These free B.Pharm notes on limit test for heavy metals cover chapter 6 of Pharmaceutical Inorganic and Analytical Chemistry (BP106T), a first-semester subject. Read the key points below, then study the complete chapter PDF. You can browse every chapter in the Pharmaceutical Inorganic and Analytical Chemistry category.
Table of Contents
Read the following document for full information
Overview of limit test for heavy metals
These notes cover the detection of heavy metal impurities by precipitation and colour reactions, and the limit test for heavy metals using thioacetamide. They include a table of acute and chronic toxicity of lead, mercury and arsenic, and identification tests for barium, calcium, lithium, magnesium, potassium and sodium. The notes end with the modified limit tests for chloride and sulphate in coloured samples.
Key points on limit test for heavy metals
- The limit test for heavy metals shows that metallic impurities coloured by hydrogen sulphide do not exceed the limit, given as micrograms of lead per gram.
- Thioacetamide gives hydrogen sulphide in an acetate buffer of pH 3.5, and the metals then form dark sulphides that are compared with standard lead.
- Lead is the commonest heavy metal, and its chronic toxicity includes anaemia, kidney dysfunction and encephalopathy.
- In the modified limit test for chloride, coloured KMnO4 is first decolourised with ethanol and the manganese dioxide is filtered off before testing.
- In the modified limit test for sulphate, ethanol removes colour and prevents supersaturation, while K2SO4 raises the sensitivity of the barium sulphate turbidity.
Relevance to pharmacy practice
Heavy metals such as lead, mercury and arsenic cause serious acute and chronic toxicity, so their levels in medicines must be strictly limited. The modified tests show a pharmacist how to apply limit tests to coloured substances that the normal method cannot handle.
Every chapter in this subject builds on the one before it, so do not skip topics that look simple. Definitions, principles, procedures and examples are the parts that examiners return to most often, and a clear understanding here will also make later subjects easier to follow in the coming semesters.
Frequently asked questions on limit test for heavy metals
Which reagent is used in the limit test for heavy metals?
Thioacetamide is used, because it gives hydrogen sulphide in an acetate buffer of pH 3.5, and the heavy metals then form dark brown sulphides.
How is barium identified?
Barium gives a yellow precipitate of barium chromate with potassium chromate in acetic acid, or a white barium sulphate precipitate with dilute sulphuric acid that is insoluble in nitric acid.
Why is a modified limit test needed for chloride in KMnO4?
KMnO4 is coloured and cannot be tested by the normal method, so it is decolourised with ethanol and the manganese dioxide is filtered off first.
Quick revision checklist
Use this self-test after reading the chapter on limit test for heavy metals. If you can do all of the following without looking at your notes, you are ready for the exam.
- I can explain: the principle of the heavy metals limit test
- I can explain: toxicity of lead, mercury and arsenic
- I can explain: identification tests for barium and calcium
- I can explain: tests for sodium, potassium and magnesium
- I can explain: modified limit tests for coloured samples
Study and exam tips
Start by reading the chapter once for the overall idea, then revise limit test for heavy metals using headings, definitions, principles, reactions and examples. Short-answer questions in the university exam usually ask for definitions, principles, procedures, reactions and uses, so write these in your own words and practise the equations, tables and flow charts that the chapter has.
For wider reading, standard books and open resources such as the NCBI Bookshelf help you confirm terms and see more examples. Revise limit test for heavy metals again a day later and again a week later, because spaced revision improves long-term memory.
This page is part of the Pharmaceutical Inorganic and Analytical Chemistry series for first-semester B.Pharm students on B.Pharm Guru, where every chapter has a short summary and a full PDF. Bookmark this page and share it with classmates who are preparing for the same exam.
Related notes: 8. MAJOR EXTRA AND INTRACELLULAR ELECTROLYTES, 9. ACID-BASE TITRATIONS.
Next: 7. ACID-BASE CHEMISTRY AND BUFFER SYSTEMS IN PHARMACY. Back to the Pharmaceutical Inorganic and Analytical Chemistry category.
