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Adsorption indicators in the titrations of hexacyanoferrate(II) and oxalate with thorium ions
Short communications
765
Tahnta. 1966.Vol. 13.pp. 765to 766. PergamonPressLtd. printedin Northernlrdmd
Adsorption indicators in the titrations of hexacyanoferrate(II) and oxalate with thorium ions (Received 9 July 1965. Accepted 16 November 1965) THERBare a number of available titrimetric methods for the determination of thorium. Willard and Winter’ titrated fluoride with thorium nitrate in presence of 50 % ethanol using a lake of zirconium and Alizarin Red S as indicator, and thii method has been studied by a number of workers.a-5 An indirect method@ in which thorium is precipitated as thorium oxalate, and the oxalate determined by permanganate has been proposed. EDTA has also been successfully used.‘** The titrations of thorium against hexacyanoferrate(I1) and oxalate ions have not been previously described. Mehrotra and Tandon have recently discussed the mechanism of the action of adsorption indicators, and laid down the essential requirements for the successful application of adsorption indicators to precipitation titrations. The successful applications of Alizarln Red S, Pyrocatechol Violet and Xylenol Orange to the precipitation titrations of hexacyanoferrate(II) and oxalate against thorium ions lends further support to the theory. The dyes are not adsorbed by the precipitates of thorium hexacyanoferrate(II) and oxalate, whilst the hexacyanoferrate(I1) and oxalate ions remain in excess in the supematant solution. When the equivalence point is just passed the dyes are adsorbed on to the surface of the precipitates with a simultaneous change of colour which is quite sharp and reversible. Hence it is possible to titrate both hexacyanoferrate(I1) and oxalate ions separately against thorium ions in weakly acidic solutions. These titrations when carried out in pure solutions are comparable in accuracy to the titration of thorium with EDTA; but, in common with most precipitation titrations, the method is not suitable in presence of ions which form slightly soluble compounds either with the cation or anion of the main precipitate or which react with the dye. Nevertheless, the proposed method is of value in that it demonstrates the utility of adsorption indicators for the determination of thorium, oxalate and hexacyanoferrate(I1) ions. EXPERIMENTAL Reagents Potassium hexacyanoferrate(ZZ) solution. Prepare and standardise by standard methods.BJO~ll Stabilise1aJ8 by addition of 3 ml. of 1M KOH/l. Sodium oxalate solution. Prepare and standard& by standard methods.B~loJ1 Thorium nitrate solution. Prepare and standardise by standard methods.BJoJ1 Add 34 ml of O*lM acid/l. of solution. Indicators. 0.2 % w/v of Alizarin Red S, Pyrocatechol Violet and Xylenol Orange. Procedure The pH of the solution to be titrated [(oxalate or hexacyanoferrate(I1) Jwas adjusted to therequired value by the addition of O.lM or O.OlM acid or alkali (Table I). One drop of the indicator (either Alizarin Red S or Xylenol Orange or Pyrocatechol Violet) for every 10 ml of the solution was added. The solution was then titrated with a solution of thorium nitrate of corresponding strength. During the addition of the titrant a white suspension was produced and as the equivalence point approached, the titrant was added slowly with constant shaking. At the end-point, which occurs just after the equivalence point, the dye is transferred from the supematant solution to the precipitate with a sudden TABLE I.-SUMMARY
Indicator Alizarin Red S. Xylenol Orange Pyrocatechol
9
Violet
OF CONDITIONS FOR suctxwu~
Transition
at the end-point
Yellowish white suspension -. Pink suspension Yellowish white suspension -+ Pink suspension Yellowish white suspension + bluish violet suspension
APPLICATION OF INDICATORS
Conditions Applicable in the pH range 25-4 Applicable in the pH range 254 Applicable in the pH range 3-5
Short communications
766
change of colour. The precipitate turned pink in case of Alizarin Red S and Xylenol Orange and bluish violet in case of Pyrocatechol Violet. The change of colour was quite sharp and reversible. Coagulation did not occur even in 002M solutions. Solutions of oxalate and hexacyanoferrate(I1) can be titrated up to a dilution of M/500 within an experimental error of 0.3 %. The colour change at the end-point and the conditions in which the indicators are applicable in the titration of oxalate and hexacyanoferrate(I1) with thorium ions are described in Table I. A study of interferences has shown that the above titrations can be carried out even in the presence of appreciable quantities of chloride, bromide, iodide, acetate and nitrates of alkali metals. The ions that interfere are those which form either a slightly soluble compound with the cation or anion of the main precipitate or react with the dye either to form a complex or to oxidise it. These interfering ions include silver, lead, zinc, cadmium, copper, nickel, cobalt, hexacyanoferrate(III), phosphate, chromate, iodate and fluoride. Acknowledgement-The author’s thank are due to Prof. R. C. Mehrotra, Department, for providing laboratory facilities and encouragement.
Head of the Chemistry
P. S. DUBBY K. N. TAND~N
Chemical Laboratories University of Rajusthan Jaipur, Rajasthan, India
Summary-Alizarin Red S, Pyrocatechol Violet and Xylenol Orange am shown to be effective adsorption indicators for the titrations of hexacyanoferrate(I1) and oxalate with Thorium(IV) solution. The action of the indicators can be explained by the theory of surface compound formation.
Zusammenfasmng-Es wird gezeigt, dal3 Alizarinrot S, Brenzcatechinviolett und Xylenolorange bei der Titration von Hexacyanoferrat(I1) und Oxalat mit Thorium(IV)-Lissung brauchbare Adsorptionsindikatoren darstellen. Die Wirkung der Indikatoren kann durch die Theorie der Bildung einer Oberfhichenverbindung erkliirt werden.
R&mm&--On montre que le rouge d’alizarine S, le violet de pyrocatechol et le xylenol orange sont des indicateurs d’adsorption efficaces pour les dosages et l’hexacyanoferrate(I1) et l’oxalate au moyen dune solution de thorium(IV). On peut expliquer l’action des indicateurs par la theorie de la formation de compost de surface.
REFERENCES 1 H. H. Willard and 0. B. Winter, Ind. Eng. Chem., Anal. Ed., 1933, 5, 7. e W. D. Armstrong, J. Am. Chem. Sot., 1933,55,1741. 3 W. M. Hoskins and C. A. Ferris, Znd. Erg. Chem., Anal. Ed., 1936, 8,6. 4 W. D. Armstrong, ibid., 1936, 8, 384. 5 R. J. Rowley and H. V. Churchill, ibid., 1937, 9, 551. BR. Belcher and A. J. Nutten, Quantitative Inorganic Analysis, p. 261. Butterworth, London, 1960. ’ J. KBrbl and R. Piibil, Chemist-Analyst, 1956,45,102; 1957,46,28. 8 M. Malat, V. Suk and 0. Ryba, Chem. Listy, 1954,48,533. s R. C. Mehrota and K. N. Tandon, Talanta, 1964,11,10!93. lo I. A. Vogel, A Text Book of Quantitative Inorganic Analysis, 2nd. Ed., pp. 541, 280. Longmans Green, London, 1962. I* Cumming and Kay, Text Book of Quantitative Analysis, 11th Ed, p. 150. Oliver & Boyd, Edinburgh, 1956. la S. Jimori, Z. Anorg. Chem., 1927,167, 145. la P. S. Dubey and K. N. Tandon, Z. Anal. Chem., communicated.
765
Tahnta. 1966.Vol. 13.pp. 765to 766. PergamonPressLtd. printedin Northernlrdmd
Adsorption indicators in the titrations of hexacyanoferrate(II) and oxalate with thorium ions (Received 9 July 1965. Accepted 16 November 1965) THERBare a number of available titrimetric methods for the determination of thorium. Willard and Winter’ titrated fluoride with thorium nitrate in presence of 50 % ethanol using a lake of zirconium and Alizarin Red S as indicator, and thii method has been studied by a number of workers.a-5 An indirect method@ in which thorium is precipitated as thorium oxalate, and the oxalate determined by permanganate has been proposed. EDTA has also been successfully used.‘** The titrations of thorium against hexacyanoferrate(I1) and oxalate ions have not been previously described. Mehrotra and Tandon have recently discussed the mechanism of the action of adsorption indicators, and laid down the essential requirements for the successful application of adsorption indicators to precipitation titrations. The successful applications of Alizarln Red S, Pyrocatechol Violet and Xylenol Orange to the precipitation titrations of hexacyanoferrate(II) and oxalate against thorium ions lends further support to the theory. The dyes are not adsorbed by the precipitates of thorium hexacyanoferrate(II) and oxalate, whilst the hexacyanoferrate(I1) and oxalate ions remain in excess in the supematant solution. When the equivalence point is just passed the dyes are adsorbed on to the surface of the precipitates with a simultaneous change of colour which is quite sharp and reversible. Hence it is possible to titrate both hexacyanoferrate(I1) and oxalate ions separately against thorium ions in weakly acidic solutions. These titrations when carried out in pure solutions are comparable in accuracy to the titration of thorium with EDTA; but, in common with most precipitation titrations, the method is not suitable in presence of ions which form slightly soluble compounds either with the cation or anion of the main precipitate or which react with the dye. Nevertheless, the proposed method is of value in that it demonstrates the utility of adsorption indicators for the determination of thorium, oxalate and hexacyanoferrate(I1) ions. EXPERIMENTAL Reagents Potassium hexacyanoferrate(ZZ) solution. Prepare and standardise by standard methods.BJO~ll Stabilise1aJ8 by addition of 3 ml. of 1M KOH/l. Sodium oxalate solution. Prepare and standard& by standard methods.B~loJ1 Thorium nitrate solution. Prepare and standardise by standard methods.BJoJ1 Add 34 ml of O*lM acid/l. of solution. Indicators. 0.2 % w/v of Alizarin Red S, Pyrocatechol Violet and Xylenol Orange. Procedure The pH of the solution to be titrated [(oxalate or hexacyanoferrate(I1) Jwas adjusted to therequired value by the addition of O.lM or O.OlM acid or alkali (Table I). One drop of the indicator (either Alizarin Red S or Xylenol Orange or Pyrocatechol Violet) for every 10 ml of the solution was added. The solution was then titrated with a solution of thorium nitrate of corresponding strength. During the addition of the titrant a white suspension was produced and as the equivalence point approached, the titrant was added slowly with constant shaking. At the end-point, which occurs just after the equivalence point, the dye is transferred from the supematant solution to the precipitate with a sudden TABLE I.-SUMMARY
Indicator Alizarin Red S. Xylenol Orange Pyrocatechol
9
Violet
OF CONDITIONS FOR suctxwu~
Transition
at the end-point
Yellowish white suspension -. Pink suspension Yellowish white suspension -+ Pink suspension Yellowish white suspension + bluish violet suspension
APPLICATION OF INDICATORS
Conditions Applicable in the pH range 25-4 Applicable in the pH range 254 Applicable in the pH range 3-5
Short communications
766
change of colour. The precipitate turned pink in case of Alizarin Red S and Xylenol Orange and bluish violet in case of Pyrocatechol Violet. The change of colour was quite sharp and reversible. Coagulation did not occur even in 002M solutions. Solutions of oxalate and hexacyanoferrate(I1) can be titrated up to a dilution of M/500 within an experimental error of 0.3 %. The colour change at the end-point and the conditions in which the indicators are applicable in the titration of oxalate and hexacyanoferrate(I1) with thorium ions are described in Table I. A study of interferences has shown that the above titrations can be carried out even in the presence of appreciable quantities of chloride, bromide, iodide, acetate and nitrates of alkali metals. The ions that interfere are those which form either a slightly soluble compound with the cation or anion of the main precipitate or react with the dye either to form a complex or to oxidise it. These interfering ions include silver, lead, zinc, cadmium, copper, nickel, cobalt, hexacyanoferrate(III), phosphate, chromate, iodate and fluoride. Acknowledgement-The author’s thank are due to Prof. R. C. Mehrotra, Department, for providing laboratory facilities and encouragement.
Head of the Chemistry
P. S. DUBBY K. N. TAND~N
Chemical Laboratories University of Rajusthan Jaipur, Rajasthan, India
Summary-Alizarin Red S, Pyrocatechol Violet and Xylenol Orange am shown to be effective adsorption indicators for the titrations of hexacyanoferrate(I1) and oxalate with Thorium(IV) solution. The action of the indicators can be explained by the theory of surface compound formation.
Zusammenfasmng-Es wird gezeigt, dal3 Alizarinrot S, Brenzcatechinviolett und Xylenolorange bei der Titration von Hexacyanoferrat(I1) und Oxalat mit Thorium(IV)-Lissung brauchbare Adsorptionsindikatoren darstellen. Die Wirkung der Indikatoren kann durch die Theorie der Bildung einer Oberfhichenverbindung erkliirt werden.
R&mm&--On montre que le rouge d’alizarine S, le violet de pyrocatechol et le xylenol orange sont des indicateurs d’adsorption efficaces pour les dosages et l’hexacyanoferrate(I1) et l’oxalate au moyen dune solution de thorium(IV). On peut expliquer l’action des indicateurs par la theorie de la formation de compost de surface.
REFERENCES 1 H. H. Willard and 0. B. Winter, Ind. Eng. Chem., Anal. Ed., 1933, 5, 7. e W. D. Armstrong, J. Am. Chem. Sot., 1933,55,1741. 3 W. M. Hoskins and C. A. Ferris, Znd. Erg. Chem., Anal. Ed., 1936, 8,6. 4 W. D. Armstrong, ibid., 1936, 8, 384. 5 R. J. Rowley and H. V. Churchill, ibid., 1937, 9, 551. BR. Belcher and A. J. Nutten, Quantitative Inorganic Analysis, p. 261. Butterworth, London, 1960. ’ J. KBrbl and R. Piibil, Chemist-Analyst, 1956,45,102; 1957,46,28. 8 M. Malat, V. Suk and 0. Ryba, Chem. Listy, 1954,48,533. s R. C. Mehrota and K. N. Tandon, Talanta, 1964,11,10!93. lo I. A. Vogel, A Text Book of Quantitative Inorganic Analysis, 2nd. Ed., pp. 541, 280. Longmans Green, London, 1962. I* Cumming and Kay, Text Book of Quantitative Analysis, 11th Ed, p. 150. Oliver & Boyd, Edinburgh, 1956. la S. Jimori, Z. Anorg. Chem., 1927,167, 145. la P. S. Dubey and K. N. Tandon, Z. Anal. Chem., communicated.