Category: Limit Tests

Title: Limit Test for Iron  

 

1. Objective: The limit test for iron is provided to demonstrate that the content of iron does not exceed the limit given in the individual monograph in terms of micrograms of iron per gram of the test substance. The standard solution against which the comparison of colour is made contains 40 μg of iron.

 

2. Principle: Limit test of Iron is based on the reaction of iron in ammonical solution with thioglycollic acid to form iron thioglycolate which is pink-reddish purple in colour.

 

3. Procedure:

• Preparation of Standard Solutions:

Note: All solutions should be prepared in Distilled/Purified Water.

• Ferric Ammonium Sulphate (0.1726%w/v): Dissolve 0.1726 g of Ferric Ammonium Sulphate in 05M Sulphuric acid and dilute to 100 ml with 0.05M Sulphuric acid.
• Iron Standard Solution (20 ppm Fe): Dilute 1 volume of Ferric Ammonium Sulphate (0.1726%w/v) to 10 volumes with water immediately before use.
• 05M Sulphuric acid: Add 2.7 ml of Sulphuric acid carefully with equal volume of water, cool and dilute to 1000 ml with water.
• Citric acid Solution (20% w/v): Dissolve 20.0 g of Citric acid in water and make up to 100 ml with water.

 

• Method:

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  and allow to stand for 5 minutes.

Limit: Any colour produced is not more intense than that obtained by treating in the same manner, using 2.0 ml of Iron Standard Solution (20 ppm Fe) in place of the solution under examination.

 

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Title: Limit Test for Potassium  

 

1. Objective: The limit test for potassium is provided to demonstrate that the content of Potassium does not exceed the limit given in the individual monograph in terms of ppm of potassium per gram of the test substance. The standard solution against which the comparison of opalescence is made contains 20 ppm of Potassium.

 

2. Principle: Limit test of potassium is based on the reaction of potassium and Sodium tetraphenylborate to form opalescence.

 3. Procedure:

• Preparation of Standard Solutions:

Note: All solutions should be prepared in Distilled/Purified Water.

• Potassium Standard Solution (100 ppm K): Dissolve 0.446 g of Potassium Sulfate in sufficient water and dilute to 100 ml with water. Immediately before use, dilute Dilute 1 volume of this solution to 20 volumes with water.
• Potassium Standard Solution (20 ppm K): Immediately before use, dilute Dilute 1 volume of Potassium Standard Solution (100 ppm K) to 5 volumes with water.

 

• Sodium Tetraphenylborate (1% w/v): Dissolve 1.0 g of Sodium Tetraphenylborate in sufficient water and dilute to 100 ml with water.
• Method:

To 10 ml of the prescribed solution add 2 ml of freshly prepared 1% w/v solution of Sodium tetraphenylborate. Prepare a standard solution in same manner using a mixture of Potassium Standard solution (20 ppm K) and 5 ml of water.

Limit: After five minutes any opalescence in the test solution is not more intense than in the standard solution.

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Title: Limit Test for Sulphates

1. Objective: The limit test for Sulphates is provided to demonstrate that the content of sulphate does not exceed the limit given in the individual monograph in terms of ppm of sulphate per gram of the test substance. The standard solution against which the comparison of opalescence is made contains 10 ppm of Sulphate.

 

2. Principle: Limit test of sulphate is based on the reaction between sulphate and barium chloride solution in presence of Ethanolic sulphate standard solution to form opalescence.

 3. Procedure:

• Preparation of Standard Solutions:

Note: All solutions should be prepared in Distilled/Purified Water.

• Potassium Sulphate (181% w/v): Dissolve 0.181 g of Potassium Sulphate in sufficient water and dilute to 100 ml with water.
• Sulphate Standard Solution (10 ppm SO₄): Dilute 1 volume of Potassium Sulphate (181% w/v) in distilled water to 100 volumes with distilled water.
• Ethanolic Sulphate Standard Solution (10 ppm SO₄): Dilute 1 volume of Potassium Sulphate (181% w/v) in ethanol (30%v/v) to 100 volumes with ethanol (30%v/v).
• Barium Chloride Solution (25% w/v): Dissolve 25.0 g of Barium Chloride in sufficient water and dilute to 100 ml with water.
• 5M Acetic acid: Dilute 28.5 ml of Glacial Acetic Acid to 100 ml with water.

 

• Method:

To 1.0 ml of a 25 % w/v solution of Barium Chloride in a Nessler cylinder add 1.5 ml of Ethanolic Sulphate Standard Solution (10 ppm SO₄), mix and allow to stand for one minute. Add 15 ml of the solution prepared as directed in the monograph or a solution of the specified quantity of substance under examination in 15 ml of water and 0.15 ml of 5M Acetic Acid. Add sufficient water to produced 50 ml, stir immediately with a glass rod and allow to stand for five minutes.

Limit: When viewed transversely against a black background any opalescence produced in not more intense than that obtained by treating in same manner 15 ml of Sulphate Standard Solution (10 ppm SO₄) in place of the solution under examination.

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Title: Limit Test for Heavy Metals 

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

 

2. Principle: Limit test of heavy metals is based on the reaction of metallic impurities with hydrogen sulphide in acidic medium to form colored solution. Metals that response to this test are lead, mercury, bismuth, arsenic, antimony, tin, cadmium, silver, copper, and molybdenum.

 

 3. Procedure:

• Preparation of Standard Solutions:

Note: All solutions should be prepared in Distilled/Purified Water.

• Lead Standard Solution (1% Pb): Dissolve 0.4 g of lead nitrate in water containing 2 ml conc. nitric acid and add sufficient water to produce 250.0 ml.
• Lead Standard Solution (100 ppm Pb): Dilute 1 volume of lead standard solution (1% Pb) to 10 volumes with water immediately before use.
• Lead Standard Solution (20 ppm Pb): Dilute 2 volume of lead standard solution (100 ppm Pb) to 10 volumes with water immediately before use.
• Lead Standard Solution (10 ppm Pb): Dilute 1 volume of lead standard solution (100 ppm Pb) to 10 volumes with water immediately before use.
• Lead Standard Solution (2 ppm Pb): Dilute 1 volume of lead standard solution (10 ppm Pb) to 5 volumes with water immediately before use.
• Lead Standard Solution (1 ppm Pb): Dilute 1 volume of lead standard solution (10 ppm Pb) to 10 volumes with water immediately before use.
• Thioacetamide Reagent: Add 1 ml of a mixture of 15 ml of 1M sodium hydroxide, 5 ml of water  and 20 ml of glycerol  (85%) to 0.2 ml of thioacetamide solution, heat in a water  bath for 20 seconds, cool and use immediately.
• Thioacetamide Solution (4% w/v): Dissolve 4g of Thioacetamide in water and dilute up to 100 ml with water.
• Hydrogen Sulphide Solution: A freshly prepared saturated solution of Hydrogen Sulphide in water.
• Acetate Buffer pH 3.5: Dissolve 25 g of ammonium acetate in 25 ml of water and add 38 ml of 7M Hydrochloric acid. Adjust the pH to 3.5 with either 2M Hydrochloric acid or 6M Ammonia and dilute to 100 ml with water.
• Dilute Ammonia Solution: Dilute 42.5 ml of strong ammonia solution to 100 ml with
• Dilute Sodium Hydroxide (5.0%w/v): Dissolve 5.0 g of Sodium Hydroxide in sufficient water and dilute to 100 ml with water.
• 1M sodium hydroxide: Dissolve 4.0 g of Sodium Hydroxide in sufficient water and dilute to 100 ml with water.
• 7M Hydrochloric acid: Dilute 59.5 ml of Conc. Hydrochloric acid to 100 ml with
• 2M Hydrochloric acid: Dilute 17 ml of Conc. Hydrochloric acid to 100 ml with
• 6M Ammonia: Dilute 45 ml of strong ammonia solution to 100 ml with

 

• Method:

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.

Method A is generally applicable only when the amount of heavy metals in the weight of test substance used exceeds 5 μg.

 

• 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 being examined 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.

Limit: The colour 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 Conc. Nitric Acid and 5 drops of Conc. 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 drop wise 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.

Limit: The colour produced with the test solution is not more intense than that produced with the Standard solution.

 

• 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 being examined 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.

Limit: The colour 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.

Limit: The colour produced with the test solution is not more intense than that produced with the standard solution.

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Title: Limit Test for Chloride

1. Objective: The limit test for chlorides is provided to demonstrate that the content of chlorides does not exceed the limit given in the individual monograph in terms of micrograms of chloride ions per gram of the substance being tested. The standard solution against which the comparison of opalescence is made contains 250 μg of Cl^–.

 

2. Principle: Limit test of Chloride is based on the reaction of soluble chloride with Silver Nitrate in presence of dilute Nitric Acid to form silver chloride, which appears as solid particles (Opalescence) in the solution.

 3. Procedure:

• Definitions:
• Matched Tubes: Tubes that are matched as closely as possible in internal diameter and in all other respects.
• Preparation of Standard Solutions:

Note: All solutions should be prepared in Distilled/Purified Water.

 

 

• Chloride Standard Solution (25 ppm Cl): Weigh 0.0824 g of Sodium Chloride dissolve in about 70 ml of water and dilute to 100 ml with water (0.0824%w/v). Dilute 5 volume of 0.0824% w/v solution of Sodium Chloride to 100 volumes with water immediately before use.
• Dilute Nitric Acid: Dilute 10.6 ml of Nitric acid to 100 ml with
• 1 M Silver Nitrate: Dissolve 1.7 g Silver Nitrate in water and dilute up to 100 ml with water. (Preserve in Amber coloured bottle.)

 

• Method:

Carry out the test in matched flat-bottomed transparent glass Nessler cylinders of about 70 ml capacity and about 23 mm internal diameter bearing a 25-ml and a 50-ml mark. Dissolve the specified quantity of the substance under examination in purified water, or prepare a solution as directed in the individual monograph and transfer to a nessler cylinder. Add 10 ml of Dilute Nitric Acid (if Nitric Acid not used in test preparation) and dilute up to 50 ml with water. Add 1.0 ml of 0.1 M Silver Nitrate solution. Stir immediately with glass rod and allow to stand for 5 minutes. Protect from light.

Limit: When viewed transversely against black background any opalescence in the test solution is not more intense than that obtained by treating a mixture of 10.0 ml of Chloride standard solution (25 ppm) and 5 ml of water in the same manner.

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Title: Limit Test for Arsenic

 

1. Objective: The limit test for arsenic is provided to demonstrate that the content of arsenic does not exceed the limit given in the individual monograph in terms of micrograms of arsenic per gram of the test substance.

 

2. Principle: Limit test of Arsenic is based on the reaction of arsenic gas with hydrogen ion to form yellow stain on mercuric chloride paper in presence of reducing agents like potassium iodide. It is also called as Gutzeit test and requires special apparatus.

3. Procedure:

• Definitions:
• Matched Tubes: Tubes that are matched as closely as possible in internal diameter and in all other respects.

 

• Preparation of Standard Solutions:

Note: All solutions should be prepared in Distilled/Purified Water.

• Arsenic Standard Solution (10 ppm As): Dissolve 0.330 g of Arsenic trioxide in 5 ml of 2M Sodium Hydroxide and dilute to 250.0 ml with Purified water. Dilute 1 volume of this solution to 100 volumes with water immediately before use.
• Mercury (II) Chloride Paper: In a rectangular dish place a 5% w/v solution of Mercury(II)  Chloride in absolute ethanol  and immerse in it pieces of white  filter paper weighing 60 g m^–2 to 120 g m^–2 (Whatman No. 1 is suitable),  each measuring 20 cm × 15 mm and folded in two. Decant the excess liquid and allow the papers to dry, protected from light, by suspending them over a non-metallic thread. Cut away the folded edges to a width of 1 cm and similarly remove the outer edges. Cut the remaining strips into 15-mm squares or discs of 15 mm diameter.   Store in a glass-stoppered container wrapped with black paper.
• Lead Acetate Cotton: Immerse absorbent cotton in a mixture of 10 volumes of Lead Acetate solution and 1 volume of 2M Acetic Acid. Drain off the excess of liquid by placing it on several layers of filter paper without squeezing the cotton. Allow to dry at room temperature.  Store in an airtight container.
• Potassium Iodide Solution (6% w/v) (1M): Dissolve 16.6 g of potassium iodide and dilute up to 100 ml with Purified water.
• 2M Sodium Hydroxide: Dissolve 8.0 g of Sodium Hydroxide in sufficient water and dilute to 100 ml with water.
• 5% w/v solution of Mercury (II) Chloride: Dissolve 5.0 g of Mercury (II) Chloride in sufficient water and dilute to 100 ml with water.
• Lead Acetate solution (10%w/v): Dissolve 10.0 g of Lead Acetate in sufficient water and dilute to 100 ml with water.
• 2M Acetic Acid: Add 11.4 ml of glacial acetic acid carefully with equal volume of water and dilute to 100 ml with water.
• Apparatus:

The apparatus consists of 100 ml conical flask or bottle closed with  ground-glass stopper through which passes a glass tube about 200 mm long and of  internal diameter 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 mm 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 mg 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 mg 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 × 15 mm).

• Precautions:

Rate of evolution of the gas varies somewhat with different batches of granulated zinc Ast, the temperature may be adjusted to obtain a regular, but not too violent, evolution of gas.

The reaction may be accelerated by placing the apparatus on a warm surface, care being taken to ensure that the Mercuric Chloride paper remains quite dry throughout the test.

Between successive tests, the tube must be washed with dilute hydrochloric acid solution Ast, rinsed with water, and dried.

• Method:

To carry out the limit test for Arsenic, a solution is prepared from the test substance by a procedure specified in the monograph. This procedure assures that the solution in every case contains the whole of the arsenic (if any) present in the substance.

The standard stain against which the comparison is made contains 10 μg of As.

A stain equivalent to the 1 ml standard stain produced by operating on 10 g of a substance indicates that the amount of arsenic is 1 μg/g.

In the statements of Arsenic limits, the permitted amount of arsenic is expressed as As.

In the conical flask or bottle introduce the test solution prepared as directed in individual monograph. Add 5 ml of 1 M potassium Iodide solution and 10 g Zinc. Immediately assemble the two parts of the apparatus and immerse the flask in a bath of water at a temperature such that a uniform evolution of gas is maintained.

Limit: After 40 minutes any stain produced on the Mercuric Chloride paper is not more intense than that obtained by treating in a same manner 1.0 ml of Arsenic Standard Solution (10 ppm) diluted to 50 ml with water.

 

 

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Title: Limit Test for Aluminium 

 

1. Objective: The limit test for Aluminium is provided to demonstrate that the content of aluminium does not exceed the limit given in the individual monograph in terms of ppm, i.e., the parts of aluminum of the substance being examined.

 

2. Principle: Limit test of Aluminium is based on the intensity of the fluorescence of the test solution, standard solution and the blank using an excitant beam and a secondary filter with a transmission band or a monochromator set to transmit at this wavelength.

 

3. Procedure:

• Preparation of Standard Solutions:

Note: All solutions should be prepared in Distilled/Purified Water.

• Aluminium Potassium Sulphate (0.176% w/v): Dissolve 0.176 g of Aluminium Potassium Sulphate in water and dilute to 100 ml with water.
• Aluminium Standard Solution (2 ppm Al): Dilute 2 volumes of Aluminium Potassium Sulphate (0.176% w/v) in 0.1M Sulphuric acid to 100 volumes with water immediately before use.
• Aluminium Standard Solution (10 ppm Al): Dilute 1 volume of a 1.39% w/v solution of Aluminium Nitrate to 100 volumes with
• Hydroxyquinoline Solution (5% w/v): Dissolve 0.5 g of Hydroxyquinoline in Chloroform and dilute up to 100 ml with Chloroform.
• 1M Sulphuric acid: Add 5.4 ml of Sulphuric acid carefully with equal volume of water, cool and dilute to 1000 ml with water.
• 1M Sulphuric acid: Add 54 ml of Sulphuric acid carefully with equal volume of water, cool and dilute to 1000 ml with water.

 

• Method:

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 0.5% w/v solution of Hydroxyquinoline in Chloroform. Dilute the combined Chloroform solutions to 50.0 ml with Chloroform (Test Solution).

Prepare a standard 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 of the test solution (f₁), of the standard (f₂) and of the blank (f₃) using an excitant beam at 392 nm and a secondary filter with a transmission band centered on 518 nm or a monochromator set to transmit at this wavelength.

The fluorescence of the test solution: (f₁– f₃).

The fluorescence of the standard solution: (f₂– f₃).  

Limit: The fluorescence of the test solution is not greater than that of the standard solution.

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