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2.2.22. Atomic emission spectrometry EUROPEAN PHARMACOPOEIA 5.0 electrode or 2 indicator electrodes) immersed in the solution to be examined and maintained at a constant potential difference as a function of the quantity of titrant added. The potential of the measuring electrode is sufficient to ensure a diffusion current for the electroactive substance. Apparatus. The apparatus comprises an adjustable voltage source and a sensitive microammeter ; the detection system generally consists of an indicator electrode (for example, a platinum electrode, a dropping-mercury electrode, a rotating-disc electrode or a carbon electrode) and a reference electrode (for example, a calomel electrode or a silver-silver chloride electrode). A three-electrode apparatus is sometimes used, consisting of an indicator electrode, a reference electrode and a polarised auxiliary electrode. Method. Set the potential of the indicator electrode as prescribed and plot a graph of the initial current and the values obtained during the titration as functions of the quantity of titrant added. Add the titrant in not fewer than 3 successive quantities equal to a total of about 80 per cent of the theoretical volume corresponding to the presumed equivalence point. The 3 values must fall on a straight line. Continue adding the titrant beyond the presumed equivalence point in not fewer than 3 successive quantities. The values obtained must fall on a straight line. The point of intersection of the 2 lines represents the end-point of the titration. For amperometric titration with 2 indicator electrodes, the whole titration curve is recorded and used to determine the end-point. Method. Dissolve the substance to be examined in the solvent or mixture of solvents prescribed in the monograph, transfer the solution to the cell or the tube of the fluorimeter and illuminate it with an excitant light beam of the wavelength prescribed in the monograph and as near as possible monochromatic. Measure the intensity of the emitted light at an angle of 90° to the excitant beam, after passing it through a filter which transmits predominantly light of the wavelength of the fluorescence. Other types of apparatus may be used provided that the results obtained are identical. For quantitative determinations, first introduce into the apparatus the solvent or mixture of solvents used to dissolve the substance to be examined and set the instrument to zero. Introduce the standard solution and adjust the sensitivity of the instrument so that the reading is greater than 50. If the second adjustment is made by altering the width of the slits, a new zero setting must be made and the intensity of the standard must be measured again. Finally introduce the solution of unknown concentration and read the result on the instrument. Calculate the concentration cx of the substance in the solution to be examined, using the formula : cx = concentration of the solution to be examined, cs = concentration of the standard solution, Ix = intensity of the light emitted by the solution to be examined, 01/2005:20220 Is = intensity of the light emitted by the standard solution. 2.2.20. POTENTIOMETRIC TITRATION In a potentiometric titration the end-point of the titration is determined by following the variation of the potential difference between 2 electrodes (either one indicator electrode and one reference electrode or 2 indicator electrodes) immersed in the solution to be examined as a function of the quantity of titrant added. The potential is usually measured at zero or practically zero current. Apparatus. The apparatus used (a simple potentiometer or electronic device) comprises a voltmeter allowing readings to the nearest millivolt. The indicator electrode to be used depends on the substance to be determined and may be a glass or metal electrode (for example, platinum, gold, silver or mercury). The reference electrode is generally a calomel or a silver-silver chloride electrode. For acid-base titrations and unless otherwise prescribed, a glass-calomel or glass-silver-silver chloride electrode combination is used. Method. Plot a graph of the variation of potential difference as a function of the quantity of the titrant added, continuing the addition of the titrant beyond the presumed equivalence point. The end-point corresponds to a sharp variation of potential difference. If the intensity of the fluorescence is not strictly proportional to the concentration, the measurement may be effected using a calibration curve. In some cases, measurement can be made with reference to a fixed standard (for example a fluorescent glass or a solution of another fluorescent substance). In such cases, the concentration of the substance to be examined must be determined using a previously drawn calibration curve under the same conditions. 01/2005:20222 2.2.22. ATOMIC EMISSION SPECTROMETRY Atomic emission spectrometry is a method for determining the concentration of an element in a substance by measuring the intensity of one of the emission lines of the atomic vapour of the element generated from the substance. The determination is carried out at the wavelength corresponding to this emission line. Apparatus. This consists essentially of an atomic generator of the element to be determined (flame, plasma, arc, etc.), a monochromator and a detector. If the generator is a flame, water R is the solvent of choice for preparing test and reference solutions, although organic solvents may also be 01/2005:20221 used if precautions are taken to ensure that the solvent does not interfere with the stability of the flame. Method. Operate an atomic emission spectrometer in 2.2.21. FLUORIMETRY accordance with the manufacturer’s instructions at the prescribed wavelength setting. Introduce a blank solution Fluorimetry is a procedure which uses the measurement into the atomic generator and adjust the instrument reading of the intensity of the fluorescent light emitted by the to zero. Introduce the most concentrated reference solution substance to be examined in relation to that emitted by a and adjust the sensitivity to obtain a suitable reading. given standard. General Notices (1) apply to all monographs and other texts 35