Talonm, Vol. 32, No. 7, Pp. Q-530, 1985 0039-9140/85 $3.00 + 0.00 Pnnted in Great Britain Pergamon Press Ltd

FURTHER DEVELOPMENTS IN THE HIGH-PRECISION COULOMETRIC TITRATION OF URANIUM

TATSUHIKO TANAKA,* GEORGE MARINENKO~ and WILLIAM F. KOCH Center for Analytical Chemistry, National Bureau of Standards, Washington, D.C. 20234, U.S.A.

(Received 2 August 1984. Revised 16 January 1985. Accepted 8 February 1985)

Summary-An experimental study of the current efficiency in the coulometric generation of Ti(III), as a function of electrolyte composition, current density and electrode material, has been performed. The cathodes investigated include platinum, mercury and graphite. The first two are suitable for high-precision determination of uranium. The graphite surface is readily poisoned, rendering it useless for high-accuracy work. The use of mercury requires thorough removal of chloride from the system. The precision and error obtained are comparable for both the mercury and platinum cathodes, and are of the order of 50 ppm.

In aqueous solutions titanium(II1) is a powerful reductant, and was first introduced as a coulometric titrant by Arthur and Donahue.’ E” for the Ti(IV)-Ti(II1) couple, + 0.1 V, is very nearly equal to that of hydrogen. Hence, in any coulometric experi- ment there is a possibility of co-reduction of hydro- gen ion along with Ti(IV), resulting in a significant loss of current efficiency. A high current efficiency for the electrogeneration of titanium(II1) can easily be obtained if mercury, which has a high hydrogen overpotential, is used as the cathode, and the hydrogen-ion concentration in the supporting elec- trolyte is low.

For these reasons, Lingane and Iwamoto2 cou- lometrically titrated uranium(V1) with titanium(II1) electro-generated at a mercury-pool cathode in 0.2-1M citrate medium, containing 0.08M titanium tetrachloride, at pH 0.5-1.5 and 85”. The elevated temperature was required to increase the rate of reduction of U(V) with Ti(III), because at room temperature (25”) the rate is impracticably low. Later, Kennedy and Lingane successfully determined ura- nium(V1) by using a platinum cathode, which is more convenient to use than a mercury cathode. In their work, a medium of 6-9M sulphuric acid and 0.6M titanium(IV) was used as the supporting electrolyte. Under these conditions, titanium(II1) can be gener- ated with 100% efficiency for current densities as high as 3 mA/cm2, but at elevated temperatures the efficiency decreases.’ Kennedy and Lingane also found that iron(H) catalyses the reduction very effectively, eliminating the need for elevated tem- peratures.’

Takeuchi et ~1.~ determined uranium(V1) in ura- nium(IV) oxide by use of Kennedy and Lingane’s method. Marinenko et aLs improved the method and

*Guest worker at the National Bureau of Standards from Science University of Tokyo, Department of Industrial Chemistry, Kagurazaka, Shinjuka-Ku, Tokyo, 162 Ja- pan.

tTo whom correspondence should be addressed.

applied it to the analysis of Standard Reference Material (SRM) 960 Uranium Metal, issued by the National Bureau of Standards (NBS).

The generation of titanium(II1) in a sulphuric acid medium, at a platinum cathode, is known to be less than 100% efficient. s*6 Bishop and Hitchcock’ in- vestigated the electrode processes occurring at the platinum cathode during the reduction of ti- tanium(IV) and found that trace impurities in the sulphuric acid can cause electrode poisoning, and emphasized the need not only to activate or condition the electrode, but also to purify the sulphuric acid. With 0.6M titanium(IV) sulphate/7M sulphuric acid purified by electrosorption, the loss of current efficiency is only 1 ppm at a current density of 59.2 mA/cm2. However, according to Bishop and Hitchcock’ it is difficult to purify sulphuric acid, and furthermore, after purification, the acid is soon recontaminated even when it is kept in an all-glass apparatus.

As mentioned above, the current efficiency for generation of titanium(II1) has been extensively stud- ied for the platinum cathode but not for other cathodes. Hence, systematic study of the efficiency of coulometric generation of titanium(II1) at platinum, mercury and graphite electrodes was undertaken.

Coulometric generation eficiency for titanium(ZZZ)

The Ti(IV)-Ti(II1) couple is reversible in 1-15M phosphoric acid,* but the Ti(IV) concentration can- not exceed O.OlM because of solution instability. Such a low concentration of Ti(IV) makes the system unusable for the coulometric generation of Ti(II1) titrant. The titanium couple is also reversible in L 4M sulphuric acid,* which is a preferred support- ing electrolyte in cathodic titrations. Hydrochloric acid has also been employed as the medium for the generation of titanium(III), but the titanium couple is less reversible than in the same concentration of sulphuric acid.8 Furthermore, both uranium(V1) and titanium(IV) are reduced by mercury in hydrochloric

525

526 TATSUHIKO TANAKA et al.

Table 1. Electrode potentials (V vs. SCE) on different cathodes, as a function of current density*

Current Hg Graphite Pt density

mA/cm2 E”, V E’, V AE, V , E” V E’, V AE, V E”, V E’, V

1O-4 +0.25 -0.35 0.60 WO.25 +0.05 0.20 +0.35 -0.10 10-j i-O.23 -0.13 0.96 +0.20 -0.40 0.60 f0.25 -0.15 10 +0.15 -0.90 1.05 0.00 -0.65 0.65 0.00 -0.18

*E’ determined in 9M H,SO,. E” determined in 9M H,SO,/lM Ti(IV).

BE, V

0.45 0.40 0.18

acid.’ In perchloric acid the titanium couple is irreversible’ because titanium(II1) immediately re- duces perchlorate to chloride. Consequently, of all the common acids considered, only sulphuric acid is suitable for the investigation.

The coulometric titration cell used throughout this work has already been described in detail.5*‘0v” Plat- inum gauze, high-purity mercury and spectrographic grade graphite rod were employed as the cathode materials. The areas of these electrodes were 80, 12 and 1.5 cm2, respectively. The graphite rod was pol- ished with fine Carborundum paper and then rubbed with filter paper before each use. The anode was a piece of platinum gauze. The potential of the cathode was measured with respect to a saturated calomel electrode (SCE). All the work was done with 100 ml of electrolyte solution, containing 100 mg of ferrous ammonium sulphate hexahydrate as catalyst for the uranium(V1) titration, and stirred at constant rate with a Teflon-coated magnetic bar. The electrode potentials for the three types of cathode in different electrolytes, at three levels of current density (10e4, 10-i and 10 mA/cm2), are listed in Table 1.

10' 1

I I 1 I I I ,

0.4 0.2 0.0 -0.4 -0.6 -0.6 -1.0

E ca,hod. vs. SCE (V)

Fig. 1. Potential of the platinum cathode as a function of

I

The current density us. working electrode potential diagrams obtained are shown in Figs. l-3, and the current efficiency for the generation of Ti(III) from Ti(IV) can be readily estimated from them.

The current due to the reduction of other species in the supporting electrolyte (e.g., of protons) is subtracted from the total current to compute the current efficiency. For example, consider the curves a and b in Fig. 2. At a current density of 20 mA/cm2 the potential of the graphite cathode in 9M sulphuric acid/l M titanium,(IV) is - 0.05 V vs. SCE (point x on curve b). At the same potential the corresponding current density in the absence of titanium(N) is 3 x 10e3 mA/cm* (point x’ on curve a). Then

current efficiency =

(20 mA/cm*) - (3.0 x 10m3 mA/cm2) x 1oo

20 mA/cm2

= 99.985%

Current efficiencies calculated for different elec- trodes, different current densities and various electro- lyte compositions are summarized in Table 2.

IO2 k

IO’ -

- 100 q “E < a !t 16 I

a .c- r d

-2 - E 10 - ?! 5 ”

1oe q

I I I I I I I I I 0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.6 -1.0 -1.2

E c,,,hod, vs. SCE (VI current density: (a) 2M H,SO,; (b) 2M H,S04/0.2M Ti(IV); (c) 9M H,SO,; (d) 9M H,SO,/O.SM Ti(IV); (e) 9M Fig. 2. Potential of the graphite cathode as a function of

H,SO,/l .OM Ti(IV). current density: (a) 9M H,SO,; (b) 9M H,SO,/l .OM Ti(IV).

High-pr~ision coulometric titration of ~an~urn 527

E ca,“*Ys. SCE (VI

Fig. 3. Potential of the mercury cathode as a function of current density: (a) 2M or 9M H,SO,; (b) 2M H,SO,/O.ZM

Ti(IV); (c) 9M H,SOJO.SM Ti(IV).

It is of interest to consider the potential of each of the cathodes investigated, at various current densi- ties, for reactions in 9M sulphuric acid in the presence and absence of titanium(IV) (1M). In the latter case the primary potential-determining reaction is the reduction of protons to hydrogen gas (potential E’), and in the former it is the reduction of titanium(W) (potential E”). A summary of the current- density/potential data is given in Table 1, and indi- cates that, between current densities of 10e4 and 10 mA/cm2, AE = (E” - E’) increases dramatically for Hg and graphite cathodes (by 0.45 V). For platinum, under the same conditions, AC\E decreases by 0.27 V. The reason is evident from the values of E’. On both the mercury and graphite cathodes, the hydrogen overvoltage increases very markedly as a

function of current density (0.55 V on mercury and 0.70 V on graphite), whereas on the platinum cath- ode the overvoltage increases by only 0.08 V over the same current-density range. The increase in over- voltage for the reduction of titani~(I~ on all three cathodes is comparable: 0.10 V for mercury, 0.25 V for graphite and 0.35 V for platinum over the 10-4-10 mA/cm* current-density range. On the basis of only these data, mercury is the preferred cathode for 100% efficient generation of Ti(II1) from ti- tanium(IV) sulphate electroiyte, followed by graphite and platinum in order of decreasing efficiency. How- ever, other considerations make platinum at least as attractive as the other two materials. As noted earlier, use of the mercury cathode results in significant titration errors when halides are present in the elec- trolyte. The addition of uranium(V1) chloride causes spontaneous oxidation of mercury to form merc~~us chloride, in a reaction which is not reversible under these experimental conditions, so results for U(V1) are low. The graphite electrode used in our experi- ments became “poisoned” after a 30-min electrolysis and gave copious liberation of hydrogen with a concurrent loss of current efhciency. Thus, graphite was abandoned as cathode material for generation of Ti(II1). For 1M titanium(IV) sulphate in 9M sul- phuric acid the results obtained with the platinum cathode agree with those of Kennedy and Lingane.3

It was found earlier by Kennedy and Lingane3 that the slow reaction between U(V1) and Ti(III) at room temperature can be accelerated by the addition of small amounts of iron(H), but even then the reaction is quite slow as long as U(W) is in excess. Only when excess of Ti(II1) is present does the reaction proceed rapidly to completion.

This point is illustrated in Fig. 4 by three indicator- current curves measured in the equivalence-point region of the uranium titration. For each curve, 10~~eq of Ti(II1) were coulometrically generated in the electrolyte [9M sulphuric acidjIM titanium(IV) sulphate containing 1 mg of ferrous ammonium sulphate hexahydrate per ml]. Curve 1 corresponds to

Table 2. Current efficiency of generation of Ti(II1) on different electrode materials

Platinum Graphite Current

Mercury ___

density, [H,SOJ 2M 9M 9M 9M 9M 2M 2M 9M PM 9M mA/cmZ II?(W)] 0.2M 0.5M 1M 0.1M iit4 O.iM 0.2M O.lM 0.5M 1M

0.01 100 100 100 100 100 100 0.05 100 100 0.1 0.5 100 100 100

1 100 99.98 100 100 100 3 99.33 99.98 99.91 99.999 5 95 100 100 99.84 99.994 8 36.25 99.77 99.96

10 15 99.13 99.94 20 99.997 99.49 99.87 30 99.996 98.50 99.72 99.72 100 100

528 TATSUHIKO TANAKA et al.

ar

=& -2 1 I , I 0 2 4 6 8

Time (min )

Fig. 4. Amperometric indicator current due to Ti(III), as a function of time. Electrolyte composition: PM H,SO.,/lM Ti(IV)/l-m&ml Fe(NHJ,(S0&.6H,O, (1) 10 peq Ti(III) + 5 peq U(V1); (2) 10 ,ueq

Ti(II1) + 10 peq U(X); (3) 10 yeq Ti(II1) + 15 peq U(V1).

the region after the equivalence point. This curve was obtained by adding 5peq of U(VI) to the 10peq of Ti(III) generated. It can be seen that equilibrium is reached quite rapidly. The initial exponential decay of the indicator current reaches a constant value after 24 min. At the equivalence point (curve 2), the rate of reaction is significantly lower when 10 peq of U(V1) are added to the generated 10 peq of Ti(III), equi- librium being reached 7 min after addition of the U(V1) aliquot. Before the equivalence point (curve 3), the reaction rate when 15 peq of U(W) are added to the generated 10 peq of Ti(III) is impracticably slow.

It is clear that, for stoichiometric reaction mea- surements, the system must be overtitrated to allow equilibration to take place at a reasonable rate and then the excess of titrant must be determined. On the basis of these studies the following experiments on the coulometric assay of uranium were performed.

EXPERIMENTAL

Coulometric assay qf uranium metal

The instrumentation for coulometric generation of the t&rant was similar to that described meviou~l~.~~‘~~” The electrolysis cell was also of the type described earlier,5*‘o*” with a few m~ifications. A silicic acid plug was cast in the bottom of the anode chamber to cover the glass frit. The remainder of the chamber was filled with 2M sulphuric acid and a glass tube with its bottom closed by a glass frit and a silicic acid alug. and filled with 2M sulphuric acid. The platinum anode and the SCE were immeised in the tube.

The end-point of the titration was determined am- perometrically with a platinum foil indicator electrode (area approximately 1.2 cm? immersed in the cathode chamber. A polarograph was used to apply a constant potential between the&indicator electrode and the SCE, and to record the indicator current. Argon, purified by passage through chro- mium(I1) chloride and then PM sulphuric acid, was used to purge oxygen from the cell and the electrolyte. The Faraday constant was taken as 96486.5 C/~uivalent and the atomic weight of uranium as 238.0289.

Analytical-grade chemicals were used without further purification. Titanium(IV) sulphate solution was prepared by slowly adding 50% titanium tetrachloride solution to concentrated sulphuric acid, and removing all the hydro- chloric acid by purging the solution overnight with argon.

The final concentrations in the stock solution were 1M titanium(IV) and 9N sulphuric acid.

Uranium sample solution was prepared from uranium metal (NBS SRM 960) cu into approximately l-g pieces.

L Immediately before the sa ple was weighed, the surface oxide was removedt2 by dipping the pieces in warm nitric acid (1 + 1) for 10 min, rinsing with distilled water, etching in hydrochloric acid (1 + 3) for 5 min, rinsing with distilled water again, immersing in acetone, air-drying for a few minutes, then placing them in a vacuum desiccator. The sample was weighed to the nearest microgram before any significant amount of surface oxide could form (i.e., within 30 min), and the weight was corrected for air buoyancy. The sample was then dissolved in hydr~hlo~c acid (1 + 1) in an inclined 125-ml Erlenmeyer flask (10 ml of acid per g of sample). After dissolution was complete, 0.5-l ml of 30% hydrogen peroxide was added slowly to ensure that all uranium was in the hexavalent state and to dissolve any traces of black residue (ostensibly I&O&. The solution was then evaporated nearly to dryness under infrared lamps, the residue was taken up with water and the evaporation was repeated, to destroy the excess of hydrogen peroxide. Five ml of concentrated sulphuric acid were then pipetted slowly down the wall of the flask and the solution was evaporated till fuming. Finally the solution was diluted to about 15 ml with distilled water and then titrated. Reagent blank sam- ples were prepared by the same procedure.

Titration procedure

The procedure was similar to that developed for the assay of NBS SRM 960 Uranium Metal by Marinenko et al.*

About 75 ml of titanium(IV) sulphate catholyte and 100 mg of ferrous ammonium sulphate hexahydrate were placed in the cathode chamber of the cell. Argon, purified from traces of oxygen by passage through chromous chloride solution, was passed through the contents of the cell for about 30 min, the solution being vigorously stirred with the magnetic stirrer. After purging, the catholyte was permitted to flow into the intermediate compartments of the cou- lometric cell until it just covered the bottom of each compartment. A current-voltage curve was run before each titration to determine the optimum potential for the indi- cator system.

For pretitration, ea. 10 peq of Urania were added to the cathode chamber and titrated at 3.22 mA by passage of small increments of charge, each corresponding to 1 peq. After a slight excess of titanium(II1) had been generated, as evidenced by the indicator-current increase, the solution was allowed to equilibrate for 30min. Additional amounts of titanium(II1) were then generated and the indicator current

High-precision coulometric titration of uranium 529

was measured after each charge increment. The linear portion of the indicator-current curve was extrapolated graphically, and its intersection with the zero-current line taken as the end-point.

After the pretitration, the intermediate cell compartments were rinsed by repeated emptying and filling with catholyte, by suction or argon pressure. The final reading of the indicator current was used to establish the amount of overtitration.

Next the intermediate compartments were filled with catholyte and the sample was transferred from the Erlen- meyer flask to the cathode chamber. The major part of the titration was then done at a constant current of about 102 mA for a precalculated time corresponding to a few peq in excess of the theoretical amount. The intermediate com- partment and the sample flask were then rinsed three times with the titrated solution, by suction or argon pressure. After equilibration of the solution for 30 min. the end-point was located by the procedure used for the pretitration.

ANALYTICAL RESULTS AND DISCUSSION

In view of the studies of the efficiency of generation of Ti(II1) at different types of cathode it was of interest to verify independently the performance of the various cathodes in coulometric titrations. Hence, NBS SRM 960 Uranium Metal was analysed by the procedure of Marinenko et aL5

As mentioned earlier, work with the graphite cath- ode was abandoned because hydrogen was liberated and the current efficiency decreased. The results of analyses with the platinum cathode are shown in Table 3. The oxidimetric assay value is 99.981% (s = 0.0048). When this value is corrected for the iron and vanadium present, which are titrated along with uranium, the assay becomes 99.970% U, which is in excellent agreement with the certified value of 99.975%. It should be noted here that though the titration procedure was the same in this work as for the certification, the current densities were different (2.5 mA/cm’ for certification, 1.27 mA/cm* in this work). The agreement of the results obtained at current densities differing by a factor of 2 is further evidence of the consistency of the titration process under selected conditions. Both current densities should provide 100% efficient generation of Ti(II1) according to the data in Table 1.

Table 3. Coulometric assay of NBS SRM 960 Uranium Metal at a platinum cathode (supporting electrolyte IM titanium(IV) sulphate/9M sulphuric acid; current density

I .27 mA/cm’)

U metal, mg

Taken

865.384 1033.580 1028.656 1075.75, 990.340

1033.714 1287.81,

Found Assay, % w/w

865.22, 99.98 1 1033.42, 99.984 1028.49, 99.984 1075.60, 99.986 990.10, 99.977

1033.43, 99.973 1287.60, 99.984

Average 99.981 Std. dev. 0.004,

As a further test of the accuracy of the current efficiencies predicted from the current density/potential diagrams, uranium metal SRM 960 was coulometrically titrated with Ti(II1) electro- generated at the mercury cathode at a current density of 8.3 mA/cm2. The electrolyte was O.lM ti- tanium(IV) sulphate/9M sulphuric acid. At a current density of 8.3 mA/cm*, the predicted current efficiency is 99.96%. The assay of the uranium metal was 99.999%, which is O.Ol’% higher than the value obtained under 100% efficient generation conditions with a platinum cathode. The uranium assay ex- pected from the current density data alone should be 100.029%. However, the titration “efficiency” should be higher than the current efficiency because uranium is directly reduced in the initial stages of the titration.

When the Ti(IV) concentration in the electrolyte was increased to 0.5h4, the uranium assay was 99.987% (n = 3, s = 0.0087%). The current-density studies indicate that this electrolyte should provide 100% efficient generation of Ti(III), which is borne out by the uranium assay.

A set of uranium metal samples, ranging in weight from 30 mg to 1.2 g, was analysed with the mercury cathode system with 0.2M titanium(IV) sulphate/9M sulphuric acid electrolyte. The results are presented in Table 4. The assay value for Uranium SRM 960 obtained with the platinum cathode system (99.981%) is in excellent agreement with the value obtained with the mercury cathode system (99.982x), under experi- mental conditions which are significantly different with respect to not only the nature of the cathode material but also the electrolyte composition and current density.

A word of caution is in order regarding the use of the mercury cathode for uranium determination. The use of the mercury cathode requires that great care be taken in preparing the supporting electrolyte. The absence of chloride is essential. When the 9M sul- phuric acid supporting electrolyte contains chloride,

Table 4. Coulometric assay of NBS SRM 960 Uranium metal at a mercury cathode, (0.2M titanium(W) sulphate/9M sulphuric acid; current density 8.3 mA/cn?)

U metal, mg

Taken Found Assay, % w/w

1078.13, 1077.91, 99.980 I 176.48, 1176.30, 99.985 1169.49, 1169.30, 99.984 1180.50, 1180.25, 99.979

Average 99.982 Std. dev. 0.003,

504.45, 504.36, 99.984 421.31, 421.27, 99.991 235.43, 235.40, 99.989 136.66, 136.64, 99.983 70.23, 70.21, 99.977 33.00, 32.99, 99.966

Average 99.982 Std. dev. 0.009,

530 TATSUH~KO TANAKA et al.

the introduction of uranium(V1) causes the spontane- excellent agreement with the certified value of ous oxidation of mercury to form mercurous chloride 99.975%. on the surface of the cathode. In principle this is reversible, i.e., Hg(1) is reduced back to metallic mercury, and should not cause any loss of generation efficiency as far as charge balance is concerned, but in practice complete electrochemical reduction of the calomel is not feasible. Thus, in the presence of chloride, the uranium assay is low.

REFERENCES

1. P. Arthur and J. F. Donahue, Anal. Chem., 1952, 24, 1612.

2.

3. 4.

The values of the reductometric assay of uranium metal reported in Tables 3 and 4 include all impurities which would oxidize Ti(II1). In SRM 960 Uranium Metal two principal electroactive impurities are present: 42.1 ppm of iron and 4 ppm of vanadium, which are equivalent to 109 ppm of uranium. There- fore, to establish the uranium assay of SRM 960, a correction of 109 ppm must be applied to the reduc- tometric assay. Thus, the corrected coulometric assay of SRM 960 is 99.970% (s = 0.0048) with the plat- inum cathode and 99.971% (s = 0.0030) with the mercury cathode. The two values, for all practical

J. J. Lingane and R. T. Iwamoto, Anal. Chim. Acta, 1955, 13, 465. J. H. Kennedy and J. J. Lingane ibid., 1958, 18, 240. T. Tekeuchi, T. Yoshimori and T. Kato, Bunseki Ka- gaku, 1963,12, 840.

5. G. Marinenko. W. F. Koch and E. S. Etz. J. Res. Natl.

6. Bur. Stds., 1983, 88, 117. T. Takahashi and H. Sakurai, 10th Annual Meeting Japan Society for Analytical Chemistry, B107, Tokyo, 1961.

7. 8.

9.

10.

E. Bishop and P. H. Hitchcock, Analyst, 1973,98, 625. J. J. Lingane and J. H. Kennedy, Anal. Chim. Acta, 1956, 15, 294. E. R. Caley and L. B. Rogers, J. Am. Chem. SOL, 1946, 68, 2202. G. Marinenko and J. K. Taylor, J. Res. Natl. Bur. St& 1963, 67A, 31.

11 . Idem, ibid., 1963, 67A 453. 12. H. Hashitani, A. Hoshino and T. Adachi, JAERI-M

purposes, are identical. This uranium assay is in 5343, 1973.