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2.2.20. Potentiometric Titration 2.2.21. Fluorimetry

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2.2.20. Potentiometric Titration 2.2.21. Fluorimetry EUROPEAN PHARMACOPOEIA 5.0 2.2.22. Atomic emission spectrometry electrode or 2 indicator electrodes) immersed in the solution Method.Dissolvethesubstancetobeexaminedinthe to be examined and maintained at a constant potential solvent or mixture of solvents prescribed in the monograph, difference as a function of the quantity of titrant added. transferthesolutiontothecellorthetubeofthefluorimeter The potential of the measuring electrode is sufficient to and illuminate it with an excitant light beam of the ensure a diffusion current for the electroactive substance. wavelength prescribed in the monograph and as near as Apparatus. The apparatus comprises an adjustable voltage possible monochromatic. source and a sensitive microammeter; the detection system Measure the intensity of the emitted light at an angle of generally consists of an indicator electrode (for example, 90° to the excitant beam, after passing it through a filter a platinum electrode, a dropping-mercury electrode, a which transmits predominantly light of the wavelength of rotating-disc electrode or a carbon electrode) and a reference the fluorescence. Other types of apparatus may be used electrode (for example, a calomel electrode or a silver-silver provided that the results obtained are identical. chloride electrode). For quantitative determinations, first introduce into the A three-electrode apparatus is sometimes used, consisting of apparatus the solvent or mixture of solvents used to dissolve an indicator electrode, a reference electrode and a polarised the substance to be examined and set the instrument to zero. auxiliary electrode. Introduce the standard solution and adjust the sensitivity Method. Set the potential of the indicator electrode as of the instrument so that the reading is greater than 50. If prescribed and plot a graph of the initial current and the the second adjustment is made by altering the width of the values obtained during the titration as functions of the slits, a new zero setting must be made and the intensity of quantity of titrant added. Add the titrant in not fewer than the standard must be measured again. Finally introduce 3 successive quantities equal to a total of about 80 per cent the solution of unknown concentration and read the result of the theoretical volume corresponding to the presumed on the instrument. Calculate the concentration cx of the equivalence point. The 3 values must fall on a straight substance in the solution to be examined, using the formula: 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 c titration. x = concentration of the solution to be examined, For amperometric titration with 2 indicator electrodes, the cs = concentration of the standard solution, whole titration curve is recorded and used to determine the Ix = intensity of the light emitted by the solution to end-point. be examined, 01/2005:20220 Is = intensity of the light emitted by the standard solution. 2.2.20. POTENTIOMETRIC TITRATION Iftheintensityofthefluorescenceisnotstrictlyproportional to the concentration, the measurement may be effected using In a potentiometric titration the end-point of the titration a calibration curve. is determined by following the variation of the potential difference between 2 electrodes (either one indicator In some cases, measurement can be made with reference electrode and one reference electrode or 2 indicator to a fixed standard (for example a fluorescent glass or a electrodes) immersed in the solution to be examined as a solution of another fluorescent substance). In such cases, function of the quantity of titrant added. the concentration of the substance to be examined must be determined using a previously drawn calibration curve The potential is usually measured at zero or practically zero under the same conditions. current. Apparatus. The apparatus used (a simple potentiometer or electronic device) comprises a voltmeter allowing readings 01/2005:20222 to the nearest millivolt. The indicator electrode to be used depends on the substance 2.2.22. ATOMIC EMISSION to be determined and may be a glass or metal electrode (for SPECTROMETRY example, platinum, gold, silver or mercury). The reference electrode is generally a calomel or a silver-silver chloride Atomic emission spectrometry is a method for determining electrode. the concentration of an element in a substance by measuring For acid-base titrations and unless otherwise prescribed, the intensity of one of the emission lines of the atomic a glass-calomel or glass-silver-silver chloride electrode vapour of the element generated from the substance. The combination is used. determination is carried out at the wavelength corresponding to this emission line. Method. Plot a graph of the variation of potential difference as a function of the quantity of the titrant added, continuing Apparatus. This consists essentially of an atomic generator the addition of the titrant beyond the presumed equivalence of the element to be determined (flame, plasma, arc, etc.), point. The end-point corresponds to a sharp variation of a monochromator and a detector. If the generator is a potential difference. 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. 2.2.21. FLUORIMETRY Method. Operate an atomic emission spectrometer in accordance with the manufacturer’s instructions at the Fluorimetry is a procedure which uses the measurement prescribed wavelength setting. Introduce a blank solution of the intensity of the fluorescent light emitted by the into the atomic generator and adjust the instrument reading substance to be examined in relation to that emitted by a to zero. Introduce the most concentrated reference solution given standard. and adjust the sensitivity to obtain a suitable reading. GeneralNotices(1)applytoallmonographsandothertexts 35 2.2.23. Atomic absorption spectrometry EUROPEAN PHARMACOPOEIA 5.0 Determinations are made by comparison with reference ensure that the solvent does not interfere with the stability solutions with known concentrations of the element to oftheflame.Whenafurnaceisused,substancesmaybe be determined either by the Method of Direct Calibration introduced dissolved in water R or an organic solvent, but (Method I) or the Method of Standard Additions (Method II). with this technique, solid sampling is also possible. METHOD I - DIRECT CALIBRATION The atomic vapour may also be generated outside the spectrometer, for example, the cold vapour method for Prepare the solution of the substance to be examined (test mercury or certain hydrides. For mercury, atoms are solution) as prescribed. Prepare not fewer than 3 reference generated by chemical reduction and the atomic vapour is solutions of the element to be determined the concentrations swept by a stream of an inert gas into an absorption cell of which span the expected value in the test solution. Any mounted in the optical path of the instrument. Hydrides are reagents used in the preparation of the test solution are either mixed with the gas feeding the burner or swept by added to the reference solutions at the same concentration. an inert gas into a heated cell in which they are dissociated Introduce the test solution and each reference solution into into atoms. the instrument at least 3 times and record the steady reading. Method. Operate an atomic absorption spectrometer in Rinse the apparatus with blank solution each time and accordance with the manufacturer’s instructions at the ascertain that the reading returns to its initial blank value. prescribed wavelength setting. Introduce a blank solution Prepare a calibration curve from the mean of the readings into the atomic generator and adjust the instrument reading obtained with the reference solutions and determine the so that it indicates maximum transmission. Introduce concentration of the element in the test solution from the the most concentrated reference solution and adjust the curve so obtained. sensitivity to obtain a suitable absorbance reading. METHOD II - STANDARD ADDITIONS Determinations are made by comparison with reference solutions with known concentrations of the element to Add to at least 3 similar volumetric flasks equal volumes be determined either by the Method of Direct Calibration of the solution of the substance to be examined (test (Method I) or the Method of Standard Additions (Method II). solution) prepared as prescribed. Add to all but one of the flasks progressively larger volumes of a reference solution METHOD I - DIRECT CALIBRATION containing a known concentration of the element to be Prepare the solution of the substance to be examined (test determined to produce a series of solutions containing solution) as prescribed. Prepare not fewer than 3 reference steadily increasing concentrations of that element known solutions of the element to be determined the concentrations to give responses in the linear part of the curve. Dilute the of which span the expected value in the test solution. Any contents of each flask to volume with solvent. reagents used in the preparation of the test solution are Introduce each of the solutions into the instrument at least added to the reference and blank solutions at the same 3 times and record the steady reading.
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