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Limit Tests

Limit Test for Arsenic, Limit Test for Heavy Metals, Limit Test for Aluminium, Limit Test for Potassium and Sulphates, Limit Test for Iron and Lead.
Limit Test for Arsenic
The limit for arsenic is indicated in the individual monographs in terms of ppm, i.e., the parts of arsenic, As, per million parts (by weight) of the substance under examination. All reagents used for the test should have as low a content of arsenic as possible.

The apparatus (Fig.) consists of a 100-ml bottle or conical flask closed with a rubber or ground glass stopper through which passes a glass tube (about 20 cm x 5 mm). 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 3mm 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 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 surface, is placed in contact with the first and is held in position by two spiral springs or clips. 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 mercuric chloride paper large enough to cover the orifice of the tube (15 mm x 15 mm)

Method
Into the bottle or conical flask introduce the test solution prepared as directed in the individual monograph, add 5 ml of 1 M potassium iodide and 10 g of zinc AsT. Immediately assemble the apparatus and immerse the flask in a water-bath at a temperature such that a uniform evolution of gas is maintained. After 40 minutes any stain produced on the mercuric chloride paper is not more intense than that obtained by treating in the same manner 1.0 ml of arsenic standard solution (10 ppm As) diluted to 50 ml with water.

Limit Test for Heavy Metals
The limit for heavy metals is indicated in the individual monographs in terms of ppm, i.e., the parts of lead, Pb, per million parts (by weight) of the substance under examination.

Method A
Standard solution

Into a 50-ml Nessler cylinder pipette 1.0 ml of lead standard solution (20 ppm Pb) and dilute with water to 25 ml. Adjust with dilute acetic acid or dilute ammonia solution to a pH between 3.0 and 4.0, dilute with water to about 35 ml and mix.

Test solution
Into a 50-ml Nessler cylinder place 25 ml of the solution prepared for the test as directed in the individual monograph or dissolve the specified quantity of the substance under examination in sufficient water to produce 25 ml. Adjust with dilute acetic acid or dilute ammonia solution to a pH between 3.0 and 4.0, dilute with water to about 35 ml and mix.

Procedure
To each of the cylinders containing the standard solution and test solution respectively add 10 ml of freshly prepared hydrogen sulphide solution, mix, dilute to 50 ml with water, allow to stand for 5 minutes and view downwards over a white surface; the color produced with the test solution is not more intense than that produced with the standard solution.

Method B
Standard solution
Proceed as directed under Method A.

Test solution
Weigh in a suitable crucible the quantity of the substance specified in the individual monograph, add sufficient sulphuric acid to wet the sample, ignite carefully at a low temperature until thoroughly charred. Add to the charred mass 2 ml of nitric acid and 5 drops of sulphuric acid and heat cautiously until white fumes are no longer evolved. Ignite, preferably in a muffle furnace, at 500° to 600°, until the carbon is completely burnt off. Cool, add 4 ml of hydrochloric acid, cover, digest on a water-bath for 15 minutes, uncover and slowly evaporate to dryness on a water-bath. Moisten the residue with 1 drop of hydrochloric acid, add 10 ml of hot water and digest for 2 minutes. Add ammonia solution dropwise until the solution is just alkaline to litmus paper, dilute to 25 ml with water and adjust with dilute acetic acid to a pH between 3.0 and 4.0. Filter, if necessary, rinse the crucible and the filter with 10 ml of water, combine the filtrate and washings in a 50-ml Nessler cylinder, dilute with water to about 35 ml and mix.

Procedure
Proceed as directed under Method A.

Method C
Standard solution
Into a 50-ml Nessler cylinder pipette 1.0 ml of lead standard solution (20 ppm Pb), add 5 ml of dilute sodium hydroxide solution, dilute with water to 50 ml and mix.

Test solution
Into a 50-ml Nessler cylinder place 25 ml of the solution prepared for the test as directed in the individual monograph, or dissolve the specified quantity of the substance under examination in a mixture of 20 ml of water and 5 ml of dilute sodium hydroxide solution. Dilute with water to 50 ml and mix.

Procedure
To each of the cylinders containing the standard solution and the test solution respectively add 5 drops of sodium sulphide solution, mix, allow to stand for 5 minutes and view downwards over a white surface; the color produced with the test solution is not more intense than that produced with the standard solution.

Method D
Standard solution

Into a small Nessler cylinder pipette 10.0 ml of either lead standard solution (1 ppm Pb) or lead standard solution (2 ppm Pb).

Test solution
Prepare as directed in the individual monograph and pipette 12 ml into a small Nessler cylinder.

Procedure
To the cylinder containing the standard solution add 2.0 ml of the test solution and mix. To each of the cylinders add 2 ml of acetate buffer pH 3.5, mix, add 1.2 ml of thioacetamide reagent, allow to stand for 2 minutes and view downwards over a white surface; the color produced with the test solution is not more intense than that produced with the standard solution.

Limit Test for Aluminium
Place the test solution in a separating funnel and shake with 2 quantities, each of 20 ml, and then with one 10 ml quantity of a 0.5 per w/v solution of hydroxyquinoline in chloroform.

Dilute the combined chloroform solutions to 50.0 ml with chloroform. Prepare a standard solution, in the same manner, using the prescribed reference solution. Prepare a blank in the same manner using the prescribed blank solution.

Measure the intensity of the fluorescence (204.5) of the test solution (fl), of the standard solution (f2) and of the blank (f3), using an excitant beam at 392 nm and a secondary filter with a transmission band centred on 518 nm or a monochromator set to transmit at this wavelength.

The fluorescence of the test solution (fl- f3) of the test solution is not greater than that of the standard solution (f2- f3).

Limit Test for Potassium and Sulphates
Potassium

To 10 ml of the prescribed solution add 2 ml of a freshly prepared 1 percent w/v solution of sodium tetraphenyl borate. Prepare a standard solution, in the same manner, using a mixture of potassium standard solution (20 ppm K) and 5 ml of water. After 5 minutes, any opalescence in the test solution is not more intense than in the standard solution.

Sulfates
NOTE - The solutions used for this test should be prepared with distilled water.
To 1.0 ml of a 25.0 per cent w/v solution of barium chloride in a Nessler cylinder add 1.5 ml of ethanolic sulfate standard solution (10 ppm SO4 ), mix and allow to stand for 1 minute. Add 15 ml of the solution prepared as directed in the monograph or a solution of the specified quantity of the substance under examination in 15 ml of water and 0.15 ml of 5 M acetic acid. Add sufficient water to produce 50 ml, stir immediately with a glass rod and allow standing for 5 minutes.

When viewed transversely against a black background any opalescence produced is not more intense than that obtained by treating in the same manner 15 ml of sulfate standard solution (10 ppm SO4) in place of the solution under test.

Limit Test for Iron and Lead
Iron

Dissolve the specified quantity of the substance under examination in water, or prepare a solution as directed in the monograph, and transfer to a Nessler cylinder. Add 2 ml of a 20% w/v solution of iron-free citric acid and 0.1 ml of thioglycollic acid, mix, make alkaline with iron-free ammonia solution, dilute to 50 ml with water and allow standing for 5 minutes. Any color produced is not more intense than that obtained by treating in the same manner 2.0 ml of iron standard solution (20 ppm Fe) in place of the solution under examination.

Lead
The limit for lead is indicated in the individual monograph in terms of ppm, i.e., the parts of lead, Pb, per million parts (by weight) of the substance under examination.
The following method is based on the extraction of lead by solutions of dithizone.

All reagents used for the test should have as low a content of lead as practicable. All reagent solutions should be stored in containers of borosilicate glass. Glassware should be rinsed thoroughly with warm dilute nitric acid followed by water.

Method
Transfer the volume of the prepared sample directed in the monograph to a separator and, unless otherwise directed in monograph, add 6 ml of ammonium citrate solution and 2 ml of hydroxylamine hydrochloride solution. (For the determination of lead in iron salts use 10 ml of ammonium citrate solution). Add two drops of phenol red solution and make the solution just alkaline (red in color) by the addition of strong ammonia solution. Cool the solution if necessary and add 2 ml of potassium cyanide solution.

Immediately extract the solution with several quantities, each of 5 ml, of dithizone extraction solution, draining off each extract into another separating funnel, until the dithizone extraction solution retains its green color. Shake the combined dithizone solution for 30 seconds with 30 ml of a 1 percent v/v solution of nitric acid and discard the chloroform layer.

Add to the acid solution exactly 5 ml of dithizone standard solution and shake for 30 seconds; the color of the chloroform layer is not more intense than that obtained by treating, in the same manner, a volume of lead standard solution (1ppm Pb) equivalent to the amount of lead permitted in the substance under examination, in place of the solution under examination.
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