Chapter 1. General methods and techniques of pharmaceutical drugs quality control

Identification means confirming with an acceptable degree of certainty that a drug, excipient, or material corresponds to the information on the label. Сhemical and physicochemical methods of analysis are used for this purpose.

Chemical methods (qualitative reactions) have a long history of use in pharmaceutical analysis. However, due to the rapid development of instrumental methods, chemical tests were largely displaced from practice. Nowadays, infrared spectroscopy is the priority method for confirming the identity of drugs. UV spectroscopy and chromatographic techniques are also widely used.

Chemical identification tests included up to date in Pharmacopoeias are relatively simple, they do not require expensive equipment, the conditions of their implementation are well studied and described. Therefore, qualitative chemical reactions are often more simple and reliable compare to instrumental methods of analysis.

Identification requires a chemical reaction to be specific, i.e., able to detect a substance or ion in the presence of other substances unambiguously. However, highly specific reactions are relatively rare, so group-selective tests are used more often. In this case, the test provides for additional steps to identify the substance unambiguously. For example, we can determine solubility or insolubility of the precipitate, color change or decolorization after a certain reagent is added. The test sensitivity is the minimum amount of a substance or an ion in a sample that can be detected under the specified test conditions. In the pharmaceutical analysis, when performing an identification test, sensitivity is less important than specificity and selectivity because the substance or the ion is usually present in high concentration in the sample.

Minor fluctuations and changes in any reaction conditions inevitably arise during laboratory practice. Reproducibility and ruggedness of the method are highly required. Therefore, the Pharmacopoeia always clearly and unambiguously regulates the order of reagents addition, concentrations of solutions and other reaction conditions. Moreover, the Pharmacopoeia provides short, but clear and comprehensive expected results descriptions.

Many drug substances contain the same ions or functional groups. There are unified methods of identification for these ions and groups. The general monograph “General identification tests” has been introduced to avoid repeating the same methodology in each monograph. This general monograph exists in all Pharmacopoeias. For example, if a drug contains an ion or a functional group with a unified method of identification, a monograph will refer to the methodology in the general monograph. In some cases, there is also a brief explanation for the test solution preparation.

Further, we will discuss the tests included in the general monograph “General identification tests” in SPhRF XIV, Ph. Eur. 10, and Ph. EAEU. Additionally, there will be given some remarks about similar tests particularities according to USP 43 and Ph. Int 10.

Acetates. Acetates are identified after esterification with ethanol. Ethyl acetate has the specific apple odor.

The test is described only in the SPhRF. This reaction identifies both acetate ion and organic acetyl group.

Another test for acetate ion is included in the SPhRF, the Ph. EAEU and the USP. Ferric (III) chloride solution is added to the neutral acetate solution that yields red solution of ferric acetate or hydroxyacetate, which is destroyed by the addition of mineral acids.

Fe³⁺ + 3CH₃COO^- ⇄ Fe(CH₃COO)₃

Fe(CH₃COO)₃ + H₂O → Fe(CH₃COO)₂(OH)↓ + CH₃COOH

There is one more test for acetate ion included in the USP, the Ph. Eur. and the EAEU. If lanthanum nitrate solution, diluted ammonia solution and iodine are added to acetate test solution, then heated until boiling, the formation of a blue precipitate or a blue coloration of the solution indicates a positive reaction. The mechanism of the reaction is not quite clear, but since iodine forms blue complexes with many colloidal solutions of substances, including starch, one theory is that the physical colloidal structure of the lanthanum basic acetate precipitate is responsible for the ability to form the complex.

The Ph. Eur. and the Ph. EAEU also suggest the reaction with oxalic acid. The test solution heated with the oxalic acid solution liberates acetic acid vapors indicated by the specific odor and acidic reaction.

Acetyl (acetylated substances). The test identifies the substance to be examined as a compound containing an acetyl group that upon acidic hydrolysis yields acetic acid. The substance undergoes hydrolysis when heated with phosphoric acid solution. The acetic acid vapor is condensed followed by the reaction with lanthanum nitrate and iodine (see Acetates). This test is included in the Ph. Eur., the Ph. Int. and the Ph. EAEU.

Aluminum. In the first step of analysis, aluminum salt solution is acidified with hydrochloric acid, and thioacetamide solution is added. No precipitate is formed. In the second step, sodium hydroxide solution is added dropwise. A gelatinous white precipitate is formed which dissolves on further addition of sodium hydroxide. Then ammonium chloride solution is added gradually. The gelatinous white precipitate is re-formed.

In the acidic environment, thioacetamide will liberate sulfide ions by hydrolyze, thus it is used as a source of S^2- ions.

Aluminum cations do not react with sulfide ions at the low pH, but heavy metal cations would react and corresponding sulfide will be precipitated.