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Pharmacy Studies: Topical agent and GIT
Pharmacy Studies: Limits tests
- Limit test for chloride
- Limit test for sulphate
- Limit test for iron
- Limit test for lead
- Limit test for arsenic
Limit test for sulphate
Limit test of sulphate is based on the reaction of soluble sulphate with barium chloride in presence of dilute hydrochloric acid to form barium sulphate which appears as solid particles (turbidity) in the solution.
Procedure:
| Test sample | Standard compound |
| Specific weight of compound is dissolved in water or solution is prepared as directed in the pharmacopoeia and transferred in Nessler cylinder | Take 1ml of 0.1089 % W/V solution of potassium sulphate in Nessler cylinder |
| Add 2ml of dilute hydrochloric acid | Add 2ml of dilute hydrochloric acid |
| Dilute to 45 ml in Nessler cylinder | Dilute to 45 ml in Nessler cylinder |
| Add 5ml of barium sulphate reagent | Add 5ml of barium sulphate reagent |
| Keep aside for 5 min | Keep aside for 5 min |
| Observe the Turbidity | Observe the Turbidity |
Barium sulphate reagent contains barium chloride, sulphate free alcohol and small amount of potassium sulphate.
Observation:
The turbidity produce in sample solution should not be greater than standard solution. If turbidity produces in sample solution is less than the standard solution, the sample will pass the limit test of sulphate and vice versa.
Reasons:
Hydrochloric acid helps to make solution acidic.
Potassium sulphate is used to increase the sensitivity of the test by giving ionic concentration in the reagent
Alcohol helps to prevent super saturation.
Limit test for iron
Principle:
Limit test of Iron is based on the reaction of iron in ammonical solution with thioglycollic acid in presence of citric acid to form iron thioglycolate which is pale pink to deep reddish purple in color.
Procedure:
Test sample
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Standard compound
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Sample is dissolved in specific amount of water and then volume is made up to 40 ml
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2 ml of standard solution of iron diluted with water upto 40ml
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Add 2 ml of 20 % w/v of citric acid (iron free)
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Add 2 ml of 20 % w/v of citric acid (iron free)
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Add 2 drops of thioglycollic acid
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Add 2 drops of thioglycollic acid
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Add ammonia to make the solution alkaline and adjust the volume to 50 ml
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Add ammonia to make the solution alkaline and adjust the volume to 50 ml
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Keep aside for 5 min
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Keep aside for 5 min
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Color developed is viewed vertically and compared with standard solution
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Color developed is viewed vertically and compared with standard solution
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Earlier aamonium thiocyanate reagent was used for the limit test of iron. Since thioglycolic acid is more sensitive reagent, it has replaced ammonium thiocyanate in the test.
Observation:
The purple color produce in sample solution should not be greater than standard solution. If purple color produces in sample solution is less than the standard solution, the sample will pass the limit test of iron and vice versa.
Reasons:
Citric acid helps precipitation of iron by ammonia by forming a complex with it.
Thioglycolic acid helps to oxidize iron (II) to iron (III).
Ammonia to make solution alkaline
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.
Arsenic, present as arsenic acid in the sample is reduced to arsenious acid by reducing agents like potassium iodide, stannous acid, zinc, hydrochloric acid, etc. Arsenious acid is further reduced to arsine (gas) by hydrogen and reacts with mercuric chloride paper to give a yellow stain.
H3AsO4 + H2SnO2 → H3AsO3 + H2SnO3
Arsenic acid Arsenious acid
H3AsO3 + 3H2 → AsH3 + 3H2O
Arsenious acid Arsine
The depth of yellow stain on mercuric chloride paper will depend upon the quality of arsenic present in the sample.
Procedure:
Test solution:
The test solution is prepared by dissolving specific amount in water and stannated HCl (arsenic free) and kept in a wide mouthed bottle.
To this solution 1 gm of KI, 5 ml of stannous chloride acid solution and 10 gm of zinc is added (all this reagents must be arsenic free)
Keep the solution aside for 40 min and stain obtained on mercuric chloride paper is compared with standard solution.
Standard solution:
A known quantity of dilute arsenic solution is kept in wide mouthed bottle and rest procedure is followed as described in test solution.
B ： glass tube with 6.5 mm inner diameter
C and D ： a ground joint glass tube with 6.5 mm inner diameter and 18 mm outer diameter at the joint. Inner joint and the outer joint form a concentric circle.
E ： rubber stopper
F ： narrow part of the glass tube B. Glass wool is inserted up to this part.
G ： rubber board (Lead acetate cotton plug)
H ： clamp
Reasons:
Stannous chloride is used for complete evolution of arsine
Zinc, potassium iodide and stannous chloride is used as a reducing aget
Hydrochlorid acid is used to make the solution acidic
Lead acetate pledger or papers are used to trap any hydrogen sulphide which may be evolved along with arsine.
Limit test for chloride:
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.
Procedure:
| Test sample | Standard compound |
| Specific weight of compound is dissolved in water or solution is prepared as directed in the pharmacopoeia and transferred in Nessler cylinder | Take 1ml of 0.05845 % W/V solution of sodium chloride in Nessler cylinder |
| Add 1ml of nitric acid | Add 1ml of nitric acid |
| Dilute to 50ml in Nessler cylinder | Dilute to 50ml in Nessler cylinder |
| Add 1ml of AgNO3 solution | Add 1ml of AgNO3 solution |
| Keep aside for 5 min | Keep aside for 5 min |
| Observe the Opalescence/Turbidity | Observe the Opalescence/Turbidity |
Observation:
The opalescence produce in sample solution should not be greater than standard solution. If opalescence produces in sample solution is less than the standard solution, the sample will pass the limit test of chloride and visa versa.
Reasons:
Nitric acid is added in the limit test of chloride to make solution acidic and helps silver chloride precipitate to make solution turbid at the end of process.
Limit test for lead
Lead is a most undesirable impurity in medical compounds and comes through use of sulphuric acid, lead lined apparatus and glass bottles use for storage of chemicals.
Principle:
Limit test of lead is based on the reaction of lead and diphenyl thiocabazone (dithizone) in alkaline solution to form lead dithizone complex which is read in color.
Dithizone is green in color in chloroform and lead-dithizone complex is violet in color, so the resulting color at the end of process is red.
Procedure:
Observation:
The intensity of the color of complex, is depends on the amount of lead in the solution. The color produce in sample solution should not be greater than standard solution. If color produces in sample solution is less than the standard solution, the sample will pass the limit test of lead and vice versa.
Reasons:
Ammonium citrate, potassium cyanide, hydroxylamine hydrochloride is used to make pH optimum so interference and influence of other impurities have been eliminated.
Phenol red is used as indicator to develop the color at the end of process
Lead present as an impurities in the substance, gets separated bye extracting an alkaline solution with a dithizone extraction solution.
Limit test of lead is based on the reaction of lead and diphenyl thiocabazone (dithizone) in alkaline solution to form lead dithizone complex which is read in color.
Dithizone is green in color in chloroform and lead-dithizone complex is violet in color, so the resulting color at the end of process is red.
Procedure:
| Test sample | Standard compound |
| A known quantity of sample solution is transferred in a separating funnel | A standard lead solution is prepared equivalent to the amount of lead permitted in the sample under examination |
| Add 6ml of ammonium citrate | Add 6ml of ammonium citrate |
| Add 2 ml of potassium cyanide and 2 ml of hydroxylamine hydrochloride | Add 2 ml of potassium cyanide and 2 ml of hydroxylamine hydrochloride |
| Add 2 drops of phenol red | Add 2 drops of phenol red |
| Make solution alkaline by adding ammonia solution. | Make solution alkaline by adding ammonia solution. |
| Extract with 5 ml of dithizone until it becomes green | Extract with 5 ml of dithizone until it becomes green |
| Combine dithizone extracts are shaken for 30 mins with 30 ml of nitric acid and the chloroform layer is discarded | Combine dithizone extracts are shaken for 30 mins with 30 ml of nitric acid and the chloroform layer is discarded |
| To the acid solution add 5 ml of standard dithizone solution | To the acid solution add 5 ml of standard dithizone solution |
| Add 4 ml of ammonium cyanide | Add 4 ml of ammonium cyanide |
| Shake for 30 mins | Shake for 30 mins |
| Observe the color | Observe the color |
Observation:
The intensity of the color of complex, is depends on the amount of lead in the solution. The color produce in sample solution should not be greater than standard solution. If color produces in sample solution is less than the standard solution, the sample will pass the limit test of lead and vice versa.
Reasons:
Ammonium citrate, potassium cyanide, hydroxylamine hydrochloride is used to make pH optimum so interference and influence of other impurities have been eliminated.
Phenol red is used as indicator to develop the color at the end of process
Lead present as an impurities in the substance, gets separated bye extracting an alkaline solution with a dithizone extraction solution.
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