Physical Chemistry - Lab Report Example

Quantitative Determination of Cu and Pb in Brass Alloy Partner’s TA’s Sample ID Introduction Primarily, the main purpose of this lab experiment involved quantitative determination of heavy metals in the brass alloy, specifically copper and lead metals. In the context of material chemistry, determining the quantitative composition of alloys is significant in evaluating the actual properties of an alloy; hence deducing the most suitable uses of the material in either mechanical or chemical applications. Brass is a widely used alloy comprising mainly of copper and zinc. In an effort to enhance certain properties of the brass alloy, traces of metals like lead are occasionally added. Technically, incorporation of lead into the brass increases the mechanical properties the alloy. On the other hand, substantial concentration of copper imparts germicidal and antimicrobial properties into the brass alloyi. In laboratory settings, methods used to quantitatively determine the amounts of heavy elements in alloys include Gravimetric analysis, Anodic Stripping Voltammetry, and Atomic Absorbance Spectroscopy among others. Each of these quantitative determination methods possesses particular advantages, setbacks, and accuracy and precision levels. In the experiment, Atomic Absorption Spectroscopy was used to measure the amounts of lead and copper in brass. Atomic Absorption Spectroscopy is a versatile quantitative technique capable of analyzing presence of over 70 elements. Technically, this technique functions by exploiting the atomization properties in form of electronic transition of elements’ atoms in gaseous phases. After atomization, light absorption level of atoms is detected and Beer’s law of absorbance vs. concentration is used to determine the quantity of analytes in the atomized samples. Besides Atomic Absorbance Spectroscopy, the experiment employed Gravimetric analysis in the determination of lead in brass. Despite being one of the oldest methods in quantitative analysis, the gravimetric technique remains accurate and precise when used for determination of metals with insoluble salts like lead. Finally, the experiment employed Anodic Stripping Voltammetry in determination of copper. Anodic Stripping Voltammetry relies on the electric conductivity of analytes, and is specifically suited for quantitative analysis of minute traces of heavy metalsii. Subsequent sections of this lab report contain procedural steps, results and discussions pertaining to application of the three aforementioned quantitative determination techniques. Experimental Procedures i. Determination of Cu and Pb in Brass using Atomic Absorbance Spectroscopy Based on theoretical procedures provided, stock solutions of unknown brass, lead and copper were prepared. First, 1.0001g of brass was weighed and diluted with 10 ml of water and 15 ml of nitric acid. The mixture was heated until all brass dissolved. Traces of hydrated stannic oxide were removed by filtration. Finally, the solution with a dilution factor of 10 was quantitatively transferred into a 500 ml volumetric flask, and 3 replicate samples obtained for subsequent steps. In a similar manner, stock solutions of Pb2+ and Cu2+ were prepared. The Pb2+ and Cu2+ stock solutions containing lead and copper within the ranges of 0-50 ppm were used to create calibration curves of absorbance vs. concentration. In the cathode lamps, absorbance of lead was monitored at 283.2 nm while that of copper was monitored at 324.8 nm. Once the standard absorbance of Pb2+ and Cu2+ had been established, samples of the unknown brass solution were introduced into the hollow cathode lamps, and absorbance monitored. Subsequently, linear regression analysis was used to derive the slope and intercept of the absorbance vs. concentration curves of Pb2+ and Cu2+ in the unknown brass solution. Finally, concentrations of Pb2+ and Cu2+ in the unknown brass samples were calculated using the calibration curves. All data and results were documented and tabulated within the results section. ii. Determination of Pb in Brass using Gravimetric Analysis First, unknown brass was digested using concentrated nitric acid. Three samples of unknown brass weighing 1.0021 g, 1.0043g and 1.0016g were weighed and diluted with 15 ml of hot nitric acid until all brass dissolved. Subsequently, 4 ml of sulfuric acid were added to each dissolved sample, and heated to remove fumes of SO3. At this step, addition of sulfuric acid selectively precipitate lead sulfate in solid form, leaving the sulfates of copper and other metals in aqueous formsiii. Finally, the solid lead sulfate was filtered out and transferred into dried filter crucibles. Masses of each crucible and Celite before and after placement of individual lead sulfate samples were recorded. Subsequently, mass percent of lead in the lead sulfate and the unknown brass were calculated and tabulated within the report’s results section. iii. Determination of CU in Brass using Anodic Stripping Voltammetry First, stock solutions of copper and unknown brass were prepared by diluting the solid samples with 4% acetic acid. Next, calibration curve for copper was developed by plotting concentration vs. area of copper samples within the range of 10-40 ppm. Approximately 25 ml of the unknown brass solution was transferred into a 50 ml beaker. Subsequently, few drops of Hg2+ were added, and the mixture transferred into the assembled electrochemistry apparatus. From the apparatus, voltage peaks for the solution’s conductivity were scanned and used to construct a voltammogram calibration curve at different copper concentrations. Linear regression was used to determine the slope and intercept of the calibration line. Finally, the previously developed calibration curve was used to determine the concentration of copper in brass unknown. Mass percentage of copper in the brass and other relevant data from the procedures were recorded and tabulated within the results section. Data and Results Table 1: Determination of Cu in Brass using Atomic Absorbance Spectroscopy Mass of brass 1.0001 g Sample dilution factor 10 Volume of stock 500 ml Number of replicates 3 Calibration values Cu/ppm Abs 0 0.0005 10 0.2351 20 0.4434 30 0.6298 40 0.8008 50 0.9762 Cu/ppm in unknown brass = 30.85 Absorbance = 0.6273 Calibration curve Standard deviation 0.9392 Relative standard deviation 0.0304 Mass of Cu in unknown brass 0.1542 Mass % 15.4229% 95% confidence level 1.3037 Tables 2: Determination of Pb in Brass using Atomic Absorbance Spectroscopy Mass of brass 1.0001 g Sample dilution factor 10 Volume of stock 500 ml Number of replicates 3 Calibration values Pb/ppm Absorbance 0 -0.0023 10 0.0500 20 0.1012 30 0.1499 40 0.1912 50 0.2383 Pb/ppm in unknown brass = 33.57 Absorbance = 0.1624 Calibration curve Standard deviation 0.6098 Relative standard deviation 0.0182 Mass of Pb in unknown brass 0.1678 Mass % 16.7829% 95% confidence level 0.8464 Table 3: Determination of Pb in Brass using Gravimetric Analysis Trial Brass mass (g) Mass of empty crucible (with Celite) Mass of crucible + sample (with Celite) Mass of Pb (g) Mass % of Pb 1 1.0021 19.9829 20.1474 0.1654 16.4155 2 1.0043 30.2762 30.4227 0.1465 14.5873 3 1.0016 35.2804 35.4362 0.1558 15.5551 Averages Average = 0.1556 Average = 15.5193 Standard deviation 0.7207 Relative standard deviation 0.0300 95% confidence level 1.0119 Table 4: Determination of Cu in Brass using Anodic Stripping Voltammetry Mass of brass 1.0006 g Sample dilution factor 15.38 Volume of stock 500 ml Number of replicates 3 Calibration values Cu/ppm Area 10 62.24 20 223.3 30 314.3 35 419.0 40 483.7 Cu/ppm in unknown brass = 21.59 Area = 226.2 Calibration curve Standard deviation 1.1264 Relative standard deviation 0.0522 Mass of Cu in unknown brass 0.1660 Mass % 16.5903 95% confidence level 2.7549 Discussion Cu and Pb in Brass using Atomic Absorbance Spectroscopy Based on the results obtained from the Atomic Absorbance Spectroscopy method, the concentration of Cu in brass was 30.85 ppm, while the same method indicated that the concentration of Pb in brass was 33.57 ppm. In this case, the mass percent of copper in the unknown brass was 15.4229%, whereas the mass percent of lead in the brass sample was 16.7829%. In both analytes, values were reported at 95% confidence level. The 95% confidence intervals for Cu and Pb were 1.3037 and 0.8464 respectively. In this context, the Atomic Absorbance Spectroscopy indicated that the actual mass of copper in brass lies between 0.1542 g ± 1.3037, while the actual mass of Pb in brass lies between 0.1678 g ± 0.8464. The relatively small confidence intervals in both metals signified presence of few experimental errorsiv. Standard deviation of Cu in brass was 0.9392, while the resultant relative standard deviation was 0.0304 or 3.04%. In Pb, the values of standard deviation and relative standard deviation were 0.6098 and 0.0182 or 1.82% respectively. The small SD and RSD indicates that individual concentration values obtained through the Atomic Absorbance Spectroscopy technique were closely clustered around the means; hence the technique had a high level of precision. Pb in Brass using Gravimetric Analysis Gravimetric analysis indicated that the mass percent of lead in the unknown brass was 15.5193%. At 95% confidence level, the interval for this technique was 1.0119. In this context, the actual mass of Pb in brass lies between 0.1556 g ± 0.6119. Also, the sizable confidence interval indicates the effects of significant errors during performance of the gravimetric analysis experiment. The standard deviation is 0.7207, indicating high relative closeness of individual values from the mean. Cu in Brass using Anodic Stripping Voltammetry Based on the calibration curve, it emerged that the average concentration of copper in brass was 21.59 ppm. The Anodic Stripping Voltammetry technique yielded a mass percent of 16.5903%. The average mass of copper in brass was found to be 0.1660. At 95% confidence level, the interval for values obtained stood at 2.7549. In this context, Anodic Stripping Voltammetry technique showed that the actual mass of copper in brass lies between 0.1660 g ± 2.7549. Undeniably, the large confidence interval portrayed the severe effects of experimental errors. Values for the standard deviation and relative standard deviation are 1.1264 and 0.0522 or 5.22% respectively, indicating substantial level of dispersion from the mean, and a high level of imprecision in the data set. Comparisons Atomic Absorbance Spectroscopy vs. Anodic Stripping Voltammetry With respect to evaluation of results conducted above, it emerged that the concentrations and mass percent of Cu and Pb from each method varied significantly. For example, the Atomic Absorbance Spectroscopy showed that the concentration of Cu in brass was 30.85 ppm while its mass percent was 15.4229%. Contrarily, the Anodic Stripping Voltammetry method showed that the concentration of Cu in brass was 21.59 ppm, while the mass percent of Cu in brass was 16.5903%. Apparently, Atomic Absorbance Spectroscopy showed a higher concentration of Cu in brass compared to the Anodic Stripping Voltammetry method. In the determination of Cu in brass, the Atomic Absorbance Spectroscopy, which yielded a 95% confidence interval of ± 1.3037 and a relative standard deviation of 3.04%, was more accurate and precise compared to Anodic Stripping Voltammetry, which had a 95% confidence interval of ± 2.7549 and a relative standard deviation of 5.22%. In this context, the small confidence interval coupled with the minute relative standard deviation supports the high reliability and precision level of values obtained through the Atomic Absorbance Spectroscopy methodv. Atomic Absorbance Spectroscopy vs. Gravimetric Analysis Gravimetric analysis indicated that the mass percent of lead in brass was 15.5193%, while Atomic Absorbance Spectroscopy showed that the mass percent of lead in brass was 15.4229%. In addition, the Gravimetric method yielded confidence interval and relative standard deviation values of ± 1.0119 and 3.006% respectively. Contrarily, Atomic Absorbance Spectroscopy yielded confidence interval and relative standard deviation values of ± 0.8464 and 1.82% respectively. Technically, a RSD of 1.82% indicates a high level of precision compared to a RSD of 3.006%. Moreover, a 95% confidence interval of ± 0.8464 is more convincing that an interval of ± 1.0119. In this case, Atomic Absorbance Spectroscopy technique proved to be more reliable and precise compared to the Gravimetric analysis method. Experiment’s Limitations and Probable Sources of Errors Based on the comparison section, it emerged that Atomic Absorbance Spectroscopy was more precise and accurate compared to either Gravimetric analysis or Anodic Stripping Voltammetry. On the other hand, Gravimetric analysis yielded low values of confidence interval and standard deviations compared to the Anodic Stripping Voltammetry method. Therefore, Gravimetric analysis was more superior in the quantitative determination process compared to the Anodic Stripping Voltammetry method. One limitation for this experiment was time. It took approximately 28 days to sufficiently complete all procedures. In this case, time was a key limiting factor. Probable sources of errors which enhanced the imprecision of Anodic Stripping Voltammetry over the other methods include but not limited to parallax during acquisition of voltage readings, and effects of environmental parameters like temperature and contaminants on conductivity of copper in the unknown brass solutionvi. Apparently, Atomic Absorbance Spectroscopy, which relies on light intensity during acquisition of readings, did not suffer considerable effects from parallax and environmental sources of errors. In addition, gravimetric analysis was free from most errors, except the inevitable effects of instrumental resolution. Conclusion In conclusion, it is evident that each technique used in quantitative determination of heavy metals in the brass alloy possesses unique merits and demerits. Expectedly, the level of accuracy and precision obtained from each method vary considerably. In this regard, the merits possessed by Atomic Absorbance Spectroscopy enhanced its level of reliability in quantitative determination of copper and lead contained by brass. On the other hand, the other two methods; Gravimetric analysis and Anodic Stripping Voltammetry, were relatively instrumental in the experiment, but yielded results with lower levels of precision compared to the Atomic Absorbance Spectroscopy method. Otherwise, all the three methods were responsive in indicating the quantities of copper and lead in brass. Reference List Abigail, S.; Chang, N. Quantitative Analysis: Techniques in Environmental Chemistry. Elsevier: New York, 2013. Eider, G. Basic Laboratory Principles in General Chemistry: With Quantitative Techniques. Hunt Publishing Company: Indianapolis, 2010. Lindsey, P. Trace Environmental Quantitative Analysis: Principles, Techniques and Applications, 2nd Ed.; CRC Press: Pittsburg, 2010. Maria, C. Environmental Sampling and Analysis for Technicians and Analytical Chemists. Cengage Learning: New York, 2008. Smith, D. Determination of Heavy Metals in Unknown Samples by Spectroscopic Methods: Theory and Practice. Journal of Micro-Chemistry, 2013, 74 (3), 289-297. Stoddard, T. J. Quantitative Experiments in General Chemistry: An Illustrative Guide. Green and Company: London, 2008. Read More