LIMIT TEST

Limit test-

Limit = Means a value or amount that is likely to be present in a substance 
Test = To examine or investigate.

Defination- 

Limit test is defined as quantitative test designed to identify and control small quantities of impurity which is present in the pharmaceutical sample/substance.

Importance of limit test-

i.       Mainly it’s used for purity testing.

ii.       Identify impurities present in substance

iii.     Control the impurities which are present in compound

iv.     Determine the amount of impurity which are present in compound

 Limit test of chloride-

Limit test of chloride is used for identifying the amount of chloride ion present in a standard solution.

Apparatus-

• Nessler cylinder
• Beaker
• Glass rod
• Nessler's cylinder stand

Chemical-

• 1.0gm sample
• Distilled water
• Nitric acid
• Silver nitrate (5 Percent W/V)
• 0.01N HCL
• Sodium chloride (0.05845 Percent)

Principle-

The limit test for chloride is based on a reaction between silver nitrate and sodium chloride where silver chloride is insoluble in dilute nitric acid.

 

 In this test, a standard solution and test solution is prepared and then appearance compared with each other solution.

Test solution- Turbidity form (Silver chloride + dil. Nitric acid) amount of chloride show thick turbidity.

Where known amount of chloride ions is obtained and compared with a standard solution on black background in nessler’s cylinder.

The sample passed if the test solution is turbid than the standard solution or fails then vice versa.

Procedure-

Result- The sample passed if it is less turbid than the standard solution.

Limit test for sulphate-

This test is applied for controlling sulphate impurity in organic substance.

 Apparatus-

• Nessler’s cylinder
• Glass rod
• Nessler cylinder stand

Chemical requirement-

• Hcl
• Barium Sulphate reagent
• Potassium Sulphate
• Distilled water

Principle- This test is based on a reaction between barium chloride and sulphate in the presence of dilute Hydrochloric acid turbidity formed and then compared with standard solution.

The test solution passes if turbidity is less than standard solution vice versa.

Procedure-

Result- Test solution is passed if less turbidity than the standard solution on black background.

Preparation of barium sulphate reagent-

• 15ml of 0.5M barium chloride(122.1gm/1000ml) + 55 ml of water

• Add 20ml sulphate free alcohol + 5ml 0.0181%w/v K₂SO₄

•  Dilute up to 100ml water

Limit Test of Iron-

This test is applied for controlling iron impurity in organic substance.

Principle-

In which reaction iron react with thioglycolic acid with ammonium citrate buffer.

Resulting in the formation of purple colour solution due to the formation of ferrous mercapto acetate and ferric ion.

This purple colour is compared with the standard colour.

Chemical Reaction- 

Glassware-

• Test tube
• Test tube stand
• Pipette

Chemical requirement-

• Citric acid
• Thioglycolic acid
• Ammonia solution
• Iron solution
• distilled water
• Standard iron solution (ferric ammonium sulphate solution)

Procedure-

Result- Colour intensity are compared with both test, Test solution should be less than the standard solution.

NOTE- Turbidity not develop

Compared both solution immediate after 5 minute.

Limit Test for Arsenic (Gutzeit Test) -

This test is done for controlling arsenic impurity in organic substances.

Principle-

In this test arsenic is converted to arsine (A_sH₃). By transfigure reduction with zinc and hydrochloride acid ( stannated hydrochloride acid).  

When arsine comes in contact with dry mercuric chloride/bromide paper then its produce yellow/ brown stain.

The test solution is compared with standard solution with known amount of arsenic.
Chemical requirements-
·       Potassium iodide
·       Stannous chloride
·       Zinc AST ( Aspartate aminotransferase)
·       Arsenic trioxide
·       Sodium hydroxide
Apparatus-
·       120ml Wide mouthed flask/bottle with rubber bang
·       200mm long glass test tube ( internal diameter 6.5mm, external diameter 8mm)
·       At lower end of a test tube 1mm upper side a 2mm hole in diameter inside of          the tube. The upper end of the tube is rounded.
·       Two rubber bungs (25mm), are fitted on a center of test tube hole 6.5 mm               diameter. A spring clips holds the both bungs tightly together.
·       The gas released passes through the side hole and the lower hole act as an exit        for water in test tube.

Procedure-

 Result- Stain produced and both are compared with each other the test sample is passes if less intense than standard solution. 
Precautions-

• The standard stain should be compared immediate
• Arsenic chloride paper should be store in dark place.
• Test tube should be cleaned with hydrochloric acid AST.

 LIMIT TEST FOR HEAVY METALS-

This test is carried out for determining the metallic impurities by sulphide ion under specific condition.

Principle-

In this reaction heavy metals involves the reaction between a solution of heavy 
metals and saturated metals ( hydrogen sulphide) in acidic medium. 

A reddish/black colour is obtained and compared with standard lead nitrate solution.

Chemical requirements-

• Lead solution
• Acetic acid
• Hydrogen sulphide
• Nitric acid
• Sulfuric acid
• Hydro chloric acid
• Dil. acetic acid

Glassware and instrument:

• Nessler’s cylinder
• Nessler’s cylinder stand
• Crucible
• Muffle furnace
• Water bath
• Dropper

Procedure-

1.    Method A – This method is applicable for those substance which produce clear and colourless solution.   

2) Method B:- This method is suitable for those substances which are not producing clear and colourless solution.