Polarography of Niobium-EDTA Complexes ROBERT KIRBY' and HENRY FREISER Department of Chemistry, University of Arizona, Tucson, Ariz.
b The polarography of niobium(V) in EDTA solutions was studied in detail, Two reduction waves were obtained. The first, a reversible 1electron reduction to niobium(lV), involved varying amounts of hydrogen ion, depending on the pH region. The diffusion current of the Nb(V)EDTA complex varied linearly with concentration between the limits of 0.50 and 1000 fig. of niobium per ml. with a precision of 4~2.0%. Preliminary heating of the solution was required to obtain the maximum diffusion current. An analytical procedure based upon this behavior was developed, capable of an accuracy of f2.0%. Tantalum may be tolerated t o o ratioof 2:1,Ta:Nb.
T (a, s),
HE niobium-EDTA complexes were first reported by Ferrett and Milner who reported the first wave t o be reversible only a t p H 3.2 and below. Their work also indicated that the half-wave potential varied with EDTA concentration. Brindley (1) described a method for the polarographic determination of niobium in EDTA solution a t pH 1.9. Kennedy (4) considered the EDTA medium a t pH 3.0 unsuitable for the determination of niobium, since the diffusion current did not vary linearly with niobium concentration. Apparently he did not heat the Nb-EDTA solutions to develop the maximum diffusion current fully, as was reported by Brindley and by Kirby and Freiser (6). Kennedy also reported that the half-wave potential of the Nb-EDTA complex became more positive with increasing EDTA concentration in solutions a t pH 3.0; the slope of E112 vs. log EDTA concentration indicated the addition of about one half of an EDTA group more on the niobium(1V)-EDTA complex. The work here reported was undertaken to evaluate the applicability of the polarographic behavior of NbEDTA solutions to the determination of niobium.
EXPERIMENTAL
Apparatus. All polarograms were obtained using a calibrated Sargent Model XV recording polarograph. No damping was employed. A Sargent Micro-Range Extender was used 1 Present address, Colgate-Palmolive Co., New Brunswick, N. J. 122
ANALYTICAL CHEMISTRY
as an accessory for solutions of l o w niobium concentration (0.1 to 4.0 fig. of Nb per ml.). Measurements were made using an H-cell with a saturated calomel electrode. The cell resistance was approximately 100 ohms, so that it was not necessary to apply a correction to Eli2. The cell was placed in a grounded constant temperature bath maintained a t 25.0' 0.1 O C. Purified nitrogen was used to remove dissolved oxygen from the sample solution. The coulometric apparatus was that described by Wise and Cokal (7). Reagents. Stock solutions of niobium were prepared from 99.6% Nbz06 (Electro Metallurgical Co., Division of Union Carbide Corp.). A 1.425-gram quantity of the oxide was fused in 10 grams of potassium hydroxide in a nickel crucible. The melt was leached in about 200 ml. of water containing 2 grams of KOH. The solution was made up to 1 liter in a volumetric flask and contained 1.00 mg. of Nb per ml. All other chemicals were reagent grade. Solutions of niobium in EDTA were prepared by adding aliquots of the stock solution to a solution of the disodium salt of (ethylenedinitri1o)tetraacetate dihydrate, adjusting the pH to the desired value with 1 M H2SO4,and heating to boiling for 30 seconds. Buffers and inert electrolyte were added after cooling, care being taken that the pH did not deviate by more than *0.2 pH unit from the desired value. RESULTS
Solutions of niobium in E D T A in the p H range of 0.3 to about 6 gave a well defined wave due to the reduction of Nb(V) t o Nb(1V). I n a narrower portion of this p H range, from about 3 to 6, a second wave was observed that was not as well defined because of its proximity t o the hydrogen wave. To apply polarographic observations to the quantitative study of the electrode reactions involving the reduction of Nb(V)-EDTA, it was necessary to heat the reaction mixtures to boiling in order to obtain complete reaction in a short time and, after cooling to the desired temperature, to buffer the solutions carefully before the polarograms were obtained. Under these conditions the following information about the reduction of N b o to Nb(1V) was obtained : The wave was a 1-electron, reversible wave.
The change of half-wave potential with temperature was -1.3 mv. per degree. The half-wave potential varied linearlv with BH in a manner which depended on t6e pH. The value of Ellz was independent of EDTA concentration. The diffusion current also varied with pH. From pH 1 to 4 it remained essentially constant, but decreased significantly a t higher pH values and reduced t o insignificant values a t pH 6 and above. Similarly, the wave height decreased markedly at pH under 1. The diffusion current changed in a completely linear manner with S b concentration. An investigation of the second polarographic wave of the Nb-EDTA complex revealed that the variation of half-wave potential and the diffusion current with pH was not a smooth function; however, the trend was for a nonlinear decrease of the diffusion current as the pH increased, and the general increase in half-wave potential as p H increased indicated a value of unity for the number of hydrogen ions in the reduction equation of the second wave. Analysis of the second wave was difficult, because hydrogen ions were also reduced in this potential range. A poorly characterized third wave, a t about -1.4 volts, was detected. Thorough investigation of the blank solution, treated in all respects exactly the same as the sample solution except for the absence of niobium, revealed no wave a t - 1.4 volts. The conclusion was drawn then that the third reduction wave observed in the polarogram of the Nb-EDTA complex was due to niobium, and possibly to the reduction of Nb(II1) to Nb(I1). Coulometric Verification of 1-Electron Reduction, Nb(V) to Nb(1V). The reduction of S b ( V ) to Nb(1V) was verified by a coulometric study of the reduction a t a large stirred mercury cathode. The potential of the cathode was automatically controlled a t -0.88 volt and the current that flowed was measured by an integrating coulometer. A boiled solution containing 0.825 mg. of Nb in 0.1M EDTA a t p H 3.7 was electrolyzed with nitrogen passing through it until the background current fell to a steady value of 0.8 ma., which was the same as the value of a blank solution. The
Figure 1. of pH
Effect
---- .€I/*
of Nb-EDTA complex vs. pH. Slope of AB 0.057, BC 0.1 12, C D 0.1 47 id Of Nb-EDTA complex vs. pH
-
0
1.0
2.0 -El/*
half-wave potential of the solution was -0.68 volt. The total current measured in the coulometer was equivalent to 0.785 mg. of Nb, a 1-electron reduction being assumed. The difference between the amount of niobium added and that found on this basis was about 5%. Effect of pH. Preliminary investigations of the effect of p H on the halfwave potential of the Nb-EDTA complex-that is, before using a buffer solution-indicated t h a t three hydrogen ions were involved in the reduction of the complex a t the dropping mercury electrode in the p H range of 3.5 t o 6.0, and t h a t the diffusion current decreased nonlinearly as the p H increased, approaching zero above pH 6. However, further investigation was carried out in buffer solutions in two pH ranges: 3.4 to 5.7 in sodium acetateacetic acid solutions, and 0.3 to 3.1 in potassium sulfate-sulfuric acid solutions. In the pH range of 3.4 to 5.7 the solutions were maintained a t a constant ionic strength of 1.1M with 0.33M potassium sulfate and 0.1M sodium acetate. The niobium concentration was 1 m X . The results (Figure 1) show that the diffusion current increases from p H 3.4 to 3.7, is constant to pH 4.2, and then decreases as p H increases. The slope of Eli2 us. p H gives a value of 2.4 for the number of hydrogen ions involved in the electrode reduction. In the pH range of 0.3 to 3.1 the solubility of EDTA is low. Solutions prepared for this investigation contained 0.2mM Nb, 5mM EDTA, and
3.0
4.0
5.0
VOLT I'S. S.C.E.
0.033M K 8 0 1 and, except for the two extremes-pH 0.52 and 3.05EDTA came out of solution while the solutions were on the 25.0' C. bath. A plot of pH us. E ~ /indicated z that one hydrogen ion in the p H region from 0.5 t o 1.9 and two hydrogen ions in the p H region from 2 to 3 are involved the reduction reaction a t the dropping mercury electrode. The relationship between p H and the diffusion current of the Nb-EDTA complex is not a simple function; however, in the p H range of 0.7 to 2.4 it is nearly constant. A maximum appeared on the polarographic waves beginning a t pH 1.92 and continuing down to p H 0.52. The maximum was ignored for calculating the diffusion current and the half-wave potential. This could be done because each wave had a definite plateau before the next reduction process began a t the dropping mercury electrode and a symmetrical wave could be drawn over the wave obtained. The work was repeated using a lower concentration of EDTA. The solut.ions were 0.2mM Nb, 3.0mM EDTA, and 0.33M K2S04. Even with this lower EDTA concentration some crystals came out of the solutions, except for the solutions a t the highest and the two lowest pH values. A maximum appeared in the polarograms of the solutions from pH 0.78 to 1.92. The slope of Ell2 us. p H gave the same results as previously obtained-that is, the number of hydrogen ions involved in the reduction reaction is unity in the p H region of 0.26 to 1.90, and two in the p H region of 2 to 3.1. The diffusion current rapidly increases from pH
0.26 to 0.78, then remains relatively constant from pH 0.78 to 2.95. Additional data were obtained in the p H region of 2 to 3, using solutions of O.lmM Nb, 1.5mM EDTA, and 0.033M K2SOI. Effect of EDTA Concentration. Before it was discovered t h a t stable solutions of the Nb-EDTA complex could be prepared by heating the solution of the complex, the study of half-wave potential us. E D T A concentration produced results t h a t were not useful for correlating changes in half-wave potentials with E D T A concentration. If a solution of niobium in EDTA was prepared, the pH adjusted to the intermediate region, say, p H 4, and an inert electrolyte added immediately afterward, hydrated NbnOs precipitated from the solution. However, when the solution was heated to boiling after adjustment to the desired p H value, the electrolyte was added and the solution remained clear. This enabled a study of Eliz us. log EDT.4 concentration a t constant ionic strength. Even with this treatment of the NbEDTA solutions, the half-Fare potential became more positive with increasing EDTA concentration; however, the reduction reaction was not reversible, as determined from the slope of E us. log i The reaction ( i d -i) approached reversible behavior as the EDTA concentration increased. The data indicated that the reaction conditions a t the electrode surface were not the same as in the bulk of the solution; the p H a t the electrode mas probably higher, the difference being greater at lower EDTA concentrations. The solutions of higher EDTA concentration were better buffer solutions, but still not of sufficient capacity a t p H 4.0. Accordingly, a series of solutions was prepared in which the EDTA concentration varied from O.OO5M to 0.20M. All solutions mere 1.0mX in niobium, 0.1M in sodium acetate, and 0.4M in acetic acid, and varying concentrations of KtSOI were used to maintain a constant ionic strength of 1.1M. The pH of each solution wa9
i
4.0. The slope of E us. log -gave ( Z d - 1) values of 0.0586 to 0.0616, indicating reversibility of the reaction. These results showed that the number of EDTA ligands associated with the Nb(V) complex is equal to the number associated with the Nb(1V) complex. Effect of Niobium Concentration. The half-wave potential of the XbE D T A complex did not change with niobium concentration in solutions for which all other conditions were kept constant. After suitable conditions were established for studying the Nb-EDTA VOL. 35, NO. 2, FEBRUARY 1963
123
complex, the relationship between niobium concentration and the diffusion current of the h'b-EDTA complex was studied. The ranges of concentrations studied were: 0.1 to 4 fig, of S b per ml. (0.00108 to 0.043mM Nb); 2 to 200 pg. of Nb per ml. (0.0215 to 2.15mN Nb); and 100 to 1000 pg. of h'b per nil. (1.08 to 10.8mM Kb). All solutions were 0.liV in EDTA, 0.33-Tf in &Sod, 0.1N in sodium acetate, and 0.4M in acetic acid, and at a pH of 4.0. In the range from 0.1 to 4.0 pg. of Nb per ml., the Sargent Micro Range Extender was used in conjunction with the Sargent Model XV recording polarograph. A linear relationship
Table 1.
Effect of Niobium Concentration
Wl, fig.
h'b
concn.,
a.Per ml.
pa.
'id,
0.10 0.20 0.40 0.50 1.00 2.00 3.00 4.00
o.oo5 0.0133 0.0186 0.0394 0.0771 0.113 0.153
per ml., from slope
% error ...
o:iii
-34.5 -13 2 ~- - 2.80 3.00 2.00 - 1.67 0.00
0.347 0.486 1.03 2.02 2.95 4.00
++
For range of 0.10 t o 4.00 pg. per ml., average deviation is &1.90%. Slope, 0.0383 pa. per pg. h'b per ml.
Table II.
per nil.
id,
pa.
0.075 0.138 0.366 0.572 0.684 0.72 1.38 3.48 5.58 6.94 Average deviation, f2.3 %. 2.00 4.00 10.00 16.00 20.00 20.00 40.00 100.0 160.0 200.0
Slope
id
= - = 0.0346 pa.
Nb
-Eilz,
volt
S b calcd. from slope, pg. per ml.
0.700 0.698 0 . 700 0.706 0.703 0.700 0.698 0.698 0.700 0 .704
2.16 3 96 10.50 16.43 19.65 20.63 33.64 100.0 160.0 199.4
pg. per ml., from slope
Nb, id,
100.0 200.0 300.0 700.0 800.0 900.0 1000.0
pa.
3.55 7.09 10.23 24.75 27.80 31.20 33.90 Average deviation, 11.45%. Slope, 0.0348 pa. per pg. Nb per ml.
124
of Nb-EDTA Complex
% error +8.00 -1.00 +5.00 +2.68 -1.75 +3.40 -0.90 0.00 0 00 -0.30
Effect of Niobium Concentration
S b concn.,
per ml.
id
per fig. Nb per ml.
Table 111. pg.
EDTA (1.86 grams) was added to about 10 ml. of water and enough H2S04 was added t o make the pH of the solution about 4 after addition of the niobium stock solution. This slurry of EDTA was heated and the niobium
Effect of Niobium Concentration on € 1 , ~ and
S b concn., fig.
between niobium concentration and diffusion current was obtained from 0.5 to 4.0 pg. of Nb per ml. The plot of diffusion current 11s. niobium concentration gave a slope of 0.0383 pa. per pg. of Nb per ml. The average deviation was i1.90 yo (Table I). A linear relationship was obtained for the range of 2 to 200 pg. of S b per ml. The slope of the h e , i d z's. niobium concentration, was 0.0346 pa, per pg, of Kb per ml. The average deviation \vas &2.30y0 (Table 11). In the first attempt to prepare solutions of 100 to 1000 pg. of Nb per ml., varying amounts of water were added to the 1.86 grams of EDTA, depending upon the volume of niobium stock solution to be added. To the sample containing 700 pg. of xb per ml., only 5 ml. of water were added. To the samples containing 800, 900, and 1000 pg, of Nb per mi. no water n-as added; the appropriate aliquot of niobium stock solution was added directly to the EDTA. When the pH was adjusted to 4.0 with H2S04, hydrated NbzOs precipitated from the solutions containing from 700 to 1000 pg. of iYb per ml. Samples containing from 700 to 1000 pg. of N b per ml. were then prepared.
ANALYTICAL CHEMISTRY
102.0 203.6 294.0 710.0 800.0 896.0 975.0
yo error +2.00 +1.80 -2.00 f1.43 0.00 -0.44 -2.50
Table
IV. Effect of Tantalum Determination of Niobium
on
(Each sample contained 20.00 pg. of Xb per ml.) S b found, Ta added, ,ug./ml. fig./ml. 0.00 20.00 40.00 80.00 160.00
20.00 20.10 19.60 18.43 16.01
stock bolution s l o d y added. The solutions n-ere cooled and the pH was adjusted more precisely to 4.0. Clear solutions were obtained. To each solution were then added 2.88 grams of K2S04, 0.41 grams of sodium acetate. and 1.14 ml. of acetic acid. The solutions were evaporated to 40 ml. and cooled, the p H was again adjusted t o 4.0 if necessary, and then the volume was made up to 50 ml. in a volumetric flask. The plot of diffusion current of the Sb-EDTA complex us. niobium concentration gave a straight line with a slope of 0.0348 pa. per pg. of S b per ml. The average deviation was *1.57, (Table 111). Effect of Ionic Strength. Comparison of the half-wavP potentials and diffusion currents of the NbE D T A complex were made for solutions containing 0.33.11 added K2S04 and solutions containing no added K2S04. Solution compositions otherwise were 0.4niA1fS b . 0 . 0 5 X EDTA, and 0.1M sodium acetate. The investigation was carried out for solutions over the p H range of 3.4 to 5.6. The half-wave potentials of the solutions containing no added KzSOl were, in general, 15 mv. more positive than solutions of comparable pH which were 0.33Jf in added KzSO~. The diffusion currents in the solutions containing no added weIe less by about 7% than in solutions containing 0.33M added K2S04 for comparable pH values up to 4.8. At higher pH values the difference in diffusion currents became much greater, reaching a 4Oy0decrease a t pH 5.6. Effect of Temperature. The temperature coefficient of the half-wave potential and of the diffusion current of the h'b-EDTA complex was studied over a 30" temperature range a t several p H values. I n all cases the temperature coefficient of the halfwave potential was negative and less than 2 mv. per degree centigrade; the average value was 1.3 mv. per degree. The temperature coefficient of the diffusion current was 1.8% per degree. Interference. hfolybdenum was the only element studied t h a t seemed t o interfere seriously with the polarographic determination of niobium. Brindley eliminated molybdenum interference by a prior separation procedure using 8-quinolinol.
A partially successful attempt was made in this investigation to eliminate molybdenum interference with the use of reagents that would strongly complex the molybdenum and move to its half-wave potential to more negative values, but have no effect on the NbEDTA coniplex. Reagents used were glycol dimercaptoacetate, 1,j-diphenyl3-thiocarbohydrazide, quinoxaline-2,3dithiol, and mercaptoacetic acid. Of these, the glycol dimercaptoacetate was the most promising. The tantalum-EDTA complex was prepared (6) and found to have a halfwave potential more negative than the niobium-EDTA comples by about 0.5 volt. Nevertheless, tantalum interfered
with the determination of niobium. The ratio of tantalum to niobium that may be tolerated in the polarographic determination of niobium was studied. The results are tabulated in Table IV. Each sample contained 20.00 p g . of Kb per ml. These solutions m r e 0.1M in EDTA and 0.1X in sodium acetate, and had a pH of 4.06 0.03. These results indicate that tantalum may be tolerated up to a ratio of 2:1, Ta:?;b with a deviation of 2.0y0 or less.
*
ACKNOWLEDGMENT
Financial assistance from Kright Air Development Division is gratefully acknowledged.
LITERATURE CITED
(1) Rrindley, D. T., Analyst 85, 877
(1960).
(2) Ferrett, D. J., Milner, G. W. C., J . Chem. SOC.1956, 1188. (3) Ferrett, D. J., Xilner, G. W. C.,
Nature 175,4i7 (1955). (4) Kennedy, J. H., ANAL. CHEM.33,
(1961).
( 5 ) Kirby, R. E., Freiser, H., Abstracts
for 12th Annual Pittsburgh Conference on Analytical Chemistry and Applied Spectrography, March 1961 (published November 1960). (6) Kirby, R. E., Freiser, H., J . Phys. Chem. 65, 191 (1961). (7) Wise, E. N., Cokal, E. J., ANAL. CHEM.32, 1417 (1960). RECEIVED for review September 10, 1962. Accepted December 12, 1962.
Co nt roIIe d- PotentiaI Co uIo met ric Tit r atio n of Uranium(IV) and Uranium(V1) in Sodium Tripolyphosphate Medium H. E. ZITTEL and LOUISE B. DUNLAP Analytical Chemistry Division, Oak Ridge Notional laboratory, Oak Ridge, Tenn.
b The U(IV) and U(VI) contents of a sample can be determined b y controlled-potential coulometric titration. Both the oxidation and reduction reactions take place at a mercurypool electrode in a supporting electrolyte that i s 6 w./v. % sodium tripolyphosphate solution adjusted to pH 7.5 to 9.5. The U(VI) is reduced coulometrically at a potential of - 1.35 volts vs. S.C.E. This reduction is a direct measure of the U(VI) present. The same test portion is then oxidized coulometrically at +O.l volt vs. S.C.E. This oxidation is a measure of the total uranium present; the U(IV) content i s found b y difference. In the range of total uranium content from 2 to 10 mg., the error of the method i s approximately 1 %. Various possible interferences were studied; only Cu(ll) and Fe(lll) caused any significant error.
U
RANIUM(IV) OXIDE is widely used as a nuclear fuel. either alone or mixed with other oxides, such as thorium oxide and zirconium oxide. After such use, the mole ratio of uranium to oxygen in uranium oxide may vary from 1 : 2 t o 1:3. I t is often necessar,y to determine the concentration of both U(1V) and U(V1) present in fuel solutions. The U(1V) can be determined either directly by titration with some strong oxidizing agent, such as dichromate or
permanganate (6, 8))or indirectly by titration of the U(V1) both before and after oxidation of the test sample and calculation of the U(1V) by difference. The U(V1) can be titrated either coulometrically (9) or with some suitable chemical titrant (4). Recently, Boyd and Menis (1) published a method for the direct determination of U(IV) by controlled-potential oxidation a t a platinum electrode. ,ill the abore-mentioned methods are carried out in acid media. I t is not possible to determine accurately both U(1V) and U(TrI) in the same test portion by any of these methods. Previous n-ork by the authors on the polarographic behavior of uranium in an aqueous solution of sodium tripolyphosphate (Na6P3010) indicated that, through use of this medium, both U(1V) and V(V1) can be determined coulometrically a t a mercury-pool electrode. The coulometric determination of U(V1) in an alkaline NasPdOlo medium has been reported previously (12). The results of a study of the anodic oxidation of U(1V) and the cathodic reduction of U(V1) in an alkaline solution of Na5P30j0at a mercury electrode are reported herein Conditions were established whereby both U(IV) and U(VI) can be determined in the same test portion without chemical conversion of the U(IV) to U(V1). Possible interferences were checked, and the precision
and accuracy of the method for both U(Iv) and U(V1) were established. EXPERIMENTAL DETAILS
Apparatus. Coulometric titrator, ORNL Model Q-2005, electronic, controlled-potential ( 5 ) . Potentiometer, Rubicon 0- to 1.6volt range. p H Meter, Becknian Model €1. Conventional titration cell with a mechanically stirred mercury electrode and a reference S.C.E. (9). Reagents. Standard solutions of U3Os. prepared by dissolving a known weight of National Bureau of Standards standard sample No. 950 UaOs in 5 ml. of 85% &Po,,, under an inert atmosphere, and diluting the solution to 25 ml. with 5M H3P04. The solutions were standardized as follows: The U(VI) content was determined according to the procedure described by Kubota (Yj. The U(IV) was then and the total oxidized with "03, uranium content was determined by coulometric titration in 0 . 5 M H2S04(9). Standard solution of U(IJ7 as UOz, prepared and standardized in the same manner as was the U30ssolution. Sodium tripolyphosphate supporting electrolyte, 6 w./v. yo Xa5P3Ol0prepared by dissolving 6 grams of commercial grade Na6P3Olo (obtained from Monsanto Chemical Co.) in water and diluting the solution to 100 ml. with water. All other reagents were prepared from A.C.S. grade chemicals and, when necesVOL 35, NO. 2, FEBRUARY 1963
125
