Control de legalidad de los actos que declaran la responsabilidad fiscal y otros actos administrativos (Artículos 237 y 238 constitucionales; Ley 610 de 2000 y Código

of thallium into MIBK from a solution of 0.2 g/mL

in

8 1

v . Extraction Shaking Time

Vari ous shaking t imes were tested by ana lys i ng the MIBK pha s e a fter extract ing from aqueous s o lut i on . The results are p l otted i n Fig . I I I - 2 3 and demonstrated that after one m inute shaki ng t ime about 9 5 % of the tha l l ium ( I ) was extracted a nd that a fter 1 . 5 - 3 . 0 minutes extract ion was a lmost 1 0 0 % . When the shaking t ime great ly exceeded 3 minutes , the percent extracti on started to reduce . In a l l future work a shaking t ime o f two minutes was adopted .

vi . stab i l ity of the I odo Tha l l ium ( ! ) Comp lex

The GFAAS absorbance of the MIBK pha s e separated from the aqueous so lution a fter extract ion was determined at d i ff erent t imes a fter phase separat ion in order to i nvestigate the

stab i l ity of the iodo tha l l ium ( I ) comp lex in the orga n i c phase . Experiments showed the iodo tha l l ium complex proved t o b e extreme ly stabl e and rema ined unchanged even s i x hours after phase separat ion ( see Fig . I I I - 2 4 ) . For rout ine ana lys i s i t was appropr iate t o complete the GFAAS measurements within two hours of phase separat ion .

vi i . The D i s tr ibuti on Rat io for Extract ion o f Tha l l ium

The d istr ibut ion ratio for the extract ion i nto MIBK o f the iodo-tha l l ium ( I ) comp lex i n 3 M sulphuri c a c i d s o luti on was i nvest igated by add ition of var ious amounts o f potas s ium

iodide to a so lut ion of tha l l ium ( I ) i n 3 M sulphur i c acid . The orga n i c j aqueous pha s e ratio was taken at 1 : 1 0 . The va lue of D was 6 4 5 0 { 9 9 . 9 8 % extraction) .

v i i i Effect o f Other E l ements

I t i s we l l known that one of the maj or probl ems i n GFAAS is the presence of coextracted elements that s erve to depress the s ignal o f the ana lyte . However , for pract i c a l purposes the maj or i nterferent present in s i l icate rocks and meteorites i s iron . Potass ium iod ide was a ls o used for reducing iron to

,... � ...- z 0 - E-< u --< 0::: E-< >< � 75 50 25 1 .0 2;0 3.0

Shaking time I min

4.0

0.6 u.:l u z --< CO 0.4 � 0 U) CO --< 0.2 0 _{1 .0 2.0 3.0} Time / hours

Fig.III-24 Stability of thallium iodo complex in the organic phase after extraction from the aqueous phase

8 2

the ferrous state , but even then about 2 % ( 1 0 mgj mL iron i n origina l aqueous s olution) o f the iron s ti l l extracted into the MIBK phas e that gave a pos itive absorbance s igna l to confuse with the tha l l ium respons e . For overcoming the problem , a backextraction step was empl oyed by use of 3 M sulphuric acid i n the presence o f 0 . 5 g j mL o f potass ium

iodide . After backextraction , the iron content of the organ i c phas e w a s l owered to < 2 0 �gjmL at which l evel there was no interference w ith the GFAAS tha l l ium determination .

I I I - 5 -2 L im i t o f Detect ion

The l imit of detect ion of the method d epended on the

aqueous-organ i c phas e ratios as we l l as the number of mul t i p l e loadings empl oyed during the GFAAS procedure . As with ga l l ium and indium , f our MIBK phases ( one extracted reagent b l a nk and extracts of three known concentrations of s tandard tha l l ium

( I ) ) were ana lysed by GFAAS after shaking and phas e

separations . The l imit of detect i on ( C1 ) represented about 4 . 5 ng j g and 1 . 5 ngj g in the origina l s o l id s amp l e using 6 and 1 0 mul t i p l e loadihgs respective ly , accord ing to the calculat ions by the IUPAC method 1931 .

The cal ibrat ion graph i s shown in F ig . I I I - 2 5 and it i s l inear ( r=0 . 9 9 9 , n=7 ) between 0 . 5 and 1 0 n g i n the MIBK pha s e , the h ighest concentrat ion of 1 0 ng jmL g iving a peak he ight absorbance of 0 . 1 0 2 . Th is a ssumes a s ingle 4 �L loading and an aqueous-organ i c phase rat io of 1 : 1 .

Assuming a l O O - fo l d d i luti on o f the s o l id s amp l e and 1 0 mul t i p l e loadings , the tha l l ium content o f the s o l id s amp l e i s

1 0 t imes that o f the aqueous s olut ion f o r a 1 : 1 phas e rat i o . However if a phas e rat io o f 4 : 1 i s used , the tha l l ium

concentrat ion i n the origina l s amp l e i s only 2 . 5 t imes greater than the va lues used in the above c a l ibrati on graph s o that 2 ngj g_ tha l l ium in the rock corresponds to an absorbance o f abo.ut 0 . 0 0 8 .

The l imit o f detect ion could i n theory be decreased sti l l further by increas ing the aqueou s- organ i c phas e rat i o and j or increas ing the number of mul t i p l e l oadings . However , h igher phas e ratios wi l l result in a decrease of the over a l l

� u z <t: � � 0 c:() � <t: 0.08 0.06 0.04 0.02 0 2.0 4.0 6.0 8.0 10.0

Tl concentration I [ng/mL]

Fig.III-25 Calibration curve for GFAAS absorbance as a