Limit Test’s for Acid Radical Impurities

LIMIT TEST’S FOR ACID RADICAL IMPURITIES

Acid radical impurities constitute a serious but unavoidable source of impurities in a large number of pharmaceutical chemicals. However, the two most commonly found acid radical impurities are chloride (Cl^– ) and sulphate (SO₄^2–) that evidently arise from the inevitable use of raw tap-water in various manufacturing operations. As these two acid radical impurities are found in abundance due to contamination, the Pharmaco-poeia categorically stipulates limit tests for them which after due minor modifications are applicable to a number of pharmaceutical substances.

In addition to the above two commonly found impurities, there are a number of other acid radical impurities which exist in pharmaceutical substances, namely : arsenate, carbonate, cyanide, nitrate, oxalate, phosphate and silicate.

All these acid radical impurities shall be discussed briefly as under :

1. Limit Test for Chlorides

The limit test for chlorides is based on its precipitation with silver nitrate in the presence of dilute HNO₃, and comparing the opalescence produced due to the formation of AgCl with a standard opalescence achieved with a known quantity of Cl^– ions.

The equation may be expressed as :

Materials Required : Nessler cylinder 1 pair ; dilute nitric acid (10% w/w of HNO₃) 10.0 ml ; silver nitrate solution (5.0% w/v in DW) 1.0 ml.

Standard Opalescence : Place 1.0 ml of a 0.05845% w/v solution of NaCI in 10 ml of dilute HNO₃ in a Nessler cylinder. Dilute to 50 ml with DW and add 1 ml of AgNO₃ solution. Stir immediately with a glass rod and allow to stand for 5 minutes.

Procedure : Dissolve the specified quantity for the substance in DW, or prepare a solution as directed in the text and transfer to a Nessler cylinder. Add 10 ml of dilute nitric acid, except when it is used in the preparation of the solution, dilute to 50 ml with DW, and add 1 ml of AgNO₃ solution. Stir immediately with a glass rod and allow to stand for 5 minutes. The opalescence produced is not greater than the standard opalescence, when viewed transversely.

A few typical examples of this test representing a wide spectrum of pharmaceutical substances are enumerated below :

2. Limit Test for Sulphates

Theory : The limit test for sulphates is based upon its precipitation as barium sulphate in the presence of barium chloride, hydrochloric acid and traces of barium sulphate. In this combination, hydrochloric acid exerts its common ion effect whereas traces of BaSO₄ aids in the rapid and complete precipitation by seeding. Thus, the opalescence caused by the sample is compared immediately with a standard turbidity produced with a known amount of the SO₄^2– ion.

The main objective of this test is to provide a rigid control of sulphate as an impurity present primarily in inorganic pharmaceutical substances.

Materials Required : Nessler cylinders 1 pair ; dilute hydrochloric acid (10% w/v of HCl) 2.0 ml.

Barium Sulphate Reagent : Mix 15 ml of 0.5 M barium chloride, 55 ml of DW, and 20 ml of sulphate free alcohol, add 5 ml of a 0.0181% w/v soln. potassium sulphate dilute to 100 ml with DW, and mix. It should always be prepared fresh.

0.5 M Barium Chloride : BaCl₂ dissolved in DW to contain in 1 Litre 122.1 g of BaCl₂. 2H₂O.

Standard Turbidity : Place 1.0 ml of a 0.1089% w/v soln. of K₂SO₄ and 2 ml of dilute HCl in a Nessler cylinder, dilute to 45 ml with DW, add 5 ml BaSO₄ reagent, stir immediately with a glass rod and allow to stand for 5 minutes.

Procedure : Dissolve the specified quantity of the substance in DW, transfer to a Nessler cylinder, and the preparation of the solution. Dilute to 45 ml with DW, add 5 ml barium sulphate reagent, stir immediately with a glass rod, and allow to stand for 5 minutes. The turbidity is not greater than the standard turbidity, when viewed transversely.

A few examples of this test consisting of a cross-section of pharmaceutical substances are stated below :

3. Limit Test for Arsenate

Acetarsol : An organic arsenic compound, being therapeutically active when administered orally, that might be of value in the treatment of spirochaetal or protozoal diseases, for instance : syphilis, yaws, relapsing fever, sleeping sickness and amoebic dysentry.

It is made from p-hydroxyphenylarsonic acid, which may be prepared either by straight forward meth-ods from phenol or from p-aminophenylarsonic acid. The resulting compound obtained from either of these reactions is nitrated, reduced and the base is finally acetylated to afford acetarsol.

Inorganic arsenates are found to be extremely toxic in nature and hence careful control is maintained by the addition of magnesium ammonio-sulphate solution to an aqueous solution of the sample, thereby producing an instant white precipitate.

4. Limit Test for Carbonate

Carbonate impurity in pharmaceutical chemicals usually arise from contamination with atmospheric CO₂.

Examples of a few official compounds subject to this test from the Pharmacopoeia are given below :

5. Limit Test for Cyanide

Cyanide present in Edetate Disodium is assayed by titration with AgNO₃ in neutral solution employing dimethylaminobenzylidenerhodamine as an adsorption indicator with a colour change from yellow to orange.

A few typical examples are illustrated below :

A. Edetate Disodium

Materials Required : Edetate disodium 30.0 g ; sodium hydroxide solution (20% w/v in DW) 35.0 ml ; dimethylaminobenzylidenerhodamine solution (0.02% w/v in acetone) 1.0 ml ; 0.01 N AgNO₃ solution (1.699 g in 1 litre of DW) 100 ml.

Procedure : Dissolve 30.0 g in a mixture of 100 ml DW and 35 ml NaOH solution, add 1 ml dimethylaminobenzylidenerhodamine and titrate with 0.01N silver nitrate until the colour of the solution changes from yellow to orange. Repeat the operation without the disodium edetate. The difference between the titrations is not more than 1.25 ml.

B. Iodine

Materials Required : Iodine 3.5 g ; zinc powder 10 g ; ferrous sulphate solution (2.0% w/v in boiled and cooled DW) 1.0 ml ; sodium hydroxide solution (20% w/v in DW) 1 ml ; hydrochloric acid (~ 11.5 N) 20 ml.

Procedure : Triturate 3.5 g thoroughly with 35 ml DW, filter and decolorise the filtrate by the addition of a little zinc powder. To 5.0 ml of the filtrate add a few drops of ferrous sulphate solution and 1 ml NaOH solution ; warm gently and acidify with HCl, no blue colour or green colour is produced.

C. Potassium Iodide

Materials Required : Potassium iodide 0.5 g ; ferrous sulphate solution (2.0% w/v in boiled and cooled DW) 1 drop ; NaOH solution (20% w/v in DW) 0.5 ml ; HCl 20.0 ml.

Procedure : Dissolve 0.5 g in 5 ml warm DW, add 1 drop of ferrous sulphate solution and 0.5 ml NaOH solution and acidify with HCl, no blue colour is produced.

6. Limit Test for Nitrate

Basic nitrate is usually found as an impurity in bismuth salts (e.g., bismuth subcarbonate), very often due to the mode of preparation from the metal via bismuth nitrate.

BP (1914) first described a limit test, based upon the production of coloured nitro-compounds by the interaction of traces of nitrates with phenol-2, 4-disulphonic acid, and the conversion of these subsequently into dark-yellow ammonium salts. However, this test has a serious disadvantage of correctly matching the yellow colours with great difficulty.

BP (1932) put forward a more reliable test for nitrate based upon the oxidation of indigocarmine to colourless substances by the action of traces of nitrates in presence of hot and fairly concentrated sulphuric acid, and the reaction may be expressed as follows :

The quantities as specifed in the Pharmacopoeia allow an official limit of nitrate equivalent to about 0.29% BiONO₃.

A few typical instances of pharmaceutical substances are enumerated below :

7. Limit Test for Oxalate

Oxalate is found to be a frequent impurity in pharmaceutical substances belonging to the category of either organic acids e.g., anhydrous citric acid, tartaric acid; or salts of organic acids e.g., ferrous gluconate, sodium citrate, potassium citrate and sodium cromoglycate. The presence of this impurity is due to the following two prime factors, namely :

(a) The use of oxalic acid to get rid of Ca²⁺ during various manufacturing processes.

(b) The use of oxalic acid in the isolation and purification of organic bases e.g., ephedrine (thereby resulting into the formation of well defined crystalline oxalates).

A few typical examples are cited below :

8. Limit Test for Phosphate

The limit test for phosphate is based upon the formation of a yellow colour reaction with molybdovanadic reagent (combination of ammonium vanadate and ammonium molybdate) in an acidic medium. However, the exact composition of the molybdovanadophosphoric acid complex is yet to be established.

Three typical examples of pharmaceutical substances are stated below :

Molybdovanadic Reagent : Suspend 4.0 g of finely powdered ammonium molybdate and 0.1 g of finely powdered ammonium metavanadate in 70 ml DW and grind until dissolved. Add 20 ml of HNO₃ and dilute to 100 ml with DW.

Phosphate Standard Solution (5 ppm PO₄) : Dilute 0.5 ml of a 0.143% w/v soln. of potassium dihydrogen orthophosphate (KH₂PO₄) to 100 ml with DW.

Sulphomolybdic Solution : Dissolve with heating, 25 ml ammonium molybdate in 200 ml DW. Separately, with care, add 280 ml H₂SO₄ to 500 ml DW. Cool and mix the two solutions and dilute to 1 Litre with DW.

Methylaminophenol-sulphite Solution : Dissolve 0.1 g of 4-methylaminophenol sulphate, 20 g sodium metabisulphite and 0.5 g anhydrous sodium sulphite in sufficient DW to produce 100 ml.