Methods of Analysis: 2. Chemical methods: 2.2 Limit tests: 2.2.3 Limit test for heavy metals

The limit test for heavy metals is provided to demonstrate that the content of metallic impurities that are coloured by hydrogen sulfide does not exceed the heavy metals limit given in the individual monograph in terms of micrograms of lead per gram of the test substance.

The test consists of two consecutive operations: preparation of the test solution, and the colour development by reaction with hydrogen sulfide, followed by comparison of the colour obtained with that produced with standard lead solution.

The preparation of the test solution is carried out, as specified in the monograph, according to procedures 1 to 4 described below. A blank is prepared in a similar manner.

The reaction with hydrogen sulfide is carried out by mixing the test solution with freshly prepared hydrogen sulfide TS. The comparison of the colour thus obtained is carried out either by directly comparing the coloration of the liquid in suitable comparison tubes (Method A) or by comparing the intensity of coloration of spots obtained by filtering the liquid using an appropriate apparatus (Method B).

Method A is generally applicable only when the amount of heavy metals in the weight of test substance used exceeds 5 μg; for amounts of 2-5 μg of heavy metals Method B should be used.

The standard lead solution used in the test; dilute lead PbTS contains 10 μg of lead in 1 ml. When 0.1 ml of this solution is employed to prepare the standard for comparison with a solution of 1 g of the substance being tested, the standard solution thus prepared contains 1 μg of Pb and represents the equivalent of 1 μg of lead per g of the substance tested.

Apparatus

For determination of heavy metals by Method A carry out the test in matched flat-bottomed comparison tubes of transparent glass of about 70 ml capacity and about 23 mm internal diameter bearing a 40-ml and a 50-ml mark. Nessler cylinders complying with the above dimensions are suitable. The expression "matched tubes" means tubes that are matched as closely as possible in internal diameter and in all other respects. For mixing the solution use a stirring rod preferably having a loop at the lower end.

For determination of heavy metals by Method B use a 50-ml syringe made of suitable material (usually plastic) with a detachable plunger and a male Luer conical joint of 9 mm internal diameter at the lower end (Millipore syringe XX 11 050 05 is suitable) to which an adaptor for filtration is attached.

The adaptor is made of suitable material (a filtration adapter Millipore SX00 013 00 in polypropylene is suitable) and has a female joint for connecting it with the syringe. It is devised so as to be separable into two parts to permit the exchange of filters, the lower part containing a support for membrane filters 13 mm in diameter. A suitable prefilter (Millipore prefilter AP 2001 300 is suitable) and a membrane filter made of mixed cellulose esters, 13 mm in diameter, with a pore opening of 3 μm (a Millipore filter SSWP 013 00 is suitable) are used for the filtration.

Recommended procedure

Preparation of test solution

Procedure 1. Weigh the quantity of substance specified in the monograph, dissolve it in 25 ml of water, adjust the pH of the solution to 3-4 with acetic acid (~60 g/l) PbTS, or with ammonia (~100 g/l) PbTS, as necessary, then dilute to 40 ml with water and mix.

Procedure 2. Weigh the quantity of substance specified in the monograph, dissolve it in about 30 ml of solvent specified (ethanol (~750 g/l) TS, methanol R, acetone R, or dioxan R may be used), add 0.5 ml of acetic acid (~300 g/l) TS, and dilute to 40 ml with the solvent.

Procedure 3. Place the quantity of the substance specified in the monograph in a suitable crucible, preferably made of silica, and carefully ignite at a low temperature until the contents are thoroughly charred. The crucible may be loosely covered with a lid during the charring. Add to the contents of the crucible 2 ml of nitric acid (~1000 g/l) TS and 5 drops of sulfuric acid (~1760 g/l TS), and cautiously heat until white fumes are evolved, and then ignite, preferably in a muffle furnace, at 500°C until all the carbon is burned off. Cool, add 2 ml of hydrochloric acid (~250 g/l) TS, and slowly evaporate in a water-bath to dryness. Moisten the residue with 1 drop of hydrochloric acid (~250 g/l) TS, add 10 ml of hot water, and digest for 2 minutes. Add, drop by drop, ammonia (~100 g/l) PbTS, until the pH of the solution is between 8 and 8.5, then add, drop by drop, acetic acid (~60 g/l) PbTS, to adjust the pH to between 3 and 4. Filter if necessary, wash the crucible and the filter with about 10 ml of water, dilute with water to 40 ml, and mix.

Procedure 4. Place the quantity of substance specified in the monograph in a suitable crucible, preferably made of silica, mix it well with about 0.5 g of magnesium oxide R and incinerate until a homogeneous white mass is obtained. If after 15 minutes of incineration the residue is still coloured, let the crucible cool, mix the contents well with a glass rod and resume heating. Next, dissolve the residue in hydrochloric acid (~70 g/l) TS, add, drop by drop, a solution of ammonia (~100 g/l) PbTS, until the pH of the solution is between 8 and 8.5, then add, also drop by drop, acetic acid (~60 g/l) PbTS, to adjust the pH to 3-4, filter, dilute with water to 40 ml, and mix.

Colour development and measurement

Method A

To 40 ml of the liquid contained in the comparison tube add 10 ml of freshly prepared hydrogen sulfide TS, mix and allow to stand for 5 minutes.

In another comparison tube place a volume of solution of dilute lead PbTS, containing the lead equivalent of the heavy metals limit specified in the monograph, dilute with water, adjust the pH with ammonia (~100 g/l) PbTS and acetic acid (~60 g/l) PbTS to 3-4; dilute with water or the solvent used to 40 ml, mix, add 10 ml of freshly prepared hydrogen sulfide TS, mix and allow to stand for 5 minutes.

Compare the colours by viewing down the vertical axis of the tube in diffused light against a white background, or by another suitable method. The colour of the test solution is not darker than that of the lead standard.

Method B

Take the filtration syringe, arrange the prefilter and the membrane filter as indicated in the diagram for prefiltration, remove the plunger from the syringe, place the test solution inside the syringe, replace the plunger, and filter the test solution slowly by exerting a regular pressure on the plunger. Collect the filtrate in a beaker or a test-tube. Open the adapter and check whether the membrane filter is free from impurities. If not, replace it and repeat the operation in the same manner. Then rearrange the prefilter and membrane filter as indicated in Fig. 4. Adjust the pH of the filtrate with ammonia (~100 g/l) PbTS and acetic acid (~60 g/l) PbTS to 3-4, add 10 ml of freshly prepared hydrogen sulfide TS, all the reagents previously filtered through a membrane filter, mix, allow to stand for 5 minutes, take out the plunger, place the solution inside the syringe, and filter it through the membrane filter by exerting slowly a regular and moderate pressure on the plunger. Open the adaptor and take out the membrane filter.

FIG. 4. LIMIT TEST FOR HEAVY METALS: METHOD B

Take a volume of solution of dilute lead PbTS containing the lead equivalent to the heavy metals limit specified in the monograph, dilute with water, adjust the pH with ammonia (~100 g/l) PbTS and acetic acid (~60 g/l) PbTS to 3-4, dilute with water to 40 ml, mix, and proceed as described above.

Compare the intensity of the coloration of spots obtained on the membrane filters. The colour obtained from the test solution is not more intense than that from the lead standard.

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