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These concentrations were compared against the average absorbance in the four plots illustrated in the results section. According to Beer’s Law, the slope of each of the lines shown is equivalent to the kconstant for each standard at each wavelength. These constants were found to be .001500 for manganese at 440nm and .0283 for manganese at 545nm. The chromium ion was calculated to have a kvalue of .003700 at 440nm and .00009000 at 545nm. The absorbance of the unknown at each of the wavelengths was taken for three different trials, for trials one, two, and three, the absorbance of the unknown at 440nm was recorded as .51. In trial one, the absorbance at 545nm was calculated to be .273, and to be .26 for trials two and three. Using these values, along with the kvalues derived fromeach of the linear functions, the concentration of each ion was found. The concentration of Mn in trials two and three was calculated as 8.76 mg/L , and 9.22 mg/L for trial one, giving an average of 8.913 mg/L. In trials two and three, the concentration of chromium deduced to be 134.3 mg/L, and 134.1 mg/L for trial one, giving an average of 134.2 11
mg/L. The standard deviation for manganese was .2656, giving a relative standard deviation of 2.98%. The standard deviation for chromium was .1074, resulting in a relative standard deviation of .08%. Conclusions & RecommendationsThe results analyzed in the discussion prove the hypothesis to only be partly true. The relative standard deviation for the manganese ion was 2.48% greater than hypothesized, while the relative standard deviation for the chromium ion was .42% lowerthan hypothesized. Relative standard deviation is used to measure precision by illustrating the percent difference between the data and the mean. The calculated relative standard deviations suggest that the data for the manganese concentration varied almost six times more than the data did for the chromium concentration. Despite this difference in RSD, the low percentages suggest that the experiment is most likely repeatable. An extremely significant gross error occurred while performing this experiment, preventing any real comparison an accepted value from taking place. The wrong concentration of H2SO4was used to dilute the samples, while the correct concentration was used for the blank solution. This discrepancy resulted in a spectrophotometer that was incorrectly calibrated, and solutions that were far too concentrated. As previously mentioned, this is a gross error, and in analytical analysis, gross errors require the experiment to be scratched, and for the procedure to be restarted.Aside from eliminating gross error, a number of changes to the procedure and materials used in the experiment could lead to more accurate results. The cuvettes used inthis experiment were made of plastic. Glass cuvettes have been found to give more 12
accurate absorbance readings because they interact with the incident light significantly less than plastic cuvettes. Plastic cuvettes are more likely to host impurities, which can absorb light and skew recordings. Along with this change, it is recommended that the boiling times for each of the samples be decreased by at least one minute. The excess time left to boil results in solutions that are too concentrated, leading to higher absorbances that aren’t accurate.13