A limit test is a test used to identify and control the small amount of impurities present in a substance. It does not find the exact quantity of the impurity; instead it checks whether the amount of impurity is within a permitted (allowed) limit. Limit tests are very important in pharmacy and pharmaceutical chemistry, because medicines must not contain impurities above a safe level. The test is usually done by comparing the test sample with a standard solution that contains a known, permitted amount of the impurity.
A limit test checks and controls the small amount of impurity in a substance.
It does not find the exact quantity; it checks if impurity is within the allowed limit.
It works by comparing the test sample with a standard of known impurity.
Chlorides are tested with silver nitrate (opalescence).
Sulphates are tested with barium chloride (turbidity).
The arsenic limit test is also called the Gutzeit test.
Limit tests are essential for quality control of medicines.
Definition: A limit test is a quantitative or semi-quantitative test designed to identify and control small amounts of impurities in a substance, and to check that the amount of impurity does not exceed a specified limit.
Principle: The limit test works by comparing the test sample with a standard. A known small amount of the impurity is taken as the 'standard'. The same reagents are added to both the test sample and the standard under the same conditions. Both develop a colour, turbidity (cloudiness) or opalescence.
• If the colour/turbidity of the test sample is less than or equal to that of the standard, the substance passes (impurity is within the limit). • If it is more than the standard, the substance fails (impurity is above the limit).
Common limit tests are carried out for the following impurities:
Thus, limit tests are an essential part of quality control in the pharmaceutical industry.
A limit test is a test used to identify and control the small amount of impurities present in a substance. It does not measure the exact quantity of the impurity; rather, it checks whether the amount of impurity is within a permitted limit. It is widely used in pharmacy to ensure that medicines do not contain harmful impurities above a safe level.
The principle of a limit test is comparison with a standard. The same reagents are added to both the test sample and a standard solution that contains a known, permitted amount of the impurity. Both develop a colour or turbidity. If the test sample's colour or turbidity is not more than the standard's, the substance passes; if it is more, the substance fails.
The common limit tests are for chlorides (using silver nitrate), sulphates (using barium chloride), iron (using thioglycollic acid), heavy metals (as coloured sulphides compared with a lead standard), and arsenic (the Gutzeit test, which produces a yellow stain). Each test detects a specific impurity by comparing colour or turbidity with a standard.
Limit tests are important because they ensure the purity and quality of medicines and chemicals by keeping harmful impurities within safe limits. They are part of the standards laid down in pharmacopoeias, are simple and quick to perform, do not need costly instruments, and help maintain uniform drug quality from batch to batch.
Why Are Inert Gases Placed in Group Zero (Group 18)?
Inert gases are placed in Group Zero (Group 18) because they have zero valency due to completely filled electron shells. Learn the reason with elements and FAQs.
Why Cannot Each Metal React with Its Own Salt Solution?
A metal cannot displace itself from its own salt solution because displacement requires a more reactive metal. Learn the activity series rule with examples and FAQs.
Why Is an Atom Electrically Neutral?
An atom is electrically neutral because the number of protons (positive) equals the number of electrons (negative), making the net charge zero. Learn with FAQs.
Why Is Potassium Nitrate Classified as an Electrolyte — Explained
Why is potassium nitrate classified as an electrolyte? KNO₃ dissociates completely into K⁺ and NO₃⁻ ions in water, conducting electricity. Strong electrolyte explained.
Why Is Sodium Hydrogen Carbonate (NaHCO₃) a Basic Salt?
Sodium hydrogen carbonate (NaHCO₃) is a basic salt because it gives an alkaline solution in water. HCO₃⁻ hydrolyses to form OH⁻. Learn the reason with FAQs.
Turn this guide into revision flashcards, a practice exam, or an AI-generated podcast — free, no signup required.