Unraveling Impurities in Pharmaceuticals: Understanding Their Sources, Effects, and the Significance of Limit Tests


As a pharmacy student, your journey into the world of pharmaceuticals is an exciting one. One crucial aspect of pharmaceutical manufacturing and analysis is the management of impurities. Impurities can significantly impact the safety, efficacy, and quality of pharmacopoeial substances, making it essential for pharmacy professionals to be well-versed in identifying, quantifying, and controlling these unwanted components. In this blog, we will delve into the world of impurities, their sources and effects, and the vital role limit tests play in ensuring pharmaceutical purity.

Understanding Impurities in Pharmaceuticals

Impurities in pharmaceutical substances refer to any unwanted components that accompany the active pharmaceutical ingredient (API) or finished product. These impurities can arise during the synthesis or manufacturing process, storage, or even due to environmental factors. The presence of impurities can alter the pharmacological activity, stability, and overall safety of the drug, making their control and detection of paramount importance.

Impurities in Pharmaceutical

Sources of Impurities

The type and amount of impurity present in pharmaceutical substances depend upon several factors:

  1. Raw material used in the manufacture
  2. Process used in the manufacture
  3. Material of the plant 
  4. Inadequate storage 
  5. Manufacturing hazards
  6. Deliberate adulteration

1. Raw material used in the manufacture: The raw materials used for the manufacturing of pharmaceutical products, often contain impurities. These impurities may come in the final product 


  • Metallic zinc may be present as impurity in Zinc oxide sample. 
  • Sodium chloride prepared from rock salt will almost contain traces of calcium and magnesium compounds.

2. Process used in the manufacture: Impurity comes during manufacturing process. 

For example: 

  • Tap water is frequently used in various manufacturing process. This tap water contains chloride, calcium and magnesium which may come as impurities in the final product. 
  • During manufacturing process, because of wide use of strong acids (HCl, H₂SO₄ ). Chloride and Sulphate ions are very commonly occurring impurities.

3. Material of the plant: The manufacturing equipment (or) utensils are made up of metals like copper, aluminium, iron or stainless steel. Due to solvent action on the equipment, the traces of metals are introduced as impurities.

 4. Inadequate storage: Stored products may be contaminated with dust, insects and even animal and insect excreta. Due to careless storage some chemical substances undergo chemical changes and decompose. Ex: Ferrous sulphate is slowly converted into insoluble ferric oxide by air and moisture.

5. Manufacturing hazards: Include Particulate contamination, process errors, cross contamination, microbial contamination, packing errors etc.

6. Deliberate adulteration: Mostly drug are mixed with cheaper drug. Ex: KBr is used as sedative is often mixed with NaBr

Limit Test

Limit tests play a vital role in pharmaceutical analysis by providing a simple and effective means of detecting specific impurities present in low concentrations. These tests help establish the maximum allowable level of a particular impurity that does not adversely affect the quality and safety of the drug product.

Let’s see some Example of Limit Test:

Limit Test For Chloride

Principle: Limit test for chloride is based upon the simple reaction between silver nitrate and soluble chlorides (if present in the sample) to give insoluble silver chloride in the presence of dilute nitric acid.

The insoluble silver chloride makes the solution opalescent and the extent of opalescence is compared with a standard opalescence produced in a standard solution having a known amount of chloride.

If the opalescence produced in the test is less intense  than that of standard opalescence, the sample passed the limit test for chloride and vice versa.

Chemical reaction:

Role of reagent: Dilute nitric acid is used to prevent the opalescence of other acid radicals with silver nitrate solution


Sample SolutionStandard Solution
1ml of sample is dissolved in water and transfer to a Nessler cylinder.Pipette out 1ml of standard NaCl solution and transfer into Nessler cylinder.
Add 10ml of dilute HNO3Add 10ml of dilute HNO3
Add 1ml of AgNO3 solution stir immediatedly with a glass rod.Add 1ml of AgNO3 solution stir immediately with a glass rod.
Dilute upto 45ml with water and kept aside for 5 minutes.Dilute upto 50ml with water and kept a side for 5 minutes.

Result: The opalescence produced by a given amount of the substance is compared with the standard opalescence. If the opalescence produced in the sample is less than the standard opalescence, the sample passes the limit test and vice versa

Limit Test For Sulphate

Principle: Limit test for sulphate depends upon the interaction of soluble sulphates (if present in the sample) with barium chloride in the presence of dilute hydrochloric acid to produce turbidity due to formation of insoluble barium sulphate (BaSO4) precipitate.

Chemical Reaction:

Role of Reagents: Barium Sulphate reagent (It consists of barium chloride (BaCl2), alcohol and a very small amount of Potassium Sulphate, KI)

  • Barium Chloride: To produce turbidity
  • Alcohol: Prevents super saturation and thereby produce uniform turbidity.
  • Potassium Sulphate (K2SO4) : Increase the sensitivity of the test by giving ionic concentration in the reagent.


Sample SolutionStandard Solution
Dissolve the specified quantity of a substance in water and transfer to a Nessler cylinderPipette out 1ml of 0.1089%w/v solution of Pot. Sulphate (standard solution) in Nessler cylinder
Add 2ml of dil. HCIAdd 2ml of dil. HCI
Add 5ml of Barium sulphate reagent stir immediately.Add 5ml of Barium sulphate reagent stir immediately
Dilute upto 45ml with water and kept aside for 5 minutes.Dilute upto 45ml with water and kept aside for 5 minutes.

Note: dil. HCl is added, except where HCI is used in the preparation of standard solution (or) test sample solution. Dil.HCI is added to dissolve other impurities like carbonates & phosphates are also present in the sample.

Result: The turbidity/precipitate produced by a given amount of the substances is compared with the standard turbidity. If the turbidity produced in the sample is less than the standard turbidity, the sample passes the limit test and vice versa.


In conclusion, understanding impurities in pharmaceuticals is crucial. By recognizing the sources and effects of impurities in pharmacopoeial substances and employing limit tests, pharmacy professionals can ensure the safety, efficacy, and quality of drugs they dispense to patients. The pursuit of purity in pharmaceuticals is a noble mission, and it begins with comprehensive knowledge and precise analytical techniques.
Remember, as future pharmacists, your commitment to maintaining the highest standards of pharmaceutical quality will have a significant impact on the health and well-being of patients worldwide. Embrace the responsibility with enthusiasm and always stay curious and open to learning, for the world of pharmacy is ever-evolving, and your journey has only just begun.

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