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6.3 COMPLEXOMETRIC TITRATIONS

6.3 COMPLEXOMETRIC TITRATIONS

      Complexing agents of value as titrants are aminopolycarboxylic acids that possess the characteristic group, N (CH2COOH)2. Such compounds are capable of forming chelate complexes with many cations in which the cation is bound in a ring structure. The ring results from the formation of a salt-like bond between the cation and the carboxyl groups together with a coordinate bond through the lone pair of electrons of the nitrogen atom. If the ring is five-membered, the chelate thus formed is likely to have high stability, so the most useful chelating titrants are those that favour the formation of such rings. This is the case with edetic acid (ethylenediaminetetraacetic acid, EDTA); the commonly used disodium salt is known as disodium edetate. With most metals carrying more than unit positive charge, edetic acid forms highly water-soluble

1:1 complexes of such a structure that at least three fivemembered chelate rings are formed, thus conferring high stability on the complex.

      The stability of such complexes is markedly dependent on pH. Most divalent metals form complexes that are stable in alkaline solution but alkaline earth chelates decompose below about pH 8, whereas many divalent metal complexes (zinc and lead, for example) are also stable in quite acid solution. Trivalent metal complexes, by virtue of the additional stability conferred by an increased number of chelate rings, are often stable even in strongly acid solution. In alkaline solution, however, some of these metals may be precipitated as hydroxides in the presence of edetic acid, not because of instability of the complex but because of the more powerful effect of the very low solubility product of the metal hydroxide.

      In order to determine the equivalence point in titration of metal ions with edetic acid, it is necessary to use a suitable indicator that will react to the presence of free metal ions in solution. The indicator originally used by Schwarzenbach for titration of calcium ions was murexide (ammonium purpurate) but this is now rarely used. Perhaps the most widely used of indicators has been mordant black 11 (also known under several trade names). This has a blue colour in ammoniacal solution but yields red complexes with many metal ions in such solutions; the metal complexes so formed are generally weaker than the corresponding edetic acid complexes, so that titration with edetates will readily remove the metal from the indicator complex and a colour change to full blue signifies total chelation of the metal present in solution. Mordant black 11 is frequently used as a mixture with methyl orange, which serves to provide a screened (more readily detectable) end-point.

      Many other substances have been proposed and used as indicators for complexometric titrations, but the present discussion must be limited to a consideration of those that are of potential value in pharmaceutical applications. Hydroxynaphthol blue gives a very sharp colour change from reddish pink to full blue when a calcium solution is titrated with sodium edetate in the pH range of 12 to 13. If magnesium is present it is precipitated as hydroxide at this pH and, providing the alkali is added before the indicator, does not interfere.

      Another widely used indicator is xylenol orange; this is a conventional acid-base indicator into which iminodiacetic acid groups have been introduced, thus permitting the substance to act as a metal-complexing indicator. The indicator gives a clear colour change from pink-violet to yellow at the end-point in titrations of aluminium, bismuth, lead, mercury, and zinc, and may be used from pH 2 to 6 according to the metal being titrated.

ALUMINIUM​

      Use Method I unless otherwise directed.

      Method I To 20.0 ml of the prescribed solution add 25.0 ml of 0.05 M disodium edetate VS and 10 ml of a mixture of equal volumes of 2 M ammonium acetate and 2 M acetic acid. Heat to boiling for 2 minutes, cool, and add 50 ml of ethanol and 3 ml of freshly prepared dithizone TS. Titrate the excess of disodium edetate with 0.05 M zinc sulfate VS until the colour changes from greenish blue to reddish violet. Each ml of 0.05 M disodium edetate is equivalent to 1.349 mg of Al.

      Method II Dissolve the prescribed quantity of the substance being examined in a mixture of 2 ml of 1 M hydrochloric acid and 50 ml of water, add 50.0 ml of 0.05 M disodium edetate VS and neutralize with 1 M sodium hydroxide using methyl red TS as indicator. Heat the solution to boiling, allow to stand for 10 minutes on a water-bath, cool rapidly, add about 50 mg of xylenol orange mixture and 5 g of methenamine, and titrate the excess of disodium edetate with 0.05 M lead(II) nitrate VS until the solution becomes red. Each ml of 0.05 M disodium edetate is equivalent to 1.349 mg of Al.

BISMUTH

      Method I should be used unless otherwise directed.

      Method I Dilute the prescribed solution to 250 ml with water and unless otherwise directed add, with shaking, strong ammonia solution dropwise until cloudiness is first observed. Add 0.5 ml of nitric acid and heat to 70º, maintaining the solution at this temperature until the solution becomes completely clear. Add about 50 mg of xylenol orange mixture and titrate with 0.05 M disodium edetate VS until the colour changes from pinkish violet to lemon-yellow. Each ml of 0.05 M disodium edetate is equivalent to 10.45 mg of Bi.

      Method II Dissolve the prescribed quantity of the substance being examined in the minimum volume of 2 M nitric acid, add 50 ml of water and adjust the pH to 1 to 2 by adding dropwise, with shaking, 2 M nitric acid or 5 M ammonia. Add about 30 mg of xylenol orange mixture and titrate with 0.05 M disodium edetate VS until the colour changes from pinkish violet to full yellow. Each ml of 0.05 M disodium edetate is equivalent to 10.45 mg of Bi.

CALCIUM

      Dilute the prescribed solution with water to 150 ml, add 15 ml of 1 M sodium hydroxide and 300 mg of hydroxynaphthol blue or 100 mg of calcon mixture or calconcarboxylic acid mixture may be used in place of hydroxynaphthol blue. Titrate with 0.05 M disodium edetate VS until the colour changes to full blue. Each ml of 0.05 M disodium edetate is equivalent to 2.004 mg of Ca.

LEAD

      Dilute the prescribed solution with water to 200 ml, or dissolve the prescribed quantity of the substance being examined in 5 to 10 ml of water, or in the minimum volume of 5 M acetic acid, and dilute to 50 ml with water. To the resulting solution add about 50 mg of xylenol orange mixture and sufficient methenamine to produce a violet-pink coloration. Titrate with 0.05 M disodium edetate VS until the colour changes to lemonyellow. Each ml of 0.05 M disodium edetate is equivalent to 10.36 mg of Pb.

MAGNESIUM

      Dilute the prescribed solution to 300 ml with water, or dissolve the prescribed quantity of the substance being examined in 5 to 10 ml of water, or in the minimum volume of 2 M hydrochloric acid, and dilute to 50 ml with water. To the resulting solution add 10 ml of ammonia buffer pH 10.0 and about 50 mg of mordant black 11 mixture. Titrate with 0.05 M disodium edetate VS, until the colour changes from violet to full blue. Each ml of 0.05 M disodium edetate is equivalent to 1.215 mg of Mg. 

ZINC

      Dilute the prescribed solution to 200 ml with water, or dissolve the prescribed quantity of the substance being examined in the prescribed quantity of the substance being examined in the prescribed amount of 2 M acetic acid and dilute to 50 ml with water. To the solution thus obtained add about 50 mg of xylenol orange mixture and sufficient methenamine to produce a violet-pink coloration. Add a further 2 g of methenamine, and titrate with 0.05 M disodium edetate VS until the colour changes to yellow. Each ml of 0.05 M disodium edetate is equivalent to 3.269 mg of Zn.

APPENDICES • 6.3 COMPLEXOMETRIC TITRATIONS
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หมายเหตุ / Note : TP II 2011 PAGE 490-491