1. Thioacetamide method:
Applicable to drugs dissolved in water, dilute acid and ethanol, the most commonly used method.
Principle: Pb2++H2S?PbS?+2H+. Hydrolysis of thioacetamide under weakly acidic conditions (pH=3.5) produces hydrogen sulfide, which generates a yellow to brownish-black sulfide suspension with heavy metal ions, which is compared to the color presented by a certain amount of standard lead solution treated by the same method.
The range of suitable color comparison is 10~20?gPb/27ml, and the pH value has a big influence on the color presentation.
The treatment method when the test solution has a color, commonly used external color elimination method - dilute caramel solution.
Reasons for using thioacetamide test solution: H2S test solution, there is a bad smell and toxicity; unstable; easy to be oxidized and precipitated sulfur; concentration is difficult to control. Thioacetamide in the experimental conditions of hydrolysis to produce H2S, and heavy metal ions color.
2. Thioacetamide method after incandescence:
Applicable to organic drugs containing aromatic rings, heterocycles and insoluble in water, dilute acids and ethanol.
Heavy metals and aromatic rings, heterocycles can form a strong valence bond, can be destroyed by incandescence, so that the heavy metals free, and then according to the first method of inspection. The use of sulfuric acid as an organic destructive agent, the temperature at 500 ~ 600 ℃ so that the complete ashing. The resulting residue plus nitric acid to further destroy, evaporated. After treatment with nitric acid, must be evaporated to remove all NO. Add hydrochloric acid into a water-soluble chloride, compared with the control test.
Organic drugs containing sodium or fluorine changed to platinum crucibles or hard glass (onyx) evaporating dishes, for example, norfloxacin contains fluorine.
3. Sodium sulfide method:
Applicable to drugs that are soluble in alkali and insoluble in dilute acid or in dilute acid that produces a precipitate.
Sodium sulfide as a coloring agent, Pb2 + and S2 - the role of PbS particles generated by the suspension, and a certain amount of standard lead solution by the same method of color comparison. Sodium sulfide has a certain degree of corrosiveness to the glass, should be used in a new system.
4. Microporous membrane method:
Suitable for containing 2~5?g heavy metal impurities and colored test solution check.
When the heavy metal limit is low, it is difficult to observe with the Nano colorimeter, filtered through a microporous membrane, heavy metal sulfide deposits in the filter membrane to form a color spot, compared with the standard lead spot, which can improve the sensitivity of the test.
Practice questions: (indefinite choice)
1. Check the heavy metals of the method ( ).
(A) Gouachette's method (B) Thioacetamide
(C) Sodium sulfide method (D) Microporous filter membrane method
(E) Thiocyanate method
Answer: BCD.
2. The incorrect statement about the thioacetamide method is ( ).
(A) It is a method of checking for chlorides
(B) It is a method of checking for heavy metals
(C) The reaction results in black color as background
(D) It is hydrolyzed under weakly acidic conditions and produces hydrogen sulphide
(E) The reaction should have a pH of 7-8
Answer: AE.