Gravimetric Determination of Calcium Report

Gravimetric Determination of Calcium Report CHEM 320: Gravimetric Determination of Calcium Fall 2015 Relevant Text Material: Gravimetric Analysis: Chapter 27 (Sections 27-1 to 27-3) Tools of the Trade: Chapter 2 Common Indicators: Table 11-3 (or 10-3 depending on edition) Statistics: Chapter 4 Introductory Notes: Ca2+ (aq) + C2O4 2- (aq) + H2O (l) CaC2O4 ? H2O (s) Soluble calcium can be quantitatively analyzed by precipitation as calcium oxalate monohydrate. Consider the acid dissociation equilibria of oxalic acid, H2C2O4. H2C2O4 H+ + HC2O4 - pKa1 = 1.25 HC2O4 - H+ + C2O4 2- pKa2 = 4.27 Very little precipitate will form in acidic solution since the equilibrium would favor the formation of oxalic acid. Large, easily filtered, relatively pure crystals of product will be obtained if the precipitation is carried out homogeneously by slowly raising the pH. This can be done by dissolving Ca2+ and C2O4 2- in acidic solution and increasing the pH gradually by thermal decomposition of urea. (H2N)2CO + 3 H2O CO2 + 2 NH4 + + 2 OHurea generation of base Part 1: Quantitative Analysis of Calcium in an Unknown Work individually, and perform the analysis in triplicate. Bring three Gooch crucibles to constant weight. Be sure that each crucible has a unique penmark or other distinguishing feature. Place a filter inside of each crucible, and oven-dry them for at least 2 hr at ~105 ?C. There are two ovens in the quant lab. Cool the crucibles in a desiccator for ~30 min and weigh them on the analytical balance. Repeat the procedure with 30-min heating periods until successive weighings of the same crucible-filter agree within 0.3 mg. Use a paper towel or tongs, not your fingers, to handle the crucibles. The unknown. Use a few small portions of your unknown to rinse a 25-mL transfer pipet. Discard the washings. Use a rubber bulb, not your mouth, to provide suction. Transfer exactly 25.00 mL of unknown to each of three 400-mL beakers, and dilute each with ~75 mL of 0.1 M HCl. Add 5 drops of the previously prepared methyl red indicator solution to each beaker. This indicator is red below pH 4.8 and yellow above pH 6.0 (refer to Table 10-3). Heat Precipitation. Add ~25 mL of the prepared ammonium oxalate solution (~40 g/L in 0.3 M HCl) to each beaker while stirring with a glass rod. Remove the rod and rinse it into the beaker with a wash bottle. Add ~15 g of solid urea to each sample, cover it with a watchglass, and boil gently for ~30 minutes until the indicator turns yellow. Filtration. When filtering, run a few drops of water through the filter so that it is firmly stuck to the bottom of the crucible, and do not forget to anchor the flask! Filter each hot solution through a previously weighed crucible-filter, using suction in a manner similar to that shown in Fig. 2-17. Add ~3 mL of ice-cold water to the beaker, and use a rubber policeman to quantitatively transfer the remaining solid to the funnel. Repeat this procedure with small portions of ice-cold water until all of the precipitate has been transferred. Finally, use two 10-mL portions of ice-cold water to rinse each beaker, and pour the washings over the precipitate. Weighing. Dry the precipitate, first with aspirator suction for 1 min, then in an oven at 105 ?C for ~1 hr. Bring each crucible to constant weight. The product is somewhat hygroscopic, so only one sample should be removed from the desiccator at a given time. Weighings should be done rapidly. Handle crucibles containing precipitate carefully. Remember that the oil of fingerprints can be sufficient to change the mass of a crucible by 0.0001 g or more. For each gravimetric experiment, calculate the molarity of Ca2+ in your unknown solution. Share your results with the class by writing the three concentration values on the table on the lab door. Complete this portion of the work as soon as possible! Statistics. Use a spreadsheet to facilitate your calculations. Report the average molarity, standard deviation, relative standard deviation, as well as the 95% confidence interval of the molarity for your unknown. Repeat the statistical evaluation with the data obtained by the whole class. Statistically speaking, how do your values compare with those obtained by the class? Also, can any of the individual measurements be removed from the class data set? What can you say about the true concentration of calcium in the unknown solution? Construct appropriate tables and/or graphs for summarizing the data in your report. Part 2: Gravimetric Analysis Interference Tests. When working with “real” samples, as opposed to your fabricated one, a serious concern is whether any other elements present in the sample will interfere with the gravimetric determination. To investigate candidates for interference, prepare 7 test tubes. Each labeled test tube should contain: 1 mL of a test solution [Na2CO3, KCl, MgCl2, Cu(NO3)2, Ba(NO3)2, Fe(NO3)2, Zn(NO3)2] 3 mL of 0.1 M HCl 1 drop of methyl red indicator 1 mL of ammonium oxalate solution Add ~0.65 g urea, and heat 25-30 min in beaker of gently boiling water. Which cations interfere? Compare your results with Table 27-1 in your text. GRAVIMETRIC ANALYSIS LABORATORY REPORT 40 Points, Due by the beginning of lab on Tuesday September 15 Submit your report (both Word and Excel files) electronically, and hand in the lab notebook pages at the beginning of lab. Quality always counts any time you express your ideas in writing. Use standard formatting with double-spaced lines and one-inch margins (11 or 12-point font). Your report should have progressed through at least two drafts before it is turned in. Always take pride in your work. Title: Include an original title for the experiment as well as your name and the date. Introduction: (1 paragraph) Acquaint the reader to with the subject by presenting pertinent background information. Experimental: In your own words, provide a concise description of the procedures that were carried out. By reading this section, another chemist should be able to reproduce your results. Results and Discussion: (either combined into one section or presented separately). Present and discuss the results via text, tables, and possibly figures. Investigate the relationships between your actual data and what you expected your data to look like. This section should end with a concise conclusion. Contact your instructor if you have any questions. A    B    C Crucible Wt.    15.60197 g    17.9815 g    18.1142 g Crucible + Calcium Wt.    16.2204 g    18.6020 g    18.7333 g Calcium Wt.    0.61843 g    0.6205 g    0.6191 g Calcium M    0.1693 M    0.1699 M    0.1695 M My Data Test Tubes    Is it precipitated     Color Na2CO3    No    - KCl    No    yellow Cu(NO3)2    Yes    Blue Ba(NO3)2    YES    White MgCl2    NO    - Fe(NO3)2    Yes    brown Zn(NO3)2    Yes    - Class data, Calcium gravimetric analysis: student 1    0.1686    0.1681    0.1691 2    0.1692    0.1690    0.1694 3    0.1694    0.1692    0.1694 4    0.1682    0.1698    0.1697 5    0.1632    0.1650    0.1618 6    0.1697    0.1694    0.1699 7    0.1686    0.1692    0.1666 8    0.1687    0.1678    0.1661 9    0.1687    0.1703    0.1683 10    0.16631    0.16217    0.16207 11    0.1697    0.1660    0.1667 12    0.1693    0.1699    0.1695 13    0.1674    0.1697    0.1697 14    0.1679    0.1684    0.1672 15    0.1697    0.1691    0.1667 16    0.1689    0.1686    0.1678