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Limit Test for Aluminium

June 2, 2024

Theme: Limit tests for impurities according to Pharmacopoeia.

Tubes for Comparative Tests

(Ph. Eur. method 2.1.5)

Tubes used for comparative tests are matched tubes of colourless glass with a uniform internal diameter. The base is transparent and flat.

A column of the liquid is examined down the vertical axis of the tube against a white background, or if necessary, against a black background. The examination is carried out in diffused light.

It is assumed that tubes with an internal diameter of 16 mm will be used. Tubes with a larger internal diameter may be used instead but the volume of liquid examined must then be increased so that the depth of liquid in the tubes is not less than where the prescribed volume of liquid and tubes 16 mm in internal diameter are used.

Limit Test for Aluminium

Extract the prescribed solution with successive quantities of 20, 20 and 10 ml of a 0.5% w/v solution of 8-hydroxyquinoline in chloroform and dilute the combined extracts to 50 ml with chloroform. Unless otherwise stated in the monograph use as the blank solution a mixture of 10 ml of acetate buffer pH 6.0 and 100 ml of water treated in the same manner and as the standard solution a mixture of 2 ml of aluminium standard solution (2 ppm Al), 10 ml of acetate buffer pH 6.0 and 98 ml of water treated in the same manner. Measure the fluorescence of the test solution (I₁), of the standard solution (I₂) and of the blank (I₃), Appendix II E, using an excitation wavelength of 392 nm and a secondary filter with a transmission band centred at 518 nm, or a monochromator set to transmit at this wavelength. The fluorescence of the test solution (I₁ I₃) is not greater than that of the standard solution (I₂ I₃).

Limit Tests for Ammonium

Use method A unless otherwise prescribed in the monograph.

Method A Dissolve the prescribed quantity of the substance being examined in 14 ml of water in a test tube, if necessary make alkaline with 2M sodium hydroxide and dilute to 15 ml with water. Add 0.3 ml of alkaline potassium tetraiodomercurate solution, stopper the tube, mix and allow to stand for 5 minutes. Any yellow colour produced is not more intense than that obtained by treating a mixture of 10 ml of ammonium standard solution (1 ppm NH₄) and 5 ml of water in the same manner.

Method B In a 25-ml jar fitted with a polyethylene cap place the prescribed quantity of the finely powdered substance being examined and dissolve or suspend in 1 ml of water. Add 0.3 g of heavy magnesium oxide. Close immediately after placing a piece of silver manganese paper 5 mm square, wetted with a few drops of water, under the polyethylene cap. Swirl, avoiding projections of liquid, and allow to stand at 40 for 30 minutes. If the silver manganese paper shows a grey colour, it is not more intense than that of a standard prepared at the same time and in the same manner using the prescribed quantity of ammonium standard solution, 1 ml of water and 0.3 g of heavy magnesium oxide.

MgO+H₂O=Mg(OH)₂

2NH₄⁺+Mg(OH)₂=2NH₃+Mg²⁺

2AgNO₃+MnSO₄+4NH₃+2H₂O=2Ag+MnO₂+(NH₄)₂SO₄+2NH₄NO₃

Limit Tests for Arsenic

Test A The apparatus (Fig. 7-1) consists of a 100-ml conical flask closed with a ground-glass stopper through which passes a glass tube about 200 mm long and 5 mm in internal diameter. The lower part of the tube is drawn to an internal diameter of 1.0 mm and 15 mm from its tip is a lateral orifice 2 to 3 mm in diameter. When the tube is in position in the stopper the lateral orifice should be at least 3 mm below the lower surface of the stopper. The upper end of the tube has a perfectly flat, ground surface at right angles to the axis of the tube. A second glass tube of the same internal diameter and 30 mm long, with a similar flat ground surface, is placed in contact with the first and is held in position by two spiral springs. Into the lower tube insert 50 to 60 mg of lead acetate cotton, loosely packed, or a small plug of cotton and a rolled piece of lead acetate paper weighing 50 to 60 mg. Between the flat surfaces of the tubes place a disc or a small square of mercury(II) bromide paper large enough to cover the orifice of the tube.

In the conical flask dissolve the prescribed quantity of the substance being examined in 25 ml of water or, in the case of a solution, dilute the prescribed volume to 25 ml with water. Add 15 ml of hydrochloric acid, 0.1 ml of tin(II) chloride solution AsT and 5 ml of potassium iodide solution, allow to stand for 15 minutes and add 5 g of activated zinc. Immediately assemble the two parts of the apparatus and immerse the flask in a water bath at a temperature such that a uniform evolution of gas is maintained. After not less than 2 hours any stain produced on the mercury(II) bromide paper is not more intense than that obtained by treating 1 ml of arsenic standard solution (1 ppm As) diluted to 25 ml with water in the same manner.

6Zn+As₂O₃+12HCl=2AsH₃+3H₂O+6ZnCl₂

AsH₃+3HgBr₂=As(HgBr)₃+HBr

AsH₃+ As(HgBr)₃=As₂Hg₃+3HBr

Test B Add the prescribed quantity of the substance being examined to a test tube containing 4 ml of hydrochloric acid and about 5 mg of potassium iodide and add 3 ml of hypophosphorous reagent. Heat the mixture on a water bath for 15 minutes, shaking occasionally. Any colour produced is not more intense than that obtained in a solution prepared in the same manner but using 0.5 ml of arsenic standard solution (10 ppm As) in place of the substance being examined.

NaH₂PO₂+HCl=NaCl+H₃PO₂

As₂O₃+3H₃PO₂=2As+3H₃PO₃

As₂O₅+5H₃PO₂=2As+5H₃PO₃

Limit Test for Calcium

The solutions used for this test should be prepared with distilled water.

To 0.2 ml of alcoholic calcium standard solution (100 ppm Ca) add 1 ml of ammonium oxalate solution. After 1 minute add a mixture of 1 ml of 2M acetic acid and 15 ml of a solution containing the prescribed quantity of the substance being examined and shake. After 15 minutes any opalescence produced is not more intense than that of a standard prepared in the same manner using a mixture of 10 ml of calcium standard solution (10 ppm Ca) and 5 ml of water in place of the solution of the substance being examined.

Limit Test for Chlorides

To a solution of the specified quantity of the substance being examined in 15 ml of water or to 15 ml of the prescribed solution add 1 ml of 2M nitric acid, pour the mixture as a single addition into 1 ml of silver nitrate solution R2 and allow to stand for 5 minutes protected from light. When viewed transversely against a black background any opalescence produced is not more intense than that obtained by treating a mixture of 10 ml of chloride standard solution (5 ppm Cl) and 5 ml of water in the same manner.

Limit Test for Fluorides

Introduce into the inner tube of the apparatus (Fig.) the specified quantity of the substance being examined, 0.1 g of acid-washed sand and 20 ml of sulphuric acid (50% v/v). Place tetrachloroethane in the outer jacket and heat to maintain at its boiling point (146°). Attach a steam generator and distil, collecting the distillate in a 100-ml graduated flask containing 0.3 ml of 0.1M sodium hydroxide and 0.1 ml of phenolphthalein solution. Maintain a constant volume (20 ml) in the tube during distillation and ensure that the distillate remains alkaline, adding 0.1M sodium hydroxide if necessary. Dilute the distillate to 100 ml with water. Prepare a standard by distillation in the same manner, using 5 ml of fluoride standard solution (10 ppm F) in place of the substance being examined. Into two glass-stoppered cylinders separately place 20 ml of the test solution and 20 ml of the standard and add 5 ml of aminomethylalizarindiacetic acid reagent to each solution. After 20 minutes any blue colour in the test solution (originally red) is not more intense than that in the standard solution.

6SiO₂+6HF=H₂[SiF₆]+2H₂O

H₂[SiF₆]+2NaOH= Na₂[SiF₆]+2H₂O

Limit Tests for Heavy Metals

Test A To 12 ml of the prescribed aqueous solution add 2 ml of acetate buffer pH 3.5, mix, add 1.2 ml of thioacetamide reagent, mix immediately and allow to stand for 2 minutes. Any brown colour produced is not more intense than that obtained by treating in the same manner a mixture of 10 ml of either lead standard solution (1 ppm Pb) or lead standard solution (2 ppm Pb), as prescribed, and 2 ml of the solution being examined. The standard solution exhibits a slightly brown colour when compared to a solution prepared by treating in the same manner a mixture of 10 ml of water and 2 ml of the solution being examined.

Method B

Test solution 12 ml of the prescribed solution of the substance to be examined prepared using an organic solvent containing a minimum percentage of water (for example, dioxan containing 15 per cent of water or acetone containing 15 per cent of water).

Reference solution (standard) A mixture of 10 ml of lead standard solution (1 or 2 ppm Pb), as prescribed, and 2 ml of the prescribed solution of the substance to be examined in an organic solvent. Prepare the lead standard solution (1 or 2 ppm Pb) by dilution of lead standard solution (100 ppm Pb) R with the solvent used for the substance to be examined.

Blank solution A mixture of 10 ml of the solvent used for the substance to be examined and 2 ml of the prescribed solution of the substance to be examined in an organic solvent. To each solution, add 2 ml of buffer solution pH 3.5 R. Mix. Add 1.2 ml of thioacetamide reagent R. Mix immediately. Examine the solutions after 2 min. The test is invalid if the reference solution does not show a slight brown colour compared to the blank solution. The substance to be examined complies with the test if any brown colour in the test solution is not more intense than that in the reference solution.

If the result is difficult to judge, filter the solutions through a membrane filter (pore size 3 µm; see Figure 2.4.8.-1, without the prefilter). Carry out the filtration slowly and uniformly, applying moderate and constant pressure to the piston. Compare the spots on the filters obtained with the different solutions.

Method C

Test solution Place the prescribed quantity (not more than 2 g) of the substance to be examined in a silica crucible with 4 ml of a 250 g/l solution of magnesium sulphate R in dilute sulphuric acid R. Mix using a fine glass rod. Heat cautiously. If the mixture is liquid, evaporate gently to dryness on a water-bath. Progressively heat to ignition and continue heating until an almost white or at most greyish residue is obtained. Carry out the ignition at a temperature not exceeding 800 °C. Allow to cool. Moisten the residue with a few drops of dilute sulphuric acid R. Evaporate, ignite again and allow to cool. The total period of ignition must not exceed 2 h. Take up the residue in 2 quantities, each of 5 ml, of dilute hydrochloric acid R. Add 0.1 ml of phenolphthalein solution R, then concentrated ammonia R until a pink colour is obtained. Cool, add glacial acetic acid R until the solution is decolorised and add 0.5 ml in excess. Filter if necessary and wash the filter. Dilute to 20 ml with water R.

Reference solution (standard) Prepare as described for the test solution, using the prescribed volume of lead standard solution (10 ppm Pb) R instead of the substance to be examined. To 10 ml of the solution obtained add 2 ml of the test solution.

Monitor solution Prepare as described for the test solution, adding to the substance to be examined the volume of lead standard solution (10 ppm Pb) R prescribed for preparation of the reference solution. To 10 ml of the solution obtained add 2 ml of the test solution.

Blank solution A mixture of 10 ml of water R and 2 ml of the test solution.

To 12 ml of each solution, add 2 ml of buffer solution pH 3.5 R. Mix. Add 1.2 ml of thioacetamide reagent R. Mix immediately. Examine the solutions after 2 min. The test is invalid if the reference solution does not show a slight brown colour compared to the blank solution or if the monitor solution is not comparable with the reference solution. The substance to be examined complies with the test if any brown colour in the test solution is not more intense than that in the reference solution.

Method D

Test solution In a silica crucible, mix thoroughly the prescribed quantity of the substance to be examined with 0.5 g of magnesium oxide R1. Ignite to dull redness until a homogeneous white or greyish-white mass is obtained. If after 30 min of ignition the mixture remains coloured, allow to cool, mix using a fine glass rod and repeat the ignition. If necessary repeat the operation. Heat at 800 °C for about 1 h. Take up the residue in 2 quantities, each of 5 ml, of a mixture of equal volumes of hydrochloric acid R1 and water R. Add 0.1 ml of phenolphthalein solution R and then concentrated ammonia R until a pink colour is obtained. Cool, add glacial acetic acid R until the solution is decolorised and add 0.5 ml in excess. Filter if necessary and wash the filter. Dilute to 20 ml with water R.

Reference solution (standard) Prepare as described for the test solution using the prescribed volume of lead standard solution (10 ppm Pb) R instead of the substance to be examined and drying in an oven at 100-105 °C. To 10 ml of the solution obtained add 2 ml of the test solution.

Monitor solution Prepare as described for the test solution, adding to the substance to be examined the volume of lead standard solution (10 ppm Pb) R prescribed for preparation of the reference solution and drying in an oven at 100-105 °C. To 10 ml of the solution obtained add 2 ml of the test solution.

Blank solution A mixture of 10 ml of water R and 2 ml of the test solution.

Method E

Test solution Dissolve the prescribed quantity of the substance to be examined in 30 ml of water R or the prescribed volume.

Reference solution (standard) Unless otherwise prescribed, dilute the prescribed volume of lead standard solution (1 ppm Pb) R to the same volume as the test solution. Prepare the filtration apparatus by adapting the barrel of a 50 ml syringe without its piston to a support containing, on the plate, a membrane filter (pore size 3 µm) and above it a prefilter (Figure 2.4.8.-1).

Transfer the test solution into the syringe barrel, put the piston in place and then apply an even pressure on it until the whole of the liquid has been filtered. In opening the support and removing the prefilter, check that the membrane filter remains uncontaminated with impurities. If this is not the case replace it with another membrane filter and repeat the operation under the same conditions.

To the prefiltrate or to the prescribed volume of the prefiltrate add 2 ml of buffer solution pH 3.5 R. Add to 1.2 ml of thioacetamide reagent R. Mix and allow to stand for 10 min and again filter as described above, but inverting the order of the filters, the liquid passing first through the membrane filter before passing through the prefilter (Figure 2.4.8.-1). The filtration must be carried out slowly and uniformly by applying moderate and constant pressure to the piston of the syringe. After complete filtration, open the support, remove the membrane filter, and dry using filter paper.

In parallel, treat the reference solution in the same manner as the test solution.

The colour of the spot obtained with the test solution is not more intense than that obtained with the reference solution.

Method F

Test solution Place the prescribed quantity or volume of the substance to be examined in a clean, dry, 100 ml long-necked combustion flask (a 300 ml flask may be used if the reaction foams excessively). Clamp the flask at an angle of 45°. If the substance to be examined is a solid, add a sufficient volume of a mixture of 8 ml of sulphuric acid R and 10 ml of nitric acid R to moisten the substance thoroughly; if the substance to be examined is a liquid, add a few millilitres of a mixture of 8 ml of sulphuric acid R and 10 ml of nitric acid R. Warm gently until the reaction commences, allow the reaction to subside and add additional portions of the same acid mixture, heating after each addition, until a total of 18 ml of the acid mixture has been added. Increase the amount of heat and boil gently until the solution darkens. Cool, add 2 ml of nitric acid R and heat again until the solution darkens. Continue the heating, followed by the addition of nitric acid R until no further darkening occurs, then heat strongly until dense, white fumes are produced. Cool, cautiously add 5 ml of water R, boil gently until dense, white fumes are produced and continue heating to reduce to 2-3 ml. Cool, cautiously add 5 ml of water R and examine the colour of the solution. If the colour is yellow, cautiously add 1 ml of strong hydrogen peroxide solution R and again evaporate until dense, white fumes are produced and reduce to a volume of 2-3 ml. If the solution is still yellow in colour, repeat the addition of 5 ml of water R and 1 ml of strong hydrogen peroxide solution R until the solution is colourless. Cool, dilute cautiously with water R and rinse into a 50 ml colour comparison tube, ensuring that the total volume does not exceed 25 ml. Adjust the solution to pH 3.0-4.0, using short range pH indicator paper as external indicator, with concentrated ammonia R1 (dilute ammonia R1 may be used, if desired, as the specified range is approached), dilute with water R to 40 ml and mix. Add 2 ml of buffer solution pH 3.5 R and 1.2 ml of thioacetamide reagent R. Mix immediately. Dilute to 50 ml with water R and mix.

Reference solution (standard) Prepare at the same time and in the same manner as the test solution, using the prescribed volume of lead standard solution (10 ppm Pb) R.

Monitor solution Prepare as described for the test solution, adding to the substance to be examined the volume of lead standard solution (10 ppm Pb) R prescribed for the preparation of the reference solution.

Blank solution Prepare as described for the test solution, omitting the substance to be examined.

Examine the solutions vertically against a white background. After 2 min, any brown colour in the test solution is not more intense than that in the reference solution.

The test is invalid if the reference solution does not show a brown colour compared to the blank solution or if the monitor solution is not comparable with the reference solution.

If the result is difficult to judge, filter the solutions through a membrane filter (pore size 3 µm; see Figure 2.4.8-1, without the prefilter). Carry out the filtration slowly and uniformly, applying moderate and constant pressure to the piston. Compare the spots on the filters obtained with the different solutions.

Method G

CAUTION: when using high-pressure digestion vessels the safety precautions and operating instructions given by the manufacturer must be followed. The digestion cycles have to be elaborated depending on the type of microwave oven to be used (for example, energy-controlled microwave ovens, temperature-controlled microwave ovens or high-pressure ovens). The cycle must be conform to the manufacturer’s instructions. The digestion cycle is suitable if a clear solution is obtained.

Test solution Place the prescribed amount of the substance to be examined (not more than 0.5 g) in a suitable, clean beaker. Add successively 2.7 ml of sulphuric acid R, 3.3 ml of nitric acid R and 2.0 ml of strong hydrogen peroxide solution R using a magnetic stirrer. Allow the substance to react with a reagent before adding the next one. Transfer the mixture to a dry high-pressure-resistant digestion vessel (fluoropolymer or quartz glass).

Reference solution (standard) Prepare as described for the test solution, using the prescribed volume of lead standard solution (10 ppm Pb) R instead of the substance to be examined.

Monitor solution Prepare as prescribed for the test solution, adding to the substance to be examined the volume of lead standard solution (10 ppm Pb) R prescribed for the preparation of the reference solution.

Blank solution Prepare as described for the test solution, omitting the substance to be examined.

Close the vessels and place in a laboratory microwave oven. Digest using a sequence of 2 separate suitable programmes. Design the programmes in several steps in order to control the reaction, monitoring pressure, temperature or energy depending on the type of microwave oven available. After the first programme allow the digestion vessels to cool before opening. Add to each vessel 2.0 ml of strong hydrogen peroxide solution R and digest using the second programme. After the second programme allow the digestion vessels to cool before opening. If necessary to obtain a clear solution, repeat the addition of strong hydrogen peroxide solution R and the second digestion programme.

Cool, dilute cautiously with water R and rinse into a flask, ensuring that the total volume does not exceed 25 ml.

Using short-range pH indicator paper as external indicator, adjust the solutions to pH 3.0-4.0 with concentrated ammonia R1 (dilute ammonia R1 may be used as the specified range is approached). To avoid heating of the solutions use an ice-bath and a magnetic stirrer. Dilute to 40 ml with water R and mix. Add 2 ml of buffer solution pH 3.5 R and 1.2 ml of thioacetamide reagent R. Mix immediately. Dilute to 50 ml with water R, mix and allow to stand for 2 min.

Filter the solutions through a membrane filter (pore size 3 µm; see Figure 2.4.8.-1, without the prefilter). Carry out the filtration slowly and uniformly, applying moderate and constant pressure to the piston. Compare the spots on the filters obtained with the different solutions.

Examine the spots on the filters. The brown colour from the spot of the test solution is not more intense than that from the reference solution.

The test is invalid if the reference solution spot does not show a brown colour compared to the blank spot, or if the spot from the monitor solution is not comparable with the spot from the reference solution.

Limit Test for Iron

Dissolve the specified quantity of the substance being examined in sufficient water to produce 10 ml or use 10 ml of the solution prescribed in the monograph and transfer to a Nessler cylinder. Add 2 ml of a 20% w/v solution of citric acid and 0.1 ml of mercaptoacetic acid, mix, make alkaline with 10M ammonia, dilute to 20 ml with water and allow to stand for 5 minutes. Any pink colour produced is not more intense than that obtained by treating 10 ml of iron standard solution (1 ppm Fe) in the same manner.

Fe³⁺+2HS-CH₂COOH+5NH₃*H₂O=[Fe(OH)(SCH₂COO^–)₂]^2-+5NH₄⁺+4H₂O

Limit Test for Magnesium

To 10 ml of a solution prepared as specified in the monograph add 0.1 g of sodium tetraborate. Adjust the pH of the solution, if necessary, to 8.8 to 9.2 with 2M hydrochloric acid or 2M sodium hydroxide. Shake with two 5-ml quantities of a 0.1% w/v solution of 8-hydroxyquinoline in chloroform, shaking for 1 minute, allowing to stand, separating and discarding the organic layer each time. To the aqueous layer add 0.4 ml of n-butylamine and 0.1 ml of triethanolamine. Adjust the pH of the solution, if necessary, to 10.5 to 11.5. Add 4 ml of the solution of 8-hydroxyquinoline, shake for 1 minute, allow to stand and separate. Any colour produced in the lower layer is not more intense than that obtained by treating a mixture of 1 ml of magnesium standard solution (10 ppm Mg) and 9 ml of water in the same manner.

Limit Test for Magnesium and Alkaline-earth Metals

To 200 ml of water add 0.1 g of hydroxylamine hydrochloride, 10 ml of ammonia buffer pH 10.0, 1 ml of 0.1M zinc sulphate and about 15 mg of mordant black 11 triturate. Heat to about 40° and titrate with 0.01M disodium edetate VS until the violet colour changes to a full blue. To the solution add the specified quantity of the substance being examined dissolved in 100 ml of water or use the prescribed solution. If the colour of the solution changes to violet, titrate with 0.01M disodium edetate VS until the full blue colour is again produced. The volume of 0.01M disodium edetate VS used in the second titration does not exceed the prescribed quantity.

Mordant black 11 triturate R at рН 10 (which is formed by means of addition of ammonium chloride buffer solution) react with Magnesium, Zinc, Alkaline-earth Metals and violet complex MeInd:

H₂Ind +Mg²⁺ ® MgInd + 2H⁺

blue violet

At titration impurities of Magnesium and Alkaline–earth Metals with 0.01 M sodium edetate a very strong colourless, soluble in water, complexes is formed:

Excess drop of sodium edetate destroys complex MeInd and complex H₂Ind is formed again and the full blue colour is again obtained.

Limit Test for Phosphates

To 100 ml of the solution prepared and, if necessary, neutralised as prescribed add 4 ml of sulphomolybdic reagent R3, shake, add 0.1 ml of tin(II) chloride solution R1, allow to stand for 10 minutes and examine 20 ml of the resulting solution. Any colour produced is not more intense than that produced in 20 ml of a solution obtained by treating a mixture of 2 ml of phosphate standard solution (5 ppm PO₄) and 98 ml of water in the same manner.

PO₄^3-+12(NH₄)₂MoO₄ +27H⁺=H₇[P(Mo₂O₇)₆]+24H⁺+10H₂O

Limit Test for Potassium

To 10 ml of the prescribed solution add 2 ml of a freshly prepared 1% w/v solution of sodium tetraphenylborate and allow to stand for 5 minutes. Any opalescence produced is not more intense than that obtained by treating a mixture of 5 ml of potassium standard solution (20 ppm K) and 5 ml of water in the same manner.

K⁺+NaB(C₆H₅)₄=KB(C₆H₅)₄+Na⁺

Limit Test for Sulphates

The solutions used for this test should be prepared with distilled water.

Add 1 ml of a 25% w/v solution of barium chloride to 1.5 ml of sulphate standard solution (10 ppm SO₄) R1, shake and allow to stand for 1 minute. Add 15 ml of the prescribed solution or a solution of the specified quantity of the substance being examined in 15 ml of water and 0.5 ml of 5M acetic acid and allow to stand for 5 minutes. Any opalescence produced is not more intense than that of a standard prepared in the same manner but using 15 ml of sulphate standard solution (10 ppm SO₄) in place of the solution being examined.

Zinc –––––––––––––––––––––––––––––––––––––––N (SPU)

To 10 ml of the prescribed solution add 2,0 ml hydrochloric acid solution R1 and 0,2 ml potassium ferrocyanide R.

Prepare a standard in the same manner using a mixture of 10 ml of zinc standard solution (5 ppm Zn) R.

After 10 min, any opalescence in the test solution is not more intense than that in the standard.

Zinc-ions react with potassium ferrocyanide solution K₄[Fe(CN)₆] and white precipitate or opalescence is formed:

3Zn²⁺ + 2K⁺ + 2[Fe(CN)₆]^4– ® K₂Zn₃[Fe(CN)₆]₂¯

white precipitate

Other general impurities:

Limit Test for Lead in Sugars

(Ph. Eur. method 2.4.10)

Determine the lead by atomic absorption spectrometry (2.2.23, Method II).

Test solution Dissolve 20.0 g of the substance to be examined in a mixture of equal volumes of dilute acetic acid R and water R and dilute to 100.0 ml with the same mixture of solvents. Add 2.0 ml of a clear 10 g/l solution of ammonium pyrrolidinedithiocarbamate R and 10.0 ml of methyl isobutyl ketone R and then shake for 30 s protected from bright light. Allow the layers to separate and use the methyl isobutyl ketone layer.

Reference solutions Prepare 3 reference solutions in the same manner as the test solution but adding 0.5 ml, 1.0 ml and 1.5 ml respectively of lead standard solution (10 ppm Pb) R in addition to the 20.0 g of the substance to be examined.

Set the zero of the instrument using methyl isobutyl ketone R treated as described for the test solution without the substance to be examined. Measure the absorbance at 283.3 nm using a lead hollow-cathode lamp as source of radiation and an air-acetylene flame.

The substance to be examined contains not more than 0.5 ppm of lead, unless otherwise prescribed.

Limit Test for Heavy Metals in Herbal Drugs and Fatty Oils

(Ph. Eur. method 2.4.27)

Examine by atomic absorption spectrometry(2.2.23).

CAUTION: when using closed high-pressure digestion vessels and microwave laboratory equipment, be familiar with the safety and operating instructions given by the manufacturer.

Apparatus

The apparatus typically consists of the following:

—as digestion flasks, polytetrafluoroethylene flasks with a volume of about 120 ml, fitted with an airtight closure, a valve to adjust the pressure inside the container and a polytetrafluoroethylene tube to allow release of gas,

—a system to make flasks airtight, using the same torsional force for each of them,

—a microwave oven, with a magnetron frequency of 2450 MHz, with a selectable output from 0 to 630 ± 70 W in 1 per cent increments, a programmable digital computer, a polytetrafluoroethylene-coated microwave cavity with a variable speed exhaust fan, a rotating turntable drive system and exhaust tubing to vent fumes,

—an atomic absorption spectrometer, equipped with hollow-cathode lamps as source of radiation and a deuterium lamp as background corrector; the system is fitted with:

(a) a graphite furnace as atomisation device for cadmium, copper, iron, lead, nickel and zinc.

(b) an automated continuous-flow hydride vapour generation system for arsenic and mercury.

Method

In case alternative apparatus is used, an adjustment of the instrument parameters may be necessary.

Clean all the glassware and laboratory equipment with a 10 g/l solution of nitric acid R before use.

Test solution In a digestion flask place the prescribed quantity of the substance to be examined (about 0.50 g of powdered drug (1400) or 0.50 g of fatty oil). Add 6 ml of heavy metal-free nitric acid R and 4 ml of heavy metal-free hydrochloric acid R. Make the flask airtight.

Place the digestion flasks in the microwave oven. Carry out the digestion in 3 steps according to the following programme, used for 7 flasks each containing the test solution: 80 per cent power for 15 min, 100 per cent power for 5 min, 80 per cent power for 20 min.

At the end of the cycle allow the flasks to cool in air and to each add 4 ml of heavy metal-free sulphuric acid R. Repeat the digestion programme. After cooling in air, open each digestion flask and introduce the clear, colourless solution obtained into a 50 ml volumetric flask. Rinse each digestion flask with 2 quantities, each of 15 ml, of water R and collect the rinsings in the volumetric flask. Add 1.0 ml of a 10 g/l solution of magnesium nitrate R and 1.0 ml of a 100 g/l solution of ammonium dihydrogen phosphate R and dilute to 50.0 ml with water R.

Blank solution Mix 6 ml of heavy metal-free nitric acid R and 4 ml of heavy metal-free hydrochloric acid R in a digestion flask. Carry out the digestion in the same manner as for the test solution.

Cadmium, copper, iron, lead, nickel and zinc

Measure the content of cadmium, copper, iron, lead, nickel and zinc by the standard additions method (2.2.23, Method II), using reference solutions of each heavy metal and the instrumental parameters described in Table 2.4.27.-1.

The absorbance value of the blank solution is automatically subtracted from the value obtained with the test solution.

Arsenic and mercury

Measure the content of arsenic and mercury in comparison with the reference solutions of arsenic or mercury at a known concentration by direct calibration (2.2.23, Method I) using an automated continuous-flow hydride vapour generation system.

The absorbance value of the blank solution is automatically subtracted from the value obtained with the test solution.

Arsenic

Sample solution To 19.0 ml of the test solution or of the blank solution as prescribed above, add 1 ml of a 200 g/l solution of potassium iodide R. Allow the test solution to stand at room temperature for about 50 min or at 70 °C for about 4 min.

Acid reagent heavy metal-free hydrochloric acid R.

Reducing reagent A 6 g/l solution of sodium tetrahydroborate R in a 5 g/l solution of sodium hydroxide R.

The instrumental parameters in Table 2.4.27.-2 may be used.

Mercury

Sample solution Test solution or blank solution, as prescribed above.

Acid reagent A 515 g/l solution of heavy metal-free hydrochloric acid R.

Reducing reagent A 10 g/l solution of stannous chloride R in dilute heavy metal-free hydrochloric acid R.

The instrumental parameters in Table 2.4.27.-2 may be used.

Limit Test for Nickel in Polyols

(Ph. Eur. method 2.4.15)

Determine the nickel by atomic absorption spectrometry (2.2.23, Method II).

Test solution Dissolve 20.0 g of the substance to be examined in a mixture of equal volumes of dilute acetic acid R and water R and dilute to 100.0 ml with the same mixture of solvents. Add 2.0 ml of a saturated solution of ammonium pyrrolidinedithiocarbamate R (about 10 g/l) and 10.0 ml of methyl isobutyl ketone R and then shake for 30 s protected from bright light. Allow the layers to separate and use the methyl isobutyl ketone layer.

Reference solutions Prepare 3 reference solutions in the same manner as the test solution but adding 0.5 ml, 1.0 ml and 1.5 ml respectively of nickel standard solution (10 ppm Ni) R in addition to the 20.0 g of the substance to be examined.

Set the zero of the instrument using methyl isobutyl ketone R treated as described for preparation of the test solution omitting the substance to be examined. Measure the absorbance at 232.0 nm using a nickel hollow-cathode lamp as source of radiation and an air-acetylene flame.

The substance to be examined contains not more than 1 ppm of nickel, unless otherwise prescribed.