1119
Anal. Chem. 1982, 54, 1119-1121
Successive Determination of Praseodymium and Ytterbium by Coulometric: Complexometric Titration Shl Chun-Nian,” Lu Jing-Ci, Ni Qi-Dao, and Chang Mou-Sen Department of Chemhtty, Chlna University of Science and Technology, Hefei Anhu;, People’s Republic of China
A method Is described lor the successive coulometric complexometric titration of microgram levels of light and heavy rare earth elements. The total amount of llght and heavy rare earth elements Is tltrated by electrogeneratlng titrant EDTA on the mercury pool as a cathode. Then, the mercury pool is swltched as an anod43, electrogeneratingHg2+,and Is used to tltrate EDTA whlch Is displaced from its light rare earth elements complex by malic acld that has been added prior to the electrogeneratlng Hg2+. The heavy rare earth elements can be determined from the total rare earth elements minus the result of dlsplacemont titration of EDTA which is equivalent to the light rare earth elements. A 10-30 pg sample of llght and heavy rare earth elements (Pr and Yb, La and Lu, Nd and Tm) can be dotermlned successlvely. Errors and standard devlatlon are not more than 2.5% and 0.20, respectively.
The coulometric complexometric titrations of Eu, Nd, Pr, and Ce have been reported by Simakin (1) and Jin-Hen (2). Because the light and heavy rare earth (R.E.) elements are so similar in nature, it is very difficult to distinguish one from the other in a mixed solution. The authors have attempted to find a method for successive determinations in a single sample of a pair of R.E. elements, one of which is a light element and the other a heavy element, based upon the difference in stability between the complexes of R.E. with malic acid and EDTA. Neither the theoretical explanation nor the optimum condition of determination, as established in this paper, has yet been discussed in the literature.
THEORETICAL SECTION Malic acid has been reported as a masking reagent for the light R.E. elements in complexometric titration (3). It can form complexes with both light and heavy R.E.; but the stability constants of complexes of heavy R.E., such as Yb, with malic acid have not been reported except for the first stability constant (9). We have determined the stability constants of complexes Yb-malic acid with pH-potentiometry. The successive stability constants are Kl = 104.78,K z = 103.98,and K3 - 102.60(at 20 “C in 0.12 M KC1). In test solutions containing both malic acid and EDTA the conditional stability constants of P r or Yb (log K’), according to Ringbom (4), can be calculated a t various pHs and a t various concentrations of either malic acid or EDTA. Figure 1 represents the relationship of the conditional stability constants of complexes RE.-malic acid a t various concentrations of EDTA and those of complexes R.E.-EDTA a t various concentrations of malic acid, with respect to pH. The curves Al, Az, B1, and B2 (log K’R.E,-m&c varies with pH) and varies with pH) intersect each curves I and I1 (log K’R,E.-~DTA other. Left of intersection point, log K’R,E,-,,,alic is larger than log K’RE,--A, and right of the intersection point log K’R,E,-dc is less than log K’R,E.-EDT>~ T o ensure that the conditional stability constant of Pr-malic is always larger than that of Pr-EDTA and the conditional stability constant of Yb-malic 0003-2700/82/0354-1119$01.25/0
Table I. The Optimum Region of pH in Various Concentrations of Malic Acid in the Presence of Both Pr and Yb amt of region of pH malic acid ___ used, M [EDTA] = M [EDTA] = 0.001 0.01
0.05 0.10
1.00
pH 4 4.0 < pH 4.0 < pH 4.3 < pH 5.1 < pH
< 5.1 < 5.8 < 6.1 < 1.2
4.0 4.0 4.5 5.0 6.0
M
< pH < 4.4 < pH < 5.4 < pH < 6.1 < pH < 6.4 < pH < 7.6
is always less than that of Yb-EDTA, the optimum pH should be between the point P1 (the intersection of curves Al and 1) and the point Pz(the intersection of curves B1and 2). The concentrationi of EDTA will varv during the titration. If the original concentration for Pr is I F 5 M, the concentration of EDTA displaced by malic acid is also M. If the error of determination is 1%, then the concentration a t the end point should be M. Figure 1shows the requirements for pH: for Pr, pH 4.5. As stated above, during the titration the optimum region of pH should be 6.1 > p H > 4.5. But the optimum region of pH corresponding with various concentrations of EDTA and malic acid is different. The optimum region of pH a t various concentrations of EDTA and malic acid is listed in Table I (take the principle of common complexometric titration into account for the highest tolerable acidity). The calculated results in Table I are in agreement with the following experiment.
EXPERIMENTAL SECTION Reagents. Mercuric EDTA Solution. A 2.167-g portion of mercuric oxide (G.R.) is dissolved in 25 mL of 1M nitric acid and added to a 100 mL solution containing 3.700 g of disodium salt of EDTA (A.R.). The solution is adjusted to pH 5.0 with 6 M sodium hydroxide solution and diluted to 250.0 mL. This solution contains a slight excess of Hg2+. The concentration of Hg-EDTA is about 0.04 M. Standard Solutions of Pr, Yb, La, Lu, Nd, and Tm. The stoichiometric amounts of corresponding oxides (S.P.)are dissolved in 5 mL of 1M nitric acid and diluted to 50 mL with water to obtain 0.01 M stock solutions. The working solution, which is prepared by diluting the stock solution, contains 0.15-0.20 mg/mL R.E. A 10% Malic Acid Solution. A 10-g portion of purified malic acid (5) is dissolved in 90 mL of water. HAC-NaAc BufferSolution. Acetic acid (0.1 M) and sodium acetate (0.1 M) are mixed in various ratios to obtain buffer solutions of various pHs. Apparatus. p H Meter: A Model S-2 pH meter made in Shanghai, China, was used. Constant Current Source. A set of 225-V B batteries is used with a relatively large series resistance (1.5 MQ) for constant current of about 0.15-0.20 mA. Constant Current Source of Polarized Indicator Electrodes. A 22.5-V B battery with a series resistance of 25 MQ is used for the constant about 0.9 uA. 0 1982 American Chemical Society
1120
ANALYTICAL CHEMISTRY, VOL. 54, NO. 7,JUNE 1982 LOG K:
20i Yb
4:
20
x Id[MALI 40 C1
0
Flgure 3. The amount of malic acid as a function of recovery of Pr and Yb: Pr, 16.25 pg; Yb, 13.85 pg.
\
0 1
2
3
4
5
6
7
8
f
l
Flgure 1. Conditional stability constants of R.E.-mallc acid and RE.-EDTA complexes as a function of pH: curve A,, log KrPr-mlic in in the presence the presence of M EDTA; curve A, log KrR-ma,k of M EDTA; curve E?,, log K'Yb-mallc in the presence of lo-' M EDTA; curve B, log Krybmakin the presence of M EDTA; curve 1, log KfPr-EDTA in the presence of 0.05 M malic acld; curve 2, log KfYb-EDTA in the presence of 0.05 M malic acid.
U'
'
'
'
250
520 ti3
Figure 2. Tlration curves for successive coulometric complexometric titration of Pr and Yb.
Electrolysis Cell. The electrolysis cell has been described previously (6). Procedure. A 0.7-mL portion of pH 5.0 buffer solution and 0.3 mL of Hg-EDTA solut,ion are transferred to the cell, and the magnetic stirrer is started. At first, the mercury pool is used as a cathode and a Pt coil is used as an anode. The main electrolysis circuit is switched on and unchelated mercury ions in the solution are pretitrated to the end point. The known amounts of Pr and Yb as working solutions are added with a microsyringe. When the potential is at its lowest, titration is carried out continuously through the end point as before. Then, the electrodes are switched over, and the mercury pool is used as an anode. After the excess of EDTA has been titrated by Hg2+generated at the mercury pool anode, a specific amount of malic acid is added, and EDTA from Pr-EDTA is released and is titrated continously by Hgz+. On the basis of potential indicated by a pair of polarized mercury electrodes and the corresponding time, potential-time curves are plotted. The time for the titration of Pr and Yb can be measured by the distance between the parallel parts of the curves; and the current is determined precisely by measuring the iR drop across the precision resistor with a pH meter. A set of typical titration curves is shown in Figure 2.
RESULTS AND DISCUSSION Titration Curves. In Figure 2, curve 1 represents the pretitration of unchelated Hg2+,curve 2 is the titration of total amounts of Pr and Yb, curve 3 is the titration of excess EDTA, and curve 4 is the titration of the EDTA corresponding to equivalent amounts of Pr. The curves are similar in shape
Figure 4. The polarized current as a function of recovery of Pr and potential break at end point: (1)% found-i; (2)€4.
to those obtained by Martin and Reilly (7) for the titration of copper. Although a maximum is obtained at the equivalent point, we prefer to measure the definite potential in the steep portion of the curves (6). The pretitration and main titration curves must be the same shape, so that the steep portions parallel each other. Current Efficiency. The experiment is carried out by Lingane's method (8). When the concentration of Hg-EDTA M and complexes in the solution is larger than 6.3 X current density is less than 1.0 mA/cm2, 100% current efficiency can be obtained by electrogenerating EDTA at the cathode. As long as the excess of EDTA exists, the reaction 2Hg 2e- = Hg;+ can be avoided and current density can be kept to not more than 5.5 mA/cm2. Even though the concentration of Hg-EDTA complexes is less than M, 100% current efficiency can be obtained by electrogenerating Hg2+at the anode (at pH 5). Effect of pH and Malic Acid. The optimum amounts of malic acid are based on obtaining the highest recovery for Pr, the lowest recovery for Yb, and the larger end point break of potential. The recoveries, which vary with amounts of malic acid a t various pH for P r and Yb, are shown in Figure 3. Figure 3 shows that for 100% recovery of Pr the lowest concentrations of malic acid are 0.020 M, 0.028 M, and 0.035 M at pH 4.5, 5.0, and 5.5, respectively. When the concentration of malic acid is less than 0.03 M, Yb cannot be found at pH 4.5, 5.0, and 5.5. Thus 0.03 M malic acid in the solution is used, corresponding with 0.04 mL of 10% malic acid in 1 mL of the test solution. In addition, Figure 3 shows that the effect of p H on P r is larger than the effect of pH on Yb. Although the higher the p€I of the buffer solution, the larger is its break; but the recovery of IJr will decrease. Thus we chose to select a medium value, i.e., p W 5.0, for the experiment. A Pair of Polarized Mercury Electrodes and Polarized Current. The method of complexometric titration using a pair of polarized mercury electrodes in the presence of HgEDTA chelate has been investigated by Martin and Reilly (7). By use of a pair of polarized mercury electrodes as indicator electrodes, the largest break and shorter response time can be obtained.
ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982
1121
--
Table 11. The Results of Successive Coulometric Measurement for Known Amount of Rare Earth Elementsa elements Pr and Yb
amt taken, p g Pr Yb
amt found, p g Pr Yb
8.13 6.88 6.88
8.08 6.97 7.05
9.23 17.2 26.1
amt taken, p g Nd Tm Nd and Tm
7.93 13.3 14.4
7.00 7.00 11.1
amt taken, p g La Lu La and Lu
10.1 10.1 8.10
9.23 17.4 18.5
errors, %
9.41 17.2 25.8
8.01 13.1 14.2
Pr
Yb
-0.62 +1.31 2.47
+1.95 0.00 -1.15
0.17 0.18 0.20
0.10 0.18 0.11
errors, %
7.17 7.05 11.2
s ( n = 5)
Nd
Tm
Nd
Tm
+LO1
+2.43 +0.71 0.90
0.15 0.20 0.14
0.20 0.10 0.12
-1.50 -1.39
amt found, p g La Lu 10.2 10.1 7.93
Yb
+
amt found, p g Nd Tm
s ( n = 5)
pr
+
errors, %
9.44 17.7 18.4
s ( n = 5)
La
Lu
La
Lu
t0.99
+ 2.28
0.08 0.20 0.17
0.09 0.12 0.20
0.00 -2.10
+1.72 -0.54
Generating current: 0.195 mA.
E
of simplicity we prefer to use sodium hydroxide rather than ammonia to adjust the pH in solutions of Hg-EDTA complexes and malic acid. Extension of the Method. The method is extended to the determination of La and Lu and Nd and Tm. The results of these determinations are listed in Table 11.
(MVh
’00 50
TIMEb)
Flgure 5. Effect of ammonia concentration on end point potential and its break. Concentration of ammonia: (1) 0.01 M; (2) 0.03 M; (3) 0.06 M; (4) 0.30 M.
The effect of polarized current on the break and recovery of Pr is shown in Figure 4. The recovery of Pr decreases as the polarized current increases. To avoid the errors arising from the indicator electrode system, we used a smaller polarized current. The 0.9 jiA constant polarized current is suited for the experiment. The exact value of the polarized current is not required but consistency is necessary. Effect of Ammonia,, From Figure 5 it can be seen that the end point potential shifts downward and the break decreases when the concentration of ammonia increases at the same pH. In order to maintain the same end point potential during pretitration and main titration, it is necessary to maintain a constant concentration of ammonia. For the sake
CONCLUSION The method is suited for successive determinations of Pr and Yb by coulometric complexometric titration as well as for La and Lu and Nd and Tm. The procedure is carried out by electrogenerating EDTA and Hg2+ at the cathode and anode, respectively. The mixed standard solutions containing 10-30 pg amounts of both light and heavy R.E. elements can be determined successively. The errors and standard deviation are not larger than 2.5% and 0.20, respectively. LITERATURE CITED Simakin, G. A.; Timofeev, G. A.; Vladimlrova, N. A. Radiokhlmiye 1977, 19, 560-564. Jln-Heng, Mo Hua Hsueh Hsueh f a 0 1980, 38 (3), 292. Yong-Zhao, Chen; Huan-Ran, Li Hua Hsueh Hsueh f a 0 1980, 38 (4), 232. Rlngbom, A “Complexatlon in Analytlcal Chemistry”; Interscience: New York, 1963; pp 144-147. Yong-Zhao, Chen Acta Sci. Nat. Unlv. Sunyatseni 1963, 4 , 63. Mank, R. G.; Steed, K. C. Anal. Chlm. Acta 1962, 26, 305. Martin, A. E.; Reilly, C. N. Anal. Chem. 1959, 3 1 , 992. Lingane, J. J. “Electroanalytical chemistry”, 2nd ed.; Intersclence: New York, 1958; p 492. Davldenko, N. K. Zh. Neorgan. Khim. 1982, 7 , 2709.
RECEIVED for review July 1982.
16, 1981. Accepted February 19,
