Vanadyl Ion as a Back-Titrant for Indirect Amperometric T it ra ti o ns w it h (Ethy Ie ne d init riI0)tet ra a c etic A c id Application to the Determination of ThA4in Fluoride-Bearing Materials GERALD GOLDSTEIN,
D.
Zrf4, a n d
1. MANNING, and H. E. ZITTEL
Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tenn.
b A rapid and precise method has been developed for indirect amperometric titrations with EDTA in which VO*2 i s used as the back-titrant. EDTA i s added in excess to the test solution, and this excess, in an acetatebuffered medium of pH 4, i s then titrated with VOt2. The equivalence point i s detected with a platinumfoil electrode maintaiced at a potential of +0.6 volt vs. S.C.E. At this potential VOY-? i s not oxidized but VO+2 is; therefore, the presence of excess titrant i s indicated b y a linear increase in current. In general, any cation that forms an EDTA chelate having a stability constant of l o L 6or greater can be determined. The relative standard deviation i s about 1%. Fluoride and phosphate were tested as masking agents to increase selectivity. The method was applied to the determination of Th+', and Zr+4 in fluoride-bearing materials.
I
(4) on the direct ainperometric titration of both rlectroactive and electroinactive ions with (ethylenedinitri1o)tetraacetic acid !EDT.4), FeiZ was used as a n indicator inn. Some elements, for example zirconium and aluminum, cannot be determined b>- this direct titrat.ion. 'Therefore, the possibility of developing :in indirect amperometric titration procedure for these ions was investigated. Tn this procedure EDTA is added in cwess to the sample, and then the excess is back-titrated. For the back-titration procedure to be widely applicable, tlie back-bitrant should behave as i'ollons: Form with EDT-4 a complex tliat has a reIativeIy low stability so t!iat as many elements as possible can hc determined; be electroactive so that electroinactive elements can be determined; be elertroactiye a t a platinum elect,rode so that the full titration curre can be recorded; be clectroactive a t Ion- p H values (1O4. The final solutions were adjusted to about 2 M in HC104. I n general, the relative standard deviation was less than 1%. Since the relative standard deviations are not significantly different, a standard solution of Zn+2, Bi+3, or Pb+2 can be used to standardize the voso4 solution. The blank titer is only about 0.7 chart division; therefore, small errors in the blank titer will not appreciably affect the precision or accuracy of the EDTA titrations.
A back-titer value of at least 20 chart divisions is desirable to minimize any small errors involved in establishing the end points of a titration. T o achieve this value, at least a twofold excess of EDTA should be added to the test solution. A twofold excess is also sufficient to ensure complete reaction of those elements that do not readily form EDTA chelates in acid solutions, for example nickel and aluminum. Effects of Fluoride and Phosphate as Masking Agents on the Determination of Various Cations. To increase the selectivity of the titration, tests were made to determine whether masking agents could prevent some cations from reacting with EDTA. The effects of F- and Po4+' on the recovery of Fe+3, 8 1 + 3 , Th+4, and Zr+4 are shown in Figures 4 and 5, respectively. Fluoride interferes seriously in the determination of Al+3, less seriously
4 ~ ~ .
0
5 10 15 MOLE RATIO, poi3 /cotion
20
Figure 5. Effect of phosphate on recovery of various cations
in the determination of Zr+4,and not a t all in that of Fe+3 within the limits tested. When the mole ratio of F- to Th+4 is greater than 4, the end point becomes less sharply defined. The Th+4 is completely masked when the mole ratio is about 14. Phosphate interferes seriously in the determination of Al+3 and Fe+3 when present in twentyfold excess.
800
I I 5 10 15 MOLE R A T I O , PO: 1 2 ~ 4
Figure 6. Effect of phosphate on determination of aluminum in presence of zirconium (zirconium aluminum, 0.057 mmole)
+
Figure 6. I n samples that contain Zr+4 in a mole ratio t o A l ~ 3of up to 1.4: 1, the Zr+'can be masked by adding P04-3 in about a 20:l mole ratio t o Zr+4. If the ratio of Zr+4 to d l f 3 is greater, a much larger amount of is necessary. dluminum was determined m HFHK03 solutions that were 0.6 to 1.031 in Al+3 and 0.3 t o 0.5.V in ZI-+~. A 1-ml, aliquot of the original sample solution was fumed strongly with 20 ml. of HC104 until only 5 ml. of H(2104 remained. This solution was transferred to a 100-ml. volumetric flask, an additional 10 ml. of HC104 was added, and the resulting solution was diluted to volume. Then a 3-ml. aliquot of the diluted sample solution \Tas taken, 5 ml. of 0.1M Na3P04was added, and the Al+3 was determined by the recommended titration procedure. The results are presented in Table 111. DETERMINATION OF Z R + ~ . The effect of F- on the determination of Zr+4 in the presence of is shown in Figure 7 . When the mole ratio of Al+3 to is not greater than 1.4:1, essentially complete recovery of Zr +4 is obtained by masking the Al+3 with F- a t a Fto Al+3 mole ratio of 15: 1. When the ratio of 81+3 t o Zr+4 is greater than 1.4:1, the A1+$ and Zr+' appear to be masked almost simultaneously; therefore, the recovery of Zr+4 depends critically on the amount of F-added. Zirconium was determined in the HF-HKO3 solutions by taking a 4-ml. aliquot of the fumed and diluted sample solution, adding 4.5 ml. of 0.1M NaF, and then titrating by the recommended
APPLICATIONS
Application to the Determination of
Zr f 4 and Al f 3 in HF-HNO, Solutions.
0
1
I 5 10 15 MOLE R A T I O , F - / c o t i a n
20
Figure 4. Effect of fluoride on recovery of various cations
Since the d a t a of Figures 4 and 5 indicate that Al+3 and can be determined in the presence of each other respectively, as by use of F- and masking agents, procedures were developed for the determination of these solutions. two elements in HF-"03 DETERMINATION OF A L + ~ . The effect of Po4+ on the determination of Alia in the presence of Zr+4 is shown in
Table II. Precision of Replicate Titrations (Five replicates) Rel. Cation titrated Std. dev., Quantity, Identity mmoles % Zn +2 Bi +a Pb+-2
0.01973 0.03072 0.02580
VOL. 35, NO. 1, JANUARY 1963
0.7 0.9 0.6
0
19
Table I l l .
Results of Determination of
Sample
A1+3, iMa (A) 0.608 0.571 0.726 0.756 0,901 0.967 0.589 0.752 0.749 0.882
1
2 3 4 5 6 7 8 9 10 a
and Zr+4 in HF-"03
Zr f4, Mb (B) 0.419 0.369 0.457 0.389 0.407 0.283 0.325 0.413 0.339 0.326
A
Solutions
AI +3 DIUSZr +4. M Determined B simultaneousl~
\SAMPLE
+
1.02 0.940 1.18 1.15 1.31 1.25 0.914 1.17 1.09 1.21
1.05 0.948 1.19 1.13 1.30 1.29 0.907 1.18 1.11 1.23
55
0.03-
B
~
d
0
u E
P04-3 present to mask Zr +4.
* F- present to mask Alf3
KO masking agent present.
Table IV. Results of Determination o f Zirconium in NaF-LiF-ZrF4 Fused Salt
Zr +4 found, mg./g. a
RR1. std. dev., %
402 404 419 414
2.1 2.9 2.8 0.3
Average of four replicates.
procedure. The data appear in Table 111. DETERMINATION OF A L + 3 PLUS z R + 4 . To determine $1 +3 plus Zr +4, no masking agent is added, and the total is determined by the recommended procedure. Comparison of these results with the sum of the individual results for Al+3 and Zr+4 (Table 111) shows good agreement. However, small errors in the determination of Zr+4 would not contribute an appreciable relative error B) . to the total of Al+3 plus Zr + 4 (A If the difference between the two sets of data is attributed t o error in the Zr+4 determination (for example, in the case of sample 6), the error in the Zr+4 determination is about - 14%. The Alfa to Zr+4 mole ratio in this sample is greater than 3; therefore, an accurate Zr+' determination is difficult (see
+
I
0
5
N
I
5 10 (5 MOLE R A T I O , F - / A b
0
20
Figure 7. Effect of fluoride on the determination of zirconium in presence of aluminum (zirconium aluminum, 0.057 mmole)
+
20
0
ANALYTICAL CHEMISTRY
Figure 7). The mole ratio of ,41+3 to in the other samples was less than 3, and the error in the Zr+4 determination does not appear to be greater than 1.5%. Application to NaF-LiF-ZrF4 Fused Salt. Zirconium(1V) was determined in NaF-LiF-ZrF4 fused salt t h a t contained approximately 400 mg. of zirconium, 80 mg. of sodium, and 20 mg. of lithium per gram of sampIe by t h e following procedure. +40.5-gram sample was transferred to a 50-ml. beaker. Then 10 ml. of 1:l HCl, 1 gram of H&03, and 20 ml. of concd. HClO4 were added. By moderate heating, the solution was evaporated t o fumes of HClO4 and the fuming continued until all the sample dissolved and only 5 ml. of HClO, remained. The side of the beaker was washed down and the solution was evaporated to fumes of HC104 again. The solution was transferred to a 100ml. volumetric flask, an additional 15 ml. of concd. HC104 was added, and the solution was diluted to volume with water. Then a 1-ml. aliquot was transferred t o a 50-ml. beaker and the general titration procedure followed. The results are shown in Table IV. Since some F- can be tolerated in the determination of Zr+4,no effort was made t o remove the last traces from the test solutions. The relative standard deviation for the determination of Zr+4 in this type material is about 2%. Application to LiF-BeF2-UF4-ThFr ZrFd Fused Salt. T h e fused salt LiF-BeF2-UF4-ThF4-ZrF4 is being considered as a possible fuel for a molten-salt reactor. A gram of t h e mixture contains about 100 mg. each of lithium and zirconium a n d 50 mg. each of beryllium, thorium, and uranium. Initial attempts to determine the sum of thorium plus zirconium resulted in recoveries of only about 50 to 80%. However, when N a F was added t o the solutions before the addition of EDTA, recoveries of Zr+4 plus Th+' increased until the mole ratio of F- to Zr+' exceeded about 1O:l; then
the recoveries decreased because of the complexing action of F-. This effect is shown in Figure 8 for two different samples. The low recovery in the absence of fluoride is attributed t o polymerization of zirconium as previously discussed. It would appear that fluoride ion depolymerizes zirconium solutions very effectively. Recently, in studies of the extraction of Zr+4 with thenoyltrifluoroacetone (TTA) (9), it was also shown that pretreatment of zirconium samples with F- was necessary in some cases t o break up polymerized Zr+4 species and to make possible the complete extraction of the Zr+4. The results of determinations by the following procedure, together with the results of analyses by other methods in which thorium and zirconium were first separated and then determined separately by titration with EDTA, are given in Table V for comparison. The relative standard deviation of the results of duplicate determinations was 1to 2%.
Table V. Results of Determination o f Thorium plus Zirconium in LiF-BeFr UF4-ThF4-ZrF4 Fused Salt
Th+' plus Zr+4,mmoles/g. AmDerometric Other method methods
For instructions regarding the preparation of the sample solution, see reference (4). Transfer a I-ml. aliquot of the sample solution to a 50-ml. beaker. Add 5 ml. of 0.1M NaF; then heat the solution t o boiling. Add 5 ml. of 3.02M EDTA solution and again boil the solution. Cool the soIution to room temperature; add 2 ml. of 3M NaC2H302solution
Adjust the pH and complete the titration as instructed in the recommended procedure I
LITERATURE CITED
(1) Barnard, A. J., Jr., et ul., “The EDTA Titration,” p. 30, J. T. Baker Chemical Co.. November 1957. (2) Bobtelsky, ’M., Rafailoff, R., Anal. Chim. -4cta 17, 308 (1957).
(3) Fritz, J. S., Johnson. M., .~N.IL. CHEM.27, 1653 (1955). (4) Goldstein, G., Manning, D. L., Zittel, H. E., Zbid., 34, 358 (1962).
(5) Johnson, J. S., Kraus, K. A., J. Am. Chem. SOC.78,3937 (1956). ( 6 ) Kelley, M. T., Miller, H. H., ANAL. CHEM.24, 1895 (1952). (7) Lukyanov, V. F., Knyazeva, E. M., J . Anal. Chem. (U.S.S.R.) 15, 71 (1960). (8) M.anning, D. I,., Meyer, A. S., Jr., White, J. C., U S. Atomic Energy Comm. Rept. ORNL-1950 (Aug. 12, 1955). (9) Marsh, S. F., Maeck, W. J., Booman, G. L., Rein, J. E.: ANAL. CHEM.33, 870 (1961). (10) Morgan, L. O., Justus, N. L., J . Am. Chem. SOC.78, 38 (1956).
(11) Pecsok, R. L., Sawyer, D. T., Zbid., 78, 5496 (1956). (12) Ringbom, A,, Siitonen, S., Skrifvars, B., Acta Chem. Scund. 11,551 (1957). (13) Schwarzenbach, G., Sandera, J., Helu. Chim. Acta 36, 1089 (1953). (14) Zielen, A. J., Connick, R..E . J . Am. Chem. Soc. 78, 5785 (1956).
RECEIVEDfor review July 26, 1962. Accepted October 29, 1962. Division of Analytical Chemistry, 141st Meeting, ACS, Washington, 13. C., September: 1962. Oak Ridge National Laboratory is operated by Union Carbide Corp. fGr the Atomic Energy Commission.
A n Automatic Potentiometric Reaction Rate Method for Cystine Using the Azide-iodine Reaction HARRY
L.
PARDUE and SANDRA SHEPHERD
Department o f Chemistry, Purdue University, West lafayette, Ind.
A rapid automatic method for the determination of micro amounts of cystine is described. The method i s based on the experimental observation that early in the reaction time the rate of reduction of iodine to iodide by sodium azide i s proportional to cystine concentration. The rate of disappearance of iodine i s measured b y following continuously the rate of change of the e.m.f. of a concentration cell sensitive to iodine concentration. Commercial equipment i s easily modified to present automatically the rate data directly on a readout dial. Cystine is determined in 2-ml. samples in the concentration range from 0.25 to 25 p.p.m. The error of the method i s of the order of 0.005 pg. of cystine or 1% relative, whichever i s larger. The measurement time for a particular sample depends upon the sample concentration. It varies from about 20 seconds for high concentrations to about 200 seconds for low concentrations.
T
A x o c m s of bii-alent sulfur compounds catalyze the reduction of iodine by sodium azide. The rate of the reaction depends upon catalyst concentration and in many cases can be used for its quantitative determination. The importance of cystine in food and dairy products and in protein research has resulted in several applications of this reaction t o micro methods for cystine. Holter and Ldvtrup (1) based a method on the manometric measurement of the volume of nitrogen produced after a 4-hour reaction time. Recently Strickland, Mack, and Childs RACE
( 5 ) described a method for cystine in protein hydrolyzates based on the cxtraction and qpectrophotometric nieasurem m t of the iodine remaining unreacted after a 1-hour reaction time. These and other similar methods provide good sensitivity for cystine but the procedures are laborious and time consuming. I n the present work it has been observed that during the early part of the reaction the rate of reduction of iodine is a simple and reproducible function of cystine concentration. This property has been used to develop a rapid method for the quantitative determination of trace amounts of cystine. The method is based on the continuous potentiometric detection of the decrease in iodine concentration during the early seconds of reaction time. Automatic measurement equipment pre7 iously described for the enzymatic determination of glucose ( 3 ) is used t o provide direct readout of rate data. The reaction takes place in a small test tube which is immersed in the electrolyte of a stable reference electrode. An asbestos fiber sealed in the bottom of the test tube provides electrical contact between the reaction mixture and reference electrode without significant mixing. The potential of a bright platinum electrode immersed in the reaction mixture is compared with that of the reference electrode. Within seconds after the addition of the sample containing cystine to the rapidly stirred reagents in the sample compartment a uniform decrease in the potential of the sample electrode is observed. The time required for the cell voltage to change
over a small predetermined interval is
measured and related t o cystine concentration. PRINCIPLES OF THE METHOD
The net reaction on which the method is based is
The concentrations of azide and iodide are large compared to that of iodine and remain essentially constant during the measurement interval. Under these conditions and at low cystine concentrations the rate of the reaction is proportional to the instantaneous concentrations of cystine and iodine as shown in Equation 2 .
where k l is a rate constant depending upon solution conditions, C is the cystine concentration, and [ 1 2 I t is the iodine concentration a t time t. If cystine undergoes no permanent alteration during the measurement interval then for a given value of C the reaction follows pseudo first-order kinetics. -4ny time interval, A t = t z tl, and the iodine concentrations a t the beginning, [I2I1, and end, [IZI2,of the interval are related by Equation 3. (3)
The only variable in the concentration cell is the iodine concentration in the sample solution. The time dependent VOL. 35, NO. 1, JANUARY 1963
21
