R. M. Sawant

Stabilisation of tetravalent cerium in perchloric acid medium and measurement of the stability constants of its fluoride complexes using ion selective potentiometry

The stability constants of the fluoride complexes of cerium(IV) in 1 M (HClO(4), NaClO(4)) medium have been measured potentiometrically using a fluoride ion-selective electrode. Quantitative oxidation of cerium to its tetravalent state and its stabilisation in the perchlorate medium were accomplished by oxidation with AgO followed by quick addition of a known amount of fluoride ion. This procedure ensures stability of the oxidation state and prevents hydrolysis and polymerisation of Ce(IV). Logarithms of the average values of beta(1), beta(2), beta(3) and beta(4) were estimated to be 7.57+/-0.04, 14.50+/-0.03, 20.13+/-0.37 and 24.14+/-0.10 respectively.

Modified pyrohydrolysis apparatus for the separation of fluorine and chlorine trace impurities from nuclear fuel samples for quality control analysis

A quartz pyrohydrolysis apparatus designed for the safe handling of pyrophoric carbide nuclear fuel samples while separating C1 and F trace impurities for quality control analysis is described. It has several advantages over the commonly used pyrohydrolysis apparatus. Performance of the apparatus during routine analysis of a large number of nuclear fuel samples has been found to be quite satisfactory.

Studies on fluoride complexing of hexavalent actinides using a fluoride ion selective electrode

Complex formation between actinide(VI) and fluoride ions in aqueous solutions has been investigated using a fluoride ion selective electrode (F-ISE). As fairly high acidity was used to suppress hydrolysis of the actinide(VI) ions, significant liquid junction potentials (Ej) existed in the systems. An iterative procedure was developed for computing free hydrogen ion concentration [H+], as it could not be measured directly, using data obtained with F-ISE. Ej values were estimated from known [H+] and the stability constants of fluoride complexes of actinide(VI) ions were calculated following KING and GALLAGHERs method using a computer program. The stability constants were found to follow the order U(VI)>Np(VI)>Pu(VI).

A critical review on the stability constants of the fluoride complexes of actinides in aqueous solution and their correlation with fundamental properties of the ions

Literature data on the stability constants of the fluoride complexes of the actinides in different oxidation states have been compiled. In order to have a reasonable inter-comparison, the stability constant (β1) values obtained in diverse ionic strength media are converted to so called thermodynamic stability constants, β10 using the DAVIES equation (a modification of Debye-Huckel equation). A correlation of the β10 values with the fundamental properties of the actinide ions using various models available in the literature has been attempted. Using the values of inonic radii and best available values of the stability constants of a large number of metal ions from recent compilations a comparative study of the various models or relations available in the literature has been tried. For metal ions in general, the semi-empirical relation recently developed by BROWN, SYLVA and ELLIS (BSE equation)_gives the best correlation. In an attempt to accommodate the unusual trend in the stability constants of the tetravalent actinides a modification in a parameter of the BSE equation has been proposed. Good agreement between the theoretically calculated and experimentally determined values for actinides in different oxidation states is obtained in most of the cases. Further improvements in theoretical relation as well as experimental data are required for better correlation.

POTENTIOMETRIC STUDIES ON AQUEOUS FLUORIDE COMPLEXES OF ACTINIDES: STABILITY CONSTANTS OF Th(IV)-, U(IV)-, Np(IV)- AND Pu(IV)-FLUORIDES

Complex formation of Th(IV), U(IV), Np(IV), and Pu(IV) with fluoride ions in aqueous perchloric acid solution of 1M ionic strength has been studied potentiometrically, using a fluoride ion selective electrode. A highly acidic medium was essential to prevent hydrolysis and disproportionation of the tetravalent actinide ions. A procedure developed earlier in this laboratory for studying the fluoride complexes of hexavalent actinides was successfully extended to this work. The β1 values follow the trend Th(IV)<U(IV)>Np(IV)>Pu(IV), and β2 values the trend Th(IV)<U(IV)>Np(IV)<Pu(IV), while β3 and β4 values followed an increasing order.

Study on the extraction of U(IV) relevant to PUREX process

Extraction of U(IV) by an organic phase containing 30% tri-n-butyl phosphate (TBP) in dodecane from aqueous nitric acid solution containing large amount of U(VI), relevant to nuclear fuel reprocessing by PUREX process, was studied. While acquiring distribution data, an argon atmosphere was maintained. A potentiometric method was used for the determination of U(IV) in the two phases after equilibration. Hydrazine used as stabiliser of U(IV) in the stock solution was destroyed and sulphamic acid was added just before equilibration to avoid the interference due to hydrazine in the subsequent determination of U(IV). Uranium(IV) distribution data between 30% TBP in dodecane and aqueous solution of uranium(VI) and nitric acid of varying concentrations have been presented and compared with available literature data.

A study on the fluoride complexes of plutonium(III), samarium(III) and bismuth(III) using fluoride ion-selective potentiometry

Stability constants of the fluoride complexes of Pu(III), Sm(III) and Bi(III) in 1.0M NaClO4/HClO4 medium at 23±1°C have been determined by potentiometry using a fluoride ion-selective electrode. Plutonium was reduced to the trivalent state using quinhydrone with an excess to serve as holding reductant. The log values of concentration stability constants log β1, log β2, and log β3 are 3.58, 6.40 and 12.61 for Pu(III), 3.23, 5.81 and 10.54 for Sm(III) and 3.69, 6.13 and 11.04 for Bi(III), respectively. The log β2 values in all these cases have very large deviation and may be taken only as rough estimates.

Determination of the stability constant of Np(V) fluoride complex using a fluoride ion selective electrode

Fluoride complexing of Np(V) has been studied using fluoride ion selective electrode (F-ISE). Free fluoride ion concentrations in the presence of Np(V) were measured at 0.1 and 1.0M ionic strength. The data were used to calculate the stability constant of the fluoride complex of Np(V) and the values obtained are reported here.

Sequential determination of urea and HMTA in the process solution of sol-gel route of advanced nuclear fuel fabrication

Determinations of hexamethylene tetramine (HMTA) and urea in the process solutions are required to optimize their concentrations for obtaining high quality ceramic oxide microspheres, for monitoring the washing procedure and for their subsequent recovery, recycling or waste disposal. Determination of urea in the feed solution by conventional procedures is difficult as it contains HMTA. It is more so in the effluent as it contains hydrolytic products like formaldehyde, methylol derivatives of urea, ammonium nitrate and ammonium hydroxide used for washing the gel microspheres. This work describes a derivative potentiometric method using a microprocessor-based autotitrator. Peaks on the first derivative of the titration plot corresponded to constituents of different basicities. Urea was selectively hydrolyzed at room temperature by the catalytic action of urease enzyme leaving HMTA unaffected. Ammonium hydroxide and ammonium bicarbonate produced from urea and HMTA were sequentially titrated for the analysis of the feed solution to obtain the three corresponding peaks respectively. Two separate titrations were required for the analysis of the effluent solution, which contained free ammonia also. One aliquot was first titrated directly without adding urease (for free ammonia and HMTA) and another aliquot was titrated after treatment with urease. The end points due to the ammonia used for washing and that from urea hydrolysis merged resulting in the appearance of three peaks again. Using this sequential method the relative standard deviations were found to be 0.81% and 1.38% for urea and HMTA, respectively, in eight determinations when the aliquots contained 50 to 75 mg of urea and 75 to 125 mg of HMTA. Feed and effluent solutions of the process stream were analyzed.

Ensuring complete absence of Ce(IV) and measurement of the stability constant of the fluoride complex of Ce(III)

Literature survey revealed a wide variation in the measured stability constant values of the aqueous fluoride complexes of trivalent cerium. This could be due to inadequate care for full conversion and maintenance of the oxidation state of cerium to trivalent state. In the present work quinhydrone has been used to ensure complete absence of Ce(IV) and the stability constant of CeF(2+) in 1 M NaClO(4) has been measured potentiometrically using a fluoride ion selective electrode. Log beta(1) obtained in this work was 2.936+/-0.024 and fitted well with the general trend of stability constants of the lanthanide (rare earths) fluorides in aqueous solution.