Pharma Easy Notes @Pharmacy

  🧪 Limit Test for Lead (Heavy Metals) Subject: Pharmaceutical Inorganic Chemistry Experiment Type: Qualitative Analysis Aim: To carry out the Limit Test for Lead in a given sample using dithizone reagent and to ensure that lead impurities are within the permissible limit as per I.P. standards. ✨ Principle Lead (Pb²⁺) in solution reacts with dithizone (diphenylthiocarbazone) in chloroform. A pink to violet color develops if lead is present. The intensity of color in the sample is compared with that of a standard lead solution. If the test solution shows less or no color compared to the standard, the sample passes the test. Reaction: P b 2 + + D i t h i z o n e ⟶ L e a d - D i t h i z o n a t e   C o m p l e x ( P i n k / V i o l e t ) Pb^{2+} + Dithizone \longrightarrow Lead\text{-}Dithizonate\ Complex (Pink/Violet) P b 2 + + D i t hi zo n e ⟶ L e a d - D i t hi zo na t e   C o m pl e x ( P ink / Vi o l e t ) ⚗️ Materials Required Test sample ...

📌 Aim: To carry out the limit test for heavy metals present in pharmaceutical substances as an impurity. 🎯 Principle The test is based on the reaction of heavy metals (like lead) with hydrogen sulfide (H₂S) in an acidic medium, producing a brownish-black coloration of metal sulfides . The intensity of this color is compared with a standard lead solution prepared simultaneously. 🧰 Materials Required Nessler cylinders Test sample solution Standard lead solution (Pb²⁺) Dilute acetic acid Hydrogen sulfide (H₂S) water or thioacetamide reagent 🧪 Procedure Take the sample solution in a Nessler cylinder. Add dilute acetic acid to adjust the pH. Add a few drops of thioacetamide reagent (or pass H₂S gas) . Prepare a standard solution of lead in another Nessler cylinder. Keep both cylinders side by side for comparison. 🔬 Reaction Involved P b 2 + + H 2 S    →    P b S ↓    ( b r o w n i s h − b l a c k p r e c i p i t a t e ) Pb^{2+} + H_2S \; \...