# Distribution of Picric Acid into Various Diluents

Authors: Yoshihiro Kudo; Yuu Takahashi; Shoichi Katsuta
Journal: Journal of Chemistry
 (2013)

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## Abstract

Extraction constants, defined asKex=HPico/[H+][Pic−], for picric acid (HPic) into ten diluents were spectrophotometrically determined at 298 K, 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 –CH3, 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 CdPic2 and 18-crown-6 ether, into the diluents.


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## Body

## 1. Introduction

Picric acid (HPic) has been widely used for the source of pairing anions in solvent extraction [1–5], application to selective sensors [6], sequential injection analysis with it [7], its separation science [8, 9], theoretical study for its partition [10], its distribution into ionic liquids [11], and so on. Also, many authors have studied extraction and separation of many metal ions by crown compounds from water (w) into various diluents [1–3, 12–17]. In particular, the extraction data of HPic are important for studying the extraction of easily hydrolyzed metal ions, such as Ag+ and Pb2+, by the compounds into the diluents [1, 2, 18]. However, few systematic studies of the HPic extraction have been reported [5].In the present paper, we systematically determined at 298 K extraction constants (Kex/mol−1 dm3) of HPic into ten diluents having lower polarities, such as m-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 Kex values, distribution constants (KD,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 KD,HA were discussed. The same discussion was applied for the distribution of the ion pair of CdPic2 with 18-crown-6 ether (18C6) into the diluents.


## 2. Materials and Methods

### 2.1. Materials

Picric acid (99.5%, HPic·xH2O) 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·xH2O 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.


### 2.2. Extraction Procedures and Data Handling

Aqueous HPic solutions of (0.15–3.4) × 10−3mol dm−3 and diluents were mixed at equivalent volume (12 cm3) in glass tubes of about 30 cm3, 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 7 minutes. The pH values of the w phases separated were measured at 298 K: pH readings were in the range of 2.57 to 4.10. A Horiba pH/ion meter (type F-23) equipped with a glass electrode (Horiba, type 9615-10D) was employed for the pH measurements. While the o phases or the diluents separated were transferred into other glass tubes and the aqueous NaOH solutions of 0.1 mol dm−3 were added to them, volume ratios (Vo/V) of the separated o phases against the added aqueous solutions with NaOH were 1/1, 1/2, 1/4, 1/5, 1/8, 1/10, 1/12, or 1/25. As similar to the above procedure, the amounts of HPic in the o phases were back-extracted into the aqueous NaOH solutions. Using a 1 cm quartz cell, concentrations of Pic− in these aqueous solutions were determined spectrophotometrically at 355.0 nm and 298 K, where a spectrophotometer used was of a Hitachi U-2001 type. The operation of the back-extraction was repeated in some cases.These Pic− concentrations which were determined at 355.0 nm due to the absorption of Pic− were assumed to equal the concentrations, [HPic]o, of HPic in the o phases. Here, the subscript “o” denotes the o phase. From these values and total concentrations, [HPic]t, of HPic, distribution ratios, DPic, for Pic− were calculated, assuming that [HPic]o≫[Pic-]o+[(HPic)2]o [19] and then using the equation DPic=[HPic]o/([HPic]t-[HPic]o) (see Section 3.1).As a software for the analysis of the plots oflogDPic versus pH (see Figure 1), a KaleidaGraph (ver. 3.501, Hulinks Inc., Tokyo) was used.Figure 1
Plots oflogDPicversus pH for six diluents. Circle: DCE; triangle: DCM; full circle: CBu; diamond: BBz; inverted triangle: Bz; square: mX. A broken line is the regression one for the DCE system based on (2): logDPic=2.489–pH–log(1+3.86×10-pH) at R=0.998.


## 2.1. Materials

Picric acid (99.5%, HPic·xH2O) 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·xH2O 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.


## 2.2. Extraction Procedures and Data Handling

Aqueous HPic solutions of (0.15–3.4) × 10−3mol dm−3 and diluents were mixed at equivalent volume (12 cm3) in glass tubes of about 30 cm3, 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 7 minutes. The pH values of the w phases separated were measured at 298 K: pH readings were in the range of 2.57 to 4.10. A Horiba pH/ion meter (type F-23) equipped with a glass electrode (Horiba, type 9615-10D) was employed for the pH measurements. While the o phases or the diluents separated were transferred into other glass tubes and the aqueous NaOH solutions of 0.1 mol dm−3 were added to them, volume ratios (Vo/V) of the separated o phases against the added aqueous solutions with NaOH were 1/1, 1/2, 1/4, 1/5, 1/8, 1/10, 1/12, or 1/25. As similar to the above procedure, the amounts of HPic in the o phases were back-extracted into the aqueous NaOH solutions. Using a 1 cm quartz cell, concentrations of Pic− in these aqueous solutions were determined spectrophotometrically at 355.0 nm and 298 K, where a spectrophotometer used was of a Hitachi U-2001 type. The operation of the back-extraction was repeated in some cases.These Pic− concentrations which were determined at 355.0 nm due to the absorption of Pic− were assumed to equal the concentrations, [HPic]o, of HPic in the o phases. Here, the subscript “o” denotes the o phase. From these values and total concentrations, [HPic]t, of HPic, distribution ratios, DPic, for Pic− were calculated, assuming that [HPic]o≫[Pic-]o+[(HPic)2]o [19] and then using the equation DPic=[HPic]o/([HPic]t-[HPic]o) (see Section 3.1).As a software for the analysis of the plots oflogDPic versus pH (see Figure 1), a KaleidaGraph (ver. 3.501, Hulinks Inc., Tokyo) was used.Figure 1
Plots oflogDPicversus pH for six diluents. Circle: DCE; triangle: DCM; full circle: CBu; diamond: BBz; inverted triangle: Bz; square: mX. A broken line is the regression one for the DCE system based on (2): logDPic=2.489–pH–log(1+3.86×10-pH) at R=0.998.


## 3. Results and Discussion

### 3.1. A Concise Theoretical Treatment of Extraction Equilibria

We assumed here the following equilibria:H++A-⇌HA and HA⇌HAo for the overall extraction equilibrium of which the constant was expressed as Kex=[HA]o/[H+][A-]. For this extraction system [9, 10, 19], the distribution ratio was defined as
(1)DA=[HA]o[A-]+[HA]=Kex[H+]1+KHA[H+].
Then, taking logarithms of both sides, the following equation was given:
(2)logDA=logKex-pH-log(1+KHA[H+])=logKex-pH-log(1+KHA10-pH).
Assuming that the KHA values are a constant in a given experimental pH range (see Section 2.2), then we can immediately obtain the logKex value from the plot of logDA versus pH. Actually, the KHA values which were averaged for all the values of ionic strength (I) were employed for the analysis of the plots, where I=(1/2)([H+]+[A-])≈[H+]. Additionally, the logKD,HA value can be estimated from the thermodynamic relation: logKex=logKD,HA+logKHA. Also, (2) can approximate to logDA≈logKex − pH in the present experimental ranges of pH (see Section 2.2).


### 3.2. Determination ofKex and KD,HPic

Figure1 shows examples for plots of logDA versus pH at A− = Pic−. Thus, the plots yielded linear relationships between logDA 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 Table 1. The logKex 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 logKex value (=2.28 [10]) estimated previously for the dichloromethane, DCM, system was also close to our value (=2.462). From the logKHPic values in Table 1, one can see easily that differences among the present I- values, the average I ones, have no objection to comparisons among the Kex values or component equilibrium constants, except for the cyclohexane, cHex, system [8].Table 1
Extraction constants for HPic into various diluents and their component equilibrium constants (KHA, KD,HA) at 298 K.
Diluentsa
I
-
b/mol dm−3
logKexc
logKD,HA
logKHAd,e
DCE
7.8
×
10
-
4
2.489
±
0.006
1.90
±
0.01
0.59
oDCBz
1.0
×
10
-
3
2.170
±
0.002
1.59
±
0.01
0.58
DCM
8.4
×
10
-
4
2.462
±
0.002
1.88
±
0.01
0.59
CBu
1.0
×
10
-
3
1.474
±
0.004
0.89
±
0.01
0.58
CBz
8.5
×
10
-
4
2.310
±
0.002
1.72
±
0.01
0.59
BBz
8.7
×
10
-
4
2.300
±
0.007
1.72
±
0.01
0.59
CF
3.7
×
10
-
4
1.941
±
0.003
,
1.940
f
1.347
±
0.006
,
1.65
0
f
0.594
±
0.005
,
0.29
0
f,g
cHexh
1.0i
0.66
−0.16
0.82
Bz
8.1
×
10
-
4
2.396
±
0.005
,
2.393
j
1.81
±
0.01
,
2.10
3
j
0.59,0.290j,g
TE
6.3
×
10
-
4
2.473
±
0.006
1.88
±
0.01
0.59
mX
7.9
×
10
-
4
2.552
±
0.005
1.97
±
0.01
0.59
aDCE: 1,2-dichloroethane; oDCBz: o-dichlorobenzene; DCM: dichloromethane; CBu: 1-chlorobutane; CBz: chlorobenzene; BBz: bromobenzene; CF: chloroform; cHex: cyclohexane; Bz: benzene; TE: toluene; mX: m-xylene:.  These diluents are lined up downwards from the high-polar DCE to the low-polar mX.  bAverage values of I in aqueous solutions.  cKex/mol−1 dm3.  dAverage values evaluated at I and expressed by KHA/mol dm−3.  eErrors of all the logKHA values evaluated here were ±0.01 except for the CF system.  fReference [2].  gLogKHPic value at (I/mol dm−3) = 0.1.  See [21].  hReference [8].  iConcentration of HCl.  jReference [1].


### 3.3. Extraction Abilities of the Diluents for HPic

According to the relationlogKex=logKD,HA+logKHA, the extraction ability of the diluents employed for HPic is obviously controlled by the logKD,HA values (see Table 1). This is supported by the following facts. A plot of logKex versus logKD,HPic for the ten diluents gave a straight line with the slope of 1.001 and the intercept of 0.584 at the correlation coefficient (R) of 0.999. This intercept was also in good agreement with the logKHPic values, listed in Table 1, within the experimental errors of ±0.01.


### 3.4. For Interaction of the Diluents Molecules with HPic

ThelogKD,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.ThelogKD,HA values were in the order Bz < TE < mX (Table 1). This fact indicates that substitution of –H by –CH3 increases the interaction of the diluents molecules with HPic. Similar results were obtained for the substitution of –Cl or –Br by –CH3: 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 –CH2CH2CH3 and –CH2CH3 decreased the interaction with HPic. The corresponding examples were DCM (ClCH2–Cl) ≫ CBu (ClCH2–CH2CH2CH3) and DCE (ClCH2CH2–Cl) ≫ CBu (ClCH2CH2–CH2CH3), 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 –CH2CH2CH3, –CH2CH3≪ –Cl, –Br < –H ≤ –CH3.The case of cHex ≪ Bz suggests that the substitution of –CH2CH2– 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 logKD,HA between CBz and BBz (Table 1).


### 3.5. Application for the Distribution of Ion Pairs with 18C6

ThelogKD,MLA2 values reported previously [20] at MLA2 = Cd(18C6)Pic2 were in the order Bz (−6.18) ≪ TE (−4.85) < CBu (−4.48) < mX (−4.13) ≪ CBz (−2.70) < CF (−2.62) < DCE (−2.53) ≤ DCM (−2.52). Comparing these values with the logKD,HPic ones for given diluents, we can easily see that Cd(18C6)Pic2 more weakly (or more strongly) interacts with the diluents (or water) molecules than HPic does. The high polarity [20] of the Cd(18C6)Pic2 structure is also supported by this comparison. Similarly, the order was divided into Bz ≪ TE < mX ≪ CBz for aromatic diluents and CBu ≪ CF < DCE ≤ DCM for aliphatic diluents.The same analysis as that described in Section3.4 gave the order of –H ≤ –CH3 ≪ –Cl for the former, while it did that of –CH2CH2CH3, –CH2CH3 ≪ –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)Pic2 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)Pic2, being contrary to the HPic system. The same is true of the substitution of –CH3 by –Cl, altering TE into CBz.


## 3.1. A Concise Theoretical Treatment of Extraction Equilibria

We assumed here the following equilibria:H++A-⇌HA and HA⇌HAo for the overall extraction equilibrium of which the constant was expressed as Kex=[HA]o/[H+][A-]. For this extraction system [9, 10, 19], the distribution ratio was defined as
(1)DA=[HA]o[A-]+[HA]=Kex[H+]1+KHA[H+].
Then, taking logarithms of both sides, the following equation was given:
(2)logDA=logKex-pH-log(1+KHA[H+])=logKex-pH-log(1+KHA10-pH).
Assuming that the KHA values are a constant in a given experimental pH range (see Section 2.2), then we can immediately obtain the logKex value from the plot of logDA versus pH. Actually, the KHA values which were averaged for all the values of ionic strength (I) were employed for the analysis of the plots, where I=(1/2)([H+]+[A-])≈[H+]. Additionally, the logKD,HA value can be estimated from the thermodynamic relation: logKex=logKD,HA+logKHA. Also, (2) can approximate to logDA≈logKex − pH in the present experimental ranges of pH (see Section 2.2).


## 3.2. Determination ofKex and KD,HPic

Figure1 shows examples for plots of logDA versus pH at A− = Pic−. Thus, the plots yielded linear relationships between logDA 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 Table 1. The logKex 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 logKex value (=2.28 [10]) estimated previously for the dichloromethane, DCM, system was also close to our value (=2.462). From the logKHPic values in Table 1, one can see easily that differences among the present I- values, the average I ones, have no objection to comparisons among the Kex values or component equilibrium constants, except for the cyclohexane, cHex, system [8].Table 1
Extraction constants for HPic into various diluents and their component equilibrium constants (KHA, KD,HA) at 298 K.
Diluentsa
I
-
b/mol dm−3
logKexc
logKD,HA
logKHAd,e
DCE
7.8
×
10
-
4
2.489
±
0.006
1.90
±
0.01
0.59
oDCBz
1.0
×
10
-
3
2.170
±
0.002
1.59
±
0.01
0.58
DCM
8.4
×
10
-
4
2.462
±
0.002
1.88
±
0.01
0.59
CBu
1.0
×
10
-
3
1.474
±
0.004
0.89
±
0.01
0.58
CBz
8.5
×
10
-
4
2.310
±
0.002
1.72
±
0.01
0.59
BBz
8.7
×
10
-
4
2.300
±
0.007
1.72
±
0.01
0.59
CF
3.7
×
10
-
4
1.941
±
0.003
,
1.940
f
1.347
±
0.006
,
1.65
0
f
0.594
±
0.005
,
0.29
0
f,g
cHexh
1.0i
0.66
−0.16
0.82
Bz
8.1
×
10
-
4
2.396
±
0.005
,
2.393
j
1.81
±
0.01
,
2.10
3
j
0.59,0.290j,g
TE
6.3
×
10
-
4
2.473
±
0.006
1.88
±
0.01
0.59
mX
7.9
×
10
-
4
2.552
±
0.005
1.97
±
0.01
0.59
aDCE: 1,2-dichloroethane; oDCBz: o-dichlorobenzene; DCM: dichloromethane; CBu: 1-chlorobutane; CBz: chlorobenzene; BBz: bromobenzene; CF: chloroform; cHex: cyclohexane; Bz: benzene; TE: toluene; mX: m-xylene:.  These diluents are lined up downwards from the high-polar DCE to the low-polar mX.  bAverage values of I in aqueous solutions.  cKex/mol−1 dm3.  dAverage values evaluated at I and expressed by KHA/mol dm−3.  eErrors of all the logKHA values evaluated here were ±0.01 except for the CF system.  fReference [2].  gLogKHPic value at (I/mol dm−3) = 0.1.  See [21].  hReference [8].  iConcentration of HCl.  jReference [1].


## 3.3. Extraction Abilities of the Diluents for HPic

According to the relationlogKex=logKD,HA+logKHA, the extraction ability of the diluents employed for HPic is obviously controlled by the logKD,HA values (see Table 1). This is supported by the following facts. A plot of logKex versus logKD,HPic for the ten diluents gave a straight line with the slope of 1.001 and the intercept of 0.584 at the correlation coefficient (R) of 0.999. This intercept was also in good agreement with the logKHPic values, listed in Table 1, within the experimental errors of ±0.01.


## 3.4. For Interaction of the Diluents Molecules with HPic

ThelogKD,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.ThelogKD,HA values were in the order Bz < TE < mX (Table 1). This fact indicates that substitution of –H by –CH3 increases the interaction of the diluents molecules with HPic. Similar results were obtained for the substitution of –Cl or –Br by –CH3: 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 –CH2CH2CH3 and –CH2CH3 decreased the interaction with HPic. The corresponding examples were DCM (ClCH2–Cl) ≫ CBu (ClCH2–CH2CH2CH3) and DCE (ClCH2CH2–Cl) ≫ CBu (ClCH2CH2–CH2CH3), 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 –CH2CH2CH3, –CH2CH3≪ –Cl, –Br < –H ≤ –CH3.The case of cHex ≪ Bz suggests that the substitution of –CH2CH2– 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 logKD,HA between CBz and BBz (Table 1).


## 3.5. Application for the Distribution of Ion Pairs with 18C6

ThelogKD,MLA2 values reported previously [20] at MLA2 = Cd(18C6)Pic2 were in the order Bz (−6.18) ≪ TE (−4.85) < CBu (−4.48) < mX (−4.13) ≪ CBz (−2.70) < CF (−2.62) < DCE (−2.53) ≤ DCM (−2.52). Comparing these values with the logKD,HPic ones for given diluents, we can easily see that Cd(18C6)Pic2 more weakly (or more strongly) interacts with the diluents (or water) molecules than HPic does. The high polarity [20] of the Cd(18C6)Pic2 structure is also supported by this comparison. Similarly, the order was divided into Bz ≪ TE < mX ≪ CBz for aromatic diluents and CBu ≪ CF < DCE ≤ DCM for aliphatic diluents.The same analysis as that described in Section3.4 gave the order of –H ≤ –CH3 ≪ –Cl for the former, while it did that of –CH2CH2CH3, –CH2CH3 ≪ –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)Pic2 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)Pic2, being contrary to the HPic system. The same is true of the substitution of –CH3 by –Cl, altering TE into CBz.


## 4. Conclusions

TheKex and KD,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 KHPic. The contributions of the functional groups constituting the diluents molecules to KD,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 KD,Cd(18C6)Pic2 were partly different from those to KD,HPic. The authors cannot now explain essential meanings of such contributions.



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*Source: 869506-2013-09-05.xml*