Unit -1 Impurities in Pharmaceutical substances

 

•  A compound is said to be impure if it is having foreign matter i.e. impurities 
•  impurities is define as the foreign particle that affect the purity of a substance .

  Source of Impurities

 1.Raw material used in pharmaceutical preparation:

            Generally most of the raw materials used in the manufacturing of pharmaceutical products are naturally occurring substances which contain other materials also, e.g. rock salt contain small amounts of  Cuso4 and Mgso4 .So NaCl prepared from this source will be impure .Hence, proper measures should be taken to ensure the purity of raw material used in the manufacture.

 

2.Methode of Manufacture:

          Manufacture process involves reactions carried out in single or multiple steps.Those step all serve as the source of impurity they are.

    a)Reagents used in starting material: 

        Many reagents are used in the process of manufacture of a  product .If the products are not washed properly ,then these reagents can be present as impurities.

       Eg:Caco3 is prepared from cacl2 and Na2co3 can remain as impurity so it should be washed & removed properly.

        

   Na2CO3  + CaCl2 → CaCO3  + 2 NaCl 

     b)Reagents used to remove other impurity:

               Potassium bromide is liable to  contain traces of barium, which is added in the course of manufacturing process to remove excess sulphate.

                                       

                            2 K + Br₂ → 2 KBr

                            2Ba + (SO₄)₂ → 2BaSO₄  

 

       c)Solvent :

                 Water is the cheapest solvent available and used however it also serves as a source of many impurities 

 

1.  Tap Water : It contain Ca++,Mg²++,Na++,Cl-,So₄-² &Co₃²𐄐 as impurities in very small amounts.
2. Softened water: It is obtain by allowing tap water to pass through the sodium from of zeolite which removes divalent cations ( Ca++,Mg²++) but still other impurities will be present.
3. Demineralized water: By this process ,all  minerals are removed but still few organic materials may be present they act as impurity.
4. Distilled Water: It is considered to be beat because it is free  from all inorganic and organic impurities and it is pure
5. Reaction Vessel: During the manufacturing process, sometimes the solvents or reagent may under go chemical reaction with the vessel and contributes to impurities in the end product .

   3.Intermediates: 

             Most of the preparations involves generation of intermediates which may also remains as impurity in the final product when they do not got converted completely.

                       

                   3 I₂ (s) + 6 KOH → KIO₃  + 5 KI + 3 H₂O

                    KIO₃ + 3C → 2KI + 3CO

4.Atomospheric condition  

    Sometimes, the atmospheric condition like heat, temperature, pressure pH are not as per required and may yield impurities  

 

5.Manufacturing Hazards:

      Hazardous and toxic substances such as dust, dirt, points,fuel or chemicals presents in the work place are capable of causing harm to the pharmaceutical products ,and hence termed as manufacturing hazards, this can occur 

• Contamination due to particular matter: The presence of unwanted particulate matter like dust, glass, metallic ions, porcelain etc. can arise in a number of ways, causing contamination of the pharmaceutical items.
• Cross-Contamination: The handling of powders granules and tablet in large-bulk frequently creates a considerable amount of air borne. dust ,which if not controlled can lead to cross-contamination of products.
• Microbial contamination: Generally products like creams, topical application to broken skin or mucous membranes are liable to bacterial mould and fungal controlled either by doing sterlized test or by other methods.
• Error during manufacturing process: During the manufacturing of an item due to use of incorrect quantities of reagent or even solvent or due to incorrect methods of mixing filling tableting the outer can be hazardous.
• Packaging errors/Errors during package: To similar shape, colour and storing vessels ,sometimes there can be mistake while packing the drug and may lead to critical or  hazards outcome if used wrongly by a patient.

6.Storage condition: 

        The hazards may arise due to problem in storage condition like: 

• Chemical instability of the product: The drugs or medicine of final product may get disturbed and undergo chemical change. Usually it occurs due to light ,air, humidity or temperature.
•  Physical changes: the occurrence of change in physical form of the drug during storage is not unknown changes in crystal size and form agglomeration may occur with no changes in chemical properties .which make the drugs hazardous
• Reaction with the container: Pharmaceutical preparation when stored in inappropriate containers react with the material of the container and undergo deterioration . e.g. salicylic acid ointment shouldn't be packed in metal containers .atropine sulphate injection should not be stored in sod.
• Temperature: The rate at which chemical and also physical changes occurs in stored products is conditioned by temperature and hence the changes in temperature may trigger the change and cause the product to be harmful.

                        Hence ,not only the impurities but all hazard causing factors should be preparedly taken cause of while manufacturing the products.

 

Effect of Impurities    

1  Impurities may bring about incompatibility with other substance
2  Impurities may cause difficulties during formulations and use of the substance .
3   Impurities may lower the shelf life of the substance
4  Sometimes impurities changes the physical and chemical properties of the substances 
5  Therapeutic effect can be decreased 
6  shows toxic effect after a certain period 
7  injurious when present above certain limits .
8  It may changes odor ,colour, taste of the substance

Principle involved in Limit test for:

 

Limit Test of 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.

$AgN{O}_3(aq)+NaCl(aq)\to AgCl(s)+NaN{O}_3(aq)$

 

$Procedure:$

 

 

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 of 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.

 

 

 

 

Test sample	Standard compound
Sample is dissolved in specific amount of water and then volume is made up to 40 ml	2 ml of standard solution of iron diluted with water upto 40ml
Add 2 ml of 20 % w/v of citric acid (iron free)	Add 2 ml of 20 % w/v of citric acid (iron free)
Add 2 drops of thioglycollic acid	Add 2 drops of thioglycollic acid
Add ammonia to make the solution alkaline and adjust the volume to 50 ml	Add ammonia to make the solution alkaline and adjust the volume to 50 ml
Keep aside for 5 min	Keep aside for 5 min
Color developed is viewed vertically and compared with standard solution	Color developed is viewed vertically and compared with standard solution

 

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 alkalin

 

 

 

Limit Test of Sulphate

Principle:

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

 

 

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 of Arsenic

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.

A ： approximately 60 ml generator bottle with 40 ml indicating line.

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 aged
Hydrochloride 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 of 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.

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.

Lead : Click here

 

 Limit Test of Heavy Metals

Principle:

Limit test of heavy metals is based on the reaction of metallic impurities with hydrogen sulfide in acidic medium to form brownish colour solution. Metals that response to this test are lead, mercury, bismuth, arsenic, antimony, tin, cadmium, silver, copper, and molybdenum. The metallic impurities in substances are expressed as parts of lead per million parts of the substance. The usual limit as per Indian Pharmacopoeia is 20 ppm

Procedure:

The Indian Pharmacopoeia has adopted three methods for the limit test of heavy metals.

Method I: Use for the substance which gives clear colorless solution under the specific condition.

Test sample	Standard compound
Solution is prepared as per the monograph and 25 ml is transferred in Nessler’s cylinder	Take 2 ml of standard lead solution and dilute to 25 ml with water
Adjust the pH between 3 to 4 by adding dilute acetic acid ‘Sp’ or dilute ammonia solution ‘Sp’	Adjust the pH between 3 to 4 by adding dilute acetic acid ‘Sp’ or dilute ammonia solution ‘Sp’
Add 2 ml of potassium cyanide and 2 ml of hydroxylamine hydrochloride	Add 2 ml of potassium cyanide and 2 ml of hydroxylamine hydrochloride
Dilute with water to 35 ml	Dilute with water to 35 ml
Add freshly prepared 10 ml of hydrogen sulphide solution	Add freshly prepared 10 ml of hydrogen sulphide solution
Dilute with water to 50 ml	Dilute with water to 50 ml
Allow to stand for five minutes	Allow to stand for five minutes
View downwards over a white surface	View downwards over a white surface

Observation:

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 heavy metals and vice versa.

Method II: Use for the substance which do not give clear colorless solution under the specific condition. 

Test sample	Standard compound
Weigh specific quantity of test substance, moisten with sulphuric acid and ignite on a low flame till completely charred Add few drops of nitric acid and heat to 500 °C Allow to cool and add 4 ml of hydrochloric acid and evaporate to dryness Moisten the residue with 10 ml of hydrochloric acid and digest for two minutes Neutralize with ammonia solution and make just acid with acetic acid	Take 2 ml of standard lead solution and dilute to 25 ml with water
Adjust the pH between 3 to 4 and filter if necessary	Adjust the pH between 3 to 4 by adding dilute acetic acid ‘Sp’ or dilute ammonia solution ‘Sp’
Dilute with water to 35 ml	Dilute with water to 35 ml
Add freshly prepared 10 ml of hydrogen sulphide solution	Add freshly prepared 10 ml of hydrogen sulphide solution
Dilute with water to 50 ml	Dilute with water to 50 ml
Allow to stand for five minutes	Allow to stand for five minutes
View downwards over a white surface	View downwards over a white surface

Observation:

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 heavy metals and vice versa.

Method III: Use for the substance which gives clear colorless solution in sodium hydroxide solution.

Test sample	Standard compound
Solution is prepared as per the monograph and 25 ml is transferred in Nessler’s cylinder or weigh specific amount of substance and dissolve in 20 ml of water and add 5 ml of dilute sodium hydroxide solution	Take 2 ml of standard lead solution
Make up the volume to 50 ml with water	Add 5 ml of dilute sodium hydroxide solution and make up the volume to 50 ml with water
Add 5 drops of sodium sulphide solution	Add 5 drops of sodium sulphide solution
Mix and set aside for 5 min	Mix and set aside for 5 min
View downwards over a white surface	View downwards over a white surface

Observation:

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 heavy metals and vice versa.

 

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