Indian Journal of ChemistryVol. 35A. January 1996, pp. 79-84

Comparative analytical determination ofthallium in different grade minerals and

ores using semimethylthymol blue

M A H Hafez", I M M Kenawy & M E M EmamChemistry Department, Faculty of Science,

Mansoura University, Mansoura, A R Egypt

Received 28 November 1994; revised and accepted 2 August1995

Semimethythymol blue (SMTB) has been used forthe spectrophotometric determination of micro-molaramounts of thallium(ill) at Amax 570 nm. The composi-tion, overall stability constant and molar absorptivityof the thallium(ill)-SMTB chelate have been deter-mined spectrophotometrically in perchloric acid-sodi-urn perchlorate solution (pH 2.5). In the thallium (Ill}SMTB system, a violet TI(SMTB) chelate is formedwith log ~ = 22.1 ± 0.1 and molar absorptivity of4.11 x 104 dm3mol-1cm-1 at 570 nm. Interferencesdue to different cations, anions and organic acids inthallium (III) determination have also been investigat-ed. Beer's law is obeyed in the range of 0.41-3.68ppm Tl(DI). The reagent has been used for the spec-trophotometric determination of thallium in .differentgrade minerals and ores.

Thallium and its compounds are extremely toxicfor plants, animals and human beings 1 but areused extensively in the form of its salts. TlBr andTlI are used in photosensitive diodes and infrareddetectors while salts of thallium are used for mak-ing optical glass of high refractive index and somephotoelectric cells. Thallium-antimony-indium al-loy is used for optical recording materials+'.

SMTB has been applied successfully as a newand promising metallochromic indicator for thecomplexometric determination of ,a number ofmetals ions in their minerals, ores and alloys suchas thorium + lanthanides and gallium + indium+ thalliurrr'". Several methods have been usedfor the spectrophotometric's" and complexometricdeterminations? 'of thallium (III) separately usingdifferent organic ligands, metallochromic indica-tors, at different conditions of pH's, temperatureand organic solvent. We report the stoichiometry,overall stability constaIit and the molar absorptiv-ity of the TP + -SMTB chelate. The optimum con-ditions for the maximum colour development of

the chelate TI(III)-SMTB have been investigatedand thallium (III) has been successfully deter-mined in naturally occurring ores and minerals ofdifferent grades by both spectrophotometric andvisual complexometric direct titration methods us-ing SMTB. The proposed spectrophotometric andcomplexometric titration methods have been com-pared with the atomic absorption spectrometric(AAS) method. Comparison between the two ap-plied analytical methods including different statis-tical tests have also been carried out.

ExperimentalThe reagents and solutions, apparatus, proce-

dure, computation of experimental data and eval-uation of statistical data were as described earli-erIO,ll.Purified sample (99%) of the free acid formof SMTB indicator was purified by liquid-liquidextraction 12 and saturated with ammonia vapourfor 1-2 min in a closed desiccator. The freshlyprepared SOlution of SMTB (0.01 mol dm-3) wasstandardized by spectrophotometric titrationagainst standard Th(N03)4 solution (pH = 2.5,A= 560 nm).

The full matrix of absorbance values of the msolutions, in which the concentration of thalliumion (CM), ligand (CL) and buffer (CH) were varied,were obtained in the range 350'-700 nm at an in-terval of 10 nm. From the experimental data (A;j'CMj, C£j' CHj, i= 1-25, j= 1-m), the molar ratios ofthe chelates formed (p:q:r), conditional stabilityconstants (J)'rr) and molar absorptivity (£pqr)' werecalculated+-' . The computation was. done using acomputer EDPS EC 1040 (500 k), (Robotrom,Germany).

The samples were crushed, ground (150 mesh),weighed (100-250 mg/250 ml), decomposed bydifferent fusion mixtures (Na2C03, Na202 and lorNa2SiOs) and dissolved as' described in litera-ture15•16, Each sample was fused with the corre-sponding fusion material (1:5) at 800-850°C, Sili-ca was completely removed in each case by treat-ing the fused ore in platinum crucible (25 ml)with an excess of hydrofluoric acid and a littlecone. sulphuric acid. Silica was expelled as thevolatile silicon tetrafluoride. The different cationspresent in the ore were first converted into thefluorides, and then to sulphates.Subsequent shorttime ignition at 800-850°C for 5-10 min convert-

80 INDIAN J CHEM. SEe. A, JANUARY 1996

ed the sulphates back into the corresponding ox-ides. The oxides were boiled with cone. nitric acidtill near dryness, cooled and diluted to 250 rnl forboth spectrophotometric and complexometric ti-tration methods. The present method can be usedfor the determination of TP + or TI + after its oxi-dation to TP + using bromine water in highly acid-ic medium (0.5 mol din-3 HCI). The excess bro-mine was removed easily by heating the solutionfor 10-15 min on a steam bath.

The samples which contained the analyte intrace amounts were preconcentrated using DowexAG SOx 8-100 ion-exchange resin (50-100 drymesh) (Aldrich, USA). Absorbance of the Tlrlll}SMTB system in HCl04-NaCI04 medium wasmeasured at 0.5, 1, 2, .... 10 h after preparation.The effect of temperature on absorbance wasstudied using an ultra-thermostat (Kotterman4130, Germany). Those samples which containedlarge amounts of the analyte were subjected toseveral dilutions. Perkin-Elmer model 2380 atom-ic absorption spectrometer (USA) was used withPye Unicam hollow cathode lamps (UK) for thal-lium. Absorbance values were taken after averag-

ing one second integration time. The optimumpractical conditions used for the AAS determina-tion of thallium were as described previously'.

In all series of thallium samples, the number ofdeterminations (n) for either spectrophotometricor visual complexometric direct titration methodwas six. The standard deviation, s, and mean va-lues of Tl203 (%) were calculated as describedpreviously 17.1 8. ~or both the methods a comparis-on between the mean with a given standard wasdone using the null hypothesis of ltl, for P= 0.05and n =6 to estimate any systematic errors. Com-parison between the means of the two investigat-ed methods was done using the null hypothesisItl2 for P= 0.05 and n = 12 (ref. 18).

Comparison between the standard deviations ofthe two methods to estimate random errors oftwo sets of data was carried out using two-tailedF-testI8.

Results and discussionThe absorbance of the complex was determined

at different Crl CSMTB molar ratio. A purple red1:1 . complex was formed between TP+ and

12D :ISO

Fig. l-Absorbance-wavelength curves of complexation of Tl(II1) with·SMTB at pH=2.5 (perchlorate medium), 1-0.1 (NaC-104), CSM'Ill = 2xlO-5 mol dm-3, d-IO mm, c,,;CsM'Ill.(against water and buffer as a 'blank), 1 = zero, 2-0.25, 3-0.5,4 = 0.75, 5 = 1.0, 6 = 1.5, 7 = 2.0, 8 = 2.5, 9 = 3.0, 10 = 3.5, 11=4.0, 12 - difference curve (curve 11 against SMfB and buffer

as a blank),

SMTB with maximum abospriton at 570 nm. Fi-gure 1 shows a clear isobestic point at 500 nmfor Tl(III) indicating a single equilibrium systembetween the free SMTB indicator and thallium-in-dicator complex, Tl(SMTB). The full matrix ofabsorbance values for all measured values of themolar ratio wf!ce taken for the analysis.

Theoretical interpretation of the data in Table 1implies that the optimum pH value for the com-plexation of Tl(III) is 6.0. However, preliminaryinvestigation at pH 6.0 showed that Tl(III) hydro-lyses at this pH value, hence the coordination ofhydroxyl group to Tl(III) is considerably strong(KTI(OH) z: 7.94 x 10-35). In such a case, theTl(Ill) cbmplex with SMTB will have a differentstructure from the one under investigation. Inves-tigation at lower pH values (4-5) revealed that thecomplexation reaction at these pH values was kin-etically very stow. Moreover, the complex formedhas a pale violet colour, which lowered the sensi-tivity of the spectrophotometric method. Experi-mental observations show pH 2.5 (HCl04-NaCI04)

to be optimum for spectrophotometric determinationof Tl(III).

Spectrophotometric determination of TP + ionA reddish violet 1:1 chelate was formed by

SMTB with TP + ion in perchlorate medium ofpH 2-3 {I= 0.1 (NaCI04)} with maximum absorp-tion at 570 nm. After directly mixing the indica-tor, thallium ion and perchlorate solution the mix-ture changed from orange to the stable violet co-lour. The reddish violet colour complex wasstable for at least six weeks. The optimum condi-tions for thallium determination corresponding tothe maximum colour development are based uponheating the complex solution with SMTB for 20-25 min at 40°C in an ultra-thermostat. The effectof organic solvent has been also studied and itwas found that 10% (v/v) dioxane-water systemgave good results.

Table 1-Logarithmic conditional and overall stability constantfor TI(SMTB) complex at different pH vaJues,.[T= 25 ± Ic,

1-0.1)

pH Buffer species1 HCI04-NaCI04

2.5 Chloroacetate-KblOj3 Chloroacetate-KffO,4 Chloroacetate-KfvO,5 Hexarnine-HN03-KN03

6 Hexamine-HN03-KN037 Hexarnine-HN03-KN03

* Log ~I:I =22.1 ± 0.1

Log WI:" TI(SMTB)*2.5±0.15.9±0.18.3 ±0.1

10.35 ±0.112.35 ±0.114.34±O.1

NOTES 81

Iruerference due to foreign speciesThe interference of different anions, cations

and organic acids on the spectrophotometric de-termination of TP + was studied. It was found thatCI04- , NO} and SO~- in excess of 100-fold mo-lar concentration did not affect the absorbancevalues. Phosphate and oxalate (100~fold) inter-fered with the determination of thallium (III). Thehalide ions, F-, CI- and Br ", exhibited no appre-ciable interference at equal molar concentration.On the other hand, they showed considerable in-terference at 100-fold molar concentration. Thisinterference 'may be ascribed to the conversion ofthe labile TI(III) into the inert [TlX4t and[TlX6P - anionic species. I - interfered at equalmolar concentration, due to the reduction ofTI(III) into the inert TI(I). Above 100-fold molarconcentration the interfering effect of the iodideion is reinforced by the precipitation of the previ-ously reduced Tl(I) as TlI (KTII = 6.5 x 10-8). Theinterfering effect of iodide could be removed bydecomposing thallous iodide using fuming nitricacid or aqua regia. It was found that acetic, citric,tartaric and sulphosalicylic acids in excess of 100-fold molar concentration did not interfere in thedetermination of thallium (III). Na2EDTA, CDTAand DTPA at even equimolar concentrations in-terfered seriously. The interference of differentcations was thoroughly investigated. Mg2+, Ca2 +,Ba2+ or Sr2+ at 100-fold molar concentration didnot interfere. Bi3", Sc3 +, Fe3+, Tl4 ", Zr4 +, In3+,Ga3 + or heavy lanthanides at equimolar concentr-ations interfered. It was also found that Cr3+,Nj2+ C02+ Pb2+ Mn2+ Cd2+ Ag " A13+ Cu2+, , , , , , ,or Zn2+ in excess of lfl-fold molar concentrationdid not interfere. La3+, Ce4+, Nd2+, Sm3+ orGd3 + at equal and lO-fold molar concentration ofthe investigated metal ion did not interfere.

Determination of Tl(IIf) in minerals and oresThe most important ores and minerals of thalli-

urn in- nature are crooksite, lorandite, hutchinson-ite, vrbaite and marcasite minerals which containthallium along with other metals. Each mineralwas weighed, pretreated and dissolved as de-scribed above. A simple method for the quantita-tive separation of thallium is based on masking allthe interfering cations present in the sample withEDtA. Thallium (III) is then reduced and precipi-tated as Tll which is dissolved and oxidized againinto TIC13 using aqua regia. The recommendedprocedure is as follows: To 2.0 ml sample solu-tion, was added 10 ml of SMTB (10 - 4 moldm-3), 2.5 ml HCI04 - NaCI04 solution, 2.5 mlof dioxane-water ~10% v/v) and doubly distilled

82 INDIAN J CHEM. SEe. A, JANUARY 1996

Samples

Table 2~Determination of thallium in ores, minerals and alloys by the spectrophotometric (SM) andvisual titration method (TM)

TIZO)

SM*, x! TM'~x2zo.su.o) 2.0.4(.0.5)66.8(.0.5) 66.9(.0.6)

22.6( -1.3) 22.7( -.0.9)34.3(.0.6) 34.2(.0.3).0.9(.0..0) .0.91.0..0)

5.0.5(.0.4) 5.0.6(.0.6)

5.7(1.8) 5.5(-1.8)

9.1(2.3) 9.2(3.4)

3.5(2.9) 3.3( -2.9)

22.1( - .0.9) 22:5(.0.9)

33.7(.0.6) 33.8(.0.9)

12.0( -1.6) 12.4(1.6)

61.8(.0.5) 61.9(.0.7)

27.6(-1.1) 27.5( -1.4)

5.5(-1.8) 5.6(.0..0)

AAS,Il(2.o.3)-(66.5)-(22.9)-(34.1)-

(.0.9)"

Crocksite MineralLorandite MineralHutchinsonite MineralVibraiteMineral·Marcasite MineralThallium (5.0.3%)-Cadmium (4.7% )-Zinc (45%) alloy (laboratory synthesizedby fusion)Thallium (5.6%)-Aluminium (55%)-Nickel (35%)-Zinc (2.4%)-Iron (2%) alloy(laboratory synthesized by fusion)Thallium (8.9%)-Indium (89.5%)-Cadmium (1.6%)-alloy (laboratory synthes-ized by fusion)Gallium (4%)-Indium (2.6%)-Thallium (3.4%)-Si02 (9.0%) (laboratory synthes-ized by fusion)Thallium (2.o%)-Aluminium (15%)-Si02 (65%) (laboratory synthesized by fu-sion)Thallium (33.5%)-ChromiUrtl (6.5%)-Si02 (6.0%) (laboratory synthesized by fu-sion) .Thallium (12%)-Bismuth (18% )-Cadmium (4.0%)-SiOP.o%), (laboratory syn-thesized by fusion)Thallium (61.5%)-Lead (13.5%)-Cadmium (2.o%)-Silver (5%) alloy (laboratorysynthesized by fusion)Thallium (3.o%)-Lead (4.0%)-Zinc (2.0%)-Aluminium (1.0%)alloy (laboratorysynthesized by fusion)Thallium (5.6%)-Indium (89.4%)-Cadmium (5%) ally (laboratory synthesized by.fusion) 5.6* Average of three determination for two weighed samples, 1.0.0-25.0mgl25.o ml, n - 6.t End-point from reddish violet to lemon yellowa Ref. (5)

5.0.3

5.6

8.9

.3.4

22.3

33.5

12.2

·61.5

27.9

Values in parentheses are % error.

Tahle 3-Statistical evaluation and tests of experirnental.data for the determination of thallium in ores, minerals and alloys bythe proposed spectrophotometric (SM) and visual titration (TM) methods

Twotailed

s., % Sr/%) sz, % srtlo) (SM) (TM) F-test (Fs.s)1 .0.2 1..0 .0.2 1..0 2.45 1.22 .0.87 1..0.02 .0.3 .0.5 .0.4 .0.6 2.45 2.45 .0.49 .0.563 .0.3 1.3 .0.2 .0.9 2.45 2.45 .0.68 2.254 .0.3 .0.9 .0.3 .0.9 1.63 .0.82 .0.58 1..0.05 .0..0 .0..0 .0..0 .0..0 .0..0.0 .0..0.0 .0..0.0 .0..0.06 .0.3 .0.6 .0.3 .0.6 1.63 2.45 .0.58 1..0.07 .0.2 3.5 .0.1 1.8 1.23 2.45 2.19 4 ..0.08 .0.1 1.1 .0.2 2.2 4.90 3.67 1.1.0 .0.259 .0.2 5.7 .0.1 3..0 1.23 2.45 2.19 4 ..0.0

1.0 .0.3 1.4 .0.4 1.8 1.63 1.23 1.96 .0.5611 .0.3 .0.9 .0.2 .0.6 1.63 3.67 .0.68 2.2512 .0.3 2.5 .0.4 3.2 1.63 1.23 1.96 .0.5613 .0.3 .0.5 .0.4 .0.7 2.45 2.45 .0.49 .0.5614 .0.4 1.5 .0.4 1.5 1.84 2.45 .0.43 1..0.015 .0.1 1.8 .0.1 1.8 2.45 .0..0.0 ·1.73 1..0.0

Itl! is for the comparison of the experimental mean with the standard value for P·=.o..o5 & n-6 (5 degrees of freedom) equal2.57.Itlz is for the comparison of the means of both SM & TM for P-.o.Q5 & n-12 for the two methods (1.0 degrees of freedom)equal 2.23.Fs,s - ~V~for p= .0..05 equal 7.146 (two tailed F-test).

r-----~(n, -I)s~ +(n2 -1)s~

sp is the pooled estimate of standard deviation -n\+n2-2

SerialNo.

Spectrophotometric method Titration method (TM) (I 1)-+-n, n2

water in a 25 ml measuring flask. The mixturewas heated in an ultra-thermostat at 40°C for 20-25 min. Using the above method thallium (III)was determined in the ores, minerals and alloyswith a percentage error of. not more than - 3%deviation (Table 2) and relativestandard deviation(range method)" of - 6% (Table 3).

Visual complexometric titration methodEach mineral or syn.thetic sample was weighed

(100-250 mg), pretreated and dissolved as above.All interfering ions were completely removed be-fore carrying out the visual complexometric titra-tion. After pretreatment of the thallium sample,thallium was precipitated as TlI using 10% (w/v)potassium iodide solution. The process of dissolu-tion in aqua regia and precipitation was repeatedtwice. The fine colloidal precipitate of TlI wasseparated by centrifugation at 2S00 rpm for 2-3min and removing the supernatant. The precipit-ate was separated by centrifugation again, dis-solved by boiling with required amount of fumingnitric acid or aqua regia, heated till near dryness,cooled and made up to 2S0 ml in a measuringflask. 10 ml of the thallium (III) solution wastransferred into a 2S0-ml conical flask. To this so-lution, was added 1-2 ml of SMTB (10-3 moldm+'), The pH of the solution was adjusted to2-2.5 using perchlorate solution. The solution wasdiluted to 25 ml and directly titrated with stand-ard N~EDTA solution. Colour change from red-dish violet to lemon yellow indicated the end-point. Thallium (III) could be determined in min-erals, ores and alloys samples with percentage er-ror of not more than 3.4% deviation (Table 2)and relative standard deviation (range method)"of 3.S% (Table 3).

Comparison of the two studied methods showthat the detection limit of the spectrophotometricmethod is 10.2 ug TP + 12S ml, while that of thevisual titration method is 0.30 mg TP + 125 ml ofthe titrating solution. This implies that the visualdirect titration has a higher detection limit ( - 29-fold higher) than that of the spectrophotometricmethod for the same volume of thallium solution.

Comparison between the experimental mean va-lue of each sample for both spectrophotometricmethod, SM and titration method,. TM with thevalue obtained by the AAS method, I..l by applyingthe null hypothesis of ltl. for p=O.OS.in order toestimate the systematic error was carried out.From Table 3, it was found that for SM the Itl1=0.0-2.45 for all thallium samples with the excep-tion of sample number 8 which has Itl1 = 4.9. Thismeans that sample number 8 is subjected to syste-

NOTES 83

matic error (the null hypothesis of ltl, for P =O.OSand n = 6 for tliis sample is rejected). For allother samples the null hypothesis of ltl, are lessthan the tabulated critical value (Itl, = 2.S7)18. ForTM the ltl. = 0.0-2.4S for all determined sam-ples with the exception of samples number 8 and11. These can be explained as mentioned before.Comparison between the experimental means ofthe two studied methods was done using the nullhypothesis of Itl2 for P = O.OSand n = 12 (Table3). It is found that for all samples (1-15) the nullhypothesis of Itl2 is between (0.0-2.19) whichmeans that Itl; are less than the calculated criticalvalue Itb = 2.23,18 so the null hypothesis is re-tained. This means that both methods under inves-tigation have right means or accepted recoveries.

Comparison between the standard deviations ofthe two investigated methods to estimate randomerrors of the two sets of data was done using twotailed F-test". From Table 3, it is clear that all theexperimental F5,5values are between 0.0-4.00 forall the determined samples. These values are lessthan the tabulated value of F55 for P = O.OS(7.146) (ref. 18). This means that there is no signi-ficant difference between the two variances(standard deviations) at 9S% confidence interval(P = O.OS) for both SM and TM or the twomethods have the same precision and are not sub-jected to any random errors.

AcknowledgementWe would like to express our sincere gratitude

to Dr. Omar H.A. Hegab, Professor of Sedimen-tology, Geology Department, Faculty of Science,Mansoura University, and Mr. M.S. EI-Din, Chief,Geological Museum, Mansoura University, forproviding us with the ores and mineral under in-vestigation.

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