Las etapas de un proceso de autoevaluación de la gestión escolar.

-G

(Uranyl Uitrate/Acetic acid)

QAlizarin

Oxine

P.A.R.

0.1

0.5

eSalicylic acid

Concentration of added acetic acid ( moles )

Decrease in

6 +

U capacity

m.moles/g

dissociated system. This would possibly explain the relatively high capacities for 0.1 and 0.05muranyl chloride solutions. At lower concentrations however the dissociated molecules for both chloride and nitrate systems will tend to be equal, and the capacities as such will be equal. Uranyl acetate has the lowest dissociation value of all three uranyl systems studied, but the concentration effect will be the governing factor.

The uses of chelate ion exchangers as viable separation techniques have been limited to commercial rather than technical difficulties. The interest lies in chelate ion exchangers of the Dowex A-l type. Dowex A-l in particular has been extensively

investigated but its applications restricted to analytical 118

procedures. Riley and co-workers used this resin for

extraction of metal ions from sea water as a preliminary concentration procedure. Both inorganic absorbents and chelate ion exchangers

have been reviewed as a means of uranium extraction from sea water. The difficulty appears to be at least three fold

f , '*! 4— (1) Uranium exists in sea water as a complexed entity jUC^CCO^)^) »

23

which has a stability constant of 1.7 + 0.6 x 10 , and the uranium is present in very low concentrations (3yg/£).

(2) The chosen extractant must be capable of working within the pH limits of sea water i.e. 7.8 to 8.2.

(3) Possibly the most important is the selectivity of the resin or absorbent. Conventional ion-exchangers are of little value in this category, and at present few very selective chelate exchangers are known.

-93-

Using synthetic solutions of known uranyl concentration, and the four XAD2 resins, the first two stipulations were investigated. The uranyl tricarbonato complexes were prepared from uranyl nitrate-sodium carbonate, uranyl chloride-sodium carbonate, and uranyl acetate-sodium carbonate. The uranium

contents of these saturated solutions were 5.89, 7.12 and 8.53 mg/ml respectively. From Table 34 the highest capacities of these resins were from the solutions containing sodium acetate. This was to be expected however as these solutions had the highest uranyl contents. The next highest should have been the solution containing sodium chloride, in fact this had the lowest capacity. This may be due to the difference in pH, or alternatively sodium chloride having an effect on the resin or uranyl complex.

Table 34 Resin U02(CO system u> u> 4> - 1 Capacity n. cone - 8 U 6 + medium pH ra.moles Oxine 295 NaN03 7.6 0.085 P »A»R. ii n 7.6 0.095 Salicylic Acid it it 7.5 0.084 Alizarin ii ii 7.6 0.100 Oxine 356 NaCl 8.5 0.037 P.A.R. it ii 8.3 0.042 Salicylic Acid ii it 8.1 0.044 Alizarin ii ii 8.4 0.046 Oxine 427 Na 7.8 0.108 P.A.R. H acetate 7.6 _0.115 C n 1 n ±.\~J _{X X V r}1 •» Acid n ti 7.5 _0.095 Alizarin I •i | SI 7.6 0.122

From graphs 43 and 44, it appears that the optimum pH range is 7.2 to 7.6, this lies slightly lower than the pH of sea water. The capacities at pH 7.6 are still approximately half the sodium acetate and sodium nitrate systems, which is due to the solutions used in the latter case being 20 times more concentrated. The efficiency of both these resins at the optimum pH is better than 50%. This efficiency will decrease however on approaching the pH value of sea water.

XAL2 azo-P.A.R.

GRAPH 43

*

o

XAL2 azo-SALICYLIC ACID. GRAPH 44 08

06 -

o

DBTEMINATION OF STABILITY COHSTMTS OP METAlr-HESIN COMPLEXES The determination of resin—metal ion stability constants has been restricted in the past to a few systems^ 3 ^ 5 li9^ The majority of these systems studied have been with resins which were soluble in a suitable water misciblo solvent9 such as dioxan. The procedure adopted was the standard potentiometric titration9 which generally gave fairly satisfactory results. A limited

success has been achieved with the resin Dowex A-l? the

25 stabilities of nickel and copper complexes have been studied ^9

n

later extended by Eger et al for various other metal ions9 and a comparison of these values with an analogous monomeric

system3 N-benzyl iminodiacetic acid9 made.

The difficulty with XAD2 azo-resins is that no such monomeric systems exist9 and as such evaluation of these

stabilities has to bo attempted on the resin itself. Several procedures were attempted5 and will be dealt with individually. (i) Direct potentiometric titration

This method has been successfully used in the past9

120

particularly by Irving and Rossotti to determine the

stability constants of oxine and salicylic acid complexes. For the oxine system the metal ion concentration is limited to the solubility of the metal oxinate in 50/50 water-dioxan. The procedure adopted here was to titrate the following systems using molar sodium hydroxide from a nicrosyringe fitted with a vernier scale.

(a) equivalent volumes of dioxan and an aqueous solution of UaC10^/HC10^.

(b) equivalent volumes of oxine (O.OlWl) in dioxan and an aqueous solution of NaC10^/HC10^.

(c) equivalent volumes of oxine in dioxan and an aqueous solution of copper sulphate (.005trj> with ITaCIO,/hCIO. present.

4 4 The concentration of HaC10„ was 0.60Q*nand the HC10,_{4 4} 0.0385m.

From the three systems studied above it was possible to determine the stability of the copper oxinate. System (c) was repeated and 0.333g of XAD2 azo-oxine resin was added. This quantity of resin has an equivalent oxine content to that present in the aqueous phase. It should have therefore been possible to titrate this system, and hence deduce the effect the chelating ion exchanger is having on the system.

Unfortunately this system was of little value , the main disadvantages with it were

t

(1) low concentration of metal ions being used and hence a large proportion of the resin would have been not co-ordinated to copper (il) ions if this system had been successful.

(2) The rate of attainment of equilibrium of XAD2 azo-oxine is approximately 6 hours, therefore after each addition of alkali this system has to equilibrate. The total time required for such a titration would be of the order 3-4 days. It is possible, however, to overcome this problem by measuring the stabilities at higher temperatures, say 70-80°C, but the

apparatus would have to be sealed to prevent evaporation losses. (2) "Anti-Schubert's" method

121

Schubert's method has been used on previous occasions but using conventional ion exchangers. However, the method now

122

connected -with his name is the modified version which allows for stepwise formation of a series of complexes. An ionic

medium is used at constant calion concentration C and variations_m of the ligand concentration are assumed not to change

activity coefficients appreciably. Constant pH and tre.ee metal concentrations are used, so that the effective ligand

concentration is proportional to CT. The resins are completely_Jj loaded with M cations. Complex formation is assumed to produce only neutral or anionic species which a.re supposed not to he absorbed by the resin. Keeping in mind the above restrictions9 the following equation can be expressed

s

_ P l , * , < * ♦ ... I i

where D°M is the distribution coefficient when ligand is absent and DM when ligand is present.

The MAnti-Schubert1 s'1 method is derived similarly but the competing chelate concentration (which in this case is oxine) was constant, and vaiying quantities of the XAD2 azo-oxine resin were employed. The results given in Table 35 indicate that this method had its limitations. The concentration and composition

of the solutions were identical with those used in the potentiometric titration procedure. The initial concentration of copper ions

was 7.96mg. The solutions were placed in a thermostat bath set at 25°C +0.1 for 1 day, after which time the resin was

introduced, and equilibration allowed to proceed for 7 days with only occasional shaking at 25°C +0.1.

Table 33 j Quantity I of resin used mg Metal ion in solution mg I Metal | ion on 1 resin mg pH 1 I 147 j 7.78 0.16 15 2.3 j 183 7.65 0.31 2.3 | 292 _7.54 0.42 2.2 < 79 7.92 0.04 i_i2.2 280 I ; ! 6.84 1.12 2.2

The resin sample which absorbed 1.12mg of copper (il) ions, had no competing oxine molecules present in solution, only

dioxan and an aqueous solution of NaClO^/HClO^ i.e.D°M. The limited success of this procedure was entirely due to the final pH of the solution being of the order 2.0. As a result of this, the distribution coefficients were extremely low and possibly unreliable. This coefficient could have been increased,

simply by raising the pH of the system, say to pH 5*0. This would have presumably resulted in distributions of 40-50$, which

are of the desired order. However, increasing the pH of the system could have affected the solubility of the copper oxinate, similarly increasing the metal ion concentration to favour the resin capacity could have produced a precipitate or suspension of metal oxinate.

If a water soluble system was used, i.e. copper and E.D.T.A. and the solutions were sufficiently dilute so as not to

contradict any assumptions made in the Schubert method, then a reasonable pH could be attained. This type of procedure was

25