DETERMINATION OF HEAVY METALS IN ALMAGEL AND ALMAGEL-A PREPARATIONS BY ANODIC INVERSION VOLTAMMETRY

Abstract

The risk of drug contamination with toxic metals during manufacture and storage necessitates the development of methods for determining trace amounts of such impurities in various preparations of mineral, plant, and synthetic origin. The State Pharmacopoeia (RSP-XI) [1] stipulates the drug analysis for heavy metals by a semiquantitative method based on the color reactions of such metals with sodium sulfide, after which the reaction mixture color is visually compared to the color of a standard solution containing a certain amount of lead ions. This method is characterized by low sensitivity, provides for a rather subjectively estimated lead content, and is incapable of detecting the presence of other metals influencing the safety of drugs. More detailed and precise analyses are usually performed by atomic absorption spectroscopy and HPLC, but these methods require expensive equipment not always available in small laboratories. In recent years, the analysis for toxic metals in various objects (environment, food products, raw materials, etc.) is frequently performed by the method of anodic inversion voltammetry (AIVA). The AIVA measurements, ensuring high sensitivity, selectivity, and reproducibility of analysis, are relatively simple and can be performed on readily available equipment [2 – 4]. In pharmaceutical analysis, the AIVA method is employed for monitoring the quality of pure water and plant raw materials with respect to heavy metals and for the quantitative determination of some antibiotics, vitamins [5, 6], and some other biologically active substances. The AIVA procedure involves two stages: (1) concentration of a substance (element) from a relatively large volume of solution within a small sample volume or on the electrode surface (with stirring at a constant potential); (2) electrochemical dissolution of the concentrated material and monitoring of the electrochemical characteristics (variation of current and potential). A useful signal in the AIVA measurements is the current variation (in the form of a peak) corresponding to dissolution of a concentrated substance deposited on the electrode surface during stage 1. This signal is recorded with the aid of a polarograph or a voltammetric analyzer. Under otherwise stationary conditions, the current peak height on the voltammetric (VA) curve is proportional to the concentration of dissolved ions, while the peak potential characterizes the nature of a given substance. An electrochemical cell for the AIVA measurements comprises a system of two electrodes (indicator and reference) in an electrolyte (background) solution containing the analyzed substance. Selection of the electrode type depends on the nature of the element to be determined. Metals such as Zn, Cd, Pb, and Cu, which are more electronegative than mercury, can be determined using a 10- to 20-m-thick mercury film electrode operating in a potential interval from + 0.15 to –1.6 V. The background electrolyte decreases the resistance of the solution, creates a buffer maintaining constant pH, and participates in the photochemical reactions of decomposition of the traces of organic substances under UV irradiation. The sensitivity of the AIVA technique with respect to microimpurities is characterized by a detection