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  Jan. 5, 2001  Vol.3 No.1 P.5 Copyright cij17logo.gif (917 bytes)

Determination of equilibrium constant of extracting rare earth metal ions with acidic organophosphorus esters

Hu Wenxiang, Peng Qingtao, Cui Hao, Yuan Chengye#, Li Shusen
(Institute of Military Medicine, Headquarters of General Equipment, Beijing 100101; #Shanghai Institute of Organic Chemistry, Chinese Academy of Science, Shanghai 200032)

Received Aug. 1, 2000; Supported by the National Natural Science Foundation of China.

Abstract The determination method of extraction equilibrium constant of the rare earth elements was described in this paper. The results of extracting lanthanum and neodymium ions with acidic organophosphorus esters showed that there were three ligands in the rare earth complexes.
Key words Acidic organophosphorus esters, Rare earth metal ions, Extraction equilibrium constant

The application of rare earth metal has become more and more important in national economy, especially in colour television industry and superconduction, and the upsurge of rare earth metal study and exploitment has been promoted greatly. The reserves of rare earth metal in China is the first in the world. Because of the character similarity of every rare earth element, the great difficulty of separation and purification has been brought out. In recent years, the solvent extraction separation of rare earth element has led to people's great interest[1,2]. In this paper, the extraction solvent was acidic organophosphorus ester, extraction equilibrium constant was obtained by determination of the concentration of rare earth metal ions and acidity of water phase after equilibration, in which volumetric analysis was used.

1 REAGENTS            
0.5 mol/L LaCl3 and NdCl3 solution was made up separately; 0.01328 mol/L ZnCl2 solution; 0.005732 mol/L EDTA solution (standardized by 0.01328 mol/L ZnCl2 solution); 0.2 mol/L NaOH solution (standardized by 0.1 mol/L potassium acid phthalate solution); 2 mol/L HCl solution (standardized by NaOH solution which had been standardized); 2 mol/L NaCl solution; 0.5% xylenol orange solution (in 50% ethanol); 5% phenolphthalein solution (in ethanol).
    Buffer solution (pH=5.4): 400g hexamethinetetramine was weighed and 1000 mL water was added in for dissolvation, then 100mL concentrated HCl solution was added in. The solution was purplish red and stored in a brown bottle.
    Masking reagent: CaCl2 saturated solution was added in 1000mL 0.1 mol/L EDTA solution (the one function of CaCl2 was avoiding the precipitation of EDTA in neutral condition), adjusted the mixture solution to neutral with HCl and NaOH solution. The function of this masking reagent was to mask the rare earth ions avoiding the base precipitation when the acidity of water phase was titrated with NaOH.
    Extraction organic phase and water phase: acidic organophosphorus ester was weighed precisely, and n-dodecane was the dilute reagent. Thus about 0.2 mol/L organic phase formed. 50mL water phase with different pH that contained 0.01 mol/L LnCl3 was made-up. Its ionic strength was adjusted by NaCl solution, and made u=1.0 mol/L (H+, Na+)Cl.

2 EXPERIMENT METHOD
2.1 Two-phase extraction equilibrium experiment
5 mL extraction water phase was taken into 25mL separatory funnel, then the equal volume organic phase contained phosphorus (the phase ratio was 1:1) was also taken into. The separatory funnel was taken onto the vibrator and vibrated for 30 min, then laid down for a while. Separated the water phase and to determine the concentration of [Ln3+] and [H+].
2.2 The determination method of the concentration of rare earth metal ion [Ln3+]
1.8mL water phase mentioned above was taken into a 40mL beaker, and 3 drops buffer solution was added in, the colour of the solution was turned into violet. Then 2 drops xylenol orange solution was added in, titrated the solution until the colour of the solution was turned into yellow with 0.005732 mol/L EDTA solution. According to the following formula, the concentration of rare earth ion , M2 , could be obtained: M1V1 = M2V2 =1.8M2 .
2.3 The determination method of [H+]
0.5-1.8 mL water phase mentioned above was taken into a beaker, 5 mL masking reagent and two drops phenolphthalein solution were added in. Titrated the solution until the colour turned to purplish red with NaOH solution that had been titrated, then the concentration of [H+] could be obtained by calculation.

3 RESULTS AND DISSCUSSION    
The extraction equation of rare earth metal ions extracted with acidic organophosphorus esters is as follows:
Ln3+ + 3(HL)203100501.gif (79 bytes)LnL3·3HL + 3[H+]                              (1)
In formula (1), HL is acidic organophosphorus esters, (HL)2 is the dimer of acidic organophosphorus esters. Then the extraction constant can be expressed as:
Kex ={ [LnL3·3HL][H+]3 /[Ln3+][(HL)2]3 } =D/[(HL)2]3¡Á [H+]3        (2)
where D is the distribution ratio. The concentration of the rare earth metal ions in organic phase can be obtained by difference-subtraction method:
[LnL3 · 3HL]= [Ln3+]1 £­ [Ln3+]2                               (3)
[(HL)2]= 1/2(C0HL - 6[LnL3·3HL])                               (4)
    In practical application, with regard to RP(O)OH(OC6H13-n), [(HL)2] can be calculated according to formula (4) when R>C4H9 , or [(HL)2] can be calculated according to formula (5) when< C4H9[3]£º
[(HL)2]= 1/16K2{-(1+1/Kd)+[(1+1/Kd)2 + 8K2C]1/2}2                             (5)
K2 is the dimeric constant of acidic organophosphorus esters, Kd is the oil-water distribution coefficient. According to formula (2) , formula (6) can be obtained:
log{ D/[(HL)2]3 } = logKex + m pH                              (6)
where m is the ligand number of acidic organophosphorus esters in rare earth metal ions complex. The extraction equilibium constant log Kex of rare earth metal ions extracted with acidic organophosphorus esters can be obtained by drawing or regression according to formula (6).
    We took acidic organophosphorus esters p-C2H5C6H4P(O)(OH)(OC6H13-n) extracted La3+(C0HL=0.2000mol/L), p-i-C3H7C6H4P(O)(OH)(OC6H13-n) extracted Nd3+(C0HL =0.1505mol/L), n-C3H7P(O)(OH)(OC6H13-n) extracted La3+(C0HL =0.2021mol/L), n-C8H17P(O)(OH)(OC6H13-n) extracted Nd3+(C0HL=0.2000mol/L) as examples, the results of the extraction equilibrium constant of rare earth metal ions La3+ and Nd3+ extracted with acidic organophosphorus esters are given in Table 1.

Table 1 Results of extraction equilibrium constant of rare earth metal ions La3+ and Nd3+ extracted with acidic organophosphorus esters

acidic organophosphorus esters

metal ions

C(HL)2

D

log(D/C3(HL)2)

pH

logKex

logKex (average)

p-C2H5C6H4P(O)(OH)(OC6H13-n)

La3+

0.07410
0.07846
0.08262
0.08672
0.08921

6.28
2.64
1.40
0.81
0.57

4.19
3.74
3.40
3.09
2.90

0.67
0.50
0.37
0.29
0.21

2.19
2.24
2.28
2.23
2.28


2.25 (n=5)

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p-i-C3H7C6H4P(O)(OH)(OC6H13-n)

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Nd3+

0.05527
0.05805
0.06081
0.06293
0.06492

1.54
1.08
0.78
0.59
0.46

3.96
3.74
3.54
3.38
3.22

0.29
0.22
0.15
0.09
0.05

3.09
3.10
3.09
3.12
3.09
3.10 (n=5)

n-C3H7P(O)(OH)(OC6H13-n)

La3+

0.07726
0.07754
0.08204
0.08405

3.83
2.97
1.76
1.30

3.92
3.80
3.50
3.34

1.48
1.43
1.33
1.28

 -0.50
-0.49
-0.48
-0.50


-0.49 (n=4)

n-C8H17P(O)(OH)(OC6H13-n)

Nd3+

0.07884
0.08096
0.08284
0.08505
0.08810

1.79
1.36
1.09
0.83
0.57

3.56
3.41
3.28
3.13
2.92

1.14
1.08
1.05
1.01
0.92

0.13
0.16
0.13
0.12
0.15


0.14 (n=5)

    According to formula (6), the monobasic linear regression between log (D/ C3(HL)2) and pH was carried out , then the slope m=3 could be obtained. It demonstrated that the ligand number of acidic organophosphorus esters was 3 with regard to extracting rare earth metal ions La3+ and Nd3+.

REFERENCES  
[1] Yuan C Y, Yan J Y, Feng H Z et al. Science in China (Zhongguo Kexue), Series B, 1986, 16: 1150-1157.
[2] Yuan C Y, Ye W Z, Ma E X et al. Science in China (Zhongguo Kexue), Series B, 1982, 12: 597-605.
[3] Hu W X. [Thesis for the Doctorate]. Shanghai: Shanghai Institute of Organic Chemistry, Chinese Academy of Science; Beijing Library ,1989: 157-165.

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