Distribution of Picric Acid into Various Diluents

Extraction constants, defined as Kex = [HPic]o/[H ][Pic], for picric acid (HPic) into ten diluents were spectrophotometrically determined at 298K, where the subscript “o” refers to an organic (o) phase which is composed of the diluent. Their values for the HPic extraction into benzene and chloroform were fairly consistent with those reported before. From the Kex values, distribution constants (KD,HPic) of HPic into the o phases were estimated using the thermodynamic relation Kex = KD,HPicKHPic, where KHPic corresponds to a protonation constant of Pic in water.Thereby, contributions of functional groups, such as –Cl and –CH 3 , involved in the diluents molecules to KD,HPic were discussed. The same discussion was also applied for the distribution of the ion pair, composed of CdPic 2 and 18-crown-6 ether, into the diluents.

In the present paper, we systematically determined at 298 K extraction constants ( ex /mol −1 dm 3 ) of HPic into ten diluents having lower polarities, such as -Xylene (mX), toluene (TE), bromobenzene (BBz), chlorobenzene (CBz), and 1-chlrorobutane (CBu).These diluents were selected on the basis of the careful consideration by Takeda et al. [3].From the  ex values, distribution constants ( D,HA ) of HPic between the w and organic (o) phases or the diluents were also calculated.Moreover, contributions of functional groups constituting the diluents molecules to  D,HA were discussed.The same discussion was applied for the distribution of the ion pair of CdPic 2 with 18-crown-6 ether (18C6) into the diluents.

Materials and Methods
2.1.Materials.Picric acid (99.5%,HPic⋅H 2 O) was purchased from Wako Pure Chemical Industries, Japan.Sodium hydroxide (97%, Kanto Chemical Co., Ltd., Japan) was used without further purification.Its purity was determined by acid-base titration with potassium hydrogen phthalate (guaranteed pure reagent: GR, 99.9 to 100.2%, Kanto).The chemicals HPic⋅H 2 O were dissolved in water and then its concentration was determined by acid-base titration with an aqueous NaOH solution of 0.1 mol dm −3 .Commercially available diluents (all GR grades, Wako and Kanto) were washed three times with water and kept under the conditions saturated with water.Other chemicals were of GR grade.A tap water was distilled once with the still of the stainless steel and then was purified by passing through the Autopure system (Yamato/Millipore, type WT101 UV).This water was employed for preparation of all aqueous solutions.

Extraction Procedures and Data
Handling.Aqueous HPic solutions of (0.1 5 -3.4) × 10 −3 mol dm −3 and diluents were mixed at equivalent volume (12 cm 3 ) in glass tubes of about 30 cm 3 , and then these stoppered glass tubes were vigorously shaken by hand for 1 minute.The tubes were agitated for 2 h by a shaker with water-bath thermostated at 298 K and then, in order to separate the two phases, centrifuged for Here, the subscript "o" denotes the o phase.From these values and total concentrations, [HPic] t , of HPic, distribution ratios,  Pic , for Pic − were calculated, assuming that [19] and then using the equation As a software for the analysis of the plots of log  Pic versus pH (see Figure 1), a KaleidaGraph (ver.3.501, Hulinks Inc., Tokyo) was used.

A Concise Theoretical Treatment of Extraction Equilibria.
We assumed here the following equilibria: H + +A −  HA and HA  HA o for the overall extraction equilibrium of which the constant was expressed as [9,10,19], the distribution ratio was defined as Then, taking logarithms of both sides, the following equation was given:  1).This is supported by the following facts.A plot of log  ex versus log  D,HPic for the ten diluents gave a straight line with the slope of 1.00 1 and the intercept of 0.58 4 at the correlation coefficient () of 0.999.This intercept was also in good agreement with the log  HPic values, listed in Table 1, within the experimental errors of ±0.01.

For Interaction of the Diluents Molecules with HPic.
The log  D,HA values were in the order cHex ≪ CBu < CF < oDCBz < BBz, CBz < Bz < DCM, TE ≤ DCE < mX (see Table 1 for these abbreviations).This order shows that an interaction with HPic becomes strengthened by going from cHex to mX.We divide here the order into (cHex ≪) oDCBz < BBz, CBz < Bz < TE < mX for aromatic diluents and CBu < CF < DCM ≤ DCE for aliphatic diluents and then will examine these orders in more detail.The log  D,HA values were in the order Bz < TE < mX (Table 1).This fact indicates that substitution of -H by -CH 3 increases the interaction of the diluents molecules with HPic.Similar results were obtained for the substitution of -Cl or -Br by -CH 3 : namely, CBz or BBz < TE and oDCBz < mX, although the position of the functional group in the latter case differs from each other.On the other hand, those of -Cl by -CH 2 CH 2 CH 3 and -CH 2 CH 3 decreased the interaction with HPic.The corresponding examples were DCM (ClCH 2 -Cl) ≫ CBu (ClCH 2 -CH 2 CH 2 CH 3 ) and DCE (ClCH 2 CH 2 -Cl) ≫ CBu (ClCH 2 CH 2 -CH 2 CH 3 ), respectively.Also, the substitution of -H by -Cl decreased their interaction.These examples were DCM > CF and Bz > CBz > oDCBz.Like the relation between Bz and CBz, the same was also true of that between Bz and BBz.The above results indicate that the functional groups constituting the diluents molecules strengthen the interaction with HPic in the order of - The case of cHex ≪ Bz suggests that the substitution of -CH 2 CH 2 -by -CH=CH-strengthens the interaction with HPic, while, there was no difference in the interaction with HPic between -Cl and -Br and accordingly that in log  D,HA between CBz and BBz (Table 1).
The same analysis as that described in Section 3.4 gave the order of -H ≤ -CH 3 ≪ -Cl for the former, while it did that of -CH 2 CH 2 CH 3 , -CH 2 CH 3 ≪ -Cl ≤ -H for the latter.A comparison of these orders with that for the HPic distribution system indicates that the interaction of the aromatic diluents molecules with Cd(18C6)Pic 2 differs from that with HPic, while the interaction of the aliphatic diluents ones is essentially similar to it.Especially, it is suggested that the substitution of -H by -Cl in the aromatic diluents molecules or altering Bz into CBz strengthens the interaction with Cd(18C6)Pic 2 , being contrary to the HPic system.The same is true of the substitution of -CH 3 by -Cl, altering TE into CBz.

Conclusions
The  ex and  D,HPic values for the HPic extraction into the ten low-polar diluents were systematically determined at 298 K.The magnitudes of the former constants were exclusively controlled by those of the latter ones from analyzing their thermodynamic relation with  HPic .The contributions of the functional groups constituting the diluents molecules to  D,HPic were clarified subsystematically.That is, it was demonstrated that the interaction of HPic with the diluents molecules is strengthened by introducing a methyl group in their molecules.Also, the contributions to  D,Cd(18C6)Pic2 were partly different from those to  D,HPic .The authors cannot now explain essential meanings of such contributions.
Assuming that the  HA values are a constant in a given experimental pH range (see Section 2.2), then we can immediately obtain the log  ex value from the plot of log  A versus pH.Actually, the  HA values which were averaged for all the values of ionic strength () were employed for the analysis of the plots, where  = (1/2)([H + ] + [A − ]) ≈ [H + ].Additionally, the log  D,HA value can be estimated from the thermodynamic relation: log  ex = log  D,HA +log  HA .Also, (2) can approximate to log  A ≈ log  ex − pH in the present experimental ranges of pH (see Section 2.2).According to the relation log  ex = log  D,HA +log  HA , the extraction ability of the diluents employed for HPic is obviously controlled by the log  D,HA values (see Table [8]]]etermination of  ex and  D,HPic .Figure1shows examples for plots of log  A versus pH at A − = Pic − .Thus, the plots yielded linear relationships between log  A and pH.The same is also true of the HPic extraction systems with the other four diluents.The extraction data thus obtained are summarized in Table1.The log  ex values of the extraction into benzene, Bz, and chloroform, CF, were in accord with those[1,2]reported before by Takeda et al.Although the measuring temperature was not described, the log  ex value (=2.28[10]) estimated previously for the dichloromethane, DCM, system was also close to our value (=2.462).From the log  HPic values in Table1, one can see easily that differences among the present  values, the average  ones, have no objection to comparisons among the  ex values or component equilibrium constants, except for the cyclohexane, cHex, system[8].3.3.Extraction Abilities of the Diluents for HPic.

Table 1 :
[21]action constants for HPic into various diluents and their component equilibrium constants ( HA ,  D,HA ) at 298 K.These diluents are lined up downwards from the high-polar DCE to the low-polar mX.bAverage values of  in aqueous solutions.cex/mol−1dm 3 .dAveragevaluesevaluatedat  and expressed by  HA /mol dm −3 .eErrors of all the log  HA values evaluated here were ±0.01 except for the CF system.fReference[2].gLogHPic value at (I/mol dm −3 ) = 0.1.See[21].