The following methods are based on the measurement of intensity of spectral lines emitted by elements such as sodium, potassium, calcium, etc. The substance containing the element is dissolved in an appropriate solvent (usually water) and subjected to excitation in a flame of appropriate temperature and composition.
Several instruments of suitable selectivity are available. The manufacturer's instructions for the operation of the instrument should be strictly followed.
Unless otherwise directed in the individual monograph, one or the other of the following methods may be used. In Method A, measurements are made by comparison of sample solutions with solutions containing a known amount of the element being analysed. Method B is suitable for samples that contain very small quantities of the element to be analysed or where there is interference from other elements.
Prepare a series of standard solutions containing the element to be determined in increasing concentration within the concentration range recommended for the particular instrument used. Choose the appropriate filters or adjust the monochromator to select the wavelength prescribed in the monograph. Spray water into the flame and adjust the galvanometer reading to zero. Spray the most concentrated standard solution into the flame and adjust the sensitivity so that a full-scale deflection of the galvanometer is recorded.
Again spray water into the flame and when the galvanometer reading is constant readjust it to zero. Spray each standard solution into the flame three times, recording the steady galvanometer readings obtained and washing the apparatus thoroughly with water after each spraying. Prepare a calibration curve by plotting the mean of each group of three readings against the concentration. Prepare the solution of the substance under examination as prescribed in the monograph and adjust the strength, if necessary, to bring it into the range of concentration recommended for the instrument used. Spray the solution into the flame three times, recording the galvanometer readings and washing the apparatus thoroughly with water after each spraying. Using the mean of the galvanometer readings, determine the concentration of the element being examined from the calibration curve. To confirm the concentration thus obtained, repeat the operations with a standard solution of the same concentration as that of the solution under examination.
Place in each of not fewer than three similar volumetric flasks equal volumes of the solution of the substance under examination prepared as prescribed in the monograph. Add to all but one of the flasks a measured amount of the prescribed standard solution to produce a series of solutions containing regularly increasing amounts of the element to be determined. Dilute the contents of each flask to the required volume with water. Prepare the flame photometer in the manner described under Method A, using water for the adjustment to zero and the solution with the largest amount of added element to adjust the sensitivity so that full scale deflection galvanometer is recorded. Examine each solution three times and plot the mean of the readings against concentration on a graph whose origin or zero reading represents zero concentration of the added element. Extrapolate the straight line joining the points until it meets the extrapolated concentration axis at a point on the negative side. The distance between this point and the intersection of the axis represents the concentration of the element in the solution being examined.
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