilibrium distribution coefficients enable the time needed for each microcomponent to reach equilibrium to be found (Table 2).

The determined cocrystallization coefficients are diverse: from <0.008 for Ca2+ to 1.20 for Fe2+and depend on the properties of both sulfate hydrates (CoSO47H2O and MSO4nH2O) and metal M2+ ions, which is discussed below.

The average value of DM/CoSO4.7H2O for microcomponents that form sulfate heptahydrates is 1.8 times greater than that for other microcomponents. The average values of cocrystallization coefficients for microcompoents having the same number of molecules of water of crystallization (Dav) decrease with the increase of the difference of number of molecules of water of crystallization of macrocomponent and microcomponent (Dn). The correlation - rxy of log Dav and Dn is significant at level a = 0.1 (Fig. 2).


Fig. 2. The dependence of log Dav on Dn.

Dav ­ average DM/CoSO4.7H2O for microcomponents having the same nm
Dn = ½nM - nm½, where nM and nm ­ number of molecules of water of
crystallization of macrocomponent and microcomponents, respectively.
rxy ­ correlation of log Dav and Dn

The average value of DM/CoSO4.7H2O for microcomponents forming, as CoSO47H2O, monoclinic sulfate hydrates (Davmon = 0.42) is very close to that of the other microcomponents (Davorth = 0.45), that is to say the similarity of crystal systems of macrocomponent and microcomponents does not affect directly average DM/CoSO4.7H2O.

The coefficients DM/CoSO4.7H2O generally increase as the solubility of MSO4nH2O in water decreases. Significant (at level a = 0.02), but relatively low correlation rxy = 0.8338 goes to show that this relationship is influenced by other factors. Simple linear regression coefficient (zxy=2.22±0,66) is close to the theoretical value for salts of AB type (zxy=2) (Fig 3). In the case of each microcomponent the Ruff's rule is fullfiled (DM/CoSO4.7H2O > 1, if CCoSO47H2O/CMSO4 nH2O >1 and vice versa).


Fig. 3. The relationship of log DM/CoSO4.7H2O and log (C01/C02)
(C01 and C02 ­ solubilities/ mol.dm-3 in water (at 20oC)
of CoSO47H2O and MSO4nH2O respectively. [28,29]
rxy ­ correlation of log DM/CoSO4.7H2O and log (C01/C02)

The cocrystallization coefficients DM/CoSO4.7H2O rises generally as the solubility of MSO4nH2O in the solid CoSO47H2O increases. Correlation rxy = 0.7570 is significant at level a = 0.05, but the values of (rxy)2 indicate that relatively small part of variability of DM/CoSO4.7H2O may be explained by variability of C [mol %]. (Fig. 4.).


Fig. 4. The dependence of cocrystallization coefficients DM/CoS4.7H2O
on the solubility of MSO4nH2O in the solid CoSO47H2O - CsMAX [10-12,21,24]

The cocrystallization coefficients DM/CoSO4.7H2O of microcomponents M2+ depend on their ionic radii (rM2+) (Fig. 5) The highest DM/CoSO4.7H2O occur for these microcomponents, whose ionic radii are most close to the radius of macrocomponent (Co2+) (except for Zn2+) and they generally decrease with the increase of the difference between rM2+ and rco2+ For the most distant ion Ca2+, its value is close to zero (D<0.008).


Fig. 5. The relationship between coefficients DM/CoSO4.7H2O of microcomponents M2+ and their ionic radii

On the basis of the presented character of the run of the curve DM/CoSO4.7H2O= f(rM2+) linear relationships have been searched between log DM/CoSO4.7H2O and ionic radii. It was assumed that log D2/1 may be dependent on ionic radius (rM2+) (absolute dimension of ion), on the converse of ionic radius (1/rM2+) (the value proportional to Cartledge ionic potential) and on the relative difference of the radii of the mutually substituting ions ½Dr/rCo½or (Dr/rCo)2. For these and some other dependences, for which the run was expected to be linear correlations have been calculated and compared with correlation of D2/1 and rM2+. The results are given in the Table 2.

For all dependences log DM/CoSO4.7H2O = f(rM2+) taken into consideration correlation (rxy) is significant at level a = 0.01 and is printed in bold. The correlation of log D2/1 and (Dr/r)2 is most significant.

The coefficients DM/CoSO4.7H2O are less closely (than in the case of ionic radii) linearly related to the difference of electronegativity of Co and other elements (De) as well as to the difference of crystal field stabilization energy of Co2+ and M2+ ions in their high spin octahedral complexes Ds = (CFSE)MACR ­ (CFSE)micr and are not linearly related to the difference of hardness of the ions Co2+ and M2+ (Dh).

The cocrystallization coefficients depend generally on electronic configuration of M2+ ions. The average values of DM/CoSO4.7H2O for open shell M2+ ions (similar in this respect to the ion of macrocomponent Co2+) are nearly four times greater than those for closed shell M2+ ions

The analysis of the influence of different chemical, physicochemical and crystal-chemical factors on DM/CoSO4.7H2O coefficients shows, that solubility in water of sulfates hydrates as well as radii of M2+ ions most influence their values. Calculation of these coefficients by means of the simplest formula DM/CoSO4.7H2O (C01/C02)2 [, (which was used in calculating D2/1 during cocrystallization of two salts forming ideal aqueous and solid solutions) leads in the case of cocrystallization of M2+ ions with CoSO47H2O to considerable deviations from experimental values and the average relative error amounts to 172%. After taking into consideration the Gibbs free energy of phase transition DGII®I of the trace component from its structure II into the monoclinic structure I of CoSO47H2O, as: 837 J/mol (for orthorhombic ZnSO47H2O, NiSO47H2O and MgSO47H2O) and 2510 J/mol (triclinic CuSO45H2O) [31], it is possible to decrease the deviations and the average relative error is lowered to 124%. However the correlation of the differences between experimental and estimated in such a way cocrystallization coefficients and relative difference of ionic radii of macrocomponent (rCo2+) and microcomponents (rM2+): [(rCo2+ - rM2+)/rCo2+]2 is rather high (rxy =- 0,9856). This enables calculating DM/CoSO4.7H2O coefficients with the equation given in the table 3. Comparison of calculated and experimental values of cocrystallization coefficients for each considered microcomponent (except Ca2+) during the crystallization of CoSO47H2O at 20oC is shown in the same table.

Comparison of calculated (Dcal.) and experimental (Dexp.) values of cocrystallization coefficients of microcomponents M2+ during the crystallization of CoSO47H2O at 20oC

CONCLUSIONS

The equilibrium distribution coefficients DM/CoSO4.7H2O of low amounts (10-3- 10-1%) of M2+ ions (M2+ = {Ni2+, Mg2+, Cu2+, Zn2+, Fe2+, Mn2+, Cd2+}) with CoSO47H2O have been determined by means of the method of long-time stirring of crushed CoSO47H2O crystals in their saturated solution at 20oC. Comparison of these coefficients with those, determined with the method of isothermal decreasing of supersaturation during 3 ­ 360 hours of stirring enabled the time needed to reach equilibrium to be found. It was different for different microcomponents.

The determined cocrystallization coefficients are diverse: from <0,008 for Ca2+ to 1,20 for Fe2+ and depend on some of the physicochemical and crystal-chemical properties of both sulfate hydrates (CoSO47H2O and MSO4nH2O) and metal M2+ ions.

DM/CSO4.7H2O values generally get greater as the difference of number of molecules of water of crystallization of CoSO47H2O and MSO4nH2O decrease and the similarity of electronic configuration of Co2+ and M2+ increases.

Cocrystallization coefficients decrease in general as the solubilities of corresponding sulfate hydrates in water increase and as their solubilities in solid CoSO47H2O decrease. They depend as well on radii (r) of M2+ ions (the dependence of log D on [(rCo2+ - rM2+)/rCo2+]2 is linear with the correlation coefficient rxy = -0,9350). These dependences are disturbed by the structure of sulfate hydrates.

The effect of electronegativity of M elements as well as crystal field stabilization energy and hardness of M2+ ions on DM/CoSO4.7H2O coefficients is rather small. They depend mainly on solubility and structure of corresponding sulfate hydrates as well as radii of M2+ ions.

Knowing solubilities (mol/L) in water of CoSO47H2O and MSO4nH2O as well radii of Co2+ and M2+ ions and taking the Gibbs free energy of the phase transition DGII-I of the trace component from its structure II into the monoclinic structure I of CoSO47H2O, as: 836,8 J/mol (for orthorhombic to monoclinic) and 2510 J/mol (for triclinic to monoclinic) it is possible to estimate D coefficients at the average relative error not exceeding 10%.

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