+1 862 207 3288 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 ìrealî 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
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