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26 excited atoms. Recorded spectra are compared with calibration lines and their intensities are converted to concentrations. The method is capable of accurate measurements at low detection limits and good precision is achieved at these low levels (30). High dilution ratios used in testing increases the accuracy and measurement range of ions. The presence of different trace ions can influence the concentration of soluble ions entering into solution, and this needs to be taken in to consideration. This method identifies all elements in the system simultaneously, and the effects of the influence of trace elements are automatically taken into consideration in the analysis. EVALUATING TECHNIQUES USED IN AASHTO SULFATE TESTING Gravimetric Techniques Sulfate concentrations determined from gravimetric procedures are highly variable and are dependent on the original sulfate levels in the soils (32). The gravimetric technique is based on a weight comparison of soil and precipitated barium sulfate, and therefore experimental errors may be high when the amount of available sulfates in the soil is low. Due to the high standard deviation at low concentrations, repeatability of the test is low and significant repetitions are required to get an accurate estimate of sulfate content (36). Barium chloride, if added quickly into the solution, can precipitate out, which affects the precipitation of sulfates in solution. In gravimetric testing, barium sulfate must be precipitated under acidic conditions where barium carbonates and phosphates can also precipitate out in the solution. The solubility of barium sulfate is also greater under acidic conditions, which can also affect the test results. Moreover, co-precipitation of barium sulfate with other ions and encapsulation of suspended colloidal clay fractions in the precipitate can exaggerate the sulfate levels in soils during gravimetric testing. Again, barium sulfates precipitated from cold solutions are finely dispersed and cannot be completely retained on a Whatman No. 42 filter paper (47). Colorimetric Techniques Sulfate tests of soils conducted based on colorimetric measurements are accurate at lower sulfate ion concentrations (36). Colorimetry is a good, repeatable test when performed under exact standards. The colorimetric technique measures the concentration of ions based on the degree of absorption of radiation of a specific wavelength and assumes the concentration to be proportional to the absorption of light. The relation holds true only at low concentrations and hence the method has a low detection limit. For higher sulfate concentrations in soil, higher dilution ratios need to be used. Solubility of sulfates also increases as water content increases and therefore the use of a higher soil to water dilution ratio when compared to field conditions is effective in extracting all available sulfates for quantification purposes. Sulfate extraction using distilled water may not exactly reflect the dissolution of sulfates in natural water. Mineral dissolution will be slightly higher with distilled water, as the former is devoid of suspended charged ions. Colorimetric measurements also cannot compensate for the dark colors in solution. These may interfere with the measurement of precipitated barium sulfate in solution. Accuracy of measurement therefore depends largely on the effective removal of suspended impurities in solution as absorption of soluble salts can induce a small error in the results (32).
