506 BETSY BIRABEN SCOTT: DETERMINATION OF TANTALUM BY THE [ArtdySt, VOl. 91 The Determination of Tantalum by the Solvent Extraction of a Tantalum - Pyrogallol Complex BY BETSY BIRABEN SCOTT (Facultad de Quimica y Farmacia, Universidad Nacional de La Plata, Argentina) A colorimetric procedure for determining up to 1.2 nig of tantalum in the presence of up to 20mg of niobium, or up to 180 mg of tungsten, has been developed. The colourless tantalum - pyrogallol complex is extracted into ethyl acetate a t pH between 4.5 to 6.0 by means of tetrahexyl or tetra- butyl ammonium iodide and back-extracted with acidified ammonium oxalate (pH 2.0). The yellow complex obtained is measured spectrophotometrically at 400 mp. TANTALUM can be determined colorimetrically when admixed with niobium and tungsten by selectively extracting a colourless tantalum - pyrogallol complex, in the presence of tetrabutyl or tetrahexyl ammonium iodide, into ethyl acetate and back-extracting with acidified ammonium oxalate.The molar extinction coefficient of the yellow complex is 2135 at 400 mp. Vanadium, chromium and molybdenum interfere less than in the original procedure of Hunt and Wells. Titanium remains as a serious interference. Nudelmanl has studied the effect of quaternary ammonium compounds on the extraction of metal - pyrogallol complexes into ethyl acetate. He observed that a colourless tantalum complex was extractable in the presence of tetrahexyl or tetraheptyl ammonium iodide. This observation has been developed into a method for the determination of tantalum in the presence of excess niobium and other ions.Usually corrections have to be applied to pyro- gallol absorptiometric methods for determining tantalum when niobium or tungsten are p r e ~ e n t , ~ , ~ but in the new procedure 1 mg of tantalum can be determined in the presence of 20 mg of niobium and 180 mg of tungsten. Titanium interferes with the determination and must be removed. EXPERIMENTAL THE EFFECT OF pH AND QUATERNARY AMMONIUM IODIDE ON EXTRACTION- Aliquots of a tantalum solution containing 0.9 mg of metal were extracted at different pH’s with additions of tetrahexyl ammonium iodide (THAI), or the tetrabutyl salt (TBAI) into ethyl acetate and then back-extracted with ammonium oxalate solution acidified to pH 2. The measured absorbances at 400mp are given in Tables I and 11.TABLE I EFFECT OF pH ON EXTRACTION OF TANTALUM (043mg) IN THE PRESENCE OF THAI AND TBAI PH 4.0 4.2 4.3 4.5 4.9 5.0 5-6 6 4 6.8 ,4bsorbance - THAI TBAI 0-490 0.500 0.515 - 0.522 0.525 0.525 - 0-520 0.527 0.535 - 0.525 - 0.522 0.532 0.500 0,518August, 19661 SOLVENT EXTRACTION OF A TANTALUM - PYROGALLOL COMPLEX TABLE I1 EFFECT OF THAI AND TBAI ON THE EXTRACTION OF TANTALUM (0.9mg) AT pH 4.5 TO 5-0 THAI, mg Absorbance TBAI, mg Absorbance 5 0.200 20 0-480 8 0.528 40 0-526 15 0.531 80 0.532 30 0-520 100 0-53 1 45 0.500 120 0.520 507 A certain amount of quaternary salt is required for extraction into the organic phase, but an excess prevents back-extraction into the aqueous phase. INTERFERENCE BY NIOBIUM- Amounts of niobium up to 4 mg are without effect; between 4 and 8 mg a larger amount of tetrahexyl ammonium iodide must be taken to ensure quantitative extraction of tantalum ; above 9 mg too much tetrahexyl ammonium iodide (required for extraction into the organic phase) prevents quantitative stripping.Tetrahexyl ammonium iodide can be used successfully with 9 mg of niobium but only over a restricted range of pH. Tetrabutyl ammonium iodide allows up to 20 mg of niobium to be present, and extraction is possible over a wider pH range than with tetrahexyl ammonium iodide (see Tables I11 and IV). TABLE I11 EFFECT OF THAI AXD TBAI ON THE IXTERFERENCE OF NIOBIUM IN THE DETERMINATION OF TANTALUM (0-9 mg) AT pH 4.5 TO 5.0 Niobium, mg 0.45 0.90 3.6 3-6 6.3 9.0 9-0 9-0 9.0 18.0 2 7.0 36.0 36.0 - THAI 15 15 15 15 30 30 8 15 40 50 - Absorbance 0.53 1 0.530 0.53 1 0.527 0.526 0.5 13 0.258 0.470 0.492 0.450 - TRAI 80 80 80 80 - - - - - - 80 80 120 200 Absorbance 0.530 0.528 0.530 0-530 - - 0.535 0.470 0.460 0.442 TABLE IV EFFECT OF pH ox THE INTERFERENCE OF 9mg OF NIOBIUM IN THE DETERMINATIOK OF TANTALCM (0.9 mg) PH 4.5 4.7 4.9 5.3 5.5 5.9 6.2 5.8 Absorbance in the presence of- r - THAI, 40 mg TBAI, 80 mg - 0.531 0.490 - 0.491 0.530 0.458 - - 0-532 0.340 - - 0.528 - 0.529 IKTERFERENCE BY TUNGSTEN- Although the tungsten - pyrogallol complex is insoluble in ethyl acetate in the presence of tetrahexyl ammonium iodide, it can interfere because extractable coloured products are formed.Spectral studies on the reaction between tungsten, pyrogallol and quaternary ammonium ions show that the existence of a definite stoicheiometric compound is dubious, and suggest the oxidative nature of the process as the colour intensity increases, not only508 BETSY BIRABEN SCOTT: DETERMINATION OF TANTALUM BY THE [Analyst, Vol.91 with acidity (Table V) and ammonium salt concentration, but also with time. Thus when niobium is present, the restricted pH allows only 11 mg of tungsten to be present without TABLE V EFFECT OF pH ON THE INTERFERENCE BY 18.4mg OF TUNGSTEN IN THE DETERMINATION OF TANTALUM (0.9 mg) Absorbance in the presence of- PH THAI, 15 mg TBAI, 60 mg 4-5 4.7 5.0 5-2 6-1 6-5 6.6 - 0.580 - 0.558 0.590 - 0.562 - 0.555 0.547 0.530 - - 0-542 interference. In the absence of niobium, up to 180 mg of tungsten do not cause interference if the pH is maintained at 6, and a minimum amount of tetrahexyl ammonium iodide is used.TABLE VI EFFECT OF TUNGSTEN IN THE DETERMINATION OF TANTALUM (0.9mg) AT A pH OF 4.5 TO 5.0 Absorbance in the presence of- Weight of I A 1 tungsten, mg THAI, 15 mg TBAI, 80 mg 5.5 0.531 0.532 9-2 0.529 0.551 11.0 0.530 0.570 12.9 0.538 - 18.4 0.590 - TABLE VII EFFECT OF TUNGSTEN ON THE DETERMINATION OF TANTALUM (0.9mg) AT A pH OF 6 Weight of tungsten, mg Absorbance in the presence of 15 mg of THAI Te t rabu t yl ammonium tractability of the tungsten OTHER INTERFERENCES- 18 0.528 92 0-527 184 0.532 202 0.548 iodide is unsuitable for use with tungsten because of the ex- - pyrogallol complex. The interferences studied only include those which usually accompany tantalum after a hydrolytic concentration starting from more complex materials.Molybdenum, vanadium and chromium interfere to a lesser extent than in Hunt and Wells’ technique (see Table VIII). Concentrations of fluoride and phosphate as high as 5 x M cause insignificant errors in the procedure. Boric acid interferes by retaining tantalum in the aqueous phase; sulphate does not interfere. TABLE VIII INTERFERENCE OF CATIOKS IN THE DETERMINATIOK OF TAKTALUM Tantalum oxide equivalent, mg 7 A \ 1 mg of metal oxide Hunt and Wells Proposed method Molybdenum trioxide . . .. 0.68 0.015 Chromic oxide . . .. .. - 0.06 Vanadium pentoxide . . . . 0.44 0.12August, 19661 SOLVENT EXTRACTION OF A T.4NTALUM - PYROGALLOL COMPLEX COLOUR PRODUCTION AND STABILITY- Colour measurements were made after 30 minutes in order to obtain perfect separation of phases; the colour was stable for a t least 1 day.The absorbance is linear up to 1.2 mg of tantalum. 509 PROCEDURE- M) and niobium ( M) solutions were prepared by dissolving the appropriate amount of spectrographically pure metal in a platinum dish with 5 ml of 48 per cent. hydrofluoric acid and a few drops of concentrated nitric acid. After concentration on a steam-bath to about 2 ml, 5 ml of concentrated sulphuric acid were added, evaporating to sulphur trioxide fumes. The diluent was 4 per cent. ammonium cxalate solution. Tungsten (5 x M), vanadium and chromium (5 x 10-2 M) solutions were prepared by dissolving the corresponding salt in distilled water. A titanium (1 x M) solution was prepared in the same way as tantalum except that titanium dioxide was used as the starting material.The molar solutions of phosphate and fluoride were prepared by dissolving the respective ammonium salt in distilled water. Test solutions-Tantalum ( M and 1 M), molybdenum (1 x PREPARATION OF CALIBRATION GRAPH- The standard curve was prepared by taking aliquots of the 0.2 mg per ml tantalum solution and proceeding as indicated in Method ( a ) or ( d ) . The curve was plotted over the range 0 to 1.2 mg of pure metal. REAGENTS- Ammonium oxalate solution, 4 per cent. w/v, aqueous. Sodium sulphite solution 30 per cent. w / v , aqueous. Sulphuric acid solution, 5 per cent. w / v , aqueous. Pyrogallol reagent (A)-Dissolve 10 g of pyrogallol in 100 ml of analytical-reagent grade ethyl acetate (this solution keeps well for several weeks).Pyrogallol reagent (B)-Dissolve 10 g of pyrogallol and 0.1 g of tetrahexyl ammonium iodide in 100 ml of analytical-reagent grade ethyl acetate. Pyrogallol reagent (C)-Dissolve 10 g of pyrogallol and 0.3 g of tetrahexyl ammonium iodide in 100ml of ethyl acetate. Tetrabutyl ammonium iodide solution-Dissolve 1 g in 100 ml of 4 per cent. ammonium oxalate solution. Acid ammonium oxalate-Acidify 4 per cent. w/v ammonium oxalate solution with con- centrated sulphuric acid until a pH of 2 is attained. METHOD- Prepare the sample solution by dissolving the metal with hydrofluoric acid and a few drops of nitric acid (see Test solutions), or by fusing the oxides with potassium bisulphate. After preparing the solution and evaporating, dilute with ammonium oxalate solution.Use an amount of sample solution containing not more than 1 mg of tantalum. ( a ) Samples containing niobium up to 4 mg and tungsten up to 11 mg-Introduce an aliquot of sample solution into a 100-ml separating funnel and dilute to 20 ml with ammonium oxalate solution after adjusting the pH to between 4-5 and 5.0 with sodium sulphite solution (if necessary correct with sulphuric acid solution). Extract with a 10-ml portion of p>.rogallol reagent (B) and shake the mixture vigorously for at least 2 minutes. Re-extract the clear aqueous phase with 5ml of the same reagent. Combine the organic layers and wash twice with 2-ml portions of 4 per cent. ammonium oxalate, waiting each time until the aqueous solution is clear.Add 20ml of acid ammonium oxalate solution to the organic layer and shake them together vigorously for 2 minutes. Wait for 30 minutes and read the absorbance at 400 mp in a 1-cm cell against a blank similarly prepared. (b) Samples containing niobium between 4 and 8 mg and tungsten up to 11 mg-All condi- tions are the same except that it is necessary to extract the first time with 10 ml of pyrogallol reagent (C). The second extraction is performed with 5 ml of reagent (B). ( c ) Samples containing tungsten up to 180 mg and niobium ztp to 1 mg-This is essentially the same procedure as in (A), but the extraction is performed at a pH of 6.0 to 6.5.510 BETSY BIRABEN SCOTT [Analyst, VOl. 91 ( d ) Sawaples containing niobium up to 20 mg and tungsten up to 6 mg-Add the sample and 5 ml of tetrabutyl ammonium iodide solution to a 100-ml separating funnel.Add ammonium oxalate solution to a total volume of about 20 ml, Correct the pH as above to between 4.5 and 6.5. Extract initially with 10 ml of pyrogallol reagent (A), and then with 5 ml of the same reagent after adding 3ml of tetrabutyl ammonium iodide solution. Treat the collected extracts in the same manner as when reagent (C) is used. REsurrs AND DISCUSSION By using the appropriate procedure the figures obtained for the determination of 0.9 mg of tantalum in the presence of excess niobium and tungsten are collected in Table IX. TABLE IX DETERMINATION OF TANTALUM (0.9mg) I N THE PRESENCE OF NIOBIUM AND TUNGSTEN Niobium, Tungsten, THA41, TBAI, mg mg mg mg pH Absorbance 4 11 15 - 4-6 0.528 7 11 35 - 4.7 0.527 0.9 180 15 - 6.2 0.530 18 5.5 - 80 5.8 0.531 The extraction of the tantalum complex into ethyl acetate is quantitative.Absorbances for the same concentration, with and without extraction, show an insignificant difference (less than 2 per cent.). An unstable yellow colour appears on acidifying the residual aqueous phase with sulphuric acid, which resembles the yellow4 complex TaO(C,O,)Yy-, but tests show that it is caused by reaction between pyrogallol and bisulphite. The residual aqueous solution remains colourless after acidification when extraction is carried out a t a lower pH of between 4.0 and 4.5. In agreement with Lucachina5 it was found that the oxalate ion must be present in the final aqueous phase ; the yellow complex was not formed when the organic phase was extracted with sulphuric acid of the same pH as the oxalate extractant. Other quaternary ammonium salts such as tetraheptyl ammonium iodide, tetrahexyl and tetrabutyl ammonium bromides give similar results. I wish to thank R. A. Wells, former Director of the National Chemical Laboratory, Teddington, for providing the facilities to carry out the work. I am also indebted to A. Woolf for his reading of the manuscript and to J. A. Catoggio and G. Smith for their helpful suggestions. REFERENCES 1. 2. 3. 4. 5. Nudelman, O., Thesis, 1964 (unpublished paper), Facultad de Quiniica y Farmacia, La Plata, Catoggio, J. A., and Rogers, L. B., Talanta, 1962, 9, 387. Hunt, E. C., and \X7ells, R. A., Afaalyst, 1954, 79, 345. Rabko, A. K., and Lucachina, V. V., Ukr. Khirn. Zh., 1962, 28, 371; Chem. Abstr., 1962, 57, 9445e. Lucachina, V. V., I b i d , , 1963, 29, 689; Chern. Abstr., 1963, 59, 14574g. Received May 24th, 1965 Argentina.